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 | ||
a0d0e21e LW |
249 | Unary "+" has no effect whatsoever, even on strings. It is useful |
250 | syntactically for separating a function name from a parenthesized expression | |
251 | that would otherwise be interpreted as the complete list of function | |
5ba421f6 | 252 | arguments. (See examples above under L<Terms and List Operators (Leftward)>.) |
d74e8afc | 253 | X<+> |
a0d0e21e | 254 | |
19799a22 GS |
255 | Unary "\" creates a reference to whatever follows it. See L<perlreftut> |
256 | and L<perlref>. Do not confuse this behavior with the behavior of | |
257 | backslash within a string, although both forms do convey the notion | |
258 | of protecting the next thing from interpolation. | |
d74e8afc | 259 | X<\> X<reference> X<backslash> |
a0d0e21e LW |
260 | |
261 | =head2 Binding Operators | |
d74e8afc | 262 | X<binding> X<operator, binding> X<=~> X<!~> |
a0d0e21e | 263 | |
c07a80fd | 264 | Binary "=~" binds a scalar expression to a pattern match. Certain operations |
cb1a09d0 AD |
265 | search or modify the string $_ by default. This operator makes that kind |
266 | of operation work on some other string. The right argument is a search | |
2c268ad5 TP |
267 | pattern, substitution, or transliteration. The left argument is what is |
268 | supposed to be searched, substituted, or transliterated instead of the default | |
f8bab1e9 | 269 | $_. When used in scalar context, the return value generally indicates the |
8ff32507 FC |
270 | success of the operation. The exceptions are substitution (s///) |
271 | and transliteration (y///) with the C</r> (non-destructive) option, | |
272 | which cause the B<r>eturn value to be the result of the substitution. | |
273 | Behavior in list context depends on the particular operator. | |
000c65fc DG |
274 | See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for |
275 | examples using these operators. | |
f8bab1e9 GS |
276 | |
277 | If the right argument is an expression rather than a search pattern, | |
2c268ad5 | 278 | substitution, or transliteration, it is interpreted as a search pattern at run |
46f8a5ea FC |
279 | time. Note that this means that its |
280 | contents will be interpolated twice, so | |
89d205f2 | 281 | |
1ca345ed | 282 | '\\' =~ q'\\'; |
89d205f2 YO |
283 | |
284 | is not ok, as the regex engine will end up trying to compile the | |
285 | pattern C<\>, which it will consider a syntax error. | |
a0d0e21e LW |
286 | |
287 | Binary "!~" is just like "=~" except the return value is negated in | |
288 | the logical sense. | |
289 | ||
8ff32507 FC |
290 | Binary "!~" with a non-destructive substitution (s///r) or transliteration |
291 | (y///r) is a syntax error. | |
4f4d7508 | 292 | |
a0d0e21e | 293 | =head2 Multiplicative Operators |
d74e8afc | 294 | X<operator, multiplicative> |
a0d0e21e LW |
295 | |
296 | Binary "*" multiplies two numbers. | |
d74e8afc | 297 | X<*> |
a0d0e21e LW |
298 | |
299 | Binary "/" divides two numbers. | |
d74e8afc | 300 | X</> X<slash> |
a0d0e21e | 301 | |
f7918450 KW |
302 | Binary "%" is the modulo operator, which computes the division |
303 | remainder of its first argument with respect to its second argument. | |
304 | Given integer | |
db691027 SF |
305 | operands C<$m> and C<$n>: If C<$n> is positive, then C<$m % $n> is |
306 | C<$m> minus the largest multiple of C<$n> less than or equal to | |
307 | C<$m>. If C<$n> is negative, then C<$m % $n> is C<$m> minus the | |
308 | smallest multiple of C<$n> that is not less than C<$m> (that is, the | |
89b4f0ad | 309 | result will be less than or equal to zero). If the operands |
db691027 SF |
310 | C<$m> and C<$n> are floating point values and the absolute value of |
311 | C<$n> (that is C<abs($n)>) is less than C<(UV_MAX + 1)>, only | |
312 | the integer portion of C<$m> and C<$n> will be used in the operation | |
4848a83b | 313 | (Note: here C<UV_MAX> means the maximum of the unsigned integer type). |
db691027 | 314 | If the absolute value of the right operand (C<abs($n)>) is greater than |
4848a83b | 315 | or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder |
db691027 | 316 | C<$r> in the equation C<($r = $m - $i*$n)> where C<$i> is a certain |
f7918450 | 317 | integer that makes C<$r> have the same sign as the right operand |
db691027 SF |
318 | C<$n> (B<not> as the left operand C<$m> like C function C<fmod()>) |
319 | and the absolute value less than that of C<$n>. | |
0412d526 | 320 | Note that when C<use integer> is in scope, "%" gives you direct access |
f7918450 | 321 | to the modulo operator as implemented by your C compiler. This |
55d729e4 GS |
322 | operator is not as well defined for negative operands, but it will |
323 | execute faster. | |
f7918450 | 324 | X<%> X<remainder> X<modulo> X<mod> |
55d729e4 | 325 | |
62d10b70 GS |
326 | Binary "x" is the repetition operator. In scalar context or if the left |
327 | operand is not enclosed in parentheses, it returns a string consisting | |
328 | of the left operand repeated the number of times specified by the right | |
329 | operand. In list context, if the left operand is enclosed in | |
3585017f | 330 | parentheses or is a list formed by C<qw/STRING/>, it repeats the list. |
31201a8e KW |
331 | If the right operand is zero or negative (raising a warning on |
332 | negative), it returns an empty string | |
3585017f | 333 | or an empty list, depending on the context. |
d74e8afc | 334 | X<x> |
a0d0e21e LW |
335 | |
336 | print '-' x 80; # print row of dashes | |
337 | ||
338 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over | |
339 | ||
340 | @ones = (1) x 80; # a list of 80 1's | |
341 | @ones = (5) x @ones; # set all elements to 5 | |
342 | ||
343 | ||
344 | =head2 Additive Operators | |
d74e8afc | 345 | X<operator, additive> |
a0d0e21e | 346 | |
1ca345ed | 347 | Binary C<+> returns the sum of two numbers. |
d74e8afc | 348 | X<+> |
a0d0e21e | 349 | |
1ca345ed | 350 | Binary C<-> returns the difference of two numbers. |
d74e8afc | 351 | X<-> |
a0d0e21e | 352 | |
1ca345ed | 353 | Binary C<.> concatenates two strings. |
d74e8afc ITB |
354 | X<string, concatenation> X<concatenation> |
355 | X<cat> X<concat> X<concatenate> X<.> | |
a0d0e21e LW |
356 | |
357 | =head2 Shift Operators | |
d74e8afc ITB |
358 | X<shift operator> X<operator, shift> X<<< << >>> |
359 | X<<< >> >>> X<right shift> X<left shift> X<bitwise shift> | |
360 | X<shl> X<shr> X<shift, right> X<shift, left> | |
a0d0e21e | 361 | |
1ca345ed | 362 | Binary C<<< << >>> returns the value of its left argument shifted left by the |
55497cff | 363 | number of bits specified by the right argument. Arguments should be |
982ce180 | 364 | integers. (See also L<Integer Arithmetic>.) |
a0d0e21e | 365 | |
1ca345ed | 366 | Binary C<<< >> >>> returns the value of its left argument shifted right by |
55497cff | 367 | the number of bits specified by the right argument. Arguments should |
982ce180 | 368 | be integers. (See also L<Integer Arithmetic>.) |
a0d0e21e | 369 | |
1ca345ed TC |
370 | Note that both C<<< << >>> and C<<< >> >>> in Perl are implemented directly using |
371 | C<<< << >>> and C<<< >> >>> in C. If C<use integer> (see L<Integer Arithmetic>) is | |
b16cf6df JH |
372 | in force then signed C integers are used, else unsigned C integers are |
373 | used. Either way, the implementation isn't going to generate results | |
374 | larger than the size of the integer type Perl was built with (32 bits | |
375 | or 64 bits). | |
376 | ||
377 | The result of overflowing the range of the integers is undefined | |
378 | because it is undefined also in C. In other words, using 32-bit | |
379 | integers, C<< 1 << 32 >> is undefined. Shifting by a negative number | |
380 | of bits is also undefined. | |
381 | ||
1ca345ed TC |
382 | If you get tired of being subject to your platform's native integers, |
383 | the C<use bigint> pragma neatly sidesteps the issue altogether: | |
384 | ||
385 | print 20 << 20; # 20971520 | |
386 | print 20 << 40; # 5120 on 32-bit machines, | |
387 | # 21990232555520 on 64-bit machines | |
388 | use bigint; | |
389 | print 20 << 100; # 25353012004564588029934064107520 | |
390 | ||
a0d0e21e | 391 | =head2 Named Unary Operators |
d74e8afc | 392 | X<operator, named unary> |
a0d0e21e LW |
393 | |
394 | The various named unary operators are treated as functions with one | |
568e6d8b | 395 | argument, with optional parentheses. |
a0d0e21e LW |
396 | |
397 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
398 | is followed by a left parenthesis as the next token, the operator and | |
399 | arguments within parentheses are taken to be of highest precedence, | |
3981b0eb | 400 | just like a normal function call. For example, |
1ca345ed | 401 | because named unary operators are higher precedence than C<||>: |
a0d0e21e LW |
402 | |
403 | chdir $foo || die; # (chdir $foo) || die | |
404 | chdir($foo) || die; # (chdir $foo) || die | |
405 | chdir ($foo) || die; # (chdir $foo) || die | |
406 | chdir +($foo) || die; # (chdir $foo) || die | |
407 | ||
3981b0eb | 408 | but, because * is higher precedence than named operators: |
a0d0e21e LW |
409 | |
410 | chdir $foo * 20; # chdir ($foo * 20) | |
411 | chdir($foo) * 20; # (chdir $foo) * 20 | |
412 | chdir ($foo) * 20; # (chdir $foo) * 20 | |
413 | chdir +($foo) * 20; # chdir ($foo * 20) | |
414 | ||
415 | rand 10 * 20; # rand (10 * 20) | |
416 | rand(10) * 20; # (rand 10) * 20 | |
417 | rand (10) * 20; # (rand 10) * 20 | |
418 | rand +(10) * 20; # rand (10 * 20) | |
419 | ||
568e6d8b RGS |
420 | Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are |
421 | treated like named unary operators, but they don't follow this functional | |
422 | parenthesis rule. That means, for example, that C<-f($file).".bak"> is | |
423 | equivalent to C<-f "$file.bak">. | |
d74e8afc | 424 | X<-X> X<filetest> X<operator, filetest> |
568e6d8b | 425 | |
5ba421f6 | 426 | See also L<"Terms and List Operators (Leftward)">. |
a0d0e21e LW |
427 | |
428 | =head2 Relational Operators | |
d74e8afc | 429 | X<relational operator> X<operator, relational> |
a0d0e21e | 430 | |
1ca345ed TC |
431 | Perl operators that return true or false generally return values |
432 | that can be safely used as numbers. For example, the relational | |
433 | operators in this section and the equality operators in the next | |
434 | one return C<1> for true and a special version of the defined empty | |
435 | string, C<"">, which counts as a zero but is exempt from warnings | |
436 | about improper numeric conversions, just as C<"0 but true"> is. | |
437 | ||
35f2feb0 | 438 | Binary "<" returns true if the left argument is numerically less than |
a0d0e21e | 439 | the right argument. |
d74e8afc | 440 | X<< < >> |
a0d0e21e | 441 | |
35f2feb0 | 442 | Binary ">" returns true if the left argument is numerically greater |
a0d0e21e | 443 | than the right argument. |
d74e8afc | 444 | X<< > >> |
a0d0e21e | 445 | |
35f2feb0 | 446 | Binary "<=" returns true if the left argument is numerically less than |
a0d0e21e | 447 | or equal to the right argument. |
d74e8afc | 448 | X<< <= >> |
a0d0e21e | 449 | |
35f2feb0 | 450 | Binary ">=" returns true if the left argument is numerically greater |
a0d0e21e | 451 | than or equal to the right argument. |
d74e8afc | 452 | X<< >= >> |
a0d0e21e LW |
453 | |
454 | Binary "lt" returns true if the left argument is stringwise less than | |
455 | the right argument. | |
d74e8afc | 456 | X<< lt >> |
a0d0e21e LW |
457 | |
458 | Binary "gt" returns true if the left argument is stringwise greater | |
459 | than the right argument. | |
d74e8afc | 460 | X<< gt >> |
a0d0e21e LW |
461 | |
462 | Binary "le" returns true if the left argument is stringwise less than | |
463 | or equal to the right argument. | |
d74e8afc | 464 | X<< le >> |
a0d0e21e LW |
465 | |
466 | Binary "ge" returns true if the left argument is stringwise greater | |
467 | than or equal to the right argument. | |
d74e8afc | 468 | X<< ge >> |
a0d0e21e LW |
469 | |
470 | =head2 Equality Operators | |
d74e8afc | 471 | X<equality> X<equal> X<equals> X<operator, equality> |
a0d0e21e LW |
472 | |
473 | Binary "==" returns true if the left argument is numerically equal to | |
474 | the right argument. | |
d74e8afc | 475 | X<==> |
a0d0e21e LW |
476 | |
477 | Binary "!=" returns true if the left argument is numerically not equal | |
478 | to the right argument. | |
d74e8afc | 479 | X<!=> |
a0d0e21e | 480 | |
35f2feb0 | 481 | Binary "<=>" returns -1, 0, or 1 depending on whether the left |
6ee5d4e7 | 482 | argument is numerically less than, equal to, or greater than the right |
d4ad863d | 483 | argument. If your platform supports NaNs (not-a-numbers) as numeric |
7d3a9d88 | 484 | values, using them with "<=>" returns undef. NaN is not "<", "==", ">", |
46f8a5ea FC |
485 | "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN |
486 | returns true, as does NaN != anything else. If your platform doesn't | |
7d3a9d88 | 487 | support NaNs then NaN is just a string with numeric value 0. |
d74e8afc | 488 | X<< <=> >> X<spaceship> |
7d3a9d88 | 489 | |
db691027 SF |
490 | $ perl -le '$x = "NaN"; print "No NaN support here" if $x == $x' |
491 | $ perl -le '$x = "NaN"; print "NaN support here" if $x != $x' | |
1ca345ed | 492 | |
db691027 | 493 | (Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all |
1ca345ed | 494 | support "NaN".) |
a0d0e21e LW |
495 | |
496 | Binary "eq" returns true if the left argument is stringwise equal to | |
497 | the right argument. | |
d74e8afc | 498 | X<eq> |
a0d0e21e LW |
499 | |
500 | Binary "ne" returns true if the left argument is stringwise not equal | |
501 | to the right argument. | |
d74e8afc | 502 | X<ne> |
a0d0e21e | 503 | |
d4ad863d JH |
504 | Binary "cmp" returns -1, 0, or 1 depending on whether the left |
505 | argument is stringwise less than, equal to, or greater than the right | |
506 | argument. | |
d74e8afc | 507 | X<cmp> |
a0d0e21e | 508 | |
1ca345ed TC |
509 | Binary "~~" does a smartmatch between its arguments. Smart matching |
510 | is described in the next section. | |
0d863452 RH |
511 | X<~~> |
512 | ||
a034a98d | 513 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
66cbab2c KW |
514 | by the current locale if a legacy C<use locale> (but not |
515 | C<use locale ':not_characters'>) is in effect. See | |
1ca345ed TC |
516 | L<perllocale>. Do not mix these with Unicode, only with legacy binary |
517 | encodings. The standard L<Unicode::Collate> and | |
518 | L<Unicode::Collate::Locale> modules offer much more powerful solutions to | |
519 | collation issues. | |
520 | ||
521 | =head2 Smartmatch Operator | |
522 | ||
523 | First available in Perl 5.10.1 (the 5.10.0 version behaved differently), | |
524 | binary C<~~> does a "smartmatch" between its arguments. This is mostly | |
525 | used implicitly in the C<when> construct described in L<perlsyn>, although | |
526 | not all C<when> clauses call the smartmatch operator. Unique among all of | |
cc08d69f RS |
527 | Perl's operators, the smartmatch operator can recurse. The smartmatch |
528 | operator is L<experimental|perlpolicy/experimental> and its behavior is | |
529 | subject to change. | |
1ca345ed TC |
530 | |
531 | It is also unique in that all other Perl operators impose a context | |
532 | (usually string or numeric context) on their operands, autoconverting | |
533 | those operands to those imposed contexts. In contrast, smartmatch | |
534 | I<infers> contexts from the actual types of its operands and uses that | |
535 | type information to select a suitable comparison mechanism. | |
536 | ||
537 | The C<~~> operator compares its operands "polymorphically", determining how | |
538 | to compare them according to their actual types (numeric, string, array, | |
539 | hash, etc.) Like the equality operators with which it shares the same | |
540 | precedence, C<~~> returns 1 for true and C<""> for false. It is often best | |
541 | read aloud as "in", "inside of", or "is contained in", because the left | |
542 | operand is often looked for I<inside> the right operand. That makes the | |
40bec8a5 | 543 | order of the operands to the smartmatch operand often opposite that of |
1ca345ed TC |
544 | the regular match operator. In other words, the "smaller" thing is usually |
545 | placed in the left operand and the larger one in the right. | |
546 | ||
547 | The behavior of a smartmatch depends on what type of things its arguments | |
548 | are, as determined by the following table. The first row of the table | |
549 | whose types apply determines the smartmatch behavior. Because what | |
550 | actually happens is mostly determined by the type of the second operand, | |
551 | the table is sorted on the right operand instead of on the left. | |
552 | ||
553 | Left Right Description and pseudocode | |
554 | =============================================================== | |
555 | Any undef check whether Any is undefined | |
556 | like: !defined Any | |
557 | ||
558 | Any Object invoke ~~ overloading on Object, or die | |
559 | ||
560 | Right operand is an ARRAY: | |
561 | ||
562 | Left Right Description and pseudocode | |
563 | =============================================================== | |
564 | ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2] | |
565 | like: (ARRAY1[0] ~~ ARRAY2[0]) | |
566 | && (ARRAY1[1] ~~ ARRAY2[1]) && ... | |
567 | HASH ARRAY any ARRAY elements exist as HASH keys | |
568 | like: grep { exists HASH->{$_} } ARRAY | |
569 | Regexp ARRAY any ARRAY elements pattern match Regexp | |
570 | like: grep { /Regexp/ } ARRAY | |
571 | undef ARRAY undef in ARRAY | |
572 | like: grep { !defined } ARRAY | |
40bec8a5 | 573 | Any ARRAY smartmatch each ARRAY element[3] |
1ca345ed TC |
574 | like: grep { Any ~~ $_ } ARRAY |
575 | ||
576 | Right operand is a HASH: | |
577 | ||
578 | Left Right Description and pseudocode | |
579 | =============================================================== | |
580 | HASH1 HASH2 all same keys in both HASHes | |
581 | like: keys HASH1 == | |
582 | grep { exists HASH2->{$_} } keys HASH1 | |
583 | ARRAY HASH any ARRAY elements exist as HASH keys | |
584 | like: grep { exists HASH->{$_} } ARRAY | |
585 | Regexp HASH any HASH keys pattern match Regexp | |
586 | like: grep { /Regexp/ } keys HASH | |
587 | undef HASH always false (undef can't be a key) | |
588 | like: 0 == 1 | |
589 | Any HASH HASH key existence | |
590 | like: exists HASH->{Any} | |
591 | ||
592 | Right operand is CODE: | |
f703fc96 | 593 | |
1ca345ed TC |
594 | Left Right Description and pseudocode |
595 | =============================================================== | |
596 | ARRAY CODE sub returns true on all ARRAY elements[1] | |
597 | like: !grep { !CODE->($_) } ARRAY | |
598 | HASH CODE sub returns true on all HASH keys[1] | |
599 | like: !grep { !CODE->($_) } keys HASH | |
600 | Any CODE sub passed Any returns true | |
601 | like: CODE->(Any) | |
602 | ||
603 | Right operand is a Regexp: | |
604 | ||
605 | Left Right Description and pseudocode | |
606 | =============================================================== | |
607 | ARRAY Regexp any ARRAY elements match Regexp | |
608 | like: grep { /Regexp/ } ARRAY | |
609 | HASH Regexp any HASH keys match Regexp | |
610 | like: grep { /Regexp/ } keys HASH | |
611 | Any Regexp pattern match | |
612 | like: Any =~ /Regexp/ | |
613 | ||
614 | Other: | |
615 | ||
616 | Left Right Description and pseudocode | |
617 | =============================================================== | |
618 | Object Any invoke ~~ overloading on Object, | |
619 | or fall back to... | |
620 | ||
621 | Any Num numeric equality | |
622 | like: Any == Num | |
623 | Num nummy[4] numeric equality | |
624 | like: Num == nummy | |
625 | undef Any check whether undefined | |
626 | like: !defined(Any) | |
627 | Any Any string equality | |
628 | like: Any eq Any | |
629 | ||
630 | ||
631 | Notes: | |
632 | ||
633 | =over | |
634 | ||
635 | =item 1. | |
636 | Empty hashes or arrays match. | |
637 | ||
638 | =item 2. | |
40bec8a5 | 639 | That is, each element smartmatches the element of the same index in the other array.[3] |
1ca345ed TC |
640 | |
641 | =item 3. | |
642 | If a circular reference is found, fall back to referential equality. | |
643 | ||
644 | =item 4. | |
645 | Either an actual number, or a string that looks like one. | |
646 | ||
647 | =back | |
648 | ||
649 | The smartmatch implicitly dereferences any non-blessed hash or array | |
650 | reference, so the C<I<HASH>> and C<I<ARRAY>> entries apply in those cases. | |
651 | For blessed references, the C<I<Object>> entries apply. Smartmatches | |
652 | involving hashes only consider hash keys, never hash values. | |
653 | ||
654 | The "like" code entry is not always an exact rendition. For example, the | |
40bec8a5 | 655 | smartmatch operator short-circuits whenever possible, but C<grep> does |
1ca345ed TC |
656 | not. Also, C<grep> in scalar context returns the number of matches, but |
657 | C<~~> returns only true or false. | |
658 | ||
659 | Unlike most operators, the smartmatch operator knows to treat C<undef> | |
660 | specially: | |
661 | ||
662 | use v5.10.1; | |
663 | @array = (1, 2, 3, undef, 4, 5); | |
664 | say "some elements undefined" if undef ~~ @array; | |
665 | ||
666 | Each operand is considered in a modified scalar context, the modification | |
667 | being that array and hash variables are passed by reference to the | |
668 | operator, which implicitly dereferences them. Both elements | |
669 | of each pair are the same: | |
670 | ||
671 | use v5.10.1; | |
672 | ||
673 | my %hash = (red => 1, blue => 2, green => 3, | |
674 | orange => 4, yellow => 5, purple => 6, | |
675 | black => 7, grey => 8, white => 9); | |
676 | ||
677 | my @array = qw(red blue green); | |
678 | ||
679 | say "some array elements in hash keys" if @array ~~ %hash; | |
680 | say "some array elements in hash keys" if \@array ~~ \%hash; | |
681 | ||
682 | say "red in array" if "red" ~~ @array; | |
683 | say "red in array" if "red" ~~ \@array; | |
684 | ||
685 | say "some keys end in e" if /e$/ ~~ %hash; | |
686 | say "some keys end in e" if /e$/ ~~ \%hash; | |
687 | ||
40bec8a5 TC |
688 | Two arrays smartmatch if each element in the first array smartmatches |
689 | (that is, is "in") the corresponding element in the second array, | |
690 | recursively. | |
1ca345ed TC |
691 | |
692 | use v5.10.1; | |
693 | my @little = qw(red blue green); | |
694 | my @bigger = ("red", "blue", [ "orange", "green" ] ); | |
695 | if (@little ~~ @bigger) { # true! | |
696 | say "little is contained in bigger"; | |
697 | } | |
698 | ||
699 | Because the smartmatch operator recurses on nested arrays, this | |
700 | will still report that "red" is in the array. | |
701 | ||
702 | use v5.10.1; | |
703 | my @array = qw(red blue green); | |
704 | my $nested_array = [[[[[[[ @array ]]]]]]]; | |
705 | say "red in array" if "red" ~~ $nested_array; | |
706 | ||
707 | If two arrays smartmatch each other, then they are deep | |
708 | copies of each others' values, as this example reports: | |
709 | ||
710 | use v5.12.0; | |
711 | my @a = (0, 1, 2, [3, [4, 5], 6], 7); | |
712 | my @b = (0, 1, 2, [3, [4, 5], 6], 7); | |
713 | ||
714 | if (@a ~~ @b && @b ~~ @a) { | |
715 | say "a and b are deep copies of each other"; | |
716 | } | |
717 | elsif (@a ~~ @b) { | |
718 | say "a smartmatches in b"; | |
719 | } | |
720 | elsif (@b ~~ @a) { | |
721 | say "b smartmatches in a"; | |
722 | } | |
723 | else { | |
724 | say "a and b don't smartmatch each other at all"; | |
725 | } | |
726 | ||
727 | ||
728 | If you were to set C<$b[3] = 4>, then instead of reporting that "a and b | |
729 | are deep copies of each other", it now reports that "b smartmatches in a". | |
730 | That because the corresponding position in C<@a> contains an array that | |
731 | (eventually) has a 4 in it. | |
732 | ||
733 | Smartmatching one hash against another reports whether both contain the | |
46f8a5ea | 734 | same keys, no more and no less. This could be used to see whether two |
1ca345ed TC |
735 | records have the same field names, without caring what values those fields |
736 | might have. For example: | |
737 | ||
738 | use v5.10.1; | |
739 | sub make_dogtag { | |
740 | state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 }; | |
741 | ||
742 | my ($class, $init_fields) = @_; | |
743 | ||
744 | die "Must supply (only) name, rank, and serial number" | |
745 | unless $init_fields ~~ $REQUIRED_FIELDS; | |
746 | ||
747 | ... | |
748 | } | |
749 | ||
750 | or, if other non-required fields are allowed, use ARRAY ~~ HASH: | |
751 | ||
752 | use v5.10.1; | |
753 | sub make_dogtag { | |
754 | state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 }; | |
755 | ||
756 | my ($class, $init_fields) = @_; | |
757 | ||
758 | die "Must supply (at least) name, rank, and serial number" | |
759 | unless [keys %{$init_fields}] ~~ $REQUIRED_FIELDS; | |
760 | ||
761 | ... | |
762 | } | |
763 | ||
764 | The smartmatch operator is most often used as the implicit operator of a | |
765 | C<when> clause. See the section on "Switch Statements" in L<perlsyn>. | |
766 | ||
767 | =head3 Smartmatching of Objects | |
768 | ||
40bec8a5 TC |
769 | To avoid relying on an object's underlying representation, if the |
770 | smartmatch's right operand is an object that doesn't overload C<~~>, | |
771 | it raises the exception "C<Smartmatching a non-overloaded object | |
46f8a5ea FC |
772 | breaks encapsulation>". That's because one has no business digging |
773 | around to see whether something is "in" an object. These are all | |
40bec8a5 | 774 | illegal on objects without a C<~~> overload: |
1ca345ed TC |
775 | |
776 | %hash ~~ $object | |
777 | 42 ~~ $object | |
778 | "fred" ~~ $object | |
779 | ||
780 | However, you can change the way an object is smartmatched by overloading | |
46f8a5ea FC |
781 | the C<~~> operator. This is allowed to |
782 | extend the usual smartmatch semantics. | |
1ca345ed TC |
783 | For objects that do have an C<~~> overload, see L<overload>. |
784 | ||
785 | Using an object as the left operand is allowed, although not very useful. | |
786 | Smartmatching rules take precedence over overloading, so even if the | |
787 | object in the left operand has smartmatch overloading, this will be | |
788 | ignored. A left operand that is a non-overloaded object falls back on a | |
789 | string or numeric comparison of whatever the C<ref> operator returns. That | |
790 | means that | |
791 | ||
792 | $object ~~ X | |
793 | ||
794 | does I<not> invoke the overload method with C<I<X>> as an argument. | |
795 | Instead the above table is consulted as normal, and based on the type of | |
796 | C<I<X>>, overloading may or may not be invoked. For simple strings or | |
797 | numbers, in becomes equivalent to this: | |
798 | ||
799 | $object ~~ $number ref($object) == $number | |
800 | $object ~~ $string ref($object) eq $string | |
801 | ||
802 | For example, this reports that the handle smells IOish | |
803 | (but please don't really do this!): | |
804 | ||
805 | use IO::Handle; | |
806 | my $fh = IO::Handle->new(); | |
807 | if ($fh ~~ /\bIO\b/) { | |
808 | say "handle smells IOish"; | |
809 | } | |
810 | ||
811 | That's because it treats C<$fh> as a string like | |
812 | C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that. | |
a034a98d | 813 | |
a0d0e21e | 814 | =head2 Bitwise And |
d74e8afc | 815 | X<operator, bitwise, and> X<bitwise and> X<&> |
a0d0e21e | 816 | |
c791a246 KW |
817 | Binary "&" returns its operands ANDed together bit by bit. Although no |
818 | warning is currently raised, the result is not well defined when this operation | |
819 | is performed on operands that aren't either numbers (see | |
820 | L<Integer Arithmetic>) or bitstrings (see L<Bitwise String Operators>). | |
a0d0e21e | 821 | |
2cdc098b | 822 | Note that "&" has lower priority than relational operators, so for example |
1ca345ed | 823 | the parentheses are essential in a test like |
2cdc098b | 824 | |
1ca345ed | 825 | print "Even\n" if ($x & 1) == 0; |
2cdc098b | 826 | |
a0d0e21e | 827 | =head2 Bitwise Or and Exclusive Or |
d74e8afc ITB |
828 | X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor> |
829 | X<bitwise xor> X<^> | |
a0d0e21e | 830 | |
2cdc098b | 831 | Binary "|" returns its operands ORed together bit by bit. |
a0d0e21e | 832 | |
2cdc098b | 833 | Binary "^" returns its operands XORed together bit by bit. |
c791a246 KW |
834 | |
835 | Although no warning is currently raised, the results are not well | |
836 | defined when these operations are performed on operands that aren't either | |
837 | numbers (see L<Integer Arithmetic>) or bitstrings (see L<Bitwise String | |
838 | Operators>). | |
a0d0e21e | 839 | |
2cdc098b MG |
840 | Note that "|" and "^" have lower priority than relational operators, so |
841 | for example the brackets are essential in a test like | |
842 | ||
1ca345ed | 843 | print "false\n" if (8 | 2) != 10; |
2cdc098b | 844 | |
a0d0e21e | 845 | =head2 C-style Logical And |
d74e8afc | 846 | X<&&> X<logical and> X<operator, logical, and> |
a0d0e21e LW |
847 | |
848 | Binary "&&" performs a short-circuit logical AND operation. That is, | |
849 | if the left operand is false, the right operand is not even evaluated. | |
850 | Scalar or list context propagates down to the right operand if it | |
851 | is evaluated. | |
852 | ||
853 | =head2 C-style Logical Or | |
d74e8afc | 854 | X<||> X<operator, logical, or> |
a0d0e21e LW |
855 | |
856 | Binary "||" performs a short-circuit logical OR operation. That is, | |
857 | if the left operand is true, the right operand is not even evaluated. | |
858 | Scalar or list context propagates down to the right operand if it | |
859 | is evaluated. | |
860 | ||
26d9d83b | 861 | =head2 Logical Defined-Or |
d74e8afc | 862 | X<//> X<operator, logical, defined-or> |
c963b151 BD |
863 | |
864 | Although it has no direct equivalent in C, Perl's C<//> operator is related | |
89d205f2 | 865 | to its C-style or. In fact, it's exactly the same as C<||>, except that it |
95bee9ba A |
866 | tests the left hand side's definedness instead of its truth. Thus, |
867 | C<< EXPR1 // EXPR2 >> returns the value of C<< EXPR1 >> if it's defined, | |
46f8a5ea FC |
868 | otherwise, the value of C<< EXPR2 >> is returned. |
869 | (C<< EXPR1 >> is evaluated in scalar context, C<< EXPR2 >> | |
870 | in the context of C<< // >> itself). Usually, | |
95bee9ba A |
871 | this is the same result as C<< defined(EXPR1) ? EXPR1 : EXPR2 >> (except that |
872 | the ternary-operator form can be used as a lvalue, while C<< EXPR1 // EXPR2 >> | |
46f8a5ea | 873 | cannot). This is very useful for |
bdc7923b | 874 | providing default values for variables. If you actually want to test if |
db691027 | 875 | at least one of C<$x> and C<$y> is defined, use C<defined($x // $y)>. |
c963b151 | 876 | |
d042e63d | 877 | The C<||>, C<//> and C<&&> operators return the last value evaluated |
46f8a5ea | 878 | (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably |
d042e63d | 879 | portable way to find out the home directory might be: |
a0d0e21e | 880 | |
c543c01b TC |
881 | $home = $ENV{HOME} |
882 | // $ENV{LOGDIR} | |
883 | // (getpwuid($<))[7] | |
884 | // die "You're homeless!\n"; | |
a0d0e21e | 885 | |
5a964f20 TC |
886 | In particular, this means that you shouldn't use this |
887 | for selecting between two aggregates for assignment: | |
888 | ||
889 | @a = @b || @c; # this is wrong | |
890 | @a = scalar(@b) || @c; # really meant this | |
891 | @a = @b ? @b : @c; # this works fine, though | |
892 | ||
1ca345ed | 893 | As alternatives to C<&&> and C<||> when used for |
f23102e2 RGS |
894 | control flow, Perl provides the C<and> and C<or> operators (see below). |
895 | The short-circuit behavior is identical. The precedence of "and" | |
c963b151 | 896 | and "or" is much lower, however, so that you can safely use them after a |
5a964f20 | 897 | list operator without the need for parentheses: |
a0d0e21e LW |
898 | |
899 | unlink "alpha", "beta", "gamma" | |
900 | or gripe(), next LINE; | |
901 | ||
902 | With the C-style operators that would have been written like this: | |
903 | ||
904 | unlink("alpha", "beta", "gamma") | |
905 | || (gripe(), next LINE); | |
906 | ||
1ca345ed TC |
907 | It would be even more readable to write that this way: |
908 | ||
909 | unless(unlink("alpha", "beta", "gamma")) { | |
910 | gripe(); | |
911 | next LINE; | |
912 | } | |
913 | ||
eeb6a2c9 | 914 | Using "or" for assignment is unlikely to do what you want; see below. |
5a964f20 TC |
915 | |
916 | =head2 Range Operators | |
d74e8afc | 917 | X<operator, range> X<range> X<..> X<...> |
a0d0e21e LW |
918 | |
919 | Binary ".." is the range operator, which is really two different | |
fb53bbb2 | 920 | operators depending on the context. In list context, it returns a |
54ae734e | 921 | list of values counting (up by ones) from the left value to the right |
2cdbc966 | 922 | value. If the left value is greater than the right value then it |
fb53bbb2 | 923 | returns the empty list. The range operator is useful for writing |
46f8a5ea | 924 | C<foreach (1..10)> loops and for doing slice operations on arrays. In |
2cdbc966 JD |
925 | the current implementation, no temporary array is created when the |
926 | range operator is used as the expression in C<foreach> loops, but older | |
927 | versions of Perl might burn a lot of memory when you write something | |
928 | like this: | |
a0d0e21e LW |
929 | |
930 | for (1 .. 1_000_000) { | |
931 | # code | |
54310121 | 932 | } |
a0d0e21e | 933 | |
8f0f46f8 | 934 | The range operator also works on strings, using the magical |
935 | auto-increment, see below. | |
54ae734e | 936 | |
5a964f20 | 937 | In scalar context, ".." returns a boolean value. The operator is |
8f0f46f8 | 938 | bistable, like a flip-flop, and emulates the line-range (comma) |
46f8a5ea | 939 | operator of B<sed>, B<awk>, and various editors. Each ".." operator |
8f0f46f8 | 940 | maintains its own boolean state, even across calls to a subroutine |
46f8a5ea | 941 | that contains it. It is false as long as its left operand is false. |
a0d0e21e LW |
942 | Once the left operand is true, the range operator stays true until the |
943 | right operand is true, I<AFTER> which the range operator becomes false | |
8f0f46f8 | 944 | again. It doesn't become false till the next time the range operator |
945 | is evaluated. It can test the right operand and become false on the | |
946 | same evaluation it became true (as in B<awk>), but it still returns | |
46f8a5ea | 947 | true once. If you don't want it to test the right operand until the |
8f0f46f8 | 948 | next evaluation, as in B<sed>, just use three dots ("...") instead of |
19799a22 GS |
949 | two. In all other regards, "..." behaves just like ".." does. |
950 | ||
951 | The right operand is not evaluated while the operator is in the | |
952 | "false" state, and the left operand is not evaluated while the | |
953 | operator is in the "true" state. The precedence is a little lower | |
954 | than || and &&. The value returned is either the empty string for | |
8f0f46f8 | 955 | false, or a sequence number (beginning with 1) for true. The sequence |
956 | number is reset for each range encountered. The final sequence number | |
957 | in a range has the string "E0" appended to it, which doesn't affect | |
958 | its numeric value, but gives you something to search for if you want | |
959 | to exclude the endpoint. You can exclude the beginning point by | |
960 | waiting for the sequence number to be greater than 1. | |
df5f8116 CW |
961 | |
962 | If either operand of scalar ".." is a constant expression, | |
963 | that operand is considered true if it is equal (C<==>) to the current | |
964 | input line number (the C<$.> variable). | |
965 | ||
966 | To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>, | |
967 | but that is only an issue if you use a floating point expression; when | |
968 | implicitly using C<$.> as described in the previous paragraph, the | |
969 | comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.> | |
970 | is set to a floating point value and you are not reading from a file. | |
971 | Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what | |
972 | you want in scalar context because each of the operands are evaluated | |
973 | using their integer representation. | |
974 | ||
975 | Examples: | |
a0d0e21e LW |
976 | |
977 | As a scalar operator: | |
978 | ||
df5f8116 | 979 | if (101 .. 200) { print; } # print 2nd hundred lines, short for |
950b09ed | 980 | # if ($. == 101 .. $. == 200) { print; } |
9f10b797 RGS |
981 | |
982 | next LINE if (1 .. /^$/); # skip header lines, short for | |
f343f960 | 983 | # next LINE if ($. == 1 .. /^$/); |
9f10b797 RGS |
984 | # (typically in a loop labeled LINE) |
985 | ||
986 | s/^/> / if (/^$/ .. eof()); # quote body | |
a0d0e21e | 987 | |
5a964f20 TC |
988 | # parse mail messages |
989 | while (<>) { | |
990 | $in_header = 1 .. /^$/; | |
df5f8116 CW |
991 | $in_body = /^$/ .. eof; |
992 | if ($in_header) { | |
f343f960 | 993 | # do something |
df5f8116 | 994 | } else { # in body |
f343f960 | 995 | # do something else |
df5f8116 | 996 | } |
5a964f20 | 997 | } continue { |
df5f8116 | 998 | close ARGV if eof; # reset $. each file |
5a964f20 TC |
999 | } |
1000 | ||
acf31ca5 SF |
1001 | Here's a simple example to illustrate the difference between |
1002 | the two range operators: | |
1003 | ||
1004 | @lines = (" - Foo", | |
1005 | "01 - Bar", | |
1006 | "1 - Baz", | |
1007 | " - Quux"); | |
1008 | ||
9f10b797 RGS |
1009 | foreach (@lines) { |
1010 | if (/0/ .. /1/) { | |
acf31ca5 SF |
1011 | print "$_\n"; |
1012 | } | |
1013 | } | |
1014 | ||
46f8a5ea | 1015 | This program will print only the line containing "Bar". If |
9f10b797 | 1016 | the range operator is changed to C<...>, it will also print the |
acf31ca5 SF |
1017 | "Baz" line. |
1018 | ||
1019 | And now some examples as a list operator: | |
a0d0e21e | 1020 | |
1ca345ed TC |
1021 | for (101 .. 200) { print } # print $_ 100 times |
1022 | @foo = @foo[0 .. $#foo]; # an expensive no-op | |
1023 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items | |
a0d0e21e | 1024 | |
5a964f20 | 1025 | The range operator (in list context) makes use of the magical |
5f05dabc | 1026 | auto-increment algorithm if the operands are strings. You |
a0d0e21e LW |
1027 | can say |
1028 | ||
c543c01b | 1029 | @alphabet = ("A" .. "Z"); |
a0d0e21e | 1030 | |
54ae734e | 1031 | to get all normal letters of the English alphabet, or |
a0d0e21e | 1032 | |
c543c01b | 1033 | $hexdigit = (0 .. 9, "a" .. "f")[$num & 15]; |
a0d0e21e LW |
1034 | |
1035 | to get a hexadecimal digit, or | |
1036 | ||
1ca345ed TC |
1037 | @z2 = ("01" .. "31"); |
1038 | print $z2[$mday]; | |
a0d0e21e | 1039 | |
ea4f5703 YST |
1040 | to get dates with leading zeros. |
1041 | ||
1042 | If the final value specified is not in the sequence that the magical | |
1043 | increment would produce, the sequence goes until the next value would | |
1044 | be longer than the final value specified. | |
1045 | ||
1046 | If the initial value specified isn't part of a magical increment | |
c543c01b | 1047 | sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>), |
ea4f5703 YST |
1048 | only the initial value will be returned. So the following will only |
1049 | return an alpha: | |
1050 | ||
c543c01b | 1051 | use charnames "greek"; |
ea4f5703 YST |
1052 | my @greek_small = ("\N{alpha}" .. "\N{omega}"); |
1053 | ||
c543c01b TC |
1054 | To get the 25 traditional lowercase Greek letters, including both sigmas, |
1055 | you could use this instead: | |
ea4f5703 | 1056 | |
c543c01b | 1057 | use charnames "greek"; |
1ca345ed TC |
1058 | my @greek_small = map { chr } ( ord("\N{alpha}") |
1059 | .. | |
1060 | ord("\N{omega}") | |
1061 | ); | |
c543c01b TC |
1062 | |
1063 | However, because there are I<many> other lowercase Greek characters than | |
1064 | just those, to match lowercase Greek characters in a regular expression, | |
47c56cc8 KW |
1065 | you could use the pattern C</(?:(?=\p{Greek})\p{Lower})+/> (or the |
1066 | L<experimental feature|perlrecharclass/Extended Bracketed Character | |
1067 | Classes> C<S</(?[ \p{Greek} & \p{Lower} ])+/>>). | |
a0d0e21e | 1068 | |
df5f8116 CW |
1069 | Because each operand is evaluated in integer form, C<2.18 .. 3.14> will |
1070 | return two elements in list context. | |
1071 | ||
1072 | @list = (2.18 .. 3.14); # same as @list = (2 .. 3); | |
1073 | ||
a0d0e21e | 1074 | =head2 Conditional Operator |
d74e8afc | 1075 | X<operator, conditional> X<operator, ternary> X<ternary> X<?:> |
a0d0e21e LW |
1076 | |
1077 | Ternary "?:" is the conditional operator, just as in C. It works much | |
1078 | like an if-then-else. If the argument before the ? is true, the | |
1079 | argument before the : is returned, otherwise the argument after the : | |
cb1a09d0 AD |
1080 | is returned. For example: |
1081 | ||
54310121 | 1082 | printf "I have %d dog%s.\n", $n, |
c543c01b | 1083 | ($n == 1) ? "" : "s"; |
cb1a09d0 AD |
1084 | |
1085 | Scalar or list context propagates downward into the 2nd | |
54310121 | 1086 | or 3rd argument, whichever is selected. |
cb1a09d0 | 1087 | |
db691027 SF |
1088 | $x = $ok ? $y : $z; # get a scalar |
1089 | @x = $ok ? @y : @z; # get an array | |
1090 | $x = $ok ? @y : @z; # oops, that's just a count! | |
cb1a09d0 AD |
1091 | |
1092 | The operator may be assigned to if both the 2nd and 3rd arguments are | |
1093 | legal lvalues (meaning that you can assign to them): | |
a0d0e21e | 1094 | |
db691027 | 1095 | ($x_or_y ? $x : $y) = $z; |
a0d0e21e | 1096 | |
5a964f20 TC |
1097 | Because this operator produces an assignable result, using assignments |
1098 | without parentheses will get you in trouble. For example, this: | |
1099 | ||
db691027 | 1100 | $x % 2 ? $x += 10 : $x += 2 |
5a964f20 TC |
1101 | |
1102 | Really means this: | |
1103 | ||
db691027 | 1104 | (($x % 2) ? ($x += 10) : $x) += 2 |
5a964f20 TC |
1105 | |
1106 | Rather than this: | |
1107 | ||
db691027 | 1108 | ($x % 2) ? ($x += 10) : ($x += 2) |
5a964f20 | 1109 | |
19799a22 GS |
1110 | That should probably be written more simply as: |
1111 | ||
db691027 | 1112 | $x += ($x % 2) ? 10 : 2; |
19799a22 | 1113 | |
4633a7c4 | 1114 | =head2 Assignment Operators |
d74e8afc | 1115 | X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=> |
5ac3b81c | 1116 | X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=> |
d74e8afc | 1117 | X<%=> X<^=> X<x=> |
a0d0e21e LW |
1118 | |
1119 | "=" is the ordinary assignment operator. | |
1120 | ||
1121 | Assignment operators work as in C. That is, | |
1122 | ||
db691027 | 1123 | $x += 2; |
a0d0e21e LW |
1124 | |
1125 | is equivalent to | |
1126 | ||
db691027 | 1127 | $x = $x + 2; |
a0d0e21e LW |
1128 | |
1129 | although without duplicating any side effects that dereferencing the lvalue | |
54310121 | 1130 | might trigger, such as from tie(). Other assignment operators work similarly. |
1131 | The following are recognized: | |
a0d0e21e LW |
1132 | |
1133 | **= += *= &= <<= &&= | |
9f10b797 RGS |
1134 | -= /= |= >>= ||= |
1135 | .= %= ^= //= | |
1136 | x= | |
a0d0e21e | 1137 | |
19799a22 | 1138 | Although these are grouped by family, they all have the precedence |
82848c10 FC |
1139 | of assignment. These combined assignment operators can only operate on |
1140 | scalars, whereas the ordinary assignment operator can assign to arrays, | |
1141 | hashes, lists and even references. (See L<"Context"|perldata/Context> | |
1142 | and L<perldata/List value constructors>, and L<perlref/Assigning to | |
1143 | References>.) | |
a0d0e21e | 1144 | |
b350dd2f GS |
1145 | Unlike in C, the scalar assignment operator produces a valid lvalue. |
1146 | Modifying an assignment is equivalent to doing the assignment and | |
1147 | then modifying the variable that was assigned to. This is useful | |
1148 | for modifying a copy of something, like this: | |
a0d0e21e | 1149 | |
1ca345ed TC |
1150 | ($tmp = $global) =~ tr/13579/24680/; |
1151 | ||
1152 | Although as of 5.14, that can be also be accomplished this way: | |
1153 | ||
1154 | use v5.14; | |
1155 | $tmp = ($global =~ tr/13579/24680/r); | |
a0d0e21e LW |
1156 | |
1157 | Likewise, | |
1158 | ||
db691027 | 1159 | ($x += 2) *= 3; |
a0d0e21e LW |
1160 | |
1161 | is equivalent to | |
1162 | ||
db691027 SF |
1163 | $x += 2; |
1164 | $x *= 3; | |
a0d0e21e | 1165 | |
b350dd2f GS |
1166 | Similarly, a list assignment in list context produces the list of |
1167 | lvalues assigned to, and a list assignment in scalar context returns | |
1168 | the number of elements produced by the expression on the right hand | |
1169 | side of the assignment. | |
1170 | ||
748a9306 | 1171 | =head2 Comma Operator |
d74e8afc | 1172 | X<comma> X<operator, comma> X<,> |
a0d0e21e | 1173 | |
5a964f20 | 1174 | Binary "," is the comma operator. In scalar context it evaluates |
a0d0e21e LW |
1175 | its left argument, throws that value away, then evaluates its right |
1176 | argument and returns that value. This is just like C's comma operator. | |
1177 | ||
5a964f20 | 1178 | In list context, it's just the list argument separator, and inserts |
ed5c6d31 PJ |
1179 | both its arguments into the list. These arguments are also evaluated |
1180 | from left to right. | |
a0d0e21e | 1181 | |
4e1988c6 FC |
1182 | The C<< => >> operator is a synonym for the comma except that it causes a |
1183 | word on its left to be interpreted as a string if it begins with a letter | |
344f2c40 IG |
1184 | or underscore and is composed only of letters, digits and underscores. |
1185 | This includes operands that might otherwise be interpreted as operators, | |
46f8a5ea | 1186 | constants, single number v-strings or function calls. If in doubt about |
c543c01b | 1187 | this behavior, the left operand can be quoted explicitly. |
344f2c40 IG |
1188 | |
1189 | Otherwise, the C<< => >> operator behaves exactly as the comma operator | |
1190 | or list argument separator, according to context. | |
1191 | ||
1192 | For example: | |
a44e5664 MS |
1193 | |
1194 | use constant FOO => "something"; | |
1195 | ||
1196 | my %h = ( FOO => 23 ); | |
1197 | ||
1198 | is equivalent to: | |
1199 | ||
1200 | my %h = ("FOO", 23); | |
1201 | ||
1202 | It is I<NOT>: | |
1203 | ||
1204 | my %h = ("something", 23); | |
1205 | ||
719b43e8 RGS |
1206 | The C<< => >> operator is helpful in documenting the correspondence |
1207 | between keys and values in hashes, and other paired elements in lists. | |
748a9306 | 1208 | |
a12b8f3c FC |
1209 | %hash = ( $key => $value ); |
1210 | login( $username => $password ); | |
a44e5664 | 1211 | |
4e1988c6 FC |
1212 | The special quoting behavior ignores precedence, and hence may apply to |
1213 | I<part> of the left operand: | |
1214 | ||
1215 | print time.shift => "bbb"; | |
1216 | ||
1217 | That example prints something like "1314363215shiftbbb", because the | |
1218 | C<< => >> implicitly quotes the C<shift> immediately on its left, ignoring | |
1219 | the fact that C<time.shift> is the entire left operand. | |
1220 | ||
a0d0e21e | 1221 | =head2 List Operators (Rightward) |
d74e8afc | 1222 | X<operator, list, rightward> X<list operator> |
a0d0e21e | 1223 | |
c543c01b | 1224 | On the right side of a list operator, the comma has very low precedence, |
a0d0e21e LW |
1225 | such that it controls all comma-separated expressions found there. |
1226 | The only operators with lower precedence are the logical operators | |
1227 | "and", "or", and "not", which may be used to evaluate calls to list | |
1ca345ed TC |
1228 | operators without the need for parentheses: |
1229 | ||
1230 | open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n"; | |
1231 | ||
1232 | However, some people find that code harder to read than writing | |
1233 | it with parentheses: | |
1234 | ||
1235 | open(HANDLE, "< :utf8", "filename") or die "Can't open: $!\n"; | |
1236 | ||
1237 | in which case you might as well just use the more customary "||" operator: | |
a0d0e21e | 1238 | |
1ca345ed | 1239 | open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n"; |
a0d0e21e | 1240 | |
5ba421f6 | 1241 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
a0d0e21e LW |
1242 | |
1243 | =head2 Logical Not | |
d74e8afc | 1244 | X<operator, logical, not> X<not> |
a0d0e21e LW |
1245 | |
1246 | Unary "not" returns the logical negation of the expression to its right. | |
1247 | It's the equivalent of "!" except for the very low precedence. | |
1248 | ||
1249 | =head2 Logical And | |
d74e8afc | 1250 | X<operator, logical, and> X<and> |
a0d0e21e LW |
1251 | |
1252 | Binary "and" returns the logical conjunction of the two surrounding | |
c543c01b TC |
1253 | expressions. It's equivalent to C<&&> except for the very low |
1254 | precedence. This means that it short-circuits: the right | |
a0d0e21e LW |
1255 | expression is evaluated only if the left expression is true. |
1256 | ||
59ab9d6e | 1257 | =head2 Logical or and Exclusive Or |
f23102e2 | 1258 | X<operator, logical, or> X<operator, logical, xor> |
59ab9d6e | 1259 | X<operator, logical, exclusive or> |
f23102e2 | 1260 | X<or> X<xor> |
a0d0e21e LW |
1261 | |
1262 | Binary "or" returns the logical disjunction of the two surrounding | |
c543c01b TC |
1263 | expressions. It's equivalent to C<||> except for the very low precedence. |
1264 | This makes it useful for control flow: | |
5a964f20 TC |
1265 | |
1266 | print FH $data or die "Can't write to FH: $!"; | |
1267 | ||
c543c01b TC |
1268 | This means that it short-circuits: the right expression is evaluated |
1269 | only if the left expression is false. Due to its precedence, you must | |
1270 | be careful to avoid using it as replacement for the C<||> operator. | |
1271 | It usually works out better for flow control than in assignments: | |
5a964f20 | 1272 | |
db691027 SF |
1273 | $x = $y or $z; # bug: this is wrong |
1274 | ($x = $y) or $z; # really means this | |
1275 | $x = $y || $z; # better written this way | |
5a964f20 | 1276 | |
19799a22 | 1277 | However, when it's a list-context assignment and you're trying to use |
c543c01b | 1278 | C<||> for control flow, you probably need "or" so that the assignment |
5a964f20 TC |
1279 | takes higher precedence. |
1280 | ||
1281 | @info = stat($file) || die; # oops, scalar sense of stat! | |
1282 | @info = stat($file) or die; # better, now @info gets its due | |
1283 | ||
c963b151 BD |
1284 | Then again, you could always use parentheses. |
1285 | ||
1ca345ed | 1286 | Binary C<xor> returns the exclusive-OR of the two surrounding expressions. |
c543c01b | 1287 | It cannot short-circuit (of course). |
a0d0e21e | 1288 | |
59ab9d6e MB |
1289 | There is no low precedence operator for defined-OR. |
1290 | ||
a0d0e21e | 1291 | =head2 C Operators Missing From Perl |
d74e8afc ITB |
1292 | X<operator, missing from perl> X<&> X<*> |
1293 | X<typecasting> X<(TYPE)> | |
a0d0e21e LW |
1294 | |
1295 | Here is what C has that Perl doesn't: | |
1296 | ||
1297 | =over 8 | |
1298 | ||
1299 | =item unary & | |
1300 | ||
1301 | Address-of operator. (But see the "\" operator for taking a reference.) | |
1302 | ||
1303 | =item unary * | |
1304 | ||
46f8a5ea | 1305 | Dereference-address operator. (Perl's prefix dereferencing |
a0d0e21e LW |
1306 | operators are typed: $, @, %, and &.) |
1307 | ||
1308 | =item (TYPE) | |
1309 | ||
19799a22 | 1310 | Type-casting operator. |
a0d0e21e LW |
1311 | |
1312 | =back | |
1313 | ||
5f05dabc | 1314 | =head2 Quote and Quote-like Operators |
89d205f2 | 1315 | X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m> |
d74e8afc ITB |
1316 | X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>> |
1317 | X<escape sequence> X<escape> | |
1318 | ||
a0d0e21e LW |
1319 | While we usually think of quotes as literal values, in Perl they |
1320 | function as operators, providing various kinds of interpolating and | |
1321 | pattern matching capabilities. Perl provides customary quote characters | |
1322 | for these behaviors, but also provides a way for you to choose your | |
1323 | quote character for any of them. In the following table, a C<{}> represents | |
9f10b797 | 1324 | any pair of delimiters you choose. |
a0d0e21e | 1325 | |
2c268ad5 TP |
1326 | Customary Generic Meaning Interpolates |
1327 | '' q{} Literal no | |
1328 | "" qq{} Literal yes | |
af9219ee | 1329 | `` qx{} Command yes* |
2c268ad5 | 1330 | qw{} Word list no |
af9219ee MG |
1331 | // m{} Pattern match yes* |
1332 | qr{} Pattern yes* | |
1333 | s{}{} Substitution yes* | |
2c268ad5 | 1334 | tr{}{} Transliteration no (but see below) |
c543c01b | 1335 | y{}{} Transliteration no (but see below) |
7e3b091d | 1336 | <<EOF here-doc yes* |
a0d0e21e | 1337 | |
af9219ee MG |
1338 | * unless the delimiter is ''. |
1339 | ||
87275199 | 1340 | Non-bracketing delimiters use the same character fore and aft, but the four |
c543c01b | 1341 | sorts of ASCII brackets (round, angle, square, curly) all nest, which means |
9f10b797 | 1342 | that |
87275199 | 1343 | |
c543c01b | 1344 | q{foo{bar}baz} |
35f2feb0 | 1345 | |
9f10b797 | 1346 | is the same as |
87275199 | 1347 | |
c543c01b | 1348 | 'foo{bar}baz' |
87275199 GS |
1349 | |
1350 | Note, however, that this does not always work for quoting Perl code: | |
1351 | ||
db691027 | 1352 | $s = q{ if($x eq "}") ... }; # WRONG |
87275199 | 1353 | |
46f8a5ea | 1354 | is a syntax error. The C<Text::Balanced> module (standard as of v5.8, |
c543c01b | 1355 | and from CPAN before then) is able to do this properly. |
87275199 | 1356 | |
19799a22 | 1357 | There can be whitespace between the operator and the quoting |
fb73857a | 1358 | characters, except when C<#> is being used as the quoting character. |
19799a22 GS |
1359 | C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the |
1360 | operator C<q> followed by a comment. Its argument will be taken | |
1361 | from the next line. This allows you to write: | |
fb73857a | 1362 | |
1363 | s {foo} # Replace foo | |
1364 | {bar} # with bar. | |
1365 | ||
c543c01b TC |
1366 | The following escape sequences are available in constructs that interpolate, |
1367 | and in transliterations: | |
5691ca5f | 1368 | 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 | 1369 | X<\o{}> |
5691ca5f | 1370 | |
2c4c1ff2 KW |
1371 | Sequence Note Description |
1372 | \t tab (HT, TAB) | |
1373 | \n newline (NL) | |
1374 | \r return (CR) | |
1375 | \f form feed (FF) | |
1376 | \b backspace (BS) | |
1377 | \a alarm (bell) (BEL) | |
1378 | \e escape (ESC) | |
c543c01b | 1379 | \x{263A} [1,8] hex char (example: SMILEY) |
2c4c1ff2 | 1380 | \x1b [2,8] restricted range hex char (example: ESC) |
fb121860 | 1381 | \N{name} [3] named Unicode character or character sequence |
2c4c1ff2 KW |
1382 | \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON) |
1383 | \c[ [5] control char (example: chr(27)) | |
1384 | \o{23072} [6,8] octal char (example: SMILEY) | |
1385 | \033 [7,8] restricted range octal char (example: ESC) | |
5691ca5f KW |
1386 | |
1387 | =over 4 | |
1388 | ||
1389 | =item [1] | |
1390 | ||
2c4c1ff2 KW |
1391 | The result is the character specified by the hexadecimal number between |
1392 | the braces. See L</[8]> below for details on which character. | |
96448467 | 1393 | |
46f8a5ea | 1394 | Only hexadecimal digits are valid between the braces. If an invalid |
96448467 DG |
1395 | character is encountered, a warning will be issued and the invalid |
1396 | character and all subsequent characters (valid or invalid) within the | |
1397 | braces will be discarded. | |
1398 | ||
1399 | If there are no valid digits between the braces, the generated character is | |
1400 | the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>) | |
c543c01b | 1401 | will not cause a warning (currently). |
40687185 KW |
1402 | |
1403 | =item [2] | |
1404 | ||
2c4c1ff2 KW |
1405 | The result is the character specified by the hexadecimal number in the range |
1406 | 0x00 to 0xFF. See L</[8]> below for details on which character. | |
96448467 DG |
1407 | |
1408 | Only hexadecimal digits are valid following C<\x>. When C<\x> is followed | |
2c4c1ff2 | 1409 | by fewer than two valid digits, any valid digits will be zero-padded. This |
c543c01b | 1410 | means that C<\x7> will be interpreted as C<\x07>, and a lone <\x> will be |
2c4c1ff2 | 1411 | interpreted as C<\x00>. Except at the end of a string, having fewer than |
c543c01b | 1412 | two valid digits will result in a warning. Note that although the warning |
96448467 DG |
1413 | says the illegal character is ignored, it is only ignored as part of the |
1414 | escape and will still be used as the subsequent character in the string. | |
1415 | For example: | |
1416 | ||
1417 | Original Result Warns? | |
1418 | "\x7" "\x07" no | |
1419 | "\x" "\x00" no | |
1420 | "\x7q" "\x07q" yes | |
1421 | "\xq" "\x00q" yes | |
1422 | ||
40687185 KW |
1423 | =item [3] |
1424 | ||
fb121860 | 1425 | The result is the Unicode character or character sequence given by I<name>. |
2c4c1ff2 | 1426 | See L<charnames>. |
40687185 KW |
1427 | |
1428 | =item [4] | |
1429 | ||
2c4c1ff2 KW |
1430 | C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code |
1431 | point is I<hexadecimal number>. | |
40687185 KW |
1432 | |
1433 | =item [5] | |
1434 | ||
5691ca5f KW |
1435 | The character following C<\c> is mapped to some other character as shown in the |
1436 | table: | |
1437 | ||
1438 | Sequence Value | |
1439 | \c@ chr(0) | |
1440 | \cA chr(1) | |
1441 | \ca chr(1) | |
1442 | \cB chr(2) | |
1443 | \cb chr(2) | |
1444 | ... | |
1445 | \cZ chr(26) | |
1446 | \cz chr(26) | |
1447 | \c[ chr(27) | |
1448 | \c] chr(29) | |
1449 | \c^ chr(30) | |
c3e9d7a9 KW |
1450 | \c_ chr(31) |
1451 | \c? chr(127) # (on ASCII platforms) | |
5691ca5f | 1452 | |
d813941f | 1453 | In other words, it's the character whose code point has had 64 xor'd with |
c3e9d7a9 KW |
1454 | its uppercase. C<\c?> is DELETE on ASCII platforms because |
1455 | S<C<ord("?") ^ 64>> is 127, and | |
d813941f KW |
1456 | C<\c@> is NULL because the ord of "@" is 64, so xor'ing 64 itself produces 0. |
1457 | ||
5691ca5f KW |
1458 | Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the |
1459 | end of a string, because the backslash would be parsed as escaping the end | |
1460 | quote. | |
1461 | ||
1462 | On ASCII platforms, the resulting characters from the list above are the | |
1463 | complete set of ASCII controls. This isn't the case on EBCDIC platforms; see | |
c3e9d7a9 KW |
1464 | L<perlebcdic/OPERATOR DIFFERENCES> for a full discussion of the |
1465 | differences between these for ASCII versus EBCDIC platforms. | |
5691ca5f | 1466 | |
c3e9d7a9 | 1467 | Use of any other character following the C<"c"> besides those listed above is |
63a63d81 KW |
1468 | discouraged, and as of Perl v5.20, the only characters actually allowed |
1469 | are the printable ASCII ones, minus the left brace C<"{">. What happens | |
1470 | for any of the allowed other characters is that the value is derived by | |
1471 | xor'ing with the seventh bit, which is 64, and a warning raised if | |
1472 | enabled. Using the non-allowed characters generates a fatal error. | |
5691ca5f KW |
1473 | |
1474 | To get platform independent controls, you can use C<\N{...}>. | |
1475 | ||
40687185 KW |
1476 | =item [6] |
1477 | ||
2c4c1ff2 KW |
1478 | The result is the character specified by the octal number between the braces. |
1479 | See L</[8]> below for details on which character. | |
04341565 DG |
1480 | |
1481 | If a character that isn't an octal digit is encountered, a warning is raised, | |
1482 | and the value is based on the octal digits before it, discarding it and all | |
1483 | following characters up to the closing brace. It is a fatal error if there are | |
1484 | no octal digits at all. | |
1485 | ||
1486 | =item [7] | |
1487 | ||
c543c01b | 1488 | The result is the character specified by the three-digit octal number in the |
2c4c1ff2 KW |
1489 | range 000 to 777 (but best to not use above 077, see next paragraph). See |
1490 | L</[8]> below for details on which character. | |
1491 | ||
1492 | Some contexts allow 2 or even 1 digit, but any usage without exactly | |
40687185 | 1493 | three digits, the first being a zero, may give unintended results. (For |
5db3e519 FC |
1494 | example, in a regular expression it may be confused with a backreference; |
1495 | see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may | |
c543c01b | 1496 | use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to |
04341565 DG |
1497 | use this construct only for ordinals C<\077> and below, remembering to pad to |
1498 | the left with zeros to make three digits. For larger ordinals, either use | |
9fef6a0d | 1499 | C<\o{}>, or convert to something else, such as to hex and use C<\x{}> |
04341565 | 1500 | instead. |
40687185 | 1501 | |
2c4c1ff2 KW |
1502 | =item [8] |
1503 | ||
c543c01b | 1504 | Several constructs above specify a character by a number. That number |
2c4c1ff2 | 1505 | gives the character's position in the character set encoding (indexed from 0). |
c543c01b | 1506 | This is called synonymously its ordinal, code position, or code point. Perl |
2c4c1ff2 KW |
1507 | works on platforms that have a native encoding currently of either ASCII/Latin1 |
1508 | or EBCDIC, each of which allow specification of 256 characters. In general, if | |
1509 | the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's | |
1510 | native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets | |
c543c01b | 1511 | it as a Unicode code point and the result is the corresponding Unicode |
2c4c1ff2 KW |
1512 | character. For example C<\x{50}> and C<\o{120}> both are the number 80 in |
1513 | decimal, which is less than 256, so the number is interpreted in the native | |
1514 | character set encoding. In ASCII the character in the 80th position (indexed | |
1515 | from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&". | |
1516 | C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted | |
1517 | as a Unicode code point no matter what the native encoding is. The name of the | |
9fef6a0d | 1518 | character in the 256th position (indexed by 0) in Unicode is |
2c4c1ff2 KW |
1519 | C<LATIN CAPITAL LETTER A WITH MACRON>. |
1520 | ||
9fef6a0d | 1521 | There are a couple of exceptions to the above rule. S<C<\N{U+I<hex number>}>> is |
2c4c1ff2 KW |
1522 | always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even |
1523 | on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the | |
1524 | number is considered to be in that encoding, and is translated from that into | |
1525 | the platform's native encoding if there is a corresponding native character; | |
1526 | otherwise to Unicode. | |
1527 | ||
5691ca5f | 1528 | =back |
4c77eaa2 | 1529 | |
e526e8bb | 1530 | B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for |
8b312c40 KW |
1531 | the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may |
1532 | use C<\ck> or | |
1533 | C<\x0b>. (C<\v> | |
e526e8bb KW |
1534 | does have meaning in regular expression patterns in Perl, see L<perlre>.) |
1535 | ||
1536 | The following escape sequences are available in constructs that interpolate, | |
904501ec | 1537 | but not in transliterations. |
628253b8 | 1538 | X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> X<\F> |
904501ec | 1539 | |
c543c01b TC |
1540 | \l lowercase next character only |
1541 | \u titlecase (not uppercase!) next character only | |
e4d34742 EB |
1542 | \L lowercase all characters till \E or end of string |
1543 | \U uppercase all characters till \E or end of string | |
628253b8 | 1544 | \F foldcase all characters till \E or end of string |
736fe711 KW |
1545 | \Q quote (disable) pattern metacharacters till \E or |
1546 | end of string | |
7e31b643 | 1547 | \E end either case modification or quoted section |
c543c01b TC |
1548 | (whichever was last seen) |
1549 | ||
736fe711 KW |
1550 | See L<perlfunc/quotemeta> for the exact definition of characters that |
1551 | are quoted by C<\Q>. | |
1552 | ||
628253b8 | 1553 | C<\L>, C<\U>, C<\F>, and C<\Q> can stack, in which case you need one |
c543c01b TC |
1554 | C<\E> for each. For example: |
1555 | ||
9fef6a0d KW |
1556 | say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?"; |
1557 | This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it? | |
a0d0e21e | 1558 | |
66cbab2c KW |
1559 | If C<use locale> is in effect (but not C<use locale ':not_characters'>), |
1560 | the case map used by C<\l>, C<\L>, | |
c543c01b | 1561 | C<\u>, and C<\U> is taken from the current locale. See L<perllocale>. |
b6538e4f | 1562 | If Unicode (for example, C<\N{}> or code points of 0x100 or |
c543c01b TC |
1563 | beyond) is being used, the case map used by C<\l>, C<\L>, C<\u>, and |
1564 | C<\U> is as defined by Unicode. That means that case-mapping | |
1565 | a single character can sometimes produce several characters. | |
31f05a37 KW |
1566 | Under C<use locale>, C<\F> produces the same results as C<\L> |
1567 | for all locales but a UTF-8 one, where it instead uses the Unicode | |
1568 | definition. | |
a034a98d | 1569 | |
5a964f20 TC |
1570 | All systems use the virtual C<"\n"> to represent a line terminator, |
1571 | called a "newline". There is no such thing as an unvarying, physical | |
19799a22 | 1572 | newline character. It is only an illusion that the operating system, |
5a964f20 TC |
1573 | device drivers, C libraries, and Perl all conspire to preserve. Not all |
1574 | systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example, | |
c543c01b TC |
1575 | on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed, |
1576 | and on systems without line terminator, | |
1577 | printing C<"\n"> might emit no actual data. In general, use C<"\n"> when | |
5a964f20 TC |
1578 | you mean a "newline" for your system, but use the literal ASCII when you |
1579 | need an exact character. For example, most networking protocols expect | |
2a380090 | 1580 | and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators, |
5a964f20 TC |
1581 | and although they often accept just C<"\012">, they seldom tolerate just |
1582 | C<"\015">. If you get in the habit of using C<"\n"> for networking, | |
1583 | you may be burned some day. | |
d74e8afc ITB |
1584 | X<newline> X<line terminator> X<eol> X<end of line> |
1585 | X<\n> X<\r> X<\r\n> | |
5a964f20 | 1586 | |
904501ec MG |
1587 | For constructs that do interpolate, variables beginning with "C<$>" |
1588 | or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or | |
ad0f383a A |
1589 | C<< $href->{key}[0] >> are also interpolated, as are array and hash slices. |
1590 | But method calls such as C<< $obj->meth >> are not. | |
af9219ee MG |
1591 | |
1592 | Interpolating an array or slice interpolates the elements in order, | |
1593 | separated by the value of C<$">, so is equivalent to interpolating | |
c543c01b TC |
1594 | C<join $", @array>. "Punctuation" arrays such as C<@*> are usually |
1595 | interpolated only if the name is enclosed in braces C<@{*}>, but the | |
1596 | arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces. | |
af9219ee | 1597 | |
bc7b91c6 EB |
1598 | For double-quoted strings, the quoting from C<\Q> is applied after |
1599 | interpolation and escapes are processed. | |
1600 | ||
1601 | "abc\Qfoo\tbar$s\Exyz" | |
1602 | ||
1603 | is equivalent to | |
1604 | ||
1605 | "abc" . quotemeta("foo\tbar$s") . "xyz" | |
1606 | ||
1607 | For the pattern of regex operators (C<qr//>, C<m//> and C<s///>), | |
1608 | the quoting from C<\Q> is applied after interpolation is processed, | |
46f8a5ea FC |
1609 | but before escapes are processed. This allows the pattern to match |
1610 | literally (except for C<$> and C<@>). For example, the following matches: | |
bc7b91c6 EB |
1611 | |
1612 | '\s\t' =~ /\Q\s\t/ | |
1613 | ||
1614 | Because C<$> or C<@> trigger interpolation, you'll need to use something | |
1615 | like C</\Quser\E\@\Qhost/> to match them literally. | |
1d2dff63 | 1616 | |
a0d0e21e LW |
1617 | Patterns are subject to an additional level of interpretation as a |
1618 | regular expression. This is done as a second pass, after variables are | |
1619 | interpolated, so that regular expressions may be incorporated into the | |
1620 | pattern from the variables. If this is not what you want, use C<\Q> to | |
1621 | interpolate a variable literally. | |
1622 | ||
19799a22 GS |
1623 | Apart from the behavior described above, Perl does not expand |
1624 | multiple levels of interpolation. In particular, contrary to the | |
1625 | expectations of shell programmers, back-quotes do I<NOT> interpolate | |
1626 | within double quotes, nor do single quotes impede evaluation of | |
1627 | variables when used within double quotes. | |
a0d0e21e | 1628 | |
5f05dabc | 1629 | =head2 Regexp Quote-Like Operators |
d74e8afc | 1630 | X<operator, regexp> |
cb1a09d0 | 1631 | |
5f05dabc | 1632 | Here are the quote-like operators that apply to pattern |
cb1a09d0 AD |
1633 | matching and related activities. |
1634 | ||
a0d0e21e LW |
1635 | =over 8 |
1636 | ||
33be4c61 | 1637 | =item qr/STRING/msixpodualn |
01c6f5f4 | 1638 | X<qr> X</i> X</m> X</o> X</s> X</x> X</p> |
a0d0e21e | 1639 | |
87e95b7f YO |
1640 | This operator quotes (and possibly compiles) its I<STRING> as a regular |
1641 | expression. I<STRING> is interpolated the same way as I<PATTERN> | |
1642 | in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation | |
1643 | is done. Returns a Perl value which may be used instead of the | |
33be4c61 | 1644 | corresponding C</STRING/msixpodualn> expression. The returned value is a |
46f8a5ea | 1645 | normalized version of the original pattern. It magically differs from |
1c8ee595 CO |
1646 | a string containing the same characters: C<ref(qr/x/)> returns "Regexp"; |
1647 | however, dereferencing it is not well defined (you currently get the | |
1648 | normalized version of the original pattern, but this may change). | |
1649 | ||
a0d0e21e | 1650 | |
87e95b7f YO |
1651 | For example, |
1652 | ||
1653 | $rex = qr/my.STRING/is; | |
85dd5c8b | 1654 | print $rex; # prints (?si-xm:my.STRING) |
87e95b7f YO |
1655 | s/$rex/foo/; |
1656 | ||
1657 | is equivalent to | |
1658 | ||
1659 | s/my.STRING/foo/is; | |
1660 | ||
1661 | The result may be used as a subpattern in a match: | |
1662 | ||
1663 | $re = qr/$pattern/; | |
7188ca43 KW |
1664 | $string =~ /foo${re}bar/; # can be interpolated in other |
1665 | # patterns | |
87e95b7f YO |
1666 | $string =~ $re; # or used standalone |
1667 | $string =~ /$re/; # or this way | |
1668 | ||
f6050459 | 1669 | Since Perl may compile the pattern at the moment of execution of the qr() |
87e95b7f YO |
1670 | operator, using qr() may have speed advantages in some situations, |
1671 | notably if the result of qr() is used standalone: | |
1672 | ||
1673 | sub match { | |
1674 | my $patterns = shift; | |
1675 | my @compiled = map qr/$_/i, @$patterns; | |
1676 | grep { | |
1677 | my $success = 0; | |
1678 | foreach my $pat (@compiled) { | |
1679 | $success = 1, last if /$pat/; | |
1680 | } | |
1681 | $success; | |
1682 | } @_; | |
5a964f20 TC |
1683 | } |
1684 | ||
87e95b7f YO |
1685 | Precompilation of the pattern into an internal representation at |
1686 | the moment of qr() avoids a need to recompile the pattern every | |
1687 | time a match C</$pat/> is attempted. (Perl has many other internal | |
1688 | optimizations, but none would be triggered in the above example if | |
1689 | we did not use qr() operator.) | |
1690 | ||
765fa144 | 1691 | Options (specified by the following modifiers) are: |
87e95b7f YO |
1692 | |
1693 | m Treat string as multiple lines. | |
1694 | s Treat string as single line. (Make . match a newline) | |
1695 | i Do case-insensitive pattern matching. | |
1696 | x Use extended regular expressions. | |
1697 | p When matching preserve a copy of the matched string so | |
7188ca43 KW |
1698 | that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be |
1699 | defined. | |
87e95b7f | 1700 | o Compile pattern only once. |
7188ca43 KW |
1701 | a ASCII-restrict: Use ASCII for \d, \s, \w; specifying two |
1702 | a's further restricts /i matching so that no ASCII | |
48cbae4f SK |
1703 | character will match a non-ASCII one. |
1704 | l Use the locale. | |
1705 | u Use Unicode rules. | |
1706 | d Use Unicode or native charset, as in 5.12 and earlier. | |
33be4c61 | 1707 | n Non-capture mode. Don't let () fill in $1, $2, etc... |
87e95b7f YO |
1708 | |
1709 | If a precompiled pattern is embedded in a larger pattern then the effect | |
33be4c61 | 1710 | of "msixpluadn" will be propagated appropriately. The effect the "o" |
87e95b7f YO |
1711 | modifier has is not propagated, being restricted to those patterns |
1712 | explicitly using it. | |
1713 | ||
b6fa137b | 1714 | The last four modifiers listed above, added in Perl 5.14, |
850b7ec9 | 1715 | control the character set rules, but C</a> is the only one you are likely |
18509dec KW |
1716 | to want to specify explicitly; the other three are selected |
1717 | automatically by various pragmas. | |
da392a17 | 1718 | |
87e95b7f | 1719 | See L<perlre> for additional information on valid syntax for STRING, and |
5e2aa8f5 | 1720 | for a detailed look at the semantics of regular expressions. In |
1ca345ed TC |
1721 | particular, all modifiers except the largely obsolete C</o> are further |
1722 | explained in L<perlre/Modifiers>. C</o> is described in the next section. | |
a0d0e21e | 1723 | |
33be4c61 | 1724 | =item m/PATTERN/msixpodualngc |
89d205f2 YO |
1725 | X<m> X<operator, match> |
1726 | X<regexp, options> X<regexp> X<regex, options> X<regex> | |
01c6f5f4 | 1727 | X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> |
a0d0e21e | 1728 | |
33be4c61 | 1729 | =item /PATTERN/msixpodualngc |
a0d0e21e | 1730 | |
5a964f20 | 1731 | Searches a string for a pattern match, and in scalar context returns |
19799a22 GS |
1732 | true if it succeeds, false if it fails. If no string is specified |
1733 | via the C<=~> or C<!~> operator, the $_ string is searched. (The | |
1734 | string specified with C<=~> need not be an lvalue--it may be the | |
1735 | result of an expression evaluation, but remember the C<=~> binds | |
006671a6 | 1736 | rather tightly.) See also L<perlre>. |
a0d0e21e | 1737 | |
f6050459 | 1738 | Options are as described in C<qr//> above; in addition, the following match |
01c6f5f4 | 1739 | process modifiers are available: |
a0d0e21e | 1740 | |
950b09ed | 1741 | g Match globally, i.e., find all occurrences. |
7188ca43 KW |
1742 | c Do not reset search position on a failed match when /g is |
1743 | in effect. | |
a0d0e21e | 1744 | |
725a61d7 | 1745 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> |
c543c01b | 1746 | you can use any pair of non-whitespace (ASCII) characters |
725a61d7 Z |
1747 | as delimiters. This is particularly useful for matching path names |
1748 | that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is | |
1749 | the delimiter, then a match-only-once rule applies, | |
46f8a5ea | 1750 | described in C<m?PATTERN?> below. If "'" (single quote) is the delimiter, |
6ca3c6c6 | 1751 | no interpolation is performed on the PATTERN. |
ed02a3bf DN |
1752 | When using a character valid in an identifier, whitespace is required |
1753 | after the C<m>. | |
a0d0e21e | 1754 | |
532c9e80 KW |
1755 | PATTERN may contain variables, which will be interpolated |
1756 | every time the pattern search is evaluated, except | |
1f247705 GS |
1757 | for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and |
1758 | C<$|> are not interpolated because they look like end-of-string tests.) | |
532c9e80 KW |
1759 | Perl will not recompile the pattern unless an interpolated |
1760 | variable that it contains changes. You can force Perl to skip the | |
1761 | test and never recompile by adding a C</o> (which stands for "once") | |
1762 | after the trailing delimiter. | |
1763 | Once upon a time, Perl would recompile regular expressions | |
1764 | unnecessarily, and this modifier was useful to tell it not to do so, in the | |
5cc41653 | 1765 | interests of speed. But now, the only reasons to use C</o> are one of: |
532c9e80 KW |
1766 | |
1767 | =over | |
1768 | ||
1769 | =item 1 | |
1770 | ||
1771 | The variables are thousands of characters long and you know that they | |
1772 | don't change, and you need to wring out the last little bit of speed by | |
1773 | having Perl skip testing for that. (There is a maintenance penalty for | |
1774 | doing this, as mentioning C</o> constitutes a promise that you won't | |
18509dec | 1775 | change the variables in the pattern. If you do change them, Perl won't |
532c9e80 KW |
1776 | even notice.) |
1777 | ||
1778 | =item 2 | |
1779 | ||
1780 | you want the pattern to use the initial values of the variables | |
1781 | regardless of whether they change or not. (But there are saner ways | |
1782 | of accomplishing this than using C</o>.) | |
1783 | ||
fa9b8686 DM |
1784 | =item 3 |
1785 | ||
1786 | If the pattern contains embedded code, such as | |
1787 | ||
1788 | use re 'eval'; | |
1789 | $code = 'foo(?{ $x })'; | |
1790 | /$code/ | |
1791 | ||
1792 | then perl will recompile each time, even though the pattern string hasn't | |
1793 | changed, to ensure that the current value of C<$x> is seen each time. | |
1794 | Use C</o> if you want to avoid this. | |
1795 | ||
532c9e80 | 1796 | =back |
a0d0e21e | 1797 | |
18509dec KW |
1798 | The bottom line is that using C</o> is almost never a good idea. |
1799 | ||
e9d89077 DN |
1800 | =item The empty pattern // |
1801 | ||
5a964f20 | 1802 | If the PATTERN evaluates to the empty string, the last |
46f8a5ea | 1803 | I<successfully> matched regular expression is used instead. In this |
c543c01b | 1804 | case, only the C<g> and C<c> flags on the empty pattern are honored; |
46f8a5ea | 1805 | the other flags are taken from the original pattern. If no match has |
d65afb4b HS |
1806 | previously succeeded, this will (silently) act instead as a genuine |
1807 | empty pattern (which will always match). | |
a0d0e21e | 1808 | |
89d205f2 YO |
1809 | Note that it's possible to confuse Perl into thinking C<//> (the empty |
1810 | regex) is really C<//> (the defined-or operator). Perl is usually pretty | |
1811 | good about this, but some pathological cases might trigger this, such as | |
db691027 | 1812 | C<$x///> (is that C<($x) / (//)> or C<$x // />?) and C<print $fh //> |
89d205f2 YO |
1813 | (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl |
1814 | will assume you meant defined-or. If you meant the empty regex, just | |
1815 | use parentheses or spaces to disambiguate, or even prefix the empty | |
c963b151 BD |
1816 | regex with an C<m> (so C<//> becomes C<m//>). |
1817 | ||
e9d89077 DN |
1818 | =item Matching in list context |
1819 | ||
19799a22 | 1820 | If the C</g> option is not used, C<m//> in list context returns a |
a0d0e21e | 1821 | list consisting of the subexpressions matched by the parentheses in the |
3ff8ecf9 BF |
1822 | pattern, that is, (C<$1>, C<$2>, C<$3>...) (Note that here C<$1> etc. are |
1823 | also set). When there are no parentheses in the pattern, the return | |
1824 | value is the list C<(1)> for success. | |
1825 | With or without parentheses, an empty list is returned upon failure. | |
a0d0e21e LW |
1826 | |
1827 | Examples: | |
1828 | ||
7188ca43 KW |
1829 | open(TTY, "+</dev/tty") |
1830 | || die "can't access /dev/tty: $!"; | |
c543c01b | 1831 | |
7188ca43 | 1832 | <TTY> =~ /^y/i && foo(); # do foo if desired |
a0d0e21e | 1833 | |
7188ca43 | 1834 | if (/Version: *([0-9.]*)/) { $version = $1; } |
a0d0e21e | 1835 | |
7188ca43 | 1836 | next if m#^/usr/spool/uucp#; |
a0d0e21e | 1837 | |
7188ca43 KW |
1838 | # poor man's grep |
1839 | $arg = shift; | |
1840 | while (<>) { | |
1841 | print if /$arg/o; # compile only once (no longer needed!) | |
1842 | } | |
a0d0e21e | 1843 | |
7188ca43 | 1844 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) |
a0d0e21e LW |
1845 | |
1846 | This last example splits $foo into the first two words and the | |
5f05dabc | 1847 | remainder of the line, and assigns those three fields to $F1, $F2, and |
c543c01b TC |
1848 | $Etc. The conditional is true if any variables were assigned; that is, |
1849 | if the pattern matched. | |
a0d0e21e | 1850 | |
19799a22 | 1851 | The C</g> modifier specifies global pattern matching--that is, |
46f8a5ea FC |
1852 | matching as many times as possible within the string. How it behaves |
1853 | depends on the context. In list context, it returns a list of the | |
19799a22 | 1854 | substrings matched by any capturing parentheses in the regular |
46f8a5ea | 1855 | expression. If there are no parentheses, it returns a list of all |
19799a22 GS |
1856 | the matched strings, as if there were parentheses around the whole |
1857 | pattern. | |
a0d0e21e | 1858 | |
7e86de3e | 1859 | In scalar context, each execution of C<m//g> finds the next match, |
19799a22 | 1860 | returning true if it matches, and false if there is no further match. |
3dd93342 | 1861 | The position after the last match can be read or set using the C<pos()> |
46f8a5ea | 1862 | function; see L<perlfunc/pos>. A failed match normally resets the |
7e86de3e | 1863 | search position to the beginning of the string, but you can avoid that |
46f8a5ea | 1864 | by adding the C</c> modifier (for example, C<m//gc>). Modifying the target |
7e86de3e | 1865 | string also resets the search position. |
c90c0ff4 | 1866 | |
e9d89077 DN |
1867 | =item \G assertion |
1868 | ||
c90c0ff4 | 1869 | You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a |
3dd93342 | 1870 | zero-width assertion that matches the exact position where the |
46f8a5ea | 1871 | previous C<m//g>, if any, left off. Without the C</g> modifier, the |
3dd93342 | 1872 | C<\G> assertion still anchors at C<pos()> as it was at the start of |
1873 | the operation (see L<perlfunc/pos>), but the match is of course only | |
46f8a5ea | 1874 | attempted once. Using C<\G> without C</g> on a target string that has |
3dd93342 | 1875 | not previously had a C</g> match applied to it is the same as using |
1876 | the C<\A> assertion to match the beginning of the string. Note also | |
1877 | that, currently, C<\G> is only properly supported when anchored at the | |
1878 | very beginning of the pattern. | |
c90c0ff4 | 1879 | |
1880 | Examples: | |
a0d0e21e LW |
1881 | |
1882 | # list context | |
1883 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); | |
1884 | ||
1885 | # scalar context | |
c543c01b TC |
1886 | local $/ = ""; |
1887 | while ($paragraph = <>) { | |
1888 | while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) { | |
19799a22 | 1889 | $sentences++; |
a0d0e21e LW |
1890 | } |
1891 | } | |
c543c01b TC |
1892 | say $sentences; |
1893 | ||
1894 | Here's another way to check for sentences in a paragraph: | |
1895 | ||
7188ca43 KW |
1896 | my $sentence_rx = qr{ |
1897 | (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or | |
1898 | # whitespace | |
1899 | \p{Lu} # capital letter | |
1900 | .*? # a bunch of anything | |
1901 | (?<= \S ) # that ends in non- | |
1902 | # whitespace | |
1903 | (?<! \b [DMS]r ) # but isn't a common abbr. | |
1904 | (?<! \b Mrs ) | |
1905 | (?<! \b Sra ) | |
1906 | (?<! \b St ) | |
1907 | [.?!] # followed by a sentence | |
1908 | # ender | |
1909 | (?= $ | \s ) # in front of end-of-string | |
1910 | # or whitespace | |
1911 | }sx; | |
1912 | local $/ = ""; | |
1913 | while (my $paragraph = <>) { | |
1914 | say "NEW PARAGRAPH"; | |
1915 | my $count = 0; | |
1916 | while ($paragraph =~ /($sentence_rx)/g) { | |
1917 | printf "\tgot sentence %d: <%s>\n", ++$count, $1; | |
c543c01b | 1918 | } |
7188ca43 | 1919 | } |
c543c01b TC |
1920 | |
1921 | Here's how to use C<m//gc> with C<\G>: | |
a0d0e21e | 1922 | |
137443ea | 1923 | $_ = "ppooqppqq"; |
44a8e56a | 1924 | while ($i++ < 2) { |
1925 | print "1: '"; | |
c90c0ff4 | 1926 | print $1 while /(o)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1927 | print "2: '"; |
c90c0ff4 | 1928 | print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1929 | print "3: '"; |
c90c0ff4 | 1930 | print $1 while /(p)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1931 | } |
5d43e42d | 1932 | print "Final: '$1', pos=",pos,"\n" if /\G(.)/; |
44a8e56a | 1933 | |
1934 | The last example should print: | |
1935 | ||
1936 | 1: 'oo', pos=4 | |
137443ea | 1937 | 2: 'q', pos=5 |
44a8e56a | 1938 | 3: 'pp', pos=7 |
1939 | 1: '', pos=7 | |
137443ea | 1940 | 2: 'q', pos=8 |
1941 | 3: '', pos=8 | |
5d43e42d DC |
1942 | Final: 'q', pos=8 |
1943 | ||
1944 | Notice that the final match matched C<q> instead of C<p>, which a match | |
46f8a5ea FC |
1945 | without the C<\G> anchor would have done. Also note that the final match |
1946 | did not update C<pos>. C<pos> is only updated on a C</g> match. If the | |
c543c01b TC |
1947 | final match did indeed match C<p>, it's a good bet that you're running a |
1948 | very old (pre-5.6.0) version of Perl. | |
44a8e56a | 1949 | |
c90c0ff4 | 1950 | A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can |
e7ea3e70 | 1951 | combine several regexps like this to process a string part-by-part, |
c90c0ff4 | 1952 | doing different actions depending on which regexp matched. Each |
1953 | regexp tries to match where the previous one leaves off. | |
e7ea3e70 | 1954 | |
3fe9a6f1 | 1955 | $_ = <<'EOL'; |
7188ca43 KW |
1956 | $url = URI::URL->new( "http://example.com/" ); |
1957 | die if $url eq "xXx"; | |
3fe9a6f1 | 1958 | EOL |
c543c01b TC |
1959 | |
1960 | LOOP: { | |
950b09ed | 1961 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; |
7188ca43 KW |
1962 | print(" lowercase"), redo LOOP |
1963 | if /\G\p{Ll}+\b[,.;]?\s*/gc; | |
1964 | print(" UPPERCASE"), redo LOOP | |
1965 | if /\G\p{Lu}+\b[,.;]?\s*/gc; | |
1966 | print(" Capitalized"), redo LOOP | |
1967 | if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc; | |
c543c01b | 1968 | print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc; |
7188ca43 KW |
1969 | print(" alphanumeric"), redo LOOP |
1970 | if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc; | |
c543c01b | 1971 | print(" line-noise"), redo LOOP if /\G\W+/gc; |
950b09ed | 1972 | print ". That's all!\n"; |
c543c01b | 1973 | } |
e7ea3e70 IZ |
1974 | |
1975 | Here is the output (split into several lines): | |
1976 | ||
7188ca43 KW |
1977 | line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE |
1978 | line-noise lowercase line-noise lowercase line-noise lowercase | |
1979 | lowercase line-noise lowercase lowercase line-noise lowercase | |
1980 | lowercase line-noise MiXeD line-noise. That's all! | |
44a8e56a | 1981 | |
33be4c61 | 1982 | =item m?PATTERN?msixpodualngc |
725a61d7 | 1983 | X<?> X<operator, match-once> |
87e95b7f | 1984 | |
33be4c61 | 1985 | =item ?PATTERN?msixpodualngc |
55d389e7 | 1986 | |
725a61d7 Z |
1987 | This is just like the C<m/PATTERN/> search, except that it matches |
1988 | only once between calls to the reset() operator. This is a useful | |
87e95b7f | 1989 | optimization when you want to see only the first occurrence of |
ceb131e8 | 1990 | something in each file of a set of files, for instance. Only C<m??> |
87e95b7f YO |
1991 | patterns local to the current package are reset. |
1992 | ||
1993 | while (<>) { | |
ceb131e8 | 1994 | if (m?^$?) { |
87e95b7f YO |
1995 | # blank line between header and body |
1996 | } | |
1997 | } continue { | |
725a61d7 | 1998 | reset if eof; # clear m?? status for next file |
87e95b7f YO |
1999 | } |
2000 | ||
c543c01b TC |
2001 | Another example switched the first "latin1" encoding it finds |
2002 | to "utf8" in a pod file: | |
2003 | ||
2004 | s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x; | |
2005 | ||
2006 | The match-once behavior is controlled by the match delimiter being | |
4932eeca | 2007 | C<?>; with any other delimiter this is the normal C<m//> operator. |
725a61d7 | 2008 | |
0381ecf1 MH |
2009 | In the past, the leading C<m> in C<m?PATTERN?> was optional, but omitting it |
2010 | would produce a deprecation warning. As of v5.22.0, omitting it produces a | |
2011 | syntax error. If you encounter this construct in older code, you can just add | |
2012 | C<m>. | |
87e95b7f | 2013 | |
33be4c61 | 2014 | =item s/PATTERN/REPLACEMENT/msixpodualngcer |
87e95b7f | 2015 | X<substitute> X<substitution> X<replace> X<regexp, replace> |
4f4d7508 | 2016 | 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 |
2017 | |
2018 | Searches a string for a pattern, and if found, replaces that pattern | |
2019 | with the replacement text and returns the number of substitutions | |
2020 | made. Otherwise it returns false (specifically, the empty string). | |
2021 | ||
c543c01b | 2022 | If the C</r> (non-destructive) option is used then it runs the |
679563bb KW |
2023 | substitution on a copy of the string and instead of returning the |
2024 | number of substitutions, it returns the copy whether or not a | |
c543c01b TC |
2025 | substitution occurred. The original string is never changed when |
2026 | C</r> is used. The copy will always be a plain string, even if the | |
2027 | input is an object or a tied variable. | |
4f4d7508 | 2028 | |
87e95b7f | 2029 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> |
c543c01b TC |
2030 | variable is searched and modified. Unless the C</r> option is used, |
2031 | the string specified must be a scalar variable, an array element, a | |
2032 | hash element, or an assignment to one of those; that is, some sort of | |
2033 | scalar lvalue. | |
87e95b7f YO |
2034 | |
2035 | If the delimiter chosen is a single quote, no interpolation is | |
2036 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the | |
2037 | PATTERN contains a $ that looks like a variable rather than an | |
2038 | end-of-string test, the variable will be interpolated into the pattern | |
2039 | at run-time. If you want the pattern compiled only once the first time | |
2040 | the variable is interpolated, use the C</o> option. If the pattern | |
2041 | evaluates to the empty string, the last successfully executed regular | |
2042 | expression is used instead. See L<perlre> for further explanation on these. | |
87e95b7f YO |
2043 | |
2044 | Options are as with m// with the addition of the following replacement | |
2045 | specific options: | |
2046 | ||
2047 | e Evaluate the right side as an expression. | |
7188ca43 KW |
2048 | ee Evaluate the right side as a string then eval the |
2049 | result. | |
2050 | r Return substitution and leave the original string | |
2051 | untouched. | |
87e95b7f | 2052 | |
ed02a3bf DN |
2053 | Any non-whitespace delimiter may replace the slashes. Add space after |
2054 | the C<s> when using a character allowed in identifiers. If single quotes | |
2055 | are used, no interpretation is done on the replacement string (the C</e> | |
3ff8ecf9 | 2056 | modifier overrides this, however). Note that Perl treats backticks |
ed02a3bf DN |
2057 | as normal delimiters; the replacement text is not evaluated as a command. |
2058 | If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has | |
1ca345ed | 2059 | its own pair of quotes, which may or may not be bracketing quotes, for example, |
87e95b7f YO |
2060 | C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the |
2061 | replacement portion to be treated as a full-fledged Perl expression | |
2062 | and evaluated right then and there. It is, however, syntax checked at | |
46f8a5ea | 2063 | compile-time. A second C<e> modifier will cause the replacement portion |
87e95b7f YO |
2064 | to be C<eval>ed before being run as a Perl expression. |
2065 | ||
2066 | Examples: | |
2067 | ||
7188ca43 | 2068 | s/\bgreen\b/mauve/g; # don't change wintergreen |
87e95b7f YO |
2069 | |
2070 | $path =~ s|/usr/bin|/usr/local/bin|; | |
2071 | ||
2072 | s/Login: $foo/Login: $bar/; # run-time pattern | |
2073 | ||
7188ca43 KW |
2074 | ($foo = $bar) =~ s/this/that/; # copy first, then |
2075 | # change | |
2076 | ($foo = "$bar") =~ s/this/that/; # convert to string, | |
2077 | # copy, then change | |
4f4d7508 DC |
2078 | $foo = $bar =~ s/this/that/r; # Same as above using /r |
2079 | $foo = $bar =~ s/this/that/r | |
7188ca43 KW |
2080 | =~ s/that/the other/r; # Chained substitutes |
2081 | # using /r | |
2082 | @foo = map { s/this/that/r } @bar # /r is very useful in | |
2083 | # maps | |
87e95b7f | 2084 | |
7188ca43 | 2085 | $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-cnt |
87e95b7f YO |
2086 | |
2087 | $_ = 'abc123xyz'; | |
2088 | s/\d+/$&*2/e; # yields 'abc246xyz' | |
2089 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' | |
2090 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' | |
2091 | ||
2092 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e | |
2093 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e | |
2094 | s/^=(\w+)/pod($1)/ge; # use function call | |
2095 | ||
4f4d7508 | 2096 | $_ = 'abc123xyz'; |
db691027 | 2097 | $x = s/abc/def/r; # $x is 'def123xyz' and |
4f4d7508 DC |
2098 | # $_ remains 'abc123xyz'. |
2099 | ||
87e95b7f YO |
2100 | # expand variables in $_, but dynamics only, using |
2101 | # symbolic dereferencing | |
2102 | s/\$(\w+)/${$1}/g; | |
2103 | ||
2104 | # Add one to the value of any numbers in the string | |
2105 | s/(\d+)/1 + $1/eg; | |
2106 | ||
c543c01b TC |
2107 | # Titlecase words in the last 30 characters only |
2108 | substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g; | |
2109 | ||
87e95b7f YO |
2110 | # This will expand any embedded scalar variable |
2111 | # (including lexicals) in $_ : First $1 is interpolated | |
2112 | # to the variable name, and then evaluated | |
2113 | s/(\$\w+)/$1/eeg; | |
2114 | ||
2115 | # Delete (most) C comments. | |
2116 | $program =~ s { | |
2117 | /\* # Match the opening delimiter. | |
2118 | .*? # Match a minimal number of characters. | |
2119 | \*/ # Match the closing delimiter. | |
2120 | } []gsx; | |
2121 | ||
7188ca43 KW |
2122 | s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, |
2123 | # expensively | |
87e95b7f | 2124 | |
7188ca43 KW |
2125 | for ($variable) { # trim whitespace in $variable, |
2126 | # cheap | |
87e95b7f YO |
2127 | s/^\s+//; |
2128 | s/\s+$//; | |
2129 | } | |
2130 | ||
2131 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields | |
2132 | ||
2133 | Note the use of $ instead of \ in the last example. Unlike | |
2134 | B<sed>, we use the \<I<digit>> form in only the left hand side. | |
2135 | Anywhere else it's $<I<digit>>. | |
2136 | ||
2137 | Occasionally, you can't use just a C</g> to get all the changes | |
2138 | to occur that you might want. Here are two common cases: | |
2139 | ||
2140 | # put commas in the right places in an integer | |
2141 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; | |
2142 | ||
2143 | # expand tabs to 8-column spacing | |
2144 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; | |
2145 | ||
2146 | =back | |
2147 | ||
2148 | =head2 Quote-Like Operators | |
2149 | X<operator, quote-like> | |
2150 | ||
01c6f5f4 RGS |
2151 | =over 4 |
2152 | ||
a0d0e21e | 2153 | =item q/STRING/ |
5d44bfff | 2154 | X<q> X<quote, single> X<'> X<''> |
a0d0e21e | 2155 | |
5d44bfff | 2156 | =item 'STRING' |
a0d0e21e | 2157 | |
19799a22 | 2158 | A single-quoted, literal string. A backslash represents a backslash |
68dc0745 | 2159 | unless followed by the delimiter or another backslash, in which case |
2160 | the delimiter or backslash is interpolated. | |
a0d0e21e LW |
2161 | |
2162 | $foo = q!I said, "You said, 'She said it.'"!; | |
2163 | $bar = q('This is it.'); | |
68dc0745 | 2164 | $baz = '\n'; # a two-character string |
a0d0e21e LW |
2165 | |
2166 | =item qq/STRING/ | |
d74e8afc | 2167 | X<qq> X<quote, double> X<"> X<""> |
a0d0e21e LW |
2168 | |
2169 | =item "STRING" | |
2170 | ||
2171 | A double-quoted, interpolated string. | |
2172 | ||
2173 | $_ .= qq | |
2174 | (*** The previous line contains the naughty word "$1".\n) | |
19799a22 | 2175 | if /\b(tcl|java|python)\b/i; # :-) |
68dc0745 | 2176 | $baz = "\n"; # a one-character string |
a0d0e21e LW |
2177 | |
2178 | =item qx/STRING/ | |
d74e8afc | 2179 | X<qx> X<`> X<``> X<backtick> |
a0d0e21e LW |
2180 | |
2181 | =item `STRING` | |
2182 | ||
43dd4d21 | 2183 | A string which is (possibly) interpolated and then executed as a |
f703fc96 | 2184 | system command with F</bin/sh> or its equivalent. Shell wildcards, |
43dd4d21 JH |
2185 | pipes, and redirections will be honored. The collected standard |
2186 | output of the command is returned; standard error is unaffected. In | |
2187 | scalar context, it comes back as a single (potentially multi-line) | |
2188 | string, or undef if the command failed. In list context, returns a | |
2189 | list of lines (however you've defined lines with $/ or | |
2190 | $INPUT_RECORD_SEPARATOR), or an empty list if the command failed. | |
5a964f20 TC |
2191 | |
2192 | Because backticks do not affect standard error, use shell file descriptor | |
2193 | syntax (assuming the shell supports this) if you care to address this. | |
2194 | To capture a command's STDERR and STDOUT together: | |
a0d0e21e | 2195 | |
5a964f20 TC |
2196 | $output = `cmd 2>&1`; |
2197 | ||
2198 | To capture a command's STDOUT but discard its STDERR: | |
2199 | ||
2200 | $output = `cmd 2>/dev/null`; | |
2201 | ||
2202 | To capture a command's STDERR but discard its STDOUT (ordering is | |
2203 | important here): | |
2204 | ||
2205 | $output = `cmd 2>&1 1>/dev/null`; | |
2206 | ||
2207 | To exchange a command's STDOUT and STDERR in order to capture the STDERR | |
2208 | but leave its STDOUT to come out the old STDERR: | |
2209 | ||
2210 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; | |
2211 | ||
2212 | To read both a command's STDOUT and its STDERR separately, it's easiest | |
2359510d SD |
2213 | to redirect them separately to files, and then read from those files |
2214 | when the program is done: | |
5a964f20 | 2215 | |
2359510d | 2216 | system("program args 1>program.stdout 2>program.stderr"); |
5a964f20 | 2217 | |
30398227 SP |
2218 | The STDIN filehandle used by the command is inherited from Perl's STDIN. |
2219 | For example: | |
2220 | ||
c543c01b TC |
2221 | open(SPLAT, "stuff") || die "can't open stuff: $!"; |
2222 | open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!"; | |
40bbb707 | 2223 | print STDOUT `sort`; |
30398227 | 2224 | |
40bbb707 | 2225 | will print the sorted contents of the file named F<"stuff">. |
30398227 | 2226 | |
5a964f20 TC |
2227 | Using single-quote as a delimiter protects the command from Perl's |
2228 | double-quote interpolation, passing it on to the shell instead: | |
2229 | ||
2230 | $perl_info = qx(ps $$); # that's Perl's $$ | |
2231 | $shell_info = qx'ps $$'; # that's the new shell's $$ | |
2232 | ||
19799a22 | 2233 | How that string gets evaluated is entirely subject to the command |
5a964f20 TC |
2234 | interpreter on your system. On most platforms, you will have to protect |
2235 | shell metacharacters if you want them treated literally. This is in | |
2236 | practice difficult to do, as it's unclear how to escape which characters. | |
2237 | See L<perlsec> for a clean and safe example of a manual fork() and exec() | |
2238 | to emulate backticks safely. | |
a0d0e21e | 2239 | |
bb32b41a GS |
2240 | On some platforms (notably DOS-like ones), the shell may not be |
2241 | capable of dealing with multiline commands, so putting newlines in | |
2242 | the string may not get you what you want. You may be able to evaluate | |
2243 | multiple commands in a single line by separating them with the command | |
1ca345ed TC |
2244 | separator character, if your shell supports that (for example, C<;> on |
2245 | many Unix shells and C<&> on the Windows NT C<cmd> shell). | |
bb32b41a | 2246 | |
3ff8ecf9 | 2247 | Perl will attempt to flush all files opened for |
0f897271 GS |
2248 | output before starting the child process, but this may not be supported |
2249 | on some platforms (see L<perlport>). To be safe, you may need to set | |
2250 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of | |
2251 | C<IO::Handle> on any open handles. | |
2252 | ||
bb32b41a GS |
2253 | Beware that some command shells may place restrictions on the length |
2254 | of the command line. You must ensure your strings don't exceed this | |
2255 | limit after any necessary interpolations. See the platform-specific | |
2256 | release notes for more details about your particular environment. | |
2257 | ||
5a964f20 TC |
2258 | Using this operator can lead to programs that are difficult to port, |
2259 | because the shell commands called vary between systems, and may in | |
2260 | fact not be present at all. As one example, the C<type> command under | |
2261 | the POSIX shell is very different from the C<type> command under DOS. | |
2262 | That doesn't mean you should go out of your way to avoid backticks | |
2263 | when they're the right way to get something done. Perl was made to be | |
2264 | a glue language, and one of the things it glues together is commands. | |
2265 | Just understand what you're getting yourself into. | |
bb32b41a | 2266 | |
da87341d | 2267 | See L</"I/O Operators"> for more discussion. |
a0d0e21e | 2268 | |
945c54fd | 2269 | =item qw/STRING/ |
d74e8afc | 2270 | X<qw> X<quote, list> X<quote, words> |
945c54fd JH |
2271 | |
2272 | Evaluates to a list of the words extracted out of STRING, using embedded | |
2273 | whitespace as the word delimiters. It can be understood as being roughly | |
2274 | equivalent to: | |
2275 | ||
c543c01b | 2276 | split(" ", q/STRING/); |
945c54fd | 2277 | |
efb1e162 CW |
2278 | the differences being that it generates a real list at compile time, and |
2279 | in scalar context it returns the last element in the list. So | |
945c54fd JH |
2280 | this expression: |
2281 | ||
2282 | qw(foo bar baz) | |
2283 | ||
2284 | is semantically equivalent to the list: | |
2285 | ||
c543c01b | 2286 | "foo", "bar", "baz" |
945c54fd JH |
2287 | |
2288 | Some frequently seen examples: | |
2289 | ||
2290 | use POSIX qw( setlocale localeconv ) | |
2291 | @EXPORT = qw( foo bar baz ); | |
2292 | ||
2293 | A common mistake is to try to separate the words with comma or to | |
2294 | put comments into a multi-line C<qw>-string. For this reason, the | |
89d205f2 | 2295 | C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable) |
945c54fd JH |
2296 | produces warnings if the STRING contains the "," or the "#" character. |
2297 | ||
8ff32507 | 2298 | =item tr/SEARCHLIST/REPLACEMENTLIST/cdsr |
d74e8afc | 2299 | X<tr> X<y> X<transliterate> X</c> X</d> X</s> |
a0d0e21e | 2300 | |
8ff32507 | 2301 | =item y/SEARCHLIST/REPLACEMENTLIST/cdsr |
a0d0e21e | 2302 | |
2c268ad5 | 2303 | Transliterates all occurrences of the characters found in the search list |
a0d0e21e LW |
2304 | with the corresponding character in the replacement list. It returns |
2305 | the number of characters replaced or deleted. If no string is | |
c543c01b TC |
2306 | specified via the C<=~> or C<!~> operator, the $_ string is transliterated. |
2307 | ||
2308 | If the C</r> (non-destructive) option is present, a new copy of the string | |
2309 | is made and its characters transliterated, and this copy is returned no | |
2310 | matter whether it was modified or not: the original string is always | |
2311 | left unchanged. The new copy is always a plain string, even if the input | |
2312 | string is an object or a tied variable. | |
8ada0baa | 2313 | |
c543c01b TC |
2314 | Unless the C</r> option is used, the string specified with C<=~> must be a |
2315 | scalar variable, an array element, a hash element, or an assignment to one | |
2316 | of those; in other words, an lvalue. | |
8ff32507 | 2317 | |
89d205f2 | 2318 | A character range may be specified with a hyphen, so C<tr/A-J/0-9/> |
2c268ad5 | 2319 | does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. |
54310121 | 2320 | For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the |
2321 | SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has | |
c543c01b TC |
2322 | its own pair of quotes, which may or may not be bracketing quotes; |
2323 | for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>. | |
2324 | ||
2325 | Note that C<tr> does B<not> do regular expression character classes such as | |
2326 | C<\d> or C<\pL>. The C<tr> operator is not equivalent to the tr(1) | |
2327 | utility. If you want to map strings between lower/upper cases, see | |
2328 | L<perlfunc/lc> and L<perlfunc/uc>, and in general consider using the C<s> | |
2329 | operator if you need regular expressions. The C<\U>, C<\u>, C<\L>, and | |
2330 | C<\l> string-interpolation escapes on the right side of a substitution | |
2331 | operator will perform correct case-mappings, but C<tr[a-z][A-Z]> will not | |
2332 | (except sometimes on legacy 7-bit data). | |
cc255d5f | 2333 | |
8ada0baa JH |
2334 | Note also that the whole range idea is rather unportable between |
2335 | character sets--and even within character sets they may cause results | |
2336 | you probably didn't expect. A sound principle is to use only ranges | |
2337 | that begin from and end at either alphabets of equal case (a-e, A-E), | |
2338 | or digits (0-4). Anything else is unsafe. If in doubt, spell out the | |
2339 | character sets in full. | |
2340 | ||
a0d0e21e LW |
2341 | Options: |
2342 | ||
2343 | c Complement the SEARCHLIST. | |
2344 | d Delete found but unreplaced characters. | |
2345 | s Squash duplicate replaced characters. | |
8ff32507 FC |
2346 | r Return the modified string and leave the original string |
2347 | untouched. | |
a0d0e21e | 2348 | |
19799a22 GS |
2349 | If the C</c> modifier is specified, the SEARCHLIST character set |
2350 | is complemented. If the C</d> modifier is specified, any characters | |
2351 | specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. | |
2352 | (Note that this is slightly more flexible than the behavior of some | |
2353 | B<tr> programs, which delete anything they find in the SEARCHLIST, | |
46f8a5ea | 2354 | period.) If the C</s> modifier is specified, sequences of characters |
19799a22 GS |
2355 | that were transliterated to the same character are squashed down |
2356 | to a single instance of the character. | |
a0d0e21e LW |
2357 | |
2358 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted | |
2359 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter | |
2360 | than the SEARCHLIST, the final character is replicated till it is long | |
5a964f20 | 2361 | enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. |
a0d0e21e LW |
2362 | This latter is useful for counting characters in a class or for |
2363 | squashing character sequences in a class. | |
2364 | ||
2365 | Examples: | |
2366 | ||
c543c01b | 2367 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII |
a0d0e21e LW |
2368 | |
2369 | $cnt = tr/*/*/; # count the stars in $_ | |
2370 | ||
2371 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky | |
2372 | ||
2373 | $cnt = tr/0-9//; # count the digits in $_ | |
2374 | ||
2375 | tr/a-zA-Z//s; # bookkeeper -> bokeper | |
2376 | ||
2377 | ($HOST = $host) =~ tr/a-z/A-Z/; | |
c543c01b | 2378 | $HOST = $host =~ tr/a-z/A-Z/r; # same thing |
8ff32507 | 2379 | |
c543c01b | 2380 | $HOST = $host =~ tr/a-z/A-Z/r # chained with s///r |
8ff32507 | 2381 | =~ s/:/ -p/r; |
a0d0e21e LW |
2382 | |
2383 | tr/a-zA-Z/ /cs; # change non-alphas to single space | |
2384 | ||
8ff32507 FC |
2385 | @stripped = map tr/a-zA-Z/ /csr, @original; |
2386 | # /r with map | |
2387 | ||
a0d0e21e | 2388 | tr [\200-\377] |
c543c01b | 2389 | [\000-\177]; # wickedly delete 8th bit |
a0d0e21e | 2390 | |
19799a22 GS |
2391 | If multiple transliterations are given for a character, only the |
2392 | first one is used: | |
748a9306 LW |
2393 | |
2394 | tr/AAA/XYZ/ | |
2395 | ||
2c268ad5 | 2396 | will transliterate any A to X. |
748a9306 | 2397 | |
19799a22 | 2398 | Because the transliteration table is built at compile time, neither |
a0d0e21e | 2399 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote |
19799a22 GS |
2400 | interpolation. That means that if you want to use variables, you |
2401 | must use an eval(): | |
a0d0e21e LW |
2402 | |
2403 | eval "tr/$oldlist/$newlist/"; | |
2404 | die $@ if $@; | |
2405 | ||
2406 | eval "tr/$oldlist/$newlist/, 1" or die $@; | |
2407 | ||
7e3b091d | 2408 | =item <<EOF |
d74e8afc | 2409 | X<here-doc> X<heredoc> X<here-document> X<<< << >>> |
7e3b091d DA |
2410 | |
2411 | A line-oriented form of quoting is based on the shell "here-document" | |
2412 | syntax. Following a C<< << >> you specify a string to terminate | |
2413 | the quoted material, and all lines following the current line down to | |
89d205f2 YO |
2414 | the terminating string are the value of the item. |
2415 | ||
2416 | The terminating string may be either an identifier (a word), or some | |
2417 | quoted text. An unquoted identifier works like double quotes. | |
2418 | There may not be a space between the C<< << >> and the identifier, | |
2419 | unless the identifier is explicitly quoted. (If you put a space it | |
2420 | will be treated as a null identifier, which is valid, and matches the | |
2421 | first empty line.) The terminating string must appear by itself | |
2422 | (unquoted and with no surrounding whitespace) on the terminating line. | |
2423 | ||
2424 | If the terminating string is quoted, the type of quotes used determine | |
2425 | the treatment of the text. | |
2426 | ||
2427 | =over 4 | |
2428 | ||
2429 | =item Double Quotes | |
2430 | ||
2431 | Double quotes indicate that the text will be interpolated using exactly | |
2432 | the same rules as normal double quoted strings. | |
7e3b091d DA |
2433 | |
2434 | print <<EOF; | |
2435 | The price is $Price. | |
2436 | EOF | |
2437 | ||
2438 | print << "EOF"; # same as above | |
2439 | The price is $Price. | |
2440 | EOF | |
2441 | ||
89d205f2 YO |
2442 | |
2443 | =item Single Quotes | |
2444 | ||
2445 | Single quotes indicate the text is to be treated literally with no | |
46f8a5ea | 2446 | interpolation of its content. This is similar to single quoted |
89d205f2 YO |
2447 | strings except that backslashes have no special meaning, with C<\\> |
2448 | being treated as two backslashes and not one as they would in every | |
2449 | other quoting construct. | |
2450 | ||
c543c01b TC |
2451 | Just as in the shell, a backslashed bareword following the C<<< << >>> |
2452 | means the same thing as a single-quoted string does: | |
2453 | ||
2454 | $cost = <<'VISTA'; # hasta la ... | |
2455 | That'll be $10 please, ma'am. | |
2456 | VISTA | |
2457 | ||
2458 | $cost = <<\VISTA; # Same thing! | |
2459 | That'll be $10 please, ma'am. | |
2460 | VISTA | |
2461 | ||
89d205f2 YO |
2462 | This is the only form of quoting in perl where there is no need |
2463 | to worry about escaping content, something that code generators | |
2464 | can and do make good use of. | |
2465 | ||
2466 | =item Backticks | |
2467 | ||
2468 | The content of the here doc is treated just as it would be if the | |
46f8a5ea | 2469 | string were embedded in backticks. Thus the content is interpolated |
89d205f2 YO |
2470 | as though it were double quoted and then executed via the shell, with |
2471 | the results of the execution returned. | |
2472 | ||
2473 | print << `EOC`; # execute command and get results | |
7e3b091d | 2474 | echo hi there |
7e3b091d DA |
2475 | EOC |
2476 | ||
89d205f2 YO |
2477 | =back |
2478 | ||
2479 | It is possible to stack multiple here-docs in a row: | |
2480 | ||
7e3b091d DA |
2481 | print <<"foo", <<"bar"; # you can stack them |
2482 | I said foo. | |
2483 | foo | |
2484 | I said bar. | |
2485 | bar | |
2486 | ||
2487 | myfunc(<< "THIS", 23, <<'THAT'); | |
2488 | Here's a line | |
2489 | or two. | |
2490 | THIS | |
2491 | and here's another. | |
2492 | THAT | |
2493 | ||
2494 | Just don't forget that you have to put a semicolon on the end | |
2495 | to finish the statement, as Perl doesn't know you're not going to | |
2496 | try to do this: | |
2497 | ||
2498 | print <<ABC | |
2499 | 179231 | |
2500 | ABC | |
2501 | + 20; | |
2502 | ||
872d7e53 TS |
2503 | If you want to remove the line terminator from your here-docs, |
2504 | use C<chomp()>. | |
2505 | ||
2506 | chomp($string = <<'END'); | |
2507 | This is a string. | |
2508 | END | |
2509 | ||
2510 | If you want your here-docs to be indented with the rest of the code, | |
2511 | you'll need to remove leading whitespace from each line manually: | |
7e3b091d DA |
2512 | |
2513 | ($quote = <<'FINIS') =~ s/^\s+//gm; | |
89d205f2 | 2514 | The Road goes ever on and on, |
7e3b091d DA |
2515 | down from the door where it began. |
2516 | FINIS | |
2517 | ||
2518 | If you use a here-doc within a delimited construct, such as in C<s///eg>, | |
1bf48760 FC |
2519 | the quoted material must still come on the line following the |
2520 | C<<< <<FOO >>> marker, which means it may be inside the delimited | |
2521 | construct: | |
7e3b091d DA |
2522 | |
2523 | s/this/<<E . 'that' | |
2524 | the other | |
2525 | E | |
2526 | . 'more '/eg; | |
2527 | ||
1bf48760 FC |
2528 | It works this way as of Perl 5.18. Historically, it was inconsistent, and |
2529 | you would have to write | |
7e3b091d | 2530 | |
89d205f2 YO |
2531 | s/this/<<E . 'that' |
2532 | . 'more '/eg; | |
2533 | the other | |
2534 | E | |
7e3b091d | 2535 | |
1bf48760 FC |
2536 | outside of string evals. |
2537 | ||
c543c01b | 2538 | Additionally, quoting rules for the end-of-string identifier are |
46f8a5ea | 2539 | unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not |
89d205f2 YO |
2540 | supported in place of C<''> and C<"">, and the only interpolation is for |
2541 | backslashing the quoting character: | |
7e3b091d DA |
2542 | |
2543 | print << "abc\"def"; | |
2544 | testing... | |
2545 | abc"def | |
2546 | ||
2547 | Finally, quoted strings cannot span multiple lines. The general rule is | |
2548 | that the identifier must be a string literal. Stick with that, and you | |
2549 | should be safe. | |
2550 | ||
a0d0e21e LW |
2551 | =back |
2552 | ||
75e14d17 | 2553 | =head2 Gory details of parsing quoted constructs |
d74e8afc | 2554 | X<quote, gory details> |
75e14d17 | 2555 | |
19799a22 GS |
2556 | When presented with something that might have several different |
2557 | interpretations, Perl uses the B<DWIM> (that's "Do What I Mean") | |
2558 | principle to pick the most probable interpretation. This strategy | |
2559 | is so successful that Perl programmers often do not suspect the | |
2560 | ambivalence of what they write. But from time to time, Perl's | |
2561 | notions differ substantially from what the author honestly meant. | |
2562 | ||
2563 | This section hopes to clarify how Perl handles quoted constructs. | |
2564 | Although the most common reason to learn this is to unravel labyrinthine | |
2565 | regular expressions, because the initial steps of parsing are the | |
2566 | same for all quoting operators, they are all discussed together. | |
2567 | ||
2568 | The most important Perl parsing rule is the first one discussed | |
2569 | below: when processing a quoted construct, Perl first finds the end | |
2570 | of that construct, then interprets its contents. If you understand | |
2571 | this rule, you may skip the rest of this section on the first | |
2572 | reading. The other rules are likely to contradict the user's | |
2573 | expectations much less frequently than this first one. | |
2574 | ||
2575 | Some passes discussed below are performed concurrently, but because | |
2576 | their results are the same, we consider them individually. For different | |
2577 | quoting constructs, Perl performs different numbers of passes, from | |
6deea57f | 2578 | one to four, but these passes are always performed in the same order. |
75e14d17 | 2579 | |
13a2d996 | 2580 | =over 4 |
75e14d17 IZ |
2581 | |
2582 | =item Finding the end | |
2583 | ||
6deea57f TS |
2584 | The first pass is finding the end of the quoted construct, where |
2585 | the information about the delimiters is used in parsing. | |
2586 | During this search, text between the starting and ending delimiters | |
46f8a5ea | 2587 | is copied to a safe location. The text copied gets delimiter-independent. |
6deea57f TS |
2588 | |
2589 | If the construct is a here-doc, the ending delimiter is a line | |
46f8a5ea | 2590 | that has a terminating string as the content. Therefore C<<<EOF> is |
6deea57f TS |
2591 | terminated by C<EOF> immediately followed by C<"\n"> and starting |
2592 | from the first column of the terminating line. | |
2593 | When searching for the terminating line of a here-doc, nothing | |
46f8a5ea | 2594 | is skipped. In other words, lines after the here-doc syntax |
6deea57f TS |
2595 | are compared with the terminating string line by line. |
2596 | ||
2597 | For the constructs except here-docs, single characters are used as starting | |
46f8a5ea | 2598 | and ending delimiters. If the starting delimiter is an opening punctuation |
6deea57f TS |
2599 | (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the |
2600 | corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>). | |
2601 | If the starting delimiter is an unpaired character like C</> or a closing | |
2602 | punctuation, the ending delimiter is same as the starting delimiter. | |
2603 | Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates | |
fc693347 | 2604 | both C<qq[]> and C<qq]]> constructs. |
6deea57f TS |
2605 | |
2606 | When searching for single-character delimiters, escaped delimiters | |
1ca345ed | 2607 | and C<\\> are skipped. For example, while searching for terminating C</>, |
6deea57f TS |
2608 | combinations of C<\\> and C<\/> are skipped. If the delimiters are |
2609 | bracketing, nested pairs are also skipped. For example, while searching | |
2610 | for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>, | |
2611 | and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well. | |
2612 | However, when backslashes are used as the delimiters (like C<qq\\> and | |
2613 | C<tr\\\>), nothing is skipped. | |
32581033 | 2614 | During the search for the end, backslashes that escape delimiters or |
7188ca43 | 2615 | other backslashes are removed (exactly speaking, they are not copied to the |
32581033 | 2616 | safe location). |
75e14d17 | 2617 | |
19799a22 GS |
2618 | For constructs with three-part delimiters (C<s///>, C<y///>, and |
2619 | C<tr///>), the search is repeated once more. | |
fc693347 | 2620 | If the first delimiter is not an opening punctuation, the three delimiters must |
d74605e5 FC |
2621 | be the same, such as C<s!!!> and C<tr)))>, |
2622 | in which case the second delimiter | |
6deea57f | 2623 | terminates the left part and starts the right part at once. |
b6538e4f | 2624 | If the left part is delimited by bracketing punctuation (that is C<()>, |
6deea57f | 2625 | C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of |
b6538e4f | 2626 | delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace |
fc693347 | 2627 | and comments are allowed between the two parts, though the comment must follow |
b6538e4f TC |
2628 | at least one whitespace character; otherwise a character expected as the |
2629 | start of the comment may be regarded as the starting delimiter of the right part. | |
75e14d17 | 2630 | |
19799a22 GS |
2631 | During this search no attention is paid to the semantics of the construct. |
2632 | Thus: | |
75e14d17 IZ |
2633 | |
2634 | "$hash{"$foo/$bar"}" | |
2635 | ||
2a94b7ce | 2636 | or: |
75e14d17 | 2637 | |
89d205f2 | 2638 | m/ |
2a94b7ce | 2639 | bar # NOT a comment, this slash / terminated m//! |
75e14d17 IZ |
2640 | /x |
2641 | ||
19799a22 GS |
2642 | do not form legal quoted expressions. The quoted part ends on the |
2643 | first C<"> and C</>, and the rest happens to be a syntax error. | |
2644 | Because the slash that terminated C<m//> was followed by a C<SPACE>, | |
2645 | the example above is not C<m//x>, but rather C<m//> with no C</x> | |
2646 | modifier. So the embedded C<#> is interpreted as a literal C<#>. | |
75e14d17 | 2647 | |
89d205f2 | 2648 | Also no attention is paid to C<\c\> (multichar control char syntax) during |
46f8a5ea | 2649 | this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part |
89d205f2 | 2650 | of C<\/>, and the following C</> is not recognized as a delimiter. |
0d594e51 TS |
2651 | Instead, use C<\034> or C<\x1c> at the end of quoted constructs. |
2652 | ||
75e14d17 | 2653 | =item Interpolation |
d74e8afc | 2654 | X<interpolation> |
75e14d17 | 2655 | |
19799a22 | 2656 | The next step is interpolation in the text obtained, which is now |
89d205f2 | 2657 | delimiter-independent. There are multiple cases. |
75e14d17 | 2658 | |
13a2d996 | 2659 | =over 4 |
75e14d17 | 2660 | |
89d205f2 | 2661 | =item C<<<'EOF'> |
75e14d17 IZ |
2662 | |
2663 | No interpolation is performed. | |
6deea57f TS |
2664 | Note that the combination C<\\> is left intact, since escaped delimiters |
2665 | are not available for here-docs. | |
75e14d17 | 2666 | |
6deea57f | 2667 | =item C<m''>, the pattern of C<s'''> |
89d205f2 | 2668 | |
6deea57f TS |
2669 | No interpolation is performed at this stage. |
2670 | Any backslashed sequences including C<\\> are treated at the stage | |
2671 | to L</"parsing regular expressions">. | |
89d205f2 | 2672 | |
6deea57f | 2673 | =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''> |
75e14d17 | 2674 | |
89d205f2 | 2675 | The only interpolation is removal of C<\> from pairs of C<\\>. |
6deea57f TS |
2676 | Therefore C<-> in C<tr'''> and C<y'''> is treated literally |
2677 | as a hyphen and no character range is available. | |
2678 | C<\1> in the replacement of C<s'''> does not work as C<$1>. | |
89d205f2 YO |
2679 | |
2680 | =item C<tr///>, C<y///> | |
2681 | ||
6deea57f TS |
2682 | No variable interpolation occurs. String modifying combinations for |
2683 | case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized. | |
2684 | The other escape sequences such as C<\200> and C<\t> and backslashed | |
2685 | characters such as C<\\> and C<\-> are converted to appropriate literals. | |
89d205f2 YO |
2686 | The character C<-> is treated specially and therefore C<\-> is treated |
2687 | as a literal C<->. | |
75e14d17 | 2688 | |
89d205f2 | 2689 | =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF"> |
75e14d17 | 2690 | |
628253b8 | 2691 | C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> (possibly paired with C<\E>) are |
19799a22 GS |
2692 | converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar"> |
2693 | is converted to C<$foo . (quotemeta("baz" . $bar))> internally. | |
6deea57f TS |
2694 | The other escape sequences such as C<\200> and C<\t> and backslashed |
2695 | characters such as C<\\> and C<\-> are replaced with appropriate | |
2696 | expansions. | |
2a94b7ce | 2697 | |
19799a22 GS |
2698 | Let it be stressed that I<whatever falls between C<\Q> and C<\E>> |
2699 | is interpolated in the usual way. Something like C<"\Q\\E"> has | |
48cbae4f | 2700 | no C<\E> inside. Instead, it has C<\Q>, C<\\>, and C<E>, so the |
19799a22 GS |
2701 | result is the same as for C<"\\\\E">. As a general rule, backslashes |
2702 | between C<\Q> and C<\E> may lead to counterintuitive results. So, | |
2703 | C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same | |
2704 | as C<"\\\t"> (since TAB is not alphanumeric). Note also that: | |
2a94b7ce IZ |
2705 | |
2706 | $str = '\t'; | |
2707 | return "\Q$str"; | |
2708 | ||
2709 | may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. | |
2710 | ||
19799a22 | 2711 | Interpolated scalars and arrays are converted internally to the C<join> and |
92d29cee | 2712 | C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes: |
75e14d17 | 2713 | |
19799a22 | 2714 | $foo . " XXX '" . (join $", @arr) . "'"; |
75e14d17 | 2715 | |
19799a22 | 2716 | All operations above are performed simultaneously, left to right. |
75e14d17 | 2717 | |
19799a22 GS |
2718 | Because the result of C<"\Q STRING \E"> has all metacharacters |
2719 | quoted, there is no way to insert a literal C<$> or C<@> inside a | |
2720 | C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became | |
2721 | C<"\\\$">; if not, it is interpreted as the start of an interpolated | |
2722 | scalar. | |
75e14d17 | 2723 | |
19799a22 | 2724 | Note also that the interpolation code needs to make a decision on |
89d205f2 | 2725 | where the interpolated scalar ends. For instance, whether |
db691027 | 2726 | C<< "a $x -> {c}" >> really means: |
75e14d17 | 2727 | |
db691027 | 2728 | "a " . $x . " -> {c}"; |
75e14d17 | 2729 | |
2a94b7ce | 2730 | or: |
75e14d17 | 2731 | |
db691027 | 2732 | "a " . $x -> {c}; |
75e14d17 | 2733 | |
19799a22 GS |
2734 | Most of the time, the longest possible text that does not include |
2735 | spaces between components and which contains matching braces or | |
2736 | brackets. because the outcome may be determined by voting based | |
2737 | on heuristic estimators, the result is not strictly predictable. | |
2738 | Fortunately, it's usually correct for ambiguous cases. | |
75e14d17 | 2739 | |
6deea57f | 2740 | =item the replacement of C<s///> |
75e14d17 | 2741 | |
628253b8 | 2742 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> and interpolation |
6deea57f TS |
2743 | happens as with C<qq//> constructs. |
2744 | ||
2745 | It is at this step that C<\1> is begrudgingly converted to C<$1> in | |
2746 | the replacement text of C<s///>, in order to correct the incorrigible | |
2747 | I<sed> hackers who haven't picked up the saner idiom yet. A warning | |
2748 | is emitted if the C<use warnings> pragma or the B<-w> command-line flag | |
2749 | (that is, the C<$^W> variable) was set. | |
2750 | ||
2751 | =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, | |
2752 | ||
628253b8 | 2753 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F>, C<\E>, |
cc74c5bd TS |
2754 | and interpolation happens (almost) as with C<qq//> constructs. |
2755 | ||
5d03b57c KW |
2756 | Processing of C<\N{...}> is also done here, and compiled into an intermediate |
2757 | form for the regex compiler. (This is because, as mentioned below, the regex | |
2758 | compilation may be done at execution time, and C<\N{...}> is a compile-time | |
2759 | construct.) | |
2760 | ||
cc74c5bd TS |
2761 | However any other combinations of C<\> followed by a character |
2762 | are not substituted but only skipped, in order to parse them | |
2763 | as regular expressions at the following step. | |
6deea57f | 2764 | As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly |
1749ea0d | 2765 | treated as an array symbol (for example C<@foo>), |
6deea57f | 2766 | even though the same text in C<qq//> gives interpolation of C<\c@>. |
6deea57f | 2767 | |
e128ab2c DM |
2768 | Code blocks such as C<(?{BLOCK})> are handled by temporarily passing control |
2769 | back to the perl parser, in a similar way that an interpolated array | |
2770 | subscript expression such as C<"foo$array[1+f("[xyz")]bar"> would be. | |
2771 | ||
6deea57f | 2772 | Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and |
19799a22 GS |
2773 | a C<#>-comment in a C<//x>-regular expression, no processing is |
2774 | performed whatsoever. This is the first step at which the presence | |
2775 | of the C<//x> modifier is relevant. | |
2776 | ||
1749ea0d TS |
2777 | Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+> |
2778 | and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are | |
2779 | voted (by several different estimators) to be either an array element | |
2780 | or C<$var> followed by an RE alternative. This is where the notation | |
19799a22 GS |
2781 | C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as |
2782 | array element C<-9>, not as a regular expression from the variable | |
2783 | C<$arr> followed by a digit, which would be the interpretation of | |
2784 | C</$arr[0-9]/>. Since voting among different estimators may occur, | |
2785 | the result is not predictable. | |
2786 | ||
19799a22 GS |
2787 | The lack of processing of C<\\> creates specific restrictions on |
2788 | the post-processed text. If the delimiter is C</>, one cannot get | |
2789 | the combination C<\/> into the result of this step. C</> will | |
2790 | finish the regular expression, C<\/> will be stripped to C</> on | |
2791 | the previous step, and C<\\/> will be left as is. Because C</> is | |
2792 | equivalent to C<\/> inside a regular expression, this does not | |
2793 | matter unless the delimiter happens to be character special to the | |
2794 | RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an | |
2795 | alphanumeric char, as in: | |
2a94b7ce IZ |
2796 | |
2797 | m m ^ a \s* b mmx; | |
2798 | ||
19799a22 | 2799 | In the RE above, which is intentionally obfuscated for illustration, the |
6deea57f | 2800 | delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the |
89d205f2 | 2801 | RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one |
19799a22 GS |
2802 | reason you're encouraged to restrict your delimiters to non-alphanumeric, |
2803 | non-whitespace choices. | |
75e14d17 IZ |
2804 | |
2805 | =back | |
2806 | ||
19799a22 | 2807 | This step is the last one for all constructs except regular expressions, |
75e14d17 IZ |
2808 | which are processed further. |
2809 | ||
6deea57f TS |
2810 | =item parsing regular expressions |
2811 | X<regexp, parse> | |
75e14d17 | 2812 | |
19799a22 | 2813 | Previous steps were performed during the compilation of Perl code, |
ac036724 | 2814 | but this one happens at run time, although it may be optimized to |
19799a22 | 2815 | be calculated at compile time if appropriate. After preprocessing |
6deea57f | 2816 | described above, and possibly after evaluation if concatenation, |
19799a22 GS |
2817 | joining, casing translation, or metaquoting are involved, the |
2818 | resulting I<string> is passed to the RE engine for compilation. | |
2819 | ||
2820 | Whatever happens in the RE engine might be better discussed in L<perlre>, | |
2821 | but for the sake of continuity, we shall do so here. | |
2822 | ||
2823 | This is another step where the presence of the C<//x> modifier is | |
2824 | relevant. The RE engine scans the string from left to right and | |
2825 | converts it to a finite automaton. | |
2826 | ||
2827 | Backslashed characters are either replaced with corresponding | |
2828 | literal strings (as with C<\{>), or else they generate special nodes | |
2829 | in the finite automaton (as with C<\b>). Characters special to the | |
2830 | RE engine (such as C<|>) generate corresponding nodes or groups of | |
2831 | nodes. C<(?#...)> comments are ignored. All the rest is either | |
2832 | converted to literal strings to match, or else is ignored (as is | |
2833 | whitespace and C<#>-style comments if C<//x> is present). | |
2834 | ||
2835 | Parsing of the bracketed character class construct, C<[...]>, is | |
2836 | rather different than the rule used for the rest of the pattern. | |
2837 | The terminator of this construct is found using the same rules as | |
2838 | for finding the terminator of a C<{}>-delimited construct, the only | |
2839 | exception being that C<]> immediately following C<[> is treated as | |
e128ab2c DM |
2840 | though preceded by a backslash. |
2841 | ||
2842 | The terminator of runtime C<(?{...})> is found by temporarily switching | |
2843 | control to the perl parser, which should stop at the point where the | |
2844 | logically balancing terminating C<}> is found. | |
19799a22 GS |
2845 | |
2846 | It is possible to inspect both the string given to RE engine and the | |
2847 | resulting finite automaton. See the arguments C<debug>/C<debugcolor> | |
2848 | in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line | |
4a4eefd0 | 2849 | switch documented in L<perlrun/"Command Switches">. |
75e14d17 IZ |
2850 | |
2851 | =item Optimization of regular expressions | |
d74e8afc | 2852 | X<regexp, optimization> |
75e14d17 | 2853 | |
7522fed5 | 2854 | This step is listed for completeness only. Since it does not change |
75e14d17 | 2855 | semantics, details of this step are not documented and are subject |
19799a22 GS |
2856 | to change without notice. This step is performed over the finite |
2857 | automaton that was generated during the previous pass. | |
2a94b7ce | 2858 | |
19799a22 GS |
2859 | It is at this stage that C<split()> silently optimizes C</^/> to |
2860 | mean C</^/m>. | |
75e14d17 IZ |
2861 | |
2862 | =back | |
2863 | ||
a0d0e21e | 2864 | =head2 I/O Operators |
d74e8afc | 2865 | X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle> |
80a96bfc | 2866 | X<< <> >> X<< <<>> >> X<@ARGV> |
a0d0e21e | 2867 | |
54310121 | 2868 | There are several I/O operators you should know about. |
fbad3eb5 | 2869 | |
7b8d334a | 2870 | A string enclosed by backticks (grave accents) first undergoes |
19799a22 GS |
2871 | double-quote interpolation. It is then interpreted as an external |
2872 | command, and the output of that command is the value of the | |
e9c56f9b JH |
2873 | backtick string, like in a shell. In scalar context, a single string |
2874 | consisting of all output is returned. In list context, a list of | |
2875 | values is returned, one per line of output. (You can set C<$/> to use | |
2876 | a different line terminator.) The command is executed each time the | |
2877 | pseudo-literal is evaluated. The status value of the command is | |
2878 | returned in C<$?> (see L<perlvar> for the interpretation of C<$?>). | |
2879 | Unlike in B<csh>, no translation is done on the return data--newlines | |
2880 | remain newlines. Unlike in any of the shells, single quotes do not | |
2881 | hide variable names in the command from interpretation. To pass a | |
2882 | literal dollar-sign through to the shell you need to hide it with a | |
2883 | backslash. The generalized form of backticks is C<qx//>. (Because | |
2884 | backticks always undergo shell expansion as well, see L<perlsec> for | |
2885 | security concerns.) | |
d74e8afc | 2886 | X<qx> X<`> X<``> X<backtick> X<glob> |
19799a22 GS |
2887 | |
2888 | In scalar context, evaluating a filehandle in angle brackets yields | |
2889 | the next line from that file (the newline, if any, included), or | |
2890 | C<undef> at end-of-file or on error. When C<$/> is set to C<undef> | |
2891 | (sometimes known as file-slurp mode) and the file is empty, it | |
2892 | returns C<''> the first time, followed by C<undef> subsequently. | |
2893 | ||
2894 | Ordinarily you must assign the returned value to a variable, but | |
2895 | there is one situation where an automatic assignment happens. If | |
2896 | and only if the input symbol is the only thing inside the conditional | |
2897 | of a C<while> statement (even if disguised as a C<for(;;)> loop), | |
2898 | the value is automatically assigned to the global variable $_, | |
2899 | destroying whatever was there previously. (This may seem like an | |
2900 | odd thing to you, but you'll use the construct in almost every Perl | |
17b829fa | 2901 | script you write.) The $_ variable is not implicitly localized. |
19799a22 GS |
2902 | You'll have to put a C<local $_;> before the loop if you want that |
2903 | to happen. | |
2904 | ||
2905 | The following lines are equivalent: | |
a0d0e21e | 2906 | |
748a9306 | 2907 | while (defined($_ = <STDIN>)) { print; } |
7b8d334a | 2908 | while ($_ = <STDIN>) { print; } |
a0d0e21e LW |
2909 | while (<STDIN>) { print; } |
2910 | for (;<STDIN>;) { print; } | |
748a9306 | 2911 | print while defined($_ = <STDIN>); |
7b8d334a | 2912 | print while ($_ = <STDIN>); |
a0d0e21e LW |
2913 | print while <STDIN>; |
2914 | ||
1ca345ed TC |
2915 | This also behaves similarly, but assigns to a lexical variable |
2916 | instead of to C<$_>: | |
7b8d334a | 2917 | |
89d205f2 | 2918 | while (my $line = <STDIN>) { print $line } |
7b8d334a | 2919 | |
19799a22 GS |
2920 | In these loop constructs, the assigned value (whether assignment |
2921 | is automatic or explicit) is then tested to see whether it is | |
1ca345ed TC |
2922 | defined. The defined test avoids problems where the line has a string |
2923 | value that would be treated as false by Perl; for example a "" or | |
19799a22 GS |
2924 | a "0" with no trailing newline. If you really mean for such values |
2925 | to terminate the loop, they should be tested for explicitly: | |
7b8d334a GS |
2926 | |
2927 | while (($_ = <STDIN>) ne '0') { ... } | |
2928 | while (<STDIN>) { last unless $_; ... } | |
2929 | ||
1ca345ed | 2930 | In other boolean contexts, C<< <FILEHANDLE> >> without an |
5ef4d93e | 2931 | explicit C<defined> test or comparison elicits a warning if the |
9f1b1f2d | 2932 | C<use warnings> pragma or the B<-w> |
19799a22 | 2933 | command-line switch (the C<$^W> variable) is in effect. |
7b8d334a | 2934 | |
5f05dabc | 2935 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
19799a22 GS |
2936 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except |
2937 | in packages, where they would be interpreted as local identifiers | |
2938 | rather than global.) Additional filehandles may be created with | |
2939 | the open() function, amongst others. See L<perlopentut> and | |
2940 | L<perlfunc/open> for details on this. | |
d74e8afc | 2941 | X<stdin> X<stdout> X<sterr> |
a0d0e21e | 2942 | |
35f2feb0 | 2943 | If a <FILEHANDLE> is used in a context that is looking for |
19799a22 GS |
2944 | a list, a list comprising all input lines is returned, one line per |
2945 | list element. It's easy to grow to a rather large data space this | |
2946 | way, so use with care. | |
a0d0e21e | 2947 | |
35f2feb0 | 2948 | <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>. |
19799a22 | 2949 | See L<perlfunc/readline>. |
fbad3eb5 | 2950 | |
35f2feb0 | 2951 | The null filehandle <> is special: it can be used to emulate the |
1ca345ed TC |
2952 | behavior of B<sed> and B<awk>, and any other Unix filter program |
2953 | that takes a list of filenames, doing the same to each line | |
2954 | of input from all of them. Input from <> comes either from | |
a0d0e21e | 2955 | standard input, or from each file listed on the command line. Here's |
35f2feb0 | 2956 | how it works: the first time <> is evaluated, the @ARGV array is |
5a964f20 | 2957 | checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened |
a0d0e21e LW |
2958 | gives you standard input. The @ARGV array is then processed as a list |
2959 | of filenames. The loop | |
2960 | ||
2961 | while (<>) { | |
2962 | ... # code for each line | |
2963 | } | |
2964 | ||
2965 | is equivalent to the following Perl-like pseudo code: | |
2966 | ||
3e3baf6d | 2967 | unshift(@ARGV, '-') unless @ARGV; |
a0d0e21e LW |
2968 | while ($ARGV = shift) { |
2969 | open(ARGV, $ARGV); | |
2970 | while (<ARGV>) { | |
2971 | ... # code for each line | |
2972 | } | |
2973 | } | |
2974 | ||
19799a22 GS |
2975 | except that it isn't so cumbersome to say, and will actually work. |
2976 | It really does shift the @ARGV array and put the current filename | |
2977 | into the $ARGV variable. It also uses filehandle I<ARGV> | |
46f8a5ea | 2978 | internally. <> is just a synonym for <ARGV>, which |
19799a22 | 2979 | is magical. (The pseudo code above doesn't work because it treats |
35f2feb0 | 2980 | <ARGV> as non-magical.) |
a0d0e21e | 2981 | |
48ab5743 ML |
2982 | Since the null filehandle uses the two argument form of L<perlfunc/open> |
2983 | it interprets special characters, so if you have a script like this: | |
2984 | ||
2985 | while (<>) { | |
2986 | print; | |
2987 | } | |
2988 | ||
2989 | and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a | |
2990 | pipe, executes the C<rm> command and reads C<rm>'s output from that pipe. | |
2991 | If you want all items in C<@ARGV> to be interpreted as file names, you | |
1033ba6e PM |
2992 | can use the module C<ARGV::readonly> from CPAN, or use the double bracket: |
2993 | ||
2994 | while (<<>>) { | |
2995 | print; | |
2996 | } | |
2997 | ||
2998 | Using double angle brackets inside of a while causes the open to use the | |
2999 | three argument form (with the second argument being C<< < >>), so all | |
80a96bfc RGS |
3000 | arguments in ARGV are treated as literal filenames (including "-"). |
3001 | (Note that for convenience, if you use C<< <<>> >> and if @ARGV is | |
3002 | empty, it will still read from the standard input.) | |
48ab5743 | 3003 | |
35f2feb0 | 3004 | You can modify @ARGV before the first <> as long as the array ends up |
a0d0e21e | 3005 | containing the list of filenames you really want. Line numbers (C<$.>) |
19799a22 GS |
3006 | continue as though the input were one big happy file. See the example |
3007 | in L<perlfunc/eof> for how to reset line numbers on each file. | |
5a964f20 | 3008 | |
89d205f2 | 3009 | If you want to set @ARGV to your own list of files, go right ahead. |
5a964f20 TC |
3010 | This sets @ARGV to all plain text files if no @ARGV was given: |
3011 | ||
3012 | @ARGV = grep { -f && -T } glob('*') unless @ARGV; | |
a0d0e21e | 3013 | |
5a964f20 TC |
3014 | You can even set them to pipe commands. For example, this automatically |
3015 | filters compressed arguments through B<gzip>: | |
3016 | ||
3017 | @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; | |
3018 | ||
3019 | If you want to pass switches into your script, you can use one of the | |
a0d0e21e LW |
3020 | Getopts modules or put a loop on the front like this: |
3021 | ||
3022 | while ($_ = $ARGV[0], /^-/) { | |
3023 | shift; | |
3024 | last if /^--$/; | |
3025 | if (/^-D(.*)/) { $debug = $1 } | |
3026 | if (/^-v/) { $verbose++ } | |
5a964f20 | 3027 | # ... # other switches |
a0d0e21e | 3028 | } |
5a964f20 | 3029 | |
a0d0e21e | 3030 | while (<>) { |
5a964f20 | 3031 | # ... # code for each line |
a0d0e21e LW |
3032 | } |
3033 | ||
89d205f2 YO |
3034 | The <> symbol will return C<undef> for end-of-file only once. |
3035 | If you call it again after this, it will assume you are processing another | |
19799a22 | 3036 | @ARGV list, and if you haven't set @ARGV, will read input from STDIN. |
a0d0e21e | 3037 | |
1ca345ed | 3038 | If what the angle brackets contain is a simple scalar variable (for example, |
35f2feb0 | 3039 | <$foo>), then that variable contains the name of the |
19799a22 GS |
3040 | filehandle to input from, or its typeglob, or a reference to the |
3041 | same. For example: | |
cb1a09d0 AD |
3042 | |
3043 | $fh = \*STDIN; | |
3044 | $line = <$fh>; | |
a0d0e21e | 3045 | |
5a964f20 TC |
3046 | If what's within the angle brackets is neither a filehandle nor a simple |
3047 | scalar variable containing a filehandle name, typeglob, or typeglob | |
3048 | reference, it is interpreted as a filename pattern to be globbed, and | |
3049 | either a list of filenames or the next filename in the list is returned, | |
19799a22 | 3050 | depending on context. This distinction is determined on syntactic |
35f2feb0 GS |
3051 | grounds alone. That means C<< <$x> >> is always a readline() from |
3052 | an indirect handle, but C<< <$hash{key}> >> is always a glob(). | |
5a964f20 | 3053 | That's because $x is a simple scalar variable, but C<$hash{key}> is |
ef191992 YST |
3054 | not--it's a hash element. Even C<< <$x > >> (note the extra space) |
3055 | is treated as C<glob("$x ")>, not C<readline($x)>. | |
5a964f20 TC |
3056 | |
3057 | One level of double-quote interpretation is done first, but you can't | |
35f2feb0 | 3058 | say C<< <$foo> >> because that's an indirect filehandle as explained |
5a964f20 TC |
3059 | in the previous paragraph. (In older versions of Perl, programmers |
3060 | would insert curly brackets to force interpretation as a filename glob: | |
35f2feb0 | 3061 | C<< <${foo}> >>. These days, it's considered cleaner to call the |
5a964f20 | 3062 | internal function directly as C<glob($foo)>, which is probably the right |
19799a22 | 3063 | way to have done it in the first place.) For example: |
a0d0e21e LW |
3064 | |
3065 | while (<*.c>) { | |
3066 | chmod 0644, $_; | |
3067 | } | |
3068 | ||
3a4b19e4 | 3069 | is roughly equivalent to: |
a0d0e21e LW |
3070 | |
3071 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); | |
3072 | while (<FOO>) { | |
5b3eff12 | 3073 | chomp; |
a0d0e21e LW |
3074 | chmod 0644, $_; |
3075 | } | |
3076 | ||
3a4b19e4 GS |
3077 | except that the globbing is actually done internally using the standard |
3078 | C<File::Glob> extension. Of course, the shortest way to do the above is: | |
a0d0e21e LW |
3079 | |
3080 | chmod 0644, <*.c>; | |
3081 | ||
19799a22 GS |
3082 | A (file)glob evaluates its (embedded) argument only when it is |
3083 | starting a new list. All values must be read before it will start | |
3084 | over. In list context, this isn't important because you automatically | |
3085 | get them all anyway. However, in scalar context the operator returns | |
069e01df | 3086 | the next value each time it's called, or C<undef> when the list has |
19799a22 GS |
3087 | run out. As with filehandle reads, an automatic C<defined> is |
3088 | generated when the glob occurs in the test part of a C<while>, | |
1ca345ed TC |
3089 | because legal glob returns (for example, |
3090 | a file called F<0>) would otherwise | |
19799a22 GS |
3091 | terminate the loop. Again, C<undef> is returned only once. So if |
3092 | you're expecting a single value from a glob, it is much better to | |
3093 | say | |
4633a7c4 LW |
3094 | |
3095 | ($file) = <blurch*>; | |
3096 | ||
3097 | than | |
3098 | ||
3099 | $file = <blurch*>; | |
3100 | ||
3101 | because the latter will alternate between returning a filename and | |
19799a22 | 3102 | returning false. |
4633a7c4 | 3103 | |
b159ebd3 | 3104 | If you're trying to do variable interpolation, it's definitely better |
4633a7c4 | 3105 | to use the glob() function, because the older notation can cause people |
e37d713d | 3106 | to become confused with the indirect filehandle notation. |
4633a7c4 LW |
3107 | |
3108 | @files = glob("$dir/*.[ch]"); | |
3109 | @files = glob($files[$i]); | |
3110 | ||
a0d0e21e | 3111 | =head2 Constant Folding |
d74e8afc | 3112 | X<constant folding> X<folding> |
a0d0e21e LW |
3113 | |
3114 | Like C, Perl does a certain amount of expression evaluation at | |
19799a22 | 3115 | compile time whenever it determines that all arguments to an |
a0d0e21e LW |
3116 | operator are static and have no side effects. In particular, string |
3117 | concatenation happens at compile time between literals that don't do | |
19799a22 | 3118 | variable substitution. Backslash interpolation also happens at |
a0d0e21e LW |
3119 | compile time. You can say |
3120 | ||
1ca345ed TC |
3121 | 'Now is the time for all' |
3122 | . "\n" | |
3123 | . 'good men to come to.' | |
a0d0e21e | 3124 | |
54310121 | 3125 | and this all reduces to one string internally. Likewise, if |
a0d0e21e LW |
3126 | you say |
3127 | ||
3128 | foreach $file (@filenames) { | |
5a964f20 | 3129 | if (-s $file > 5 + 100 * 2**16) { } |
54310121 | 3130 | } |
a0d0e21e | 3131 | |
1ca345ed | 3132 | the compiler precomputes the number which that expression |
19799a22 | 3133 | represents so that the interpreter won't have to. |
a0d0e21e | 3134 | |
fd1abbef | 3135 | =head2 No-ops |
d74e8afc | 3136 | X<no-op> X<nop> |
fd1abbef DN |
3137 | |
3138 | Perl doesn't officially have a no-op operator, but the bare constants | |
1ca345ed | 3139 | C<0> and C<1> are special-cased not to produce a warning in void |
fd1abbef DN |
3140 | context, so you can for example safely do |
3141 | ||
3142 | 1 while foo(); | |
3143 | ||
2c268ad5 | 3144 | =head2 Bitwise String Operators |
d74e8afc | 3145 | X<operator, bitwise, string> |
2c268ad5 TP |
3146 | |
3147 | Bitstrings of any size may be manipulated by the bitwise operators | |
3148 | (C<~ | & ^>). | |
3149 | ||
19799a22 GS |
3150 | If the operands to a binary bitwise op are strings of different |
3151 | sizes, B<|> and B<^> ops act as though the shorter operand had | |
3152 | additional zero bits on the right, while the B<&> op acts as though | |
3153 | the longer operand were truncated to the length of the shorter. | |
3154 | The granularity for such extension or truncation is one or more | |
3155 | bytes. | |
2c268ad5 | 3156 | |
89d205f2 | 3157 | # ASCII-based examples |
2c268ad5 TP |
3158 | print "j p \n" ^ " a h"; # prints "JAPH\n" |
3159 | print "JA" | " ph\n"; # prints "japh\n" | |
3160 | print "japh\nJunk" & '_____'; # prints "JAPH\n"; | |
3161 | print 'p N$' ^ " E<H\n"; # prints "Perl\n"; | |
3162 | ||
19799a22 | 3163 | If you are intending to manipulate bitstrings, be certain that |
2c268ad5 | 3164 | you're supplying bitstrings: If an operand is a number, that will imply |
19799a22 | 3165 | a B<numeric> bitwise operation. You may explicitly show which type of |
2c268ad5 TP |
3166 | operation you intend by using C<""> or C<0+>, as in the examples below. |
3167 | ||
4358a253 SS |
3168 | $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF) |
3169 | $foo = '150' | 105; # yields 255 | |
2c268ad5 TP |
3170 | $foo = 150 | '105'; # yields 255 |
3171 | $foo = '150' | '105'; # yields string '155' (under ASCII) | |
3172 | ||
3173 | $baz = 0+$foo & 0+$bar; # both ops explicitly numeric | |
3174 | $biz = "$foo" ^ "$bar"; # both ops explicitly stringy | |
a0d0e21e | 3175 | |
1ae175c8 GS |
3176 | See L<perlfunc/vec> for information on how to manipulate individual bits |
3177 | in a bit vector. | |
3178 | ||
55497cff | 3179 | =head2 Integer Arithmetic |
d74e8afc | 3180 | X<integer> |
a0d0e21e | 3181 | |
19799a22 | 3182 | By default, Perl assumes that it must do most of its arithmetic in |
a0d0e21e LW |
3183 | floating point. But by saying |
3184 | ||
3185 | use integer; | |
3186 | ||
3eab78e3 CW |
3187 | you may tell the compiler to use integer operations |
3188 | (see L<integer> for a detailed explanation) from here to the end of | |
3189 | the enclosing BLOCK. An inner BLOCK may countermand this by saying | |
a0d0e21e LW |
3190 | |
3191 | no integer; | |
3192 | ||
19799a22 | 3193 | which lasts until the end of that BLOCK. Note that this doesn't |
3eab78e3 CW |
3194 | mean everything is an integer, merely that Perl will use integer |
3195 | operations for arithmetic, comparison, and bitwise operators. For | |
3196 | example, even under C<use integer>, if you take the C<sqrt(2)>, you'll | |
3197 | still get C<1.4142135623731> or so. | |
19799a22 GS |
3198 | |
3199 | Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", | |
89d205f2 | 3200 | and ">>") always produce integral results. (But see also |
13a2d996 | 3201 | L<Bitwise String Operators>.) However, C<use integer> still has meaning for |
19799a22 GS |
3202 | them. By default, their results are interpreted as unsigned integers, but |
3203 | if C<use integer> is in effect, their results are interpreted | |
3204 | as signed integers. For example, C<~0> usually evaluates to a large | |
0be96356 | 3205 | integral value. However, C<use integer; ~0> is C<-1> on two's-complement |
19799a22 | 3206 | machines. |
68dc0745 | 3207 | |
3208 | =head2 Floating-point Arithmetic | |
06ce2fa3 | 3209 | |
d74e8afc | 3210 | X<floating-point> X<floating point> X<float> X<real> |
68dc0745 | 3211 | |
3212 | While C<use integer> provides integer-only arithmetic, there is no | |
19799a22 GS |
3213 | analogous mechanism to provide automatic rounding or truncation to a |
3214 | certain number of decimal places. For rounding to a certain number | |
3215 | of digits, sprintf() or printf() is usually the easiest route. | |
3216 | See L<perlfaq4>. | |
68dc0745 | 3217 | |
5a964f20 TC |
3218 | Floating-point numbers are only approximations to what a mathematician |
3219 | would call real numbers. There are infinitely more reals than floats, | |
3220 | so some corners must be cut. For example: | |
3221 | ||
3222 | printf "%.20g\n", 123456789123456789; | |
3223 | # produces 123456789123456784 | |
3224 | ||
8548cb57 RGS |
3225 | Testing for exact floating-point equality or inequality is not a |
3226 | good idea. Here's a (relatively expensive) work-around to compare | |
5a964f20 TC |
3227 | whether two floating-point numbers are equal to a particular number of |
3228 | decimal places. See Knuth, volume II, for a more robust treatment of | |
3229 | this topic. | |
3230 | ||
3231 | sub fp_equal { | |
3232 | my ($X, $Y, $POINTS) = @_; | |
3233 | my ($tX, $tY); | |
3234 | $tX = sprintf("%.${POINTS}g", $X); | |
3235 | $tY = sprintf("%.${POINTS}g", $Y); | |
3236 | return $tX eq $tY; | |
3237 | } | |
3238 | ||
68dc0745 | 3239 | The POSIX module (part of the standard perl distribution) implements |
19799a22 GS |
3240 | ceil(), floor(), and other mathematical and trigonometric functions. |
3241 | The Math::Complex module (part of the standard perl distribution) | |
3242 | defines mathematical functions that work on both the reals and the | |
3243 | imaginary numbers. Math::Complex not as efficient as POSIX, but | |
68dc0745 | 3244 | POSIX can't work with complex numbers. |
3245 | ||
3246 | Rounding in financial applications can have serious implications, and | |
3247 | the rounding method used should be specified precisely. In these | |
3248 | cases, it probably pays not to trust whichever system rounding is | |
3249 | being used by Perl, but to instead implement the rounding function you | |
3250 | need yourself. | |
5a964f20 TC |
3251 | |
3252 | =head2 Bigger Numbers | |
d74e8afc | 3253 | X<number, arbitrary precision> |
5a964f20 | 3254 | |
c543c01b | 3255 | The standard C<Math::BigInt>, C<Math::BigRat>, and C<Math::BigFloat> modules, |
fb1a95c6 | 3256 | along with the C<bignum>, C<bigint>, and C<bigrat> pragmas, provide |
19799a22 | 3257 | variable-precision arithmetic and overloaded operators, although |
46f8a5ea | 3258 | they're currently pretty slow. At the cost of some space and |
19799a22 GS |
3259 | considerable speed, they avoid the normal pitfalls associated with |
3260 | limited-precision representations. | |
5a964f20 | 3261 | |
c543c01b TC |
3262 | use 5.010; |
3263 | use bigint; # easy interface to Math::BigInt | |
3264 | $x = 123456789123456789; | |
3265 | say $x * $x; | |
3266 | +15241578780673678515622620750190521 | |
3267 | ||
3268 | Or with rationals: | |
3269 | ||
db691027 SF |
3270 | use 5.010; |
3271 | use bigrat; | |
3272 | $x = 3/22; | |
3273 | $y = 4/6; | |
3274 | say "x/y is ", $x/$y; | |
3275 | say "x*y is ", $x*$y; | |
3276 | x/y is 9/44 | |
3277 | x*y is 1/11 | |
c543c01b TC |
3278 | |
3279 | Several modules let you calculate with (bound only by memory and CPU time) | |
46f8a5ea FC |
3280 | unlimited or fixed precision. There |
3281 | are also some non-standard modules that | |
c543c01b | 3282 | provide faster implementations via external C libraries. |
cd5c4fce T |
3283 | |
3284 | Here is a short, but incomplete summary: | |
3285 | ||
950b09ed KW |
3286 | Math::String treat string sequences like numbers |
3287 | Math::FixedPrecision calculate with a fixed precision | |
3288 | Math::Currency for currency calculations | |
3289 | Bit::Vector manipulate bit vectors fast (uses C) | |
3290 | Math::BigIntFast Bit::Vector wrapper for big numbers | |
3291 | Math::Pari provides access to the Pari C library | |
70c45be3 FC |
3292 | Math::Cephes uses the external Cephes C library (no |
3293 | big numbers) | |
950b09ed KW |
3294 | Math::Cephes::Fraction fractions via the Cephes library |
3295 | Math::GMP another one using an external C library | |
70c45be3 FC |
3296 | Math::GMPz an alternative interface to libgmp's big ints |
3297 | Math::GMPq an interface to libgmp's fraction numbers | |
3298 | Math::GMPf an interface to libgmp's floating point numbers | |
cd5c4fce T |
3299 | |
3300 | Choose wisely. | |
16070b82 GS |
3301 | |
3302 | =cut |