Commit | Line | Data |
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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, |
db691027 SF |
9 | independent of the type of the operands. For example C<$x + $y> |
10 | is always a numeric addition, and if C<$x> or C<$y> do not contain | |
ae3f7391 ML |
11 | numbers, an attempt is made to convert them to numbers first. |
12 | ||
13 | This is in contrast to many other dynamic languages, where the | |
46f8a5ea | 14 | operation is determined by the type of the first argument. It also |
ae3f7391 | 15 | means that Perl has two versions of some operators, one for numeric |
db691027 SF |
16 | and one for string comparison. For example C<$x == $y> compares |
17 | two numbers for equality, and C<$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 | |
31 | others. For example, in C<2 + 4 * 5>, the multiplication has higher | |
32 | precedence so C<4 * 5> is evaluated first yielding C<2 + 20 == | |
33 | 22> and not C<6 * 5 == 30>. | |
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 | |
37 | evaluate the left operations first or the right. For example, in C<8 | |
38 | - 4 - 2>, subtraction is left associative so Perl evaluates the | |
39 | expression left to right. C<8 - 4> is evaluated first making the | |
40 | expression C<4 - 2 == 2> and not 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 LW |
73 | |
74 | In the following sections, these operators are covered in precedence order. | |
75 | ||
5a964f20 TC |
76 | Many operators can be overloaded for objects. See L<overload>. |
77 | ||
a0d0e21e | 78 | =head2 Terms and List Operators (Leftward) |
d74e8afc | 79 | X<list operator> X<operator, list> X<term> |
a0d0e21e | 80 | |
62c18ce2 | 81 | A TERM has the highest precedence in Perl. They include variables, |
5f05dabc | 82 | quote and quote-like operators, any expression in parentheses, |
a0d0e21e LW |
83 | and any function whose arguments are parenthesized. Actually, there |
84 | aren't really functions in this sense, just list operators and unary | |
85 | operators behaving as functions because you put parentheses around | |
86 | the arguments. These are all documented in L<perlfunc>. | |
87 | ||
88 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
89 | is followed by a left parenthesis as the next token, the operator and | |
90 | arguments within parentheses are taken to be of highest precedence, | |
91 | just like a normal function call. | |
92 | ||
93 | In the absence of parentheses, the precedence of list operators such as | |
94 | C<print>, C<sort>, or C<chmod> is either very high or very low depending on | |
54310121 | 95 | whether you are looking at the left side or the right side of the operator. |
a0d0e21e LW |
96 | For example, in |
97 | ||
98 | @ary = (1, 3, sort 4, 2); | |
99 | print @ary; # prints 1324 | |
100 | ||
19799a22 GS |
101 | the commas on the right of the sort are evaluated before the sort, |
102 | but the commas on the left are evaluated after. In other words, | |
103 | list operators tend to gobble up all arguments that follow, and | |
a0d0e21e | 104 | then act like a simple TERM with regard to the preceding expression. |
19799a22 | 105 | Be careful with parentheses: |
a0d0e21e LW |
106 | |
107 | # These evaluate exit before doing the print: | |
108 | print($foo, exit); # Obviously not what you want. | |
109 | print $foo, exit; # Nor is this. | |
110 | ||
111 | # These do the print before evaluating exit: | |
112 | (print $foo), exit; # This is what you want. | |
113 | print($foo), exit; # Or this. | |
114 | print ($foo), exit; # Or even this. | |
115 | ||
116 | Also note that | |
117 | ||
118 | print ($foo & 255) + 1, "\n"; | |
119 | ||
d042e63d MS |
120 | probably doesn't do what you expect at first glance. The parentheses |
121 | enclose the argument list for C<print> which is evaluated (printing | |
122 | the result of C<$foo & 255>). Then one is added to the return value | |
123 | of C<print> (usually 1). The result is something like this: | |
124 | ||
125 | 1 + 1, "\n"; # Obviously not what you meant. | |
126 | ||
127 | To do what you meant properly, you must write: | |
128 | ||
129 | print(($foo & 255) + 1, "\n"); | |
130 | ||
131 | See L<Named Unary Operators> for more discussion of this. | |
a0d0e21e LW |
132 | |
133 | Also parsed as terms are the C<do {}> and C<eval {}> constructs, as | |
54310121 | 134 | well as subroutine and method calls, and the anonymous |
a0d0e21e LW |
135 | constructors C<[]> and C<{}>. |
136 | ||
2ae324a7 | 137 | See also L<Quote and Quote-like Operators> toward the end of this section, |
da87341d | 138 | as well as L</"I/O Operators">. |
a0d0e21e LW |
139 | |
140 | =head2 The Arrow Operator | |
d74e8afc | 141 | X<arrow> X<dereference> X<< -> >> |
a0d0e21e | 142 | |
35f2feb0 | 143 | "C<< -> >>" is an infix dereference operator, just as it is in C |
19799a22 GS |
144 | and C++. If the right side is either a C<[...]>, C<{...}>, or a |
145 | C<(...)> subscript, then the left side must be either a hard or | |
146 | symbolic reference to an array, a hash, or a subroutine respectively. | |
147 | (Or technically speaking, a location capable of holding a hard | |
148 | reference, if it's an array or hash reference being used for | |
149 | assignment.) See L<perlreftut> and L<perlref>. | |
a0d0e21e | 150 | |
19799a22 GS |
151 | Otherwise, the right side is a method name or a simple scalar |
152 | variable containing either the method name or a subroutine reference, | |
153 | and the left side must be either an object (a blessed reference) | |
154 | or a class name (that is, a package name). See L<perlobj>. | |
a0d0e21e | 155 | |
821361b6 RS |
156 | The dereferencing cases (as opposed to method-calling cases) are |
157 | somewhat extended by the experimental C<postderef> feature. For the | |
158 | details of that feature, consult L<perlref/Postfix Dereference Syntax>. | |
159 | ||
5f05dabc | 160 | =head2 Auto-increment and Auto-decrement |
d74e8afc | 161 | X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<--> |
a0d0e21e | 162 | |
d042e63d MS |
163 | "++" and "--" work as in C. That is, if placed before a variable, |
164 | they increment or decrement the variable by one before returning the | |
165 | value, and if placed after, increment or decrement after returning the | |
166 | value. | |
167 | ||
168 | $i = 0; $j = 0; | |
169 | print $i++; # prints 0 | |
170 | print ++$j; # prints 1 | |
a0d0e21e | 171 | |
b033823e | 172 | Note that just as in C, Perl doesn't define B<when> the variable is |
46f8a5ea FC |
173 | incremented or decremented. You just know it will be done sometime |
174 | before or after the value is returned. This also means that modifying | |
c543c01b | 175 | a variable twice in the same statement will lead to undefined behavior. |
b033823e A |
176 | Avoid statements like: |
177 | ||
178 | $i = $i ++; | |
179 | print ++ $i + $i ++; | |
180 | ||
181 | Perl will not guarantee what the result of the above statements is. | |
182 | ||
54310121 | 183 | The auto-increment operator has a little extra builtin magic to it. If |
a0d0e21e LW |
184 | you increment a variable that is numeric, or that has ever been used in |
185 | a numeric context, you get a normal increment. If, however, the | |
5f05dabc | 186 | variable has been used in only string contexts since it was set, and |
5a964f20 | 187 | has a value that is not the empty string and matches the pattern |
9c0670e1 | 188 | C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each |
a0d0e21e LW |
189 | character within its range, with carry: |
190 | ||
c543c01b TC |
191 | print ++($foo = "99"); # prints "100" |
192 | print ++($foo = "a0"); # prints "a1" | |
193 | print ++($foo = "Az"); # prints "Ba" | |
194 | print ++($foo = "zz"); # prints "aaa" | |
a0d0e21e | 195 | |
6a61d433 HS |
196 | C<undef> is always treated as numeric, and in particular is changed |
197 | to C<0> before incrementing (so that a post-increment of an undef value | |
198 | will return C<0> rather than C<undef>). | |
199 | ||
5f05dabc | 200 | The auto-decrement operator is not magical. |
a0d0e21e LW |
201 | |
202 | =head2 Exponentiation | |
d74e8afc | 203 | X<**> X<exponentiation> X<power> |
a0d0e21e | 204 | |
19799a22 | 205 | Binary "**" is the exponentiation operator. It binds even more |
46f8a5ea | 206 | tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is |
cb1a09d0 AD |
207 | implemented using C's pow(3) function, which actually works on doubles |
208 | internally.) | |
a0d0e21e | 209 | |
44a465b3 JH |
210 | Note that certain exponentiation expressions are ill-defined: |
211 | these include C<0**0>, C<1**Inf>, and C<Inf**0>. Do not expect | |
212 | any particular results from these special cases, the results | |
213 | are platform-dependent. | |
214 | ||
a0d0e21e | 215 | =head2 Symbolic Unary Operators |
d74e8afc | 216 | X<unary operator> X<operator, unary> |
a0d0e21e | 217 | |
1ca345ed | 218 | Unary "!" performs logical negation, that is, "not". See also C<not> for a lower |
a0d0e21e | 219 | precedence version of this. |
d74e8afc | 220 | X<!> |
a0d0e21e | 221 | |
da2f94c5 FC |
222 | Unary "-" performs arithmetic negation if the operand is numeric, |
223 | including any string that looks like a number. If the operand is | |
224 | an identifier, a string consisting of a minus sign concatenated | |
225 | with the identifier is returned. Otherwise, if the string starts | |
226 | with a plus or minus, a string starting with the opposite sign is | |
227 | returned. One effect of these rules is that -bareword is equivalent | |
8705167b | 228 | to the string "-bareword". If, however, the string begins with a |
353c6505 | 229 | non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert |
46f8a5ea | 230 | the string to a numeric and the arithmetic negation is performed. If the |
06705523 SP |
231 | string cannot be cleanly converted to a numeric, Perl will give the warning |
232 | B<Argument "the string" isn't numeric in negation (-) at ...>. | |
d74e8afc | 233 | X<-> X<negation, arithmetic> |
a0d0e21e | 234 | |
1ca345ed | 235 | Unary "~" performs bitwise negation, that is, 1's complement. For |
972b05a9 JH |
236 | example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and |
237 | L<Bitwise String Operators>.) Note that the width of the result is | |
238 | platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 | |
239 | bits wide on a 64-bit platform, so if you are expecting a certain bit | |
f113cf86 | 240 | width, remember to use the "&" operator to mask off the excess bits. |
d74e8afc | 241 | X<~> X<negation, binary> |
a0d0e21e | 242 | |
f113cf86 TC |
243 | When complementing strings, if all characters have ordinal values under |
244 | 256, then their complements will, also. But if they do not, all | |
245 | characters will be in either 32- or 64-bit complements, depending on your | |
246 | architecture. So for example, C<~"\x{3B1}"> is C<"\x{FFFF_FC4E}"> on | |
247 | 32-bit machines and C<"\x{FFFF_FFFF_FFFF_FC4E}"> on 64-bit machines. | |
248 | ||
fb7054ba FC |
249 | If the experimental "bitwise" feature is enabled via C<use feature |
250 | 'bitwise'>, then unary "~" always treats its argument as a number, and an | |
251 | alternate form of the operator, "~.", always treats its argument as a | |
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 | |
254 | produces a warning unless you use C<no warnings 'experimental::bitwise'>. | |
255 | ||
a0d0e21e LW |
256 | Unary "+" has no effect whatsoever, even on strings. It is useful |
257 | syntactically for separating a function name from a parenthesized expression | |
258 | that would otherwise be interpreted as the complete list of function | |
5ba421f6 | 259 | arguments. (See examples above under L<Terms and List Operators (Leftward)>.) |
d74e8afc | 260 | X<+> |
a0d0e21e | 261 | |
19799a22 GS |
262 | Unary "\" creates a reference to whatever follows it. See L<perlreftut> |
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 | |
c07a80fd | 271 | Binary "=~" binds a scalar expression to a pattern match. Certain operations |
cb1a09d0 AD |
272 | search or modify the string $_ by default. This operator makes that kind |
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 | |
f8bab1e9 | 276 | $_. When used in scalar context, the return value generally indicates the |
8ff32507 FC |
277 | success of the operation. The exceptions are substitution (s///) |
278 | and transliteration (y///) with the C</r> (non-destructive) option, | |
279 | which cause the B<r>eturn value to be the result of the substitution. | |
280 | Behavior in list context depends on the particular operator. | |
000c65fc DG |
281 | See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for |
282 | examples using these operators. | |
f8bab1e9 GS |
283 | |
284 | If the right argument is an expression rather than a search pattern, | |
2c268ad5 | 285 | substitution, or transliteration, it is interpreted as a search pattern at run |
46f8a5ea FC |
286 | time. Note that this means that its |
287 | contents will be interpolated twice, so | |
89d205f2 | 288 | |
1ca345ed | 289 | '\\' =~ q'\\'; |
89d205f2 YO |
290 | |
291 | is not ok, as the regex engine will end up trying to compile the | |
292 | pattern C<\>, which it will consider a syntax error. | |
a0d0e21e LW |
293 | |
294 | Binary "!~" is just like "=~" except the return value is negated in | |
295 | the logical sense. | |
296 | ||
8ff32507 FC |
297 | Binary "!~" with a non-destructive substitution (s///r) or transliteration |
298 | (y///r) is a syntax error. | |
4f4d7508 | 299 | |
a0d0e21e | 300 | =head2 Multiplicative Operators |
d74e8afc | 301 | X<operator, multiplicative> |
a0d0e21e LW |
302 | |
303 | Binary "*" multiplies two numbers. | |
d74e8afc | 304 | X<*> |
a0d0e21e LW |
305 | |
306 | Binary "/" divides two numbers. | |
d74e8afc | 307 | X</> X<slash> |
a0d0e21e | 308 | |
f7918450 KW |
309 | Binary "%" is the modulo operator, which computes the division |
310 | remainder of its first argument with respect to its second argument. | |
311 | Given integer | |
db691027 SF |
312 | operands C<$m> and C<$n>: If C<$n> is positive, then C<$m % $n> is |
313 | C<$m> minus the largest multiple of C<$n> less than or equal to | |
314 | C<$m>. If C<$n> is negative, then C<$m % $n> is C<$m> minus the | |
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 SF |
317 | C<$m> and C<$n> are floating point values and the absolute value of |
318 | C<$n> (that is C<abs($n)>) is less than C<(UV_MAX + 1)>, only | |
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($n)>) is greater than |
4848a83b | 322 | or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder |
db691027 | 323 | C<$r> in the equation 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<$n>. | |
0412d526 | 327 | Note that when C<use integer> is in scope, "%" gives you direct access |
f7918450 | 328 | to the modulo operator as implemented by your C compiler. This |
55d729e4 GS |
329 | operator is not as well defined for negative operands, but it will |
330 | execute faster. | |
f7918450 | 331 | X<%> X<remainder> X<modulo> X<mod> |
55d729e4 | 332 | |
62d10b70 GS |
333 | Binary "x" is the repetition operator. In scalar context or if the left |
334 | operand is not enclosed in parentheses, it returns a string consisting | |
335 | of the left operand repeated the number of times specified by the right | |
336 | operand. In list context, if the left operand is enclosed in | |
3585017f | 337 | parentheses or is a list formed by C<qw/STRING/>, it repeats the list. |
31201a8e KW |
338 | If the right operand is zero or negative (raising a warning on |
339 | negative), it returns an empty string | |
3585017f | 340 | or an empty list, depending on the context. |
d74e8afc | 341 | X<x> |
a0d0e21e LW |
342 | |
343 | print '-' x 80; # print row of dashes | |
344 | ||
345 | print "\t" x ($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 | |
1ca345ed | 354 | Binary C<+> returns the sum of two numbers. |
d74e8afc | 355 | X<+> |
a0d0e21e | 356 | |
1ca345ed | 357 | Binary C<-> returns the difference of two numbers. |
d74e8afc | 358 | X<-> |
a0d0e21e | 359 | |
1ca345ed | 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 | |
1ca345ed | 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 |
982ce180 | 371 | integers. (See also L<Integer Arithmetic>.) |
a0d0e21e | 372 | |
1ca345ed | 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 |
982ce180 | 375 | be integers. (See also L<Integer Arithmetic>.) |
a0d0e21e | 376 | |
1ca345ed TC |
377 | Note that both C<<< << >>> and C<<< >> >>> in Perl are implemented directly using |
378 | C<<< << >>> and C<<< >> >>> in C. If C<use integer> (see L<Integer Arithmetic>) is | |
b16cf6df JH |
379 | in force then signed C integers are used, else unsigned C integers are |
380 | used. Either way, the implementation isn't going to generate results | |
381 | larger than the size of the integer type Perl was built with (32 bits | |
382 | or 64 bits). | |
383 | ||
384 | The result of overflowing the range of the integers is undefined | |
385 | because it is undefined also in C. In other words, using 32-bit | |
386 | integers, C<< 1 << 32 >> is undefined. Shifting by a negative number | |
387 | of bits is also undefined. | |
388 | ||
1ca345ed TC |
389 | If you get tired of being subject to your platform's native integers, |
390 | the C<use bigint> pragma neatly sidesteps the issue altogether: | |
391 | ||
392 | print 20 << 20; # 20971520 | |
393 | print 20 << 40; # 5120 on 32-bit machines, | |
394 | # 21990232555520 on 64-bit machines | |
395 | use bigint; | |
396 | print 20 << 100; # 25353012004564588029934064107520 | |
397 | ||
a0d0e21e | 398 | =head2 Named Unary Operators |
d74e8afc | 399 | X<operator, named unary> |
a0d0e21e LW |
400 | |
401 | The various named unary operators are treated as functions with one | |
568e6d8b | 402 | argument, with optional parentheses. |
a0d0e21e LW |
403 | |
404 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
405 | is followed by a left parenthesis as the next token, the operator and | |
406 | arguments within parentheses are taken to be of highest precedence, | |
3981b0eb | 407 | just like a normal function call. For example, |
1ca345ed | 408 | because named unary operators are higher precedence than C<||>: |
a0d0e21e LW |
409 | |
410 | chdir $foo || die; # (chdir $foo) || die | |
411 | chdir($foo) || die; # (chdir $foo) || die | |
412 | chdir ($foo) || die; # (chdir $foo) || die | |
413 | chdir +($foo) || die; # (chdir $foo) || die | |
414 | ||
3981b0eb | 415 | but, because * is higher precedence than named operators: |
a0d0e21e LW |
416 | |
417 | chdir $foo * 20; # chdir ($foo * 20) | |
418 | chdir($foo) * 20; # (chdir $foo) * 20 | |
419 | chdir ($foo) * 20; # (chdir $foo) * 20 | |
420 | chdir +($foo) * 20; # chdir ($foo * 20) | |
421 | ||
422 | rand 10 * 20; # rand (10 * 20) | |
423 | rand(10) * 20; # (rand 10) * 20 | |
424 | rand (10) * 20; # (rand 10) * 20 | |
425 | rand +(10) * 20; # rand (10 * 20) | |
426 | ||
568e6d8b RGS |
427 | Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are |
428 | treated like named unary operators, but they don't follow this functional | |
429 | parenthesis rule. That means, for example, that C<-f($file).".bak"> is | |
430 | equivalent to C<-f "$file.bak">. | |
d74e8afc | 431 | X<-X> X<filetest> X<operator, filetest> |
568e6d8b | 432 | |
5ba421f6 | 433 | See also L<"Terms and List Operators (Leftward)">. |
a0d0e21e LW |
434 | |
435 | =head2 Relational Operators | |
d74e8afc | 436 | X<relational operator> X<operator, relational> |
a0d0e21e | 437 | |
1ca345ed TC |
438 | Perl operators that return true or false generally return values |
439 | that can be safely used as numbers. For example, the relational | |
440 | operators in this section and the equality operators in the next | |
441 | one return C<1> for true and a special version of the defined empty | |
442 | string, C<"">, which counts as a zero but is exempt from warnings | |
443 | about improper numeric conversions, just as C<"0 but true"> is. | |
444 | ||
35f2feb0 | 445 | Binary "<" returns true if the left argument is numerically less than |
a0d0e21e | 446 | the right argument. |
d74e8afc | 447 | X<< < >> |
a0d0e21e | 448 | |
35f2feb0 | 449 | Binary ">" returns true if the left argument is numerically greater |
a0d0e21e | 450 | than the right argument. |
d74e8afc | 451 | X<< > >> |
a0d0e21e | 452 | |
35f2feb0 | 453 | Binary "<=" returns true if the left argument is numerically less than |
a0d0e21e | 454 | or equal to the right argument. |
d74e8afc | 455 | X<< <= >> |
a0d0e21e | 456 | |
35f2feb0 | 457 | Binary ">=" returns true if the left argument is numerically greater |
a0d0e21e | 458 | than or equal to the right argument. |
d74e8afc | 459 | X<< >= >> |
a0d0e21e LW |
460 | |
461 | Binary "lt" returns true if the left argument is stringwise less than | |
462 | the right argument. | |
d74e8afc | 463 | X<< lt >> |
a0d0e21e LW |
464 | |
465 | Binary "gt" returns true if the left argument is stringwise greater | |
466 | than the right argument. | |
d74e8afc | 467 | X<< gt >> |
a0d0e21e LW |
468 | |
469 | Binary "le" returns true if the left argument is stringwise less than | |
470 | or equal to the right argument. | |
d74e8afc | 471 | X<< le >> |
a0d0e21e LW |
472 | |
473 | Binary "ge" returns true if the left argument is stringwise greater | |
474 | than or equal to the right argument. | |
d74e8afc | 475 | X<< ge >> |
a0d0e21e LW |
476 | |
477 | =head2 Equality Operators | |
d74e8afc | 478 | X<equality> X<equal> X<equals> X<operator, equality> |
a0d0e21e LW |
479 | |
480 | Binary "==" returns true if the left argument is numerically equal to | |
481 | the right argument. | |
d74e8afc | 482 | X<==> |
a0d0e21e LW |
483 | |
484 | Binary "!=" returns true if the left argument is numerically not equal | |
485 | to the right argument. | |
d74e8afc | 486 | X<!=> |
a0d0e21e | 487 | |
35f2feb0 | 488 | Binary "<=>" returns -1, 0, or 1 depending on whether the left |
6ee5d4e7 | 489 | argument is numerically less than, equal to, or greater than the right |
d4ad863d | 490 | argument. If your platform supports NaNs (not-a-numbers) as numeric |
7d3a9d88 | 491 | values, using them with "<=>" returns undef. NaN is not "<", "==", ">", |
46f8a5ea FC |
492 | "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN |
493 | returns true, as does NaN != anything else. If your platform doesn't | |
7d3a9d88 | 494 | support NaNs then NaN is just a string with numeric value 0. |
d74e8afc | 495 | X<< <=> >> X<spaceship> |
7d3a9d88 | 496 | |
db691027 SF |
497 | $ perl -le '$x = "NaN"; print "No NaN support here" if $x == $x' |
498 | $ perl -le '$x = "NaN"; print "NaN support here" if $x != $x' | |
1ca345ed | 499 | |
db691027 | 500 | (Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all |
1ca345ed | 501 | support "NaN".) |
a0d0e21e LW |
502 | |
503 | Binary "eq" returns true if the left argument is stringwise equal to | |
504 | the right argument. | |
d74e8afc | 505 | X<eq> |
a0d0e21e LW |
506 | |
507 | Binary "ne" returns true if the left argument is stringwise not equal | |
508 | to the right argument. | |
d74e8afc | 509 | X<ne> |
a0d0e21e | 510 | |
d4ad863d JH |
511 | Binary "cmp" returns -1, 0, or 1 depending on whether the left |
512 | argument is stringwise less than, equal to, or greater than the right | |
513 | argument. | |
d74e8afc | 514 | X<cmp> |
a0d0e21e | 515 | |
1ca345ed TC |
516 | Binary "~~" does a smartmatch between its arguments. Smart matching |
517 | is described in the next section. | |
0d863452 RH |
518 | X<~~> |
519 | ||
a034a98d | 520 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
66cbab2c KW |
521 | by the current locale if a legacy C<use locale> (but not |
522 | C<use locale ':not_characters'>) is in effect. See | |
1ca345ed TC |
523 | L<perllocale>. Do not mix these with Unicode, only with legacy binary |
524 | encodings. The standard L<Unicode::Collate> and | |
525 | L<Unicode::Collate::Locale> modules offer much more powerful solutions to | |
526 | collation issues. | |
527 | ||
82365311 DG |
528 | For case-insensitive comparisions, look at the L<perlfunc/fc> case-folding |
529 | function, available in Perl v5.16 or later: | |
530 | ||
531 | if ( fc($x) eq fc($y) ) { ... } | |
532 | ||
1ca345ed TC |
533 | =head2 Smartmatch Operator |
534 | ||
535 | First available in Perl 5.10.1 (the 5.10.0 version behaved differently), | |
536 | binary C<~~> does a "smartmatch" between its arguments. This is mostly | |
537 | used implicitly in the C<when> construct described in L<perlsyn>, although | |
538 | not all C<when> clauses call the smartmatch operator. Unique among all of | |
cc08d69f RS |
539 | Perl's operators, the smartmatch operator can recurse. The smartmatch |
540 | operator is L<experimental|perlpolicy/experimental> and its behavior is | |
541 | subject to change. | |
1ca345ed TC |
542 | |
543 | It is also unique in that all other Perl operators impose a context | |
544 | (usually string or numeric context) on their operands, autoconverting | |
545 | those operands to those imposed contexts. In contrast, smartmatch | |
546 | I<infers> contexts from the actual types of its operands and uses that | |
547 | type information to select a suitable comparison mechanism. | |
548 | ||
549 | The C<~~> operator compares its operands "polymorphically", determining how | |
550 | to compare them according to their actual types (numeric, string, array, | |
551 | hash, etc.) Like the equality operators with which it shares the same | |
552 | precedence, C<~~> returns 1 for true and C<""> for false. It is often best | |
553 | read aloud as "in", "inside of", or "is contained in", because the left | |
554 | operand is often looked for I<inside> the right operand. That makes the | |
40bec8a5 | 555 | order of the operands to the smartmatch operand often opposite that of |
1ca345ed TC |
556 | the regular match operator. In other words, the "smaller" thing is usually |
557 | placed in the left operand and the larger one in the right. | |
558 | ||
559 | The behavior of a smartmatch depends on what type of things its arguments | |
560 | are, as determined by the following table. The first row of the table | |
561 | whose types apply determines the smartmatch behavior. Because what | |
562 | actually happens is mostly determined by the type of the second operand, | |
563 | the table is sorted on the right operand instead of on the left. | |
564 | ||
565 | Left Right Description and pseudocode | |
566 | =============================================================== | |
567 | Any undef check whether Any is undefined | |
568 | like: !defined Any | |
569 | ||
570 | Any Object invoke ~~ overloading on Object, or die | |
571 | ||
572 | Right operand is an ARRAY: | |
573 | ||
574 | Left Right Description and pseudocode | |
575 | =============================================================== | |
576 | ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2] | |
577 | like: (ARRAY1[0] ~~ ARRAY2[0]) | |
578 | && (ARRAY1[1] ~~ ARRAY2[1]) && ... | |
579 | HASH ARRAY any ARRAY elements exist as HASH keys | |
580 | like: grep { exists HASH->{$_} } ARRAY | |
581 | Regexp ARRAY any ARRAY elements pattern match Regexp | |
582 | like: grep { /Regexp/ } ARRAY | |
583 | undef ARRAY undef in ARRAY | |
584 | like: grep { !defined } ARRAY | |
40bec8a5 | 585 | Any ARRAY smartmatch each ARRAY element[3] |
1ca345ed TC |
586 | like: grep { Any ~~ $_ } ARRAY |
587 | ||
588 | Right operand is a HASH: | |
589 | ||
590 | Left Right Description and pseudocode | |
591 | =============================================================== | |
592 | HASH1 HASH2 all same keys in both HASHes | |
593 | like: keys HASH1 == | |
594 | grep { exists HASH2->{$_} } keys HASH1 | |
595 | ARRAY HASH any ARRAY elements exist as HASH keys | |
596 | like: grep { exists HASH->{$_} } ARRAY | |
597 | Regexp HASH any HASH keys pattern match Regexp | |
598 | like: grep { /Regexp/ } keys HASH | |
599 | undef HASH always false (undef can't be a key) | |
600 | like: 0 == 1 | |
601 | Any HASH HASH key existence | |
602 | like: exists HASH->{Any} | |
603 | ||
604 | Right operand is CODE: | |
f703fc96 | 605 | |
1ca345ed TC |
606 | Left Right Description and pseudocode |
607 | =============================================================== | |
608 | ARRAY CODE sub returns true on all ARRAY elements[1] | |
609 | like: !grep { !CODE->($_) } ARRAY | |
610 | HASH CODE sub returns true on all HASH keys[1] | |
611 | like: !grep { !CODE->($_) } keys HASH | |
612 | Any CODE sub passed Any returns true | |
613 | like: CODE->(Any) | |
614 | ||
615 | Right operand is a Regexp: | |
616 | ||
617 | Left Right Description and pseudocode | |
618 | =============================================================== | |
619 | ARRAY Regexp any ARRAY elements match Regexp | |
620 | like: grep { /Regexp/ } ARRAY | |
621 | HASH Regexp any HASH keys match Regexp | |
622 | like: grep { /Regexp/ } keys HASH | |
623 | Any Regexp pattern match | |
624 | like: Any =~ /Regexp/ | |
625 | ||
626 | Other: | |
627 | ||
628 | Left Right Description and pseudocode | |
629 | =============================================================== | |
630 | Object Any invoke ~~ overloading on Object, | |
631 | or fall back to... | |
632 | ||
633 | Any Num numeric equality | |
634 | like: Any == Num | |
635 | Num nummy[4] numeric equality | |
636 | like: Num == nummy | |
637 | undef Any check whether undefined | |
638 | like: !defined(Any) | |
639 | Any Any string equality | |
640 | like: Any eq Any | |
641 | ||
642 | ||
643 | Notes: | |
644 | ||
645 | =over | |
646 | ||
647 | =item 1. | |
648 | Empty hashes or arrays match. | |
649 | ||
650 | =item 2. | |
40bec8a5 | 651 | That is, each element smartmatches the element of the same index in the other array.[3] |
1ca345ed TC |
652 | |
653 | =item 3. | |
654 | If a circular reference is found, fall back to referential equality. | |
655 | ||
656 | =item 4. | |
657 | Either an actual number, or a string that looks like one. | |
658 | ||
659 | =back | |
660 | ||
661 | The smartmatch implicitly dereferences any non-blessed hash or array | |
662 | reference, so the C<I<HASH>> and C<I<ARRAY>> entries apply in those cases. | |
663 | For blessed references, the C<I<Object>> entries apply. Smartmatches | |
664 | involving hashes only consider hash keys, never hash values. | |
665 | ||
666 | The "like" code entry is not always an exact rendition. For example, the | |
40bec8a5 | 667 | smartmatch operator short-circuits whenever possible, but C<grep> does |
1ca345ed TC |
668 | not. Also, C<grep> in scalar context returns the number of matches, but |
669 | C<~~> returns only true or false. | |
670 | ||
671 | Unlike most operators, the smartmatch operator knows to treat C<undef> | |
672 | specially: | |
673 | ||
674 | use v5.10.1; | |
675 | @array = (1, 2, 3, undef, 4, 5); | |
676 | say "some elements undefined" if undef ~~ @array; | |
677 | ||
678 | Each operand is considered in a modified scalar context, the modification | |
679 | being that array and hash variables are passed by reference to the | |
680 | operator, which implicitly dereferences them. Both elements | |
681 | of each pair are the same: | |
682 | ||
683 | use v5.10.1; | |
684 | ||
685 | my %hash = (red => 1, blue => 2, green => 3, | |
686 | orange => 4, yellow => 5, purple => 6, | |
687 | black => 7, grey => 8, white => 9); | |
688 | ||
689 | my @array = qw(red blue green); | |
690 | ||
691 | say "some array elements in hash keys" if @array ~~ %hash; | |
692 | say "some array elements in hash keys" if \@array ~~ \%hash; | |
693 | ||
694 | say "red in array" if "red" ~~ @array; | |
695 | say "red in array" if "red" ~~ \@array; | |
696 | ||
697 | say "some keys end in e" if /e$/ ~~ %hash; | |
698 | say "some keys end in e" if /e$/ ~~ \%hash; | |
699 | ||
40bec8a5 TC |
700 | Two arrays smartmatch if each element in the first array smartmatches |
701 | (that is, is "in") the corresponding element in the second array, | |
702 | recursively. | |
1ca345ed TC |
703 | |
704 | use v5.10.1; | |
705 | my @little = qw(red blue green); | |
706 | my @bigger = ("red", "blue", [ "orange", "green" ] ); | |
707 | if (@little ~~ @bigger) { # true! | |
708 | say "little is contained in bigger"; | |
709 | } | |
710 | ||
711 | Because the smartmatch operator recurses on nested arrays, this | |
712 | will still report that "red" is in the array. | |
713 | ||
714 | use v5.10.1; | |
715 | my @array = qw(red blue green); | |
716 | my $nested_array = [[[[[[[ @array ]]]]]]]; | |
717 | say "red in array" if "red" ~~ $nested_array; | |
718 | ||
719 | If two arrays smartmatch each other, then they are deep | |
720 | copies of each others' values, as this example reports: | |
721 | ||
722 | use v5.12.0; | |
723 | my @a = (0, 1, 2, [3, [4, 5], 6], 7); | |
724 | my @b = (0, 1, 2, [3, [4, 5], 6], 7); | |
725 | ||
726 | if (@a ~~ @b && @b ~~ @a) { | |
727 | say "a and b are deep copies of each other"; | |
728 | } | |
729 | elsif (@a ~~ @b) { | |
730 | say "a smartmatches in b"; | |
731 | } | |
732 | elsif (@b ~~ @a) { | |
733 | say "b smartmatches in a"; | |
734 | } | |
735 | else { | |
736 | say "a and b don't smartmatch each other at all"; | |
737 | } | |
738 | ||
739 | ||
740 | If you were to set C<$b[3] = 4>, then instead of reporting that "a and b | |
741 | are deep copies of each other", it now reports that "b smartmatches in a". | |
742 | That because the corresponding position in C<@a> contains an array that | |
743 | (eventually) has a 4 in it. | |
744 | ||
745 | Smartmatching one hash against another reports whether both contain the | |
46f8a5ea | 746 | same keys, no more and no less. This could be used to see whether two |
1ca345ed TC |
747 | records have the same field names, without caring what values those fields |
748 | might have. For example: | |
749 | ||
750 | use v5.10.1; | |
751 | sub make_dogtag { | |
752 | state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 }; | |
753 | ||
754 | my ($class, $init_fields) = @_; | |
755 | ||
756 | die "Must supply (only) name, rank, and serial number" | |
757 | unless $init_fields ~~ $REQUIRED_FIELDS; | |
758 | ||
759 | ... | |
760 | } | |
761 | ||
762 | or, if other non-required fields are allowed, use ARRAY ~~ HASH: | |
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, $init_fields) = @_; | |
769 | ||
770 | die "Must supply (at least) name, rank, and serial number" | |
771 | unless [keys %{$init_fields}] ~~ $REQUIRED_FIELDS; | |
772 | ||
773 | ... | |
774 | } | |
775 | ||
776 | The smartmatch operator is most often used as the implicit operator of a | |
777 | C<when> clause. See the section on "Switch Statements" in L<perlsyn>. | |
778 | ||
779 | =head3 Smartmatching of Objects | |
780 | ||
40bec8a5 TC |
781 | To avoid relying on an object's underlying representation, if the |
782 | smartmatch's right operand is an object that doesn't overload C<~~>, | |
783 | it raises the exception "C<Smartmatching a non-overloaded object | |
46f8a5ea FC |
784 | breaks encapsulation>". That's because one has no business digging |
785 | around to see whether something is "in" an object. These are all | |
40bec8a5 | 786 | illegal on objects without a C<~~> overload: |
1ca345ed TC |
787 | |
788 | %hash ~~ $object | |
789 | 42 ~~ $object | |
790 | "fred" ~~ $object | |
791 | ||
792 | However, you can change the way an object is smartmatched by overloading | |
46f8a5ea FC |
793 | the C<~~> operator. This is allowed to |
794 | extend the usual smartmatch semantics. | |
1ca345ed TC |
795 | For objects that do have an C<~~> overload, see L<overload>. |
796 | ||
797 | Using an object as the left operand is allowed, although not very useful. | |
798 | Smartmatching rules take precedence over overloading, so even if the | |
799 | object in the left operand has smartmatch overloading, this will be | |
800 | ignored. A left operand that is a non-overloaded object falls back on a | |
801 | string or numeric comparison of whatever the C<ref> operator returns. That | |
802 | means that | |
803 | ||
804 | $object ~~ X | |
805 | ||
806 | does I<not> invoke the overload method with C<I<X>> as an argument. | |
807 | Instead the above table is consulted as normal, and based on the type of | |
808 | C<I<X>>, overloading may or may not be invoked. For simple strings or | |
809 | numbers, in becomes equivalent to this: | |
810 | ||
811 | $object ~~ $number ref($object) == $number | |
812 | $object ~~ $string ref($object) eq $string | |
813 | ||
814 | For example, this reports that the handle smells IOish | |
815 | (but please don't really do this!): | |
816 | ||
817 | use IO::Handle; | |
818 | my $fh = IO::Handle->new(); | |
819 | if ($fh ~~ /\bIO\b/) { | |
820 | say "handle smells IOish"; | |
821 | } | |
822 | ||
823 | That's because it treats C<$fh> as a string like | |
824 | C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that. | |
a034a98d | 825 | |
a0d0e21e | 826 | =head2 Bitwise And |
d74e8afc | 827 | X<operator, bitwise, and> X<bitwise and> X<&> |
a0d0e21e | 828 | |
c791a246 KW |
829 | Binary "&" returns its operands ANDed together bit by bit. Although no |
830 | warning is currently raised, the result is not well defined when this operation | |
831 | is performed on operands that aren't either numbers (see | |
832 | L<Integer Arithmetic>) or bitstrings (see L<Bitwise String Operators>). | |
a0d0e21e | 833 | |
2cdc098b | 834 | Note that "&" has lower priority than relational operators, so for example |
1ca345ed | 835 | the parentheses are essential in a test like |
2cdc098b | 836 | |
1ca345ed | 837 | print "Even\n" if ($x & 1) == 0; |
2cdc098b | 838 | |
fb7054ba FC |
839 | If the experimental "bitwise" feature is enabled via C<use feature |
840 | 'bitwise'>, then this operator always treats its operand as numbers. This | |
841 | feature produces a warning unless you use C<no warnings | |
842 | 'experimental::bitwise'>. | |
843 | ||
a0d0e21e | 844 | =head2 Bitwise Or and Exclusive Or |
d74e8afc ITB |
845 | X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor> |
846 | X<bitwise xor> X<^> | |
a0d0e21e | 847 | |
2cdc098b | 848 | Binary "|" returns its operands ORed together bit by bit. |
a0d0e21e | 849 | |
2cdc098b | 850 | Binary "^" returns its operands XORed together bit by bit. |
c791a246 KW |
851 | |
852 | Although no warning is currently raised, the results are not well | |
853 | defined when these operations are performed on operands that aren't either | |
854 | numbers (see L<Integer Arithmetic>) or bitstrings (see L<Bitwise String | |
855 | Operators>). | |
a0d0e21e | 856 | |
2cdc098b MG |
857 | Note that "|" and "^" have lower priority than relational operators, so |
858 | for example the brackets are essential in a test like | |
859 | ||
1ca345ed | 860 | print "false\n" if (8 | 2) != 10; |
2cdc098b | 861 | |
fb7054ba FC |
862 | If the experimental "bitwise" feature is enabled via C<use feature |
863 | 'bitwise'>, then this operator always treats its operand as numbers. This | |
864 | feature produces a warning unless you use C<no warnings | |
865 | 'experimental::bitwise'>. | |
866 | ||
a0d0e21e | 867 | =head2 C-style Logical And |
d74e8afc | 868 | X<&&> X<logical and> X<operator, logical, and> |
a0d0e21e LW |
869 | |
870 | Binary "&&" performs a short-circuit logical AND operation. That is, | |
871 | if the left operand is false, the right operand is not even evaluated. | |
872 | Scalar or list context propagates down to the right operand if it | |
873 | is evaluated. | |
874 | ||
875 | =head2 C-style Logical Or | |
d74e8afc | 876 | X<||> X<operator, logical, or> |
a0d0e21e LW |
877 | |
878 | Binary "||" performs a short-circuit logical OR operation. That is, | |
879 | if the left operand is true, the right operand is not even evaluated. | |
880 | Scalar or list context propagates down to the right operand if it | |
881 | is evaluated. | |
882 | ||
26d9d83b | 883 | =head2 Logical Defined-Or |
d74e8afc | 884 | X<//> X<operator, logical, defined-or> |
c963b151 BD |
885 | |
886 | Although it has no direct equivalent in C, Perl's C<//> operator is related | |
89d205f2 | 887 | to its C-style or. In fact, it's exactly the same as C<||>, except that it |
95bee9ba A |
888 | tests the left hand side's definedness instead of its truth. Thus, |
889 | C<< EXPR1 // EXPR2 >> returns the value of C<< EXPR1 >> if it's defined, | |
46f8a5ea FC |
890 | otherwise, the value of C<< EXPR2 >> is returned. |
891 | (C<< EXPR1 >> is evaluated in scalar context, C<< EXPR2 >> | |
892 | in the context of C<< // >> itself). Usually, | |
95bee9ba A |
893 | this is the same result as C<< defined(EXPR1) ? EXPR1 : EXPR2 >> (except that |
894 | the ternary-operator form can be used as a lvalue, while C<< EXPR1 // EXPR2 >> | |
46f8a5ea | 895 | cannot). This is very useful for |
bdc7923b | 896 | providing default values for variables. If you actually want to test if |
db691027 | 897 | at least one of C<$x> and C<$y> is defined, use C<defined($x // $y)>. |
c963b151 | 898 | |
d042e63d | 899 | The C<||>, C<//> and C<&&> operators return the last value evaluated |
46f8a5ea | 900 | (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably |
d042e63d | 901 | portable way to find out the home directory might be: |
a0d0e21e | 902 | |
c543c01b TC |
903 | $home = $ENV{HOME} |
904 | // $ENV{LOGDIR} | |
905 | // (getpwuid($<))[7] | |
906 | // die "You're homeless!\n"; | |
a0d0e21e | 907 | |
5a964f20 TC |
908 | In particular, this means that you shouldn't use this |
909 | for selecting between two aggregates for assignment: | |
910 | ||
911 | @a = @b || @c; # this is wrong | |
912 | @a = scalar(@b) || @c; # really meant this | |
913 | @a = @b ? @b : @c; # this works fine, though | |
914 | ||
1ca345ed | 915 | As alternatives to C<&&> and C<||> when used for |
f23102e2 RGS |
916 | control flow, Perl provides the C<and> and C<or> operators (see below). |
917 | The short-circuit behavior is identical. The precedence of "and" | |
c963b151 | 918 | and "or" is much lower, however, so that you can safely use them after a |
5a964f20 | 919 | list operator without the need for parentheses: |
a0d0e21e LW |
920 | |
921 | unlink "alpha", "beta", "gamma" | |
922 | or gripe(), next LINE; | |
923 | ||
924 | With the C-style operators that would have been written like this: | |
925 | ||
926 | unlink("alpha", "beta", "gamma") | |
927 | || (gripe(), next LINE); | |
928 | ||
1ca345ed TC |
929 | It would be even more readable to write that this way: |
930 | ||
931 | unless(unlink("alpha", "beta", "gamma")) { | |
932 | gripe(); | |
933 | next LINE; | |
934 | } | |
935 | ||
eeb6a2c9 | 936 | Using "or" for assignment is unlikely to do what you want; see below. |
5a964f20 TC |
937 | |
938 | =head2 Range Operators | |
d74e8afc | 939 | X<operator, range> X<range> X<..> X<...> |
a0d0e21e LW |
940 | |
941 | Binary ".." is the range operator, which is really two different | |
fb53bbb2 | 942 | operators depending on the context. In list context, it returns a |
54ae734e | 943 | list of values counting (up by ones) from the left value to the right |
2cdbc966 | 944 | value. If the left value is greater than the right value then it |
fb53bbb2 | 945 | returns the empty list. The range operator is useful for writing |
46f8a5ea | 946 | C<foreach (1..10)> loops and for doing slice operations on arrays. In |
2cdbc966 JD |
947 | the current implementation, no temporary array is created when the |
948 | range operator is used as the expression in C<foreach> loops, but older | |
949 | versions of Perl might burn a lot of memory when you write something | |
950 | like this: | |
a0d0e21e LW |
951 | |
952 | for (1 .. 1_000_000) { | |
953 | # code | |
54310121 | 954 | } |
a0d0e21e | 955 | |
8f0f46f8 | 956 | The range operator also works on strings, using the magical |
957 | auto-increment, see below. | |
54ae734e | 958 | |
5a964f20 | 959 | In scalar context, ".." returns a boolean value. The operator is |
8f0f46f8 | 960 | bistable, like a flip-flop, and emulates the line-range (comma) |
46f8a5ea | 961 | operator of B<sed>, B<awk>, and various editors. Each ".." operator |
8f0f46f8 | 962 | maintains its own boolean state, even across calls to a subroutine |
46f8a5ea | 963 | that contains it. It is false as long as its left operand is false. |
a0d0e21e LW |
964 | Once the left operand is true, the range operator stays true until the |
965 | right operand is true, I<AFTER> which the range operator becomes false | |
8f0f46f8 | 966 | again. It doesn't become false till the next time the range operator |
967 | is evaluated. It can test the right operand and become false on the | |
968 | same evaluation it became true (as in B<awk>), but it still returns | |
46f8a5ea | 969 | true once. If you don't want it to test the right operand until the |
8f0f46f8 | 970 | next evaluation, as in B<sed>, just use three dots ("...") instead of |
19799a22 GS |
971 | two. In all other regards, "..." behaves just like ".." does. |
972 | ||
973 | The right operand is not evaluated while the operator is in the | |
974 | "false" state, and the left operand is not evaluated while the | |
975 | operator is in the "true" state. The precedence is a little lower | |
976 | than || and &&. The value returned is either the empty string for | |
8f0f46f8 | 977 | false, or a sequence number (beginning with 1) for true. The sequence |
978 | number is reset for each range encountered. The final sequence number | |
979 | in a range has the string "E0" appended to it, which doesn't affect | |
980 | its numeric value, but gives you something to search for if you want | |
981 | to exclude the endpoint. You can exclude the beginning point by | |
982 | waiting for the sequence number to be greater than 1. | |
df5f8116 CW |
983 | |
984 | If either operand of scalar ".." is a constant expression, | |
985 | that operand is considered true if it is equal (C<==>) to the current | |
986 | input line number (the C<$.> variable). | |
987 | ||
988 | To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>, | |
989 | but that is only an issue if you use a floating point expression; when | |
990 | implicitly using C<$.> as described in the previous paragraph, the | |
991 | comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.> | |
992 | is set to a floating point value and you are not reading from a file. | |
993 | Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what | |
994 | you want in scalar context because each of the operands are evaluated | |
995 | using their integer representation. | |
996 | ||
997 | Examples: | |
a0d0e21e LW |
998 | |
999 | As a scalar operator: | |
1000 | ||
df5f8116 | 1001 | if (101 .. 200) { print; } # print 2nd hundred lines, short for |
950b09ed | 1002 | # if ($. == 101 .. $. == 200) { print; } |
9f10b797 RGS |
1003 | |
1004 | next LINE if (1 .. /^$/); # skip header lines, short for | |
f343f960 | 1005 | # next LINE if ($. == 1 .. /^$/); |
9f10b797 RGS |
1006 | # (typically in a loop labeled LINE) |
1007 | ||
1008 | s/^/> / if (/^$/ .. eof()); # quote body | |
a0d0e21e | 1009 | |
5a964f20 TC |
1010 | # parse mail messages |
1011 | while (<>) { | |
1012 | $in_header = 1 .. /^$/; | |
df5f8116 CW |
1013 | $in_body = /^$/ .. eof; |
1014 | if ($in_header) { | |
f343f960 | 1015 | # do something |
df5f8116 | 1016 | } else { # in body |
f343f960 | 1017 | # do something else |
df5f8116 | 1018 | } |
5a964f20 | 1019 | } continue { |
df5f8116 | 1020 | close ARGV if eof; # reset $. each file |
5a964f20 TC |
1021 | } |
1022 | ||
acf31ca5 SF |
1023 | Here's a simple example to illustrate the difference between |
1024 | the two range operators: | |
1025 | ||
1026 | @lines = (" - Foo", | |
1027 | "01 - Bar", | |
1028 | "1 - Baz", | |
1029 | " - Quux"); | |
1030 | ||
9f10b797 RGS |
1031 | foreach (@lines) { |
1032 | if (/0/ .. /1/) { | |
acf31ca5 SF |
1033 | print "$_\n"; |
1034 | } | |
1035 | } | |
1036 | ||
46f8a5ea | 1037 | This program will print only the line containing "Bar". If |
9f10b797 | 1038 | the range operator is changed to C<...>, it will also print the |
acf31ca5 SF |
1039 | "Baz" line. |
1040 | ||
1041 | And now some examples as a list operator: | |
a0d0e21e | 1042 | |
1ca345ed TC |
1043 | for (101 .. 200) { print } # print $_ 100 times |
1044 | @foo = @foo[0 .. $#foo]; # an expensive no-op | |
1045 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items | |
a0d0e21e | 1046 | |
5a964f20 | 1047 | The range operator (in list context) makes use of the magical |
5f05dabc | 1048 | auto-increment algorithm if the operands are strings. You |
a0d0e21e LW |
1049 | can say |
1050 | ||
c543c01b | 1051 | @alphabet = ("A" .. "Z"); |
a0d0e21e | 1052 | |
54ae734e | 1053 | to get all normal letters of the English alphabet, or |
a0d0e21e | 1054 | |
c543c01b | 1055 | $hexdigit = (0 .. 9, "a" .. "f")[$num & 15]; |
a0d0e21e LW |
1056 | |
1057 | to get a hexadecimal digit, or | |
1058 | ||
1ca345ed TC |
1059 | @z2 = ("01" .. "31"); |
1060 | print $z2[$mday]; | |
a0d0e21e | 1061 | |
ea4f5703 YST |
1062 | to get dates with leading zeros. |
1063 | ||
1064 | If the final value specified is not in the sequence that the magical | |
1065 | increment would produce, the sequence goes until the next value would | |
1066 | be longer than the final value specified. | |
1067 | ||
1068 | If the initial value specified isn't part of a magical increment | |
c543c01b | 1069 | sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>), |
ea4f5703 YST |
1070 | only the initial value will be returned. So the following will only |
1071 | return an alpha: | |
1072 | ||
c543c01b | 1073 | use charnames "greek"; |
ea4f5703 YST |
1074 | my @greek_small = ("\N{alpha}" .. "\N{omega}"); |
1075 | ||
c543c01b TC |
1076 | To get the 25 traditional lowercase Greek letters, including both sigmas, |
1077 | you could use this instead: | |
ea4f5703 | 1078 | |
c543c01b | 1079 | use charnames "greek"; |
1ca345ed TC |
1080 | my @greek_small = map { chr } ( ord("\N{alpha}") |
1081 | .. | |
1082 | ord("\N{omega}") | |
1083 | ); | |
c543c01b TC |
1084 | |
1085 | However, because there are I<many> other lowercase Greek characters than | |
1086 | just those, to match lowercase Greek characters in a regular expression, | |
47c56cc8 KW |
1087 | you could use the pattern C</(?:(?=\p{Greek})\p{Lower})+/> (or the |
1088 | L<experimental feature|perlrecharclass/Extended Bracketed Character | |
1089 | Classes> C<S</(?[ \p{Greek} & \p{Lower} ])+/>>). | |
a0d0e21e | 1090 | |
df5f8116 CW |
1091 | Because each operand is evaluated in integer form, C<2.18 .. 3.14> will |
1092 | return two elements in list context. | |
1093 | ||
1094 | @list = (2.18 .. 3.14); # same as @list = (2 .. 3); | |
1095 | ||
a0d0e21e | 1096 | =head2 Conditional Operator |
d74e8afc | 1097 | X<operator, conditional> X<operator, ternary> X<ternary> X<?:> |
a0d0e21e LW |
1098 | |
1099 | Ternary "?:" is the conditional operator, just as in C. It works much | |
1100 | like an if-then-else. If the argument before the ? is true, the | |
1101 | argument before the : is returned, otherwise the argument after the : | |
cb1a09d0 AD |
1102 | is returned. For example: |
1103 | ||
54310121 | 1104 | printf "I have %d dog%s.\n", $n, |
c543c01b | 1105 | ($n == 1) ? "" : "s"; |
cb1a09d0 AD |
1106 | |
1107 | Scalar or list context propagates downward into the 2nd | |
54310121 | 1108 | or 3rd argument, whichever is selected. |
cb1a09d0 | 1109 | |
db691027 SF |
1110 | $x = $ok ? $y : $z; # get a scalar |
1111 | @x = $ok ? @y : @z; # get an array | |
1112 | $x = $ok ? @y : @z; # oops, that's just a count! | |
cb1a09d0 AD |
1113 | |
1114 | The operator may be assigned to if both the 2nd and 3rd arguments are | |
1115 | legal lvalues (meaning that you can assign to them): | |
a0d0e21e | 1116 | |
db691027 | 1117 | ($x_or_y ? $x : $y) = $z; |
a0d0e21e | 1118 | |
5a964f20 TC |
1119 | Because this operator produces an assignable result, using assignments |
1120 | without parentheses will get you in trouble. For example, this: | |
1121 | ||
db691027 | 1122 | $x % 2 ? $x += 10 : $x += 2 |
5a964f20 TC |
1123 | |
1124 | Really means this: | |
1125 | ||
db691027 | 1126 | (($x % 2) ? ($x += 10) : $x) += 2 |
5a964f20 TC |
1127 | |
1128 | Rather than this: | |
1129 | ||
db691027 | 1130 | ($x % 2) ? ($x += 10) : ($x += 2) |
5a964f20 | 1131 | |
19799a22 GS |
1132 | That should probably be written more simply as: |
1133 | ||
db691027 | 1134 | $x += ($x % 2) ? 10 : 2; |
19799a22 | 1135 | |
4633a7c4 | 1136 | =head2 Assignment Operators |
d74e8afc | 1137 | X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=> |
5ac3b81c | 1138 | X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=> |
fb7054ba | 1139 | X<%=> X<^=> X<x=> X<&.=> X<|.=> X<^.=> |
a0d0e21e LW |
1140 | |
1141 | "=" is the ordinary assignment operator. | |
1142 | ||
1143 | Assignment operators work as in C. That is, | |
1144 | ||
db691027 | 1145 | $x += 2; |
a0d0e21e LW |
1146 | |
1147 | is equivalent to | |
1148 | ||
db691027 | 1149 | $x = $x + 2; |
a0d0e21e LW |
1150 | |
1151 | although without duplicating any side effects that dereferencing the lvalue | |
54310121 | 1152 | might trigger, such as from tie(). Other assignment operators work similarly. |
1153 | The following are recognized: | |
a0d0e21e | 1154 | |
fb7054ba FC |
1155 | **= += *= &= &.= <<= &&= |
1156 | -= /= |= |.= >>= ||= | |
1157 | .= %= ^= ^.= //= | |
9f10b797 | 1158 | x= |
a0d0e21e | 1159 | |
19799a22 | 1160 | Although these are grouped by family, they all have the precedence |
82848c10 FC |
1161 | of assignment. These combined assignment operators can only operate on |
1162 | scalars, whereas the ordinary assignment operator can assign to arrays, | |
1163 | hashes, lists and even references. (See L<"Context"|perldata/Context> | |
1164 | and L<perldata/List value constructors>, and L<perlref/Assigning to | |
1165 | References>.) | |
a0d0e21e | 1166 | |
b350dd2f GS |
1167 | Unlike in C, the scalar assignment operator produces a valid lvalue. |
1168 | Modifying an assignment is equivalent to doing the assignment and | |
1169 | then modifying the variable that was assigned to. This is useful | |
1170 | for modifying a copy of something, like this: | |
a0d0e21e | 1171 | |
1ca345ed TC |
1172 | ($tmp = $global) =~ tr/13579/24680/; |
1173 | ||
1174 | Although as of 5.14, that can be also be accomplished this way: | |
1175 | ||
1176 | use v5.14; | |
1177 | $tmp = ($global =~ tr/13579/24680/r); | |
a0d0e21e LW |
1178 | |
1179 | Likewise, | |
1180 | ||
db691027 | 1181 | ($x += 2) *= 3; |
a0d0e21e LW |
1182 | |
1183 | is equivalent to | |
1184 | ||
db691027 SF |
1185 | $x += 2; |
1186 | $x *= 3; | |
a0d0e21e | 1187 | |
b350dd2f GS |
1188 | Similarly, a list assignment in list context produces the list of |
1189 | lvalues assigned to, and a list assignment in scalar context returns | |
1190 | the number of elements produced by the expression on the right hand | |
1191 | side of the assignment. | |
1192 | ||
fb7054ba FC |
1193 | The three dotted bitwise assignment operators (C<&.= |.= ^.=>) are new in |
1194 | Perl 5.22 and experimental. See L</Bitwise String Operators>. | |
1195 | ||
748a9306 | 1196 | =head2 Comma Operator |
d74e8afc | 1197 | X<comma> X<operator, comma> X<,> |
a0d0e21e | 1198 | |
5a964f20 | 1199 | Binary "," is the comma operator. In scalar context it evaluates |
a0d0e21e LW |
1200 | its left argument, throws that value away, then evaluates its right |
1201 | argument and returns that value. This is just like C's comma operator. | |
1202 | ||
5a964f20 | 1203 | In list context, it's just the list argument separator, and inserts |
ed5c6d31 PJ |
1204 | both its arguments into the list. These arguments are also evaluated |
1205 | from left to right. | |
a0d0e21e | 1206 | |
4e1988c6 FC |
1207 | The C<< => >> operator is a synonym for the comma except that it causes a |
1208 | word on its left to be interpreted as a string if it begins with a letter | |
344f2c40 IG |
1209 | or underscore and is composed only of letters, digits and underscores. |
1210 | This includes operands that might otherwise be interpreted as operators, | |
46f8a5ea | 1211 | constants, single number v-strings or function calls. If in doubt about |
c543c01b | 1212 | this behavior, the left operand can be quoted explicitly. |
344f2c40 IG |
1213 | |
1214 | Otherwise, the C<< => >> operator behaves exactly as the comma operator | |
1215 | or list argument separator, according to context. | |
1216 | ||
1217 | For example: | |
a44e5664 MS |
1218 | |
1219 | use constant FOO => "something"; | |
1220 | ||
1221 | my %h = ( FOO => 23 ); | |
1222 | ||
1223 | is equivalent to: | |
1224 | ||
1225 | my %h = ("FOO", 23); | |
1226 | ||
1227 | It is I<NOT>: | |
1228 | ||
1229 | my %h = ("something", 23); | |
1230 | ||
719b43e8 RGS |
1231 | The C<< => >> operator is helpful in documenting the correspondence |
1232 | between keys and values in hashes, and other paired elements in lists. | |
748a9306 | 1233 | |
a12b8f3c FC |
1234 | %hash = ( $key => $value ); |
1235 | login( $username => $password ); | |
a44e5664 | 1236 | |
4e1988c6 FC |
1237 | The special quoting behavior ignores precedence, and hence may apply to |
1238 | I<part> of the left operand: | |
1239 | ||
1240 | print time.shift => "bbb"; | |
1241 | ||
1242 | That example prints something like "1314363215shiftbbb", because the | |
1243 | C<< => >> implicitly quotes the C<shift> immediately on its left, ignoring | |
1244 | the fact that C<time.shift> is the entire left operand. | |
1245 | ||
a0d0e21e | 1246 | =head2 List Operators (Rightward) |
d74e8afc | 1247 | X<operator, list, rightward> X<list operator> |
a0d0e21e | 1248 | |
c543c01b | 1249 | On the right side of a list operator, the comma has very low precedence, |
a0d0e21e LW |
1250 | such that it controls all comma-separated expressions found there. |
1251 | The only operators with lower precedence are the logical operators | |
1252 | "and", "or", and "not", which may be used to evaluate calls to list | |
1ca345ed TC |
1253 | operators without the need for parentheses: |
1254 | ||
1255 | open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n"; | |
1256 | ||
1257 | However, some people find that code harder to read than writing | |
1258 | it with parentheses: | |
1259 | ||
1260 | open(HANDLE, "< :utf8", "filename") or die "Can't open: $!\n"; | |
1261 | ||
1262 | in which case you might as well just use the more customary "||" operator: | |
a0d0e21e | 1263 | |
1ca345ed | 1264 | open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n"; |
a0d0e21e | 1265 | |
5ba421f6 | 1266 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
a0d0e21e LW |
1267 | |
1268 | =head2 Logical Not | |
d74e8afc | 1269 | X<operator, logical, not> X<not> |
a0d0e21e LW |
1270 | |
1271 | Unary "not" returns the logical negation of the expression to its right. | |
1272 | It's the equivalent of "!" except for the very low precedence. | |
1273 | ||
1274 | =head2 Logical And | |
d74e8afc | 1275 | X<operator, logical, and> X<and> |
a0d0e21e LW |
1276 | |
1277 | Binary "and" returns the logical conjunction of the two surrounding | |
c543c01b TC |
1278 | expressions. It's equivalent to C<&&> except for the very low |
1279 | precedence. This means that it short-circuits: the right | |
a0d0e21e LW |
1280 | expression is evaluated only if the left expression is true. |
1281 | ||
59ab9d6e | 1282 | =head2 Logical or and Exclusive Or |
f23102e2 | 1283 | X<operator, logical, or> X<operator, logical, xor> |
59ab9d6e | 1284 | X<operator, logical, exclusive or> |
f23102e2 | 1285 | X<or> X<xor> |
a0d0e21e LW |
1286 | |
1287 | Binary "or" returns the logical disjunction of the two surrounding | |
c543c01b TC |
1288 | expressions. It's equivalent to C<||> except for the very low precedence. |
1289 | This makes it useful for control flow: | |
5a964f20 TC |
1290 | |
1291 | print FH $data or die "Can't write to FH: $!"; | |
1292 | ||
c543c01b TC |
1293 | This means that it short-circuits: the right expression is evaluated |
1294 | only if the left expression is false. Due to its precedence, you must | |
1295 | be careful to avoid using it as replacement for the C<||> operator. | |
1296 | It usually works out better for flow control than in assignments: | |
5a964f20 | 1297 | |
db691027 SF |
1298 | $x = $y or $z; # bug: this is wrong |
1299 | ($x = $y) or $z; # really means this | |
1300 | $x = $y || $z; # better written this way | |
5a964f20 | 1301 | |
19799a22 | 1302 | However, when it's a list-context assignment and you're trying to use |
c543c01b | 1303 | C<||> for control flow, you probably need "or" so that the assignment |
5a964f20 TC |
1304 | takes higher precedence. |
1305 | ||
1306 | @info = stat($file) || die; # oops, scalar sense of stat! | |
1307 | @info = stat($file) or die; # better, now @info gets its due | |
1308 | ||
c963b151 BD |
1309 | Then again, you could always use parentheses. |
1310 | ||
1ca345ed | 1311 | Binary C<xor> returns the exclusive-OR of the two surrounding expressions. |
c543c01b | 1312 | It cannot short-circuit (of course). |
a0d0e21e | 1313 | |
59ab9d6e MB |
1314 | There is no low precedence operator for defined-OR. |
1315 | ||
a0d0e21e | 1316 | =head2 C Operators Missing From Perl |
d74e8afc ITB |
1317 | X<operator, missing from perl> X<&> X<*> |
1318 | X<typecasting> X<(TYPE)> | |
a0d0e21e LW |
1319 | |
1320 | Here is what C has that Perl doesn't: | |
1321 | ||
1322 | =over 8 | |
1323 | ||
1324 | =item unary & | |
1325 | ||
1326 | Address-of operator. (But see the "\" operator for taking a reference.) | |
1327 | ||
1328 | =item unary * | |
1329 | ||
46f8a5ea | 1330 | Dereference-address operator. (Perl's prefix dereferencing |
a0d0e21e LW |
1331 | operators are typed: $, @, %, and &.) |
1332 | ||
1333 | =item (TYPE) | |
1334 | ||
19799a22 | 1335 | Type-casting operator. |
a0d0e21e LW |
1336 | |
1337 | =back | |
1338 | ||
5f05dabc | 1339 | =head2 Quote and Quote-like Operators |
89d205f2 | 1340 | X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m> |
d74e8afc ITB |
1341 | X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>> |
1342 | X<escape sequence> X<escape> | |
1343 | ||
a0d0e21e LW |
1344 | While we usually think of quotes as literal values, in Perl they |
1345 | function as operators, providing various kinds of interpolating and | |
1346 | pattern matching capabilities. Perl provides customary quote characters | |
1347 | for these behaviors, but also provides a way for you to choose your | |
1348 | quote character for any of them. In the following table, a C<{}> represents | |
9f10b797 | 1349 | any pair of delimiters you choose. |
a0d0e21e | 1350 | |
2c268ad5 TP |
1351 | Customary Generic Meaning Interpolates |
1352 | '' q{} Literal no | |
1353 | "" qq{} Literal yes | |
af9219ee | 1354 | `` qx{} Command yes* |
2c268ad5 | 1355 | qw{} Word list no |
af9219ee MG |
1356 | // m{} Pattern match yes* |
1357 | qr{} Pattern yes* | |
1358 | s{}{} Substitution yes* | |
2c268ad5 | 1359 | tr{}{} Transliteration no (but see below) |
c543c01b | 1360 | y{}{} Transliteration no (but see below) |
7e3b091d | 1361 | <<EOF here-doc yes* |
a0d0e21e | 1362 | |
af9219ee MG |
1363 | * unless the delimiter is ''. |
1364 | ||
87275199 | 1365 | Non-bracketing delimiters use the same character fore and aft, but the four |
c543c01b | 1366 | sorts of ASCII brackets (round, angle, square, curly) all nest, which means |
9f10b797 | 1367 | that |
87275199 | 1368 | |
c543c01b | 1369 | q{foo{bar}baz} |
35f2feb0 | 1370 | |
9f10b797 | 1371 | is the same as |
87275199 | 1372 | |
c543c01b | 1373 | 'foo{bar}baz' |
87275199 GS |
1374 | |
1375 | Note, however, that this does not always work for quoting Perl code: | |
1376 | ||
db691027 | 1377 | $s = q{ if($x eq "}") ... }; # WRONG |
87275199 | 1378 | |
46f8a5ea | 1379 | is a syntax error. The C<Text::Balanced> module (standard as of v5.8, |
c543c01b | 1380 | and from CPAN before then) is able to do this properly. |
87275199 | 1381 | |
19799a22 | 1382 | There can be whitespace between the operator and the quoting |
fb73857a | 1383 | characters, except when C<#> is being used as the quoting character. |
19799a22 GS |
1384 | C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the |
1385 | operator C<q> followed by a comment. Its argument will be taken | |
1386 | from the next line. This allows you to write: | |
fb73857a | 1387 | |
1388 | s {foo} # Replace foo | |
1389 | {bar} # with bar. | |
1390 | ||
c543c01b TC |
1391 | The following escape sequences are available in constructs that interpolate, |
1392 | and in transliterations: | |
5691ca5f | 1393 | 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 | 1394 | X<\o{}> |
5691ca5f | 1395 | |
2c4c1ff2 KW |
1396 | Sequence Note Description |
1397 | \t tab (HT, TAB) | |
1398 | \n newline (NL) | |
1399 | \r return (CR) | |
1400 | \f form feed (FF) | |
1401 | \b backspace (BS) | |
1402 | \a alarm (bell) (BEL) | |
1403 | \e escape (ESC) | |
c543c01b | 1404 | \x{263A} [1,8] hex char (example: SMILEY) |
2c4c1ff2 | 1405 | \x1b [2,8] restricted range hex char (example: ESC) |
fb121860 | 1406 | \N{name} [3] named Unicode character or character sequence |
2c4c1ff2 KW |
1407 | \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON) |
1408 | \c[ [5] control char (example: chr(27)) | |
1409 | \o{23072} [6,8] octal char (example: SMILEY) | |
1410 | \033 [7,8] restricted range octal char (example: ESC) | |
5691ca5f KW |
1411 | |
1412 | =over 4 | |
1413 | ||
1414 | =item [1] | |
1415 | ||
2c4c1ff2 KW |
1416 | The result is the character specified by the hexadecimal number between |
1417 | the braces. See L</[8]> below for details on which character. | |
96448467 | 1418 | |
46f8a5ea | 1419 | Only hexadecimal digits are valid between the braces. If an invalid |
96448467 DG |
1420 | character is encountered, a warning will be issued and the invalid |
1421 | character and all subsequent characters (valid or invalid) within the | |
1422 | braces will be discarded. | |
1423 | ||
1424 | If there are no valid digits between the braces, the generated character is | |
1425 | the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>) | |
c543c01b | 1426 | will not cause a warning (currently). |
40687185 KW |
1427 | |
1428 | =item [2] | |
1429 | ||
2c4c1ff2 KW |
1430 | The result is the character specified by the hexadecimal number in the range |
1431 | 0x00 to 0xFF. See L</[8]> below for details on which character. | |
96448467 DG |
1432 | |
1433 | Only hexadecimal digits are valid following C<\x>. When C<\x> is followed | |
2c4c1ff2 | 1434 | by fewer than two valid digits, any valid digits will be zero-padded. This |
c543c01b | 1435 | means that C<\x7> will be interpreted as C<\x07>, and a lone <\x> will be |
2c4c1ff2 | 1436 | interpreted as C<\x00>. Except at the end of a string, having fewer than |
c543c01b | 1437 | two valid digits will result in a warning. Note that although the warning |
96448467 DG |
1438 | says the illegal character is ignored, it is only ignored as part of the |
1439 | escape and will still be used as the subsequent character in the string. | |
1440 | For example: | |
1441 | ||
1442 | Original Result Warns? | |
1443 | "\x7" "\x07" no | |
1444 | "\x" "\x00" no | |
1445 | "\x7q" "\x07q" yes | |
1446 | "\xq" "\x00q" yes | |
1447 | ||
40687185 KW |
1448 | =item [3] |
1449 | ||
fb121860 | 1450 | The result is the Unicode character or character sequence given by I<name>. |
2c4c1ff2 | 1451 | See L<charnames>. |
40687185 KW |
1452 | |
1453 | =item [4] | |
1454 | ||
2c4c1ff2 KW |
1455 | C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code |
1456 | point is I<hexadecimal number>. | |
40687185 KW |
1457 | |
1458 | =item [5] | |
1459 | ||
5691ca5f KW |
1460 | The character following C<\c> is mapped to some other character as shown in the |
1461 | table: | |
1462 | ||
1463 | Sequence Value | |
1464 | \c@ chr(0) | |
1465 | \cA chr(1) | |
1466 | \ca chr(1) | |
1467 | \cB chr(2) | |
1468 | \cb chr(2) | |
1469 | ... | |
1470 | \cZ chr(26) | |
1471 | \cz chr(26) | |
1472 | \c[ chr(27) | |
1473 | \c] chr(29) | |
1474 | \c^ chr(30) | |
c3e9d7a9 KW |
1475 | \c_ chr(31) |
1476 | \c? chr(127) # (on ASCII platforms) | |
5691ca5f | 1477 | |
d813941f | 1478 | In other words, it's the character whose code point has had 64 xor'd with |
c3e9d7a9 KW |
1479 | its uppercase. C<\c?> is DELETE on ASCII platforms because |
1480 | S<C<ord("?") ^ 64>> is 127, and | |
d813941f KW |
1481 | C<\c@> is NULL because the ord of "@" is 64, so xor'ing 64 itself produces 0. |
1482 | ||
5691ca5f KW |
1483 | Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the |
1484 | end of a string, because the backslash would be parsed as escaping the end | |
1485 | quote. | |
1486 | ||
1487 | On ASCII platforms, the resulting characters from the list above are the | |
1488 | complete set of ASCII controls. This isn't the case on EBCDIC platforms; see | |
c3e9d7a9 KW |
1489 | L<perlebcdic/OPERATOR DIFFERENCES> for a full discussion of the |
1490 | differences between these for ASCII versus EBCDIC platforms. | |
5691ca5f | 1491 | |
c3e9d7a9 | 1492 | Use of any other character following the C<"c"> besides those listed above is |
63a63d81 KW |
1493 | discouraged, and as of Perl v5.20, the only characters actually allowed |
1494 | are the printable ASCII ones, minus the left brace C<"{">. What happens | |
1495 | for any of the allowed other characters is that the value is derived by | |
1496 | xor'ing with the seventh bit, which is 64, and a warning raised if | |
1497 | enabled. Using the non-allowed characters generates a fatal error. | |
5691ca5f KW |
1498 | |
1499 | To get platform independent controls, you can use C<\N{...}>. | |
1500 | ||
40687185 KW |
1501 | =item [6] |
1502 | ||
2c4c1ff2 KW |
1503 | The result is the character specified by the octal number between the braces. |
1504 | See L</[8]> below for details on which character. | |
04341565 DG |
1505 | |
1506 | If a character that isn't an octal digit is encountered, a warning is raised, | |
1507 | and the value is based on the octal digits before it, discarding it and all | |
1508 | following characters up to the closing brace. It is a fatal error if there are | |
1509 | no octal digits at all. | |
1510 | ||
1511 | =item [7] | |
1512 | ||
c543c01b | 1513 | The result is the character specified by the three-digit octal number in the |
2c4c1ff2 KW |
1514 | range 000 to 777 (but best to not use above 077, see next paragraph). See |
1515 | L</[8]> below for details on which character. | |
1516 | ||
1517 | Some contexts allow 2 or even 1 digit, but any usage without exactly | |
40687185 | 1518 | three digits, the first being a zero, may give unintended results. (For |
5db3e519 FC |
1519 | example, in a regular expression it may be confused with a backreference; |
1520 | see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may | |
c543c01b | 1521 | use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to |
04341565 DG |
1522 | use this construct only for ordinals C<\077> and below, remembering to pad to |
1523 | the left with zeros to make three digits. For larger ordinals, either use | |
9fef6a0d | 1524 | C<\o{}>, or convert to something else, such as to hex and use C<\x{}> |
04341565 | 1525 | instead. |
40687185 | 1526 | |
2c4c1ff2 KW |
1527 | =item [8] |
1528 | ||
c543c01b | 1529 | Several constructs above specify a character by a number. That number |
2c4c1ff2 | 1530 | gives the character's position in the character set encoding (indexed from 0). |
c543c01b | 1531 | This is called synonymously its ordinal, code position, or code point. Perl |
2c4c1ff2 KW |
1532 | works on platforms that have a native encoding currently of either ASCII/Latin1 |
1533 | or EBCDIC, each of which allow specification of 256 characters. In general, if | |
1534 | the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's | |
1535 | native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets | |
c543c01b | 1536 | it as a Unicode code point and the result is the corresponding Unicode |
2c4c1ff2 KW |
1537 | character. For example C<\x{50}> and C<\o{120}> both are the number 80 in |
1538 | decimal, which is less than 256, so the number is interpreted in the native | |
1539 | character set encoding. In ASCII the character in the 80th position (indexed | |
1540 | from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&". | |
1541 | C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted | |
1542 | as a Unicode code point no matter what the native encoding is. The name of the | |
9fef6a0d | 1543 | character in the 256th position (indexed by 0) in Unicode is |
2c4c1ff2 KW |
1544 | C<LATIN CAPITAL LETTER A WITH MACRON>. |
1545 | ||
9fef6a0d | 1546 | There are a couple of exceptions to the above rule. S<C<\N{U+I<hex number>}>> is |
2c4c1ff2 KW |
1547 | always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even |
1548 | on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the | |
1549 | number is considered to be in that encoding, and is translated from that into | |
1550 | the platform's native encoding if there is a corresponding native character; | |
1551 | otherwise to Unicode. | |
1552 | ||
5691ca5f | 1553 | =back |
4c77eaa2 | 1554 | |
e526e8bb | 1555 | B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for |
8b312c40 KW |
1556 | the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may |
1557 | use C<\ck> or | |
1558 | C<\x0b>. (C<\v> | |
e526e8bb KW |
1559 | does have meaning in regular expression patterns in Perl, see L<perlre>.) |
1560 | ||
1561 | The following escape sequences are available in constructs that interpolate, | |
904501ec | 1562 | but not in transliterations. |
628253b8 | 1563 | X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> X<\F> |
904501ec | 1564 | |
c543c01b TC |
1565 | \l lowercase next character only |
1566 | \u titlecase (not uppercase!) next character only | |
e4d34742 EB |
1567 | \L lowercase all characters till \E or end of string |
1568 | \U uppercase all characters till \E or end of string | |
628253b8 | 1569 | \F foldcase all characters till \E or end of string |
736fe711 KW |
1570 | \Q quote (disable) pattern metacharacters till \E or |
1571 | end of string | |
7e31b643 | 1572 | \E end either case modification or quoted section |
c543c01b TC |
1573 | (whichever was last seen) |
1574 | ||
736fe711 KW |
1575 | See L<perlfunc/quotemeta> for the exact definition of characters that |
1576 | are quoted by C<\Q>. | |
1577 | ||
628253b8 | 1578 | C<\L>, C<\U>, C<\F>, and C<\Q> can stack, in which case you need one |
c543c01b TC |
1579 | C<\E> for each. For example: |
1580 | ||
9fef6a0d KW |
1581 | say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?"; |
1582 | This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it? | |
a0d0e21e | 1583 | |
66cbab2c KW |
1584 | If C<use locale> is in effect (but not C<use locale ':not_characters'>), |
1585 | the case map used by C<\l>, C<\L>, | |
c543c01b | 1586 | C<\u>, and C<\U> is taken from the current locale. See L<perllocale>. |
b6538e4f | 1587 | If Unicode (for example, C<\N{}> or code points of 0x100 or |
c543c01b TC |
1588 | beyond) is being used, the case map used by C<\l>, C<\L>, C<\u>, and |
1589 | C<\U> is as defined by Unicode. That means that case-mapping | |
1590 | a single character can sometimes produce several characters. | |
31f05a37 KW |
1591 | Under C<use locale>, C<\F> produces the same results as C<\L> |
1592 | for all locales but a UTF-8 one, where it instead uses the Unicode | |
1593 | definition. | |
a034a98d | 1594 | |
5a964f20 TC |
1595 | All systems use the virtual C<"\n"> to represent a line terminator, |
1596 | called a "newline". There is no such thing as an unvarying, physical | |
19799a22 | 1597 | newline character. It is only an illusion that the operating system, |
5a964f20 TC |
1598 | device drivers, C libraries, and Perl all conspire to preserve. Not all |
1599 | systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example, | |
c543c01b TC |
1600 | on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed, |
1601 | and on systems without line terminator, | |
1602 | printing C<"\n"> might emit no actual data. In general, use C<"\n"> when | |
5a964f20 TC |
1603 | you mean a "newline" for your system, but use the literal ASCII when you |
1604 | need an exact character. For example, most networking protocols expect | |
2a380090 | 1605 | and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators, |
5a964f20 TC |
1606 | and although they often accept just C<"\012">, they seldom tolerate just |
1607 | C<"\015">. If you get in the habit of using C<"\n"> for networking, | |
1608 | you may be burned some day. | |
d74e8afc ITB |
1609 | X<newline> X<line terminator> X<eol> X<end of line> |
1610 | X<\n> X<\r> X<\r\n> | |
5a964f20 | 1611 | |
904501ec MG |
1612 | For constructs that do interpolate, variables beginning with "C<$>" |
1613 | or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or | |
ad0f383a A |
1614 | C<< $href->{key}[0] >> are also interpolated, as are array and hash slices. |
1615 | But method calls such as C<< $obj->meth >> are not. | |
af9219ee MG |
1616 | |
1617 | Interpolating an array or slice interpolates the elements in order, | |
1618 | separated by the value of C<$">, so is equivalent to interpolating | |
c543c01b TC |
1619 | C<join $", @array>. "Punctuation" arrays such as C<@*> are usually |
1620 | interpolated only if the name is enclosed in braces C<@{*}>, but the | |
1621 | arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces. | |
af9219ee | 1622 | |
bc7b91c6 EB |
1623 | For double-quoted strings, the quoting from C<\Q> is applied after |
1624 | interpolation and escapes are processed. | |
1625 | ||
1626 | "abc\Qfoo\tbar$s\Exyz" | |
1627 | ||
1628 | is equivalent to | |
1629 | ||
1630 | "abc" . quotemeta("foo\tbar$s") . "xyz" | |
1631 | ||
1632 | For the pattern of regex operators (C<qr//>, C<m//> and C<s///>), | |
1633 | the quoting from C<\Q> is applied after interpolation is processed, | |
46f8a5ea FC |
1634 | but before escapes are processed. This allows the pattern to match |
1635 | literally (except for C<$> and C<@>). For example, the following matches: | |
bc7b91c6 EB |
1636 | |
1637 | '\s\t' =~ /\Q\s\t/ | |
1638 | ||
1639 | Because C<$> or C<@> trigger interpolation, you'll need to use something | |
1640 | like C</\Quser\E\@\Qhost/> to match them literally. | |
1d2dff63 | 1641 | |
a0d0e21e LW |
1642 | Patterns are subject to an additional level of interpretation as a |
1643 | regular expression. This is done as a second pass, after variables are | |
1644 | interpolated, so that regular expressions may be incorporated into the | |
1645 | pattern from the variables. If this is not what you want, use C<\Q> to | |
1646 | interpolate a variable literally. | |
1647 | ||
19799a22 GS |
1648 | Apart from the behavior described above, Perl does not expand |
1649 | multiple levels of interpolation. In particular, contrary to the | |
1650 | expectations of shell programmers, back-quotes do I<NOT> interpolate | |
1651 | within double quotes, nor do single quotes impede evaluation of | |
1652 | variables when used within double quotes. | |
a0d0e21e | 1653 | |
5f05dabc | 1654 | =head2 Regexp Quote-Like Operators |
d74e8afc | 1655 | X<operator, regexp> |
cb1a09d0 | 1656 | |
5f05dabc | 1657 | Here are the quote-like operators that apply to pattern |
cb1a09d0 AD |
1658 | matching and related activities. |
1659 | ||
a0d0e21e LW |
1660 | =over 8 |
1661 | ||
33be4c61 | 1662 | =item qr/STRING/msixpodualn |
01c6f5f4 | 1663 | X<qr> X</i> X</m> X</o> X</s> X</x> X</p> |
a0d0e21e | 1664 | |
87e95b7f YO |
1665 | This operator quotes (and possibly compiles) its I<STRING> as a regular |
1666 | expression. I<STRING> is interpolated the same way as I<PATTERN> | |
1667 | in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation | |
1668 | is done. Returns a Perl value which may be used instead of the | |
33be4c61 | 1669 | corresponding C</STRING/msixpodualn> expression. The returned value is a |
46f8a5ea | 1670 | normalized version of the original pattern. It magically differs from |
1c8ee595 CO |
1671 | a string containing the same characters: C<ref(qr/x/)> returns "Regexp"; |
1672 | however, dereferencing it is not well defined (you currently get the | |
1673 | normalized version of the original pattern, but this may change). | |
1674 | ||
a0d0e21e | 1675 | |
87e95b7f YO |
1676 | For example, |
1677 | ||
1678 | $rex = qr/my.STRING/is; | |
85dd5c8b | 1679 | print $rex; # prints (?si-xm:my.STRING) |
87e95b7f YO |
1680 | s/$rex/foo/; |
1681 | ||
1682 | is equivalent to | |
1683 | ||
1684 | s/my.STRING/foo/is; | |
1685 | ||
1686 | The result may be used as a subpattern in a match: | |
1687 | ||
1688 | $re = qr/$pattern/; | |
7188ca43 KW |
1689 | $string =~ /foo${re}bar/; # can be interpolated in other |
1690 | # patterns | |
87e95b7f YO |
1691 | $string =~ $re; # or used standalone |
1692 | $string =~ /$re/; # or this way | |
1693 | ||
f6050459 | 1694 | Since Perl may compile the pattern at the moment of execution of the qr() |
87e95b7f YO |
1695 | operator, using qr() may have speed advantages in some situations, |
1696 | notably if the result of qr() is used standalone: | |
1697 | ||
1698 | sub match { | |
1699 | my $patterns = shift; | |
1700 | my @compiled = map qr/$_/i, @$patterns; | |
1701 | grep { | |
1702 | my $success = 0; | |
1703 | foreach my $pat (@compiled) { | |
1704 | $success = 1, last if /$pat/; | |
1705 | } | |
1706 | $success; | |
1707 | } @_; | |
5a964f20 TC |
1708 | } |
1709 | ||
87e95b7f YO |
1710 | Precompilation of the pattern into an internal representation at |
1711 | the moment of qr() avoids a need to recompile the pattern every | |
1712 | time a match C</$pat/> is attempted. (Perl has many other internal | |
1713 | optimizations, but none would be triggered in the above example if | |
1714 | we did not use qr() operator.) | |
1715 | ||
765fa144 | 1716 | Options (specified by the following modifiers) are: |
87e95b7f YO |
1717 | |
1718 | m Treat string as multiple lines. | |
1719 | s Treat string as single line. (Make . match a newline) | |
1720 | i Do case-insensitive pattern matching. | |
1721 | x Use extended regular expressions. | |
1722 | p When matching preserve a copy of the matched string so | |
7188ca43 KW |
1723 | that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be |
1724 | defined. | |
87e95b7f | 1725 | o Compile pattern only once. |
7188ca43 KW |
1726 | a ASCII-restrict: Use ASCII for \d, \s, \w; specifying two |
1727 | a's further restricts /i matching so that no ASCII | |
48cbae4f SK |
1728 | character will match a non-ASCII one. |
1729 | l Use the locale. | |
1730 | u Use Unicode rules. | |
1731 | d Use Unicode or native charset, as in 5.12 and earlier. | |
33be4c61 | 1732 | n Non-capture mode. Don't let () fill in $1, $2, etc... |
87e95b7f YO |
1733 | |
1734 | If a precompiled pattern is embedded in a larger pattern then the effect | |
33be4c61 | 1735 | of "msixpluadn" will be propagated appropriately. The effect the "o" |
87e95b7f YO |
1736 | modifier has is not propagated, being restricted to those patterns |
1737 | explicitly using it. | |
1738 | ||
b6fa137b | 1739 | The last four modifiers listed above, added in Perl 5.14, |
850b7ec9 | 1740 | control the character set rules, but C</a> is the only one you are likely |
18509dec KW |
1741 | to want to specify explicitly; the other three are selected |
1742 | automatically by various pragmas. | |
da392a17 | 1743 | |
87e95b7f | 1744 | See L<perlre> for additional information on valid syntax for STRING, and |
5e2aa8f5 | 1745 | for a detailed look at the semantics of regular expressions. In |
1ca345ed TC |
1746 | particular, all modifiers except the largely obsolete C</o> are further |
1747 | explained in L<perlre/Modifiers>. C</o> is described in the next section. | |
a0d0e21e | 1748 | |
33be4c61 | 1749 | =item m/PATTERN/msixpodualngc |
89d205f2 YO |
1750 | X<m> X<operator, match> |
1751 | X<regexp, options> X<regexp> X<regex, options> X<regex> | |
01c6f5f4 | 1752 | X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> |
a0d0e21e | 1753 | |
33be4c61 | 1754 | =item /PATTERN/msixpodualngc |
a0d0e21e | 1755 | |
5a964f20 | 1756 | Searches a string for a pattern match, and in scalar context returns |
19799a22 GS |
1757 | true if it succeeds, false if it fails. If no string is specified |
1758 | via the C<=~> or C<!~> operator, the $_ string is searched. (The | |
1759 | string specified with C<=~> need not be an lvalue--it may be the | |
1760 | result of an expression evaluation, but remember the C<=~> binds | |
006671a6 | 1761 | rather tightly.) See also L<perlre>. |
a0d0e21e | 1762 | |
f6050459 | 1763 | Options are as described in C<qr//> above; in addition, the following match |
01c6f5f4 | 1764 | process modifiers are available: |
a0d0e21e | 1765 | |
950b09ed | 1766 | g Match globally, i.e., find all occurrences. |
7188ca43 KW |
1767 | c Do not reset search position on a failed match when /g is |
1768 | in effect. | |
a0d0e21e | 1769 | |
725a61d7 | 1770 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> |
c543c01b | 1771 | you can use any pair of non-whitespace (ASCII) characters |
725a61d7 Z |
1772 | as delimiters. This is particularly useful for matching path names |
1773 | that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is | |
1774 | the delimiter, then a match-only-once rule applies, | |
46f8a5ea | 1775 | described in C<m?PATTERN?> below. If "'" (single quote) is the delimiter, |
6ca3c6c6 | 1776 | no interpolation is performed on the PATTERN. |
ed02a3bf DN |
1777 | When using a character valid in an identifier, whitespace is required |
1778 | after the C<m>. | |
a0d0e21e | 1779 | |
532c9e80 KW |
1780 | PATTERN may contain variables, which will be interpolated |
1781 | every time the pattern search is evaluated, except | |
1f247705 GS |
1782 | for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and |
1783 | C<$|> are not interpolated because they look like end-of-string tests.) | |
532c9e80 KW |
1784 | Perl will not recompile the pattern unless an interpolated |
1785 | variable that it contains changes. You can force Perl to skip the | |
1786 | test and never recompile by adding a C</o> (which stands for "once") | |
1787 | after the trailing delimiter. | |
1788 | Once upon a time, Perl would recompile regular expressions | |
1789 | unnecessarily, and this modifier was useful to tell it not to do so, in the | |
5cc41653 | 1790 | interests of speed. But now, the only reasons to use C</o> are one of: |
532c9e80 KW |
1791 | |
1792 | =over | |
1793 | ||
1794 | =item 1 | |
1795 | ||
1796 | The variables are thousands of characters long and you know that they | |
1797 | don't change, and you need to wring out the last little bit of speed by | |
1798 | having Perl skip testing for that. (There is a maintenance penalty for | |
1799 | doing this, as mentioning C</o> constitutes a promise that you won't | |
18509dec | 1800 | change the variables in the pattern. If you do change them, Perl won't |
532c9e80 KW |
1801 | even notice.) |
1802 | ||
1803 | =item 2 | |
1804 | ||
1805 | you want the pattern to use the initial values of the variables | |
1806 | regardless of whether they change or not. (But there are saner ways | |
1807 | of accomplishing this than using C</o>.) | |
1808 | ||
fa9b8686 DM |
1809 | =item 3 |
1810 | ||
1811 | If the pattern contains embedded code, such as | |
1812 | ||
1813 | use re 'eval'; | |
1814 | $code = 'foo(?{ $x })'; | |
1815 | /$code/ | |
1816 | ||
1817 | then perl will recompile each time, even though the pattern string hasn't | |
1818 | changed, to ensure that the current value of C<$x> is seen each time. | |
1819 | Use C</o> if you want to avoid this. | |
1820 | ||
532c9e80 | 1821 | =back |
a0d0e21e | 1822 | |
18509dec KW |
1823 | The bottom line is that using C</o> is almost never a good idea. |
1824 | ||
e9d89077 DN |
1825 | =item The empty pattern // |
1826 | ||
5a964f20 | 1827 | If the PATTERN evaluates to the empty string, the last |
46f8a5ea | 1828 | I<successfully> matched regular expression is used instead. In this |
c543c01b | 1829 | case, only the C<g> and C<c> flags on the empty pattern are honored; |
46f8a5ea | 1830 | the other flags are taken from the original pattern. If no match has |
d65afb4b HS |
1831 | previously succeeded, this will (silently) act instead as a genuine |
1832 | empty pattern (which will always match). | |
a0d0e21e | 1833 | |
89d205f2 YO |
1834 | Note that it's possible to confuse Perl into thinking C<//> (the empty |
1835 | regex) is really C<//> (the defined-or operator). Perl is usually pretty | |
1836 | good about this, but some pathological cases might trigger this, such as | |
db691027 | 1837 | C<$x///> (is that C<($x) / (//)> or C<$x // />?) and C<print $fh //> |
89d205f2 YO |
1838 | (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl |
1839 | will assume you meant defined-or. If you meant the empty regex, just | |
1840 | use parentheses or spaces to disambiguate, or even prefix the empty | |
c963b151 BD |
1841 | regex with an C<m> (so C<//> becomes C<m//>). |
1842 | ||
e9d89077 DN |
1843 | =item Matching in list context |
1844 | ||
19799a22 | 1845 | If the C</g> option is not used, C<m//> in list context returns a |
a0d0e21e | 1846 | list consisting of the subexpressions matched by the parentheses in the |
3ff8ecf9 BF |
1847 | pattern, that is, (C<$1>, C<$2>, C<$3>...) (Note that here C<$1> etc. are |
1848 | also set). When there are no parentheses in the pattern, the return | |
1849 | value is the list C<(1)> for success. | |
1850 | With or without parentheses, an empty list is returned upon failure. | |
a0d0e21e LW |
1851 | |
1852 | Examples: | |
1853 | ||
7188ca43 KW |
1854 | open(TTY, "+</dev/tty") |
1855 | || die "can't access /dev/tty: $!"; | |
c543c01b | 1856 | |
7188ca43 | 1857 | <TTY> =~ /^y/i && foo(); # do foo if desired |
a0d0e21e | 1858 | |
7188ca43 | 1859 | if (/Version: *([0-9.]*)/) { $version = $1; } |
a0d0e21e | 1860 | |
7188ca43 | 1861 | next if m#^/usr/spool/uucp#; |
a0d0e21e | 1862 | |
7188ca43 KW |
1863 | # poor man's grep |
1864 | $arg = shift; | |
1865 | while (<>) { | |
1866 | print if /$arg/o; # compile only once (no longer needed!) | |
1867 | } | |
a0d0e21e | 1868 | |
7188ca43 | 1869 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) |
a0d0e21e LW |
1870 | |
1871 | This last example splits $foo into the first two words and the | |
5f05dabc | 1872 | remainder of the line, and assigns those three fields to $F1, $F2, and |
c543c01b TC |
1873 | $Etc. The conditional is true if any variables were assigned; that is, |
1874 | if the pattern matched. | |
a0d0e21e | 1875 | |
19799a22 | 1876 | The C</g> modifier specifies global pattern matching--that is, |
46f8a5ea FC |
1877 | matching as many times as possible within the string. How it behaves |
1878 | depends on the context. In list context, it returns a list of the | |
19799a22 | 1879 | substrings matched by any capturing parentheses in the regular |
46f8a5ea | 1880 | expression. If there are no parentheses, it returns a list of all |
19799a22 GS |
1881 | the matched strings, as if there were parentheses around the whole |
1882 | pattern. | |
a0d0e21e | 1883 | |
7e86de3e | 1884 | In scalar context, each execution of C<m//g> finds the next match, |
19799a22 | 1885 | returning true if it matches, and false if there is no further match. |
3dd93342 | 1886 | The position after the last match can be read or set using the C<pos()> |
46f8a5ea | 1887 | function; see L<perlfunc/pos>. A failed match normally resets the |
7e86de3e | 1888 | search position to the beginning of the string, but you can avoid that |
46f8a5ea | 1889 | by adding the C</c> modifier (for example, C<m//gc>). Modifying the target |
7e86de3e | 1890 | string also resets the search position. |
c90c0ff4 | 1891 | |
e9d89077 DN |
1892 | =item \G assertion |
1893 | ||
c90c0ff4 | 1894 | You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a |
3dd93342 | 1895 | zero-width assertion that matches the exact position where the |
46f8a5ea | 1896 | previous C<m//g>, if any, left off. Without the C</g> modifier, the |
3dd93342 | 1897 | C<\G> assertion still anchors at C<pos()> as it was at the start of |
1898 | the operation (see L<perlfunc/pos>), but the match is of course only | |
46f8a5ea | 1899 | attempted once. Using C<\G> without C</g> on a target string that has |
3dd93342 | 1900 | not previously had a C</g> match applied to it is the same as using |
1901 | the C<\A> assertion to match the beginning of the string. Note also | |
1902 | that, currently, C<\G> is only properly supported when anchored at the | |
1903 | very beginning of the pattern. | |
c90c0ff4 | 1904 | |
1905 | Examples: | |
a0d0e21e LW |
1906 | |
1907 | # list context | |
1908 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); | |
1909 | ||
1910 | # scalar context | |
c543c01b TC |
1911 | local $/ = ""; |
1912 | while ($paragraph = <>) { | |
1913 | while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) { | |
19799a22 | 1914 | $sentences++; |
a0d0e21e LW |
1915 | } |
1916 | } | |
c543c01b TC |
1917 | say $sentences; |
1918 | ||
1919 | Here's another way to check for sentences in a paragraph: | |
1920 | ||
7188ca43 KW |
1921 | my $sentence_rx = qr{ |
1922 | (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or | |
1923 | # whitespace | |
1924 | \p{Lu} # capital letter | |
1925 | .*? # a bunch of anything | |
1926 | (?<= \S ) # that ends in non- | |
1927 | # whitespace | |
1928 | (?<! \b [DMS]r ) # but isn't a common abbr. | |
1929 | (?<! \b Mrs ) | |
1930 | (?<! \b Sra ) | |
1931 | (?<! \b St ) | |
1932 | [.?!] # followed by a sentence | |
1933 | # ender | |
1934 | (?= $ | \s ) # in front of end-of-string | |
1935 | # or whitespace | |
1936 | }sx; | |
1937 | local $/ = ""; | |
1938 | while (my $paragraph = <>) { | |
1939 | say "NEW PARAGRAPH"; | |
1940 | my $count = 0; | |
1941 | while ($paragraph =~ /($sentence_rx)/g) { | |
1942 | printf "\tgot sentence %d: <%s>\n", ++$count, $1; | |
c543c01b | 1943 | } |
7188ca43 | 1944 | } |
c543c01b TC |
1945 | |
1946 | Here's how to use C<m//gc> with C<\G>: | |
a0d0e21e | 1947 | |
137443ea | 1948 | $_ = "ppooqppqq"; |
44a8e56a | 1949 | while ($i++ < 2) { |
1950 | print "1: '"; | |
c90c0ff4 | 1951 | print $1 while /(o)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1952 | print "2: '"; |
c90c0ff4 | 1953 | print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1954 | print "3: '"; |
c90c0ff4 | 1955 | print $1 while /(p)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1956 | } |
5d43e42d | 1957 | print "Final: '$1', pos=",pos,"\n" if /\G(.)/; |
44a8e56a | 1958 | |
1959 | The last example should print: | |
1960 | ||
1961 | 1: 'oo', pos=4 | |
137443ea | 1962 | 2: 'q', pos=5 |
44a8e56a | 1963 | 3: 'pp', pos=7 |
1964 | 1: '', pos=7 | |
137443ea | 1965 | 2: 'q', pos=8 |
1966 | 3: '', pos=8 | |
5d43e42d DC |
1967 | Final: 'q', pos=8 |
1968 | ||
1969 | Notice that the final match matched C<q> instead of C<p>, which a match | |
46f8a5ea FC |
1970 | without the C<\G> anchor would have done. Also note that the final match |
1971 | did not update C<pos>. C<pos> is only updated on a C</g> match. If the | |
c543c01b TC |
1972 | final match did indeed match C<p>, it's a good bet that you're running a |
1973 | very old (pre-5.6.0) version of Perl. | |
44a8e56a | 1974 | |
c90c0ff4 | 1975 | A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can |
e7ea3e70 | 1976 | combine several regexps like this to process a string part-by-part, |
c90c0ff4 | 1977 | doing different actions depending on which regexp matched. Each |
1978 | regexp tries to match where the previous one leaves off. | |
e7ea3e70 | 1979 | |
3fe9a6f1 | 1980 | $_ = <<'EOL'; |
7188ca43 KW |
1981 | $url = URI::URL->new( "http://example.com/" ); |
1982 | die if $url eq "xXx"; | |
3fe9a6f1 | 1983 | EOL |
c543c01b TC |
1984 | |
1985 | LOOP: { | |
950b09ed | 1986 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; |
7188ca43 KW |
1987 | print(" lowercase"), redo LOOP |
1988 | if /\G\p{Ll}+\b[,.;]?\s*/gc; | |
1989 | print(" UPPERCASE"), redo LOOP | |
1990 | if /\G\p{Lu}+\b[,.;]?\s*/gc; | |
1991 | print(" Capitalized"), redo LOOP | |
1992 | if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc; | |
c543c01b | 1993 | print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc; |
7188ca43 KW |
1994 | print(" alphanumeric"), redo LOOP |
1995 | if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc; | |
c543c01b | 1996 | print(" line-noise"), redo LOOP if /\G\W+/gc; |
950b09ed | 1997 | print ". That's all!\n"; |
c543c01b | 1998 | } |
e7ea3e70 IZ |
1999 | |
2000 | Here is the output (split into several lines): | |
2001 | ||
7188ca43 KW |
2002 | line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE |
2003 | line-noise lowercase line-noise lowercase line-noise lowercase | |
2004 | lowercase line-noise lowercase lowercase line-noise lowercase | |
2005 | lowercase line-noise MiXeD line-noise. That's all! | |
44a8e56a | 2006 | |
33be4c61 | 2007 | =item m?PATTERN?msixpodualngc |
725a61d7 | 2008 | X<?> X<operator, match-once> |
87e95b7f | 2009 | |
33be4c61 | 2010 | =item ?PATTERN?msixpodualngc |
55d389e7 | 2011 | |
725a61d7 Z |
2012 | This is just like the C<m/PATTERN/> search, except that it matches |
2013 | only once between calls to the reset() operator. This is a useful | |
87e95b7f | 2014 | optimization when you want to see only the first occurrence of |
ceb131e8 | 2015 | something in each file of a set of files, for instance. Only C<m??> |
87e95b7f YO |
2016 | patterns local to the current package are reset. |
2017 | ||
2018 | while (<>) { | |
ceb131e8 | 2019 | if (m?^$?) { |
87e95b7f YO |
2020 | # blank line between header and body |
2021 | } | |
2022 | } continue { | |
725a61d7 | 2023 | reset if eof; # clear m?? status for next file |
87e95b7f YO |
2024 | } |
2025 | ||
c543c01b TC |
2026 | Another example switched the first "latin1" encoding it finds |
2027 | to "utf8" in a pod file: | |
2028 | ||
2029 | s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x; | |
2030 | ||
2031 | The match-once behavior is controlled by the match delimiter being | |
4932eeca | 2032 | C<?>; with any other delimiter this is the normal C<m//> operator. |
725a61d7 | 2033 | |
0381ecf1 MH |
2034 | In the past, the leading C<m> in C<m?PATTERN?> was optional, but omitting it |
2035 | would produce a deprecation warning. As of v5.22.0, omitting it produces a | |
2036 | syntax error. If you encounter this construct in older code, you can just add | |
2037 | C<m>. | |
87e95b7f | 2038 | |
33be4c61 | 2039 | =item s/PATTERN/REPLACEMENT/msixpodualngcer |
87e95b7f | 2040 | X<substitute> X<substitution> X<replace> X<regexp, replace> |
4f4d7508 | 2041 | 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 |
2042 | |
2043 | Searches a string for a pattern, and if found, replaces that pattern | |
2044 | with the replacement text and returns the number of substitutions | |
2045 | made. Otherwise it returns false (specifically, the empty string). | |
2046 | ||
c543c01b | 2047 | If the C</r> (non-destructive) option is used then it runs the |
679563bb KW |
2048 | substitution on a copy of the string and instead of returning the |
2049 | number of substitutions, it returns the copy whether or not a | |
c543c01b TC |
2050 | substitution occurred. The original string is never changed when |
2051 | C</r> is used. The copy will always be a plain string, even if the | |
2052 | input is an object or a tied variable. | |
4f4d7508 | 2053 | |
87e95b7f | 2054 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> |
c543c01b TC |
2055 | variable is searched and modified. Unless the C</r> option is used, |
2056 | the string specified must be a scalar variable, an array element, a | |
2057 | hash element, or an assignment to one of those; that is, some sort of | |
2058 | scalar lvalue. | |
87e95b7f YO |
2059 | |
2060 | If the delimiter chosen is a single quote, no interpolation is | |
2061 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the | |
2062 | PATTERN contains a $ that looks like a variable rather than an | |
2063 | end-of-string test, the variable will be interpolated into the pattern | |
2064 | at run-time. If you want the pattern compiled only once the first time | |
2065 | the variable is interpolated, use the C</o> option. If the pattern | |
2066 | evaluates to the empty string, the last successfully executed regular | |
2067 | expression is used instead. See L<perlre> for further explanation on these. | |
87e95b7f YO |
2068 | |
2069 | Options are as with m// with the addition of the following replacement | |
2070 | specific options: | |
2071 | ||
2072 | e Evaluate the right side as an expression. | |
7188ca43 KW |
2073 | ee Evaluate the right side as a string then eval the |
2074 | result. | |
2075 | r Return substitution and leave the original string | |
2076 | untouched. | |
87e95b7f | 2077 | |
ed02a3bf DN |
2078 | Any non-whitespace delimiter may replace the slashes. Add space after |
2079 | the C<s> when using a character allowed in identifiers. If single quotes | |
2080 | are used, no interpretation is done on the replacement string (the C</e> | |
3ff8ecf9 | 2081 | modifier overrides this, however). Note that Perl treats backticks |
ed02a3bf DN |
2082 | as normal delimiters; the replacement text is not evaluated as a command. |
2083 | If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has | |
1ca345ed | 2084 | its own pair of quotes, which may or may not be bracketing quotes, for example, |
87e95b7f YO |
2085 | C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the |
2086 | replacement portion to be treated as a full-fledged Perl expression | |
2087 | and evaluated right then and there. It is, however, syntax checked at | |
46f8a5ea | 2088 | compile-time. A second C<e> modifier will cause the replacement portion |
87e95b7f YO |
2089 | to be C<eval>ed before being run as a Perl expression. |
2090 | ||
2091 | Examples: | |
2092 | ||
7188ca43 | 2093 | s/\bgreen\b/mauve/g; # don't change wintergreen |
87e95b7f YO |
2094 | |
2095 | $path =~ s|/usr/bin|/usr/local/bin|; | |
2096 | ||
2097 | s/Login: $foo/Login: $bar/; # run-time pattern | |
2098 | ||
7188ca43 KW |
2099 | ($foo = $bar) =~ s/this/that/; # copy first, then |
2100 | # change | |
2101 | ($foo = "$bar") =~ s/this/that/; # convert to string, | |
2102 | # copy, then change | |
4f4d7508 DC |
2103 | $foo = $bar =~ s/this/that/r; # Same as above using /r |
2104 | $foo = $bar =~ s/this/that/r | |
7188ca43 KW |
2105 | =~ s/that/the other/r; # Chained substitutes |
2106 | # using /r | |
2107 | @foo = map { s/this/that/r } @bar # /r is very useful in | |
2108 | # maps | |
87e95b7f | 2109 | |
7188ca43 | 2110 | $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-cnt |
87e95b7f YO |
2111 | |
2112 | $_ = 'abc123xyz'; | |
2113 | s/\d+/$&*2/e; # yields 'abc246xyz' | |
2114 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' | |
2115 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' | |
2116 | ||
2117 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e | |
2118 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e | |
2119 | s/^=(\w+)/pod($1)/ge; # use function call | |
2120 | ||
4f4d7508 | 2121 | $_ = 'abc123xyz'; |
db691027 | 2122 | $x = s/abc/def/r; # $x is 'def123xyz' and |
4f4d7508 DC |
2123 | # $_ remains 'abc123xyz'. |
2124 | ||
87e95b7f YO |
2125 | # expand variables in $_, but dynamics only, using |
2126 | # symbolic dereferencing | |
2127 | s/\$(\w+)/${$1}/g; | |
2128 | ||
2129 | # Add one to the value of any numbers in the string | |
2130 | s/(\d+)/1 + $1/eg; | |
2131 | ||
c543c01b TC |
2132 | # Titlecase words in the last 30 characters only |
2133 | substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g; | |
2134 | ||
87e95b7f YO |
2135 | # This will expand any embedded scalar variable |
2136 | # (including lexicals) in $_ : First $1 is interpolated | |
2137 | # to the variable name, and then evaluated | |
2138 | s/(\$\w+)/$1/eeg; | |
2139 | ||
2140 | # Delete (most) C comments. | |
2141 | $program =~ s { | |
2142 | /\* # Match the opening delimiter. | |
2143 | .*? # Match a minimal number of characters. | |
2144 | \*/ # Match the closing delimiter. | |
2145 | } []gsx; | |
2146 | ||
7188ca43 KW |
2147 | s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, |
2148 | # expensively | |
87e95b7f | 2149 | |
7188ca43 KW |
2150 | for ($variable) { # trim whitespace in $variable, |
2151 | # cheap | |
87e95b7f YO |
2152 | s/^\s+//; |
2153 | s/\s+$//; | |
2154 | } | |
2155 | ||
2156 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields | |
2157 | ||
2158 | Note the use of $ instead of \ in the last example. Unlike | |
2159 | B<sed>, we use the \<I<digit>> form in only the left hand side. | |
2160 | Anywhere else it's $<I<digit>>. | |
2161 | ||
2162 | Occasionally, you can't use just a C</g> to get all the changes | |
2163 | to occur that you might want. Here are two common cases: | |
2164 | ||
2165 | # put commas in the right places in an integer | |
2166 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; | |
2167 | ||
2168 | # expand tabs to 8-column spacing | |
2169 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; | |
2170 | ||
2171 | =back | |
2172 | ||
2173 | =head2 Quote-Like Operators | |
2174 | X<operator, quote-like> | |
2175 | ||
01c6f5f4 RGS |
2176 | =over 4 |
2177 | ||
a0d0e21e | 2178 | =item q/STRING/ |
5d44bfff | 2179 | X<q> X<quote, single> X<'> X<''> |
a0d0e21e | 2180 | |
5d44bfff | 2181 | =item 'STRING' |
a0d0e21e | 2182 | |
19799a22 | 2183 | A single-quoted, literal string. A backslash represents a backslash |
68dc0745 | 2184 | unless followed by the delimiter or another backslash, in which case |
2185 | the delimiter or backslash is interpolated. | |
a0d0e21e LW |
2186 | |
2187 | $foo = q!I said, "You said, 'She said it.'"!; | |
2188 | $bar = q('This is it.'); | |
68dc0745 | 2189 | $baz = '\n'; # a two-character string |
a0d0e21e LW |
2190 | |
2191 | =item qq/STRING/ | |
d74e8afc | 2192 | X<qq> X<quote, double> X<"> X<""> |
a0d0e21e LW |
2193 | |
2194 | =item "STRING" | |
2195 | ||
2196 | A double-quoted, interpolated string. | |
2197 | ||
2198 | $_ .= qq | |
2199 | (*** The previous line contains the naughty word "$1".\n) | |
19799a22 | 2200 | if /\b(tcl|java|python)\b/i; # :-) |
68dc0745 | 2201 | $baz = "\n"; # a one-character string |
a0d0e21e LW |
2202 | |
2203 | =item qx/STRING/ | |
d74e8afc | 2204 | X<qx> X<`> X<``> X<backtick> |
a0d0e21e LW |
2205 | |
2206 | =item `STRING` | |
2207 | ||
43dd4d21 | 2208 | A string which is (possibly) interpolated and then executed as a |
f703fc96 | 2209 | system command with F</bin/sh> or its equivalent. Shell wildcards, |
43dd4d21 JH |
2210 | pipes, and redirections will be honored. The collected standard |
2211 | output of the command is returned; standard error is unaffected. In | |
2212 | scalar context, it comes back as a single (potentially multi-line) | |
2213 | string, or undef if the command failed. In list context, returns a | |
2214 | list of lines (however you've defined lines with $/ or | |
2215 | $INPUT_RECORD_SEPARATOR), or an empty list if the command failed. | |
5a964f20 TC |
2216 | |
2217 | Because backticks do not affect standard error, use shell file descriptor | |
2218 | syntax (assuming the shell supports this) if you care to address this. | |
2219 | To capture a command's STDERR and STDOUT together: | |
a0d0e21e | 2220 | |
5a964f20 TC |
2221 | $output = `cmd 2>&1`; |
2222 | ||
2223 | To capture a command's STDOUT but discard its STDERR: | |
2224 | ||
2225 | $output = `cmd 2>/dev/null`; | |
2226 | ||
2227 | To capture a command's STDERR but discard its STDOUT (ordering is | |
2228 | important here): | |
2229 | ||
2230 | $output = `cmd 2>&1 1>/dev/null`; | |
2231 | ||
2232 | To exchange a command's STDOUT and STDERR in order to capture the STDERR | |
2233 | but leave its STDOUT to come out the old STDERR: | |
2234 | ||
2235 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; | |
2236 | ||
2237 | To read both a command's STDOUT and its STDERR separately, it's easiest | |
2359510d SD |
2238 | to redirect them separately to files, and then read from those files |
2239 | when the program is done: | |
5a964f20 | 2240 | |
2359510d | 2241 | system("program args 1>program.stdout 2>program.stderr"); |
5a964f20 | 2242 | |
30398227 SP |
2243 | The STDIN filehandle used by the command is inherited from Perl's STDIN. |
2244 | For example: | |
2245 | ||
c543c01b TC |
2246 | open(SPLAT, "stuff") || die "can't open stuff: $!"; |
2247 | open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!"; | |
40bbb707 | 2248 | print STDOUT `sort`; |
30398227 | 2249 | |
40bbb707 | 2250 | will print the sorted contents of the file named F<"stuff">. |
30398227 | 2251 | |
5a964f20 TC |
2252 | Using single-quote as a delimiter protects the command from Perl's |
2253 | double-quote interpolation, passing it on to the shell instead: | |
2254 | ||
2255 | $perl_info = qx(ps $$); # that's Perl's $$ | |
2256 | $shell_info = qx'ps $$'; # that's the new shell's $$ | |
2257 | ||
19799a22 | 2258 | How that string gets evaluated is entirely subject to the command |
5a964f20 TC |
2259 | interpreter on your system. On most platforms, you will have to protect |
2260 | shell metacharacters if you want them treated literally. This is in | |
2261 | practice difficult to do, as it's unclear how to escape which characters. | |
2262 | See L<perlsec> for a clean and safe example of a manual fork() and exec() | |
2263 | to emulate backticks safely. | |
a0d0e21e | 2264 | |
bb32b41a GS |
2265 | On some platforms (notably DOS-like ones), the shell may not be |
2266 | capable of dealing with multiline commands, so putting newlines in | |
2267 | the string may not get you what you want. You may be able to evaluate | |
2268 | multiple commands in a single line by separating them with the command | |
1ca345ed TC |
2269 | separator character, if your shell supports that (for example, C<;> on |
2270 | many Unix shells and C<&> on the Windows NT C<cmd> shell). | |
bb32b41a | 2271 | |
3ff8ecf9 | 2272 | Perl will attempt to flush all files opened for |
0f897271 GS |
2273 | output before starting the child process, but this may not be supported |
2274 | on some platforms (see L<perlport>). To be safe, you may need to set | |
2275 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of | |
2276 | C<IO::Handle> on any open handles. | |
2277 | ||
bb32b41a GS |
2278 | Beware that some command shells may place restrictions on the length |
2279 | of the command line. You must ensure your strings don't exceed this | |
2280 | limit after any necessary interpolations. See the platform-specific | |
2281 | release notes for more details about your particular environment. | |
2282 | ||
5a964f20 TC |
2283 | Using this operator can lead to programs that are difficult to port, |
2284 | because the shell commands called vary between systems, and may in | |
2285 | fact not be present at all. As one example, the C<type> command under | |
2286 | the POSIX shell is very different from the C<type> command under DOS. | |
2287 | That doesn't mean you should go out of your way to avoid backticks | |
2288 | when they're the right way to get something done. Perl was made to be | |
2289 | a glue language, and one of the things it glues together is commands. | |
2290 | Just understand what you're getting yourself into. | |
bb32b41a | 2291 | |
da87341d | 2292 | See L</"I/O Operators"> for more discussion. |
a0d0e21e | 2293 | |
945c54fd | 2294 | =item qw/STRING/ |
d74e8afc | 2295 | X<qw> X<quote, list> X<quote, words> |
945c54fd JH |
2296 | |
2297 | Evaluates to a list of the words extracted out of STRING, using embedded | |
2298 | whitespace as the word delimiters. It can be understood as being roughly | |
2299 | equivalent to: | |
2300 | ||
c543c01b | 2301 | split(" ", q/STRING/); |
945c54fd | 2302 | |
efb1e162 CW |
2303 | the differences being that it generates a real list at compile time, and |
2304 | in scalar context it returns the last element in the list. So | |
945c54fd JH |
2305 | this expression: |
2306 | ||
2307 | qw(foo bar baz) | |
2308 | ||
2309 | is semantically equivalent to the list: | |
2310 | ||
c543c01b | 2311 | "foo", "bar", "baz" |
945c54fd JH |
2312 | |
2313 | Some frequently seen examples: | |
2314 | ||
2315 | use POSIX qw( setlocale localeconv ) | |
2316 | @EXPORT = qw( foo bar baz ); | |
2317 | ||
2318 | A common mistake is to try to separate the words with comma or to | |
2319 | put comments into a multi-line C<qw>-string. For this reason, the | |
89d205f2 | 2320 | C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable) |
945c54fd JH |
2321 | produces warnings if the STRING contains the "," or the "#" character. |
2322 | ||
8ff32507 | 2323 | =item tr/SEARCHLIST/REPLACEMENTLIST/cdsr |
d74e8afc | 2324 | X<tr> X<y> X<transliterate> X</c> X</d> X</s> |
a0d0e21e | 2325 | |
8ff32507 | 2326 | =item y/SEARCHLIST/REPLACEMENTLIST/cdsr |
a0d0e21e | 2327 | |
2c268ad5 | 2328 | Transliterates all occurrences of the characters found in the search list |
a0d0e21e LW |
2329 | with the corresponding character in the replacement list. It returns |
2330 | the number of characters replaced or deleted. If no string is | |
c543c01b TC |
2331 | specified via the C<=~> or C<!~> operator, the $_ string is transliterated. |
2332 | ||
2333 | If the C</r> (non-destructive) option is present, a new copy of the string | |
2334 | is made and its characters transliterated, and this copy is returned no | |
2335 | matter whether it was modified or not: the original string is always | |
2336 | left unchanged. The new copy is always a plain string, even if the input | |
2337 | string is an object or a tied variable. | |
8ada0baa | 2338 | |
c543c01b TC |
2339 | Unless the C</r> option is used, the string specified with C<=~> must be a |
2340 | scalar variable, an array element, a hash element, or an assignment to one | |
2341 | of those; in other words, an lvalue. | |
8ff32507 | 2342 | |
89d205f2 | 2343 | A character range may be specified with a hyphen, so C<tr/A-J/0-9/> |
2c268ad5 | 2344 | does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. |
54310121 | 2345 | For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the |
2346 | SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has | |
c543c01b TC |
2347 | its own pair of quotes, which may or may not be bracketing quotes; |
2348 | for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>. | |
2349 | ||
2350 | Note that C<tr> does B<not> do regular expression character classes such as | |
2351 | C<\d> or C<\pL>. The C<tr> operator is not equivalent to the tr(1) | |
2352 | utility. If you want to map strings between lower/upper cases, see | |
2353 | L<perlfunc/lc> and L<perlfunc/uc>, and in general consider using the C<s> | |
2354 | operator if you need regular expressions. The C<\U>, C<\u>, C<\L>, and | |
2355 | C<\l> string-interpolation escapes on the right side of a substitution | |
2356 | operator will perform correct case-mappings, but C<tr[a-z][A-Z]> will not | |
2357 | (except sometimes on legacy 7-bit data). | |
cc255d5f | 2358 | |
8ada0baa JH |
2359 | Note also that the whole range idea is rather unportable between |
2360 | character sets--and even within character sets they may cause results | |
2361 | you probably didn't expect. A sound principle is to use only ranges | |
2362 | that begin from and end at either alphabets of equal case (a-e, A-E), | |
2363 | or digits (0-4). Anything else is unsafe. If in doubt, spell out the | |
2364 | character sets in full. | |
2365 | ||
a0d0e21e LW |
2366 | Options: |
2367 | ||
2368 | c Complement the SEARCHLIST. | |
2369 | d Delete found but unreplaced characters. | |
2370 | s Squash duplicate replaced characters. | |
8ff32507 FC |
2371 | r Return the modified string and leave the original string |
2372 | untouched. | |
a0d0e21e | 2373 | |
19799a22 GS |
2374 | If the C</c> modifier is specified, the SEARCHLIST character set |
2375 | is complemented. If the C</d> modifier is specified, any characters | |
2376 | specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. | |
2377 | (Note that this is slightly more flexible than the behavior of some | |
2378 | B<tr> programs, which delete anything they find in the SEARCHLIST, | |
46f8a5ea | 2379 | period.) If the C</s> modifier is specified, sequences of characters |
19799a22 GS |
2380 | that were transliterated to the same character are squashed down |
2381 | to a single instance of the character. | |
a0d0e21e LW |
2382 | |
2383 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted | |
2384 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter | |
2385 | than the SEARCHLIST, the final character is replicated till it is long | |
5a964f20 | 2386 | enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. |
a0d0e21e LW |
2387 | This latter is useful for counting characters in a class or for |
2388 | squashing character sequences in a class. | |
2389 | ||
2390 | Examples: | |
2391 | ||
c543c01b | 2392 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII |
a0d0e21e LW |
2393 | |
2394 | $cnt = tr/*/*/; # count the stars in $_ | |
2395 | ||
2396 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky | |
2397 | ||
2398 | $cnt = tr/0-9//; # count the digits in $_ | |
2399 | ||
2400 | tr/a-zA-Z//s; # bookkeeper -> bokeper | |
2401 | ||
2402 | ($HOST = $host) =~ tr/a-z/A-Z/; | |
c543c01b | 2403 | $HOST = $host =~ tr/a-z/A-Z/r; # same thing |
8ff32507 | 2404 | |
c543c01b | 2405 | $HOST = $host =~ tr/a-z/A-Z/r # chained with s///r |
8ff32507 | 2406 | =~ s/:/ -p/r; |
a0d0e21e LW |
2407 | |
2408 | tr/a-zA-Z/ /cs; # change non-alphas to single space | |
2409 | ||
8ff32507 FC |
2410 | @stripped = map tr/a-zA-Z/ /csr, @original; |
2411 | # /r with map | |
2412 | ||
a0d0e21e | 2413 | tr [\200-\377] |
c543c01b | 2414 | [\000-\177]; # wickedly delete 8th bit |
a0d0e21e | 2415 | |
19799a22 GS |
2416 | If multiple transliterations are given for a character, only the |
2417 | first one is used: | |
748a9306 LW |
2418 | |
2419 | tr/AAA/XYZ/ | |
2420 | ||
2c268ad5 | 2421 | will transliterate any A to X. |
748a9306 | 2422 | |
19799a22 | 2423 | Because the transliteration table is built at compile time, neither |
a0d0e21e | 2424 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote |
19799a22 GS |
2425 | interpolation. That means that if you want to use variables, you |
2426 | must use an eval(): | |
a0d0e21e LW |
2427 | |
2428 | eval "tr/$oldlist/$newlist/"; | |
2429 | die $@ if $@; | |
2430 | ||
2431 | eval "tr/$oldlist/$newlist/, 1" or die $@; | |
2432 | ||
7e3b091d | 2433 | =item <<EOF |
d74e8afc | 2434 | X<here-doc> X<heredoc> X<here-document> X<<< << >>> |
7e3b091d DA |
2435 | |
2436 | A line-oriented form of quoting is based on the shell "here-document" | |
2437 | syntax. Following a C<< << >> you specify a string to terminate | |
2438 | the quoted material, and all lines following the current line down to | |
89d205f2 YO |
2439 | the terminating string are the value of the item. |
2440 | ||
2441 | The terminating string may be either an identifier (a word), or some | |
2442 | quoted text. An unquoted identifier works like double quotes. | |
2443 | There may not be a space between the C<< << >> and the identifier, | |
2444 | unless the identifier is explicitly quoted. (If you put a space it | |
2445 | will be treated as a null identifier, which is valid, and matches the | |
2446 | first empty line.) The terminating string must appear by itself | |
2447 | (unquoted and with no surrounding whitespace) on the terminating line. | |
2448 | ||
2449 | If the terminating string is quoted, the type of quotes used determine | |
2450 | the treatment of the text. | |
2451 | ||
2452 | =over 4 | |
2453 | ||
2454 | =item Double Quotes | |
2455 | ||
2456 | Double quotes indicate that the text will be interpolated using exactly | |
2457 | the same rules as normal double quoted strings. | |
7e3b091d DA |
2458 | |
2459 | print <<EOF; | |
2460 | The price is $Price. | |
2461 | EOF | |
2462 | ||
2463 | print << "EOF"; # same as above | |
2464 | The price is $Price. | |
2465 | EOF | |
2466 | ||
89d205f2 YO |
2467 | |
2468 | =item Single Quotes | |
2469 | ||
2470 | Single quotes indicate the text is to be treated literally with no | |
46f8a5ea | 2471 | interpolation of its content. This is similar to single quoted |
89d205f2 YO |
2472 | strings except that backslashes have no special meaning, with C<\\> |
2473 | being treated as two backslashes and not one as they would in every | |
2474 | other quoting construct. | |
2475 | ||
c543c01b TC |
2476 | Just as in the shell, a backslashed bareword following the C<<< << >>> |
2477 | means the same thing as a single-quoted string does: | |
2478 | ||
2479 | $cost = <<'VISTA'; # hasta la ... | |
2480 | That'll be $10 please, ma'am. | |
2481 | VISTA | |
2482 | ||
2483 | $cost = <<\VISTA; # Same thing! | |
2484 | That'll be $10 please, ma'am. | |
2485 | VISTA | |
2486 | ||
89d205f2 YO |
2487 | This is the only form of quoting in perl where there is no need |
2488 | to worry about escaping content, something that code generators | |
2489 | can and do make good use of. | |
2490 | ||
2491 | =item Backticks | |
2492 | ||
2493 | The content of the here doc is treated just as it would be if the | |
46f8a5ea | 2494 | string were embedded in backticks. Thus the content is interpolated |
89d205f2 YO |
2495 | as though it were double quoted and then executed via the shell, with |
2496 | the results of the execution returned. | |
2497 | ||
2498 | print << `EOC`; # execute command and get results | |
7e3b091d | 2499 | echo hi there |
7e3b091d DA |
2500 | EOC |
2501 | ||
89d205f2 YO |
2502 | =back |
2503 | ||
2504 | It is possible to stack multiple here-docs in a row: | |
2505 | ||
7e3b091d DA |
2506 | print <<"foo", <<"bar"; # you can stack them |
2507 | I said foo. | |
2508 | foo | |
2509 | I said bar. | |
2510 | bar | |
2511 | ||
2512 | myfunc(<< "THIS", 23, <<'THAT'); | |
2513 | Here's a line | |
2514 | or two. | |
2515 | THIS | |
2516 | and here's another. | |
2517 | THAT | |
2518 | ||
2519 | Just don't forget that you have to put a semicolon on the end | |
2520 | to finish the statement, as Perl doesn't know you're not going to | |
2521 | try to do this: | |
2522 | ||
2523 | print <<ABC | |
2524 | 179231 | |
2525 | ABC | |
2526 | + 20; | |
2527 | ||
872d7e53 TS |
2528 | If you want to remove the line terminator from your here-docs, |
2529 | use C<chomp()>. | |
2530 | ||
2531 | chomp($string = <<'END'); | |
2532 | This is a string. | |
2533 | END | |
2534 | ||
2535 | If you want your here-docs to be indented with the rest of the code, | |
2536 | you'll need to remove leading whitespace from each line manually: | |
7e3b091d DA |
2537 | |
2538 | ($quote = <<'FINIS') =~ s/^\s+//gm; | |
89d205f2 | 2539 | The Road goes ever on and on, |
7e3b091d DA |
2540 | down from the door where it began. |
2541 | FINIS | |
2542 | ||
2543 | If you use a here-doc within a delimited construct, such as in C<s///eg>, | |
1bf48760 FC |
2544 | the quoted material must still come on the line following the |
2545 | C<<< <<FOO >>> marker, which means it may be inside the delimited | |
2546 | construct: | |
7e3b091d DA |
2547 | |
2548 | s/this/<<E . 'that' | |
2549 | the other | |
2550 | E | |
2551 | . 'more '/eg; | |
2552 | ||
1bf48760 FC |
2553 | It works this way as of Perl 5.18. Historically, it was inconsistent, and |
2554 | you would have to write | |
7e3b091d | 2555 | |
89d205f2 YO |
2556 | s/this/<<E . 'that' |
2557 | . 'more '/eg; | |
2558 | the other | |
2559 | E | |
7e3b091d | 2560 | |
1bf48760 FC |
2561 | outside of string evals. |
2562 | ||
c543c01b | 2563 | Additionally, quoting rules for the end-of-string identifier are |
46f8a5ea | 2564 | unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not |
89d205f2 YO |
2565 | supported in place of C<''> and C<"">, and the only interpolation is for |
2566 | backslashing the quoting character: | |
7e3b091d DA |
2567 | |
2568 | print << "abc\"def"; | |
2569 | testing... | |
2570 | abc"def | |
2571 | ||
2572 | Finally, quoted strings cannot span multiple lines. The general rule is | |
2573 | that the identifier must be a string literal. Stick with that, and you | |
2574 | should be safe. | |
2575 | ||
a0d0e21e LW |
2576 | =back |
2577 | ||
75e14d17 | 2578 | =head2 Gory details of parsing quoted constructs |
d74e8afc | 2579 | X<quote, gory details> |
75e14d17 | 2580 | |
19799a22 GS |
2581 | When presented with something that might have several different |
2582 | interpretations, Perl uses the B<DWIM> (that's "Do What I Mean") | |
2583 | principle to pick the most probable interpretation. This strategy | |
2584 | is so successful that Perl programmers often do not suspect the | |
2585 | ambivalence of what they write. But from time to time, Perl's | |
2586 | notions differ substantially from what the author honestly meant. | |
2587 | ||
2588 | This section hopes to clarify how Perl handles quoted constructs. | |
2589 | Although the most common reason to learn this is to unravel labyrinthine | |
2590 | regular expressions, because the initial steps of parsing are the | |
2591 | same for all quoting operators, they are all discussed together. | |
2592 | ||
2593 | The most important Perl parsing rule is the first one discussed | |
2594 | below: when processing a quoted construct, Perl first finds the end | |
2595 | of that construct, then interprets its contents. If you understand | |
2596 | this rule, you may skip the rest of this section on the first | |
2597 | reading. The other rules are likely to contradict the user's | |
2598 | expectations much less frequently than this first one. | |
2599 | ||
2600 | Some passes discussed below are performed concurrently, but because | |
2601 | their results are the same, we consider them individually. For different | |
2602 | quoting constructs, Perl performs different numbers of passes, from | |
6deea57f | 2603 | one to four, but these passes are always performed in the same order. |
75e14d17 | 2604 | |
13a2d996 | 2605 | =over 4 |
75e14d17 IZ |
2606 | |
2607 | =item Finding the end | |
2608 | ||
6deea57f TS |
2609 | The first pass is finding the end of the quoted construct, where |
2610 | the information about the delimiters is used in parsing. | |
2611 | During this search, text between the starting and ending delimiters | |
46f8a5ea | 2612 | is copied to a safe location. The text copied gets delimiter-independent. |
6deea57f TS |
2613 | |
2614 | If the construct is a here-doc, the ending delimiter is a line | |
46f8a5ea | 2615 | that has a terminating string as the content. Therefore C<<<EOF> is |
6deea57f TS |
2616 | terminated by C<EOF> immediately followed by C<"\n"> and starting |
2617 | from the first column of the terminating line. | |
2618 | When searching for the terminating line of a here-doc, nothing | |
46f8a5ea | 2619 | is skipped. In other words, lines after the here-doc syntax |
6deea57f TS |
2620 | are compared with the terminating string line by line. |
2621 | ||
2622 | For the constructs except here-docs, single characters are used as starting | |
46f8a5ea | 2623 | and ending delimiters. If the starting delimiter is an opening punctuation |
6deea57f TS |
2624 | (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the |
2625 | corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>). | |
2626 | If the starting delimiter is an unpaired character like C</> or a closing | |
2627 | punctuation, the ending delimiter is same as the starting delimiter. | |
2628 | Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates | |
fc693347 | 2629 | both C<qq[]> and C<qq]]> constructs. |
6deea57f TS |
2630 | |
2631 | When searching for single-character delimiters, escaped delimiters | |
1ca345ed | 2632 | and C<\\> are skipped. For example, while searching for terminating C</>, |
6deea57f TS |
2633 | combinations of C<\\> and C<\/> are skipped. If the delimiters are |
2634 | bracketing, nested pairs are also skipped. For example, while searching | |
2635 | for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>, | |
2636 | and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well. | |
2637 | However, when backslashes are used as the delimiters (like C<qq\\> and | |
2638 | C<tr\\\>), nothing is skipped. | |
32581033 | 2639 | During the search for the end, backslashes that escape delimiters or |
7188ca43 | 2640 | other backslashes are removed (exactly speaking, they are not copied to the |
32581033 | 2641 | safe location). |
75e14d17 | 2642 | |
19799a22 GS |
2643 | For constructs with three-part delimiters (C<s///>, C<y///>, and |
2644 | C<tr///>), the search is repeated once more. | |
fc693347 | 2645 | If the first delimiter is not an opening punctuation, the three delimiters must |
d74605e5 FC |
2646 | be the same, such as C<s!!!> and C<tr)))>, |
2647 | in which case the second delimiter | |
6deea57f | 2648 | terminates the left part and starts the right part at once. |
b6538e4f | 2649 | If the left part is delimited by bracketing punctuation (that is C<()>, |
6deea57f | 2650 | C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of |
b6538e4f | 2651 | delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace |
fc693347 | 2652 | and comments are allowed between the two parts, though the comment must follow |
b6538e4f TC |
2653 | at least one whitespace character; otherwise a character expected as the |
2654 | start of the comment may be regarded as the starting delimiter of the right part. | |
75e14d17 | 2655 | |
19799a22 GS |
2656 | During this search no attention is paid to the semantics of the construct. |
2657 | Thus: | |
75e14d17 IZ |
2658 | |
2659 | "$hash{"$foo/$bar"}" | |
2660 | ||
2a94b7ce | 2661 | or: |
75e14d17 | 2662 | |
89d205f2 | 2663 | m/ |
2a94b7ce | 2664 | bar # NOT a comment, this slash / terminated m//! |
75e14d17 IZ |
2665 | /x |
2666 | ||
19799a22 GS |
2667 | do not form legal quoted expressions. The quoted part ends on the |
2668 | first C<"> and C</>, and the rest happens to be a syntax error. | |
2669 | Because the slash that terminated C<m//> was followed by a C<SPACE>, | |
2670 | the example above is not C<m//x>, but rather C<m//> with no C</x> | |
2671 | modifier. So the embedded C<#> is interpreted as a literal C<#>. | |
75e14d17 | 2672 | |
89d205f2 | 2673 | Also no attention is paid to C<\c\> (multichar control char syntax) during |
46f8a5ea | 2674 | this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part |
89d205f2 | 2675 | of C<\/>, and the following C</> is not recognized as a delimiter. |
0d594e51 TS |
2676 | Instead, use C<\034> or C<\x1c> at the end of quoted constructs. |
2677 | ||
75e14d17 | 2678 | =item Interpolation |
d74e8afc | 2679 | X<interpolation> |
75e14d17 | 2680 | |
19799a22 | 2681 | The next step is interpolation in the text obtained, which is now |
89d205f2 | 2682 | delimiter-independent. There are multiple cases. |
75e14d17 | 2683 | |
13a2d996 | 2684 | =over 4 |
75e14d17 | 2685 | |
89d205f2 | 2686 | =item C<<<'EOF'> |
75e14d17 IZ |
2687 | |
2688 | No interpolation is performed. | |
6deea57f TS |
2689 | Note that the combination C<\\> is left intact, since escaped delimiters |
2690 | are not available for here-docs. | |
75e14d17 | 2691 | |
6deea57f | 2692 | =item C<m''>, the pattern of C<s'''> |
89d205f2 | 2693 | |
6deea57f TS |
2694 | No interpolation is performed at this stage. |
2695 | Any backslashed sequences including C<\\> are treated at the stage | |
2696 | to L</"parsing regular expressions">. | |
89d205f2 | 2697 | |
6deea57f | 2698 | =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''> |
75e14d17 | 2699 | |
89d205f2 | 2700 | The only interpolation is removal of C<\> from pairs of C<\\>. |
6deea57f TS |
2701 | Therefore C<-> in C<tr'''> and C<y'''> is treated literally |
2702 | as a hyphen and no character range is available. | |
2703 | C<\1> in the replacement of C<s'''> does not work as C<$1>. | |
89d205f2 YO |
2704 | |
2705 | =item C<tr///>, C<y///> | |
2706 | ||
6deea57f TS |
2707 | No variable interpolation occurs. String modifying combinations for |
2708 | case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized. | |
2709 | The other escape sequences such as C<\200> and C<\t> and backslashed | |
2710 | characters such as C<\\> and C<\-> are converted to appropriate literals. | |
89d205f2 YO |
2711 | The character C<-> is treated specially and therefore C<\-> is treated |
2712 | as a literal C<->. | |
75e14d17 | 2713 | |
89d205f2 | 2714 | =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF"> |
75e14d17 | 2715 | |
628253b8 | 2716 | C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> (possibly paired with C<\E>) are |
19799a22 GS |
2717 | converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar"> |
2718 | is converted to C<$foo . (quotemeta("baz" . $bar))> internally. | |
6deea57f TS |
2719 | The other escape sequences such as C<\200> and C<\t> and backslashed |
2720 | characters such as C<\\> and C<\-> are replaced with appropriate | |
2721 | expansions. | |
2a94b7ce | 2722 | |
19799a22 GS |
2723 | Let it be stressed that I<whatever falls between C<\Q> and C<\E>> |
2724 | is interpolated in the usual way. Something like C<"\Q\\E"> has | |
48cbae4f | 2725 | no C<\E> inside. Instead, it has C<\Q>, C<\\>, and C<E>, so the |
19799a22 GS |
2726 | result is the same as for C<"\\\\E">. As a general rule, backslashes |
2727 | between C<\Q> and C<\E> may lead to counterintuitive results. So, | |
2728 | C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same | |
2729 | as C<"\\\t"> (since TAB is not alphanumeric). Note also that: | |
2a94b7ce IZ |
2730 | |
2731 | $str = '\t'; | |
2732 | return "\Q$str"; | |
2733 | ||
2734 | may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. | |
2735 | ||
19799a22 | 2736 | Interpolated scalars and arrays are converted internally to the C<join> and |
92d29cee | 2737 | C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes: |
75e14d17 | 2738 | |
19799a22 | 2739 | $foo . " XXX '" . (join $", @arr) . "'"; |
75e14d17 | 2740 | |
19799a22 | 2741 | All operations above are performed simultaneously, left to right. |
75e14d17 | 2742 | |
19799a22 GS |
2743 | Because the result of C<"\Q STRING \E"> has all metacharacters |
2744 | quoted, there is no way to insert a literal C<$> or C<@> inside a | |
2745 | C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became | |
2746 | C<"\\\$">; if not, it is interpreted as the start of an interpolated | |
2747 | scalar. | |
75e14d17 | 2748 | |
19799a22 | 2749 | Note also that the interpolation code needs to make a decision on |
89d205f2 | 2750 | where the interpolated scalar ends. For instance, whether |
db691027 | 2751 | C<< "a $x -> {c}" >> really means: |
75e14d17 | 2752 | |
db691027 | 2753 | "a " . $x . " -> {c}"; |
75e14d17 | 2754 | |
2a94b7ce | 2755 | or: |
75e14d17 | 2756 | |
db691027 | 2757 | "a " . $x -> {c}; |
75e14d17 | 2758 | |
19799a22 GS |
2759 | Most of the time, the longest possible text that does not include |
2760 | spaces between components and which contains matching braces or | |
2761 | brackets. because the outcome may be determined by voting based | |
2762 | on heuristic estimators, the result is not strictly predictable. | |
2763 | Fortunately, it's usually correct for ambiguous cases. | |
75e14d17 | 2764 | |
6deea57f | 2765 | =item the replacement of C<s///> |
75e14d17 | 2766 | |
628253b8 | 2767 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> and interpolation |
6deea57f TS |
2768 | happens as with C<qq//> constructs. |
2769 | ||
2770 | It is at this step that C<\1> is begrudgingly converted to C<$1> in | |
2771 | the replacement text of C<s///>, in order to correct the incorrigible | |
2772 | I<sed> hackers who haven't picked up the saner idiom yet. A warning | |
2773 | is emitted if the C<use warnings> pragma or the B<-w> command-line flag | |
2774 | (that is, the C<$^W> variable) was set. | |
2775 | ||
2776 | =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, | |
2777 | ||
628253b8 | 2778 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F>, C<\E>, |
cc74c5bd TS |
2779 | and interpolation happens (almost) as with C<qq//> constructs. |
2780 | ||
5d03b57c KW |
2781 | Processing of C<\N{...}> is also done here, and compiled into an intermediate |
2782 | form for the regex compiler. (This is because, as mentioned below, the regex | |
2783 | compilation may be done at execution time, and C<\N{...}> is a compile-time | |
2784 | construct.) | |
2785 | ||
cc74c5bd TS |
2786 | However any other combinations of C<\> followed by a character |
2787 | are not substituted but only skipped, in order to parse them | |
2788 | as regular expressions at the following step. | |
6deea57f | 2789 | As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly |
1749ea0d | 2790 | treated as an array symbol (for example C<@foo>), |
6deea57f | 2791 | even though the same text in C<qq//> gives interpolation of C<\c@>. |
6deea57f | 2792 | |
e128ab2c DM |
2793 | Code blocks such as C<(?{BLOCK})> are handled by temporarily passing control |
2794 | back to the perl parser, in a similar way that an interpolated array | |
2795 | subscript expression such as C<"foo$array[1+f("[xyz")]bar"> would be. | |
2796 | ||
6deea57f | 2797 | Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and |
19799a22 GS |
2798 | a C<#>-comment in a C<//x>-regular expression, no processing is |
2799 | performed whatsoever. This is the first step at which the presence | |
2800 | of the C<//x> modifier is relevant. | |
2801 | ||
1749ea0d TS |
2802 | Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+> |
2803 | and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are | |
2804 | voted (by several different estimators) to be either an array element | |
2805 | or C<$var> followed by an RE alternative. This is where the notation | |
19799a22 GS |
2806 | C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as |
2807 | array element C<-9>, not as a regular expression from the variable | |
2808 | C<$arr> followed by a digit, which would be the interpretation of | |
2809 | C</$arr[0-9]/>. Since voting among different estimators may occur, | |
2810 | the result is not predictable. | |
2811 | ||
19799a22 GS |
2812 | The lack of processing of C<\\> creates specific restrictions on |
2813 | the post-processed text. If the delimiter is C</>, one cannot get | |
2814 | the combination C<\/> into the result of this step. C</> will | |
2815 | finish the regular expression, C<\/> will be stripped to C</> on | |
2816 | the previous step, and C<\\/> will be left as is. Because C</> is | |
2817 | equivalent to C<\/> inside a regular expression, this does not | |
2818 | matter unless the delimiter happens to be character special to the | |
2819 | RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an | |
2820 | alphanumeric char, as in: | |
2a94b7ce IZ |
2821 | |
2822 | m m ^ a \s* b mmx; | |
2823 | ||
19799a22 | 2824 | In the RE above, which is intentionally obfuscated for illustration, the |
6deea57f | 2825 | delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the |
89d205f2 | 2826 | RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one |
19799a22 GS |
2827 | reason you're encouraged to restrict your delimiters to non-alphanumeric, |
2828 | non-whitespace choices. | |
75e14d17 IZ |
2829 | |
2830 | =back | |
2831 | ||
19799a22 | 2832 | This step is the last one for all constructs except regular expressions, |
75e14d17 IZ |
2833 | which are processed further. |
2834 | ||
6deea57f TS |
2835 | =item parsing regular expressions |
2836 | X<regexp, parse> | |
75e14d17 | 2837 | |
19799a22 | 2838 | Previous steps were performed during the compilation of Perl code, |
ac036724 | 2839 | but this one happens at run time, although it may be optimized to |
19799a22 | 2840 | be calculated at compile time if appropriate. After preprocessing |
6deea57f | 2841 | described above, and possibly after evaluation if concatenation, |
19799a22 GS |
2842 | joining, casing translation, or metaquoting are involved, the |
2843 | resulting I<string> is passed to the RE engine for compilation. | |
2844 | ||
2845 | Whatever happens in the RE engine might be better discussed in L<perlre>, | |
2846 | but for the sake of continuity, we shall do so here. | |
2847 | ||
2848 | This is another step where the presence of the C<//x> modifier is | |
2849 | relevant. The RE engine scans the string from left to right and | |
2850 | converts it to a finite automaton. | |
2851 | ||
2852 | Backslashed characters are either replaced with corresponding | |
2853 | literal strings (as with C<\{>), or else they generate special nodes | |
2854 | in the finite automaton (as with C<\b>). Characters special to the | |
2855 | RE engine (such as C<|>) generate corresponding nodes or groups of | |
2856 | nodes. C<(?#...)> comments are ignored. All the rest is either | |
2857 | converted to literal strings to match, or else is ignored (as is | |
2858 | whitespace and C<#>-style comments if C<//x> is present). | |
2859 | ||
2860 | Parsing of the bracketed character class construct, C<[...]>, is | |
2861 | rather different than the rule used for the rest of the pattern. | |
2862 | The terminator of this construct is found using the same rules as | |
2863 | for finding the terminator of a C<{}>-delimited construct, the only | |
2864 | exception being that C<]> immediately following C<[> is treated as | |
e128ab2c DM |
2865 | though preceded by a backslash. |
2866 | ||
2867 | The terminator of runtime C<(?{...})> is found by temporarily switching | |
2868 | control to the perl parser, which should stop at the point where the | |
2869 | logically balancing terminating C<}> is found. | |
19799a22 GS |
2870 | |
2871 | It is possible to inspect both the string given to RE engine and the | |
2872 | resulting finite automaton. See the arguments C<debug>/C<debugcolor> | |
2873 | in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line | |
4a4eefd0 | 2874 | switch documented in L<perlrun/"Command Switches">. |
75e14d17 IZ |
2875 | |
2876 | =item Optimization of regular expressions | |
d74e8afc | 2877 | X<regexp, optimization> |
75e14d17 | 2878 | |
7522fed5 | 2879 | This step is listed for completeness only. Since it does not change |
75e14d17 | 2880 | semantics, details of this step are not documented and are subject |
19799a22 GS |
2881 | to change without notice. This step is performed over the finite |
2882 | automaton that was generated during the previous pass. | |
2a94b7ce | 2883 | |
19799a22 GS |
2884 | It is at this stage that C<split()> silently optimizes C</^/> to |
2885 | mean C</^/m>. | |
75e14d17 IZ |
2886 | |
2887 | =back | |
2888 | ||
a0d0e21e | 2889 | =head2 I/O Operators |
d74e8afc | 2890 | X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle> |
80a96bfc | 2891 | X<< <> >> X<< <<>> >> X<@ARGV> |
a0d0e21e | 2892 | |
54310121 | 2893 | There are several I/O operators you should know about. |
fbad3eb5 | 2894 | |
7b8d334a | 2895 | A string enclosed by backticks (grave accents) first undergoes |
19799a22 GS |
2896 | double-quote interpolation. It is then interpreted as an external |
2897 | command, and the output of that command is the value of the | |
e9c56f9b JH |
2898 | backtick string, like in a shell. In scalar context, a single string |
2899 | consisting of all output is returned. In list context, a list of | |
2900 | values is returned, one per line of output. (You can set C<$/> to use | |
2901 | a different line terminator.) The command is executed each time the | |
2902 | pseudo-literal is evaluated. The status value of the command is | |
2903 | returned in C<$?> (see L<perlvar> for the interpretation of C<$?>). | |
2904 | Unlike in B<csh>, no translation is done on the return data--newlines | |
2905 | remain newlines. Unlike in any of the shells, single quotes do not | |
2906 | hide variable names in the command from interpretation. To pass a | |
2907 | literal dollar-sign through to the shell you need to hide it with a | |
2908 | backslash. The generalized form of backticks is C<qx//>. (Because | |
2909 | backticks always undergo shell expansion as well, see L<perlsec> for | |
2910 | security concerns.) | |
d74e8afc | 2911 | X<qx> X<`> X<``> X<backtick> X<glob> |
19799a22 GS |
2912 | |
2913 | In scalar context, evaluating a filehandle in angle brackets yields | |
2914 | the next line from that file (the newline, if any, included), or | |
2915 | C<undef> at end-of-file or on error. When C<$/> is set to C<undef> | |
2916 | (sometimes known as file-slurp mode) and the file is empty, it | |
2917 | returns C<''> the first time, followed by C<undef> subsequently. | |
2918 | ||
2919 | Ordinarily you must assign the returned value to a variable, but | |
2920 | there is one situation where an automatic assignment happens. If | |
2921 | and only if the input symbol is the only thing inside the conditional | |
2922 | of a C<while> statement (even if disguised as a C<for(;;)> loop), | |
2923 | the value is automatically assigned to the global variable $_, | |
2924 | destroying whatever was there previously. (This may seem like an | |
2925 | odd thing to you, but you'll use the construct in almost every Perl | |
17b829fa | 2926 | script you write.) The $_ variable is not implicitly localized. |
19799a22 GS |
2927 | You'll have to put a C<local $_;> before the loop if you want that |
2928 | to happen. | |
2929 | ||
2930 | The following lines are equivalent: | |
a0d0e21e | 2931 | |
748a9306 | 2932 | while (defined($_ = <STDIN>)) { print; } |
7b8d334a | 2933 | while ($_ = <STDIN>) { print; } |
a0d0e21e LW |
2934 | while (<STDIN>) { print; } |
2935 | for (;<STDIN>;) { print; } | |
748a9306 | 2936 | print while defined($_ = <STDIN>); |
7b8d334a | 2937 | print while ($_ = <STDIN>); |
a0d0e21e LW |
2938 | print while <STDIN>; |
2939 | ||
1ca345ed TC |
2940 | This also behaves similarly, but assigns to a lexical variable |
2941 | instead of to C<$_>: | |
7b8d334a | 2942 | |
89d205f2 | 2943 | while (my $line = <STDIN>) { print $line } |
7b8d334a | 2944 | |
19799a22 GS |
2945 | In these loop constructs, the assigned value (whether assignment |
2946 | is automatic or explicit) is then tested to see whether it is | |
1ca345ed TC |
2947 | defined. The defined test avoids problems where the line has a string |
2948 | value that would be treated as false by Perl; for example a "" or | |
19799a22 GS |
2949 | a "0" with no trailing newline. If you really mean for such values |
2950 | to terminate the loop, they should be tested for explicitly: | |
7b8d334a GS |
2951 | |
2952 | while (($_ = <STDIN>) ne '0') { ... } | |
2953 | while (<STDIN>) { last unless $_; ... } | |
2954 | ||
1ca345ed | 2955 | In other boolean contexts, C<< <FILEHANDLE> >> without an |
5ef4d93e | 2956 | explicit C<defined> test or comparison elicits a warning if the |
9f1b1f2d | 2957 | C<use warnings> pragma or the B<-w> |
19799a22 | 2958 | command-line switch (the C<$^W> variable) is in effect. |
7b8d334a | 2959 | |
5f05dabc | 2960 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
19799a22 GS |
2961 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except |
2962 | in packages, where they would be interpreted as local identifiers | |
2963 | rather than global.) Additional filehandles may be created with | |
2964 | the open() function, amongst others. See L<perlopentut> and | |
2965 | L<perlfunc/open> for details on this. | |
d74e8afc | 2966 | X<stdin> X<stdout> X<sterr> |
a0d0e21e | 2967 | |
35f2feb0 | 2968 | If a <FILEHANDLE> is used in a context that is looking for |
19799a22 GS |
2969 | a list, a list comprising all input lines is returned, one line per |
2970 | list element. It's easy to grow to a rather large data space this | |
2971 | way, so use with care. | |
a0d0e21e | 2972 | |
35f2feb0 | 2973 | <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>. |
19799a22 | 2974 | See L<perlfunc/readline>. |
fbad3eb5 | 2975 | |
35f2feb0 | 2976 | The null filehandle <> is special: it can be used to emulate the |
1ca345ed TC |
2977 | behavior of B<sed> and B<awk>, and any other Unix filter program |
2978 | that takes a list of filenames, doing the same to each line | |
2979 | of input from all of them. Input from <> comes either from | |
a0d0e21e | 2980 | standard input, or from each file listed on the command line. Here's |
35f2feb0 | 2981 | how it works: the first time <> is evaluated, the @ARGV array is |
5a964f20 | 2982 | checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened |
a0d0e21e LW |
2983 | gives you standard input. The @ARGV array is then processed as a list |
2984 | of filenames. The loop | |
2985 | ||
2986 | while (<>) { | |
2987 | ... # code for each line | |
2988 | } | |
2989 | ||
2990 | is equivalent to the following Perl-like pseudo code: | |
2991 | ||
3e3baf6d | 2992 | unshift(@ARGV, '-') unless @ARGV; |
a0d0e21e LW |
2993 | while ($ARGV = shift) { |
2994 | open(ARGV, $ARGV); | |
2995 | while (<ARGV>) { | |
2996 | ... # code for each line | |
2997 | } | |
2998 | } | |
2999 | ||
19799a22 GS |
3000 | except that it isn't so cumbersome to say, and will actually work. |
3001 | It really does shift the @ARGV array and put the current filename | |
3002 | into the $ARGV variable. It also uses filehandle I<ARGV> | |
46f8a5ea | 3003 | internally. <> is just a synonym for <ARGV>, which |
19799a22 | 3004 | is magical. (The pseudo code above doesn't work because it treats |
35f2feb0 | 3005 | <ARGV> as non-magical.) |
a0d0e21e | 3006 | |
48ab5743 ML |
3007 | Since the null filehandle uses the two argument form of L<perlfunc/open> |
3008 | it interprets special characters, so if you have a script like this: | |
3009 | ||
3010 | while (<>) { | |
3011 | print; | |
3012 | } | |
3013 | ||
3014 | and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a | |
3015 | pipe, executes the C<rm> command and reads C<rm>'s output from that pipe. | |
3016 | If you want all items in C<@ARGV> to be interpreted as file names, you | |
1033ba6e PM |
3017 | can use the module C<ARGV::readonly> from CPAN, or use the double bracket: |
3018 | ||
3019 | while (<<>>) { | |
3020 | print; | |
3021 | } | |
3022 | ||
3023 | Using double angle brackets inside of a while causes the open to use the | |
3024 | three argument form (with the second argument being C<< < >>), so all | |
80a96bfc RGS |
3025 | arguments in ARGV are treated as literal filenames (including "-"). |
3026 | (Note that for convenience, if you use C<< <<>> >> and if @ARGV is | |
3027 | empty, it will still read from the standard input.) | |
48ab5743 | 3028 | |
35f2feb0 | 3029 | You can modify @ARGV before the first <> as long as the array ends up |
a0d0e21e | 3030 | containing the list of filenames you really want. Line numbers (C<$.>) |
19799a22 GS |
3031 | continue as though the input were one big happy file. See the example |
3032 | in L<perlfunc/eof> for how to reset line numbers on each file. | |
5a964f20 | 3033 | |
89d205f2 | 3034 | If you want to set @ARGV to your own list of files, go right ahead. |
5a964f20 TC |
3035 | This sets @ARGV to all plain text files if no @ARGV was given: |
3036 | ||
3037 | @ARGV = grep { -f && -T } glob('*') unless @ARGV; | |
a0d0e21e | 3038 | |
5a964f20 TC |
3039 | You can even set them to pipe commands. For example, this automatically |
3040 | filters compressed arguments through B<gzip>: | |
3041 | ||
3042 | @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; | |
3043 | ||
3044 | If you want to pass switches into your script, you can use one of the | |
a0d0e21e LW |
3045 | Getopts modules or put a loop on the front like this: |
3046 | ||
3047 | while ($_ = $ARGV[0], /^-/) { | |
3048 | shift; | |
3049 | last if /^--$/; | |
3050 | if (/^-D(.*)/) { $debug = $1 } | |
3051 | if (/^-v/) { $verbose++ } | |
5a964f20 | 3052 | # ... # other switches |
a0d0e21e | 3053 | } |
5a964f20 | 3054 | |
a0d0e21e | 3055 | while (<>) { |
5a964f20 | 3056 | # ... # code for each line |
a0d0e21e LW |
3057 | } |
3058 | ||
89d205f2 YO |
3059 | The <> symbol will return C<undef> for end-of-file only once. |
3060 | If you call it again after this, it will assume you are processing another | |
19799a22 | 3061 | @ARGV list, and if you haven't set @ARGV, will read input from STDIN. |
a0d0e21e | 3062 | |
1ca345ed | 3063 | If what the angle brackets contain is a simple scalar variable (for example, |
35f2feb0 | 3064 | <$foo>), then that variable contains the name of the |
19799a22 GS |
3065 | filehandle to input from, or its typeglob, or a reference to the |
3066 | same. For example: | |
cb1a09d0 AD |
3067 | |
3068 | $fh = \*STDIN; | |
3069 | $line = <$fh>; | |
a0d0e21e | 3070 | |
5a964f20 TC |
3071 | If what's within the angle brackets is neither a filehandle nor a simple |
3072 | scalar variable containing a filehandle name, typeglob, or typeglob | |
3073 | reference, it is interpreted as a filename pattern to be globbed, and | |
3074 | either a list of filenames or the next filename in the list is returned, | |
19799a22 | 3075 | depending on context. This distinction is determined on syntactic |
35f2feb0 GS |
3076 | grounds alone. That means C<< <$x> >> is always a readline() from |
3077 | an indirect handle, but C<< <$hash{key}> >> is always a glob(). | |
5a964f20 | 3078 | That's because $x is a simple scalar variable, but C<$hash{key}> is |
ef191992 YST |
3079 | not--it's a hash element. Even C<< <$x > >> (note the extra space) |
3080 | is treated as C<glob("$x ")>, not C<readline($x)>. | |
5a964f20 TC |
3081 | |
3082 | One level of double-quote interpretation is done first, but you can't | |
35f2feb0 | 3083 | say C<< <$foo> >> because that's an indirect filehandle as explained |
5a964f20 TC |
3084 | in the previous paragraph. (In older versions of Perl, programmers |
3085 | would insert curly brackets to force interpretation as a filename glob: | |
35f2feb0 | 3086 | C<< <${foo}> >>. These days, it's considered cleaner to call the |
5a964f20 | 3087 | internal function directly as C<glob($foo)>, which is probably the right |
19799a22 | 3088 | way to have done it in the first place.) For example: |
a0d0e21e LW |
3089 | |
3090 | while (<*.c>) { | |
3091 | chmod 0644, $_; | |
3092 | } | |
3093 | ||
3a4b19e4 | 3094 | is roughly equivalent to: |
a0d0e21e LW |
3095 | |
3096 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); | |
3097 | while (<FOO>) { | |
5b3eff12 | 3098 | chomp; |
a0d0e21e LW |
3099 | chmod 0644, $_; |
3100 | } | |
3101 | ||
3a4b19e4 GS |
3102 | except that the globbing is actually done internally using the standard |
3103 | C<File::Glob> extension. Of course, the shortest way to do the above is: | |
a0d0e21e LW |
3104 | |
3105 | chmod 0644, <*.c>; | |
3106 | ||
19799a22 GS |
3107 | A (file)glob evaluates its (embedded) argument only when it is |
3108 | starting a new list. All values must be read before it will start | |
3109 | over. In list context, this isn't important because you automatically | |
3110 | get them all anyway. However, in scalar context the operator returns | |
069e01df | 3111 | the next value each time it's called, or C<undef> when the list has |
19799a22 GS |
3112 | run out. As with filehandle reads, an automatic C<defined> is |
3113 | generated when the glob occurs in the test part of a C<while>, | |
1ca345ed TC |
3114 | because legal glob returns (for example, |
3115 | a file called F<0>) would otherwise | |
19799a22 GS |
3116 | terminate the loop. Again, C<undef> is returned only once. So if |
3117 | you're expecting a single value from a glob, it is much better to | |
3118 | say | |
4633a7c4 LW |
3119 | |
3120 | ($file) = <blurch*>; | |
3121 | ||
3122 | than | |
3123 | ||
3124 | $file = <blurch*>; | |
3125 | ||
3126 | because the latter will alternate between returning a filename and | |
19799a22 | 3127 | returning false. |
4633a7c4 | 3128 | |
b159ebd3 | 3129 | If you're trying to do variable interpolation, it's definitely better |
4633a7c4 | 3130 | to use the glob() function, because the older notation can cause people |
e37d713d | 3131 | to become confused with the indirect filehandle notation. |
4633a7c4 LW |
3132 | |
3133 | @files = glob("$dir/*.[ch]"); | |
3134 | @files = glob($files[$i]); | |
3135 | ||
a0d0e21e | 3136 | =head2 Constant Folding |
d74e8afc | 3137 | X<constant folding> X<folding> |
a0d0e21e LW |
3138 | |
3139 | Like C, Perl does a certain amount of expression evaluation at | |
19799a22 | 3140 | compile time whenever it determines that all arguments to an |
a0d0e21e LW |
3141 | operator are static and have no side effects. In particular, string |
3142 | concatenation happens at compile time between literals that don't do | |
19799a22 | 3143 | variable substitution. Backslash interpolation also happens at |
a0d0e21e LW |
3144 | compile time. You can say |
3145 | ||
1ca345ed TC |
3146 | 'Now is the time for all' |
3147 | . "\n" | |
3148 | . 'good men to come to.' | |
a0d0e21e | 3149 | |
54310121 | 3150 | and this all reduces to one string internally. Likewise, if |
a0d0e21e LW |
3151 | you say |
3152 | ||
3153 | foreach $file (@filenames) { | |
5a964f20 | 3154 | if (-s $file > 5 + 100 * 2**16) { } |
54310121 | 3155 | } |
a0d0e21e | 3156 | |
1ca345ed | 3157 | the compiler precomputes the number which that expression |
19799a22 | 3158 | represents so that the interpreter won't have to. |
a0d0e21e | 3159 | |
fd1abbef | 3160 | =head2 No-ops |
d74e8afc | 3161 | X<no-op> X<nop> |
fd1abbef DN |
3162 | |
3163 | Perl doesn't officially have a no-op operator, but the bare constants | |
1ca345ed | 3164 | C<0> and C<1> are special-cased not to produce a warning in void |
fd1abbef DN |
3165 | context, so you can for example safely do |
3166 | ||
3167 | 1 while foo(); | |
3168 | ||
2c268ad5 | 3169 | =head2 Bitwise String Operators |
fb7054ba | 3170 | X<operator, bitwise, string> X<&.> X<|.> X<^.> X<~.> |
2c268ad5 TP |
3171 | |
3172 | Bitstrings of any size may be manipulated by the bitwise operators | |
3173 | (C<~ | & ^>). | |
3174 | ||
19799a22 GS |
3175 | If the operands to a binary bitwise op are strings of different |
3176 | sizes, B<|> and B<^> ops act as though the shorter operand had | |
3177 | additional zero bits on the right, while the B<&> op acts as though | |
3178 | the longer operand were truncated to the length of the shorter. | |
3179 | The granularity for such extension or truncation is one or more | |
3180 | bytes. | |
2c268ad5 | 3181 | |
89d205f2 | 3182 | # ASCII-based examples |
2c268ad5 TP |
3183 | print "j p \n" ^ " a h"; # prints "JAPH\n" |
3184 | print "JA" | " ph\n"; # prints "japh\n" | |
3185 | print "japh\nJunk" & '_____'; # prints "JAPH\n"; | |
3186 | print 'p N$' ^ " E<H\n"; # prints "Perl\n"; | |
3187 | ||
19799a22 | 3188 | If you are intending to manipulate bitstrings, be certain that |
2c268ad5 | 3189 | you're supplying bitstrings: If an operand is a number, that will imply |
19799a22 | 3190 | a B<numeric> bitwise operation. You may explicitly show which type of |
2c268ad5 TP |
3191 | operation you intend by using C<""> or C<0+>, as in the examples below. |
3192 | ||
4358a253 SS |
3193 | $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF) |
3194 | $foo = '150' | 105; # yields 255 | |
2c268ad5 TP |
3195 | $foo = 150 | '105'; # yields 255 |
3196 | $foo = '150' | '105'; # yields string '155' (under ASCII) | |
3197 | ||
3198 | $baz = 0+$foo & 0+$bar; # both ops explicitly numeric | |
3199 | $biz = "$foo" ^ "$bar"; # both ops explicitly stringy | |
a0d0e21e | 3200 | |
fb7054ba FC |
3201 | This somewhat unpredictable behavior can be avoided with the experimental |
3202 | "bitwise" feature, new in Perl 5.22. You can enable it via C<use feature | |
3203 | 'bitwise'>. By default, it will warn unless the "experimental::bitwise" | |
3204 | warnings category has been disabled. (C<use experimental 'bitwise'> will | |
3205 | enable the feature and disable the warning.) Under this feature, the four | |
3206 | standard bitwise operators (C<~ | & ^>) are always numeric. Adding a dot | |
3207 | after each operator (C<~. |. &. ^.>) forces it to treat its operands as | |
3208 | strings: | |
3209 | ||
3210 | use experimental "bitwise"; | |
3211 | $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF) | |
3212 | $foo = '150' | 105; # yields 255 | |
3213 | $foo = 150 | '105'; # yields 255 | |
3214 | $foo = '150' | '105'; # yields 255 | |
3215 | $foo = 150 |. 105; # yields string '155' (under ASCII) | |
3216 | $foo = '150' |. 105; # yields string '155' | |
3217 | $foo = 150 |.'105'; # yields string '155' | |
3218 | $foo = '150' |.'105'; # yields string '155' | |
3219 | ||
3220 | $baz = $foo & $bar; # both operands numeric | |
3221 | $biz = $foo ^. $bar; # both operands stringy | |
3222 | ||
3223 | The assignment variants of these operators (C<&= |= ^= &.= |.= ^.=>) | |
3224 | behave likewise under the feature. | |
3225 | ||
1ae175c8 GS |
3226 | See L<perlfunc/vec> for information on how to manipulate individual bits |
3227 | in a bit vector. | |
3228 | ||
55497cff | 3229 | =head2 Integer Arithmetic |
d74e8afc | 3230 | X<integer> |
a0d0e21e | 3231 | |
19799a22 | 3232 | By default, Perl assumes that it must do most of its arithmetic in |
a0d0e21e LW |
3233 | floating point. But by saying |
3234 | ||
3235 | use integer; | |
3236 | ||
3eab78e3 CW |
3237 | you may tell the compiler to use integer operations |
3238 | (see L<integer> for a detailed explanation) from here to the end of | |
3239 | the enclosing BLOCK. An inner BLOCK may countermand this by saying | |
a0d0e21e LW |
3240 | |
3241 | no integer; | |
3242 | ||
19799a22 | 3243 | which lasts until the end of that BLOCK. Note that this doesn't |
3eab78e3 CW |
3244 | mean everything is an integer, merely that Perl will use integer |
3245 | operations for arithmetic, comparison, and bitwise operators. For | |
3246 | example, even under C<use integer>, if you take the C<sqrt(2)>, you'll | |
3247 | still get C<1.4142135623731> or so. | |
19799a22 GS |
3248 | |
3249 | Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", | |
89d205f2 | 3250 | and ">>") always produce integral results. (But see also |
13a2d996 | 3251 | L<Bitwise String Operators>.) However, C<use integer> still has meaning for |
19799a22 GS |
3252 | them. By default, their results are interpreted as unsigned integers, but |
3253 | if C<use integer> is in effect, their results are interpreted | |
3254 | as signed integers. For example, C<~0> usually evaluates to a large | |
0be96356 | 3255 | integral value. However, C<use integer; ~0> is C<-1> on two's-complement |
19799a22 | 3256 | machines. |
68dc0745 | 3257 | |
3258 | =head2 Floating-point Arithmetic | |
06ce2fa3 | 3259 | |
d74e8afc | 3260 | X<floating-point> X<floating point> X<float> X<real> |
68dc0745 | 3261 | |
3262 | While C<use integer> provides integer-only arithmetic, there is no | |
19799a22 GS |
3263 | analogous mechanism to provide automatic rounding or truncation to a |
3264 | certain number of decimal places. For rounding to a certain number | |
3265 | of digits, sprintf() or printf() is usually the easiest route. | |
3266 | See L<perlfaq4>. | |
68dc0745 | 3267 | |
5a964f20 TC |
3268 | Floating-point numbers are only approximations to what a mathematician |
3269 | would call real numbers. There are infinitely more reals than floats, | |
3270 | so some corners must be cut. For example: | |
3271 | ||
3272 | printf "%.20g\n", 123456789123456789; | |
3273 | # produces 123456789123456784 | |
3274 | ||
8548cb57 RGS |
3275 | Testing for exact floating-point equality or inequality is not a |
3276 | good idea. Here's a (relatively expensive) work-around to compare | |
5a964f20 TC |
3277 | whether two floating-point numbers are equal to a particular number of |
3278 | decimal places. See Knuth, volume II, for a more robust treatment of | |
3279 | this topic. | |
3280 | ||
3281 | sub fp_equal { | |
3282 | my ($X, $Y, $POINTS) = @_; | |
3283 | my ($tX, $tY); | |
3284 | $tX = sprintf("%.${POINTS}g", $X); | |
3285 | $tY = sprintf("%.${POINTS}g", $Y); | |
3286 | return $tX eq $tY; | |
3287 | } | |
3288 | ||
68dc0745 | 3289 | The POSIX module (part of the standard perl distribution) implements |
19799a22 GS |
3290 | ceil(), floor(), and other mathematical and trigonometric functions. |
3291 | The Math::Complex module (part of the standard perl distribution) | |
3292 | defines mathematical functions that work on both the reals and the | |
3293 | imaginary numbers. Math::Complex not as efficient as POSIX, but | |
68dc0745 | 3294 | POSIX can't work with complex numbers. |
3295 | ||
3296 | Rounding in financial applications can have serious implications, and | |
3297 | the rounding method used should be specified precisely. In these | |
3298 | cases, it probably pays not to trust whichever system rounding is | |
3299 | being used by Perl, but to instead implement the rounding function you | |
3300 | need yourself. | |
5a964f20 TC |
3301 | |
3302 | =head2 Bigger Numbers | |
d74e8afc | 3303 | X<number, arbitrary precision> |
5a964f20 | 3304 | |
c543c01b | 3305 | The standard C<Math::BigInt>, C<Math::BigRat>, and C<Math::BigFloat> modules, |
fb1a95c6 | 3306 | along with the C<bignum>, C<bigint>, and C<bigrat> pragmas, provide |
19799a22 | 3307 | variable-precision arithmetic and overloaded operators, although |
46f8a5ea | 3308 | they're currently pretty slow. At the cost of some space and |
19799a22 GS |
3309 | considerable speed, they avoid the normal pitfalls associated with |
3310 | limited-precision representations. | |
5a964f20 | 3311 | |
c543c01b TC |
3312 | use 5.010; |
3313 | use bigint; # easy interface to Math::BigInt | |
3314 | $x = 123456789123456789; | |
3315 | say $x * $x; | |
3316 | +15241578780673678515622620750190521 | |
3317 | ||
3318 | Or with rationals: | |
3319 | ||
db691027 SF |
3320 | use 5.010; |
3321 | use bigrat; | |
3322 | $x = 3/22; | |
3323 | $y = 4/6; | |
3324 | say "x/y is ", $x/$y; | |
3325 | say "x*y is ", $x*$y; | |
3326 | x/y is 9/44 | |
3327 | x*y is 1/11 | |
c543c01b TC |
3328 | |
3329 | Several modules let you calculate with (bound only by memory and CPU time) | |
46f8a5ea FC |
3330 | unlimited or fixed precision. There |
3331 | are also some non-standard modules that | |
c543c01b | 3332 | provide faster implementations via external C libraries. |
cd5c4fce T |
3333 | |
3334 | Here is a short, but incomplete summary: | |
3335 | ||
950b09ed KW |
3336 | Math::String treat string sequences like numbers |
3337 | Math::FixedPrecision calculate with a fixed precision | |
3338 | Math::Currency for currency calculations | |
3339 | Bit::Vector manipulate bit vectors fast (uses C) | |
3340 | Math::BigIntFast Bit::Vector wrapper for big numbers | |
3341 | Math::Pari provides access to the Pari C library | |
70c45be3 FC |
3342 | Math::Cephes uses the external Cephes C library (no |
3343 | big numbers) | |
950b09ed KW |
3344 | Math::Cephes::Fraction fractions via the Cephes library |
3345 | Math::GMP another one using an external C library | |
70c45be3 FC |
3346 | Math::GMPz an alternative interface to libgmp's big ints |
3347 | Math::GMPq an interface to libgmp's fraction numbers | |
3348 | Math::GMPf an interface to libgmp's floating point numbers | |
cd5c4fce T |
3349 | |
3350 | Choose wisely. | |
16070b82 GS |
3351 | |
3352 | =cut |