Commit | Line | Data |
---|---|---|
a0d0e21e | 1 | =head1 NAME |
d74e8afc | 2 | X<operator> |
a0d0e21e LW |
3 | |
4 | perlop - Perl operators and precedence | |
5 | ||
d042e63d MS |
6 | =head1 DESCRIPTION |
7 | ||
ae3f7391 | 8 | In Perl, the operator determines what operation is performed, |
ba7f043c | 9 | independent of the type of the operands. For example S<C<$x + $y>> |
db691027 | 10 | is always a numeric addition, and if C<$x> or C<$y> do not contain |
ae3f7391 ML |
11 | numbers, an attempt is made to convert them to numbers first. |
12 | ||
13 | This is in contrast to many other dynamic languages, where the | |
46f8a5ea | 14 | operation is determined by the type of the first argument. It also |
ae3f7391 | 15 | means that Perl has two versions of some operators, one for numeric |
ba7f043c KW |
16 | and one for string comparison. For example S<C<$x == $y>> compares |
17 | two numbers for equality, and S<C<$x eq $y>> compares two strings. | |
ae3f7391 ML |
18 | |
19 | There are a few exceptions though: C<x> can be either string | |
20 | repetition or list repetition, depending on the type of the left | |
0b55efd7 | 21 | operand, and C<&>, C<|>, C<^> and C<~> can be either string or numeric bit |
ae3f7391 ML |
22 | operations. |
23 | ||
89d205f2 | 24 | =head2 Operator Precedence and Associativity |
d74e8afc | 25 | X<operator, precedence> X<precedence> X<associativity> |
d042e63d MS |
26 | |
27 | Operator precedence and associativity work in Perl more or less like | |
28 | they do in mathematics. | |
29 | ||
30 | I<Operator precedence> means some operators are evaluated before | |
ba7f043c KW |
31 | others. For example, in S<C<2 + 4 * 5>>, the multiplication has higher |
32 | precedence so S<C<4 * 5>> is evaluated first yielding S<C<2 + 20 == | |
33 | 22>> and not S<C<6 * 5 == 30>>. | |
d042e63d MS |
34 | |
35 | I<Operator associativity> defines what happens if a sequence of the | |
36 | same operators is used one after another: whether the evaluator will | |
ba7f043c KW |
37 | evaluate the left operations first, or the right first. For example, in |
38 | S<C<8 - 4 - 2>>, subtraction is left associative so Perl evaluates the | |
39 | expression left to right. S<C<8 - 4>> is evaluated first making the | |
40 | expression S<C<4 - 2 == 2>> and not S<C<8 - 2 == 6>>. | |
a0d0e21e LW |
41 | |
42 | Perl operators have the following associativity and precedence, | |
19799a22 GS |
43 | listed from highest precedence to lowest. Operators borrowed from |
44 | C keep the same precedence relationship with each other, even where | |
45 | C's precedence is slightly screwy. (This makes learning Perl easier | |
46 | for C folks.) With very few exceptions, these all operate on scalar | |
47 | values only, not array values. | |
a0d0e21e LW |
48 | |
49 | left terms and list operators (leftward) | |
50 | left -> | |
51 | nonassoc ++ -- | |
52 | right ** | |
53 | right ! ~ \ and unary + and - | |
54310121 | 54 | left =~ !~ |
a0d0e21e LW |
55 | left * / % x |
56 | left + - . | |
57 | left << >> | |
58 | nonassoc named unary operators | |
59 | nonassoc < > <= >= lt gt le ge | |
0d863452 | 60 | nonassoc == != <=> eq ne cmp ~~ |
a0d0e21e LW |
61 | left & |
62 | left | ^ | |
63 | left && | |
c963b151 | 64 | left || // |
137443ea | 65 | nonassoc .. ... |
a0d0e21e | 66 | right ?: |
2ba1f20a | 67 | right = += -= *= etc. goto last next redo dump |
a0d0e21e LW |
68 | left , => |
69 | nonassoc list operators (rightward) | |
a5f75d66 | 70 | right not |
a0d0e21e | 71 | left and |
f23102e2 | 72 | left or xor |
a0d0e21e | 73 | |
3df91f1a DM |
74 | In the following sections, these operators are covered in detail, in the |
75 | same order in which they appear in the table above. | |
a0d0e21e | 76 | |
5a964f20 TC |
77 | Many operators can be overloaded for objects. See L<overload>. |
78 | ||
a0d0e21e | 79 | =head2 Terms and List Operators (Leftward) |
d74e8afc | 80 | X<list operator> X<operator, list> X<term> |
a0d0e21e | 81 | |
62c18ce2 | 82 | A TERM has the highest precedence in Perl. They include variables, |
5f05dabc | 83 | quote and quote-like operators, any expression in parentheses, |
a0d0e21e LW |
84 | and any function whose arguments are parenthesized. Actually, there |
85 | aren't really functions in this sense, just list operators and unary | |
86 | operators behaving as functions because you put parentheses around | |
87 | the arguments. These are all documented in L<perlfunc>. | |
88 | ||
ba7f043c | 89 | If any list operator (C<print()>, etc.) or any unary operator (C<chdir()>, etc.) |
a0d0e21e LW |
90 | is followed by a left parenthesis as the next token, the operator and |
91 | arguments within parentheses are taken to be of highest precedence, | |
92 | just like a normal function call. | |
93 | ||
94 | In the absence of parentheses, the precedence of list operators such as | |
95 | C<print>, C<sort>, or C<chmod> is either very high or very low depending on | |
54310121 | 96 | whether you are looking at the left side or the right side of the operator. |
a0d0e21e LW |
97 | For example, in |
98 | ||
99 | @ary = (1, 3, sort 4, 2); | |
100 | print @ary; # prints 1324 | |
101 | ||
ba7f043c | 102 | the commas on the right of the C<sort> are evaluated before the C<sort>, |
19799a22 GS |
103 | but the commas on the left are evaluated after. In other words, |
104 | list operators tend to gobble up all arguments that follow, and | |
a0d0e21e | 105 | then act like a simple TERM with regard to the preceding expression. |
19799a22 | 106 | Be careful with parentheses: |
a0d0e21e LW |
107 | |
108 | # These evaluate exit before doing the print: | |
109 | print($foo, exit); # Obviously not what you want. | |
110 | print $foo, exit; # Nor is this. | |
111 | ||
112 | # These do the print before evaluating exit: | |
113 | (print $foo), exit; # This is what you want. | |
114 | print($foo), exit; # Or this. | |
115 | print ($foo), exit; # Or even this. | |
116 | ||
117 | Also note that | |
118 | ||
119 | print ($foo & 255) + 1, "\n"; | |
120 | ||
d042e63d MS |
121 | probably doesn't do what you expect at first glance. The parentheses |
122 | enclose the argument list for C<print> which is evaluated (printing | |
ba7f043c | 123 | the result of S<C<$foo & 255>>). Then one is added to the return value |
d042e63d MS |
124 | of C<print> (usually 1). The result is something like this: |
125 | ||
126 | 1 + 1, "\n"; # Obviously not what you meant. | |
127 | ||
128 | To do what you meant properly, you must write: | |
129 | ||
130 | print(($foo & 255) + 1, "\n"); | |
131 | ||
5a0de581 | 132 | See L</Named Unary Operators> for more discussion of this. |
a0d0e21e | 133 | |
ba7f043c | 134 | Also parsed as terms are the S<C<do {}>> and S<C<eval {}>> constructs, as |
54310121 | 135 | well as subroutine and method calls, and the anonymous |
a0d0e21e LW |
136 | constructors C<[]> and C<{}>. |
137 | ||
5a0de581 | 138 | See also L</Quote and Quote-like Operators> toward the end of this section, |
da87341d | 139 | as well as L</"I/O Operators">. |
a0d0e21e LW |
140 | |
141 | =head2 The Arrow Operator | |
d74e8afc | 142 | X<arrow> X<dereference> X<< -> >> |
a0d0e21e | 143 | |
35f2feb0 | 144 | "C<< -> >>" is an infix dereference operator, just as it is in C |
19799a22 GS |
145 | and C++. If the right side is either a C<[...]>, C<{...}>, or a |
146 | C<(...)> subscript, then the left side must be either a hard or | |
147 | symbolic reference to an array, a hash, or a subroutine respectively. | |
148 | (Or technically speaking, a location capable of holding a hard | |
149 | reference, if it's an array or hash reference being used for | |
150 | assignment.) See L<perlreftut> and L<perlref>. | |
a0d0e21e | 151 | |
19799a22 GS |
152 | Otherwise, the right side is a method name or a simple scalar |
153 | variable containing either the method name or a subroutine reference, | |
154 | and the left side must be either an object (a blessed reference) | |
155 | or a class name (that is, a package name). See L<perlobj>. | |
a0d0e21e | 156 | |
821361b6 | 157 | The dereferencing cases (as opposed to method-calling cases) are |
2ad792cd | 158 | somewhat extended by the C<postderef> feature. For the |
821361b6 RS |
159 | details of that feature, consult L<perlref/Postfix Dereference Syntax>. |
160 | ||
5f05dabc | 161 | =head2 Auto-increment and Auto-decrement |
d74e8afc | 162 | X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<--> |
a0d0e21e | 163 | |
ba7f043c | 164 | C<"++"> and C<"--"> work as in C. That is, if placed before a variable, |
d042e63d MS |
165 | they increment or decrement the variable by one before returning the |
166 | value, and if placed after, increment or decrement after returning the | |
167 | value. | |
168 | ||
169 | $i = 0; $j = 0; | |
170 | print $i++; # prints 0 | |
171 | print ++$j; # prints 1 | |
a0d0e21e | 172 | |
b033823e | 173 | Note that just as in C, Perl doesn't define B<when> the variable is |
46f8a5ea FC |
174 | incremented or decremented. You just know it will be done sometime |
175 | before or after the value is returned. This also means that modifying | |
c543c01b | 176 | a variable twice in the same statement will lead to undefined behavior. |
b033823e A |
177 | Avoid statements like: |
178 | ||
179 | $i = $i ++; | |
180 | print ++ $i + $i ++; | |
181 | ||
182 | Perl will not guarantee what the result of the above statements is. | |
183 | ||
54310121 | 184 | The auto-increment operator has a little extra builtin magic to it. If |
a0d0e21e LW |
185 | you increment a variable that is numeric, or that has ever been used in |
186 | a numeric context, you get a normal increment. If, however, the | |
5f05dabc | 187 | variable has been used in only string contexts since it was set, and |
5a964f20 | 188 | has a value that is not the empty string and matches the pattern |
9c0670e1 | 189 | C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each |
a0d0e21e LW |
190 | character within its range, with carry: |
191 | ||
c543c01b TC |
192 | print ++($foo = "99"); # prints "100" |
193 | print ++($foo = "a0"); # prints "a1" | |
194 | print ++($foo = "Az"); # prints "Ba" | |
195 | print ++($foo = "zz"); # prints "aaa" | |
a0d0e21e | 196 | |
6a61d433 HS |
197 | C<undef> is always treated as numeric, and in particular is changed |
198 | to C<0> before incrementing (so that a post-increment of an undef value | |
199 | will return C<0> rather than C<undef>). | |
200 | ||
5f05dabc | 201 | The auto-decrement operator is not magical. |
a0d0e21e LW |
202 | |
203 | =head2 Exponentiation | |
d74e8afc | 204 | X<**> X<exponentiation> X<power> |
a0d0e21e | 205 | |
ba7f043c KW |
206 | Binary C<"**"> is the exponentiation operator. It binds even more |
207 | tightly than unary minus, so C<-2**4> is C<-(2**4)>, not C<(-2)**4>. | |
208 | (This is | |
209 | implemented using C's C<pow(3)> function, which actually works on doubles | |
cb1a09d0 | 210 | internally.) |
a0d0e21e | 211 | |
44a465b3 JH |
212 | Note that certain exponentiation expressions are ill-defined: |
213 | these include C<0**0>, C<1**Inf>, and C<Inf**0>. Do not expect | |
214 | any particular results from these special cases, the results | |
215 | are platform-dependent. | |
216 | ||
a0d0e21e | 217 | =head2 Symbolic Unary Operators |
d74e8afc | 218 | X<unary operator> X<operator, unary> |
a0d0e21e | 219 | |
4b05bc8e KW |
220 | Unary C<"!"> performs logical negation, that is, "not". See also |
221 | L<C<not>|/Logical Not> for a lower precedence version of this. | |
d74e8afc | 222 | X<!> |
a0d0e21e | 223 | |
ba7f043c | 224 | Unary C<"-"> performs arithmetic negation if the operand is numeric, |
da2f94c5 FC |
225 | including any string that looks like a number. If the operand is |
226 | an identifier, a string consisting of a minus sign concatenated | |
227 | with the identifier is returned. Otherwise, if the string starts | |
228 | with a plus or minus, a string starting with the opposite sign is | |
ba7f043c KW |
229 | returned. One effect of these rules is that C<-bareword> is equivalent |
230 | to the string C<"-bareword">. If, however, the string begins with a | |
231 | non-alphabetic character (excluding C<"+"> or C<"-">), Perl will attempt | |
232 | to convert | |
233 | the string to a numeric, and the arithmetic negation is performed. If the | |
06705523 SP |
234 | string cannot be cleanly converted to a numeric, Perl will give the warning |
235 | B<Argument "the string" isn't numeric in negation (-) at ...>. | |
d74e8afc | 236 | X<-> X<negation, arithmetic> |
a0d0e21e | 237 | |
ba7f043c | 238 | Unary C<"~"> performs bitwise negation, that is, 1's complement. For |
5a0de581 LM |
239 | example, S<C<0666 & ~027>> is 0640. (See also L</Integer Arithmetic> and |
240 | L</Bitwise String Operators>.) Note that the width of the result is | |
ba7f043c | 241 | platform-dependent: C<~0> is 32 bits wide on a 32-bit platform, but 64 |
972b05a9 | 242 | bits wide on a 64-bit platform, so if you are expecting a certain bit |
ba7f043c | 243 | width, remember to use the C<"&"> operator to mask off the excess bits. |
d74e8afc | 244 | X<~> X<negation, binary> |
a0d0e21e | 245 | |
fac71630 KW |
246 | Starting in Perl 5.28, it is a fatal error to try to complement a string |
247 | containing a character with an ordinal value above 255. | |
f113cf86 | 248 | |
ba7f043c KW |
249 | If the experimental "bitwise" feature is enabled via S<C<use feature |
250 | 'bitwise'>>, then unary C<"~"> always treats its argument as a number, and an | |
251 | alternate form of the operator, C<"~.">, always treats its argument as a | |
fb7054ba FC |
252 | string. So C<~0> and C<~"0"> will both give 2**32-1 on 32-bit platforms, |
253 | whereas C<~.0> and C<~."0"> will both yield C<"\xff">. This feature | |
ba7f043c | 254 | produces a warning unless you use S<C<no warnings 'experimental::bitwise'>>. |
fb7054ba | 255 | |
ba7f043c | 256 | Unary C<"+"> has no effect whatsoever, even on strings. It is useful |
a0d0e21e LW |
257 | syntactically for separating a function name from a parenthesized expression |
258 | that would otherwise be interpreted as the complete list of function | |
a95b3d6a | 259 | arguments. (See examples above under L</Terms and List Operators (Leftward)>.) |
d74e8afc | 260 | X<+> |
a0d0e21e | 261 | |
ba7f043c | 262 | Unary C<"\"> creates a reference to whatever follows it. See L<perlreftut> |
19799a22 GS |
263 | and L<perlref>. Do not confuse this behavior with the behavior of |
264 | backslash within a string, although both forms do convey the notion | |
265 | of protecting the next thing from interpolation. | |
d74e8afc | 266 | X<\> X<reference> X<backslash> |
a0d0e21e LW |
267 | |
268 | =head2 Binding Operators | |
d74e8afc | 269 | X<binding> X<operator, binding> X<=~> X<!~> |
a0d0e21e | 270 | |
ba7f043c KW |
271 | Binary C<"=~"> binds a scalar expression to a pattern match. Certain operations |
272 | search or modify the string C<$_> by default. This operator makes that kind | |
cb1a09d0 | 273 | of operation work on some other string. The right argument is a search |
2c268ad5 TP |
274 | pattern, substitution, or transliteration. The left argument is what is |
275 | supposed to be searched, substituted, or transliterated instead of the default | |
ba7f043c KW |
276 | C<$_>. When used in scalar context, the return value generally indicates the |
277 | success of the operation. The exceptions are substitution (C<s///>) | |
278 | and transliteration (C<y///>) with the C</r> (non-destructive) option, | |
8ff32507 FC |
279 | which cause the B<r>eturn value to be the result of the substitution. |
280 | Behavior in list context depends on the particular operator. | |
000c65fc DG |
281 | See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for |
282 | examples using these operators. | |
f8bab1e9 GS |
283 | |
284 | If the right argument is an expression rather than a search pattern, | |
2c268ad5 | 285 | substitution, or transliteration, it is interpreted as a search pattern at run |
46f8a5ea FC |
286 | time. Note that this means that its |
287 | contents will be interpolated twice, so | |
89d205f2 | 288 | |
1ca345ed | 289 | '\\' =~ q'\\'; |
89d205f2 YO |
290 | |
291 | is not ok, as the regex engine will end up trying to compile the | |
292 | pattern C<\>, which it will consider a syntax error. | |
a0d0e21e | 293 | |
ba7f043c | 294 | Binary C<"!~"> is just like C<"=~"> except the return value is negated in |
a0d0e21e LW |
295 | the logical sense. |
296 | ||
ba7f043c KW |
297 | Binary C<"!~"> with a non-destructive substitution (C<s///r>) or transliteration |
298 | (C<y///r>) is a syntax error. | |
4f4d7508 | 299 | |
a0d0e21e | 300 | =head2 Multiplicative Operators |
d74e8afc | 301 | X<operator, multiplicative> |
a0d0e21e | 302 | |
ba7f043c | 303 | Binary C<"*"> multiplies two numbers. |
d74e8afc | 304 | X<*> |
a0d0e21e | 305 | |
ba7f043c | 306 | Binary C<"/"> divides two numbers. |
d74e8afc | 307 | X</> X<slash> |
a0d0e21e | 308 | |
ba7f043c | 309 | Binary C<"%"> is the modulo operator, which computes the division |
f7918450 KW |
310 | remainder of its first argument with respect to its second argument. |
311 | Given integer | |
ba7f043c | 312 | operands C<$m> and C<$n>: If C<$n> is positive, then S<C<$m % $n>> is |
db691027 | 313 | C<$m> minus the largest multiple of C<$n> less than or equal to |
ba7f043c | 314 | C<$m>. If C<$n> is negative, then S<C<$m % $n>> is C<$m> minus the |
db691027 | 315 | smallest multiple of C<$n> that is not less than C<$m> (that is, the |
89b4f0ad | 316 | result will be less than or equal to zero). If the operands |
db691027 | 317 | C<$m> and C<$n> are floating point values and the absolute value of |
ba7f043c | 318 | C<$n> (that is C<abs($n)>) is less than S<C<(UV_MAX + 1)>>, only |
db691027 | 319 | the integer portion of C<$m> and C<$n> will be used in the operation |
4848a83b | 320 | (Note: here C<UV_MAX> means the maximum of the unsigned integer type). |
db691027 | 321 | If the absolute value of the right operand (C<abs($n)>) is greater than |
ba7f043c KW |
322 | or equal to S<C<(UV_MAX + 1)>>, C<"%"> computes the floating-point remainder |
323 | C<$r> in the equation S<C<($r = $m - $i*$n)>> where C<$i> is a certain | |
f7918450 | 324 | integer that makes C<$r> have the same sign as the right operand |
db691027 SF |
325 | C<$n> (B<not> as the left operand C<$m> like C function C<fmod()>) |
326 | and the absolute value less than that of C<$n>. | |
ba7f043c | 327 | Note that when S<C<use integer>> is in scope, C<"%"> gives you direct access |
f7918450 | 328 | to the modulo operator as implemented by your C compiler. This |
55d729e4 GS |
329 | operator is not as well defined for negative operands, but it will |
330 | execute faster. | |
f7918450 | 331 | X<%> X<remainder> X<modulo> X<mod> |
55d729e4 | 332 | |
ba7f043c | 333 | Binary C<"x"> is the repetition operator. In scalar context or if the left |
62d10b70 GS |
334 | operand is not enclosed in parentheses, it returns a string consisting |
335 | of the left operand repeated the number of times specified by the right | |
336 | operand. In list context, if the left operand is enclosed in | |
ba7f043c | 337 | parentheses or is a list formed by C<qw/I<STRING>/>, it repeats the list. |
31201a8e KW |
338 | If the right operand is zero or negative (raising a warning on |
339 | negative), it returns an empty string | |
3585017f | 340 | or an empty list, depending on the context. |
d74e8afc | 341 | X<x> |
a0d0e21e LW |
342 | |
343 | print '-' x 80; # print row of dashes | |
344 | ||
345 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over | |
346 | ||
347 | @ones = (1) x 80; # a list of 80 1's | |
348 | @ones = (5) x @ones; # set all elements to 5 | |
349 | ||
350 | ||
351 | =head2 Additive Operators | |
d74e8afc | 352 | X<operator, additive> |
a0d0e21e | 353 | |
ba7f043c | 354 | Binary C<"+"> returns the sum of two numbers. |
d74e8afc | 355 | X<+> |
a0d0e21e | 356 | |
ba7f043c | 357 | Binary C<"-"> returns the difference of two numbers. |
d74e8afc | 358 | X<-> |
a0d0e21e | 359 | |
ba7f043c | 360 | Binary C<"."> concatenates two strings. |
d74e8afc ITB |
361 | X<string, concatenation> X<concatenation> |
362 | X<cat> X<concat> X<concatenate> X<.> | |
a0d0e21e LW |
363 | |
364 | =head2 Shift Operators | |
d74e8afc ITB |
365 | X<shift operator> X<operator, shift> X<<< << >>> |
366 | X<<< >> >>> X<right shift> X<left shift> X<bitwise shift> | |
367 | X<shl> X<shr> X<shift, right> X<shift, left> | |
a0d0e21e | 368 | |
ba7f043c | 369 | Binary C<<< "<<" >>> returns the value of its left argument shifted left by the |
55497cff | 370 | number of bits specified by the right argument. Arguments should be |
5a0de581 | 371 | integers. (See also L</Integer Arithmetic>.) |
a0d0e21e | 372 | |
ba7f043c | 373 | Binary C<<< ">>" >>> returns the value of its left argument shifted right by |
55497cff | 374 | the number of bits specified by the right argument. Arguments should |
5a0de581 | 375 | be integers. (See also L</Integer Arithmetic>.) |
a0d0e21e | 376 | |
5a0de581 | 377 | If S<C<use integer>> (see L</Integer Arithmetic>) is in force then |
a63df121 JH |
378 | signed C integers are used (I<arithmetic shift>), otherwise unsigned C |
379 | integers are used (I<logical shift>), even for negative shiftees. | |
380 | In arithmetic right shift the sign bit is replicated on the left, | |
381 | in logical shift zero bits come in from the left. | |
382 | ||
383 | Either way, the implementation isn't going to generate results larger | |
384 | than the size of the integer type Perl was built with (32 bits or 64 bits). | |
385 | ||
386 | Shifting by negative number of bits means the reverse shift: left | |
387 | shift becomes right shift, right shift becomes left shift. This is | |
388 | unlike in C, where negative shift is undefined. | |
389 | ||
390 | Shifting by more bits than the size of the integers means most of the | |
391 | time zero (all bits fall off), except that under S<C<use integer>> | |
392 | right overshifting a negative shiftee results in -1. This is unlike | |
393 | in C, where shifting by too many bits is undefined. A common C | |
394 | behavior is "shift by modulo wordbits", so that for example | |
395 | ||
396 | 1 >> 64 == 1 >> (64 % 64) == 1 >> 0 == 1 # Common C behavior. | |
397 | ||
398 | but that is completely accidental. | |
b16cf6df | 399 | |
1ca345ed | 400 | If you get tired of being subject to your platform's native integers, |
ba7f043c | 401 | the S<C<use bigint>> pragma neatly sidesteps the issue altogether: |
1ca345ed TC |
402 | |
403 | print 20 << 20; # 20971520 | |
a727cfac | 404 | print 20 << 40; # 5120 on 32-bit machines, |
1ca345ed TC |
405 | # 21990232555520 on 64-bit machines |
406 | use bigint; | |
407 | print 20 << 100; # 25353012004564588029934064107520 | |
408 | ||
a0d0e21e | 409 | =head2 Named Unary Operators |
d74e8afc | 410 | X<operator, named unary> |
a0d0e21e LW |
411 | |
412 | The various named unary operators are treated as functions with one | |
568e6d8b | 413 | argument, with optional parentheses. |
a0d0e21e | 414 | |
ba7f043c | 415 | If any list operator (C<print()>, etc.) or any unary operator (C<chdir()>, etc.) |
a0d0e21e LW |
416 | is followed by a left parenthesis as the next token, the operator and |
417 | arguments within parentheses are taken to be of highest precedence, | |
3981b0eb | 418 | just like a normal function call. For example, |
1ca345ed | 419 | because named unary operators are higher precedence than C<||>: |
a0d0e21e LW |
420 | |
421 | chdir $foo || die; # (chdir $foo) || die | |
422 | chdir($foo) || die; # (chdir $foo) || die | |
423 | chdir ($foo) || die; # (chdir $foo) || die | |
424 | chdir +($foo) || die; # (chdir $foo) || die | |
425 | ||
ba7f043c | 426 | but, because C<"*"> is higher precedence than named operators: |
a0d0e21e LW |
427 | |
428 | chdir $foo * 20; # chdir ($foo * 20) | |
429 | chdir($foo) * 20; # (chdir $foo) * 20 | |
430 | chdir ($foo) * 20; # (chdir $foo) * 20 | |
431 | chdir +($foo) * 20; # chdir ($foo * 20) | |
432 | ||
433 | rand 10 * 20; # rand (10 * 20) | |
434 | rand(10) * 20; # (rand 10) * 20 | |
435 | rand (10) * 20; # (rand 10) * 20 | |
436 | rand +(10) * 20; # rand (10 * 20) | |
437 | ||
568e6d8b RGS |
438 | Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are |
439 | treated like named unary operators, but they don't follow this functional | |
440 | parenthesis rule. That means, for example, that C<-f($file).".bak"> is | |
ba7f043c | 441 | equivalent to S<C<-f "$file.bak">>. |
d74e8afc | 442 | X<-X> X<filetest> X<operator, filetest> |
568e6d8b | 443 | |
5a0de581 | 444 | See also L</"Terms and List Operators (Leftward)">. |
a0d0e21e LW |
445 | |
446 | =head2 Relational Operators | |
d74e8afc | 447 | X<relational operator> X<operator, relational> |
a0d0e21e | 448 | |
a727cfac | 449 | Perl operators that return true or false generally return values |
1ca345ed TC |
450 | that can be safely used as numbers. For example, the relational |
451 | operators in this section and the equality operators in the next | |
452 | one return C<1> for true and a special version of the defined empty | |
453 | string, C<"">, which counts as a zero but is exempt from warnings | |
ba7f043c | 454 | about improper numeric conversions, just as S<C<"0 but true">> is. |
1ca345ed | 455 | |
ba7f043c | 456 | Binary C<< "<" >> returns true if the left argument is numerically less than |
a0d0e21e | 457 | the right argument. |
d74e8afc | 458 | X<< < >> |
a0d0e21e | 459 | |
ba7f043c | 460 | Binary C<< ">" >> returns true if the left argument is numerically greater |
a0d0e21e | 461 | than the right argument. |
d74e8afc | 462 | X<< > >> |
a0d0e21e | 463 | |
ba7f043c | 464 | Binary C<< "<=" >> returns true if the left argument is numerically less than |
a0d0e21e | 465 | or equal to the right argument. |
d74e8afc | 466 | X<< <= >> |
a0d0e21e | 467 | |
ba7f043c | 468 | Binary C<< ">=" >> returns true if the left argument is numerically greater |
a0d0e21e | 469 | than or equal to the right argument. |
d74e8afc | 470 | X<< >= >> |
a0d0e21e | 471 | |
ba7f043c | 472 | Binary C<"lt"> returns true if the left argument is stringwise less than |
a0d0e21e | 473 | the right argument. |
d74e8afc | 474 | X<< lt >> |
a0d0e21e | 475 | |
ba7f043c | 476 | Binary C<"gt"> returns true if the left argument is stringwise greater |
a0d0e21e | 477 | than the right argument. |
d74e8afc | 478 | X<< gt >> |
a0d0e21e | 479 | |
ba7f043c | 480 | Binary C<"le"> returns true if the left argument is stringwise less than |
a0d0e21e | 481 | or equal to the right argument. |
d74e8afc | 482 | X<< le >> |
a0d0e21e | 483 | |
ba7f043c | 484 | Binary C<"ge"> returns true if the left argument is stringwise greater |
a0d0e21e | 485 | than or equal to the right argument. |
d74e8afc | 486 | X<< ge >> |
a0d0e21e LW |
487 | |
488 | =head2 Equality Operators | |
d74e8afc | 489 | X<equality> X<equal> X<equals> X<operator, equality> |
a0d0e21e | 490 | |
ba7f043c | 491 | Binary C<< "==" >> returns true if the left argument is numerically equal to |
a0d0e21e | 492 | the right argument. |
d74e8afc | 493 | X<==> |
a0d0e21e | 494 | |
ba7f043c | 495 | Binary C<< "!=" >> returns true if the left argument is numerically not equal |
a0d0e21e | 496 | to the right argument. |
d74e8afc | 497 | X<!=> |
a0d0e21e | 498 | |
ba7f043c | 499 | Binary C<< "<=>" >> returns -1, 0, or 1 depending on whether the left |
6ee5d4e7 | 500 | argument is numerically less than, equal to, or greater than the right |
ba7f043c KW |
501 | argument. If your platform supports C<NaN>'s (not-a-numbers) as numeric |
502 | values, using them with C<< "<=>" >> returns undef. C<NaN> is not | |
503 | C<< "<" >>, C<< "==" >>, C<< ">" >>, C<< "<=" >> or C<< ">=" >> anything | |
504 | (even C<NaN>), so those 5 return false. S<C<< NaN != NaN >>> returns | |
505 | true, as does S<C<NaN !=> I<anything else>>. If your platform doesn't | |
506 | support C<NaN>'s then C<NaN> is just a string with numeric value 0. | |
507 | X<< <=> >> | |
508 | X<spaceship> | |
7d3a9d88 | 509 | |
db691027 SF |
510 | $ perl -le '$x = "NaN"; print "No NaN support here" if $x == $x' |
511 | $ perl -le '$x = "NaN"; print "NaN support here" if $x != $x' | |
1ca345ed | 512 | |
db691027 | 513 | (Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all |
ba7f043c | 514 | support C<"NaN">.) |
a0d0e21e | 515 | |
ba7f043c | 516 | Binary C<"eq"> returns true if the left argument is stringwise equal to |
a0d0e21e | 517 | the right argument. |
d74e8afc | 518 | X<eq> |
a0d0e21e | 519 | |
ba7f043c | 520 | Binary C<"ne"> returns true if the left argument is stringwise not equal |
a0d0e21e | 521 | to the right argument. |
d74e8afc | 522 | X<ne> |
a0d0e21e | 523 | |
ba7f043c | 524 | Binary C<"cmp"> returns -1, 0, or 1 depending on whether the left |
d4ad863d JH |
525 | argument is stringwise less than, equal to, or greater than the right |
526 | argument. | |
d74e8afc | 527 | X<cmp> |
a0d0e21e | 528 | |
ba7f043c | 529 | Binary C<"~~"> does a smartmatch between its arguments. Smart matching |
1ca345ed | 530 | is described in the next section. |
0d863452 RH |
531 | X<~~> |
532 | ||
ba7f043c KW |
533 | C<"lt">, C<"le">, C<"ge">, C<"gt"> and C<"cmp"> use the collation (sort) |
534 | order specified by the current C<LC_COLLATE> locale if a S<C<use | |
535 | locale>> form that includes collation is in effect. See L<perllocale>. | |
536 | Do not mix these with Unicode, | |
537 | only use them with legacy 8-bit locale encodings. | |
538 | The standard C<L<Unicode::Collate>> and | |
539 | C<L<Unicode::Collate::Locale>> modules offer much more powerful | |
540 | solutions to collation issues. | |
1ca345ed | 541 | |
82365311 DG |
542 | For case-insensitive comparisions, look at the L<perlfunc/fc> case-folding |
543 | function, available in Perl v5.16 or later: | |
544 | ||
545 | if ( fc($x) eq fc($y) ) { ... } | |
546 | ||
1ca345ed TC |
547 | =head2 Smartmatch Operator |
548 | ||
549 | First available in Perl 5.10.1 (the 5.10.0 version behaved differently), | |
550 | binary C<~~> does a "smartmatch" between its arguments. This is mostly | |
551 | used implicitly in the C<when> construct described in L<perlsyn>, although | |
552 | not all C<when> clauses call the smartmatch operator. Unique among all of | |
cc08d69f RS |
553 | Perl's operators, the smartmatch operator can recurse. The smartmatch |
554 | operator is L<experimental|perlpolicy/experimental> and its behavior is | |
555 | subject to change. | |
1ca345ed TC |
556 | |
557 | It is also unique in that all other Perl operators impose a context | |
558 | (usually string or numeric context) on their operands, autoconverting | |
559 | those operands to those imposed contexts. In contrast, smartmatch | |
560 | I<infers> contexts from the actual types of its operands and uses that | |
561 | type information to select a suitable comparison mechanism. | |
562 | ||
563 | The C<~~> operator compares its operands "polymorphically", determining how | |
564 | to compare them according to their actual types (numeric, string, array, | |
565 | hash, etc.) Like the equality operators with which it shares the same | |
566 | precedence, C<~~> returns 1 for true and C<""> for false. It is often best | |
567 | read aloud as "in", "inside of", or "is contained in", because the left | |
568 | operand is often looked for I<inside> the right operand. That makes the | |
40bec8a5 | 569 | order of the operands to the smartmatch operand often opposite that of |
1ca345ed TC |
570 | the regular match operator. In other words, the "smaller" thing is usually |
571 | placed in the left operand and the larger one in the right. | |
572 | ||
573 | The behavior of a smartmatch depends on what type of things its arguments | |
574 | are, as determined by the following table. The first row of the table | |
575 | whose types apply determines the smartmatch behavior. Because what | |
576 | actually happens is mostly determined by the type of the second operand, | |
577 | the table is sorted on the right operand instead of on the left. | |
578 | ||
a727cfac | 579 | Left Right Description and pseudocode |
1ca345ed | 580 | =============================================================== |
a727cfac | 581 | Any undef check whether Any is undefined |
1ca345ed TC |
582 | like: !defined Any |
583 | ||
584 | Any Object invoke ~~ overloading on Object, or die | |
585 | ||
586 | Right operand is an ARRAY: | |
587 | ||
a727cfac | 588 | Left Right Description and pseudocode |
1ca345ed TC |
589 | =============================================================== |
590 | ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2] | |
591 | like: (ARRAY1[0] ~~ ARRAY2[0]) | |
592 | && (ARRAY1[1] ~~ ARRAY2[1]) && ... | |
a727cfac | 593 | HASH ARRAY any ARRAY elements exist as HASH keys |
1ca345ed TC |
594 | like: grep { exists HASH->{$_} } ARRAY |
595 | Regexp ARRAY any ARRAY elements pattern match Regexp | |
596 | like: grep { /Regexp/ } ARRAY | |
a727cfac | 597 | undef ARRAY undef in ARRAY |
1ca345ed | 598 | like: grep { !defined } ARRAY |
a727cfac | 599 | Any ARRAY smartmatch each ARRAY element[3] |
1ca345ed TC |
600 | like: grep { Any ~~ $_ } ARRAY |
601 | ||
602 | Right operand is a HASH: | |
603 | ||
a727cfac | 604 | Left Right Description and pseudocode |
1ca345ed | 605 | =============================================================== |
a727cfac | 606 | HASH1 HASH2 all same keys in both HASHes |
1ca345ed TC |
607 | like: keys HASH1 == |
608 | grep { exists HASH2->{$_} } keys HASH1 | |
a727cfac | 609 | ARRAY HASH any ARRAY elements exist as HASH keys |
1ca345ed | 610 | like: grep { exists HASH->{$_} } ARRAY |
a727cfac | 611 | Regexp HASH any HASH keys pattern match Regexp |
1ca345ed | 612 | like: grep { /Regexp/ } keys HASH |
a727cfac | 613 | undef HASH always false (undef can't be a key) |
1ca345ed | 614 | like: 0 == 1 |
a727cfac | 615 | Any HASH HASH key existence |
1ca345ed TC |
616 | like: exists HASH->{Any} |
617 | ||
618 | Right operand is CODE: | |
f703fc96 | 619 | |
a727cfac | 620 | Left Right Description and pseudocode |
1ca345ed TC |
621 | =============================================================== |
622 | ARRAY CODE sub returns true on all ARRAY elements[1] | |
623 | like: !grep { !CODE->($_) } ARRAY | |
624 | HASH CODE sub returns true on all HASH keys[1] | |
625 | like: !grep { !CODE->($_) } keys HASH | |
a727cfac | 626 | Any CODE sub passed Any returns true |
1ca345ed TC |
627 | like: CODE->(Any) |
628 | ||
629 | Right operand is a Regexp: | |
630 | ||
a727cfac | 631 | Left Right Description and pseudocode |
1ca345ed | 632 | =============================================================== |
a727cfac | 633 | ARRAY Regexp any ARRAY elements match Regexp |
1ca345ed | 634 | like: grep { /Regexp/ } ARRAY |
a727cfac | 635 | HASH Regexp any HASH keys match Regexp |
1ca345ed | 636 | like: grep { /Regexp/ } keys HASH |
a727cfac | 637 | Any Regexp pattern match |
1ca345ed TC |
638 | like: Any =~ /Regexp/ |
639 | ||
640 | Other: | |
641 | ||
a727cfac | 642 | Left Right Description and pseudocode |
1ca345ed TC |
643 | =============================================================== |
644 | Object Any invoke ~~ overloading on Object, | |
645 | or fall back to... | |
646 | ||
a727cfac | 647 | Any Num numeric equality |
1ca345ed TC |
648 | like: Any == Num |
649 | Num nummy[4] numeric equality | |
650 | like: Num == nummy | |
651 | undef Any check whether undefined | |
652 | like: !defined(Any) | |
a727cfac | 653 | Any Any string equality |
1ca345ed TC |
654 | like: Any eq Any |
655 | ||
656 | ||
657 | Notes: | |
658 | ||
659 | =over | |
660 | ||
661 | =item 1. | |
a727cfac | 662 | Empty hashes or arrays match. |
1ca345ed TC |
663 | |
664 | =item 2. | |
40bec8a5 | 665 | That is, each element smartmatches the element of the same index in the other array.[3] |
1ca345ed TC |
666 | |
667 | =item 3. | |
a727cfac | 668 | If a circular reference is found, fall back to referential equality. |
1ca345ed TC |
669 | |
670 | =item 4. | |
671 | Either an actual number, or a string that looks like one. | |
672 | ||
673 | =back | |
674 | ||
675 | The smartmatch implicitly dereferences any non-blessed hash or array | |
676 | reference, so the C<I<HASH>> and C<I<ARRAY>> entries apply in those cases. | |
677 | For blessed references, the C<I<Object>> entries apply. Smartmatches | |
678 | involving hashes only consider hash keys, never hash values. | |
679 | ||
680 | The "like" code entry is not always an exact rendition. For example, the | |
40bec8a5 | 681 | smartmatch operator short-circuits whenever possible, but C<grep> does |
1ca345ed TC |
682 | not. Also, C<grep> in scalar context returns the number of matches, but |
683 | C<~~> returns only true or false. | |
684 | ||
685 | Unlike most operators, the smartmatch operator knows to treat C<undef> | |
686 | specially: | |
687 | ||
688 | use v5.10.1; | |
689 | @array = (1, 2, 3, undef, 4, 5); | |
690 | say "some elements undefined" if undef ~~ @array; | |
691 | ||
692 | Each operand is considered in a modified scalar context, the modification | |
693 | being that array and hash variables are passed by reference to the | |
694 | operator, which implicitly dereferences them. Both elements | |
695 | of each pair are the same: | |
696 | ||
697 | use v5.10.1; | |
698 | ||
699 | my %hash = (red => 1, blue => 2, green => 3, | |
700 | orange => 4, yellow => 5, purple => 6, | |
701 | black => 7, grey => 8, white => 9); | |
702 | ||
703 | my @array = qw(red blue green); | |
704 | ||
705 | say "some array elements in hash keys" if @array ~~ %hash; | |
706 | say "some array elements in hash keys" if \@array ~~ \%hash; | |
707 | ||
708 | say "red in array" if "red" ~~ @array; | |
709 | say "red in array" if "red" ~~ \@array; | |
710 | ||
711 | say "some keys end in e" if /e$/ ~~ %hash; | |
712 | say "some keys end in e" if /e$/ ~~ \%hash; | |
713 | ||
40bec8a5 TC |
714 | Two arrays smartmatch if each element in the first array smartmatches |
715 | (that is, is "in") the corresponding element in the second array, | |
716 | recursively. | |
1ca345ed TC |
717 | |
718 | use v5.10.1; | |
719 | my @little = qw(red blue green); | |
720 | my @bigger = ("red", "blue", [ "orange", "green" ] ); | |
721 | if (@little ~~ @bigger) { # true! | |
722 | say "little is contained in bigger"; | |
a727cfac | 723 | } |
1ca345ed TC |
724 | |
725 | Because the smartmatch operator recurses on nested arrays, this | |
726 | will still report that "red" is in the array. | |
727 | ||
728 | use v5.10.1; | |
729 | my @array = qw(red blue green); | |
730 | my $nested_array = [[[[[[[ @array ]]]]]]]; | |
731 | say "red in array" if "red" ~~ $nested_array; | |
732 | ||
733 | If two arrays smartmatch each other, then they are deep | |
734 | copies of each others' values, as this example reports: | |
735 | ||
736 | use v5.12.0; | |
a727cfac SF |
737 | my @a = (0, 1, 2, [3, [4, 5], 6], 7); |
738 | my @b = (0, 1, 2, [3, [4, 5], 6], 7); | |
1ca345ed TC |
739 | |
740 | if (@a ~~ @b && @b ~~ @a) { | |
741 | say "a and b are deep copies of each other"; | |
a727cfac | 742 | } |
1ca345ed TC |
743 | elsif (@a ~~ @b) { |
744 | say "a smartmatches in b"; | |
a727cfac | 745 | } |
1ca345ed TC |
746 | elsif (@b ~~ @a) { |
747 | say "b smartmatches in a"; | |
a727cfac | 748 | } |
1ca345ed TC |
749 | else { |
750 | say "a and b don't smartmatch each other at all"; | |
a727cfac | 751 | } |
1ca345ed TC |
752 | |
753 | ||
ba7f043c KW |
754 | If you were to set S<C<$b[3] = 4>>, then instead of reporting that "a and b |
755 | are deep copies of each other", it now reports that C<"b smartmatches in a">. | |
756 | That's because the corresponding position in C<@a> contains an array that | |
1ca345ed TC |
757 | (eventually) has a 4 in it. |
758 | ||
759 | Smartmatching one hash against another reports whether both contain the | |
46f8a5ea | 760 | same keys, no more and no less. This could be used to see whether two |
1ca345ed TC |
761 | records have the same field names, without caring what values those fields |
762 | might have. For example: | |
763 | ||
764 | use v5.10.1; | |
765 | sub make_dogtag { | |
766 | state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 }; | |
767 | ||
768 | my ($class, $init_fields) = @_; | |
769 | ||
770 | die "Must supply (only) name, rank, and serial number" | |
771 | unless $init_fields ~~ $REQUIRED_FIELDS; | |
772 | ||
773 | ... | |
774 | } | |
775 | ||
1b590b38 LM |
776 | However, this only does what you mean if C<$init_fields> is indeed a hash |
777 | reference. The condition C<$init_fields ~~ $REQUIRED_FIELDS> also allows the | |
778 | strings C<"name">, C<"rank">, C<"serial_num"> as well as any array reference | |
779 | that contains C<"name"> or C<"rank"> or C<"serial_num"> anywhere to pass | |
780 | through. | |
1ca345ed TC |
781 | |
782 | The smartmatch operator is most often used as the implicit operator of a | |
783 | C<when> clause. See the section on "Switch Statements" in L<perlsyn>. | |
784 | ||
785 | =head3 Smartmatching of Objects | |
786 | ||
40bec8a5 TC |
787 | To avoid relying on an object's underlying representation, if the |
788 | smartmatch's right operand is an object that doesn't overload C<~~>, | |
789 | it raises the exception "C<Smartmatching a non-overloaded object | |
46f8a5ea FC |
790 | breaks encapsulation>". That's because one has no business digging |
791 | around to see whether something is "in" an object. These are all | |
40bec8a5 | 792 | illegal on objects without a C<~~> overload: |
1ca345ed TC |
793 | |
794 | %hash ~~ $object | |
795 | 42 ~~ $object | |
796 | "fred" ~~ $object | |
797 | ||
798 | However, you can change the way an object is smartmatched by overloading | |
46f8a5ea FC |
799 | the C<~~> operator. This is allowed to |
800 | extend the usual smartmatch semantics. | |
1ca345ed TC |
801 | For objects that do have an C<~~> overload, see L<overload>. |
802 | ||
803 | Using an object as the left operand is allowed, although not very useful. | |
804 | Smartmatching rules take precedence over overloading, so even if the | |
805 | object in the left operand has smartmatch overloading, this will be | |
806 | ignored. A left operand that is a non-overloaded object falls back on a | |
807 | string or numeric comparison of whatever the C<ref> operator returns. That | |
808 | means that | |
809 | ||
810 | $object ~~ X | |
811 | ||
812 | does I<not> invoke the overload method with C<I<X>> as an argument. | |
813 | Instead the above table is consulted as normal, and based on the type of | |
814 | C<I<X>>, overloading may or may not be invoked. For simple strings or | |
ba7f043c | 815 | numbers, "in" becomes equivalent to this: |
1ca345ed TC |
816 | |
817 | $object ~~ $number ref($object) == $number | |
a727cfac | 818 | $object ~~ $string ref($object) eq $string |
1ca345ed TC |
819 | |
820 | For example, this reports that the handle smells IOish | |
821 | (but please don't really do this!): | |
822 | ||
823 | use IO::Handle; | |
824 | my $fh = IO::Handle->new(); | |
825 | if ($fh ~~ /\bIO\b/) { | |
826 | say "handle smells IOish"; | |
a727cfac | 827 | } |
1ca345ed TC |
828 | |
829 | That's because it treats C<$fh> as a string like | |
830 | C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that. | |
a034a98d | 831 | |
a0d0e21e | 832 | =head2 Bitwise And |
d74e8afc | 833 | X<operator, bitwise, and> X<bitwise and> X<&> |
a0d0e21e | 834 | |
ba7f043c | 835 | Binary C<"&"> returns its operands ANDed together bit by bit. Although no |
c791a246 KW |
836 | warning is currently raised, the result is not well defined when this operation |
837 | is performed on operands that aren't either numbers (see | |
5a0de581 | 838 | L</Integer Arithmetic>) nor bitstrings (see L</Bitwise String Operators>). |
a0d0e21e | 839 | |
ba7f043c | 840 | Note that C<"&"> has lower priority than relational operators, so for example |
1ca345ed | 841 | the parentheses are essential in a test like |
2cdc098b | 842 | |
1ca345ed | 843 | print "Even\n" if ($x & 1) == 0; |
2cdc098b | 844 | |
ba7f043c KW |
845 | If the experimental "bitwise" feature is enabled via S<C<use feature |
846 | 'bitwise'>>, then this operator always treats its operand as numbers. This | |
847 | feature produces a warning unless you also use C<S<no warnings | |
848 | 'experimental::bitwise'>>. | |
fb7054ba | 849 | |
a0d0e21e | 850 | =head2 Bitwise Or and Exclusive Or |
d74e8afc ITB |
851 | X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor> |
852 | X<bitwise xor> X<^> | |
a0d0e21e | 853 | |
ba7f043c | 854 | Binary C<"|"> returns its operands ORed together bit by bit. |
a0d0e21e | 855 | |
ba7f043c | 856 | Binary C<"^"> returns its operands XORed together bit by bit. |
c791a246 KW |
857 | |
858 | Although no warning is currently raised, the results are not well | |
859 | defined when these operations are performed on operands that aren't either | |
5a0de581 | 860 | numbers (see L</Integer Arithmetic>) nor bitstrings (see L</Bitwise String |
c791a246 | 861 | Operators>). |
a0d0e21e | 862 | |
ba7f043c KW |
863 | Note that C<"|"> and C<"^"> have lower priority than relational operators, so |
864 | for example the parentheses are essential in a test like | |
2cdc098b | 865 | |
1ca345ed | 866 | print "false\n" if (8 | 2) != 10; |
2cdc098b | 867 | |
ba7f043c KW |
868 | If the experimental "bitwise" feature is enabled via S<C<use feature |
869 | 'bitwise'>>, then this operator always treats its operand as numbers. This | |
870 | feature produces a warning unless you also use S<C<no warnings | |
871 | 'experimental::bitwise'>>. | |
fb7054ba | 872 | |
a0d0e21e | 873 | =head2 C-style Logical And |
d74e8afc | 874 | X<&&> X<logical and> X<operator, logical, and> |
a0d0e21e | 875 | |
ba7f043c | 876 | Binary C<"&&"> performs a short-circuit logical AND operation. That is, |
a0d0e21e LW |
877 | if the left operand is false, the right operand is not even evaluated. |
878 | Scalar or list context propagates down to the right operand if it | |
879 | is evaluated. | |
880 | ||
881 | =head2 C-style Logical Or | |
d74e8afc | 882 | X<||> X<operator, logical, or> |
a0d0e21e | 883 | |
ba7f043c | 884 | Binary C<"||"> performs a short-circuit logical OR operation. That is, |
a0d0e21e LW |
885 | if the left operand is true, the right operand is not even evaluated. |
886 | Scalar or list context propagates down to the right operand if it | |
887 | is evaluated. | |
888 | ||
26d9d83b | 889 | =head2 Logical Defined-Or |
d74e8afc | 890 | X<//> X<operator, logical, defined-or> |
c963b151 BD |
891 | |
892 | Although it has no direct equivalent in C, Perl's C<//> operator is related | |
ba7f043c | 893 | to its C-style "or". In fact, it's exactly the same as C<||>, except that it |
95bee9ba | 894 | tests the left hand side's definedness instead of its truth. Thus, |
ba7f043c | 895 | S<C<< EXPR1 // EXPR2 >>> returns the value of C<< EXPR1 >> if it's defined, |
46f8a5ea FC |
896 | otherwise, the value of C<< EXPR2 >> is returned. |
897 | (C<< EXPR1 >> is evaluated in scalar context, C<< EXPR2 >> | |
898 | in the context of C<< // >> itself). Usually, | |
ba7f043c KW |
899 | this is the same result as S<C<< defined(EXPR1) ? EXPR1 : EXPR2 >>> (except that |
900 | the ternary-operator form can be used as a lvalue, while S<C<< EXPR1 // EXPR2 >>> | |
46f8a5ea | 901 | cannot). This is very useful for |
bdc7923b | 902 | providing default values for variables. If you actually want to test if |
ba7f043c | 903 | at least one of C<$x> and C<$y> is defined, use S<C<defined($x // $y)>>. |
c963b151 | 904 | |
d042e63d | 905 | The C<||>, C<//> and C<&&> operators return the last value evaluated |
46f8a5ea | 906 | (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably |
d042e63d | 907 | portable way to find out the home directory might be: |
a0d0e21e | 908 | |
c543c01b TC |
909 | $home = $ENV{HOME} |
910 | // $ENV{LOGDIR} | |
911 | // (getpwuid($<))[7] | |
912 | // die "You're homeless!\n"; | |
a0d0e21e | 913 | |
5a964f20 TC |
914 | In particular, this means that you shouldn't use this |
915 | for selecting between two aggregates for assignment: | |
916 | ||
bf55d65d LTC |
917 | @a = @b || @c; # This doesn't do the right thing |
918 | @a = scalar(@b) || @c; # because it really means this. | |
919 | @a = @b ? @b : @c; # This works fine, though. | |
5a964f20 | 920 | |
1ca345ed | 921 | As alternatives to C<&&> and C<||> when used for |
f23102e2 | 922 | control flow, Perl provides the C<and> and C<or> operators (see below). |
ba7f043c KW |
923 | The short-circuit behavior is identical. The precedence of C<"and"> |
924 | and C<"or"> is much lower, however, so that you can safely use them after a | |
5a964f20 | 925 | list operator without the need for parentheses: |
a0d0e21e LW |
926 | |
927 | unlink "alpha", "beta", "gamma" | |
928 | or gripe(), next LINE; | |
929 | ||
930 | With the C-style operators that would have been written like this: | |
931 | ||
932 | unlink("alpha", "beta", "gamma") | |
933 | || (gripe(), next LINE); | |
934 | ||
1ca345ed TC |
935 | It would be even more readable to write that this way: |
936 | ||
937 | unless(unlink("alpha", "beta", "gamma")) { | |
938 | gripe(); | |
939 | next LINE; | |
a727cfac | 940 | } |
1ca345ed | 941 | |
ba7f043c | 942 | Using C<"or"> for assignment is unlikely to do what you want; see below. |
5a964f20 TC |
943 | |
944 | =head2 Range Operators | |
d74e8afc | 945 | X<operator, range> X<range> X<..> X<...> |
a0d0e21e | 946 | |
ba7f043c | 947 | Binary C<".."> is the range operator, which is really two different |
fb53bbb2 | 948 | operators depending on the context. In list context, it returns a |
54ae734e | 949 | list of values counting (up by ones) from the left value to the right |
2cdbc966 | 950 | value. If the left value is greater than the right value then it |
fb53bbb2 | 951 | returns the empty list. The range operator is useful for writing |
ba7f043c | 952 | S<C<foreach (1..10)>> loops and for doing slice operations on arrays. In |
2cdbc966 JD |
953 | the current implementation, no temporary array is created when the |
954 | range operator is used as the expression in C<foreach> loops, but older | |
955 | versions of Perl might burn a lot of memory when you write something | |
956 | like this: | |
a0d0e21e LW |
957 | |
958 | for (1 .. 1_000_000) { | |
959 | # code | |
54310121 | 960 | } |
a0d0e21e | 961 | |
8f0f46f8 | 962 | The range operator also works on strings, using the magical |
963 | auto-increment, see below. | |
54ae734e | 964 | |
ba7f043c | 965 | In scalar context, C<".."> returns a boolean value. The operator is |
8f0f46f8 | 966 | bistable, like a flip-flop, and emulates the line-range (comma) |
ba7f043c | 967 | operator of B<sed>, B<awk>, and various editors. Each C<".."> operator |
8f0f46f8 | 968 | maintains its own boolean state, even across calls to a subroutine |
46f8a5ea | 969 | that contains it. It is false as long as its left operand is false. |
a0d0e21e LW |
970 | Once the left operand is true, the range operator stays true until the |
971 | right operand is true, I<AFTER> which the range operator becomes false | |
8f0f46f8 | 972 | again. It doesn't become false till the next time the range operator |
973 | is evaluated. It can test the right operand and become false on the | |
974 | same evaluation it became true (as in B<awk>), but it still returns | |
46f8a5ea | 975 | true once. If you don't want it to test the right operand until the |
ba7f043c KW |
976 | next evaluation, as in B<sed>, just use three dots (C<"...">) instead of |
977 | two. In all other regards, C<"..."> behaves just like C<".."> does. | |
19799a22 GS |
978 | |
979 | The right operand is not evaluated while the operator is in the | |
980 | "false" state, and the left operand is not evaluated while the | |
981 | operator is in the "true" state. The precedence is a little lower | |
982 | than || and &&. The value returned is either the empty string for | |
8f0f46f8 | 983 | false, or a sequence number (beginning with 1) for true. The sequence |
984 | number is reset for each range encountered. The final sequence number | |
ba7f043c | 985 | in a range has the string C<"E0"> appended to it, which doesn't affect |
8f0f46f8 | 986 | its numeric value, but gives you something to search for if you want |
987 | to exclude the endpoint. You can exclude the beginning point by | |
988 | waiting for the sequence number to be greater than 1. | |
df5f8116 | 989 | |
ba7f043c | 990 | If either operand of scalar C<".."> is a constant expression, |
df5f8116 CW |
991 | that operand is considered true if it is equal (C<==>) to the current |
992 | input line number (the C<$.> variable). | |
993 | ||
ba7f043c | 994 | To be pedantic, the comparison is actually S<C<int(EXPR) == int(EXPR)>>, |
df5f8116 CW |
995 | but that is only an issue if you use a floating point expression; when |
996 | implicitly using C<$.> as described in the previous paragraph, the | |
ba7f043c | 997 | comparison is S<C<int(EXPR) == int($.)>> which is only an issue when C<$.> |
df5f8116 | 998 | is set to a floating point value and you are not reading from a file. |
ba7f043c | 999 | Furthermore, S<C<"span" .. "spat">> or S<C<2.18 .. 3.14>> will not do what |
df5f8116 CW |
1000 | you want in scalar context because each of the operands are evaluated |
1001 | using their integer representation. | |
1002 | ||
1003 | Examples: | |
a0d0e21e LW |
1004 | |
1005 | As a scalar operator: | |
1006 | ||
df5f8116 | 1007 | if (101 .. 200) { print; } # print 2nd hundred lines, short for |
950b09ed | 1008 | # if ($. == 101 .. $. == 200) { print; } |
9f10b797 RGS |
1009 | |
1010 | next LINE if (1 .. /^$/); # skip header lines, short for | |
f343f960 | 1011 | # next LINE if ($. == 1 .. /^$/); |
9f10b797 RGS |
1012 | # (typically in a loop labeled LINE) |
1013 | ||
1014 | s/^/> / if (/^$/ .. eof()); # quote body | |
a0d0e21e | 1015 | |
5a964f20 TC |
1016 | # parse mail messages |
1017 | while (<>) { | |
1018 | $in_header = 1 .. /^$/; | |
df5f8116 CW |
1019 | $in_body = /^$/ .. eof; |
1020 | if ($in_header) { | |
f343f960 | 1021 | # do something |
df5f8116 | 1022 | } else { # in body |
f343f960 | 1023 | # do something else |
df5f8116 | 1024 | } |
5a964f20 | 1025 | } continue { |
df5f8116 | 1026 | close ARGV if eof; # reset $. each file |
5a964f20 TC |
1027 | } |
1028 | ||
acf31ca5 SF |
1029 | Here's a simple example to illustrate the difference between |
1030 | the two range operators: | |
1031 | ||
1032 | @lines = (" - Foo", | |
1033 | "01 - Bar", | |
1034 | "1 - Baz", | |
1035 | " - Quux"); | |
1036 | ||
9f10b797 RGS |
1037 | foreach (@lines) { |
1038 | if (/0/ .. /1/) { | |
acf31ca5 SF |
1039 | print "$_\n"; |
1040 | } | |
1041 | } | |
1042 | ||
46f8a5ea | 1043 | This program will print only the line containing "Bar". If |
9f10b797 | 1044 | the range operator is changed to C<...>, it will also print the |
acf31ca5 SF |
1045 | "Baz" line. |
1046 | ||
1047 | And now some examples as a list operator: | |
a0d0e21e | 1048 | |
1ca345ed TC |
1049 | for (101 .. 200) { print } # print $_ 100 times |
1050 | @foo = @foo[0 .. $#foo]; # an expensive no-op | |
1051 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items | |
a0d0e21e | 1052 | |
5a964f20 | 1053 | The range operator (in list context) makes use of the magical |
5f05dabc | 1054 | auto-increment algorithm if the operands are strings. You |
a0d0e21e LW |
1055 | can say |
1056 | ||
c543c01b | 1057 | @alphabet = ("A" .. "Z"); |
a0d0e21e | 1058 | |
54ae734e | 1059 | to get all normal letters of the English alphabet, or |
a0d0e21e | 1060 | |
c543c01b | 1061 | $hexdigit = (0 .. 9, "a" .. "f")[$num & 15]; |
a0d0e21e LW |
1062 | |
1063 | to get a hexadecimal digit, or | |
1064 | ||
1ca345ed TC |
1065 | @z2 = ("01" .. "31"); |
1066 | print $z2[$mday]; | |
a0d0e21e | 1067 | |
ea4f5703 YST |
1068 | to get dates with leading zeros. |
1069 | ||
1070 | If the final value specified is not in the sequence that the magical | |
1071 | increment would produce, the sequence goes until the next value would | |
1072 | be longer than the final value specified. | |
1073 | ||
d6c970c7 AC |
1074 | As of Perl 5.26, the list-context range operator on strings works as expected |
1075 | in the scope of L<< S<C<"use feature 'unicode_strings">>|feature/The | |
1076 | 'unicode_strings' feature >>. In previous versions, and outside the scope of | |
1077 | that feature, it exhibits L<perlunicode/The "Unicode Bug">: its behavior | |
1078 | depends on the internal encoding of the range endpoint. | |
1079 | ||
ea4f5703 | 1080 | If the initial value specified isn't part of a magical increment |
c543c01b | 1081 | sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>), |
ea4f5703 YST |
1082 | only the initial value will be returned. So the following will only |
1083 | return an alpha: | |
1084 | ||
c543c01b | 1085 | use charnames "greek"; |
ea4f5703 YST |
1086 | my @greek_small = ("\N{alpha}" .. "\N{omega}"); |
1087 | ||
c543c01b TC |
1088 | To get the 25 traditional lowercase Greek letters, including both sigmas, |
1089 | you could use this instead: | |
ea4f5703 | 1090 | |
c543c01b | 1091 | use charnames "greek"; |
a727cfac | 1092 | my @greek_small = map { chr } ( ord("\N{alpha}") |
1ca345ed | 1093 | .. |
a727cfac | 1094 | ord("\N{omega}") |
1ca345ed | 1095 | ); |
c543c01b TC |
1096 | |
1097 | However, because there are I<many> other lowercase Greek characters than | |
1098 | just those, to match lowercase Greek characters in a regular expression, | |
47c56cc8 KW |
1099 | you could use the pattern C</(?:(?=\p{Greek})\p{Lower})+/> (or the |
1100 | L<experimental feature|perlrecharclass/Extended Bracketed Character | |
1101 | Classes> C<S</(?[ \p{Greek} & \p{Lower} ])+/>>). | |
a0d0e21e | 1102 | |
ba7f043c | 1103 | Because each operand is evaluated in integer form, S<C<2.18 .. 3.14>> will |
df5f8116 CW |
1104 | return two elements in list context. |
1105 | ||
1106 | @list = (2.18 .. 3.14); # same as @list = (2 .. 3); | |
1107 | ||
a0d0e21e | 1108 | =head2 Conditional Operator |
d74e8afc | 1109 | X<operator, conditional> X<operator, ternary> X<ternary> X<?:> |
a0d0e21e | 1110 | |
ba7f043c KW |
1111 | Ternary C<"?:"> is the conditional operator, just as in C. It works much |
1112 | like an if-then-else. If the argument before the C<?> is true, the | |
1113 | argument before the C<:> is returned, otherwise the argument after the | |
1114 | C<:> is returned. For example: | |
cb1a09d0 | 1115 | |
54310121 | 1116 | printf "I have %d dog%s.\n", $n, |
c543c01b | 1117 | ($n == 1) ? "" : "s"; |
cb1a09d0 AD |
1118 | |
1119 | Scalar or list context propagates downward into the 2nd | |
54310121 | 1120 | or 3rd argument, whichever is selected. |
cb1a09d0 | 1121 | |
db691027 SF |
1122 | $x = $ok ? $y : $z; # get a scalar |
1123 | @x = $ok ? @y : @z; # get an array | |
1124 | $x = $ok ? @y : @z; # oops, that's just a count! | |
cb1a09d0 AD |
1125 | |
1126 | The operator may be assigned to if both the 2nd and 3rd arguments are | |
1127 | legal lvalues (meaning that you can assign to them): | |
a0d0e21e | 1128 | |
db691027 | 1129 | ($x_or_y ? $x : $y) = $z; |
a0d0e21e | 1130 | |
5a964f20 TC |
1131 | Because this operator produces an assignable result, using assignments |
1132 | without parentheses will get you in trouble. For example, this: | |
1133 | ||
db691027 | 1134 | $x % 2 ? $x += 10 : $x += 2 |
5a964f20 TC |
1135 | |
1136 | Really means this: | |
1137 | ||
db691027 | 1138 | (($x % 2) ? ($x += 10) : $x) += 2 |
5a964f20 TC |
1139 | |
1140 | Rather than this: | |
1141 | ||
db691027 | 1142 | ($x % 2) ? ($x += 10) : ($x += 2) |
5a964f20 | 1143 | |
19799a22 GS |
1144 | That should probably be written more simply as: |
1145 | ||
db691027 | 1146 | $x += ($x % 2) ? 10 : 2; |
19799a22 | 1147 | |
4633a7c4 | 1148 | =head2 Assignment Operators |
d74e8afc | 1149 | X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=> |
5ac3b81c | 1150 | X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=> |
fb7054ba | 1151 | X<%=> X<^=> X<x=> X<&.=> X<|.=> X<^.=> |
a0d0e21e | 1152 | |
ba7f043c | 1153 | C<"="> is the ordinary assignment operator. |
a0d0e21e LW |
1154 | |
1155 | Assignment operators work as in C. That is, | |
1156 | ||
db691027 | 1157 | $x += 2; |
a0d0e21e LW |
1158 | |
1159 | is equivalent to | |
1160 | ||
db691027 | 1161 | $x = $x + 2; |
a0d0e21e LW |
1162 | |
1163 | although without duplicating any side effects that dereferencing the lvalue | |
ba7f043c | 1164 | might trigger, such as from C<tie()>. Other assignment operators work similarly. |
54310121 | 1165 | The following are recognized: |
a0d0e21e | 1166 | |
fb7054ba FC |
1167 | **= += *= &= &.= <<= &&= |
1168 | -= /= |= |.= >>= ||= | |
1169 | .= %= ^= ^.= //= | |
9f10b797 | 1170 | x= |
a0d0e21e | 1171 | |
19799a22 | 1172 | Although these are grouped by family, they all have the precedence |
82848c10 FC |
1173 | of assignment. These combined assignment operators can only operate on |
1174 | scalars, whereas the ordinary assignment operator can assign to arrays, | |
1175 | hashes, lists and even references. (See L<"Context"|perldata/Context> | |
1176 | and L<perldata/List value constructors>, and L<perlref/Assigning to | |
1177 | References>.) | |
a0d0e21e | 1178 | |
b350dd2f GS |
1179 | Unlike in C, the scalar assignment operator produces a valid lvalue. |
1180 | Modifying an assignment is equivalent to doing the assignment and | |
1181 | then modifying the variable that was assigned to. This is useful | |
1182 | for modifying a copy of something, like this: | |
a0d0e21e | 1183 | |
1ca345ed TC |
1184 | ($tmp = $global) =~ tr/13579/24680/; |
1185 | ||
1186 | Although as of 5.14, that can be also be accomplished this way: | |
1187 | ||
1188 | use v5.14; | |
1189 | $tmp = ($global =~ tr/13579/24680/r); | |
a0d0e21e LW |
1190 | |
1191 | Likewise, | |
1192 | ||
db691027 | 1193 | ($x += 2) *= 3; |
a0d0e21e LW |
1194 | |
1195 | is equivalent to | |
1196 | ||
db691027 SF |
1197 | $x += 2; |
1198 | $x *= 3; | |
a0d0e21e | 1199 | |
b350dd2f GS |
1200 | Similarly, a list assignment in list context produces the list of |
1201 | lvalues assigned to, and a list assignment in scalar context returns | |
1202 | the number of elements produced by the expression on the right hand | |
1203 | side of the assignment. | |
1204 | ||
ba7f043c | 1205 | The three dotted bitwise assignment operators (C<&.=> C<|.=> C<^.=>) are new in |
fb7054ba FC |
1206 | Perl 5.22 and experimental. See L</Bitwise String Operators>. |
1207 | ||
748a9306 | 1208 | =head2 Comma Operator |
d74e8afc | 1209 | X<comma> X<operator, comma> X<,> |
a0d0e21e | 1210 | |
ba7f043c | 1211 | Binary C<","> is the comma operator. In scalar context it evaluates |
a0d0e21e LW |
1212 | its left argument, throws that value away, then evaluates its right |
1213 | argument and returns that value. This is just like C's comma operator. | |
1214 | ||
5a964f20 | 1215 | In list context, it's just the list argument separator, and inserts |
ed5c6d31 PJ |
1216 | both its arguments into the list. These arguments are also evaluated |
1217 | from left to right. | |
a0d0e21e | 1218 | |
ba7f043c KW |
1219 | The C<< => >> operator (sometimes pronounced "fat comma") is a synonym |
1220 | for the comma except that it causes a | |
4e1988c6 | 1221 | word on its left to be interpreted as a string if it begins with a letter |
344f2c40 IG |
1222 | or underscore and is composed only of letters, digits and underscores. |
1223 | This includes operands that might otherwise be interpreted as operators, | |
46f8a5ea | 1224 | constants, single number v-strings or function calls. If in doubt about |
c543c01b | 1225 | this behavior, the left operand can be quoted explicitly. |
344f2c40 IG |
1226 | |
1227 | Otherwise, the C<< => >> operator behaves exactly as the comma operator | |
1228 | or list argument separator, according to context. | |
1229 | ||
1230 | For example: | |
a44e5664 MS |
1231 | |
1232 | use constant FOO => "something"; | |
1233 | ||
1234 | my %h = ( FOO => 23 ); | |
1235 | ||
1236 | is equivalent to: | |
1237 | ||
1238 | my %h = ("FOO", 23); | |
1239 | ||
1240 | It is I<NOT>: | |
1241 | ||
1242 | my %h = ("something", 23); | |
1243 | ||
719b43e8 RGS |
1244 | The C<< => >> operator is helpful in documenting the correspondence |
1245 | between keys and values in hashes, and other paired elements in lists. | |
748a9306 | 1246 | |
a12b8f3c FC |
1247 | %hash = ( $key => $value ); |
1248 | login( $username => $password ); | |
a44e5664 | 1249 | |
4e1988c6 FC |
1250 | The special quoting behavior ignores precedence, and hence may apply to |
1251 | I<part> of the left operand: | |
1252 | ||
1253 | print time.shift => "bbb"; | |
1254 | ||
ba7f043c | 1255 | That example prints something like C<"1314363215shiftbbb">, because the |
4e1988c6 FC |
1256 | C<< => >> implicitly quotes the C<shift> immediately on its left, ignoring |
1257 | the fact that C<time.shift> is the entire left operand. | |
1258 | ||
a0d0e21e | 1259 | =head2 List Operators (Rightward) |
d74e8afc | 1260 | X<operator, list, rightward> X<list operator> |
a0d0e21e | 1261 | |
c543c01b | 1262 | On the right side of a list operator, the comma has very low precedence, |
a0d0e21e LW |
1263 | such that it controls all comma-separated expressions found there. |
1264 | The only operators with lower precedence are the logical operators | |
ba7f043c | 1265 | C<"and">, C<"or">, and C<"not">, which may be used to evaluate calls to list |
1ca345ed TC |
1266 | operators without the need for parentheses: |
1267 | ||
a8980281 P |
1268 | open HANDLE, "< :encoding(UTF-8)", "filename" |
1269 | or die "Can't open: $!\n"; | |
1ca345ed TC |
1270 | |
1271 | However, some people find that code harder to read than writing | |
1272 | it with parentheses: | |
1273 | ||
a8980281 P |
1274 | open(HANDLE, "< :encoding(UTF-8)", "filename") |
1275 | or die "Can't open: $!\n"; | |
1ca345ed | 1276 | |
ba7f043c | 1277 | in which case you might as well just use the more customary C<"||"> operator: |
a0d0e21e | 1278 | |
a8980281 P |
1279 | open(HANDLE, "< :encoding(UTF-8)", "filename") |
1280 | || die "Can't open: $!\n"; | |
a0d0e21e | 1281 | |
a95b3d6a | 1282 | See also discussion of list operators in L</Terms and List Operators (Leftward)>. |
a0d0e21e LW |
1283 | |
1284 | =head2 Logical Not | |
d74e8afc | 1285 | X<operator, logical, not> X<not> |
a0d0e21e | 1286 | |
ba7f043c KW |
1287 | Unary C<"not"> returns the logical negation of the expression to its right. |
1288 | It's the equivalent of C<"!"> except for the very low precedence. | |
a0d0e21e LW |
1289 | |
1290 | =head2 Logical And | |
d74e8afc | 1291 | X<operator, logical, and> X<and> |
a0d0e21e | 1292 | |
ba7f043c | 1293 | Binary C<"and"> returns the logical conjunction of the two surrounding |
c543c01b TC |
1294 | expressions. It's equivalent to C<&&> except for the very low |
1295 | precedence. This means that it short-circuits: the right | |
a0d0e21e LW |
1296 | expression is evaluated only if the left expression is true. |
1297 | ||
59ab9d6e | 1298 | =head2 Logical or and Exclusive Or |
f23102e2 | 1299 | X<operator, logical, or> X<operator, logical, xor> |
59ab9d6e | 1300 | X<operator, logical, exclusive or> |
f23102e2 | 1301 | X<or> X<xor> |
a0d0e21e | 1302 | |
ba7f043c | 1303 | Binary C<"or"> returns the logical disjunction of the two surrounding |
c543c01b TC |
1304 | expressions. It's equivalent to C<||> except for the very low precedence. |
1305 | This makes it useful for control flow: | |
5a964f20 TC |
1306 | |
1307 | print FH $data or die "Can't write to FH: $!"; | |
1308 | ||
c543c01b TC |
1309 | This means that it short-circuits: the right expression is evaluated |
1310 | only if the left expression is false. Due to its precedence, you must | |
1311 | be careful to avoid using it as replacement for the C<||> operator. | |
1312 | It usually works out better for flow control than in assignments: | |
5a964f20 | 1313 | |
db691027 SF |
1314 | $x = $y or $z; # bug: this is wrong |
1315 | ($x = $y) or $z; # really means this | |
1316 | $x = $y || $z; # better written this way | |
5a964f20 | 1317 | |
19799a22 | 1318 | However, when it's a list-context assignment and you're trying to use |
ba7f043c | 1319 | C<||> for control flow, you probably need C<"or"> so that the assignment |
5a964f20 TC |
1320 | takes higher precedence. |
1321 | ||
1322 | @info = stat($file) || die; # oops, scalar sense of stat! | |
1323 | @info = stat($file) or die; # better, now @info gets its due | |
1324 | ||
c963b151 BD |
1325 | Then again, you could always use parentheses. |
1326 | ||
ba7f043c | 1327 | Binary C<"xor"> returns the exclusive-OR of the two surrounding expressions. |
c543c01b | 1328 | It cannot short-circuit (of course). |
a0d0e21e | 1329 | |
59ab9d6e MB |
1330 | There is no low precedence operator for defined-OR. |
1331 | ||
a0d0e21e | 1332 | =head2 C Operators Missing From Perl |
d74e8afc ITB |
1333 | X<operator, missing from perl> X<&> X<*> |
1334 | X<typecasting> X<(TYPE)> | |
a0d0e21e LW |
1335 | |
1336 | Here is what C has that Perl doesn't: | |
1337 | ||
1338 | =over 8 | |
1339 | ||
1340 | =item unary & | |
1341 | ||
ba7f043c | 1342 | Address-of operator. (But see the C<"\"> operator for taking a reference.) |
a0d0e21e LW |
1343 | |
1344 | =item unary * | |
1345 | ||
46f8a5ea | 1346 | Dereference-address operator. (Perl's prefix dereferencing |
ba7f043c | 1347 | operators are typed: C<$>, C<@>, C<%>, and C<&>.) |
a0d0e21e LW |
1348 | |
1349 | =item (TYPE) | |
1350 | ||
19799a22 | 1351 | Type-casting operator. |
a0d0e21e LW |
1352 | |
1353 | =back | |
1354 | ||
5f05dabc | 1355 | =head2 Quote and Quote-like Operators |
89d205f2 | 1356 | X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m> |
d74e8afc ITB |
1357 | X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>> |
1358 | X<escape sequence> X<escape> | |
1359 | ||
a0d0e21e LW |
1360 | While we usually think of quotes as literal values, in Perl they |
1361 | function as operators, providing various kinds of interpolating and | |
1362 | pattern matching capabilities. Perl provides customary quote characters | |
1363 | for these behaviors, but also provides a way for you to choose your | |
1364 | quote character for any of them. In the following table, a C<{}> represents | |
9f10b797 | 1365 | any pair of delimiters you choose. |
a0d0e21e | 1366 | |
2c268ad5 TP |
1367 | Customary Generic Meaning Interpolates |
1368 | '' q{} Literal no | |
1369 | "" qq{} Literal yes | |
af9219ee | 1370 | `` qx{} Command yes* |
2c268ad5 | 1371 | qw{} Word list no |
af9219ee MG |
1372 | // m{} Pattern match yes* |
1373 | qr{} Pattern yes* | |
1374 | s{}{} Substitution yes* | |
2c268ad5 | 1375 | tr{}{} Transliteration no (but see below) |
c543c01b | 1376 | y{}{} Transliteration no (but see below) |
7e3b091d | 1377 | <<EOF here-doc yes* |
a0d0e21e | 1378 | |
af9219ee MG |
1379 | * unless the delimiter is ''. |
1380 | ||
87275199 | 1381 | Non-bracketing delimiters use the same character fore and aft, but the four |
c543c01b | 1382 | sorts of ASCII brackets (round, angle, square, curly) all nest, which means |
9f10b797 | 1383 | that |
87275199 | 1384 | |
c543c01b | 1385 | q{foo{bar}baz} |
35f2feb0 | 1386 | |
9f10b797 | 1387 | is the same as |
87275199 | 1388 | |
c543c01b | 1389 | 'foo{bar}baz' |
87275199 GS |
1390 | |
1391 | Note, however, that this does not always work for quoting Perl code: | |
1392 | ||
db691027 | 1393 | $s = q{ if($x eq "}") ... }; # WRONG |
87275199 | 1394 | |
ba7f043c | 1395 | is a syntax error. The C<L<Text::Balanced>> module (standard as of v5.8, |
c543c01b | 1396 | and from CPAN before then) is able to do this properly. |
87275199 | 1397 | |
841bfb48 KW |
1398 | There can (and in some cases, must) be whitespace between the operator |
1399 | and the quoting | |
fb73857a | 1400 | characters, except when C<#> is being used as the quoting character. |
ba7f043c | 1401 | C<q#foo#> is parsed as the string C<foo>, while S<C<q #foo#>> is the |
19799a22 GS |
1402 | operator C<q> followed by a comment. Its argument will be taken |
1403 | from the next line. This allows you to write: | |
fb73857a | 1404 | |
1405 | s {foo} # Replace foo | |
1406 | {bar} # with bar. | |
1407 | ||
841bfb48 KW |
1408 | The cases where whitespace must be used are when the quoting character |
1409 | is a word character (meaning it matches C</\w/>): | |
1410 | ||
1411 | q XfooX # Works: means the string 'foo' | |
1412 | qXfooX # WRONG! | |
1413 | ||
c543c01b TC |
1414 | The following escape sequences are available in constructs that interpolate, |
1415 | and in transliterations: | |
5691ca5f | 1416 | X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}> |
04341565 | 1417 | X<\o{}> |
5691ca5f | 1418 | |
2c4c1ff2 KW |
1419 | Sequence Note Description |
1420 | \t tab (HT, TAB) | |
1421 | \n newline (NL) | |
1422 | \r return (CR) | |
1423 | \f form feed (FF) | |
1424 | \b backspace (BS) | |
1425 | \a alarm (bell) (BEL) | |
1426 | \e escape (ESC) | |
c543c01b | 1427 | \x{263A} [1,8] hex char (example: SMILEY) |
2c4c1ff2 | 1428 | \x1b [2,8] restricted range hex char (example: ESC) |
fb121860 | 1429 | \N{name} [3] named Unicode character or character sequence |
2c4c1ff2 KW |
1430 | \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON) |
1431 | \c[ [5] control char (example: chr(27)) | |
1432 | \o{23072} [6,8] octal char (example: SMILEY) | |
1433 | \033 [7,8] restricted range octal char (example: ESC) | |
5691ca5f KW |
1434 | |
1435 | =over 4 | |
1436 | ||
1437 | =item [1] | |
1438 | ||
2c4c1ff2 KW |
1439 | The result is the character specified by the hexadecimal number between |
1440 | the braces. See L</[8]> below for details on which character. | |
96448467 | 1441 | |
46f8a5ea | 1442 | Only hexadecimal digits are valid between the braces. If an invalid |
96448467 DG |
1443 | character is encountered, a warning will be issued and the invalid |
1444 | character and all subsequent characters (valid or invalid) within the | |
1445 | braces will be discarded. | |
1446 | ||
1447 | If there are no valid digits between the braces, the generated character is | |
1448 | the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>) | |
c543c01b | 1449 | will not cause a warning (currently). |
40687185 KW |
1450 | |
1451 | =item [2] | |
1452 | ||
2c4c1ff2 KW |
1453 | The result is the character specified by the hexadecimal number in the range |
1454 | 0x00 to 0xFF. See L</[8]> below for details on which character. | |
96448467 DG |
1455 | |
1456 | Only hexadecimal digits are valid following C<\x>. When C<\x> is followed | |
2c4c1ff2 | 1457 | by fewer than two valid digits, any valid digits will be zero-padded. This |
ba7f043c | 1458 | means that C<\x7> will be interpreted as C<\x07>, and a lone C<"\x"> will be |
2c4c1ff2 | 1459 | interpreted as C<\x00>. Except at the end of a string, having fewer than |
c543c01b | 1460 | two valid digits will result in a warning. Note that although the warning |
96448467 DG |
1461 | says the illegal character is ignored, it is only ignored as part of the |
1462 | escape and will still be used as the subsequent character in the string. | |
1463 | For example: | |
1464 | ||
1465 | Original Result Warns? | |
1466 | "\x7" "\x07" no | |
1467 | "\x" "\x00" no | |
1468 | "\x7q" "\x07q" yes | |
1469 | "\xq" "\x00q" yes | |
1470 | ||
40687185 KW |
1471 | =item [3] |
1472 | ||
fb121860 | 1473 | The result is the Unicode character or character sequence given by I<name>. |
2c4c1ff2 | 1474 | See L<charnames>. |
40687185 KW |
1475 | |
1476 | =item [4] | |
1477 | ||
ba7f043c | 1478 | S<C<\N{U+I<hexadecimal number>}>> means the Unicode character whose Unicode code |
2c4c1ff2 | 1479 | point is I<hexadecimal number>. |
40687185 KW |
1480 | |
1481 | =item [5] | |
1482 | ||
5691ca5f KW |
1483 | The character following C<\c> is mapped to some other character as shown in the |
1484 | table: | |
1485 | ||
1486 | Sequence Value | |
1487 | \c@ chr(0) | |
1488 | \cA chr(1) | |
1489 | \ca chr(1) | |
1490 | \cB chr(2) | |
1491 | \cb chr(2) | |
1492 | ... | |
1493 | \cZ chr(26) | |
1494 | \cz chr(26) | |
1495 | \c[ chr(27) | |
ba7f043c | 1496 | # See below for chr(28) |
5691ca5f KW |
1497 | \c] chr(29) |
1498 | \c^ chr(30) | |
c3e9d7a9 | 1499 | \c_ chr(31) |
ba7f043c KW |
1500 | \c? chr(127) # (on ASCII platforms; see below for link to |
1501 | # EBCDIC discussion) | |
5691ca5f | 1502 | |
d813941f | 1503 | In other words, it's the character whose code point has had 64 xor'd with |
c3e9d7a9 KW |
1504 | its uppercase. C<\c?> is DELETE on ASCII platforms because |
1505 | S<C<ord("?") ^ 64>> is 127, and | |
ba7f043c | 1506 | C<\c@> is NULL because the ord of C<"@"> is 64, so xor'ing 64 itself produces 0. |
d813941f | 1507 | |
ba7f043c | 1508 | Also, C<\c\I<X>> yields S<C< chr(28) . "I<X>">> for any I<X>, but cannot come at the |
5691ca5f KW |
1509 | end of a string, because the backslash would be parsed as escaping the end |
1510 | quote. | |
1511 | ||
1512 | On ASCII platforms, the resulting characters from the list above are the | |
1513 | complete set of ASCII controls. This isn't the case on EBCDIC platforms; see | |
c3e9d7a9 KW |
1514 | L<perlebcdic/OPERATOR DIFFERENCES> for a full discussion of the |
1515 | differences between these for ASCII versus EBCDIC platforms. | |
5691ca5f | 1516 | |
c3e9d7a9 | 1517 | Use of any other character following the C<"c"> besides those listed above is |
63a63d81 KW |
1518 | discouraged, and as of Perl v5.20, the only characters actually allowed |
1519 | are the printable ASCII ones, minus the left brace C<"{">. What happens | |
1520 | for any of the allowed other characters is that the value is derived by | |
1521 | xor'ing with the seventh bit, which is 64, and a warning raised if | |
1522 | enabled. Using the non-allowed characters generates a fatal error. | |
5691ca5f KW |
1523 | |
1524 | To get platform independent controls, you can use C<\N{...}>. | |
1525 | ||
40687185 KW |
1526 | =item [6] |
1527 | ||
2c4c1ff2 KW |
1528 | The result is the character specified by the octal number between the braces. |
1529 | See L</[8]> below for details on which character. | |
04341565 DG |
1530 | |
1531 | If a character that isn't an octal digit is encountered, a warning is raised, | |
1532 | and the value is based on the octal digits before it, discarding it and all | |
1533 | following characters up to the closing brace. It is a fatal error if there are | |
1534 | no octal digits at all. | |
1535 | ||
1536 | =item [7] | |
1537 | ||
c543c01b | 1538 | The result is the character specified by the three-digit octal number in the |
2c4c1ff2 KW |
1539 | range 000 to 777 (but best to not use above 077, see next paragraph). See |
1540 | L</[8]> below for details on which character. | |
1541 | ||
1542 | Some contexts allow 2 or even 1 digit, but any usage without exactly | |
40687185 | 1543 | three digits, the first being a zero, may give unintended results. (For |
5db3e519 FC |
1544 | example, in a regular expression it may be confused with a backreference; |
1545 | see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may | |
c543c01b | 1546 | use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to |
04341565 DG |
1547 | use this construct only for ordinals C<\077> and below, remembering to pad to |
1548 | the left with zeros to make three digits. For larger ordinals, either use | |
ba7f043c KW |
1549 | C<\o{}>, or convert to something else, such as to hex and use C<\N{U+}> |
1550 | (which is portable between platforms with different character sets) or | |
1551 | C<\x{}> instead. | |
40687185 | 1552 | |
2c4c1ff2 KW |
1553 | =item [8] |
1554 | ||
c543c01b | 1555 | Several constructs above specify a character by a number. That number |
2c4c1ff2 | 1556 | gives the character's position in the character set encoding (indexed from 0). |
c543c01b | 1557 | This is called synonymously its ordinal, code position, or code point. Perl |
2c4c1ff2 KW |
1558 | works on platforms that have a native encoding currently of either ASCII/Latin1 |
1559 | or EBCDIC, each of which allow specification of 256 characters. In general, if | |
1560 | the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's | |
1561 | native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets | |
c543c01b | 1562 | it as a Unicode code point and the result is the corresponding Unicode |
2c4c1ff2 KW |
1563 | character. For example C<\x{50}> and C<\o{120}> both are the number 80 in |
1564 | decimal, which is less than 256, so the number is interpreted in the native | |
1565 | character set encoding. In ASCII the character in the 80th position (indexed | |
ba7f043c | 1566 | from 0) is the letter C<"P">, and in EBCDIC it is the ampersand symbol C<"&">. |
2c4c1ff2 KW |
1567 | C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted |
1568 | as a Unicode code point no matter what the native encoding is. The name of the | |
9fef6a0d | 1569 | character in the 256th position (indexed by 0) in Unicode is |
2c4c1ff2 KW |
1570 | C<LATIN CAPITAL LETTER A WITH MACRON>. |
1571 | ||
2dc9bc84 | 1572 | An exception to the above rule is that S<C<\N{U+I<hex number>}>> is |
ba7f043c | 1573 | always interpreted as a Unicode code point, so that C<\N{U+0050}> is C<"P"> even |
2dc9bc84 | 1574 | on EBCDIC platforms. |
2c4c1ff2 | 1575 | |
5691ca5f | 1576 | =back |
4c77eaa2 | 1577 | |
e526e8bb | 1578 | B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for |
8b312c40 | 1579 | the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may |
ba7f043c | 1580 | use C<\N{VT}>, C<\ck>, C<\N{U+0b}>, or C<\x0b>. (C<\v> |
e526e8bb KW |
1581 | does have meaning in regular expression patterns in Perl, see L<perlre>.) |
1582 | ||
1583 | The following escape sequences are available in constructs that interpolate, | |
904501ec | 1584 | but not in transliterations. |
628253b8 | 1585 | X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> X<\F> |
904501ec | 1586 | |
c543c01b TC |
1587 | \l lowercase next character only |
1588 | \u titlecase (not uppercase!) next character only | |
e4d34742 EB |
1589 | \L lowercase all characters till \E or end of string |
1590 | \U uppercase all characters till \E or end of string | |
628253b8 | 1591 | \F foldcase all characters till \E or end of string |
736fe711 KW |
1592 | \Q quote (disable) pattern metacharacters till \E or |
1593 | end of string | |
7e31b643 | 1594 | \E end either case modification or quoted section |
c543c01b TC |
1595 | (whichever was last seen) |
1596 | ||
736fe711 KW |
1597 | See L<perlfunc/quotemeta> for the exact definition of characters that |
1598 | are quoted by C<\Q>. | |
1599 | ||
628253b8 | 1600 | C<\L>, C<\U>, C<\F>, and C<\Q> can stack, in which case you need one |
c543c01b TC |
1601 | C<\E> for each. For example: |
1602 | ||
9fef6a0d KW |
1603 | say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?"; |
1604 | This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it? | |
a0d0e21e | 1605 | |
ba7f043c KW |
1606 | If a S<C<use locale>> form that includes C<LC_CTYPE> is in effect (see |
1607 | L<perllocale>), the case map used by C<\l>, C<\L>, C<\u>, and C<\U> is | |
1608 | taken from the current locale. If Unicode (for example, C<\N{}> or code | |
1609 | points of 0x100 or beyond) is being used, the case map used by C<\l>, | |
1610 | C<\L>, C<\u>, and C<\U> is as defined by Unicode. That means that | |
1611 | case-mapping a single character can sometimes produce a sequence of | |
1612 | several characters. | |
1613 | Under S<C<use locale>>, C<\F> produces the same results as C<\L> | |
31f05a37 KW |
1614 | for all locales but a UTF-8 one, where it instead uses the Unicode |
1615 | definition. | |
a034a98d | 1616 | |
5a964f20 TC |
1617 | All systems use the virtual C<"\n"> to represent a line terminator, |
1618 | called a "newline". There is no such thing as an unvarying, physical | |
19799a22 | 1619 | newline character. It is only an illusion that the operating system, |
5a964f20 TC |
1620 | device drivers, C libraries, and Perl all conspire to preserve. Not all |
1621 | systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example, | |
c543c01b | 1622 | on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed, |
ba7f043c | 1623 | and on systems without a line terminator, |
c543c01b | 1624 | printing C<"\n"> might emit no actual data. In general, use C<"\n"> when |
5a964f20 TC |
1625 | you mean a "newline" for your system, but use the literal ASCII when you |
1626 | need an exact character. For example, most networking protocols expect | |
2a380090 | 1627 | and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators, |
5a964f20 TC |
1628 | and although they often accept just C<"\012">, they seldom tolerate just |
1629 | C<"\015">. If you get in the habit of using C<"\n"> for networking, | |
1630 | you may be burned some day. | |
d74e8afc ITB |
1631 | X<newline> X<line terminator> X<eol> X<end of line> |
1632 | X<\n> X<\r> X<\r\n> | |
5a964f20 | 1633 | |
904501ec MG |
1634 | For constructs that do interpolate, variables beginning with "C<$>" |
1635 | or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or | |
ad0f383a A |
1636 | C<< $href->{key}[0] >> are also interpolated, as are array and hash slices. |
1637 | But method calls such as C<< $obj->meth >> are not. | |
af9219ee MG |
1638 | |
1639 | Interpolating an array or slice interpolates the elements in order, | |
1640 | separated by the value of C<$">, so is equivalent to interpolating | |
ba7f043c | 1641 | S<C<join $", @array>>. "Punctuation" arrays such as C<@*> are usually |
c543c01b TC |
1642 | interpolated only if the name is enclosed in braces C<@{*}>, but the |
1643 | arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces. | |
af9219ee | 1644 | |
bc7b91c6 EB |
1645 | For double-quoted strings, the quoting from C<\Q> is applied after |
1646 | interpolation and escapes are processed. | |
1647 | ||
1648 | "abc\Qfoo\tbar$s\Exyz" | |
1649 | ||
1650 | is equivalent to | |
1651 | ||
1652 | "abc" . quotemeta("foo\tbar$s") . "xyz" | |
1653 | ||
1654 | For the pattern of regex operators (C<qr//>, C<m//> and C<s///>), | |
1655 | the quoting from C<\Q> is applied after interpolation is processed, | |
46f8a5ea FC |
1656 | but before escapes are processed. This allows the pattern to match |
1657 | literally (except for C<$> and C<@>). For example, the following matches: | |
bc7b91c6 EB |
1658 | |
1659 | '\s\t' =~ /\Q\s\t/ | |
1660 | ||
1661 | Because C<$> or C<@> trigger interpolation, you'll need to use something | |
1662 | like C</\Quser\E\@\Qhost/> to match them literally. | |
1d2dff63 | 1663 | |
a0d0e21e LW |
1664 | Patterns are subject to an additional level of interpretation as a |
1665 | regular expression. This is done as a second pass, after variables are | |
1666 | interpolated, so that regular expressions may be incorporated into the | |
1667 | pattern from the variables. If this is not what you want, use C<\Q> to | |
1668 | interpolate a variable literally. | |
1669 | ||
19799a22 GS |
1670 | Apart from the behavior described above, Perl does not expand |
1671 | multiple levels of interpolation. In particular, contrary to the | |
1672 | expectations of shell programmers, back-quotes do I<NOT> interpolate | |
1673 | within double quotes, nor do single quotes impede evaluation of | |
1674 | variables when used within double quotes. | |
a0d0e21e | 1675 | |
5f05dabc | 1676 | =head2 Regexp Quote-Like Operators |
d74e8afc | 1677 | X<operator, regexp> |
cb1a09d0 | 1678 | |
5f05dabc | 1679 | Here are the quote-like operators that apply to pattern |
cb1a09d0 AD |
1680 | matching and related activities. |
1681 | ||
a0d0e21e LW |
1682 | =over 8 |
1683 | ||
ba7f043c | 1684 | =item C<qr/I<STRING>/msixpodualn> |
01c6f5f4 | 1685 | X<qr> X</i> X</m> X</o> X</s> X</x> X</p> |
a0d0e21e | 1686 | |
87e95b7f YO |
1687 | This operator quotes (and possibly compiles) its I<STRING> as a regular |
1688 | expression. I<STRING> is interpolated the same way as I<PATTERN> | |
6d314683 YO |
1689 | in C<m/I<PATTERN>/>. If C<"'"> is used as the delimiter, no variable |
1690 | interpolation is done. Returns a Perl value which may be used instead of the | |
ba7f043c | 1691 | corresponding C</I<STRING>/msixpodualn> expression. The returned value is a |
46f8a5ea | 1692 | normalized version of the original pattern. It magically differs from |
1c8ee595 | 1693 | a string containing the same characters: C<ref(qr/x/)> returns "Regexp"; |
a727cfac | 1694 | however, dereferencing it is not well defined (you currently get the |
1c8ee595 CO |
1695 | normalized version of the original pattern, but this may change). |
1696 | ||
a0d0e21e | 1697 | |
87e95b7f YO |
1698 | For example, |
1699 | ||
1700 | $rex = qr/my.STRING/is; | |
85dd5c8b | 1701 | print $rex; # prints (?si-xm:my.STRING) |
87e95b7f YO |
1702 | s/$rex/foo/; |
1703 | ||
1704 | is equivalent to | |
1705 | ||
1706 | s/my.STRING/foo/is; | |
1707 | ||
1708 | The result may be used as a subpattern in a match: | |
1709 | ||
1710 | $re = qr/$pattern/; | |
7188ca43 KW |
1711 | $string =~ /foo${re}bar/; # can be interpolated in other |
1712 | # patterns | |
87e95b7f YO |
1713 | $string =~ $re; # or used standalone |
1714 | $string =~ /$re/; # or this way | |
1715 | ||
ba7f043c KW |
1716 | Since Perl may compile the pattern at the moment of execution of the C<qr()> |
1717 | operator, using C<qr()> may have speed advantages in some situations, | |
1718 | notably if the result of C<qr()> is used standalone: | |
87e95b7f YO |
1719 | |
1720 | sub match { | |
1721 | my $patterns = shift; | |
1722 | my @compiled = map qr/$_/i, @$patterns; | |
1723 | grep { | |
1724 | my $success = 0; | |
1725 | foreach my $pat (@compiled) { | |
1726 | $success = 1, last if /$pat/; | |
1727 | } | |
1728 | $success; | |
1729 | } @_; | |
5a964f20 TC |
1730 | } |
1731 | ||
87e95b7f | 1732 | Precompilation of the pattern into an internal representation at |
ba7f043c | 1733 | the moment of C<qr()> avoids the need to recompile the pattern every |
87e95b7f YO |
1734 | time a match C</$pat/> is attempted. (Perl has many other internal |
1735 | optimizations, but none would be triggered in the above example if | |
ba7f043c | 1736 | we did not use C<qr()> operator.) |
87e95b7f | 1737 | |
765fa144 | 1738 | Options (specified by the following modifiers) are: |
87e95b7f YO |
1739 | |
1740 | m Treat string as multiple lines. | |
1741 | s Treat string as single line. (Make . match a newline) | |
1742 | i Do case-insensitive pattern matching. | |
77c8f263 KW |
1743 | x Use extended regular expressions; specifying two |
1744 | x's means \t and the SPACE character are ignored within | |
1745 | square-bracketed character classes | |
87e95b7f | 1746 | p When matching preserve a copy of the matched string so |
7188ca43 | 1747 | that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be |
ba7f043c | 1748 | defined (ignored starting in v5.20) as these are always |
1a8aad5a | 1749 | defined starting in that release |
87e95b7f | 1750 | o Compile pattern only once. |
8ef45c18 KW |
1751 | a ASCII-restrict: Use ASCII for \d, \s, \w and [[:posix:]] |
1752 | character classes; specifying two a's adds the further | |
1753 | restriction that no ASCII character will match a | |
1754 | non-ASCII one under /i. | |
ba7f043c | 1755 | l Use the current run-time locale's rules. |
48cbae4f SK |
1756 | u Use Unicode rules. |
1757 | d Use Unicode or native charset, as in 5.12 and earlier. | |
33be4c61 | 1758 | n Non-capture mode. Don't let () fill in $1, $2, etc... |
87e95b7f YO |
1759 | |
1760 | If a precompiled pattern is embedded in a larger pattern then the effect | |
ba7f043c KW |
1761 | of C<"msixpluadn"> will be propagated appropriately. The effect that the |
1762 | C</o> modifier has is not propagated, being restricted to those patterns | |
87e95b7f YO |
1763 | explicitly using it. |
1764 | ||
b6fa137b | 1765 | The last four modifiers listed above, added in Perl 5.14, |
850b7ec9 | 1766 | control the character set rules, but C</a> is the only one you are likely |
18509dec KW |
1767 | to want to specify explicitly; the other three are selected |
1768 | automatically by various pragmas. | |
da392a17 | 1769 | |
ba7f043c | 1770 | See L<perlre> for additional information on valid syntax for I<STRING>, and |
5e2aa8f5 | 1771 | for a detailed look at the semantics of regular expressions. In |
1ca345ed TC |
1772 | particular, all modifiers except the largely obsolete C</o> are further |
1773 | explained in L<perlre/Modifiers>. C</o> is described in the next section. | |
a0d0e21e | 1774 | |
ba7f043c | 1775 | =item C<m/I<PATTERN>/msixpodualngc> |
89d205f2 YO |
1776 | X<m> X<operator, match> |
1777 | X<regexp, options> X<regexp> X<regex, options> X<regex> | |
01c6f5f4 | 1778 | X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> |
a0d0e21e | 1779 | |
ba7f043c | 1780 | =item C</I<PATTERN>/msixpodualngc> |
a0d0e21e | 1781 | |
5a964f20 | 1782 | Searches a string for a pattern match, and in scalar context returns |
19799a22 | 1783 | true if it succeeds, false if it fails. If no string is specified |
ba7f043c | 1784 | via the C<=~> or C<!~> operator, the C<$_> string is searched. (The |
19799a22 GS |
1785 | string specified with C<=~> need not be an lvalue--it may be the |
1786 | result of an expression evaluation, but remember the C<=~> binds | |
006671a6 | 1787 | rather tightly.) See also L<perlre>. |
a0d0e21e | 1788 | |
f6050459 | 1789 | Options are as described in C<qr//> above; in addition, the following match |
01c6f5f4 | 1790 | process modifiers are available: |
a0d0e21e | 1791 | |
950b09ed | 1792 | g Match globally, i.e., find all occurrences. |
7188ca43 KW |
1793 | c Do not reset search position on a failed match when /g is |
1794 | in effect. | |
a0d0e21e | 1795 | |
ba7f043c | 1796 | If C<"/"> is the delimiter then the initial C<m> is optional. With the C<m> |
c543c01b | 1797 | you can use any pair of non-whitespace (ASCII) characters |
725a61d7 | 1798 | as delimiters. This is particularly useful for matching path names |
ba7f043c | 1799 | that contain C<"/">, to avoid LTS (leaning toothpick syndrome). If C<"?"> is |
725a61d7 | 1800 | the delimiter, then a match-only-once rule applies, |
ba7f043c | 1801 | described in C<m?I<PATTERN>?> below. If C<"'"> (single quote) is the delimiter, |
6d314683 | 1802 | no variable interpolation is performed on the I<PATTERN>. |
ba7f043c | 1803 | When using a delimiter character valid in an identifier, whitespace is required |
ed02a3bf | 1804 | after the C<m>. |
a0d0e21e | 1805 | |
ba7f043c | 1806 | I<PATTERN> may contain variables, which will be interpolated |
532c9e80 | 1807 | every time the pattern search is evaluated, except |
1f247705 GS |
1808 | for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and |
1809 | C<$|> are not interpolated because they look like end-of-string tests.) | |
532c9e80 KW |
1810 | Perl will not recompile the pattern unless an interpolated |
1811 | variable that it contains changes. You can force Perl to skip the | |
1812 | test and never recompile by adding a C</o> (which stands for "once") | |
1813 | after the trailing delimiter. | |
1814 | Once upon a time, Perl would recompile regular expressions | |
1815 | unnecessarily, and this modifier was useful to tell it not to do so, in the | |
5cc41653 | 1816 | interests of speed. But now, the only reasons to use C</o> are one of: |
532c9e80 KW |
1817 | |
1818 | =over | |
1819 | ||
1820 | =item 1 | |
1821 | ||
1822 | The variables are thousands of characters long and you know that they | |
1823 | don't change, and you need to wring out the last little bit of speed by | |
1824 | having Perl skip testing for that. (There is a maintenance penalty for | |
1825 | doing this, as mentioning C</o> constitutes a promise that you won't | |
18509dec | 1826 | change the variables in the pattern. If you do change them, Perl won't |
532c9e80 KW |
1827 | even notice.) |
1828 | ||
1829 | =item 2 | |
1830 | ||
1831 | you want the pattern to use the initial values of the variables | |
1832 | regardless of whether they change or not. (But there are saner ways | |
1833 | of accomplishing this than using C</o>.) | |
1834 | ||
fa9b8686 DM |
1835 | =item 3 |
1836 | ||
1837 | If the pattern contains embedded code, such as | |
1838 | ||
1839 | use re 'eval'; | |
1840 | $code = 'foo(?{ $x })'; | |
1841 | /$code/ | |
1842 | ||
1843 | then perl will recompile each time, even though the pattern string hasn't | |
1844 | changed, to ensure that the current value of C<$x> is seen each time. | |
1845 | Use C</o> if you want to avoid this. | |
1846 | ||
532c9e80 | 1847 | =back |
a0d0e21e | 1848 | |
18509dec KW |
1849 | The bottom line is that using C</o> is almost never a good idea. |
1850 | ||
ba7f043c | 1851 | =item The empty pattern C<//> |
e9d89077 | 1852 | |
ba7f043c | 1853 | If the I<PATTERN> evaluates to the empty string, the last |
46f8a5ea | 1854 | I<successfully> matched regular expression is used instead. In this |
c543c01b | 1855 | case, only the C<g> and C<c> flags on the empty pattern are honored; |
46f8a5ea | 1856 | the other flags are taken from the original pattern. If no match has |
d65afb4b HS |
1857 | previously succeeded, this will (silently) act instead as a genuine |
1858 | empty pattern (which will always match). | |
a0d0e21e | 1859 | |
89d205f2 YO |
1860 | Note that it's possible to confuse Perl into thinking C<//> (the empty |
1861 | regex) is really C<//> (the defined-or operator). Perl is usually pretty | |
1862 | good about this, but some pathological cases might trigger this, such as | |
ba7f043c KW |
1863 | C<$x///> (is that S<C<($x) / (//)>> or S<C<$x // />>?) and S<C<print $fh //>> |
1864 | (S<C<print $fh(//>> or S<C<print($fh //>>?). In all of these examples, Perl | |
89d205f2 YO |
1865 | will assume you meant defined-or. If you meant the empty regex, just |
1866 | use parentheses or spaces to disambiguate, or even prefix the empty | |
c963b151 BD |
1867 | regex with an C<m> (so C<//> becomes C<m//>). |
1868 | ||
e9d89077 DN |
1869 | =item Matching in list context |
1870 | ||
19799a22 | 1871 | If the C</g> option is not used, C<m//> in list context returns a |
a0d0e21e | 1872 | list consisting of the subexpressions matched by the parentheses in the |
3ff8ecf9 BF |
1873 | pattern, that is, (C<$1>, C<$2>, C<$3>...) (Note that here C<$1> etc. are |
1874 | also set). When there are no parentheses in the pattern, the return | |
a727cfac | 1875 | value is the list C<(1)> for success. |
3ff8ecf9 | 1876 | With or without parentheses, an empty list is returned upon failure. |
a0d0e21e LW |
1877 | |
1878 | Examples: | |
1879 | ||
7188ca43 KW |
1880 | open(TTY, "+</dev/tty") |
1881 | || die "can't access /dev/tty: $!"; | |
c543c01b | 1882 | |
7188ca43 | 1883 | <TTY> =~ /^y/i && foo(); # do foo if desired |
a0d0e21e | 1884 | |
7188ca43 | 1885 | if (/Version: *([0-9.]*)/) { $version = $1; } |
a0d0e21e | 1886 | |
7188ca43 | 1887 | next if m#^/usr/spool/uucp#; |
a0d0e21e | 1888 | |
7188ca43 KW |
1889 | # poor man's grep |
1890 | $arg = shift; | |
1891 | while (<>) { | |
1892 | print if /$arg/o; # compile only once (no longer needed!) | |
1893 | } | |
a0d0e21e | 1894 | |
7188ca43 | 1895 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) |
a0d0e21e | 1896 | |
ba7f043c KW |
1897 | This last example splits C<$foo> into the first two words and the |
1898 | remainder of the line, and assigns those three fields to C<$F1>, C<$F2>, and | |
1899 | C<$Etc>. The conditional is true if any variables were assigned; that is, | |
c543c01b | 1900 | if the pattern matched. |
a0d0e21e | 1901 | |
19799a22 | 1902 | The C</g> modifier specifies global pattern matching--that is, |
46f8a5ea FC |
1903 | matching as many times as possible within the string. How it behaves |
1904 | depends on the context. In list context, it returns a list of the | |
19799a22 | 1905 | substrings matched by any capturing parentheses in the regular |
46f8a5ea | 1906 | expression. If there are no parentheses, it returns a list of all |
19799a22 GS |
1907 | the matched strings, as if there were parentheses around the whole |
1908 | pattern. | |
a0d0e21e | 1909 | |
7e86de3e | 1910 | In scalar context, each execution of C<m//g> finds the next match, |
19799a22 | 1911 | returning true if it matches, and false if there is no further match. |
3dd93342 | 1912 | The position after the last match can be read or set using the C<pos()> |
46f8a5ea | 1913 | function; see L<perlfunc/pos>. A failed match normally resets the |
7e86de3e | 1914 | search position to the beginning of the string, but you can avoid that |
46f8a5ea | 1915 | by adding the C</c> modifier (for example, C<m//gc>). Modifying the target |
7e86de3e | 1916 | string also resets the search position. |
c90c0ff4 | 1917 | |
ba7f043c | 1918 | =item C<\G I<assertion>> |
e9d89077 | 1919 | |
c90c0ff4 | 1920 | You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a |
3dd93342 | 1921 | zero-width assertion that matches the exact position where the |
46f8a5ea | 1922 | previous C<m//g>, if any, left off. Without the C</g> modifier, the |
3dd93342 | 1923 | C<\G> assertion still anchors at C<pos()> as it was at the start of |
1924 | the operation (see L<perlfunc/pos>), but the match is of course only | |
46f8a5ea | 1925 | attempted once. Using C<\G> without C</g> on a target string that has |
3dd93342 | 1926 | not previously had a C</g> match applied to it is the same as using |
1927 | the C<\A> assertion to match the beginning of the string. Note also | |
1928 | that, currently, C<\G> is only properly supported when anchored at the | |
1929 | very beginning of the pattern. | |
c90c0ff4 | 1930 | |
1931 | Examples: | |
a0d0e21e LW |
1932 | |
1933 | # list context | |
1934 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); | |
1935 | ||
1936 | # scalar context | |
c543c01b TC |
1937 | local $/ = ""; |
1938 | while ($paragraph = <>) { | |
1939 | while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) { | |
19799a22 | 1940 | $sentences++; |
a0d0e21e LW |
1941 | } |
1942 | } | |
c543c01b TC |
1943 | say $sentences; |
1944 | ||
1945 | Here's another way to check for sentences in a paragraph: | |
1946 | ||
7188ca43 KW |
1947 | my $sentence_rx = qr{ |
1948 | (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or | |
1949 | # whitespace | |
1950 | \p{Lu} # capital letter | |
1951 | .*? # a bunch of anything | |
1952 | (?<= \S ) # that ends in non- | |
1953 | # whitespace | |
1954 | (?<! \b [DMS]r ) # but isn't a common abbr. | |
1955 | (?<! \b Mrs ) | |
1956 | (?<! \b Sra ) | |
1957 | (?<! \b St ) | |
1958 | [.?!] # followed by a sentence | |
1959 | # ender | |
1960 | (?= $ | \s ) # in front of end-of-string | |
1961 | # or whitespace | |
1962 | }sx; | |
1963 | local $/ = ""; | |
1964 | while (my $paragraph = <>) { | |
1965 | say "NEW PARAGRAPH"; | |
1966 | my $count = 0; | |
1967 | while ($paragraph =~ /($sentence_rx)/g) { | |
1968 | printf "\tgot sentence %d: <%s>\n", ++$count, $1; | |
c543c01b | 1969 | } |
7188ca43 | 1970 | } |
c543c01b TC |
1971 | |
1972 | Here's how to use C<m//gc> with C<\G>: | |
a0d0e21e | 1973 | |
137443ea | 1974 | $_ = "ppooqppqq"; |
44a8e56a | 1975 | while ($i++ < 2) { |
1976 | print "1: '"; | |
c90c0ff4 | 1977 | print $1 while /(o)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1978 | print "2: '"; |
c90c0ff4 | 1979 | print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1980 | print "3: '"; |
c90c0ff4 | 1981 | print $1 while /(p)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1982 | } |
5d43e42d | 1983 | print "Final: '$1', pos=",pos,"\n" if /\G(.)/; |
44a8e56a | 1984 | |
1985 | The last example should print: | |
1986 | ||
1987 | 1: 'oo', pos=4 | |
137443ea | 1988 | 2: 'q', pos=5 |
44a8e56a | 1989 | 3: 'pp', pos=7 |
1990 | 1: '', pos=7 | |
137443ea | 1991 | 2: 'q', pos=8 |
1992 | 3: '', pos=8 | |
5d43e42d DC |
1993 | Final: 'q', pos=8 |
1994 | ||
1995 | Notice that the final match matched C<q> instead of C<p>, which a match | |
46f8a5ea FC |
1996 | without the C<\G> anchor would have done. Also note that the final match |
1997 | did not update C<pos>. C<pos> is only updated on a C</g> match. If the | |
c543c01b TC |
1998 | final match did indeed match C<p>, it's a good bet that you're running a |
1999 | very old (pre-5.6.0) version of Perl. | |
44a8e56a | 2000 | |
c90c0ff4 | 2001 | A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can |
e7ea3e70 | 2002 | combine several regexps like this to process a string part-by-part, |
c90c0ff4 | 2003 | doing different actions depending on which regexp matched. Each |
2004 | regexp tries to match where the previous one leaves off. | |
e7ea3e70 | 2005 | |
3fe9a6f1 | 2006 | $_ = <<'EOL'; |
7188ca43 KW |
2007 | $url = URI::URL->new( "http://example.com/" ); |
2008 | die if $url eq "xXx"; | |
3fe9a6f1 | 2009 | EOL |
c543c01b TC |
2010 | |
2011 | LOOP: { | |
950b09ed | 2012 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; |
7188ca43 KW |
2013 | print(" lowercase"), redo LOOP |
2014 | if /\G\p{Ll}+\b[,.;]?\s*/gc; | |
2015 | print(" UPPERCASE"), redo LOOP | |
2016 | if /\G\p{Lu}+\b[,.;]?\s*/gc; | |
2017 | print(" Capitalized"), redo LOOP | |
2018 | if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc; | |
c543c01b | 2019 | print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc; |
7188ca43 KW |
2020 | print(" alphanumeric"), redo LOOP |
2021 | if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc; | |
c543c01b | 2022 | print(" line-noise"), redo LOOP if /\G\W+/gc; |
950b09ed | 2023 | print ". That's all!\n"; |
c543c01b | 2024 | } |
e7ea3e70 IZ |
2025 | |
2026 | Here is the output (split into several lines): | |
2027 | ||
7188ca43 KW |
2028 | line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE |
2029 | line-noise lowercase line-noise lowercase line-noise lowercase | |
2030 | lowercase line-noise lowercase lowercase line-noise lowercase | |
2031 | lowercase line-noise MiXeD line-noise. That's all! | |
44a8e56a | 2032 | |
ba7f043c | 2033 | =item C<m?I<PATTERN>?msixpodualngc> |
725a61d7 | 2034 | X<?> X<operator, match-once> |
87e95b7f | 2035 | |
ba7f043c KW |
2036 | This is just like the C<m/I<PATTERN>/> search, except that it matches |
2037 | only once between calls to the C<reset()> operator. This is a useful | |
87e95b7f | 2038 | optimization when you want to see only the first occurrence of |
ceb131e8 | 2039 | something in each file of a set of files, for instance. Only C<m??> |
87e95b7f YO |
2040 | patterns local to the current package are reset. |
2041 | ||
2042 | while (<>) { | |
ceb131e8 | 2043 | if (m?^$?) { |
87e95b7f YO |
2044 | # blank line between header and body |
2045 | } | |
2046 | } continue { | |
725a61d7 | 2047 | reset if eof; # clear m?? status for next file |
87e95b7f YO |
2048 | } |
2049 | ||
c543c01b TC |
2050 | Another example switched the first "latin1" encoding it finds |
2051 | to "utf8" in a pod file: | |
2052 | ||
2053 | s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x; | |
2054 | ||
2055 | The match-once behavior is controlled by the match delimiter being | |
4932eeca | 2056 | C<?>; with any other delimiter this is the normal C<m//> operator. |
725a61d7 | 2057 | |
ba7f043c | 2058 | In the past, the leading C<m> in C<m?I<PATTERN>?> was optional, but omitting it |
0381ecf1 MH |
2059 | would produce a deprecation warning. As of v5.22.0, omitting it produces a |
2060 | syntax error. If you encounter this construct in older code, you can just add | |
2061 | C<m>. | |
87e95b7f | 2062 | |
ba7f043c | 2063 | =item C<s/I<PATTERN>/I<REPLACEMENT>/msixpodualngcer> |
0a31ee11 | 2064 | X<s> X<substitute> X<substitution> X<replace> X<regexp, replace> |
4f4d7508 | 2065 | X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r> |
87e95b7f YO |
2066 | |
2067 | Searches a string for a pattern, and if found, replaces that pattern | |
2068 | with the replacement text and returns the number of substitutions | |
2069 | made. Otherwise it returns false (specifically, the empty string). | |
2070 | ||
c543c01b | 2071 | If the C</r> (non-destructive) option is used then it runs the |
679563bb KW |
2072 | substitution on a copy of the string and instead of returning the |
2073 | number of substitutions, it returns the copy whether or not a | |
c543c01b TC |
2074 | substitution occurred. The original string is never changed when |
2075 | C</r> is used. The copy will always be a plain string, even if the | |
2076 | input is an object or a tied variable. | |
4f4d7508 | 2077 | |
87e95b7f | 2078 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> |
c543c01b TC |
2079 | variable is searched and modified. Unless the C</r> option is used, |
2080 | the string specified must be a scalar variable, an array element, a | |
2081 | hash element, or an assignment to one of those; that is, some sort of | |
2082 | scalar lvalue. | |
87e95b7f | 2083 | |
6d314683 | 2084 | If the delimiter chosen is a single quote, no variable interpolation is |
ba7f043c KW |
2085 | done on either the I<PATTERN> or the I<REPLACEMENT>. Otherwise, if the |
2086 | I<PATTERN> contains a C<$> that looks like a variable rather than an | |
87e95b7f YO |
2087 | end-of-string test, the variable will be interpolated into the pattern |
2088 | at run-time. If you want the pattern compiled only once the first time | |
2089 | the variable is interpolated, use the C</o> option. If the pattern | |
2090 | evaluates to the empty string, the last successfully executed regular | |
2091 | expression is used instead. See L<perlre> for further explanation on these. | |
87e95b7f | 2092 | |
ba7f043c | 2093 | Options are as with C<m//> with the addition of the following replacement |
87e95b7f YO |
2094 | specific options: |
2095 | ||
2096 | e Evaluate the right side as an expression. | |
7188ca43 KW |
2097 | ee Evaluate the right side as a string then eval the |
2098 | result. | |
2099 | r Return substitution and leave the original string | |
2100 | untouched. | |
87e95b7f | 2101 | |
ed02a3bf DN |
2102 | Any non-whitespace delimiter may replace the slashes. Add space after |
2103 | the C<s> when using a character allowed in identifiers. If single quotes | |
2104 | are used, no interpretation is done on the replacement string (the C</e> | |
3ff8ecf9 | 2105 | modifier overrides this, however). Note that Perl treats backticks |
ed02a3bf | 2106 | as normal delimiters; the replacement text is not evaluated as a command. |
ba7f043c | 2107 | If the I<PATTERN> is delimited by bracketing quotes, the I<REPLACEMENT> has |
1ca345ed | 2108 | its own pair of quotes, which may or may not be bracketing quotes, for example, |
87e95b7f YO |
2109 | C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the |
2110 | replacement portion to be treated as a full-fledged Perl expression | |
2111 | and evaluated right then and there. It is, however, syntax checked at | |
46f8a5ea | 2112 | compile-time. A second C<e> modifier will cause the replacement portion |
87e95b7f YO |
2113 | to be C<eval>ed before being run as a Perl expression. |
2114 | ||
2115 | Examples: | |
2116 | ||
7188ca43 | 2117 | s/\bgreen\b/mauve/g; # don't change wintergreen |
87e95b7f YO |
2118 | |
2119 | $path =~ s|/usr/bin|/usr/local/bin|; | |
2120 | ||
2121 | s/Login: $foo/Login: $bar/; # run-time pattern | |
2122 | ||
7188ca43 KW |
2123 | ($foo = $bar) =~ s/this/that/; # copy first, then |
2124 | # change | |
2125 | ($foo = "$bar") =~ s/this/that/; # convert to string, | |
2126 | # copy, then change | |
4f4d7508 DC |
2127 | $foo = $bar =~ s/this/that/r; # Same as above using /r |
2128 | $foo = $bar =~ s/this/that/r | |
7188ca43 KW |
2129 | =~ s/that/the other/r; # Chained substitutes |
2130 | # using /r | |
2131 | @foo = map { s/this/that/r } @bar # /r is very useful in | |
2132 | # maps | |
87e95b7f | 2133 | |
7188ca43 | 2134 | $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-cnt |
87e95b7f YO |
2135 | |
2136 | $_ = 'abc123xyz'; | |
2137 | s/\d+/$&*2/e; # yields 'abc246xyz' | |
2138 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' | |
2139 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' | |
2140 | ||
2141 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e | |
2142 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e | |
2143 | s/^=(\w+)/pod($1)/ge; # use function call | |
2144 | ||
4f4d7508 | 2145 | $_ = 'abc123xyz'; |
db691027 | 2146 | $x = s/abc/def/r; # $x is 'def123xyz' and |
4f4d7508 DC |
2147 | # $_ remains 'abc123xyz'. |
2148 | ||
87e95b7f YO |
2149 | # expand variables in $_, but dynamics only, using |
2150 | # symbolic dereferencing | |
2151 | s/\$(\w+)/${$1}/g; | |
2152 | ||
2153 | # Add one to the value of any numbers in the string | |
2154 | s/(\d+)/1 + $1/eg; | |
2155 | ||
c543c01b TC |
2156 | # Titlecase words in the last 30 characters only |
2157 | substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g; | |
2158 | ||
87e95b7f YO |
2159 | # This will expand any embedded scalar variable |
2160 | # (including lexicals) in $_ : First $1 is interpolated | |
2161 | # to the variable name, and then evaluated | |
2162 | s/(\$\w+)/$1/eeg; | |
2163 | ||
2164 | # Delete (most) C comments. | |
2165 | $program =~ s { | |
2166 | /\* # Match the opening delimiter. | |
2167 | .*? # Match a minimal number of characters. | |
2168 | \*/ # Match the closing delimiter. | |
2169 | } []gsx; | |
2170 | ||
7188ca43 KW |
2171 | s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, |
2172 | # expensively | |
87e95b7f | 2173 | |
7188ca43 KW |
2174 | for ($variable) { # trim whitespace in $variable, |
2175 | # cheap | |
87e95b7f YO |
2176 | s/^\s+//; |
2177 | s/\s+$//; | |
2178 | } | |
2179 | ||
2180 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields | |
2181 | ||
ba7f043c KW |
2182 | Note the use of C<$> instead of C<\> in the last example. Unlike |
2183 | B<sed>, we use the \<I<digit>> form only in the left hand side. | |
87e95b7f YO |
2184 | Anywhere else it's $<I<digit>>. |
2185 | ||
2186 | Occasionally, you can't use just a C</g> to get all the changes | |
2187 | to occur that you might want. Here are two common cases: | |
2188 | ||
2189 | # put commas in the right places in an integer | |
2190 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; | |
2191 | ||
2192 | # expand tabs to 8-column spacing | |
2193 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; | |
2194 | ||
2195 | =back | |
2196 | ||
2197 | =head2 Quote-Like Operators | |
2198 | X<operator, quote-like> | |
2199 | ||
01c6f5f4 RGS |
2200 | =over 4 |
2201 | ||
ba7f043c | 2202 | =item C<q/I<STRING>/> |
5d44bfff | 2203 | X<q> X<quote, single> X<'> X<''> |
a0d0e21e | 2204 | |
ba7f043c | 2205 | =item C<'I<STRING>'> |
a0d0e21e | 2206 | |
19799a22 | 2207 | A single-quoted, literal string. A backslash represents a backslash |
68dc0745 | 2208 | unless followed by the delimiter or another backslash, in which case |
2209 | the delimiter or backslash is interpolated. | |
a0d0e21e LW |
2210 | |
2211 | $foo = q!I said, "You said, 'She said it.'"!; | |
2212 | $bar = q('This is it.'); | |
68dc0745 | 2213 | $baz = '\n'; # a two-character string |
a0d0e21e | 2214 | |
ba7f043c | 2215 | =item C<qq/I<STRING>/> |
d74e8afc | 2216 | X<qq> X<quote, double> X<"> X<""> |
a0d0e21e | 2217 | |
ba7f043c | 2218 | =item "I<STRING>" |
a0d0e21e LW |
2219 | |
2220 | A double-quoted, interpolated string. | |
2221 | ||
2222 | $_ .= qq | |
2223 | (*** The previous line contains the naughty word "$1".\n) | |
19799a22 | 2224 | if /\b(tcl|java|python)\b/i; # :-) |
68dc0745 | 2225 | $baz = "\n"; # a one-character string |
a0d0e21e | 2226 | |
ba7f043c | 2227 | =item C<qx/I<STRING>/> |
d74e8afc | 2228 | X<qx> X<`> X<``> X<backtick> |
a0d0e21e | 2229 | |
ba7f043c | 2230 | =item C<`I<STRING>`> |
a0d0e21e | 2231 | |
43dd4d21 | 2232 | A string which is (possibly) interpolated and then executed as a |
f703fc96 | 2233 | system command with F</bin/sh> or its equivalent. Shell wildcards, |
43dd4d21 JH |
2234 | pipes, and redirections will be honored. The collected standard |
2235 | output of the command is returned; standard error is unaffected. In | |
2236 | scalar context, it comes back as a single (potentially multi-line) | |
ba7f043c KW |
2237 | string, or C<undef> if the command failed. In list context, returns a |
2238 | list of lines (however you've defined lines with C<$/> or | |
2239 | C<$INPUT_RECORD_SEPARATOR>), or an empty list if the command failed. | |
5a964f20 TC |
2240 | |
2241 | Because backticks do not affect standard error, use shell file descriptor | |
2242 | syntax (assuming the shell supports this) if you care to address this. | |
2243 | To capture a command's STDERR and STDOUT together: | |
a0d0e21e | 2244 | |
5a964f20 TC |
2245 | $output = `cmd 2>&1`; |
2246 | ||
2247 | To capture a command's STDOUT but discard its STDERR: | |
2248 | ||
2249 | $output = `cmd 2>/dev/null`; | |
2250 | ||
2251 | To capture a command's STDERR but discard its STDOUT (ordering is | |
2252 | important here): | |
2253 | ||
2254 | $output = `cmd 2>&1 1>/dev/null`; | |
2255 | ||
2256 | To exchange a command's STDOUT and STDERR in order to capture the STDERR | |
2257 | but leave its STDOUT to come out the old STDERR: | |
2258 | ||
2259 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; | |
2260 | ||
2261 | To read both a command's STDOUT and its STDERR separately, it's easiest | |
2359510d SD |
2262 | to redirect them separately to files, and then read from those files |
2263 | when the program is done: | |
5a964f20 | 2264 | |
2359510d | 2265 | system("program args 1>program.stdout 2>program.stderr"); |
5a964f20 | 2266 | |
30398227 SP |
2267 | The STDIN filehandle used by the command is inherited from Perl's STDIN. |
2268 | For example: | |
2269 | ||
c543c01b TC |
2270 | open(SPLAT, "stuff") || die "can't open stuff: $!"; |
2271 | open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!"; | |
40bbb707 | 2272 | print STDOUT `sort`; |
30398227 | 2273 | |
40bbb707 | 2274 | will print the sorted contents of the file named F<"stuff">. |
30398227 | 2275 | |
5a964f20 TC |
2276 | Using single-quote as a delimiter protects the command from Perl's |
2277 | double-quote interpolation, passing it on to the shell instead: | |
2278 | ||
2279 | $perl_info = qx(ps $$); # that's Perl's $$ | |
2280 | $shell_info = qx'ps $$'; # that's the new shell's $$ | |
2281 | ||
19799a22 | 2282 | How that string gets evaluated is entirely subject to the command |
5a964f20 TC |
2283 | interpreter on your system. On most platforms, you will have to protect |
2284 | shell metacharacters if you want them treated literally. This is in | |
2285 | practice difficult to do, as it's unclear how to escape which characters. | |
ba7f043c | 2286 | See L<perlsec> for a clean and safe example of a manual C<fork()> and C<exec()> |
5a964f20 | 2287 | to emulate backticks safely. |
a0d0e21e | 2288 | |
bb32b41a GS |
2289 | On some platforms (notably DOS-like ones), the shell may not be |
2290 | capable of dealing with multiline commands, so putting newlines in | |
2291 | the string may not get you what you want. You may be able to evaluate | |
2292 | multiple commands in a single line by separating them with the command | |
a727cfac | 2293 | separator character, if your shell supports that (for example, C<;> on |
1ca345ed | 2294 | many Unix shells and C<&> on the Windows NT C<cmd> shell). |
bb32b41a | 2295 | |
3ff8ecf9 | 2296 | Perl will attempt to flush all files opened for |
0f897271 GS |
2297 | output before starting the child process, but this may not be supported |
2298 | on some platforms (see L<perlport>). To be safe, you may need to set | |
ba7f043c KW |
2299 | C<$|> (C<$AUTOFLUSH> in C<L<English>>) or call the C<autoflush()> method of |
2300 | C<L<IO::Handle>> on any open handles. | |
0f897271 | 2301 | |
bb32b41a GS |
2302 | Beware that some command shells may place restrictions on the length |
2303 | of the command line. You must ensure your strings don't exceed this | |
2304 | limit after any necessary interpolations. See the platform-specific | |
2305 | release notes for more details about your particular environment. | |
2306 | ||
5a964f20 TC |
2307 | Using this operator can lead to programs that are difficult to port, |
2308 | because the shell commands called vary between systems, and may in | |
2309 | fact not be present at all. As one example, the C<type> command under | |
2310 | the POSIX shell is very different from the C<type> command under DOS. | |
2311 | That doesn't mean you should go out of your way to avoid backticks | |
2312 | when they're the right way to get something done. Perl was made to be | |
2313 | a glue language, and one of the things it glues together is commands. | |
2314 | Just understand what you're getting yourself into. | |
bb32b41a | 2315 | |
7cf4dd3e DB |
2316 | Like C<system>, backticks put the child process exit code in C<$?>. |
2317 | If you'd like to manually inspect failure, you can check all possible | |
2318 | failure modes by inspecting C<$?> like this: | |
2319 | ||
2320 | if ($? == -1) { | |
2321 | print "failed to execute: $!\n"; | |
2322 | } | |
2323 | elsif ($? & 127) { | |
2324 | printf "child died with signal %d, %s coredump\n", | |
2325 | ($? & 127), ($? & 128) ? 'with' : 'without'; | |
2326 | } | |
2327 | else { | |
2328 | printf "child exited with value %d\n", $? >> 8; | |
2329 | } | |
2330 | ||
fe43a9cc TC |
2331 | Use the L<open> pragma to control the I/O layers used when reading the |
2332 | output of the command, for example: | |
2333 | ||
2334 | use open IN => ":encoding(UTF-8)"; | |
2335 | my $x = `cmd-producing-utf-8`; | |
2336 | ||
da87341d | 2337 | See L</"I/O Operators"> for more discussion. |
a0d0e21e | 2338 | |
ba7f043c | 2339 | =item C<qw/I<STRING>/> |
d74e8afc | 2340 | X<qw> X<quote, list> X<quote, words> |
945c54fd | 2341 | |
ba7f043c | 2342 | Evaluates to a list of the words extracted out of I<STRING>, using embedded |
945c54fd JH |
2343 | whitespace as the word delimiters. It can be understood as being roughly |
2344 | equivalent to: | |
2345 | ||
c543c01b | 2346 | split(" ", q/STRING/); |
945c54fd | 2347 | |
efb1e162 CW |
2348 | the differences being that it generates a real list at compile time, and |
2349 | in scalar context it returns the last element in the list. So | |
945c54fd JH |
2350 | this expression: |
2351 | ||
2352 | qw(foo bar baz) | |
2353 | ||
2354 | is semantically equivalent to the list: | |
2355 | ||
c543c01b | 2356 | "foo", "bar", "baz" |
945c54fd JH |
2357 | |
2358 | Some frequently seen examples: | |
2359 | ||
2360 | use POSIX qw( setlocale localeconv ) | |
2361 | @EXPORT = qw( foo bar baz ); | |
2362 | ||
ba7f043c | 2363 | A common mistake is to try to separate the words with commas or to |
945c54fd | 2364 | put comments into a multi-line C<qw>-string. For this reason, the |
ba7f043c KW |
2365 | S<C<use warnings>> pragma and the B<-w> switch (that is, the C<$^W> variable) |
2366 | produces warnings if the I<STRING> contains the C<","> or the C<"#"> character. | |
945c54fd | 2367 | |
ba7f043c | 2368 | =item C<tr/I<SEARCHLIST>/I<REPLACEMENTLIST>/cdsr> |
d74e8afc | 2369 | X<tr> X<y> X<transliterate> X</c> X</d> X</s> |
a0d0e21e | 2370 | |
ba7f043c | 2371 | =item C<y/I<SEARCHLIST>/I<REPLACEMENTLIST>/cdsr> |
a0d0e21e | 2372 | |
2c268ad5 | 2373 | Transliterates all occurrences of the characters found in the search list |
a0d0e21e LW |
2374 | with the corresponding character in the replacement list. It returns |
2375 | the number of characters replaced or deleted. If no string is | |
ba7f043c | 2376 | specified via the C<=~> or C<!~> operator, the C<$_> string is transliterated. |
c543c01b TC |
2377 | |
2378 | If the C</r> (non-destructive) option is present, a new copy of the string | |
2379 | is made and its characters transliterated, and this copy is returned no | |
2380 | matter whether it was modified or not: the original string is always | |
2381 | left unchanged. The new copy is always a plain string, even if the input | |
2382 | string is an object or a tied variable. | |
8ada0baa | 2383 | |
c543c01b TC |
2384 | Unless the C</r> option is used, the string specified with C<=~> must be a |
2385 | scalar variable, an array element, a hash element, or an assignment to one | |
2386 | of those; in other words, an lvalue. | |
8ff32507 | 2387 | |
89d205f2 | 2388 | A character range may be specified with a hyphen, so C<tr/A-J/0-9/> |
2c268ad5 | 2389 | does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. |
54310121 | 2390 | For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the |
af2cbe4d KW |
2391 | I<SEARCHLIST> is delimited by bracketing quotes, the I<REPLACEMENTLIST> |
2392 | must have its own pair of quotes, which may or may not be bracketing | |
2393 | quotes; for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>. | |
c543c01b | 2394 | |
ba7f043c | 2395 | Characters may be literals or any of the escape sequences accepted in |
6d314683 YO |
2396 | double-quoted strings. But there is no variable interpolation, so C<"$"> |
2397 | and C<"@"> are treated as literals. A hyphen at the beginning or end, or | |
ba7f043c KW |
2398 | preceded by a backslash is considered a literal. Escape sequence |
2399 | details are in L<the table near the beginning of this section|/Quote and | |
f4240379 | 2400 | Quote-like Operators>. |
ba7f043c | 2401 | |
c543c01b | 2402 | Note that C<tr> does B<not> do regular expression character classes such as |
ba7f043c | 2403 | C<\d> or C<\pL>. The C<tr> operator is not equivalent to the C<L<tr(1)>> |
af2cbe4d KW |
2404 | utility. C<tr[a-z][A-Z]> will uppercase the 26 letters "a" through "z", |
2405 | but for case changing not confined to ASCII, use | |
2406 | L<C<lc>|perlfunc/lc>, L<C<uc>|perlfunc/uc>, | |
2407 | L<C<lcfirst>|perlfunc/lcfirst>, L<C<ucfirst>|perlfunc/ucfirst> | |
2408 | (all documented in L<perlfunc>), or the | |
2409 | L<substitution operator C<sE<sol>I<PATTERN>E<sol>I<REPLACEMENT>E<sol>>|/sE<sol>PATTERNE<sol>REPLACEMENTE<sol>msixpodualngcer> | |
2410 | (with C<\U>, C<\u>, C<\L>, and C<\l> string-interpolation escapes in the | |
2411 | I<REPLACEMENT> portion). | |
cc255d5f | 2412 | |
f4240379 KW |
2413 | Most ranges are unportable between character sets, but certain ones |
2414 | signal Perl to do special handling to make them portable. There are two | |
2415 | classes of portable ranges. The first are any subsets of the ranges | |
2416 | C<A-Z>, C<a-z>, and C<0-9>, when expressed as literal characters. | |
2417 | ||
2418 | tr/h-k/H-K/ | |
2419 | ||
2420 | capitalizes the letters C<"h">, C<"i">, C<"j">, and C<"k"> and nothing | |
2421 | else, no matter what the platform's character set is. In contrast, all | |
2422 | of | |
2423 | ||
2424 | tr/\x68-\x6B/\x48-\x4B/ | |
2425 | tr/h-\x6B/H-\x4B/ | |
2426 | tr/\x68-k/\x48-K/ | |
2427 | ||
2428 | do the same capitalizations as the previous example when run on ASCII | |
2429 | platforms, but something completely different on EBCDIC ones. | |
2430 | ||
2431 | The second class of portable ranges is invoked when one or both of the | |
2432 | range's end points are expressed as C<\N{...}> | |
2433 | ||
2434 | $string =~ tr/\N{U+20}-\N{U+7E}//d; | |
2435 | ||
2436 | removes from C<$string> all the platform's characters which are | |
2437 | equivalent to any of Unicode U+0020, U+0021, ... U+007D, U+007E. This | |
2438 | is a portable range, and has the same effect on every platform it is | |
2439 | run on. It turns out that in this example, these are the ASCII | |
2440 | printable characters. So after this is run, C<$string> has only | |
2441 | controls and characters which have no ASCII equivalents. | |
2442 | ||
2443 | But, even for portable ranges, it is not generally obvious what is | |
2444 | included without having to look things up. A sound principle is to use | |
2445 | only ranges that begin from and end at either ASCII alphabetics of equal | |
8df98a27 | 2446 | case (C<b-e>, C<B-E>), or digits (C<1-4>). Anything else is unclear |
f4240379 | 2447 | (and unportable unless C<\N{...}> is used). If in doubt, spell out the |
8ada0baa JH |
2448 | character sets in full. |
2449 | ||
a0d0e21e LW |
2450 | Options: |
2451 | ||
2452 | c Complement the SEARCHLIST. | |
2453 | d Delete found but unreplaced characters. | |
2454 | s Squash duplicate replaced characters. | |
8ff32507 FC |
2455 | r Return the modified string and leave the original string |
2456 | untouched. | |
a0d0e21e | 2457 | |
ba7f043c | 2458 | If the C</c> modifier is specified, the I<SEARCHLIST> character set |
19799a22 | 2459 | is complemented. If the C</d> modifier is specified, any characters |
ba7f043c | 2460 | specified by I<SEARCHLIST> not found in I<REPLACEMENTLIST> are deleted. |
19799a22 | 2461 | (Note that this is slightly more flexible than the behavior of some |
ba7f043c | 2462 | B<tr> programs, which delete anything they find in the I<SEARCHLIST>, |
46f8a5ea | 2463 | period.) If the C</s> modifier is specified, sequences of characters |
19799a22 GS |
2464 | that were transliterated to the same character are squashed down |
2465 | to a single instance of the character. | |
a0d0e21e | 2466 | |
ba7f043c KW |
2467 | If the C</d> modifier is used, the I<REPLACEMENTLIST> is always interpreted |
2468 | exactly as specified. Otherwise, if the I<REPLACEMENTLIST> is shorter | |
2469 | than the I<SEARCHLIST>, the final character is replicated till it is long | |
2470 | enough. If the I<REPLACEMENTLIST> is empty, the I<SEARCHLIST> is replicated. | |
a0d0e21e LW |
2471 | This latter is useful for counting characters in a class or for |
2472 | squashing character sequences in a class. | |
2473 | ||
2474 | Examples: | |
2475 | ||
c543c01b | 2476 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII |
a0d0e21e LW |
2477 | |
2478 | $cnt = tr/*/*/; # count the stars in $_ | |
2479 | ||
2480 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky | |
2481 | ||
2482 | $cnt = tr/0-9//; # count the digits in $_ | |
2483 | ||
2484 | tr/a-zA-Z//s; # bookkeeper -> bokeper | |
2485 | ||
2486 | ($HOST = $host) =~ tr/a-z/A-Z/; | |
c543c01b | 2487 | $HOST = $host =~ tr/a-z/A-Z/r; # same thing |
8ff32507 | 2488 | |
c543c01b | 2489 | $HOST = $host =~ tr/a-z/A-Z/r # chained with s///r |
8ff32507 | 2490 | =~ s/:/ -p/r; |
a0d0e21e LW |
2491 | |
2492 | tr/a-zA-Z/ /cs; # change non-alphas to single space | |
2493 | ||
8ff32507 FC |
2494 | @stripped = map tr/a-zA-Z/ /csr, @original; |
2495 | # /r with map | |
2496 | ||
a0d0e21e | 2497 | tr [\200-\377] |
c543c01b | 2498 | [\000-\177]; # wickedly delete 8th bit |
a0d0e21e | 2499 | |
19799a22 GS |
2500 | If multiple transliterations are given for a character, only the |
2501 | first one is used: | |
748a9306 LW |
2502 | |
2503 | tr/AAA/XYZ/ | |
2504 | ||
2c268ad5 | 2505 | will transliterate any A to X. |
748a9306 | 2506 | |
19799a22 | 2507 | Because the transliteration table is built at compile time, neither |
ba7f043c | 2508 | the I<SEARCHLIST> nor the I<REPLACEMENTLIST> are subjected to double quote |
19799a22 | 2509 | interpolation. That means that if you want to use variables, you |
ba7f043c | 2510 | must use an C<eval()>: |
a0d0e21e LW |
2511 | |
2512 | eval "tr/$oldlist/$newlist/"; | |
2513 | die $@ if $@; | |
2514 | ||
2515 | eval "tr/$oldlist/$newlist/, 1" or die $@; | |
2516 | ||
ba7f043c | 2517 | =item C<< <<I<EOF> >> |
d74e8afc | 2518 | X<here-doc> X<heredoc> X<here-document> X<<< << >>> |
7e3b091d DA |
2519 | |
2520 | A line-oriented form of quoting is based on the shell "here-document" | |
2521 | syntax. Following a C<< << >> you specify a string to terminate | |
2522 | the quoted material, and all lines following the current line down to | |
89d205f2 YO |
2523 | the terminating string are the value of the item. |
2524 | ||
47eb4411 MH |
2525 | Prefixing the terminating string with a C<~> specifies that you |
2526 | want to use L</Indented Here-docs> (see below). | |
2527 | ||
89d205f2 YO |
2528 | The terminating string may be either an identifier (a word), or some |
2529 | quoted text. An unquoted identifier works like double quotes. | |
2530 | There may not be a space between the C<< << >> and the identifier, | |
2531 | unless the identifier is explicitly quoted. (If you put a space it | |
2532 | will be treated as a null identifier, which is valid, and matches the | |
2533 | first empty line.) The terminating string must appear by itself | |
2534 | (unquoted and with no surrounding whitespace) on the terminating line. | |
2535 | ||
2536 | If the terminating string is quoted, the type of quotes used determine | |
2537 | the treatment of the text. | |
2538 | ||
2539 | =over 4 | |
2540 | ||
2541 | =item Double Quotes | |
2542 | ||
2543 | Double quotes indicate that the text will be interpolated using exactly | |
2544 | the same rules as normal double quoted strings. | |
7e3b091d DA |
2545 | |
2546 | print <<EOF; | |
2547 | The price is $Price. | |
2548 | EOF | |
2549 | ||
2550 | print << "EOF"; # same as above | |
2551 | The price is $Price. | |
2552 | EOF | |
2553 | ||
89d205f2 YO |
2554 | |
2555 | =item Single Quotes | |
2556 | ||
2557 | Single quotes indicate the text is to be treated literally with no | |
46f8a5ea | 2558 | interpolation of its content. This is similar to single quoted |
89d205f2 YO |
2559 | strings except that backslashes have no special meaning, with C<\\> |
2560 | being treated as two backslashes and not one as they would in every | |
2561 | other quoting construct. | |
2562 | ||
c543c01b TC |
2563 | Just as in the shell, a backslashed bareword following the C<<< << >>> |
2564 | means the same thing as a single-quoted string does: | |
2565 | ||
2566 | $cost = <<'VISTA'; # hasta la ... | |
2567 | That'll be $10 please, ma'am. | |
2568 | VISTA | |
2569 | ||
2570 | $cost = <<\VISTA; # Same thing! | |
2571 | That'll be $10 please, ma'am. | |
2572 | VISTA | |
2573 | ||
89d205f2 YO |
2574 | This is the only form of quoting in perl where there is no need |
2575 | to worry about escaping content, something that code generators | |
2576 | can and do make good use of. | |
2577 | ||
2578 | =item Backticks | |
2579 | ||
2580 | The content of the here doc is treated just as it would be if the | |
46f8a5ea | 2581 | string were embedded in backticks. Thus the content is interpolated |
89d205f2 YO |
2582 | as though it were double quoted and then executed via the shell, with |
2583 | the results of the execution returned. | |
2584 | ||
2585 | print << `EOC`; # execute command and get results | |
7e3b091d | 2586 | echo hi there |
7e3b091d DA |
2587 | EOC |
2588 | ||
89d205f2 YO |
2589 | =back |
2590 | ||
47eb4411 MH |
2591 | =over 4 |
2592 | ||
2593 | =item Indented Here-docs | |
2594 | ||
2595 | The here-doc modifier C<~> allows you to indent your here-docs to make | |
2596 | the code more readable: | |
2597 | ||
2598 | if ($some_var) { | |
2599 | print <<~EOF; | |
2600 | This is a here-doc | |
2601 | EOF | |
2602 | } | |
2603 | ||
2604 | This will print... | |
2605 | ||
2606 | This is a here-doc | |
2607 | ||
2608 | ...with no leading whitespace. | |
2609 | ||
2610 | The delimiter is used to determine the B<exact> whitespace to | |
2611 | remove from the beginning of each line. All lines B<must> have | |
2612 | at least the same starting whitespace (except lines only | |
2613 | containing a newline) or perl will croak. Tabs and spaces can | |
2614 | be mixed, but are matched exactly. One tab will not be equal to | |
2615 | 8 spaces! | |
2616 | ||
2617 | Additional beginning whitespace (beyond what preceded the | |
2618 | delimiter) will be preserved: | |
2619 | ||
2620 | print <<~EOF; | |
2621 | This text is not indented | |
2622 | This text is indented with two spaces | |
2623 | This text is indented with two tabs | |
2624 | EOF | |
2625 | ||
2626 | Finally, the modifier may be used with all of the forms | |
2627 | mentioned above: | |
2628 | ||
2629 | <<~\EOF; | |
2630 | <<~'EOF' | |
2631 | <<~"EOF" | |
2632 | <<~`EOF` | |
2633 | ||
2634 | And whitespace may be used between the C<~> and quoted delimiters: | |
2635 | ||
2636 | <<~ 'EOF'; # ... "EOF", `EOF` | |
2637 | ||
2638 | =back | |
2639 | ||
89d205f2 YO |
2640 | It is possible to stack multiple here-docs in a row: |
2641 | ||
7e3b091d DA |
2642 | print <<"foo", <<"bar"; # you can stack them |
2643 | I said foo. | |
2644 | foo | |
2645 | I said bar. | |
2646 | bar | |
2647 | ||
2648 | myfunc(<< "THIS", 23, <<'THAT'); | |
2649 | Here's a line | |
2650 | or two. | |
2651 | THIS | |
2652 | and here's another. | |
2653 | THAT | |
2654 | ||
2655 | Just don't forget that you have to put a semicolon on the end | |
2656 | to finish the statement, as Perl doesn't know you're not going to | |
2657 | try to do this: | |
2658 | ||
2659 | print <<ABC | |
2660 | 179231 | |
2661 | ABC | |
2662 | + 20; | |
2663 | ||
872d7e53 TS |
2664 | If you want to remove the line terminator from your here-docs, |
2665 | use C<chomp()>. | |
2666 | ||
2667 | chomp($string = <<'END'); | |
2668 | This is a string. | |
2669 | END | |
2670 | ||
2671 | If you want your here-docs to be indented with the rest of the code, | |
2672 | you'll need to remove leading whitespace from each line manually: | |
7e3b091d DA |
2673 | |
2674 | ($quote = <<'FINIS') =~ s/^\s+//gm; | |
89d205f2 | 2675 | The Road goes ever on and on, |
7e3b091d DA |
2676 | down from the door where it began. |
2677 | FINIS | |
2678 | ||
2679 | If you use a here-doc within a delimited construct, such as in C<s///eg>, | |
1bf48760 FC |
2680 | the quoted material must still come on the line following the |
2681 | C<<< <<FOO >>> marker, which means it may be inside the delimited | |
2682 | construct: | |
7e3b091d DA |
2683 | |
2684 | s/this/<<E . 'that' | |
2685 | the other | |
2686 | E | |
2687 | . 'more '/eg; | |
2688 | ||
1bf48760 FC |
2689 | It works this way as of Perl 5.18. Historically, it was inconsistent, and |
2690 | you would have to write | |
7e3b091d | 2691 | |
89d205f2 YO |
2692 | s/this/<<E . 'that' |
2693 | . 'more '/eg; | |
2694 | the other | |
2695 | E | |
7e3b091d | 2696 | |
1bf48760 FC |
2697 | outside of string evals. |
2698 | ||
c543c01b | 2699 | Additionally, quoting rules for the end-of-string identifier are |
46f8a5ea | 2700 | unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not |
89d205f2 YO |
2701 | supported in place of C<''> and C<"">, and the only interpolation is for |
2702 | backslashing the quoting character: | |
7e3b091d DA |
2703 | |
2704 | print << "abc\"def"; | |
2705 | testing... | |
2706 | abc"def | |
2707 | ||
2708 | Finally, quoted strings cannot span multiple lines. The general rule is | |
2709 | that the identifier must be a string literal. Stick with that, and you | |
2710 | should be safe. | |
2711 | ||
a0d0e21e LW |
2712 | =back |
2713 | ||
75e14d17 | 2714 | =head2 Gory details of parsing quoted constructs |
d74e8afc | 2715 | X<quote, gory details> |
75e14d17 | 2716 | |
19799a22 GS |
2717 | When presented with something that might have several different |
2718 | interpretations, Perl uses the B<DWIM> (that's "Do What I Mean") | |
2719 | principle to pick the most probable interpretation. This strategy | |
2720 | is so successful that Perl programmers often do not suspect the | |
2721 | ambivalence of what they write. But from time to time, Perl's | |
2722 | notions differ substantially from what the author honestly meant. | |
2723 | ||
2724 | This section hopes to clarify how Perl handles quoted constructs. | |
2725 | Although the most common reason to learn this is to unravel labyrinthine | |
2726 | regular expressions, because the initial steps of parsing are the | |
2727 | same for all quoting operators, they are all discussed together. | |
2728 | ||
2729 | The most important Perl parsing rule is the first one discussed | |
2730 | below: when processing a quoted construct, Perl first finds the end | |
2731 | of that construct, then interprets its contents. If you understand | |
2732 | this rule, you may skip the rest of this section on the first | |
2733 | reading. The other rules are likely to contradict the user's | |
2734 | expectations much less frequently than this first one. | |
2735 | ||
2736 | Some passes discussed below are performed concurrently, but because | |
2737 | their results are the same, we consider them individually. For different | |
2738 | quoting constructs, Perl performs different numbers of passes, from | |
6deea57f | 2739 | one to four, but these passes are always performed in the same order. |
75e14d17 | 2740 | |
13a2d996 | 2741 | =over 4 |
75e14d17 IZ |
2742 | |
2743 | =item Finding the end | |
2744 | ||
ba7f043c KW |
2745 | The first pass is finding the end of the quoted construct. This results |
2746 | in saving to a safe location a copy of the text (between the starting | |
2747 | and ending delimiters), normalized as necessary to avoid needing to know | |
2748 | what the original delimiters were. | |
6deea57f TS |
2749 | |
2750 | If the construct is a here-doc, the ending delimiter is a line | |
46f8a5ea | 2751 | that has a terminating string as the content. Therefore C<<<EOF> is |
6deea57f TS |
2752 | terminated by C<EOF> immediately followed by C<"\n"> and starting |
2753 | from the first column of the terminating line. | |
2754 | When searching for the terminating line of a here-doc, nothing | |
46f8a5ea | 2755 | is skipped. In other words, lines after the here-doc syntax |
6deea57f TS |
2756 | are compared with the terminating string line by line. |
2757 | ||
2758 | For the constructs except here-docs, single characters are used as starting | |
46f8a5ea | 2759 | and ending delimiters. If the starting delimiter is an opening punctuation |
6deea57f TS |
2760 | (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the |
2761 | corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>). | |
2762 | If the starting delimiter is an unpaired character like C</> or a closing | |
ba7f043c | 2763 | punctuation, the ending delimiter is the same as the starting delimiter. |
6deea57f | 2764 | Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates |
fc693347 | 2765 | both C<qq[]> and C<qq]]> constructs. |
6deea57f TS |
2766 | |
2767 | When searching for single-character delimiters, escaped delimiters | |
1ca345ed | 2768 | and C<\\> are skipped. For example, while searching for terminating C</>, |
6deea57f TS |
2769 | combinations of C<\\> and C<\/> are skipped. If the delimiters are |
2770 | bracketing, nested pairs are also skipped. For example, while searching | |
ba7f043c | 2771 | for a closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>, |
6deea57f TS |
2772 | and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well. |
2773 | However, when backslashes are used as the delimiters (like C<qq\\> and | |
2774 | C<tr\\\>), nothing is skipped. | |
32581033 | 2775 | During the search for the end, backslashes that escape delimiters or |
7188ca43 | 2776 | other backslashes are removed (exactly speaking, they are not copied to the |
32581033 | 2777 | safe location). |
75e14d17 | 2778 | |
19799a22 GS |
2779 | For constructs with three-part delimiters (C<s///>, C<y///>, and |
2780 | C<tr///>), the search is repeated once more. | |
fc693347 | 2781 | If the first delimiter is not an opening punctuation, the three delimiters must |
d74605e5 FC |
2782 | be the same, such as C<s!!!> and C<tr)))>, |
2783 | in which case the second delimiter | |
6deea57f | 2784 | terminates the left part and starts the right part at once. |
b6538e4f | 2785 | If the left part is delimited by bracketing punctuation (that is C<()>, |
6deea57f | 2786 | C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of |
b6538e4f | 2787 | delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace |
ba7f043c | 2788 | and comments are allowed between the two parts, although the comment must follow |
a727cfac | 2789 | at least one whitespace character; otherwise a character expected as the |
b6538e4f | 2790 | start of the comment may be regarded as the starting delimiter of the right part. |
75e14d17 | 2791 | |
19799a22 GS |
2792 | During this search no attention is paid to the semantics of the construct. |
2793 | Thus: | |
75e14d17 IZ |
2794 | |
2795 | "$hash{"$foo/$bar"}" | |
2796 | ||
2a94b7ce | 2797 | or: |
75e14d17 | 2798 | |
89d205f2 | 2799 | m/ |
2a94b7ce | 2800 | bar # NOT a comment, this slash / terminated m//! |
75e14d17 IZ |
2801 | /x |
2802 | ||
19799a22 GS |
2803 | do not form legal quoted expressions. The quoted part ends on the |
2804 | first C<"> and C</>, and the rest happens to be a syntax error. | |
2805 | Because the slash that terminated C<m//> was followed by a C<SPACE>, | |
2806 | the example above is not C<m//x>, but rather C<m//> with no C</x> | |
2807 | modifier. So the embedded C<#> is interpreted as a literal C<#>. | |
75e14d17 | 2808 | |
89d205f2 | 2809 | Also no attention is paid to C<\c\> (multichar control char syntax) during |
46f8a5ea | 2810 | this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part |
89d205f2 | 2811 | of C<\/>, and the following C</> is not recognized as a delimiter. |
0d594e51 TS |
2812 | Instead, use C<\034> or C<\x1c> at the end of quoted constructs. |
2813 | ||
75e14d17 | 2814 | =item Interpolation |
d74e8afc | 2815 | X<interpolation> |
75e14d17 | 2816 | |
19799a22 | 2817 | The next step is interpolation in the text obtained, which is now |
89d205f2 | 2818 | delimiter-independent. There are multiple cases. |
75e14d17 | 2819 | |
13a2d996 | 2820 | =over 4 |
75e14d17 | 2821 | |
89d205f2 | 2822 | =item C<<<'EOF'> |
75e14d17 IZ |
2823 | |
2824 | No interpolation is performed. | |
6deea57f TS |
2825 | Note that the combination C<\\> is left intact, since escaped delimiters |
2826 | are not available for here-docs. | |
75e14d17 | 2827 | |
6deea57f | 2828 | =item C<m''>, the pattern of C<s'''> |
89d205f2 | 2829 | |
6deea57f TS |
2830 | No interpolation is performed at this stage. |
2831 | Any backslashed sequences including C<\\> are treated at the stage | |
2832 | to L</"parsing regular expressions">. | |
89d205f2 | 2833 | |
6deea57f | 2834 | =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''> |
75e14d17 | 2835 | |
89d205f2 | 2836 | The only interpolation is removal of C<\> from pairs of C<\\>. |
ba7f043c | 2837 | Therefore C<"-"> in C<tr'''> and C<y'''> is treated literally |
6deea57f TS |
2838 | as a hyphen and no character range is available. |
2839 | C<\1> in the replacement of C<s'''> does not work as C<$1>. | |
89d205f2 YO |
2840 | |
2841 | =item C<tr///>, C<y///> | |
2842 | ||
6deea57f TS |
2843 | No variable interpolation occurs. String modifying combinations for |
2844 | case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized. | |
2845 | The other escape sequences such as C<\200> and C<\t> and backslashed | |
2846 | characters such as C<\\> and C<\-> are converted to appropriate literals. | |
ba7f043c KW |
2847 | The character C<"-"> is treated specially and therefore C<\-> is treated |
2848 | as a literal C<"-">. | |
75e14d17 | 2849 | |
89d205f2 | 2850 | =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF"> |
75e14d17 | 2851 | |
628253b8 | 2852 | C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> (possibly paired with C<\E>) are |
19799a22 | 2853 | converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar"> |
ba7f043c | 2854 | is converted to S<C<$foo . (quotemeta("baz" . $bar))>> internally. |
6deea57f TS |
2855 | The other escape sequences such as C<\200> and C<\t> and backslashed |
2856 | characters such as C<\\> and C<\-> are replaced with appropriate | |
2857 | expansions. | |
2a94b7ce | 2858 | |
19799a22 GS |
2859 | Let it be stressed that I<whatever falls between C<\Q> and C<\E>> |
2860 | is interpolated in the usual way. Something like C<"\Q\\E"> has | |
48cbae4f | 2861 | no C<\E> inside. Instead, it has C<\Q>, C<\\>, and C<E>, so the |
19799a22 GS |
2862 | result is the same as for C<"\\\\E">. As a general rule, backslashes |
2863 | between C<\Q> and C<\E> may lead to counterintuitive results. So, | |
2864 | C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same | |
2865 | as C<"\\\t"> (since TAB is not alphanumeric). Note also that: | |
2a94b7ce IZ |
2866 | |
2867 | $str = '\t'; | |
2868 | return "\Q$str"; | |
2869 | ||
2870 | may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. | |
2871 | ||
19799a22 | 2872 | Interpolated scalars and arrays are converted internally to the C<join> and |
ba7f043c | 2873 | C<"."> catenation operations. Thus, S<C<"$foo XXX '@arr'">> becomes: |
75e14d17 | 2874 | |
19799a22 | 2875 | $foo . " XXX '" . (join $", @arr) . "'"; |
75e14d17 | 2876 | |
19799a22 | 2877 | All operations above are performed simultaneously, left to right. |
75e14d17 | 2878 | |
ba7f043c | 2879 | Because the result of S<C<"\Q I<STRING> \E">> has all metacharacters |
19799a22 | 2880 | quoted, there is no way to insert a literal C<$> or C<@> inside a |
ba7f043c | 2881 | C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to become |
19799a22 GS |
2882 | C<"\\\$">; if not, it is interpreted as the start of an interpolated |
2883 | scalar. | |
75e14d17 | 2884 | |
19799a22 | 2885 | Note also that the interpolation code needs to make a decision on |
89d205f2 | 2886 | where the interpolated scalar ends. For instance, whether |
ba7f043c | 2887 | S<C<< "a $x -> {c}" >>> really means: |
75e14d17 | 2888 | |
db691027 | 2889 | "a " . $x . " -> {c}"; |
75e14d17 | 2890 | |
2a94b7ce | 2891 | or: |
75e14d17 | 2892 | |
db691027 | 2893 | "a " . $x -> {c}; |
75e14d17 | 2894 | |
19799a22 GS |
2895 | Most of the time, the longest possible text that does not include |
2896 | spaces between components and which contains matching braces or | |
2897 | brackets. because the outcome may be determined by voting based | |
2898 | on heuristic estimators, the result is not strictly predictable. | |
2899 | Fortunately, it's usually correct for ambiguous cases. | |
75e14d17 | 2900 | |
6deea57f | 2901 | =item the replacement of C<s///> |
75e14d17 | 2902 | |
628253b8 | 2903 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> and interpolation |
6deea57f TS |
2904 | happens as with C<qq//> constructs. |
2905 | ||
2906 | It is at this step that C<\1> is begrudgingly converted to C<$1> in | |
2907 | the replacement text of C<s///>, in order to correct the incorrigible | |
2908 | I<sed> hackers who haven't picked up the saner idiom yet. A warning | |
ba7f043c | 2909 | is emitted if the S<C<use warnings>> pragma or the B<-w> command-line flag |
6deea57f TS |
2910 | (that is, the C<$^W> variable) was set. |
2911 | ||
9c6deb98 | 2912 | =item C<RE> in C<m?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, |
6deea57f | 2913 | |
628253b8 | 2914 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F>, C<\E>, |
cc74c5bd TS |
2915 | and interpolation happens (almost) as with C<qq//> constructs. |
2916 | ||
5d03b57c KW |
2917 | Processing of C<\N{...}> is also done here, and compiled into an intermediate |
2918 | form for the regex compiler. (This is because, as mentioned below, the regex | |
2919 | compilation may be done at execution time, and C<\N{...}> is a compile-time | |
2920 | construct.) | |
2921 | ||
cc74c5bd TS |
2922 | However any other combinations of C<\> followed by a character |
2923 | are not substituted but only skipped, in order to parse them | |
2924 | as regular expressions at the following step. | |
6deea57f | 2925 | As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly |
1749ea0d | 2926 | treated as an array symbol (for example C<@foo>), |
6deea57f | 2927 | even though the same text in C<qq//> gives interpolation of C<\c@>. |
6deea57f | 2928 | |
e128ab2c DM |
2929 | Code blocks such as C<(?{BLOCK})> are handled by temporarily passing control |
2930 | back to the perl parser, in a similar way that an interpolated array | |
2931 | subscript expression such as C<"foo$array[1+f("[xyz")]bar"> would be. | |
2932 | ||
ba7f043c KW |
2933 | Moreover, inside C<(?{BLOCK})>, S<C<(?# comment )>>, and |
2934 | a C<#>-comment in a C</x>-regular expression, no processing is | |
19799a22 | 2935 | performed whatsoever. This is the first step at which the presence |
ba7f043c | 2936 | of the C</x> modifier is relevant. |
19799a22 | 2937 | |
1749ea0d TS |
2938 | Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+> |
2939 | and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are | |
2940 | voted (by several different estimators) to be either an array element | |
2941 | or C<$var> followed by an RE alternative. This is where the notation | |
19799a22 GS |
2942 | C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as |
2943 | array element C<-9>, not as a regular expression from the variable | |
2944 | C<$arr> followed by a digit, which would be the interpretation of | |
2945 | C</$arr[0-9]/>. Since voting among different estimators may occur, | |
2946 | the result is not predictable. | |
2947 | ||
19799a22 GS |
2948 | The lack of processing of C<\\> creates specific restrictions on |
2949 | the post-processed text. If the delimiter is C</>, one cannot get | |
2950 | the combination C<\/> into the result of this step. C</> will | |
2951 | finish the regular expression, C<\/> will be stripped to C</> on | |
2952 | the previous step, and C<\\/> will be left as is. Because C</> is | |
2953 | equivalent to C<\/> inside a regular expression, this does not | |
2954 | matter unless the delimiter happens to be character special to the | |
9c6deb98 | 2955 | RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<m?foo?>; or an |
19799a22 | 2956 | alphanumeric char, as in: |
2a94b7ce IZ |
2957 | |
2958 | m m ^ a \s* b mmx; | |
2959 | ||
19799a22 | 2960 | In the RE above, which is intentionally obfuscated for illustration, the |
6deea57f | 2961 | delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the |
ba7f043c | 2962 | RE is the same as for S<C<m/ ^ a \s* b /mx>>. There's more than one |
19799a22 GS |
2963 | reason you're encouraged to restrict your delimiters to non-alphanumeric, |
2964 | non-whitespace choices. | |
75e14d17 IZ |
2965 | |
2966 | =back | |
2967 | ||
19799a22 | 2968 | This step is the last one for all constructs except regular expressions, |
75e14d17 IZ |
2969 | which are processed further. |
2970 | ||
6deea57f TS |
2971 | =item parsing regular expressions |
2972 | X<regexp, parse> | |
75e14d17 | 2973 | |
19799a22 | 2974 | Previous steps were performed during the compilation of Perl code, |
ac036724 | 2975 | but this one happens at run time, although it may be optimized to |
19799a22 | 2976 | be calculated at compile time if appropriate. After preprocessing |
6deea57f | 2977 | described above, and possibly after evaluation if concatenation, |
19799a22 GS |
2978 | joining, casing translation, or metaquoting are involved, the |
2979 | resulting I<string> is passed to the RE engine for compilation. | |
2980 | ||
2981 | Whatever happens in the RE engine might be better discussed in L<perlre>, | |
2982 | but for the sake of continuity, we shall do so here. | |
2983 | ||
ba7f043c | 2984 | This is another step where the presence of the C</x> modifier is |
19799a22 | 2985 | relevant. The RE engine scans the string from left to right and |
ba7f043c | 2986 | converts it into a finite automaton. |
19799a22 GS |
2987 | |
2988 | Backslashed characters are either replaced with corresponding | |
2989 | literal strings (as with C<\{>), or else they generate special nodes | |
2990 | in the finite automaton (as with C<\b>). Characters special to the | |
2991 | RE engine (such as C<|>) generate corresponding nodes or groups of | |
2992 | nodes. C<(?#...)> comments are ignored. All the rest is either | |
2993 | converted to literal strings to match, or else is ignored (as is | |
ba7f043c | 2994 | whitespace and C<#>-style comments if C</x> is present). |
19799a22 GS |
2995 | |
2996 | Parsing of the bracketed character class construct, C<[...]>, is | |
2997 | rather different than the rule used for the rest of the pattern. | |
2998 | The terminator of this construct is found using the same rules as | |
2999 | for finding the terminator of a C<{}>-delimited construct, the only | |
3000 | exception being that C<]> immediately following C<[> is treated as | |
e128ab2c DM |
3001 | though preceded by a backslash. |
3002 | ||
3003 | The terminator of runtime C<(?{...})> is found by temporarily switching | |
3004 | control to the perl parser, which should stop at the point where the | |
3005 | logically balancing terminating C<}> is found. | |
19799a22 GS |
3006 | |
3007 | It is possible to inspect both the string given to RE engine and the | |
3008 | resulting finite automaton. See the arguments C<debug>/C<debugcolor> | |
ba7f043c | 3009 | in the S<C<use L<re>>> pragma, as well as Perl's B<-Dr> command-line |
4a4eefd0 | 3010 | switch documented in L<perlrun/"Command Switches">. |
75e14d17 IZ |
3011 | |
3012 | =item Optimization of regular expressions | |
d74e8afc | 3013 | X<regexp, optimization> |
75e14d17 | 3014 | |
7522fed5 | 3015 | This step is listed for completeness only. Since it does not change |
75e14d17 | 3016 | semantics, details of this step are not documented and are subject |
19799a22 GS |
3017 | to change without notice. This step is performed over the finite |
3018 | automaton that was generated during the previous pass. | |
2a94b7ce | 3019 | |
19799a22 GS |
3020 | It is at this stage that C<split()> silently optimizes C</^/> to |
3021 | mean C</^/m>. | |
75e14d17 IZ |
3022 | |
3023 | =back | |
3024 | ||
a0d0e21e | 3025 | =head2 I/O Operators |
d74e8afc | 3026 | X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle> |
80a96bfc | 3027 | X<< <> >> X<< <<>> >> X<@ARGV> |
a0d0e21e | 3028 | |
54310121 | 3029 | There are several I/O operators you should know about. |
fbad3eb5 | 3030 | |
7b8d334a | 3031 | A string enclosed by backticks (grave accents) first undergoes |
19799a22 GS |
3032 | double-quote interpolation. It is then interpreted as an external |
3033 | command, and the output of that command is the value of the | |
e9c56f9b JH |
3034 | backtick string, like in a shell. In scalar context, a single string |
3035 | consisting of all output is returned. In list context, a list of | |
3036 | values is returned, one per line of output. (You can set C<$/> to use | |
3037 | a different line terminator.) The command is executed each time the | |
3038 | pseudo-literal is evaluated. The status value of the command is | |
3039 | returned in C<$?> (see L<perlvar> for the interpretation of C<$?>). | |
3040 | Unlike in B<csh>, no translation is done on the return data--newlines | |
3041 | remain newlines. Unlike in any of the shells, single quotes do not | |
3042 | hide variable names in the command from interpretation. To pass a | |
3043 | literal dollar-sign through to the shell you need to hide it with a | |
3044 | backslash. The generalized form of backticks is C<qx//>. (Because | |
3045 | backticks always undergo shell expansion as well, see L<perlsec> for | |
3046 | security concerns.) | |
d74e8afc | 3047 | X<qx> X<`> X<``> X<backtick> X<glob> |
19799a22 GS |
3048 | |
3049 | In scalar context, evaluating a filehandle in angle brackets yields | |
3050 | the next line from that file (the newline, if any, included), or | |
3051 | C<undef> at end-of-file or on error. When C<$/> is set to C<undef> | |
3052 | (sometimes known as file-slurp mode) and the file is empty, it | |
3053 | returns C<''> the first time, followed by C<undef> subsequently. | |
3054 | ||
3055 | Ordinarily you must assign the returned value to a variable, but | |
3056 | there is one situation where an automatic assignment happens. If | |
3057 | and only if the input symbol is the only thing inside the conditional | |
3058 | of a C<while> statement (even if disguised as a C<for(;;)> loop), | |
ba7f043c | 3059 | the value is automatically assigned to the global variable C<$_>, |
19799a22 GS |
3060 | destroying whatever was there previously. (This may seem like an |
3061 | odd thing to you, but you'll use the construct in almost every Perl | |
ba7f043c KW |
3062 | script you write.) The C<$_> variable is not implicitly localized. |
3063 | You'll have to put a S<C<local $_;>> before the loop if you want that | |
19799a22 GS |
3064 | to happen. |
3065 | ||
3066 | The following lines are equivalent: | |
a0d0e21e | 3067 | |
748a9306 | 3068 | while (defined($_ = <STDIN>)) { print; } |
7b8d334a | 3069 | while ($_ = <STDIN>) { print; } |
a0d0e21e LW |
3070 | while (<STDIN>) { print; } |
3071 | for (;<STDIN>;) { print; } | |
748a9306 | 3072 | print while defined($_ = <STDIN>); |
7b8d334a | 3073 | print while ($_ = <STDIN>); |
a0d0e21e LW |
3074 | print while <STDIN>; |
3075 | ||
a727cfac | 3076 | This also behaves similarly, but assigns to a lexical variable |
1ca345ed | 3077 | instead of to C<$_>: |
7b8d334a | 3078 | |
89d205f2 | 3079 | while (my $line = <STDIN>) { print $line } |
7b8d334a | 3080 | |
19799a22 GS |
3081 | In these loop constructs, the assigned value (whether assignment |
3082 | is automatic or explicit) is then tested to see whether it is | |
1ca345ed TC |
3083 | defined. The defined test avoids problems where the line has a string |
3084 | value that would be treated as false by Perl; for example a "" or | |
ba7f043c | 3085 | a C<"0"> with no trailing newline. If you really mean for such values |
19799a22 | 3086 | to terminate the loop, they should be tested for explicitly: |
7b8d334a GS |
3087 | |
3088 | while (($_ = <STDIN>) ne '0') { ... } | |
3089 | while (<STDIN>) { last unless $_; ... } | |
3090 | ||
ba7f043c | 3091 | In other boolean contexts, C<< <I<FILEHANDLE>> >> without an |
5ef4d93e | 3092 | explicit C<defined> test or comparison elicits a warning if the |
ba7f043c | 3093 | S<C<use warnings>> pragma or the B<-w> |
19799a22 | 3094 | command-line switch (the C<$^W> variable) is in effect. |
7b8d334a | 3095 | |
5f05dabc | 3096 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
19799a22 GS |
3097 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except |
3098 | in packages, where they would be interpreted as local identifiers | |
3099 | rather than global.) Additional filehandles may be created with | |
ba7f043c | 3100 | the C<open()> function, amongst others. See L<perlopentut> and |
19799a22 | 3101 | L<perlfunc/open> for details on this. |
d74e8afc | 3102 | X<stdin> X<stdout> X<sterr> |
a0d0e21e | 3103 | |
ba7f043c | 3104 | If a C<< <I<FILEHANDLE>> >> is used in a context that is looking for |
19799a22 GS |
3105 | a list, a list comprising all input lines is returned, one line per |
3106 | list element. It's easy to grow to a rather large data space this | |
3107 | way, so use with care. | |
a0d0e21e | 3108 | |
ba7f043c | 3109 | C<< <I<FILEHANDLE>> >> may also be spelled C<readline(*I<FILEHANDLE>)>. |
19799a22 | 3110 | See L<perlfunc/readline>. |
fbad3eb5 | 3111 | |
ba7f043c | 3112 | The null filehandle C<< <> >> is special: it can be used to emulate the |
1ca345ed TC |
3113 | behavior of B<sed> and B<awk>, and any other Unix filter program |
3114 | that takes a list of filenames, doing the same to each line | |
ba7f043c | 3115 | of input from all of them. Input from C<< <> >> comes either from |
a0d0e21e | 3116 | standard input, or from each file listed on the command line. Here's |
ba7f043c KW |
3117 | how it works: the first time C<< <> >> is evaluated, the C<@ARGV> array is |
3118 | checked, and if it is empty, C<$ARGV[0]> is set to C<"-">, which when opened | |
3119 | gives you standard input. The C<@ARGV> array is then processed as a list | |
a0d0e21e LW |
3120 | of filenames. The loop |
3121 | ||
3122 | while (<>) { | |
3123 | ... # code for each line | |
3124 | } | |
3125 | ||
3126 | is equivalent to the following Perl-like pseudo code: | |
3127 | ||
3e3baf6d | 3128 | unshift(@ARGV, '-') unless @ARGV; |
a0d0e21e LW |
3129 | while ($ARGV = shift) { |
3130 | open(ARGV, $ARGV); | |
3131 | while (<ARGV>) { | |
3132 | ... # code for each line | |
3133 | } | |
3134 | } | |
3135 | ||
19799a22 | 3136 | except that it isn't so cumbersome to say, and will actually work. |
ba7f043c KW |
3137 | It really does shift the C<@ARGV> array and put the current filename |
3138 | into the C<$ARGV> variable. It also uses filehandle I<ARGV> | |
3139 | internally. C<< <> >> is just a synonym for C<< <ARGV> >>, which | |
19799a22 | 3140 | is magical. (The pseudo code above doesn't work because it treats |
ba7f043c | 3141 | C<< <ARGV> >> as non-magical.) |
a0d0e21e | 3142 | |
48ab5743 ML |
3143 | Since the null filehandle uses the two argument form of L<perlfunc/open> |
3144 | it interprets special characters, so if you have a script like this: | |
3145 | ||
3146 | while (<>) { | |
3147 | print; | |
3148 | } | |
3149 | ||
ba7f043c | 3150 | and call it with S<C<perl dangerous.pl 'rm -rfv *|'>>, it actually opens a |
48ab5743 ML |
3151 | pipe, executes the C<rm> command and reads C<rm>'s output from that pipe. |
3152 | If you want all items in C<@ARGV> to be interpreted as file names, you | |
1033ba6e PM |
3153 | can use the module C<ARGV::readonly> from CPAN, or use the double bracket: |
3154 | ||
3155 | while (<<>>) { | |
3156 | print; | |
3157 | } | |
3158 | ||
3159 | Using double angle brackets inside of a while causes the open to use the | |
3160 | three argument form (with the second argument being C<< < >>), so all | |
ba7f043c KW |
3161 | arguments in C<ARGV> are treated as literal filenames (including C<"-">). |
3162 | (Note that for convenience, if you use C<< <<>> >> and if C<@ARGV> is | |
80a96bfc | 3163 | empty, it will still read from the standard input.) |
48ab5743 | 3164 | |
ba7f043c | 3165 | You can modify C<@ARGV> before the first C<< <> >> as long as the array ends up |
a0d0e21e | 3166 | containing the list of filenames you really want. Line numbers (C<$.>) |
19799a22 GS |
3167 | continue as though the input were one big happy file. See the example |
3168 | in L<perlfunc/eof> for how to reset line numbers on each file. | |
5a964f20 | 3169 | |
ba7f043c KW |
3170 | If you want to set C<@ARGV> to your own list of files, go right ahead. |
3171 | This sets C<@ARGV> to all plain text files if no C<@ARGV> was given: | |
5a964f20 TC |
3172 | |
3173 | @ARGV = grep { -f && -T } glob('*') unless @ARGV; | |
a0d0e21e | 3174 | |
5a964f20 TC |
3175 | You can even set them to pipe commands. For example, this automatically |
3176 | filters compressed arguments through B<gzip>: | |
3177 | ||
3178 | @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; | |
3179 | ||
3180 | If you want to pass switches into your script, you can use one of the | |
ba7f043c | 3181 | C<Getopts> modules or put a loop on the front like this: |
a0d0e21e LW |
3182 | |
3183 | while ($_ = $ARGV[0], /^-/) { | |
3184 | shift; | |
3185 | last if /^--$/; | |
3186 | if (/^-D(.*)/) { $debug = $1 } | |
3187 | if (/^-v/) { $verbose++ } | |
5a964f20 | 3188 | # ... # other switches |
a0d0e21e | 3189 | } |
5a964f20 | 3190 | |
a0d0e21e | 3191 | while (<>) { |
5a964f20 | 3192 | # ... # code for each line |
a0d0e21e LW |
3193 | } |
3194 | ||
ba7f043c | 3195 | The C<< <> >> symbol will return C<undef> for end-of-file only once. |
89d205f2 | 3196 | If you call it again after this, it will assume you are processing another |
ba7f043c | 3197 | C<@ARGV> list, and if you haven't set C<@ARGV>, will read input from STDIN. |
a0d0e21e | 3198 | |
1ca345ed | 3199 | If what the angle brackets contain is a simple scalar variable (for example, |
ba7f043c | 3200 | C<$foo>), then that variable contains the name of the |
19799a22 GS |
3201 | filehandle to input from, or its typeglob, or a reference to the |
3202 | same. For example: | |
cb1a09d0 AD |
3203 | |
3204 | $fh = \*STDIN; | |
3205 | $line = <$fh>; | |
a0d0e21e | 3206 | |
5a964f20 TC |
3207 | If what's within the angle brackets is neither a filehandle nor a simple |
3208 | scalar variable containing a filehandle name, typeglob, or typeglob | |
3209 | reference, it is interpreted as a filename pattern to be globbed, and | |
3210 | either a list of filenames or the next filename in the list is returned, | |
19799a22 | 3211 | depending on context. This distinction is determined on syntactic |
ba7f043c KW |
3212 | grounds alone. That means C<< <$x> >> is always a C<readline()> from |
3213 | an indirect handle, but C<< <$hash{key}> >> is always a C<glob()>. | |
3214 | That's because C<$x> is a simple scalar variable, but C<$hash{key}> is | |
ef191992 YST |
3215 | not--it's a hash element. Even C<< <$x > >> (note the extra space) |
3216 | is treated as C<glob("$x ")>, not C<readline($x)>. | |
5a964f20 TC |
3217 | |
3218 | One level of double-quote interpretation is done first, but you can't | |
35f2feb0 | 3219 | say C<< <$foo> >> because that's an indirect filehandle as explained |
5a964f20 TC |
3220 | in the previous paragraph. (In older versions of Perl, programmers |
3221 | would insert curly brackets to force interpretation as a filename glob: | |
35f2feb0 | 3222 | C<< <${foo}> >>. These days, it's considered cleaner to call the |
5a964f20 | 3223 | internal function directly as C<glob($foo)>, which is probably the right |
19799a22 | 3224 | way to have done it in the first place.) For example: |
a0d0e21e LW |
3225 | |
3226 | while (<*.c>) { | |
3227 | chmod 0644, $_; | |
3228 | } | |
3229 | ||
3a4b19e4 | 3230 | is roughly equivalent to: |
a0d0e21e LW |
3231 | |
3232 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); | |
3233 | while (<FOO>) { | |
5b3eff12 | 3234 | chomp; |
a0d0e21e LW |
3235 | chmod 0644, $_; |
3236 | } | |
3237 | ||
3a4b19e4 | 3238 | except that the globbing is actually done internally using the standard |
ba7f043c | 3239 | C<L<File::Glob>> extension. Of course, the shortest way to do the above is: |
a0d0e21e LW |
3240 | |
3241 | chmod 0644, <*.c>; | |
3242 | ||
19799a22 GS |
3243 | A (file)glob evaluates its (embedded) argument only when it is |
3244 | starting a new list. All values must be read before it will start | |
3245 | over. In list context, this isn't important because you automatically | |
3246 | get them all anyway. However, in scalar context the operator returns | |
069e01df | 3247 | the next value each time it's called, or C<undef> when the list has |
19799a22 GS |
3248 | run out. As with filehandle reads, an automatic C<defined> is |
3249 | generated when the glob occurs in the test part of a C<while>, | |
1ca345ed TC |
3250 | because legal glob returns (for example, |
3251 | a file called F<0>) would otherwise | |
19799a22 GS |
3252 | terminate the loop. Again, C<undef> is returned only once. So if |
3253 | you're expecting a single value from a glob, it is much better to | |
3254 | say | |
4633a7c4 LW |
3255 | |
3256 | ($file) = <blurch*>; | |
3257 | ||
3258 | than | |
3259 | ||
3260 | $file = <blurch*>; | |
3261 | ||
3262 | because the latter will alternate between returning a filename and | |
19799a22 | 3263 | returning false. |
4633a7c4 | 3264 | |
b159ebd3 | 3265 | If you're trying to do variable interpolation, it's definitely better |
ba7f043c | 3266 | to use the C<glob()> function, because the older notation can cause people |
e37d713d | 3267 | to become confused with the indirect filehandle notation. |
4633a7c4 LW |
3268 | |
3269 | @files = glob("$dir/*.[ch]"); | |
3270 | @files = glob($files[$i]); | |
3271 | ||
a0d0e21e | 3272 | =head2 Constant Folding |
d74e8afc | 3273 | X<constant folding> X<folding> |
a0d0e21e LW |
3274 | |
3275 | Like C, Perl does a certain amount of expression evaluation at | |
19799a22 | 3276 | compile time whenever it determines that all arguments to an |
a0d0e21e LW |
3277 | operator are static and have no side effects. In particular, string |
3278 | concatenation happens at compile time between literals that don't do | |
19799a22 | 3279 | variable substitution. Backslash interpolation also happens at |
a0d0e21e LW |
3280 | compile time. You can say |
3281 | ||
1ca345ed | 3282 | 'Now is the time for all' |
a727cfac | 3283 | . "\n" |
1ca345ed | 3284 | . 'good men to come to.' |
a0d0e21e | 3285 | |
54310121 | 3286 | and this all reduces to one string internally. Likewise, if |
a0d0e21e LW |
3287 | you say |
3288 | ||
3289 | foreach $file (@filenames) { | |
5a964f20 | 3290 | if (-s $file > 5 + 100 * 2**16) { } |
54310121 | 3291 | } |
a0d0e21e | 3292 | |
1ca345ed | 3293 | the compiler precomputes the number which that expression |
19799a22 | 3294 | represents so that the interpreter won't have to. |
a0d0e21e | 3295 | |
fd1abbef | 3296 | =head2 No-ops |
d74e8afc | 3297 | X<no-op> X<nop> |
fd1abbef DN |
3298 | |
3299 | Perl doesn't officially have a no-op operator, but the bare constants | |
1ca345ed | 3300 | C<0> and C<1> are special-cased not to produce a warning in void |
fd1abbef DN |
3301 | context, so you can for example safely do |
3302 | ||
3303 | 1 while foo(); | |
3304 | ||
2c268ad5 | 3305 | =head2 Bitwise String Operators |
fb7054ba | 3306 | X<operator, bitwise, string> X<&.> X<|.> X<^.> X<~.> |
2c268ad5 TP |
3307 | |
3308 | Bitstrings of any size may be manipulated by the bitwise operators | |
3309 | (C<~ | & ^>). | |
3310 | ||
19799a22 GS |
3311 | If the operands to a binary bitwise op are strings of different |
3312 | sizes, B<|> and B<^> ops act as though the shorter operand had | |
3313 | additional zero bits on the right, while the B<&> op acts as though | |
3314 | the longer operand were truncated to the length of the shorter. | |
3315 | The granularity for such extension or truncation is one or more | |
3316 | bytes. | |
2c268ad5 | 3317 | |
89d205f2 | 3318 | # ASCII-based examples |
2c268ad5 TP |
3319 | print "j p \n" ^ " a h"; # prints "JAPH\n" |
3320 | print "JA" | " ph\n"; # prints "japh\n" | |
3321 | print "japh\nJunk" & '_____'; # prints "JAPH\n"; | |
3322 | print 'p N$' ^ " E<H\n"; # prints "Perl\n"; | |
3323 | ||
19799a22 | 3324 | If you are intending to manipulate bitstrings, be certain that |
2c268ad5 | 3325 | you're supplying bitstrings: If an operand is a number, that will imply |
19799a22 | 3326 | a B<numeric> bitwise operation. You may explicitly show which type of |
2c268ad5 TP |
3327 | operation you intend by using C<""> or C<0+>, as in the examples below. |
3328 | ||
4358a253 SS |
3329 | $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF) |
3330 | $foo = '150' | 105; # yields 255 | |
2c268ad5 TP |
3331 | $foo = 150 | '105'; # yields 255 |
3332 | $foo = '150' | '105'; # yields string '155' (under ASCII) | |
3333 | ||
3334 | $baz = 0+$foo & 0+$bar; # both ops explicitly numeric | |
3335 | $biz = "$foo" ^ "$bar"; # both ops explicitly stringy | |
a0d0e21e | 3336 | |
fb7054ba | 3337 | This somewhat unpredictable behavior can be avoided with the experimental |
ba7f043c KW |
3338 | "bitwise" feature, new in Perl 5.22. You can enable it via S<C<use feature |
3339 | 'bitwise'>>. By default, it will warn unless the C<"experimental::bitwise"> | |
3340 | warnings category has been disabled. (S<C<use experimental 'bitwise'>> will | |
fb7054ba FC |
3341 | enable the feature and disable the warning.) Under this feature, the four |
3342 | standard bitwise operators (C<~ | & ^>) are always numeric. Adding a dot | |
3343 | after each operator (C<~. |. &. ^.>) forces it to treat its operands as | |
3344 | strings: | |
3345 | ||
3346 | use experimental "bitwise"; | |
3347 | $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF) | |
3348 | $foo = '150' | 105; # yields 255 | |
3349 | $foo = 150 | '105'; # yields 255 | |
3350 | $foo = '150' | '105'; # yields 255 | |
9f1b8172 | 3351 | $foo = 150 |. 105; # yields string '155' |
fb7054ba FC |
3352 | $foo = '150' |. 105; # yields string '155' |
3353 | $foo = 150 |.'105'; # yields string '155' | |
3354 | $foo = '150' |.'105'; # yields string '155' | |
3355 | ||
3356 | $baz = $foo & $bar; # both operands numeric | |
3357 | $biz = $foo ^. $bar; # both operands stringy | |
3358 | ||
3359 | The assignment variants of these operators (C<&= |= ^= &.= |.= ^.=>) | |
3360 | behave likewise under the feature. | |
3361 | ||
fac71630 KW |
3362 | It is a fatal error if an operand contains a character whose ordinal |
3363 | value is above 0xFF, and hence not expressible except in UTF-8. The | |
3364 | operation is performed on a non-UTF-8 copy for other operands encoded in | |
3365 | UTF-8. See L<perlunicode/Byte and Character Semantics>. | |
737f7534 | 3366 | |
1ae175c8 GS |
3367 | See L<perlfunc/vec> for information on how to manipulate individual bits |
3368 | in a bit vector. | |
3369 | ||
55497cff | 3370 | =head2 Integer Arithmetic |
d74e8afc | 3371 | X<integer> |
a0d0e21e | 3372 | |
19799a22 | 3373 | By default, Perl assumes that it must do most of its arithmetic in |
a0d0e21e LW |
3374 | floating point. But by saying |
3375 | ||
3376 | use integer; | |
3377 | ||
3eab78e3 CW |
3378 | you may tell the compiler to use integer operations |
3379 | (see L<integer> for a detailed explanation) from here to the end of | |
3380 | the enclosing BLOCK. An inner BLOCK may countermand this by saying | |
a0d0e21e LW |
3381 | |
3382 | no integer; | |
3383 | ||
19799a22 | 3384 | which lasts until the end of that BLOCK. Note that this doesn't |
3eab78e3 CW |
3385 | mean everything is an integer, merely that Perl will use integer |
3386 | operations for arithmetic, comparison, and bitwise operators. For | |
ba7f043c | 3387 | example, even under S<C<use integer>>, if you take the C<sqrt(2)>, you'll |
3eab78e3 | 3388 | still get C<1.4142135623731> or so. |
19799a22 | 3389 | |
ba7f043c KW |
3390 | Used on numbers, the bitwise operators (C<&> C<|> C<^> C<~> C<< << >> |
3391 | C<< >> >>) always produce integral results. (But see also | |
5a0de581 | 3392 | L</Bitwise String Operators>.) However, S<C<use integer>> still has meaning for |
19799a22 | 3393 | them. By default, their results are interpreted as unsigned integers, but |
ba7f043c | 3394 | if S<C<use integer>> is in effect, their results are interpreted |
19799a22 | 3395 | as signed integers. For example, C<~0> usually evaluates to a large |
ba7f043c | 3396 | integral value. However, S<C<use integer; ~0>> is C<-1> on two's-complement |
19799a22 | 3397 | machines. |
68dc0745 | 3398 | |
3399 | =head2 Floating-point Arithmetic | |
06ce2fa3 | 3400 | |
d74e8afc | 3401 | X<floating-point> X<floating point> X<float> X<real> |
68dc0745 | 3402 | |
ba7f043c | 3403 | While S<C<use integer>> provides integer-only arithmetic, there is no |
19799a22 GS |
3404 | analogous mechanism to provide automatic rounding or truncation to a |
3405 | certain number of decimal places. For rounding to a certain number | |
ba7f043c | 3406 | of digits, C<sprintf()> or C<printf()> is usually the easiest route. |
19799a22 | 3407 | See L<perlfaq4>. |
68dc0745 | 3408 | |
5a964f20 TC |
3409 | Floating-point numbers are only approximations to what a mathematician |
3410 | would call real numbers. There are infinitely more reals than floats, | |
3411 | so some corners must be cut. For example: | |
3412 | ||
3413 | printf "%.20g\n", 123456789123456789; | |
3414 | # produces 123456789123456784 | |
3415 | ||
8548cb57 RGS |
3416 | Testing for exact floating-point equality or inequality is not a |
3417 | good idea. Here's a (relatively expensive) work-around to compare | |
5a964f20 TC |
3418 | whether two floating-point numbers are equal to a particular number of |
3419 | decimal places. See Knuth, volume II, for a more robust treatment of | |
3420 | this topic. | |
3421 | ||
3422 | sub fp_equal { | |
3423 | my ($X, $Y, $POINTS) = @_; | |
3424 | my ($tX, $tY); | |
3425 | $tX = sprintf("%.${POINTS}g", $X); | |
3426 | $tY = sprintf("%.${POINTS}g", $Y); | |
3427 | return $tX eq $tY; | |
3428 | } | |
3429 | ||
68dc0745 | 3430 | The POSIX module (part of the standard perl distribution) implements |
ba7f043c KW |
3431 | C<ceil()>, C<floor()>, and other mathematical and trigonometric functions. |
3432 | The C<L<Math::Complex>> module (part of the standard perl distribution) | |
19799a22 | 3433 | defines mathematical functions that work on both the reals and the |
ba7f043c | 3434 | imaginary numbers. C<Math::Complex> is not as efficient as POSIX, but |
68dc0745 | 3435 | POSIX can't work with complex numbers. |
3436 | ||
3437 | Rounding in financial applications can have serious implications, and | |
3438 | the rounding method used should be specified precisely. In these | |
3439 | cases, it probably pays not to trust whichever system rounding is | |
3440 | being used by Perl, but to instead implement the rounding function you | |
3441 | need yourself. | |
5a964f20 TC |
3442 | |
3443 | =head2 Bigger Numbers | |
d74e8afc | 3444 | X<number, arbitrary precision> |
5a964f20 | 3445 | |
ba7f043c KW |
3446 | The standard C<L<Math::BigInt>>, C<L<Math::BigRat>>, and |
3447 | C<L<Math::BigFloat>> modules, | |
fb1a95c6 | 3448 | along with the C<bignum>, C<bigint>, and C<bigrat> pragmas, provide |
19799a22 | 3449 | variable-precision arithmetic and overloaded operators, although |
46f8a5ea | 3450 | they're currently pretty slow. At the cost of some space and |
19799a22 GS |
3451 | considerable speed, they avoid the normal pitfalls associated with |
3452 | limited-precision representations. | |
5a964f20 | 3453 | |
c543c01b TC |
3454 | use 5.010; |
3455 | use bigint; # easy interface to Math::BigInt | |
3456 | $x = 123456789123456789; | |
3457 | say $x * $x; | |
3458 | +15241578780673678515622620750190521 | |
3459 | ||
3460 | Or with rationals: | |
3461 | ||
db691027 SF |
3462 | use 5.010; |
3463 | use bigrat; | |
3464 | $x = 3/22; | |
3465 | $y = 4/6; | |
3466 | say "x/y is ", $x/$y; | |
3467 | say "x*y is ", $x*$y; | |
3468 | x/y is 9/44 | |
3469 | x*y is 1/11 | |
c543c01b | 3470 | |
ba7f043c KW |
3471 | Several modules let you calculate with unlimited or fixed precision |
3472 | (bound only by memory and CPU time). There | |
46f8a5ea | 3473 | are also some non-standard modules that |
c543c01b | 3474 | provide faster implementations via external C libraries. |
cd5c4fce T |
3475 | |
3476 | Here is a short, but incomplete summary: | |
3477 | ||
950b09ed KW |
3478 | Math::String treat string sequences like numbers |
3479 | Math::FixedPrecision calculate with a fixed precision | |
3480 | Math::Currency for currency calculations | |
3481 | Bit::Vector manipulate bit vectors fast (uses C) | |
3482 | Math::BigIntFast Bit::Vector wrapper for big numbers | |
3483 | Math::Pari provides access to the Pari C library | |
70c45be3 FC |
3484 | Math::Cephes uses the external Cephes C library (no |
3485 | big numbers) | |
950b09ed KW |
3486 | Math::Cephes::Fraction fractions via the Cephes library |
3487 | Math::GMP another one using an external C library | |
70c45be3 FC |
3488 | Math::GMPz an alternative interface to libgmp's big ints |
3489 | Math::GMPq an interface to libgmp's fraction numbers | |
3490 | Math::GMPf an interface to libgmp's floating point numbers | |
cd5c4fce T |
3491 | |
3492 | Choose wisely. | |
16070b82 GS |
3493 | |
3494 | =cut |