Commit | Line | Data |
---|---|---|
a0d0e21e | 1 | =head1 NAME |
d74e8afc | 2 | X<operator> |
a0d0e21e LW |
3 | |
4 | perlop - Perl operators and precedence | |
5 | ||
d042e63d MS |
6 | =head1 DESCRIPTION |
7 | ||
89d205f2 | 8 | =head2 Operator Precedence and Associativity |
d74e8afc | 9 | X<operator, precedence> X<precedence> X<associativity> |
d042e63d MS |
10 | |
11 | Operator precedence and associativity work in Perl more or less like | |
12 | they do in mathematics. | |
13 | ||
14 | I<Operator precedence> means some operators are evaluated before | |
15 | others. For example, in C<2 + 4 * 5>, the multiplication has higher | |
16 | precedence so C<4 * 5> is evaluated first yielding C<2 + 20 == | |
17 | 22> and not C<6 * 5 == 30>. | |
18 | ||
19 | I<Operator associativity> defines what happens if a sequence of the | |
20 | same operators is used one after another: whether the evaluator will | |
21 | evaluate the left operations first or the right. For example, in C<8 | |
22 | - 4 - 2>, subtraction is left associative so Perl evaluates the | |
23 | expression left to right. C<8 - 4> is evaluated first making the | |
24 | expression C<4 - 2 == 2> and not C<8 - 2 == 6>. | |
a0d0e21e LW |
25 | |
26 | Perl operators have the following associativity and precedence, | |
19799a22 GS |
27 | listed from highest precedence to lowest. Operators borrowed from |
28 | C keep the same precedence relationship with each other, even where | |
29 | C's precedence is slightly screwy. (This makes learning Perl easier | |
30 | for C folks.) With very few exceptions, these all operate on scalar | |
31 | values only, not array values. | |
a0d0e21e LW |
32 | |
33 | left terms and list operators (leftward) | |
34 | left -> | |
35 | nonassoc ++ -- | |
36 | right ** | |
37 | right ! ~ \ and unary + and - | |
54310121 | 38 | left =~ !~ |
a0d0e21e LW |
39 | left * / % x |
40 | left + - . | |
41 | left << >> | |
42 | nonassoc named unary operators | |
43 | nonassoc < > <= >= lt gt le ge | |
0d863452 | 44 | nonassoc == != <=> eq ne cmp ~~ |
a0d0e21e LW |
45 | left & |
46 | left | ^ | |
47 | left && | |
c963b151 | 48 | left || // |
137443ea | 49 | nonassoc .. ... |
a0d0e21e LW |
50 | right ?: |
51 | right = += -= *= etc. | |
52 | left , => | |
53 | nonassoc list operators (rightward) | |
a5f75d66 | 54 | right not |
a0d0e21e | 55 | left and |
c963b151 | 56 | left or xor err |
a0d0e21e LW |
57 | |
58 | In the following sections, these operators are covered in precedence order. | |
59 | ||
5a964f20 TC |
60 | Many operators can be overloaded for objects. See L<overload>. |
61 | ||
a0d0e21e | 62 | =head2 Terms and List Operators (Leftward) |
d74e8afc | 63 | X<list operator> X<operator, list> X<term> |
a0d0e21e | 64 | |
62c18ce2 | 65 | A TERM has the highest precedence in Perl. They include variables, |
5f05dabc | 66 | quote and quote-like operators, any expression in parentheses, |
a0d0e21e LW |
67 | and any function whose arguments are parenthesized. Actually, there |
68 | aren't really functions in this sense, just list operators and unary | |
69 | operators behaving as functions because you put parentheses around | |
70 | the arguments. These are all documented in L<perlfunc>. | |
71 | ||
72 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
73 | is followed by a left parenthesis as the next token, the operator and | |
74 | arguments within parentheses are taken to be of highest precedence, | |
75 | just like a normal function call. | |
76 | ||
77 | In the absence of parentheses, the precedence of list operators such as | |
78 | C<print>, C<sort>, or C<chmod> is either very high or very low depending on | |
54310121 | 79 | whether you are looking at the left side or the right side of the operator. |
a0d0e21e LW |
80 | For example, in |
81 | ||
82 | @ary = (1, 3, sort 4, 2); | |
83 | print @ary; # prints 1324 | |
84 | ||
19799a22 GS |
85 | the commas on the right of the sort are evaluated before the sort, |
86 | but the commas on the left are evaluated after. In other words, | |
87 | list operators tend to gobble up all arguments that follow, and | |
a0d0e21e | 88 | then act like a simple TERM with regard to the preceding expression. |
19799a22 | 89 | Be careful with parentheses: |
a0d0e21e LW |
90 | |
91 | # These evaluate exit before doing the print: | |
92 | print($foo, exit); # Obviously not what you want. | |
93 | print $foo, exit; # Nor is this. | |
94 | ||
95 | # These do the print before evaluating exit: | |
96 | (print $foo), exit; # This is what you want. | |
97 | print($foo), exit; # Or this. | |
98 | print ($foo), exit; # Or even this. | |
99 | ||
100 | Also note that | |
101 | ||
102 | print ($foo & 255) + 1, "\n"; | |
103 | ||
d042e63d MS |
104 | probably doesn't do what you expect at first glance. The parentheses |
105 | enclose the argument list for C<print> which is evaluated (printing | |
106 | the result of C<$foo & 255>). Then one is added to the return value | |
107 | of C<print> (usually 1). The result is something like this: | |
108 | ||
109 | 1 + 1, "\n"; # Obviously not what you meant. | |
110 | ||
111 | To do what you meant properly, you must write: | |
112 | ||
113 | print(($foo & 255) + 1, "\n"); | |
114 | ||
115 | See L<Named Unary Operators> for more discussion of this. | |
a0d0e21e LW |
116 | |
117 | Also parsed as terms are the C<do {}> and C<eval {}> constructs, as | |
54310121 | 118 | well as subroutine and method calls, and the anonymous |
a0d0e21e LW |
119 | constructors C<[]> and C<{}>. |
120 | ||
2ae324a7 | 121 | See also L<Quote and Quote-like Operators> toward the end of this section, |
da87341d | 122 | as well as L</"I/O Operators">. |
a0d0e21e LW |
123 | |
124 | =head2 The Arrow Operator | |
d74e8afc | 125 | X<arrow> X<dereference> X<< -> >> |
a0d0e21e | 126 | |
35f2feb0 | 127 | "C<< -> >>" is an infix dereference operator, just as it is in C |
19799a22 GS |
128 | and C++. If the right side is either a C<[...]>, C<{...}>, or a |
129 | C<(...)> subscript, then the left side must be either a hard or | |
130 | symbolic reference to an array, a hash, or a subroutine respectively. | |
131 | (Or technically speaking, a location capable of holding a hard | |
132 | reference, if it's an array or hash reference being used for | |
133 | assignment.) See L<perlreftut> and L<perlref>. | |
a0d0e21e | 134 | |
19799a22 GS |
135 | Otherwise, the right side is a method name or a simple scalar |
136 | variable containing either the method name or a subroutine reference, | |
137 | and the left side must be either an object (a blessed reference) | |
138 | or a class name (that is, a package name). See L<perlobj>. | |
a0d0e21e | 139 | |
5f05dabc | 140 | =head2 Auto-increment and Auto-decrement |
d74e8afc | 141 | X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<--> |
a0d0e21e | 142 | |
d042e63d MS |
143 | "++" and "--" work as in C. That is, if placed before a variable, |
144 | they increment or decrement the variable by one before returning the | |
145 | value, and if placed after, increment or decrement after returning the | |
146 | value. | |
147 | ||
148 | $i = 0; $j = 0; | |
149 | print $i++; # prints 0 | |
150 | print ++$j; # prints 1 | |
a0d0e21e | 151 | |
b033823e | 152 | Note that just as in C, Perl doesn't define B<when> the variable is |
89d205f2 | 153 | incremented or decremented. You just know it will be done sometime |
b033823e A |
154 | before or after the value is returned. This also means that modifying |
155 | a variable twice in the same statement will lead to undefined behaviour. | |
156 | Avoid statements like: | |
157 | ||
158 | $i = $i ++; | |
159 | print ++ $i + $i ++; | |
160 | ||
161 | Perl will not guarantee what the result of the above statements is. | |
162 | ||
54310121 | 163 | The auto-increment operator has a little extra builtin magic to it. If |
a0d0e21e LW |
164 | you increment a variable that is numeric, or that has ever been used in |
165 | a numeric context, you get a normal increment. If, however, the | |
5f05dabc | 166 | variable has been used in only string contexts since it was set, and |
5a964f20 | 167 | has a value that is not the empty string and matches the pattern |
9c0670e1 | 168 | C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each |
a0d0e21e LW |
169 | character within its range, with carry: |
170 | ||
171 | print ++($foo = '99'); # prints '100' | |
172 | print ++($foo = 'a0'); # prints 'a1' | |
173 | print ++($foo = 'Az'); # prints 'Ba' | |
174 | print ++($foo = 'zz'); # prints 'aaa' | |
175 | ||
6a61d433 HS |
176 | C<undef> is always treated as numeric, and in particular is changed |
177 | to C<0> before incrementing (so that a post-increment of an undef value | |
178 | will return C<0> rather than C<undef>). | |
179 | ||
5f05dabc | 180 | The auto-decrement operator is not magical. |
a0d0e21e LW |
181 | |
182 | =head2 Exponentiation | |
d74e8afc | 183 | X<**> X<exponentiation> X<power> |
a0d0e21e | 184 | |
19799a22 | 185 | Binary "**" is the exponentiation operator. It binds even more |
cb1a09d0 AD |
186 | tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is |
187 | implemented using C's pow(3) function, which actually works on doubles | |
188 | internally.) | |
a0d0e21e LW |
189 | |
190 | =head2 Symbolic Unary Operators | |
d74e8afc | 191 | X<unary operator> X<operator, unary> |
a0d0e21e | 192 | |
5f05dabc | 193 | Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower |
a0d0e21e | 194 | precedence version of this. |
d74e8afc | 195 | X<!> |
a0d0e21e LW |
196 | |
197 | Unary "-" performs arithmetic negation if the operand is numeric. If | |
198 | the operand is an identifier, a string consisting of a minus sign | |
199 | concatenated with the identifier is returned. Otherwise, if the string | |
200 | starts with a plus or minus, a string starting with the opposite sign | |
bff5667c | 201 | is returned. One effect of these rules is that -bareword is equivalent |
8705167b | 202 | to the string "-bareword". If, however, the string begins with a |
06705523 SP |
203 | non-alphabetic character (exluding "+" or "-"), Perl will attempt to convert |
204 | the string to a numeric and the arithmetic negation is performed. If the | |
205 | string cannot be cleanly converted to a numeric, Perl will give the warning | |
206 | B<Argument "the string" isn't numeric in negation (-) at ...>. | |
d74e8afc | 207 | X<-> X<negation, arithmetic> |
a0d0e21e | 208 | |
972b05a9 JH |
209 | Unary "~" performs bitwise negation, i.e., 1's complement. For |
210 | example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and | |
211 | L<Bitwise String Operators>.) Note that the width of the result is | |
212 | platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 | |
213 | bits wide on a 64-bit platform, so if you are expecting a certain bit | |
d042e63d | 214 | width, remember to use the & operator to mask off the excess bits. |
d74e8afc | 215 | X<~> X<negation, binary> |
a0d0e21e LW |
216 | |
217 | Unary "+" has no effect whatsoever, even on strings. It is useful | |
218 | syntactically for separating a function name from a parenthesized expression | |
219 | that would otherwise be interpreted as the complete list of function | |
5ba421f6 | 220 | arguments. (See examples above under L<Terms and List Operators (Leftward)>.) |
d74e8afc | 221 | X<+> |
a0d0e21e | 222 | |
19799a22 GS |
223 | Unary "\" creates a reference to whatever follows it. See L<perlreftut> |
224 | and L<perlref>. Do not confuse this behavior with the behavior of | |
225 | backslash within a string, although both forms do convey the notion | |
226 | of protecting the next thing from interpolation. | |
d74e8afc | 227 | X<\> X<reference> X<backslash> |
a0d0e21e LW |
228 | |
229 | =head2 Binding Operators | |
d74e8afc | 230 | X<binding> X<operator, binding> X<=~> X<!~> |
a0d0e21e | 231 | |
c07a80fd | 232 | Binary "=~" binds a scalar expression to a pattern match. Certain operations |
cb1a09d0 AD |
233 | search or modify the string $_ by default. This operator makes that kind |
234 | of operation work on some other string. The right argument is a search | |
2c268ad5 TP |
235 | pattern, substitution, or transliteration. The left argument is what is |
236 | supposed to be searched, substituted, or transliterated instead of the default | |
f8bab1e9 GS |
237 | $_. When used in scalar context, the return value generally indicates the |
238 | success of the operation. Behavior in list context depends on the particular | |
89d205f2 | 239 | operator. See L</"Regexp Quote-Like Operators"> for details and |
d7782e69 | 240 | L<perlretut> for examples using these operators. |
f8bab1e9 GS |
241 | |
242 | If the right argument is an expression rather than a search pattern, | |
2c268ad5 | 243 | substitution, or transliteration, it is interpreted as a search pattern at run |
89d205f2 YO |
244 | time. Note that this means that its contents will be interpolated twice, so |
245 | ||
246 | '\\' =~ q'\\'; | |
247 | ||
248 | is not ok, as the regex engine will end up trying to compile the | |
249 | pattern C<\>, which it will consider a syntax error. | |
a0d0e21e LW |
250 | |
251 | Binary "!~" is just like "=~" except the return value is negated in | |
252 | the logical sense. | |
253 | ||
254 | =head2 Multiplicative Operators | |
d74e8afc | 255 | X<operator, multiplicative> |
a0d0e21e LW |
256 | |
257 | Binary "*" multiplies two numbers. | |
d74e8afc | 258 | X<*> |
a0d0e21e LW |
259 | |
260 | Binary "/" divides two numbers. | |
d74e8afc | 261 | X</> X<slash> |
a0d0e21e | 262 | |
54310121 | 263 | Binary "%" computes the modulus of two numbers. Given integer |
264 | operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is | |
265 | C<$a> minus the largest multiple of C<$b> that is not greater than | |
266 | C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the | |
267 | smallest multiple of C<$b> that is not less than C<$a> (i.e. the | |
89b4f0ad | 268 | result will be less than or equal to zero). If the operands |
4848a83b TS |
269 | C<$a> and C<$b> are floating point values and the absolute value of |
270 | C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only | |
271 | the integer portion of C<$a> and C<$b> will be used in the operation | |
272 | (Note: here C<UV_MAX> means the maximum of the unsigned integer type). | |
273 | If the absolute value of the right operand (C<abs($b)>) is greater than | |
274 | or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder | |
275 | C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain | |
276 | integer that makes C<$r> should have the same sign as the right operand | |
277 | C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>) | |
278 | and the absolute value less than that of C<$b>. | |
0412d526 | 279 | Note that when C<use integer> is in scope, "%" gives you direct access |
55d729e4 GS |
280 | to the modulus operator as implemented by your C compiler. This |
281 | operator is not as well defined for negative operands, but it will | |
282 | execute faster. | |
d74e8afc | 283 | X<%> X<remainder> X<modulus> X<mod> |
55d729e4 | 284 | |
62d10b70 GS |
285 | Binary "x" is the repetition operator. In scalar context or if the left |
286 | operand is not enclosed in parentheses, it returns a string consisting | |
287 | of the left operand repeated the number of times specified by the right | |
288 | operand. In list context, if the left operand is enclosed in | |
3585017f YST |
289 | parentheses or is a list formed by C<qw/STRING/>, it repeats the list. |
290 | If the right operand is zero or negative, it returns an empty string | |
291 | or an empty list, depending on the context. | |
d74e8afc | 292 | X<x> |
a0d0e21e LW |
293 | |
294 | print '-' x 80; # print row of dashes | |
295 | ||
296 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over | |
297 | ||
298 | @ones = (1) x 80; # a list of 80 1's | |
299 | @ones = (5) x @ones; # set all elements to 5 | |
300 | ||
301 | ||
302 | =head2 Additive Operators | |
d74e8afc | 303 | X<operator, additive> |
a0d0e21e LW |
304 | |
305 | Binary "+" returns the sum of two numbers. | |
d74e8afc | 306 | X<+> |
a0d0e21e LW |
307 | |
308 | Binary "-" returns the difference of two numbers. | |
d74e8afc | 309 | X<-> |
a0d0e21e LW |
310 | |
311 | Binary "." concatenates two strings. | |
d74e8afc ITB |
312 | X<string, concatenation> X<concatenation> |
313 | X<cat> X<concat> X<concatenate> X<.> | |
a0d0e21e LW |
314 | |
315 | =head2 Shift Operators | |
d74e8afc ITB |
316 | X<shift operator> X<operator, shift> X<<< << >>> |
317 | X<<< >> >>> X<right shift> X<left shift> X<bitwise shift> | |
318 | X<shl> X<shr> X<shift, right> X<shift, left> | |
a0d0e21e | 319 | |
55497cff | 320 | Binary "<<" returns the value of its left argument shifted left by the |
321 | number of bits specified by the right argument. Arguments should be | |
982ce180 | 322 | integers. (See also L<Integer Arithmetic>.) |
a0d0e21e | 323 | |
55497cff | 324 | Binary ">>" returns the value of its left argument shifted right by |
325 | the number of bits specified by the right argument. Arguments should | |
982ce180 | 326 | be integers. (See also L<Integer Arithmetic>.) |
a0d0e21e | 327 | |
b16cf6df JH |
328 | Note that both "<<" and ">>" in Perl are implemented directly using |
329 | "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is | |
330 | in force then signed C integers are used, else unsigned C integers are | |
331 | used. Either way, the implementation isn't going to generate results | |
332 | larger than the size of the integer type Perl was built with (32 bits | |
333 | or 64 bits). | |
334 | ||
335 | The result of overflowing the range of the integers is undefined | |
336 | because it is undefined also in C. In other words, using 32-bit | |
337 | integers, C<< 1 << 32 >> is undefined. Shifting by a negative number | |
338 | of bits is also undefined. | |
339 | ||
a0d0e21e | 340 | =head2 Named Unary Operators |
d74e8afc | 341 | X<operator, named unary> |
a0d0e21e LW |
342 | |
343 | The various named unary operators are treated as functions with one | |
568e6d8b | 344 | argument, with optional parentheses. |
a0d0e21e LW |
345 | |
346 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
347 | is followed by a left parenthesis as the next token, the operator and | |
348 | arguments within parentheses are taken to be of highest precedence, | |
3981b0eb JA |
349 | just like a normal function call. For example, |
350 | because named unary operators are higher precedence than ||: | |
a0d0e21e LW |
351 | |
352 | chdir $foo || die; # (chdir $foo) || die | |
353 | chdir($foo) || die; # (chdir $foo) || die | |
354 | chdir ($foo) || die; # (chdir $foo) || die | |
355 | chdir +($foo) || die; # (chdir $foo) || die | |
356 | ||
3981b0eb | 357 | but, because * is higher precedence than named operators: |
a0d0e21e LW |
358 | |
359 | chdir $foo * 20; # chdir ($foo * 20) | |
360 | chdir($foo) * 20; # (chdir $foo) * 20 | |
361 | chdir ($foo) * 20; # (chdir $foo) * 20 | |
362 | chdir +($foo) * 20; # chdir ($foo * 20) | |
363 | ||
364 | rand 10 * 20; # rand (10 * 20) | |
365 | rand(10) * 20; # (rand 10) * 20 | |
366 | rand (10) * 20; # (rand 10) * 20 | |
367 | rand +(10) * 20; # rand (10 * 20) | |
368 | ||
568e6d8b RGS |
369 | Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are |
370 | treated like named unary operators, but they don't follow this functional | |
371 | parenthesis rule. That means, for example, that C<-f($file).".bak"> is | |
372 | equivalent to C<-f "$file.bak">. | |
d74e8afc | 373 | X<-X> X<filetest> X<operator, filetest> |
568e6d8b | 374 | |
5ba421f6 | 375 | See also L<"Terms and List Operators (Leftward)">. |
a0d0e21e LW |
376 | |
377 | =head2 Relational Operators | |
d74e8afc | 378 | X<relational operator> X<operator, relational> |
a0d0e21e | 379 | |
35f2feb0 | 380 | Binary "<" returns true if the left argument is numerically less than |
a0d0e21e | 381 | the right argument. |
d74e8afc | 382 | X<< < >> |
a0d0e21e | 383 | |
35f2feb0 | 384 | Binary ">" returns true if the left argument is numerically greater |
a0d0e21e | 385 | than the right argument. |
d74e8afc | 386 | X<< > >> |
a0d0e21e | 387 | |
35f2feb0 | 388 | Binary "<=" returns true if the left argument is numerically less than |
a0d0e21e | 389 | or equal to the right argument. |
d74e8afc | 390 | X<< <= >> |
a0d0e21e | 391 | |
35f2feb0 | 392 | Binary ">=" returns true if the left argument is numerically greater |
a0d0e21e | 393 | than or equal to the right argument. |
d74e8afc | 394 | X<< >= >> |
a0d0e21e LW |
395 | |
396 | Binary "lt" returns true if the left argument is stringwise less than | |
397 | the right argument. | |
d74e8afc | 398 | X<< lt >> |
a0d0e21e LW |
399 | |
400 | Binary "gt" returns true if the left argument is stringwise greater | |
401 | than the right argument. | |
d74e8afc | 402 | X<< gt >> |
a0d0e21e LW |
403 | |
404 | Binary "le" returns true if the left argument is stringwise less than | |
405 | or equal to the right argument. | |
d74e8afc | 406 | X<< le >> |
a0d0e21e LW |
407 | |
408 | Binary "ge" returns true if the left argument is stringwise greater | |
409 | than or equal to the right argument. | |
d74e8afc | 410 | X<< ge >> |
a0d0e21e LW |
411 | |
412 | =head2 Equality Operators | |
d74e8afc | 413 | X<equality> X<equal> X<equals> X<operator, equality> |
a0d0e21e LW |
414 | |
415 | Binary "==" returns true if the left argument is numerically equal to | |
416 | the right argument. | |
d74e8afc | 417 | X<==> |
a0d0e21e LW |
418 | |
419 | Binary "!=" returns true if the left argument is numerically not equal | |
420 | to the right argument. | |
d74e8afc | 421 | X<!=> |
a0d0e21e | 422 | |
35f2feb0 | 423 | Binary "<=>" returns -1, 0, or 1 depending on whether the left |
6ee5d4e7 | 424 | argument is numerically less than, equal to, or greater than the right |
d4ad863d | 425 | argument. If your platform supports NaNs (not-a-numbers) as numeric |
7d3a9d88 NC |
426 | values, using them with "<=>" returns undef. NaN is not "<", "==", ">", |
427 | "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN | |
428 | returns true, as does NaN != anything else. If your platform doesn't | |
429 | support NaNs then NaN is just a string with numeric value 0. | |
d74e8afc | 430 | X<< <=> >> X<spaceship> |
7d3a9d88 | 431 | |
2b54f59f YST |
432 | perl -le '$a = "NaN"; print "No NaN support here" if $a == $a' |
433 | perl -le '$a = "NaN"; print "NaN support here" if $a != $a' | |
a0d0e21e LW |
434 | |
435 | Binary "eq" returns true if the left argument is stringwise equal to | |
436 | the right argument. | |
d74e8afc | 437 | X<eq> |
a0d0e21e LW |
438 | |
439 | Binary "ne" returns true if the left argument is stringwise not equal | |
440 | to the right argument. | |
d74e8afc | 441 | X<ne> |
a0d0e21e | 442 | |
d4ad863d JH |
443 | Binary "cmp" returns -1, 0, or 1 depending on whether the left |
444 | argument is stringwise less than, equal to, or greater than the right | |
445 | argument. | |
d74e8afc | 446 | X<cmp> |
a0d0e21e | 447 | |
0d863452 | 448 | Binary "~~" does a smart match between its arguments. Smart matching |
0f7107a0 | 449 | is described in L<perlsyn/"Smart matching in detail">. |
0d863452 RH |
450 | This operator is only available if you enable the "~~" feature: |
451 | see L<feature> for more information. | |
452 | X<~~> | |
453 | ||
a034a98d DD |
454 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
455 | by the current locale if C<use locale> is in effect. See L<perllocale>. | |
456 | ||
a0d0e21e | 457 | =head2 Bitwise And |
d74e8afc | 458 | X<operator, bitwise, and> X<bitwise and> X<&> |
a0d0e21e | 459 | |
2cdc098b | 460 | Binary "&" returns its operands ANDed together bit by bit. |
2c268ad5 | 461 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e | 462 | |
2cdc098b MG |
463 | Note that "&" has lower priority than relational operators, so for example |
464 | the brackets are essential in a test like | |
465 | ||
466 | print "Even\n" if ($x & 1) == 0; | |
467 | ||
a0d0e21e | 468 | =head2 Bitwise Or and Exclusive Or |
d74e8afc ITB |
469 | X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor> |
470 | X<bitwise xor> X<^> | |
a0d0e21e | 471 | |
2cdc098b | 472 | Binary "|" returns its operands ORed together bit by bit. |
2c268ad5 | 473 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e | 474 | |
2cdc098b | 475 | Binary "^" returns its operands XORed together bit by bit. |
2c268ad5 | 476 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e | 477 | |
2cdc098b MG |
478 | Note that "|" and "^" have lower priority than relational operators, so |
479 | for example the brackets are essential in a test like | |
480 | ||
481 | print "false\n" if (8 | 2) != 10; | |
482 | ||
a0d0e21e | 483 | =head2 C-style Logical And |
d74e8afc | 484 | X<&&> X<logical and> X<operator, logical, and> |
a0d0e21e LW |
485 | |
486 | Binary "&&" performs a short-circuit logical AND operation. That is, | |
487 | if the left operand is false, the right operand is not even evaluated. | |
488 | Scalar or list context propagates down to the right operand if it | |
489 | is evaluated. | |
490 | ||
491 | =head2 C-style Logical Or | |
d74e8afc | 492 | X<||> X<operator, logical, or> |
a0d0e21e LW |
493 | |
494 | Binary "||" performs a short-circuit logical OR operation. That is, | |
495 | if the left operand is true, the right operand is not even evaluated. | |
496 | Scalar or list context propagates down to the right operand if it | |
497 | is evaluated. | |
498 | ||
c963b151 | 499 | =head2 C-style Logical Defined-Or |
d74e8afc | 500 | X<//> X<operator, logical, defined-or> |
c963b151 BD |
501 | |
502 | Although it has no direct equivalent in C, Perl's C<//> operator is related | |
89d205f2 | 503 | to its C-style or. In fact, it's exactly the same as C<||>, except that it |
c963b151 | 504 | tests the left hand side's definedness instead of its truth. Thus, C<$a // $b> |
89d205f2 YO |
505 | is similar to C<defined($a) || $b> (except that it returns the value of C<$a> |
506 | rather than the value of C<defined($a)>) and is exactly equivalent to | |
c963b151 | 507 | C<defined($a) ? $a : $b>. This is very useful for providing default values |
89d205f2 | 508 | for variables. If you actually want to test if at least one of C<$a> and |
d042e63d | 509 | C<$b> is defined, use C<defined($a // $b)>. |
c963b151 | 510 | |
d042e63d MS |
511 | The C<||>, C<//> and C<&&> operators return the last value evaluated |
512 | (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably | |
513 | portable way to find out the home directory might be: | |
a0d0e21e | 514 | |
c963b151 BD |
515 | $home = $ENV{'HOME'} // $ENV{'LOGDIR'} // |
516 | (getpwuid($<))[7] // die "You're homeless!\n"; | |
a0d0e21e | 517 | |
5a964f20 TC |
518 | In particular, this means that you shouldn't use this |
519 | for selecting between two aggregates for assignment: | |
520 | ||
521 | @a = @b || @c; # this is wrong | |
522 | @a = scalar(@b) || @c; # really meant this | |
523 | @a = @b ? @b : @c; # this works fine, though | |
524 | ||
c963b151 BD |
525 | As more readable alternatives to C<&&>, C<//> and C<||> when used for |
526 | control flow, Perl provides C<and>, C<err> and C<or> operators (see below). | |
89d205f2 | 527 | The short-circuit behavior is identical. The precedence of "and", "err" |
c963b151 | 528 | and "or" is much lower, however, so that you can safely use them after a |
5a964f20 | 529 | list operator without the need for parentheses: |
a0d0e21e LW |
530 | |
531 | unlink "alpha", "beta", "gamma" | |
532 | or gripe(), next LINE; | |
533 | ||
534 | With the C-style operators that would have been written like this: | |
535 | ||
536 | unlink("alpha", "beta", "gamma") | |
537 | || (gripe(), next LINE); | |
538 | ||
eeb6a2c9 | 539 | Using "or" for assignment is unlikely to do what you want; see below. |
5a964f20 TC |
540 | |
541 | =head2 Range Operators | |
d74e8afc | 542 | X<operator, range> X<range> X<..> X<...> |
a0d0e21e LW |
543 | |
544 | Binary ".." is the range operator, which is really two different | |
fb53bbb2 | 545 | operators depending on the context. In list context, it returns a |
54ae734e | 546 | list of values counting (up by ones) from the left value to the right |
2cdbc966 | 547 | value. If the left value is greater than the right value then it |
fb53bbb2 | 548 | returns the empty list. The range operator is useful for writing |
54ae734e | 549 | C<foreach (1..10)> loops and for doing slice operations on arrays. In |
2cdbc966 JD |
550 | the current implementation, no temporary array is created when the |
551 | range operator is used as the expression in C<foreach> loops, but older | |
552 | versions of Perl might burn a lot of memory when you write something | |
553 | like this: | |
a0d0e21e LW |
554 | |
555 | for (1 .. 1_000_000) { | |
556 | # code | |
54310121 | 557 | } |
a0d0e21e | 558 | |
54ae734e MG |
559 | The range operator also works on strings, using the magical auto-increment, |
560 | see below. | |
561 | ||
5a964f20 | 562 | In scalar context, ".." returns a boolean value. The operator is |
a0d0e21e LW |
563 | bistable, like a flip-flop, and emulates the line-range (comma) operator |
564 | of B<sed>, B<awk>, and various editors. Each ".." operator maintains its | |
565 | own boolean state. It is false as long as its left operand is false. | |
566 | Once the left operand is true, the range operator stays true until the | |
567 | right operand is true, I<AFTER> which the range operator becomes false | |
19799a22 | 568 | again. It doesn't become false till the next time the range operator is |
a0d0e21e LW |
569 | evaluated. It can test the right operand and become false on the same |
570 | evaluation it became true (as in B<awk>), but it still returns true once. | |
19799a22 GS |
571 | If you don't want it to test the right operand till the next |
572 | evaluation, as in B<sed>, just use three dots ("...") instead of | |
573 | two. In all other regards, "..." behaves just like ".." does. | |
574 | ||
575 | The right operand is not evaluated while the operator is in the | |
576 | "false" state, and the left operand is not evaluated while the | |
577 | operator is in the "true" state. The precedence is a little lower | |
578 | than || and &&. The value returned is either the empty string for | |
579 | false, or a sequence number (beginning with 1) for true. The | |
580 | sequence number is reset for each range encountered. The final | |
581 | sequence number in a range has the string "E0" appended to it, which | |
582 | doesn't affect its numeric value, but gives you something to search | |
583 | for if you want to exclude the endpoint. You can exclude the | |
584 | beginning point by waiting for the sequence number to be greater | |
df5f8116 CW |
585 | than 1. |
586 | ||
587 | If either operand of scalar ".." is a constant expression, | |
588 | that operand is considered true if it is equal (C<==>) to the current | |
589 | input line number (the C<$.> variable). | |
590 | ||
591 | To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>, | |
592 | but that is only an issue if you use a floating point expression; when | |
593 | implicitly using C<$.> as described in the previous paragraph, the | |
594 | comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.> | |
595 | is set to a floating point value and you are not reading from a file. | |
596 | Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what | |
597 | you want in scalar context because each of the operands are evaluated | |
598 | using their integer representation. | |
599 | ||
600 | Examples: | |
a0d0e21e LW |
601 | |
602 | As a scalar operator: | |
603 | ||
df5f8116 CW |
604 | if (101 .. 200) { print; } # print 2nd hundred lines, short for |
605 | # if ($. == 101 .. $. == 200) ... | |
9f10b797 RGS |
606 | |
607 | next LINE if (1 .. /^$/); # skip header lines, short for | |
df5f8116 | 608 | # ... if ($. == 1 .. /^$/); |
9f10b797 RGS |
609 | # (typically in a loop labeled LINE) |
610 | ||
611 | s/^/> / if (/^$/ .. eof()); # quote body | |
a0d0e21e | 612 | |
5a964f20 TC |
613 | # parse mail messages |
614 | while (<>) { | |
615 | $in_header = 1 .. /^$/; | |
df5f8116 CW |
616 | $in_body = /^$/ .. eof; |
617 | if ($in_header) { | |
618 | # ... | |
619 | } else { # in body | |
620 | # ... | |
621 | } | |
5a964f20 | 622 | } continue { |
df5f8116 | 623 | close ARGV if eof; # reset $. each file |
5a964f20 TC |
624 | } |
625 | ||
acf31ca5 SF |
626 | Here's a simple example to illustrate the difference between |
627 | the two range operators: | |
628 | ||
629 | @lines = (" - Foo", | |
630 | "01 - Bar", | |
631 | "1 - Baz", | |
632 | " - Quux"); | |
633 | ||
9f10b797 RGS |
634 | foreach (@lines) { |
635 | if (/0/ .. /1/) { | |
acf31ca5 SF |
636 | print "$_\n"; |
637 | } | |
638 | } | |
639 | ||
9f10b797 RGS |
640 | This program will print only the line containing "Bar". If |
641 | the range operator is changed to C<...>, it will also print the | |
acf31ca5 SF |
642 | "Baz" line. |
643 | ||
644 | And now some examples as a list operator: | |
a0d0e21e LW |
645 | |
646 | for (101 .. 200) { print; } # print $_ 100 times | |
3e3baf6d | 647 | @foo = @foo[0 .. $#foo]; # an expensive no-op |
a0d0e21e LW |
648 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items |
649 | ||
5a964f20 | 650 | The range operator (in list context) makes use of the magical |
5f05dabc | 651 | auto-increment algorithm if the operands are strings. You |
a0d0e21e LW |
652 | can say |
653 | ||
654 | @alphabet = ('A' .. 'Z'); | |
655 | ||
54ae734e | 656 | to get all normal letters of the English alphabet, or |
a0d0e21e LW |
657 | |
658 | $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; | |
659 | ||
660 | to get a hexadecimal digit, or | |
661 | ||
662 | @z2 = ('01' .. '31'); print $z2[$mday]; | |
663 | ||
ea4f5703 YST |
664 | to get dates with leading zeros. |
665 | ||
666 | If the final value specified is not in the sequence that the magical | |
667 | increment would produce, the sequence goes until the next value would | |
668 | be longer than the final value specified. | |
669 | ||
670 | If the initial value specified isn't part of a magical increment | |
671 | sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"), | |
672 | only the initial value will be returned. So the following will only | |
673 | return an alpha: | |
674 | ||
675 | use charnames 'greek'; | |
676 | my @greek_small = ("\N{alpha}" .. "\N{omega}"); | |
677 | ||
678 | To get lower-case greek letters, use this instead: | |
679 | ||
680 | my @greek_small = map { chr } ( ord("\N{alpha}") .. ord("\N{omega}") ); | |
a0d0e21e | 681 | |
df5f8116 CW |
682 | Because each operand is evaluated in integer form, C<2.18 .. 3.14> will |
683 | return two elements in list context. | |
684 | ||
685 | @list = (2.18 .. 3.14); # same as @list = (2 .. 3); | |
686 | ||
a0d0e21e | 687 | =head2 Conditional Operator |
d74e8afc | 688 | X<operator, conditional> X<operator, ternary> X<ternary> X<?:> |
a0d0e21e LW |
689 | |
690 | Ternary "?:" is the conditional operator, just as in C. It works much | |
691 | like an if-then-else. If the argument before the ? is true, the | |
692 | argument before the : is returned, otherwise the argument after the : | |
cb1a09d0 AD |
693 | is returned. For example: |
694 | ||
54310121 | 695 | printf "I have %d dog%s.\n", $n, |
cb1a09d0 AD |
696 | ($n == 1) ? '' : "s"; |
697 | ||
698 | Scalar or list context propagates downward into the 2nd | |
54310121 | 699 | or 3rd argument, whichever is selected. |
cb1a09d0 AD |
700 | |
701 | $a = $ok ? $b : $c; # get a scalar | |
702 | @a = $ok ? @b : @c; # get an array | |
703 | $a = $ok ? @b : @c; # oops, that's just a count! | |
704 | ||
705 | The operator may be assigned to if both the 2nd and 3rd arguments are | |
706 | legal lvalues (meaning that you can assign to them): | |
a0d0e21e LW |
707 | |
708 | ($a_or_b ? $a : $b) = $c; | |
709 | ||
5a964f20 TC |
710 | Because this operator produces an assignable result, using assignments |
711 | without parentheses will get you in trouble. For example, this: | |
712 | ||
713 | $a % 2 ? $a += 10 : $a += 2 | |
714 | ||
715 | Really means this: | |
716 | ||
717 | (($a % 2) ? ($a += 10) : $a) += 2 | |
718 | ||
719 | Rather than this: | |
720 | ||
721 | ($a % 2) ? ($a += 10) : ($a += 2) | |
722 | ||
19799a22 GS |
723 | That should probably be written more simply as: |
724 | ||
725 | $a += ($a % 2) ? 10 : 2; | |
726 | ||
4633a7c4 | 727 | =head2 Assignment Operators |
d74e8afc | 728 | X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=> |
5ac3b81c | 729 | X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=> |
d74e8afc | 730 | X<%=> X<^=> X<x=> |
a0d0e21e LW |
731 | |
732 | "=" is the ordinary assignment operator. | |
733 | ||
734 | Assignment operators work as in C. That is, | |
735 | ||
736 | $a += 2; | |
737 | ||
738 | is equivalent to | |
739 | ||
740 | $a = $a + 2; | |
741 | ||
742 | although without duplicating any side effects that dereferencing the lvalue | |
54310121 | 743 | might trigger, such as from tie(). Other assignment operators work similarly. |
744 | The following are recognized: | |
a0d0e21e LW |
745 | |
746 | **= += *= &= <<= &&= | |
9f10b797 RGS |
747 | -= /= |= >>= ||= |
748 | .= %= ^= //= | |
749 | x= | |
a0d0e21e | 750 | |
19799a22 | 751 | Although these are grouped by family, they all have the precedence |
a0d0e21e LW |
752 | of assignment. |
753 | ||
b350dd2f GS |
754 | Unlike in C, the scalar assignment operator produces a valid lvalue. |
755 | Modifying an assignment is equivalent to doing the assignment and | |
756 | then modifying the variable that was assigned to. This is useful | |
757 | for modifying a copy of something, like this: | |
a0d0e21e LW |
758 | |
759 | ($tmp = $global) =~ tr [A-Z] [a-z]; | |
760 | ||
761 | Likewise, | |
762 | ||
763 | ($a += 2) *= 3; | |
764 | ||
765 | is equivalent to | |
766 | ||
767 | $a += 2; | |
768 | $a *= 3; | |
769 | ||
b350dd2f GS |
770 | Similarly, a list assignment in list context produces the list of |
771 | lvalues assigned to, and a list assignment in scalar context returns | |
772 | the number of elements produced by the expression on the right hand | |
773 | side of the assignment. | |
774 | ||
748a9306 | 775 | =head2 Comma Operator |
d74e8afc | 776 | X<comma> X<operator, comma> X<,> |
a0d0e21e | 777 | |
5a964f20 | 778 | Binary "," is the comma operator. In scalar context it evaluates |
a0d0e21e LW |
779 | its left argument, throws that value away, then evaluates its right |
780 | argument and returns that value. This is just like C's comma operator. | |
781 | ||
5a964f20 | 782 | In list context, it's just the list argument separator, and inserts |
ed5c6d31 PJ |
783 | both its arguments into the list. These arguments are also evaluated |
784 | from left to right. | |
a0d0e21e | 785 | |
d042e63d | 786 | The C<< => >> operator is a synonym for the comma, but forces any word |
719b43e8 | 787 | (consisting entirely of word characters) to its left to be interpreted |
a44e5664 MS |
788 | as a string (as of 5.001). This includes words that might otherwise be |
789 | considered a constant or function call. | |
790 | ||
791 | use constant FOO => "something"; | |
792 | ||
793 | my %h = ( FOO => 23 ); | |
794 | ||
795 | is equivalent to: | |
796 | ||
797 | my %h = ("FOO", 23); | |
798 | ||
799 | It is I<NOT>: | |
800 | ||
801 | my %h = ("something", 23); | |
802 | ||
803 | If the argument on the left is not a word, it is first interpreted as | |
804 | an expression, and then the string value of that is used. | |
719b43e8 RGS |
805 | |
806 | The C<< => >> operator is helpful in documenting the correspondence | |
807 | between keys and values in hashes, and other paired elements in lists. | |
748a9306 | 808 | |
a44e5664 MS |
809 | %hash = ( $key => $value ); |
810 | login( $username => $password ); | |
811 | ||
a0d0e21e | 812 | =head2 List Operators (Rightward) |
d74e8afc | 813 | X<operator, list, rightward> X<list operator> |
a0d0e21e LW |
814 | |
815 | On the right side of a list operator, it has very low precedence, | |
816 | such that it controls all comma-separated expressions found there. | |
817 | The only operators with lower precedence are the logical operators | |
818 | "and", "or", and "not", which may be used to evaluate calls to list | |
819 | operators without the need for extra parentheses: | |
820 | ||
821 | open HANDLE, "filename" | |
822 | or die "Can't open: $!\n"; | |
823 | ||
5ba421f6 | 824 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
a0d0e21e LW |
825 | |
826 | =head2 Logical Not | |
d74e8afc | 827 | X<operator, logical, not> X<not> |
a0d0e21e LW |
828 | |
829 | Unary "not" returns the logical negation of the expression to its right. | |
830 | It's the equivalent of "!" except for the very low precedence. | |
831 | ||
832 | =head2 Logical And | |
d74e8afc | 833 | X<operator, logical, and> X<and> |
a0d0e21e LW |
834 | |
835 | Binary "and" returns the logical conjunction of the two surrounding | |
836 | expressions. It's equivalent to && except for the very low | |
5f05dabc | 837 | precedence. This means that it short-circuits: i.e., the right |
a0d0e21e LW |
838 | expression is evaluated only if the left expression is true. |
839 | ||
c963b151 | 840 | =head2 Logical or, Defined or, and Exclusive Or |
d74e8afc ITB |
841 | X<operator, logical, or> X<operator, logical, xor> X<operator, logical, err> |
842 | X<operator, logical, defined or> X<operator, logical, exclusive or> | |
843 | X<or> X<xor> X<err> | |
a0d0e21e LW |
844 | |
845 | Binary "or" returns the logical disjunction of the two surrounding | |
5a964f20 TC |
846 | expressions. It's equivalent to || except for the very low precedence. |
847 | This makes it useful for control flow | |
848 | ||
849 | print FH $data or die "Can't write to FH: $!"; | |
850 | ||
851 | This means that it short-circuits: i.e., the right expression is evaluated | |
852 | only if the left expression is false. Due to its precedence, you should | |
853 | probably avoid using this for assignment, only for control flow. | |
854 | ||
855 | $a = $b or $c; # bug: this is wrong | |
856 | ($a = $b) or $c; # really means this | |
857 | $a = $b || $c; # better written this way | |
858 | ||
19799a22 | 859 | However, when it's a list-context assignment and you're trying to use |
5a964f20 TC |
860 | "||" for control flow, you probably need "or" so that the assignment |
861 | takes higher precedence. | |
862 | ||
863 | @info = stat($file) || die; # oops, scalar sense of stat! | |
864 | @info = stat($file) or die; # better, now @info gets its due | |
865 | ||
c963b151 BD |
866 | Then again, you could always use parentheses. |
867 | ||
9f10b797 RGS |
868 | Binary "err" is equivalent to C<//>--it's just like binary "or", except it |
869 | tests its left argument's definedness instead of its truth. There are two | |
870 | ways to remember "err": either because many functions return C<undef> on | |
871 | an B<err>or, or as a sort of correction: C<$a = ($b err 'default')>. This | |
872 | keyword is only available when the 'err' feature is enabled: see | |
873 | L<feature> for more information. | |
a0d0e21e LW |
874 | |
875 | Binary "xor" returns the exclusive-OR of the two surrounding expressions. | |
876 | It cannot short circuit, of course. | |
877 | ||
878 | =head2 C Operators Missing From Perl | |
d74e8afc ITB |
879 | X<operator, missing from perl> X<&> X<*> |
880 | X<typecasting> X<(TYPE)> | |
a0d0e21e LW |
881 | |
882 | Here is what C has that Perl doesn't: | |
883 | ||
884 | =over 8 | |
885 | ||
886 | =item unary & | |
887 | ||
888 | Address-of operator. (But see the "\" operator for taking a reference.) | |
889 | ||
890 | =item unary * | |
891 | ||
54310121 | 892 | Dereference-address operator. (Perl's prefix dereferencing |
a0d0e21e LW |
893 | operators are typed: $, @, %, and &.) |
894 | ||
895 | =item (TYPE) | |
896 | ||
19799a22 | 897 | Type-casting operator. |
a0d0e21e LW |
898 | |
899 | =back | |
900 | ||
5f05dabc | 901 | =head2 Quote and Quote-like Operators |
89d205f2 | 902 | X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m> |
d74e8afc ITB |
903 | X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>> |
904 | X<escape sequence> X<escape> | |
905 | ||
a0d0e21e LW |
906 | |
907 | While we usually think of quotes as literal values, in Perl they | |
908 | function as operators, providing various kinds of interpolating and | |
909 | pattern matching capabilities. Perl provides customary quote characters | |
910 | for these behaviors, but also provides a way for you to choose your | |
911 | quote character for any of them. In the following table, a C<{}> represents | |
9f10b797 | 912 | any pair of delimiters you choose. |
a0d0e21e | 913 | |
2c268ad5 TP |
914 | Customary Generic Meaning Interpolates |
915 | '' q{} Literal no | |
916 | "" qq{} Literal yes | |
af9219ee | 917 | `` qx{} Command yes* |
2c268ad5 | 918 | qw{} Word list no |
af9219ee MG |
919 | // m{} Pattern match yes* |
920 | qr{} Pattern yes* | |
921 | s{}{} Substitution yes* | |
2c268ad5 | 922 | tr{}{} Transliteration no (but see below) |
7e3b091d | 923 | <<EOF here-doc yes* |
a0d0e21e | 924 | |
af9219ee MG |
925 | * unless the delimiter is ''. |
926 | ||
87275199 GS |
927 | Non-bracketing delimiters use the same character fore and aft, but the four |
928 | sorts of brackets (round, angle, square, curly) will all nest, which means | |
9f10b797 | 929 | that |
87275199 | 930 | |
9f10b797 | 931 | q{foo{bar}baz} |
35f2feb0 | 932 | |
9f10b797 | 933 | is the same as |
87275199 GS |
934 | |
935 | 'foo{bar}baz' | |
936 | ||
937 | Note, however, that this does not always work for quoting Perl code: | |
938 | ||
939 | $s = q{ if($a eq "}") ... }; # WRONG | |
940 | ||
83df6a1d JH |
941 | is a syntax error. The C<Text::Balanced> module (from CPAN, and |
942 | starting from Perl 5.8 part of the standard distribution) is able | |
943 | to do this properly. | |
87275199 | 944 | |
19799a22 | 945 | There can be whitespace between the operator and the quoting |
fb73857a | 946 | characters, except when C<#> is being used as the quoting character. |
19799a22 GS |
947 | C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the |
948 | operator C<q> followed by a comment. Its argument will be taken | |
949 | from the next line. This allows you to write: | |
fb73857a | 950 | |
951 | s {foo} # Replace foo | |
952 | {bar} # with bar. | |
953 | ||
904501ec MG |
954 | The following escape sequences are available in constructs that interpolate |
955 | and in transliterations. | |
d74e8afc | 956 | X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> |
a0d0e21e | 957 | |
6ee5d4e7 | 958 | \t tab (HT, TAB) |
5a964f20 | 959 | \n newline (NL) |
6ee5d4e7 | 960 | \r return (CR) |
961 | \f form feed (FF) | |
962 | \b backspace (BS) | |
963 | \a alarm (bell) (BEL) | |
964 | \e escape (ESC) | |
a0ed51b3 LW |
965 | \033 octal char (ESC) |
966 | \x1b hex char (ESC) | |
967 | \x{263a} wide hex char (SMILEY) | |
19799a22 | 968 | \c[ control char (ESC) |
95cc3e0c | 969 | \N{name} named Unicode character |
2c268ad5 | 970 | |
4c77eaa2 AE |
971 | B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for |
972 | the vertical tab (VT - ASCII 11). | |
973 | ||
904501ec MG |
974 | The following escape sequences are available in constructs that interpolate |
975 | but not in transliterations. | |
d74e8afc | 976 | X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> |
904501ec | 977 | |
a0d0e21e LW |
978 | \l lowercase next char |
979 | \u uppercase next char | |
980 | \L lowercase till \E | |
981 | \U uppercase till \E | |
982 | \E end case modification | |
1d2dff63 | 983 | \Q quote non-word characters till \E |
a0d0e21e | 984 | |
95cc3e0c JH |
985 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, |
986 | C<\u> and C<\U> is taken from the current locale. See L<perllocale>. | |
987 | If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or | |
988 | beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and | |
989 | C<\U> is as defined by Unicode. For documentation of C<\N{name}>, | |
990 | see L<charnames>. | |
a034a98d | 991 | |
5a964f20 TC |
992 | All systems use the virtual C<"\n"> to represent a line terminator, |
993 | called a "newline". There is no such thing as an unvarying, physical | |
19799a22 | 994 | newline character. It is only an illusion that the operating system, |
5a964f20 TC |
995 | device drivers, C libraries, and Perl all conspire to preserve. Not all |
996 | systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example, | |
997 | on a Mac, these are reversed, and on systems without line terminator, | |
998 | printing C<"\n"> may emit no actual data. In general, use C<"\n"> when | |
999 | you mean a "newline" for your system, but use the literal ASCII when you | |
1000 | need an exact character. For example, most networking protocols expect | |
2a380090 | 1001 | and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators, |
5a964f20 TC |
1002 | and although they often accept just C<"\012">, they seldom tolerate just |
1003 | C<"\015">. If you get in the habit of using C<"\n"> for networking, | |
1004 | you may be burned some day. | |
d74e8afc ITB |
1005 | X<newline> X<line terminator> X<eol> X<end of line> |
1006 | X<\n> X<\r> X<\r\n> | |
5a964f20 | 1007 | |
904501ec MG |
1008 | For constructs that do interpolate, variables beginning with "C<$>" |
1009 | or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or | |
ad0f383a A |
1010 | C<< $href->{key}[0] >> are also interpolated, as are array and hash slices. |
1011 | But method calls such as C<< $obj->meth >> are not. | |
af9219ee MG |
1012 | |
1013 | Interpolating an array or slice interpolates the elements in order, | |
1014 | separated by the value of C<$">, so is equivalent to interpolating | |
6deea57f TS |
1015 | C<join $", @array>. "Punctuation" arrays such as C<@*> are only |
1016 | interpolated if the name is enclosed in braces C<@{*}>, but special | |
1017 | arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces. | |
af9219ee | 1018 | |
89d205f2 YO |
1019 | You cannot include a literal C<$> or C<@> within a C<\Q> sequence. |
1020 | An unescaped C<$> or C<@> interpolates the corresponding variable, | |
1d2dff63 | 1021 | while escaping will cause the literal string C<\$> to be inserted. |
89d205f2 | 1022 | You'll need to write something like C<m/\Quser\E\@\Qhost/>. |
1d2dff63 | 1023 | |
a0d0e21e LW |
1024 | Patterns are subject to an additional level of interpretation as a |
1025 | regular expression. This is done as a second pass, after variables are | |
1026 | interpolated, so that regular expressions may be incorporated into the | |
1027 | pattern from the variables. If this is not what you want, use C<\Q> to | |
1028 | interpolate a variable literally. | |
1029 | ||
19799a22 GS |
1030 | Apart from the behavior described above, Perl does not expand |
1031 | multiple levels of interpolation. In particular, contrary to the | |
1032 | expectations of shell programmers, back-quotes do I<NOT> interpolate | |
1033 | within double quotes, nor do single quotes impede evaluation of | |
1034 | variables when used within double quotes. | |
a0d0e21e | 1035 | |
5f05dabc | 1036 | =head2 Regexp Quote-Like Operators |
d74e8afc | 1037 | X<operator, regexp> |
cb1a09d0 | 1038 | |
5f05dabc | 1039 | Here are the quote-like operators that apply to pattern |
cb1a09d0 AD |
1040 | matching and related activities. |
1041 | ||
a0d0e21e LW |
1042 | =over 8 |
1043 | ||
1044 | =item ?PATTERN? | |
d74e8afc | 1045 | X<?> |
a0d0e21e LW |
1046 | |
1047 | This is just like the C</pattern/> search, except that it matches only | |
1048 | once between calls to the reset() operator. This is a useful | |
5f05dabc | 1049 | optimization when you want to see only the first occurrence of |
a0d0e21e LW |
1050 | something in each file of a set of files, for instance. Only C<??> |
1051 | patterns local to the current package are reset. | |
1052 | ||
5a964f20 TC |
1053 | while (<>) { |
1054 | if (?^$?) { | |
1055 | # blank line between header and body | |
1056 | } | |
1057 | } continue { | |
1058 | reset if eof; # clear ?? status for next file | |
1059 | } | |
1060 | ||
483b4840 | 1061 | This usage is vaguely deprecated, which means it just might possibly |
19799a22 GS |
1062 | be removed in some distant future version of Perl, perhaps somewhere |
1063 | around the year 2168. | |
a0d0e21e | 1064 | |
fb73857a | 1065 | =item m/PATTERN/cgimosx |
89d205f2 YO |
1066 | X<m> X<operator, match> |
1067 | X<regexp, options> X<regexp> X<regex, options> X<regex> | |
d74e8afc | 1068 | X</c> X</i> X</m> X</o> X</s> X</x> |
a0d0e21e | 1069 | |
cde0cee5 | 1070 | =item /PATTERN/cgimosxk |
a0d0e21e | 1071 | |
5a964f20 | 1072 | Searches a string for a pattern match, and in scalar context returns |
19799a22 GS |
1073 | true if it succeeds, false if it fails. If no string is specified |
1074 | via the C<=~> or C<!~> operator, the $_ string is searched. (The | |
1075 | string specified with C<=~> need not be an lvalue--it may be the | |
1076 | result of an expression evaluation, but remember the C<=~> binds | |
1077 | rather tightly.) See also L<perlre>. See L<perllocale> for | |
1078 | discussion of additional considerations that apply when C<use locale> | |
1079 | is in effect. | |
a0d0e21e LW |
1080 | |
1081 | Options are: | |
1082 | ||
a0d0e21e LW |
1083 | i Do case-insensitive pattern matching. |
1084 | m Treat string as multiple lines. | |
a0d0e21e LW |
1085 | s Treat string as single line. |
1086 | x Use extended regular expressions. | |
cde0cee5 YO |
1087 | g Match globally, i.e., find all occurrences. |
1088 | c Do not reset search position on a failed match when /g is in effect. | |
1089 | o Compile pattern only once. | |
1090 | k Keep a copy of the matched string so that ${^MATCH} and friends | |
1091 | will be defined. | |
a0d0e21e LW |
1092 | |
1093 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> | |
89d205f2 | 1094 | you can use any pair of non-alphanumeric, non-whitespace characters |
19799a22 GS |
1095 | as delimiters. This is particularly useful for matching path names |
1096 | that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is | |
7bac28a0 | 1097 | the delimiter, then the match-only-once rule of C<?PATTERN?> applies. |
19799a22 | 1098 | If "'" is the delimiter, no interpolation is performed on the PATTERN. |
a0d0e21e LW |
1099 | |
1100 | PATTERN may contain variables, which will be interpolated (and the | |
f70b4f9c | 1101 | pattern recompiled) every time the pattern search is evaluated, except |
1f247705 GS |
1102 | for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and |
1103 | C<$|> are not interpolated because they look like end-of-string tests.) | |
f70b4f9c AB |
1104 | If you want such a pattern to be compiled only once, add a C</o> after |
1105 | the trailing delimiter. This avoids expensive run-time recompilations, | |
1106 | and is useful when the value you are interpolating won't change over | |
1107 | the life of the script. However, mentioning C</o> constitutes a promise | |
1108 | that you won't change the variables in the pattern. If you change them, | |
13a2d996 | 1109 | Perl won't even notice. See also L<"qr/STRING/imosx">. |
a0d0e21e | 1110 | |
5a964f20 | 1111 | If the PATTERN evaluates to the empty string, the last |
d65afb4b HS |
1112 | I<successfully> matched regular expression is used instead. In this |
1113 | case, only the C<g> and C<c> flags on the empty pattern is honoured - | |
1114 | the other flags are taken from the original pattern. If no match has | |
1115 | previously succeeded, this will (silently) act instead as a genuine | |
1116 | empty pattern (which will always match). | |
a0d0e21e | 1117 | |
89d205f2 YO |
1118 | Note that it's possible to confuse Perl into thinking C<//> (the empty |
1119 | regex) is really C<//> (the defined-or operator). Perl is usually pretty | |
1120 | good about this, but some pathological cases might trigger this, such as | |
1121 | C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //> | |
1122 | (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl | |
1123 | will assume you meant defined-or. If you meant the empty regex, just | |
1124 | use parentheses or spaces to disambiguate, or even prefix the empty | |
c963b151 BD |
1125 | regex with an C<m> (so C<//> becomes C<m//>). |
1126 | ||
19799a22 | 1127 | If the C</g> option is not used, C<m//> in list context returns a |
a0d0e21e | 1128 | list consisting of the subexpressions matched by the parentheses in the |
f7e33566 GS |
1129 | pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are |
1130 | also set, and that this differs from Perl 4's behavior.) When there are | |
1131 | no parentheses in the pattern, the return value is the list C<(1)> for | |
1132 | success. With or without parentheses, an empty list is returned upon | |
1133 | failure. | |
a0d0e21e LW |
1134 | |
1135 | Examples: | |
1136 | ||
1137 | open(TTY, '/dev/tty'); | |
1138 | <TTY> =~ /^y/i && foo(); # do foo if desired | |
1139 | ||
1140 | if (/Version: *([0-9.]*)/) { $version = $1; } | |
1141 | ||
1142 | next if m#^/usr/spool/uucp#; | |
1143 | ||
1144 | # poor man's grep | |
1145 | $arg = shift; | |
1146 | while (<>) { | |
1147 | print if /$arg/o; # compile only once | |
1148 | } | |
1149 | ||
1150 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) | |
1151 | ||
1152 | This last example splits $foo into the first two words and the | |
5f05dabc | 1153 | remainder of the line, and assigns those three fields to $F1, $F2, and |
1154 | $Etc. The conditional is true if any variables were assigned, i.e., if | |
a0d0e21e LW |
1155 | the pattern matched. |
1156 | ||
19799a22 GS |
1157 | The C</g> modifier specifies global pattern matching--that is, |
1158 | matching as many times as possible within the string. How it behaves | |
1159 | depends on the context. In list context, it returns a list of the | |
1160 | substrings matched by any capturing parentheses in the regular | |
1161 | expression. If there are no parentheses, it returns a list of all | |
1162 | the matched strings, as if there were parentheses around the whole | |
1163 | pattern. | |
a0d0e21e | 1164 | |
7e86de3e | 1165 | In scalar context, each execution of C<m//g> finds the next match, |
19799a22 | 1166 | returning true if it matches, and false if there is no further match. |
7e86de3e MG |
1167 | The position after the last match can be read or set using the pos() |
1168 | function; see L<perlfunc/pos>. A failed match normally resets the | |
1169 | search position to the beginning of the string, but you can avoid that | |
1170 | by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target | |
1171 | string also resets the search position. | |
c90c0ff4 | 1172 | |
1173 | You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a | |
1174 | zero-width assertion that matches the exact position where the previous | |
5d43e42d DC |
1175 | C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion |
1176 | still anchors at pos(), but the match is of course only attempted once. | |
1177 | Using C<\G> without C</g> on a target string that has not previously had a | |
1178 | C</g> match applied to it is the same as using the C<\A> assertion to match | |
fe4b3f22 RGS |
1179 | the beginning of the string. Note also that, currently, C<\G> is only |
1180 | properly supported when anchored at the very beginning of the pattern. | |
c90c0ff4 | 1181 | |
1182 | Examples: | |
a0d0e21e LW |
1183 | |
1184 | # list context | |
1185 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); | |
1186 | ||
1187 | # scalar context | |
5d43e42d | 1188 | $/ = ""; |
19799a22 GS |
1189 | while (defined($paragraph = <>)) { |
1190 | while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { | |
1191 | $sentences++; | |
a0d0e21e LW |
1192 | } |
1193 | } | |
1194 | print "$sentences\n"; | |
1195 | ||
c90c0ff4 | 1196 | # using m//gc with \G |
137443ea | 1197 | $_ = "ppooqppqq"; |
44a8e56a | 1198 | while ($i++ < 2) { |
1199 | print "1: '"; | |
c90c0ff4 | 1200 | print $1 while /(o)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1201 | print "2: '"; |
c90c0ff4 | 1202 | print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1203 | print "3: '"; |
c90c0ff4 | 1204 | print $1 while /(p)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1205 | } |
5d43e42d | 1206 | print "Final: '$1', pos=",pos,"\n" if /\G(.)/; |
44a8e56a | 1207 | |
1208 | The last example should print: | |
1209 | ||
1210 | 1: 'oo', pos=4 | |
137443ea | 1211 | 2: 'q', pos=5 |
44a8e56a | 1212 | 3: 'pp', pos=7 |
1213 | 1: '', pos=7 | |
137443ea | 1214 | 2: 'q', pos=8 |
1215 | 3: '', pos=8 | |
5d43e42d DC |
1216 | Final: 'q', pos=8 |
1217 | ||
1218 | Notice that the final match matched C<q> instead of C<p>, which a match | |
1219 | without the C<\G> anchor would have done. Also note that the final match | |
1220 | did not update C<pos> -- C<pos> is only updated on a C</g> match. If the | |
1221 | final match did indeed match C<p>, it's a good bet that you're running an | |
1222 | older (pre-5.6.0) Perl. | |
44a8e56a | 1223 | |
c90c0ff4 | 1224 | A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can |
e7ea3e70 | 1225 | combine several regexps like this to process a string part-by-part, |
c90c0ff4 | 1226 | doing different actions depending on which regexp matched. Each |
1227 | regexp tries to match where the previous one leaves off. | |
e7ea3e70 | 1228 | |
3fe9a6f1 | 1229 | $_ = <<'EOL'; |
63acfd00 | 1230 | $url = URI::URL->new( "http://www/" ); die if $url eq "xXx"; |
3fe9a6f1 | 1231 | EOL |
1232 | LOOP: | |
e7ea3e70 | 1233 | { |
c90c0ff4 | 1234 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; |
1235 | print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc; | |
1236 | print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc; | |
1237 | print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc; | |
1238 | print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc; | |
1239 | print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc; | |
1240 | print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc; | |
e7ea3e70 IZ |
1241 | print ". That's all!\n"; |
1242 | } | |
1243 | ||
1244 | Here is the output (split into several lines): | |
1245 | ||
1246 | line-noise lowercase line-noise lowercase UPPERCASE line-noise | |
1247 | UPPERCASE line-noise lowercase line-noise lowercase line-noise | |
1248 | lowercase lowercase line-noise lowercase lowercase line-noise | |
1249 | MiXeD line-noise. That's all! | |
44a8e56a | 1250 | |
a0d0e21e | 1251 | =item q/STRING/ |
5d44bfff | 1252 | X<q> X<quote, single> X<'> X<''> |
a0d0e21e | 1253 | |
5d44bfff | 1254 | =item 'STRING' |
a0d0e21e | 1255 | |
19799a22 | 1256 | A single-quoted, literal string. A backslash represents a backslash |
68dc0745 | 1257 | unless followed by the delimiter or another backslash, in which case |
1258 | the delimiter or backslash is interpolated. | |
a0d0e21e LW |
1259 | |
1260 | $foo = q!I said, "You said, 'She said it.'"!; | |
1261 | $bar = q('This is it.'); | |
68dc0745 | 1262 | $baz = '\n'; # a two-character string |
a0d0e21e LW |
1263 | |
1264 | =item qq/STRING/ | |
d74e8afc | 1265 | X<qq> X<quote, double> X<"> X<""> |
a0d0e21e LW |
1266 | |
1267 | =item "STRING" | |
1268 | ||
1269 | A double-quoted, interpolated string. | |
1270 | ||
1271 | $_ .= qq | |
1272 | (*** The previous line contains the naughty word "$1".\n) | |
19799a22 | 1273 | if /\b(tcl|java|python)\b/i; # :-) |
68dc0745 | 1274 | $baz = "\n"; # a one-character string |
a0d0e21e | 1275 | |
eec2d3df | 1276 | =item qr/STRING/imosx |
d74e8afc | 1277 | X<qr> X</i> X</m> X</o> X</s> X</x> |
eec2d3df | 1278 | |
322edccd | 1279 | This operator quotes (and possibly compiles) its I<STRING> as a regular |
19799a22 GS |
1280 | expression. I<STRING> is interpolated the same way as I<PATTERN> |
1281 | in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation | |
1282 | is done. Returns a Perl value which may be used instead of the | |
1283 | corresponding C</STRING/imosx> expression. | |
4b6a7270 IZ |
1284 | |
1285 | For example, | |
1286 | ||
1287 | $rex = qr/my.STRING/is; | |
1288 | s/$rex/foo/; | |
1289 | ||
1290 | is equivalent to | |
1291 | ||
1292 | s/my.STRING/foo/is; | |
1293 | ||
1294 | The result may be used as a subpattern in a match: | |
eec2d3df GS |
1295 | |
1296 | $re = qr/$pattern/; | |
0a92e3a8 GS |
1297 | $string =~ /foo${re}bar/; # can be interpolated in other patterns |
1298 | $string =~ $re; # or used standalone | |
4b6a7270 IZ |
1299 | $string =~ /$re/; # or this way |
1300 | ||
1301 | Since Perl may compile the pattern at the moment of execution of qr() | |
19799a22 | 1302 | operator, using qr() may have speed advantages in some situations, |
4b6a7270 IZ |
1303 | notably if the result of qr() is used standalone: |
1304 | ||
1305 | sub match { | |
1306 | my $patterns = shift; | |
1307 | my @compiled = map qr/$_/i, @$patterns; | |
1308 | grep { | |
1309 | my $success = 0; | |
a7665c5e | 1310 | foreach my $pat (@compiled) { |
4b6a7270 IZ |
1311 | $success = 1, last if /$pat/; |
1312 | } | |
1313 | $success; | |
1314 | } @_; | |
1315 | } | |
1316 | ||
19799a22 GS |
1317 | Precompilation of the pattern into an internal representation at |
1318 | the moment of qr() avoids a need to recompile the pattern every | |
1319 | time a match C</$pat/> is attempted. (Perl has many other internal | |
1320 | optimizations, but none would be triggered in the above example if | |
1321 | we did not use qr() operator.) | |
eec2d3df GS |
1322 | |
1323 | Options are: | |
1324 | ||
1325 | i Do case-insensitive pattern matching. | |
1326 | m Treat string as multiple lines. | |
1327 | o Compile pattern only once. | |
1328 | s Treat string as single line. | |
1329 | x Use extended regular expressions. | |
1330 | ||
0a92e3a8 GS |
1331 | See L<perlre> for additional information on valid syntax for STRING, and |
1332 | for a detailed look at the semantics of regular expressions. | |
1333 | ||
a0d0e21e | 1334 | =item qx/STRING/ |
d74e8afc | 1335 | X<qx> X<`> X<``> X<backtick> |
a0d0e21e LW |
1336 | |
1337 | =item `STRING` | |
1338 | ||
43dd4d21 JH |
1339 | A string which is (possibly) interpolated and then executed as a |
1340 | system command with C</bin/sh> or its equivalent. Shell wildcards, | |
1341 | pipes, and redirections will be honored. The collected standard | |
1342 | output of the command is returned; standard error is unaffected. In | |
1343 | scalar context, it comes back as a single (potentially multi-line) | |
1344 | string, or undef if the command failed. In list context, returns a | |
1345 | list of lines (however you've defined lines with $/ or | |
1346 | $INPUT_RECORD_SEPARATOR), or an empty list if the command failed. | |
5a964f20 TC |
1347 | |
1348 | Because backticks do not affect standard error, use shell file descriptor | |
1349 | syntax (assuming the shell supports this) if you care to address this. | |
1350 | To capture a command's STDERR and STDOUT together: | |
a0d0e21e | 1351 | |
5a964f20 TC |
1352 | $output = `cmd 2>&1`; |
1353 | ||
1354 | To capture a command's STDOUT but discard its STDERR: | |
1355 | ||
1356 | $output = `cmd 2>/dev/null`; | |
1357 | ||
1358 | To capture a command's STDERR but discard its STDOUT (ordering is | |
1359 | important here): | |
1360 | ||
1361 | $output = `cmd 2>&1 1>/dev/null`; | |
1362 | ||
1363 | To exchange a command's STDOUT and STDERR in order to capture the STDERR | |
1364 | but leave its STDOUT to come out the old STDERR: | |
1365 | ||
1366 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; | |
1367 | ||
1368 | To read both a command's STDOUT and its STDERR separately, it's easiest | |
2359510d SD |
1369 | to redirect them separately to files, and then read from those files |
1370 | when the program is done: | |
5a964f20 | 1371 | |
2359510d | 1372 | system("program args 1>program.stdout 2>program.stderr"); |
5a964f20 | 1373 | |
30398227 SP |
1374 | The STDIN filehandle used by the command is inherited from Perl's STDIN. |
1375 | For example: | |
1376 | ||
1377 | open BLAM, "blam" || die "Can't open: $!"; | |
1378 | open STDIN, "<&BLAM"; | |
1379 | print `sort`; | |
1380 | ||
1381 | will print the sorted contents of the file "blam". | |
1382 | ||
5a964f20 TC |
1383 | Using single-quote as a delimiter protects the command from Perl's |
1384 | double-quote interpolation, passing it on to the shell instead: | |
1385 | ||
1386 | $perl_info = qx(ps $$); # that's Perl's $$ | |
1387 | $shell_info = qx'ps $$'; # that's the new shell's $$ | |
1388 | ||
19799a22 | 1389 | How that string gets evaluated is entirely subject to the command |
5a964f20 TC |
1390 | interpreter on your system. On most platforms, you will have to protect |
1391 | shell metacharacters if you want them treated literally. This is in | |
1392 | practice difficult to do, as it's unclear how to escape which characters. | |
1393 | See L<perlsec> for a clean and safe example of a manual fork() and exec() | |
1394 | to emulate backticks safely. | |
a0d0e21e | 1395 | |
bb32b41a GS |
1396 | On some platforms (notably DOS-like ones), the shell may not be |
1397 | capable of dealing with multiline commands, so putting newlines in | |
1398 | the string may not get you what you want. You may be able to evaluate | |
1399 | multiple commands in a single line by separating them with the command | |
1400 | separator character, if your shell supports that (e.g. C<;> on many Unix | |
1401 | shells; C<&> on the Windows NT C<cmd> shell). | |
1402 | ||
0f897271 GS |
1403 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
1404 | output before starting the child process, but this may not be supported | |
1405 | on some platforms (see L<perlport>). To be safe, you may need to set | |
1406 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of | |
1407 | C<IO::Handle> on any open handles. | |
1408 | ||
bb32b41a GS |
1409 | Beware that some command shells may place restrictions on the length |
1410 | of the command line. You must ensure your strings don't exceed this | |
1411 | limit after any necessary interpolations. See the platform-specific | |
1412 | release notes for more details about your particular environment. | |
1413 | ||
5a964f20 TC |
1414 | Using this operator can lead to programs that are difficult to port, |
1415 | because the shell commands called vary between systems, and may in | |
1416 | fact not be present at all. As one example, the C<type> command under | |
1417 | the POSIX shell is very different from the C<type> command under DOS. | |
1418 | That doesn't mean you should go out of your way to avoid backticks | |
1419 | when they're the right way to get something done. Perl was made to be | |
1420 | a glue language, and one of the things it glues together is commands. | |
1421 | Just understand what you're getting yourself into. | |
bb32b41a | 1422 | |
da87341d | 1423 | See L</"I/O Operators"> for more discussion. |
a0d0e21e | 1424 | |
945c54fd | 1425 | =item qw/STRING/ |
d74e8afc | 1426 | X<qw> X<quote, list> X<quote, words> |
945c54fd JH |
1427 | |
1428 | Evaluates to a list of the words extracted out of STRING, using embedded | |
1429 | whitespace as the word delimiters. It can be understood as being roughly | |
1430 | equivalent to: | |
1431 | ||
1432 | split(' ', q/STRING/); | |
1433 | ||
efb1e162 CW |
1434 | the differences being that it generates a real list at compile time, and |
1435 | in scalar context it returns the last element in the list. So | |
945c54fd JH |
1436 | this expression: |
1437 | ||
1438 | qw(foo bar baz) | |
1439 | ||
1440 | is semantically equivalent to the list: | |
1441 | ||
1442 | 'foo', 'bar', 'baz' | |
1443 | ||
1444 | Some frequently seen examples: | |
1445 | ||
1446 | use POSIX qw( setlocale localeconv ) | |
1447 | @EXPORT = qw( foo bar baz ); | |
1448 | ||
1449 | A common mistake is to try to separate the words with comma or to | |
1450 | put comments into a multi-line C<qw>-string. For this reason, the | |
89d205f2 | 1451 | C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable) |
945c54fd JH |
1452 | produces warnings if the STRING contains the "," or the "#" character. |
1453 | ||
cde0cee5 | 1454 | =item s/PATTERN/REPLACEMENT/egimosxk |
d74e8afc ITB |
1455 | X<substitute> X<substitution> X<replace> X<regexp, replace> |
1456 | X<regexp, substitute> X</e> X</g> X</i> X</m> X</o> X</s> X</x> | |
a0d0e21e LW |
1457 | |
1458 | Searches a string for a pattern, and if found, replaces that pattern | |
1459 | with the replacement text and returns the number of substitutions | |
e37d713d | 1460 | made. Otherwise it returns false (specifically, the empty string). |
a0d0e21e LW |
1461 | |
1462 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> | |
1463 | variable is searched and modified. (The string specified with C<=~> must | |
5a964f20 | 1464 | be scalar variable, an array element, a hash element, or an assignment |
5f05dabc | 1465 | to one of those, i.e., an lvalue.) |
a0d0e21e | 1466 | |
19799a22 | 1467 | If the delimiter chosen is a single quote, no interpolation is |
a0d0e21e LW |
1468 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the |
1469 | PATTERN contains a $ that looks like a variable rather than an | |
1470 | end-of-string test, the variable will be interpolated into the pattern | |
5f05dabc | 1471 | at run-time. If you want the pattern compiled only once the first time |
a0d0e21e | 1472 | the variable is interpolated, use the C</o> option. If the pattern |
5a964f20 | 1473 | evaluates to the empty string, the last successfully executed regular |
a0d0e21e | 1474 | expression is used instead. See L<perlre> for further explanation on these. |
5a964f20 | 1475 | See L<perllocale> for discussion of additional considerations that apply |
a034a98d | 1476 | when C<use locale> is in effect. |
a0d0e21e LW |
1477 | |
1478 | Options are: | |
1479 | ||
a0d0e21e LW |
1480 | i Do case-insensitive pattern matching. |
1481 | m Treat string as multiple lines. | |
a0d0e21e LW |
1482 | s Treat string as single line. |
1483 | x Use extended regular expressions. | |
cde0cee5 YO |
1484 | g Replace globally, i.e., all occurrences. |
1485 | o Compile pattern only once. | |
1486 | k Keep a copy of the original string so ${^MATCH} and friends | |
1487 | will be defined. | |
1488 | e Evaluate the right side as an expression. | |
1489 | ||
a0d0e21e LW |
1490 | |
1491 | Any non-alphanumeric, non-whitespace delimiter may replace the | |
1492 | slashes. If single quotes are used, no interpretation is done on the | |
e37d713d | 1493 | replacement string (the C</e> modifier overrides this, however). Unlike |
54310121 | 1494 | Perl 4, Perl 5 treats backticks as normal delimiters; the replacement |
e37d713d | 1495 | text is not evaluated as a command. If the |
a0d0e21e | 1496 | PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own |
5f05dabc | 1497 | pair of quotes, which may or may not be bracketing quotes, e.g., |
35f2feb0 | 1498 | C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the |
cec88af6 GS |
1499 | replacement portion to be treated as a full-fledged Perl expression |
1500 | and evaluated right then and there. It is, however, syntax checked at | |
1501 | compile-time. A second C<e> modifier will cause the replacement portion | |
1502 | to be C<eval>ed before being run as a Perl expression. | |
a0d0e21e LW |
1503 | |
1504 | Examples: | |
1505 | ||
1506 | s/\bgreen\b/mauve/g; # don't change wintergreen | |
1507 | ||
1508 | $path =~ s|/usr/bin|/usr/local/bin|; | |
1509 | ||
1510 | s/Login: $foo/Login: $bar/; # run-time pattern | |
1511 | ||
5a964f20 | 1512 | ($foo = $bar) =~ s/this/that/; # copy first, then change |
a0d0e21e | 1513 | |
5a964f20 | 1514 | $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count |
a0d0e21e LW |
1515 | |
1516 | $_ = 'abc123xyz'; | |
1517 | s/\d+/$&*2/e; # yields 'abc246xyz' | |
1518 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' | |
1519 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' | |
1520 | ||
1521 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e | |
1522 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e | |
023a2b40 | 1523 | s/^=(\w+)/pod($1)/ge; # use function call |
a0d0e21e | 1524 | |
5a964f20 TC |
1525 | # expand variables in $_, but dynamics only, using |
1526 | # symbolic dereferencing | |
1527 | s/\$(\w+)/${$1}/g; | |
1528 | ||
cec88af6 GS |
1529 | # Add one to the value of any numbers in the string |
1530 | s/(\d+)/1 + $1/eg; | |
1531 | ||
1532 | # This will expand any embedded scalar variable | |
1533 | # (including lexicals) in $_ : First $1 is interpolated | |
1534 | # to the variable name, and then evaluated | |
a0d0e21e LW |
1535 | s/(\$\w+)/$1/eeg; |
1536 | ||
5a964f20 | 1537 | # Delete (most) C comments. |
a0d0e21e | 1538 | $program =~ s { |
4633a7c4 LW |
1539 | /\* # Match the opening delimiter. |
1540 | .*? # Match a minimal number of characters. | |
1541 | \*/ # Match the closing delimiter. | |
a0d0e21e LW |
1542 | } []gsx; |
1543 | ||
6b0ac556 | 1544 | s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively |
5a964f20 | 1545 | |
6b0ac556 | 1546 | for ($variable) { # trim whitespace in $variable, cheap |
5a964f20 TC |
1547 | s/^\s+//; |
1548 | s/\s+$//; | |
1549 | } | |
a0d0e21e LW |
1550 | |
1551 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields | |
1552 | ||
54310121 | 1553 | Note the use of $ instead of \ in the last example. Unlike |
35f2feb0 GS |
1554 | B<sed>, we use the \<I<digit>> form in only the left hand side. |
1555 | Anywhere else it's $<I<digit>>. | |
a0d0e21e | 1556 | |
5f05dabc | 1557 | Occasionally, you can't use just a C</g> to get all the changes |
19799a22 | 1558 | to occur that you might want. Here are two common cases: |
a0d0e21e LW |
1559 | |
1560 | # put commas in the right places in an integer | |
89d205f2 | 1561 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; |
a0d0e21e LW |
1562 | |
1563 | # expand tabs to 8-column spacing | |
1564 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; | |
1565 | ||
6940069f | 1566 | =item tr/SEARCHLIST/REPLACEMENTLIST/cds |
d74e8afc | 1567 | X<tr> X<y> X<transliterate> X</c> X</d> X</s> |
a0d0e21e | 1568 | |
6940069f | 1569 | =item y/SEARCHLIST/REPLACEMENTLIST/cds |
a0d0e21e | 1570 | |
2c268ad5 | 1571 | Transliterates all occurrences of the characters found in the search list |
a0d0e21e LW |
1572 | with the corresponding character in the replacement list. It returns |
1573 | the number of characters replaced or deleted. If no string is | |
2c268ad5 | 1574 | specified via the =~ or !~ operator, the $_ string is transliterated. (The |
54310121 | 1575 | string specified with =~ must be a scalar variable, an array element, a |
1576 | hash element, or an assignment to one of those, i.e., an lvalue.) | |
8ada0baa | 1577 | |
89d205f2 | 1578 | A character range may be specified with a hyphen, so C<tr/A-J/0-9/> |
2c268ad5 | 1579 | does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. |
54310121 | 1580 | For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the |
1581 | SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has | |
1582 | its own pair of quotes, which may or may not be bracketing quotes, | |
2c268ad5 | 1583 | e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>. |
a0d0e21e | 1584 | |
cc255d5f | 1585 | Note that C<tr> does B<not> do regular expression character classes |
e0c83546 | 1586 | such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to |
cc255d5f JH |
1587 | the tr(1) utility. If you want to map strings between lower/upper |
1588 | cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider | |
1589 | using the C<s> operator if you need regular expressions. | |
1590 | ||
8ada0baa JH |
1591 | Note also that the whole range idea is rather unportable between |
1592 | character sets--and even within character sets they may cause results | |
1593 | you probably didn't expect. A sound principle is to use only ranges | |
1594 | that begin from and end at either alphabets of equal case (a-e, A-E), | |
1595 | or digits (0-4). Anything else is unsafe. If in doubt, spell out the | |
1596 | character sets in full. | |
1597 | ||
a0d0e21e LW |
1598 | Options: |
1599 | ||
1600 | c Complement the SEARCHLIST. | |
1601 | d Delete found but unreplaced characters. | |
1602 | s Squash duplicate replaced characters. | |
1603 | ||
19799a22 GS |
1604 | If the C</c> modifier is specified, the SEARCHLIST character set |
1605 | is complemented. If the C</d> modifier is specified, any characters | |
1606 | specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. | |
1607 | (Note that this is slightly more flexible than the behavior of some | |
1608 | B<tr> programs, which delete anything they find in the SEARCHLIST, | |
1609 | period.) If the C</s> modifier is specified, sequences of characters | |
1610 | that were transliterated to the same character are squashed down | |
1611 | to a single instance of the character. | |
a0d0e21e LW |
1612 | |
1613 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted | |
1614 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter | |
1615 | than the SEARCHLIST, the final character is replicated till it is long | |
5a964f20 | 1616 | enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. |
a0d0e21e LW |
1617 | This latter is useful for counting characters in a class or for |
1618 | squashing character sequences in a class. | |
1619 | ||
1620 | Examples: | |
1621 | ||
1622 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case | |
1623 | ||
1624 | $cnt = tr/*/*/; # count the stars in $_ | |
1625 | ||
1626 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky | |
1627 | ||
1628 | $cnt = tr/0-9//; # count the digits in $_ | |
1629 | ||
1630 | tr/a-zA-Z//s; # bookkeeper -> bokeper | |
1631 | ||
1632 | ($HOST = $host) =~ tr/a-z/A-Z/; | |
1633 | ||
1634 | tr/a-zA-Z/ /cs; # change non-alphas to single space | |
1635 | ||
1636 | tr [\200-\377] | |
1637 | [\000-\177]; # delete 8th bit | |
1638 | ||
19799a22 GS |
1639 | If multiple transliterations are given for a character, only the |
1640 | first one is used: | |
748a9306 LW |
1641 | |
1642 | tr/AAA/XYZ/ | |
1643 | ||
2c268ad5 | 1644 | will transliterate any A to X. |
748a9306 | 1645 | |
19799a22 | 1646 | Because the transliteration table is built at compile time, neither |
a0d0e21e | 1647 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote |
19799a22 GS |
1648 | interpolation. That means that if you want to use variables, you |
1649 | must use an eval(): | |
a0d0e21e LW |
1650 | |
1651 | eval "tr/$oldlist/$newlist/"; | |
1652 | die $@ if $@; | |
1653 | ||
1654 | eval "tr/$oldlist/$newlist/, 1" or die $@; | |
1655 | ||
7e3b091d | 1656 | =item <<EOF |
d74e8afc | 1657 | X<here-doc> X<heredoc> X<here-document> X<<< << >>> |
7e3b091d DA |
1658 | |
1659 | A line-oriented form of quoting is based on the shell "here-document" | |
1660 | syntax. Following a C<< << >> you specify a string to terminate | |
1661 | the quoted material, and all lines following the current line down to | |
89d205f2 YO |
1662 | the terminating string are the value of the item. |
1663 | ||
1664 | The terminating string may be either an identifier (a word), or some | |
1665 | quoted text. An unquoted identifier works like double quotes. | |
1666 | There may not be a space between the C<< << >> and the identifier, | |
1667 | unless the identifier is explicitly quoted. (If you put a space it | |
1668 | will be treated as a null identifier, which is valid, and matches the | |
1669 | first empty line.) The terminating string must appear by itself | |
1670 | (unquoted and with no surrounding whitespace) on the terminating line. | |
1671 | ||
1672 | If the terminating string is quoted, the type of quotes used determine | |
1673 | the treatment of the text. | |
1674 | ||
1675 | =over 4 | |
1676 | ||
1677 | =item Double Quotes | |
1678 | ||
1679 | Double quotes indicate that the text will be interpolated using exactly | |
1680 | the same rules as normal double quoted strings. | |
7e3b091d DA |
1681 | |
1682 | print <<EOF; | |
1683 | The price is $Price. | |
1684 | EOF | |
1685 | ||
1686 | print << "EOF"; # same as above | |
1687 | The price is $Price. | |
1688 | EOF | |
1689 | ||
89d205f2 YO |
1690 | |
1691 | =item Single Quotes | |
1692 | ||
1693 | Single quotes indicate the text is to be treated literally with no | |
1694 | interpolation of its content. This is similar to single quoted | |
1695 | strings except that backslashes have no special meaning, with C<\\> | |
1696 | being treated as two backslashes and not one as they would in every | |
1697 | other quoting construct. | |
1698 | ||
1699 | This is the only form of quoting in perl where there is no need | |
1700 | to worry about escaping content, something that code generators | |
1701 | can and do make good use of. | |
1702 | ||
1703 | =item Backticks | |
1704 | ||
1705 | The content of the here doc is treated just as it would be if the | |
1706 | string were embedded in backticks. Thus the content is interpolated | |
1707 | as though it were double quoted and then executed via the shell, with | |
1708 | the results of the execution returned. | |
1709 | ||
1710 | print << `EOC`; # execute command and get results | |
7e3b091d | 1711 | echo hi there |
7e3b091d DA |
1712 | EOC |
1713 | ||
89d205f2 YO |
1714 | =back |
1715 | ||
1716 | It is possible to stack multiple here-docs in a row: | |
1717 | ||
7e3b091d DA |
1718 | print <<"foo", <<"bar"; # you can stack them |
1719 | I said foo. | |
1720 | foo | |
1721 | I said bar. | |
1722 | bar | |
1723 | ||
1724 | myfunc(<< "THIS", 23, <<'THAT'); | |
1725 | Here's a line | |
1726 | or two. | |
1727 | THIS | |
1728 | and here's another. | |
1729 | THAT | |
1730 | ||
1731 | Just don't forget that you have to put a semicolon on the end | |
1732 | to finish the statement, as Perl doesn't know you're not going to | |
1733 | try to do this: | |
1734 | ||
1735 | print <<ABC | |
1736 | 179231 | |
1737 | ABC | |
1738 | + 20; | |
1739 | ||
872d7e53 TS |
1740 | If you want to remove the line terminator from your here-docs, |
1741 | use C<chomp()>. | |
1742 | ||
1743 | chomp($string = <<'END'); | |
1744 | This is a string. | |
1745 | END | |
1746 | ||
1747 | If you want your here-docs to be indented with the rest of the code, | |
1748 | you'll need to remove leading whitespace from each line manually: | |
7e3b091d DA |
1749 | |
1750 | ($quote = <<'FINIS') =~ s/^\s+//gm; | |
89d205f2 | 1751 | The Road goes ever on and on, |
7e3b091d DA |
1752 | down from the door where it began. |
1753 | FINIS | |
1754 | ||
1755 | If you use a here-doc within a delimited construct, such as in C<s///eg>, | |
1756 | the quoted material must come on the lines following the final delimiter. | |
1757 | So instead of | |
1758 | ||
1759 | s/this/<<E . 'that' | |
1760 | the other | |
1761 | E | |
1762 | . 'more '/eg; | |
1763 | ||
1764 | you have to write | |
1765 | ||
89d205f2 YO |
1766 | s/this/<<E . 'that' |
1767 | . 'more '/eg; | |
1768 | the other | |
1769 | E | |
7e3b091d DA |
1770 | |
1771 | If the terminating identifier is on the last line of the program, you | |
1772 | must be sure there is a newline after it; otherwise, Perl will give the | |
1773 | warning B<Can't find string terminator "END" anywhere before EOF...>. | |
1774 | ||
89d205f2 YO |
1775 | Additionally, the quoting rules for the end of string identifier are not |
1776 | related to Perl's quoting rules -- C<q()>, C<qq()>, and the like are not | |
1777 | supported in place of C<''> and C<"">, and the only interpolation is for | |
1778 | backslashing the quoting character: | |
7e3b091d DA |
1779 | |
1780 | print << "abc\"def"; | |
1781 | testing... | |
1782 | abc"def | |
1783 | ||
1784 | Finally, quoted strings cannot span multiple lines. The general rule is | |
1785 | that the identifier must be a string literal. Stick with that, and you | |
1786 | should be safe. | |
1787 | ||
a0d0e21e LW |
1788 | =back |
1789 | ||
75e14d17 | 1790 | =head2 Gory details of parsing quoted constructs |
d74e8afc | 1791 | X<quote, gory details> |
75e14d17 | 1792 | |
19799a22 GS |
1793 | When presented with something that might have several different |
1794 | interpretations, Perl uses the B<DWIM> (that's "Do What I Mean") | |
1795 | principle to pick the most probable interpretation. This strategy | |
1796 | is so successful that Perl programmers often do not suspect the | |
1797 | ambivalence of what they write. But from time to time, Perl's | |
1798 | notions differ substantially from what the author honestly meant. | |
1799 | ||
1800 | This section hopes to clarify how Perl handles quoted constructs. | |
1801 | Although the most common reason to learn this is to unravel labyrinthine | |
1802 | regular expressions, because the initial steps of parsing are the | |
1803 | same for all quoting operators, they are all discussed together. | |
1804 | ||
1805 | The most important Perl parsing rule is the first one discussed | |
1806 | below: when processing a quoted construct, Perl first finds the end | |
1807 | of that construct, then interprets its contents. If you understand | |
1808 | this rule, you may skip the rest of this section on the first | |
1809 | reading. The other rules are likely to contradict the user's | |
1810 | expectations much less frequently than this first one. | |
1811 | ||
1812 | Some passes discussed below are performed concurrently, but because | |
1813 | their results are the same, we consider them individually. For different | |
1814 | quoting constructs, Perl performs different numbers of passes, from | |
6deea57f | 1815 | one to four, but these passes are always performed in the same order. |
75e14d17 | 1816 | |
13a2d996 | 1817 | =over 4 |
75e14d17 IZ |
1818 | |
1819 | =item Finding the end | |
1820 | ||
6deea57f TS |
1821 | The first pass is finding the end of the quoted construct, where |
1822 | the information about the delimiters is used in parsing. | |
1823 | During this search, text between the starting and ending delimiters | |
1824 | is copied to a safe location. The text copied gets delimiter-independent. | |
1825 | ||
1826 | If the construct is a here-doc, the ending delimiter is a line | |
1827 | that has a terminating string as the content. Therefore C<<<EOF> is | |
1828 | terminated by C<EOF> immediately followed by C<"\n"> and starting | |
1829 | from the first column of the terminating line. | |
1830 | When searching for the terminating line of a here-doc, nothing | |
1831 | is skipped. In other words, lines after the here-doc syntax | |
1832 | are compared with the terminating string line by line. | |
1833 | ||
1834 | For the constructs except here-docs, single characters are used as starting | |
1835 | and ending delimiters. If the starting delimiter is an opening punctuation | |
1836 | (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the | |
1837 | corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>). | |
1838 | If the starting delimiter is an unpaired character like C</> or a closing | |
1839 | punctuation, the ending delimiter is same as the starting delimiter. | |
1840 | Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates | |
1841 | C<qq[]> and C<qq]]> constructs. | |
1842 | ||
1843 | When searching for single-character delimiters, escaped delimiters | |
1844 | and C<\\> are skipped. For example, while searching for terminating C</>, | |
1845 | combinations of C<\\> and C<\/> are skipped. If the delimiters are | |
1846 | bracketing, nested pairs are also skipped. For example, while searching | |
1847 | for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>, | |
1848 | and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well. | |
1849 | However, when backslashes are used as the delimiters (like C<qq\\> and | |
1850 | C<tr\\\>), nothing is skipped. | |
1851 | During the search for the end, backslashes that escape delimiters | |
1852 | are removed (exactly speaking, they are not copied to the safe location). | |
75e14d17 | 1853 | |
19799a22 GS |
1854 | For constructs with three-part delimiters (C<s///>, C<y///>, and |
1855 | C<tr///>), the search is repeated once more. | |
6deea57f TS |
1856 | If the first delimiter is not an opening punctuation, three delimiters must |
1857 | be same such as C<s!!!> and C<tr)))>, in which case the second delimiter | |
1858 | terminates the left part and starts the right part at once. | |
1859 | If the left part is delimited by bracketing punctuations (that is C<()>, | |
1860 | C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of | |
1861 | delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces | |
1862 | and comments are allowed between both parts, though the comment must follow | |
1863 | at least one whitespace; otherwise a character expected as the start of | |
1864 | the comment may be regarded as the starting delimiter of the right part. | |
75e14d17 | 1865 | |
19799a22 GS |
1866 | During this search no attention is paid to the semantics of the construct. |
1867 | Thus: | |
75e14d17 IZ |
1868 | |
1869 | "$hash{"$foo/$bar"}" | |
1870 | ||
2a94b7ce | 1871 | or: |
75e14d17 | 1872 | |
89d205f2 | 1873 | m/ |
2a94b7ce | 1874 | bar # NOT a comment, this slash / terminated m//! |
75e14d17 IZ |
1875 | /x |
1876 | ||
19799a22 GS |
1877 | do not form legal quoted expressions. The quoted part ends on the |
1878 | first C<"> and C</>, and the rest happens to be a syntax error. | |
1879 | Because the slash that terminated C<m//> was followed by a C<SPACE>, | |
1880 | the example above is not C<m//x>, but rather C<m//> with no C</x> | |
1881 | modifier. So the embedded C<#> is interpreted as a literal C<#>. | |
75e14d17 | 1882 | |
89d205f2 YO |
1883 | Also no attention is paid to C<\c\> (multichar control char syntax) during |
1884 | this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part | |
1885 | of C<\/>, and the following C</> is not recognized as a delimiter. | |
0d594e51 TS |
1886 | Instead, use C<\034> or C<\x1c> at the end of quoted constructs. |
1887 | ||
75e14d17 | 1888 | =item Interpolation |
d74e8afc | 1889 | X<interpolation> |
75e14d17 | 1890 | |
19799a22 | 1891 | The next step is interpolation in the text obtained, which is now |
89d205f2 | 1892 | delimiter-independent. There are multiple cases. |
75e14d17 | 1893 | |
13a2d996 | 1894 | =over 4 |
75e14d17 | 1895 | |
89d205f2 | 1896 | =item C<<<'EOF'> |
75e14d17 IZ |
1897 | |
1898 | No interpolation is performed. | |
6deea57f TS |
1899 | Note that the combination C<\\> is left intact, since escaped delimiters |
1900 | are not available for here-docs. | |
75e14d17 | 1901 | |
6deea57f | 1902 | =item C<m''>, the pattern of C<s'''> |
89d205f2 | 1903 | |
6deea57f TS |
1904 | No interpolation is performed at this stage. |
1905 | Any backslashed sequences including C<\\> are treated at the stage | |
1906 | to L</"parsing regular expressions">. | |
89d205f2 | 1907 | |
6deea57f | 1908 | =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''> |
75e14d17 | 1909 | |
89d205f2 | 1910 | The only interpolation is removal of C<\> from pairs of C<\\>. |
6deea57f TS |
1911 | Therefore C<-> in C<tr'''> and C<y'''> is treated literally |
1912 | as a hyphen and no character range is available. | |
1913 | C<\1> in the replacement of C<s'''> does not work as C<$1>. | |
89d205f2 YO |
1914 | |
1915 | =item C<tr///>, C<y///> | |
1916 | ||
6deea57f TS |
1917 | No variable interpolation occurs. String modifying combinations for |
1918 | case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized. | |
1919 | The other escape sequences such as C<\200> and C<\t> and backslashed | |
1920 | characters such as C<\\> and C<\-> are converted to appropriate literals. | |
89d205f2 YO |
1921 | The character C<-> is treated specially and therefore C<\-> is treated |
1922 | as a literal C<->. | |
75e14d17 | 1923 | |
89d205f2 | 1924 | =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF"> |
75e14d17 | 1925 | |
19799a22 GS |
1926 | C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are |
1927 | converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar"> | |
1928 | is converted to C<$foo . (quotemeta("baz" . $bar))> internally. | |
6deea57f TS |
1929 | The other escape sequences such as C<\200> and C<\t> and backslashed |
1930 | characters such as C<\\> and C<\-> are replaced with appropriate | |
1931 | expansions. | |
2a94b7ce | 1932 | |
19799a22 GS |
1933 | Let it be stressed that I<whatever falls between C<\Q> and C<\E>> |
1934 | is interpolated in the usual way. Something like C<"\Q\\E"> has | |
1935 | no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the | |
1936 | result is the same as for C<"\\\\E">. As a general rule, backslashes | |
1937 | between C<\Q> and C<\E> may lead to counterintuitive results. So, | |
1938 | C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same | |
1939 | as C<"\\\t"> (since TAB is not alphanumeric). Note also that: | |
2a94b7ce IZ |
1940 | |
1941 | $str = '\t'; | |
1942 | return "\Q$str"; | |
1943 | ||
1944 | may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. | |
1945 | ||
19799a22 | 1946 | Interpolated scalars and arrays are converted internally to the C<join> and |
92d29cee | 1947 | C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes: |
75e14d17 | 1948 | |
19799a22 | 1949 | $foo . " XXX '" . (join $", @arr) . "'"; |
75e14d17 | 1950 | |
19799a22 | 1951 | All operations above are performed simultaneously, left to right. |
75e14d17 | 1952 | |
19799a22 GS |
1953 | Because the result of C<"\Q STRING \E"> has all metacharacters |
1954 | quoted, there is no way to insert a literal C<$> or C<@> inside a | |
1955 | C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became | |
1956 | C<"\\\$">; if not, it is interpreted as the start of an interpolated | |
1957 | scalar. | |
75e14d17 | 1958 | |
19799a22 | 1959 | Note also that the interpolation code needs to make a decision on |
89d205f2 | 1960 | where the interpolated scalar ends. For instance, whether |
35f2feb0 | 1961 | C<< "a $b -> {c}" >> really means: |
75e14d17 IZ |
1962 | |
1963 | "a " . $b . " -> {c}"; | |
1964 | ||
2a94b7ce | 1965 | or: |
75e14d17 IZ |
1966 | |
1967 | "a " . $b -> {c}; | |
1968 | ||
19799a22 GS |
1969 | Most of the time, the longest possible text that does not include |
1970 | spaces between components and which contains matching braces or | |
1971 | brackets. because the outcome may be determined by voting based | |
1972 | on heuristic estimators, the result is not strictly predictable. | |
1973 | Fortunately, it's usually correct for ambiguous cases. | |
75e14d17 | 1974 | |
6deea57f | 1975 | =item the replacement of C<s///> |
75e14d17 | 1976 | |
19799a22 | 1977 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation |
6deea57f TS |
1978 | happens as with C<qq//> constructs. |
1979 | ||
1980 | It is at this step that C<\1> is begrudgingly converted to C<$1> in | |
1981 | the replacement text of C<s///>, in order to correct the incorrigible | |
1982 | I<sed> hackers who haven't picked up the saner idiom yet. A warning | |
1983 | is emitted if the C<use warnings> pragma or the B<-w> command-line flag | |
1984 | (that is, the C<$^W> variable) was set. | |
1985 | ||
1986 | =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, | |
1987 | ||
cc74c5bd TS |
1988 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>, |
1989 | and interpolation happens (almost) as with C<qq//> constructs. | |
1990 | ||
1991 | However any other combinations of C<\> followed by a character | |
1992 | are not substituted but only skipped, in order to parse them | |
1993 | as regular expressions at the following step. | |
6deea57f | 1994 | As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly |
1749ea0d | 1995 | treated as an array symbol (for example C<@foo>), |
6deea57f | 1996 | even though the same text in C<qq//> gives interpolation of C<\c@>. |
6deea57f TS |
1997 | |
1998 | Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and | |
19799a22 GS |
1999 | a C<#>-comment in a C<//x>-regular expression, no processing is |
2000 | performed whatsoever. This is the first step at which the presence | |
2001 | of the C<//x> modifier is relevant. | |
2002 | ||
1749ea0d TS |
2003 | Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+> |
2004 | and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are | |
2005 | voted (by several different estimators) to be either an array element | |
2006 | or C<$var> followed by an RE alternative. This is where the notation | |
19799a22 GS |
2007 | C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as |
2008 | array element C<-9>, not as a regular expression from the variable | |
2009 | C<$arr> followed by a digit, which would be the interpretation of | |
2010 | C</$arr[0-9]/>. Since voting among different estimators may occur, | |
2011 | the result is not predictable. | |
2012 | ||
19799a22 GS |
2013 | The lack of processing of C<\\> creates specific restrictions on |
2014 | the post-processed text. If the delimiter is C</>, one cannot get | |
2015 | the combination C<\/> into the result of this step. C</> will | |
2016 | finish the regular expression, C<\/> will be stripped to C</> on | |
2017 | the previous step, and C<\\/> will be left as is. Because C</> is | |
2018 | equivalent to C<\/> inside a regular expression, this does not | |
2019 | matter unless the delimiter happens to be character special to the | |
2020 | RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an | |
2021 | alphanumeric char, as in: | |
2a94b7ce IZ |
2022 | |
2023 | m m ^ a \s* b mmx; | |
2024 | ||
19799a22 | 2025 | In the RE above, which is intentionally obfuscated for illustration, the |
6deea57f | 2026 | delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the |
89d205f2 | 2027 | RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one |
19799a22 GS |
2028 | reason you're encouraged to restrict your delimiters to non-alphanumeric, |
2029 | non-whitespace choices. | |
75e14d17 IZ |
2030 | |
2031 | =back | |
2032 | ||
19799a22 | 2033 | This step is the last one for all constructs except regular expressions, |
75e14d17 IZ |
2034 | which are processed further. |
2035 | ||
6deea57f TS |
2036 | =item parsing regular expressions |
2037 | X<regexp, parse> | |
75e14d17 | 2038 | |
19799a22 GS |
2039 | Previous steps were performed during the compilation of Perl code, |
2040 | but this one happens at run time--although it may be optimized to | |
2041 | be calculated at compile time if appropriate. After preprocessing | |
6deea57f | 2042 | described above, and possibly after evaluation if concatenation, |
19799a22 GS |
2043 | joining, casing translation, or metaquoting are involved, the |
2044 | resulting I<string> is passed to the RE engine for compilation. | |
2045 | ||
2046 | Whatever happens in the RE engine might be better discussed in L<perlre>, | |
2047 | but for the sake of continuity, we shall do so here. | |
2048 | ||
2049 | This is another step where the presence of the C<//x> modifier is | |
2050 | relevant. The RE engine scans the string from left to right and | |
2051 | converts it to a finite automaton. | |
2052 | ||
2053 | Backslashed characters are either replaced with corresponding | |
2054 | literal strings (as with C<\{>), or else they generate special nodes | |
2055 | in the finite automaton (as with C<\b>). Characters special to the | |
2056 | RE engine (such as C<|>) generate corresponding nodes or groups of | |
2057 | nodes. C<(?#...)> comments are ignored. All the rest is either | |
2058 | converted to literal strings to match, or else is ignored (as is | |
2059 | whitespace and C<#>-style comments if C<//x> is present). | |
2060 | ||
2061 | Parsing of the bracketed character class construct, C<[...]>, is | |
2062 | rather different than the rule used for the rest of the pattern. | |
2063 | The terminator of this construct is found using the same rules as | |
2064 | for finding the terminator of a C<{}>-delimited construct, the only | |
2065 | exception being that C<]> immediately following C<[> is treated as | |
2066 | though preceded by a backslash. Similarly, the terminator of | |
2067 | C<(?{...})> is found using the same rules as for finding the | |
2068 | terminator of a C<{}>-delimited construct. | |
2069 | ||
2070 | It is possible to inspect both the string given to RE engine and the | |
2071 | resulting finite automaton. See the arguments C<debug>/C<debugcolor> | |
2072 | in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line | |
4a4eefd0 | 2073 | switch documented in L<perlrun/"Command Switches">. |
75e14d17 IZ |
2074 | |
2075 | =item Optimization of regular expressions | |
d74e8afc | 2076 | X<regexp, optimization> |
75e14d17 | 2077 | |
7522fed5 | 2078 | This step is listed for completeness only. Since it does not change |
75e14d17 | 2079 | semantics, details of this step are not documented and are subject |
19799a22 GS |
2080 | to change without notice. This step is performed over the finite |
2081 | automaton that was generated during the previous pass. | |
2a94b7ce | 2082 | |
19799a22 GS |
2083 | It is at this stage that C<split()> silently optimizes C</^/> to |
2084 | mean C</^/m>. | |
75e14d17 IZ |
2085 | |
2086 | =back | |
2087 | ||
a0d0e21e | 2088 | =head2 I/O Operators |
d74e8afc ITB |
2089 | X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle> |
2090 | X<< <> >> X<@ARGV> | |
a0d0e21e | 2091 | |
54310121 | 2092 | There are several I/O operators you should know about. |
fbad3eb5 | 2093 | |
7b8d334a | 2094 | A string enclosed by backticks (grave accents) first undergoes |
19799a22 GS |
2095 | double-quote interpolation. It is then interpreted as an external |
2096 | command, and the output of that command is the value of the | |
e9c56f9b JH |
2097 | backtick string, like in a shell. In scalar context, a single string |
2098 | consisting of all output is returned. In list context, a list of | |
2099 | values is returned, one per line of output. (You can set C<$/> to use | |
2100 | a different line terminator.) The command is executed each time the | |
2101 | pseudo-literal is evaluated. The status value of the command is | |
2102 | returned in C<$?> (see L<perlvar> for the interpretation of C<$?>). | |
2103 | Unlike in B<csh>, no translation is done on the return data--newlines | |
2104 | remain newlines. Unlike in any of the shells, single quotes do not | |
2105 | hide variable names in the command from interpretation. To pass a | |
2106 | literal dollar-sign through to the shell you need to hide it with a | |
2107 | backslash. The generalized form of backticks is C<qx//>. (Because | |
2108 | backticks always undergo shell expansion as well, see L<perlsec> for | |
2109 | security concerns.) | |
d74e8afc | 2110 | X<qx> X<`> X<``> X<backtick> X<glob> |
19799a22 GS |
2111 | |
2112 | In scalar context, evaluating a filehandle in angle brackets yields | |
2113 | the next line from that file (the newline, if any, included), or | |
2114 | C<undef> at end-of-file or on error. When C<$/> is set to C<undef> | |
2115 | (sometimes known as file-slurp mode) and the file is empty, it | |
2116 | returns C<''> the first time, followed by C<undef> subsequently. | |
2117 | ||
2118 | Ordinarily you must assign the returned value to a variable, but | |
2119 | there is one situation where an automatic assignment happens. If | |
2120 | and only if the input symbol is the only thing inside the conditional | |
2121 | of a C<while> statement (even if disguised as a C<for(;;)> loop), | |
2122 | the value is automatically assigned to the global variable $_, | |
2123 | destroying whatever was there previously. (This may seem like an | |
2124 | odd thing to you, but you'll use the construct in almost every Perl | |
17b829fa | 2125 | script you write.) The $_ variable is not implicitly localized. |
19799a22 GS |
2126 | You'll have to put a C<local $_;> before the loop if you want that |
2127 | to happen. | |
2128 | ||
2129 | The following lines are equivalent: | |
a0d0e21e | 2130 | |
748a9306 | 2131 | while (defined($_ = <STDIN>)) { print; } |
7b8d334a | 2132 | while ($_ = <STDIN>) { print; } |
a0d0e21e LW |
2133 | while (<STDIN>) { print; } |
2134 | for (;<STDIN>;) { print; } | |
748a9306 | 2135 | print while defined($_ = <STDIN>); |
7b8d334a | 2136 | print while ($_ = <STDIN>); |
a0d0e21e LW |
2137 | print while <STDIN>; |
2138 | ||
19799a22 | 2139 | This also behaves similarly, but avoids $_ : |
7b8d334a | 2140 | |
89d205f2 | 2141 | while (my $line = <STDIN>) { print $line } |
7b8d334a | 2142 | |
19799a22 GS |
2143 | In these loop constructs, the assigned value (whether assignment |
2144 | is automatic or explicit) is then tested to see whether it is | |
2145 | defined. The defined test avoids problems where line has a string | |
2146 | value that would be treated as false by Perl, for example a "" or | |
2147 | a "0" with no trailing newline. If you really mean for such values | |
2148 | to terminate the loop, they should be tested for explicitly: | |
7b8d334a GS |
2149 | |
2150 | while (($_ = <STDIN>) ne '0') { ... } | |
2151 | while (<STDIN>) { last unless $_; ... } | |
2152 | ||
35f2feb0 | 2153 | In other boolean contexts, C<< <I<filehandle>> >> without an |
89d205f2 | 2154 | explicit C<defined> test or comparison elicit a warning if the |
9f1b1f2d | 2155 | C<use warnings> pragma or the B<-w> |
19799a22 | 2156 | command-line switch (the C<$^W> variable) is in effect. |
7b8d334a | 2157 | |
5f05dabc | 2158 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
19799a22 GS |
2159 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except |
2160 | in packages, where they would be interpreted as local identifiers | |
2161 | rather than global.) Additional filehandles may be created with | |
2162 | the open() function, amongst others. See L<perlopentut> and | |
2163 | L<perlfunc/open> for details on this. | |
d74e8afc | 2164 | X<stdin> X<stdout> X<sterr> |
a0d0e21e | 2165 | |
35f2feb0 | 2166 | If a <FILEHANDLE> is used in a context that is looking for |
19799a22 GS |
2167 | a list, a list comprising all input lines is returned, one line per |
2168 | list element. It's easy to grow to a rather large data space this | |
2169 | way, so use with care. | |
a0d0e21e | 2170 | |
35f2feb0 | 2171 | <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>. |
19799a22 | 2172 | See L<perlfunc/readline>. |
fbad3eb5 | 2173 | |
35f2feb0 GS |
2174 | The null filehandle <> is special: it can be used to emulate the |
2175 | behavior of B<sed> and B<awk>. Input from <> comes either from | |
a0d0e21e | 2176 | standard input, or from each file listed on the command line. Here's |
35f2feb0 | 2177 | how it works: the first time <> is evaluated, the @ARGV array is |
5a964f20 | 2178 | checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened |
a0d0e21e LW |
2179 | gives you standard input. The @ARGV array is then processed as a list |
2180 | of filenames. The loop | |
2181 | ||
2182 | while (<>) { | |
2183 | ... # code for each line | |
2184 | } | |
2185 | ||
2186 | is equivalent to the following Perl-like pseudo code: | |
2187 | ||
3e3baf6d | 2188 | unshift(@ARGV, '-') unless @ARGV; |
a0d0e21e LW |
2189 | while ($ARGV = shift) { |
2190 | open(ARGV, $ARGV); | |
2191 | while (<ARGV>) { | |
2192 | ... # code for each line | |
2193 | } | |
2194 | } | |
2195 | ||
19799a22 GS |
2196 | except that it isn't so cumbersome to say, and will actually work. |
2197 | It really does shift the @ARGV array and put the current filename | |
2198 | into the $ARGV variable. It also uses filehandle I<ARGV> | |
35f2feb0 | 2199 | internally--<> is just a synonym for <ARGV>, which |
19799a22 | 2200 | is magical. (The pseudo code above doesn't work because it treats |
35f2feb0 | 2201 | <ARGV> as non-magical.) |
a0d0e21e | 2202 | |
35f2feb0 | 2203 | You can modify @ARGV before the first <> as long as the array ends up |
a0d0e21e | 2204 | containing the list of filenames you really want. Line numbers (C<$.>) |
19799a22 GS |
2205 | continue as though the input were one big happy file. See the example |
2206 | in L<perlfunc/eof> for how to reset line numbers on each file. | |
5a964f20 | 2207 | |
89d205f2 | 2208 | If you want to set @ARGV to your own list of files, go right ahead. |
5a964f20 TC |
2209 | This sets @ARGV to all plain text files if no @ARGV was given: |
2210 | ||
2211 | @ARGV = grep { -f && -T } glob('*') unless @ARGV; | |
a0d0e21e | 2212 | |
5a964f20 TC |
2213 | You can even set them to pipe commands. For example, this automatically |
2214 | filters compressed arguments through B<gzip>: | |
2215 | ||
2216 | @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; | |
2217 | ||
2218 | If you want to pass switches into your script, you can use one of the | |
a0d0e21e LW |
2219 | Getopts modules or put a loop on the front like this: |
2220 | ||
2221 | while ($_ = $ARGV[0], /^-/) { | |
2222 | shift; | |
2223 | last if /^--$/; | |
2224 | if (/^-D(.*)/) { $debug = $1 } | |
2225 | if (/^-v/) { $verbose++ } | |
5a964f20 | 2226 | # ... # other switches |
a0d0e21e | 2227 | } |
5a964f20 | 2228 | |
a0d0e21e | 2229 | while (<>) { |
5a964f20 | 2230 | # ... # code for each line |
a0d0e21e LW |
2231 | } |
2232 | ||
89d205f2 YO |
2233 | The <> symbol will return C<undef> for end-of-file only once. |
2234 | If you call it again after this, it will assume you are processing another | |
19799a22 | 2235 | @ARGV list, and if you haven't set @ARGV, will read input from STDIN. |
a0d0e21e | 2236 | |
b159ebd3 | 2237 | If what the angle brackets contain is a simple scalar variable (e.g., |
35f2feb0 | 2238 | <$foo>), then that variable contains the name of the |
19799a22 GS |
2239 | filehandle to input from, or its typeglob, or a reference to the |
2240 | same. For example: | |
cb1a09d0 AD |
2241 | |
2242 | $fh = \*STDIN; | |
2243 | $line = <$fh>; | |
a0d0e21e | 2244 | |
5a964f20 TC |
2245 | If what's within the angle brackets is neither a filehandle nor a simple |
2246 | scalar variable containing a filehandle name, typeglob, or typeglob | |
2247 | reference, it is interpreted as a filename pattern to be globbed, and | |
2248 | either a list of filenames or the next filename in the list is returned, | |
19799a22 | 2249 | depending on context. This distinction is determined on syntactic |
35f2feb0 GS |
2250 | grounds alone. That means C<< <$x> >> is always a readline() from |
2251 | an indirect handle, but C<< <$hash{key}> >> is always a glob(). | |
5a964f20 | 2252 | That's because $x is a simple scalar variable, but C<$hash{key}> is |
ef191992 YST |
2253 | not--it's a hash element. Even C<< <$x > >> (note the extra space) |
2254 | is treated as C<glob("$x ")>, not C<readline($x)>. | |
5a964f20 TC |
2255 | |
2256 | One level of double-quote interpretation is done first, but you can't | |
35f2feb0 | 2257 | say C<< <$foo> >> because that's an indirect filehandle as explained |
5a964f20 TC |
2258 | in the previous paragraph. (In older versions of Perl, programmers |
2259 | would insert curly brackets to force interpretation as a filename glob: | |
35f2feb0 | 2260 | C<< <${foo}> >>. These days, it's considered cleaner to call the |
5a964f20 | 2261 | internal function directly as C<glob($foo)>, which is probably the right |
19799a22 | 2262 | way to have done it in the first place.) For example: |
a0d0e21e LW |
2263 | |
2264 | while (<*.c>) { | |
2265 | chmod 0644, $_; | |
2266 | } | |
2267 | ||
3a4b19e4 | 2268 | is roughly equivalent to: |
a0d0e21e LW |
2269 | |
2270 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); | |
2271 | while (<FOO>) { | |
5b3eff12 | 2272 | chomp; |
a0d0e21e LW |
2273 | chmod 0644, $_; |
2274 | } | |
2275 | ||
3a4b19e4 GS |
2276 | except that the globbing is actually done internally using the standard |
2277 | C<File::Glob> extension. Of course, the shortest way to do the above is: | |
a0d0e21e LW |
2278 | |
2279 | chmod 0644, <*.c>; | |
2280 | ||
19799a22 GS |
2281 | A (file)glob evaluates its (embedded) argument only when it is |
2282 | starting a new list. All values must be read before it will start | |
2283 | over. In list context, this isn't important because you automatically | |
2284 | get them all anyway. However, in scalar context the operator returns | |
069e01df | 2285 | the next value each time it's called, or C<undef> when the list has |
19799a22 GS |
2286 | run out. As with filehandle reads, an automatic C<defined> is |
2287 | generated when the glob occurs in the test part of a C<while>, | |
2288 | because legal glob returns (e.g. a file called F<0>) would otherwise | |
2289 | terminate the loop. Again, C<undef> is returned only once. So if | |
2290 | you're expecting a single value from a glob, it is much better to | |
2291 | say | |
4633a7c4 LW |
2292 | |
2293 | ($file) = <blurch*>; | |
2294 | ||
2295 | than | |
2296 | ||
2297 | $file = <blurch*>; | |
2298 | ||
2299 | because the latter will alternate between returning a filename and | |
19799a22 | 2300 | returning false. |
4633a7c4 | 2301 | |
b159ebd3 | 2302 | If you're trying to do variable interpolation, it's definitely better |
4633a7c4 | 2303 | to use the glob() function, because the older notation can cause people |
e37d713d | 2304 | to become confused with the indirect filehandle notation. |
4633a7c4 LW |
2305 | |
2306 | @files = glob("$dir/*.[ch]"); | |
2307 | @files = glob($files[$i]); | |
2308 | ||
a0d0e21e | 2309 | =head2 Constant Folding |
d74e8afc | 2310 | X<constant folding> X<folding> |
a0d0e21e LW |
2311 | |
2312 | Like C, Perl does a certain amount of expression evaluation at | |
19799a22 | 2313 | compile time whenever it determines that all arguments to an |
a0d0e21e LW |
2314 | operator are static and have no side effects. In particular, string |
2315 | concatenation happens at compile time between literals that don't do | |
19799a22 | 2316 | variable substitution. Backslash interpolation also happens at |
a0d0e21e LW |
2317 | compile time. You can say |
2318 | ||
2319 | 'Now is the time for all' . "\n" . | |
2320 | 'good men to come to.' | |
2321 | ||
54310121 | 2322 | and this all reduces to one string internally. Likewise, if |
a0d0e21e LW |
2323 | you say |
2324 | ||
2325 | foreach $file (@filenames) { | |
5a964f20 | 2326 | if (-s $file > 5 + 100 * 2**16) { } |
54310121 | 2327 | } |
a0d0e21e | 2328 | |
19799a22 GS |
2329 | the compiler will precompute the number which that expression |
2330 | represents so that the interpreter won't have to. | |
a0d0e21e | 2331 | |
fd1abbef | 2332 | =head2 No-ops |
d74e8afc | 2333 | X<no-op> X<nop> |
fd1abbef DN |
2334 | |
2335 | Perl doesn't officially have a no-op operator, but the bare constants | |
2336 | C<0> and C<1> are special-cased to not produce a warning in a void | |
2337 | context, so you can for example safely do | |
2338 | ||
2339 | 1 while foo(); | |
2340 | ||
2c268ad5 | 2341 | =head2 Bitwise String Operators |
d74e8afc | 2342 | X<operator, bitwise, string> |
2c268ad5 TP |
2343 | |
2344 | Bitstrings of any size may be manipulated by the bitwise operators | |
2345 | (C<~ | & ^>). | |
2346 | ||
19799a22 GS |
2347 | If the operands to a binary bitwise op are strings of different |
2348 | sizes, B<|> and B<^> ops act as though the shorter operand had | |
2349 | additional zero bits on the right, while the B<&> op acts as though | |
2350 | the longer operand were truncated to the length of the shorter. | |
2351 | The granularity for such extension or truncation is one or more | |
2352 | bytes. | |
2c268ad5 | 2353 | |
89d205f2 | 2354 | # ASCII-based examples |
2c268ad5 TP |
2355 | print "j p \n" ^ " a h"; # prints "JAPH\n" |
2356 | print "JA" | " ph\n"; # prints "japh\n" | |
2357 | print "japh\nJunk" & '_____'; # prints "JAPH\n"; | |
2358 | print 'p N$' ^ " E<H\n"; # prints "Perl\n"; | |
2359 | ||
19799a22 | 2360 | If you are intending to manipulate bitstrings, be certain that |
2c268ad5 | 2361 | you're supplying bitstrings: If an operand is a number, that will imply |
19799a22 | 2362 | a B<numeric> bitwise operation. You may explicitly show which type of |
2c268ad5 TP |
2363 | operation you intend by using C<""> or C<0+>, as in the examples below. |
2364 | ||
4358a253 SS |
2365 | $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF) |
2366 | $foo = '150' | 105; # yields 255 | |
2c268ad5 TP |
2367 | $foo = 150 | '105'; # yields 255 |
2368 | $foo = '150' | '105'; # yields string '155' (under ASCII) | |
2369 | ||
2370 | $baz = 0+$foo & 0+$bar; # both ops explicitly numeric | |
2371 | $biz = "$foo" ^ "$bar"; # both ops explicitly stringy | |
a0d0e21e | 2372 | |
1ae175c8 GS |
2373 | See L<perlfunc/vec> for information on how to manipulate individual bits |
2374 | in a bit vector. | |
2375 | ||
55497cff | 2376 | =head2 Integer Arithmetic |
d74e8afc | 2377 | X<integer> |
a0d0e21e | 2378 | |
19799a22 | 2379 | By default, Perl assumes that it must do most of its arithmetic in |
a0d0e21e LW |
2380 | floating point. But by saying |
2381 | ||
2382 | use integer; | |
2383 | ||
2384 | you may tell the compiler that it's okay to use integer operations | |
19799a22 GS |
2385 | (if it feels like it) from here to the end of the enclosing BLOCK. |
2386 | An inner BLOCK may countermand this by saying | |
a0d0e21e LW |
2387 | |
2388 | no integer; | |
2389 | ||
19799a22 GS |
2390 | which lasts until the end of that BLOCK. Note that this doesn't |
2391 | mean everything is only an integer, merely that Perl may use integer | |
2392 | operations if it is so inclined. For example, even under C<use | |
2393 | integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731> | |
2394 | or so. | |
2395 | ||
2396 | Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", | |
89d205f2 | 2397 | and ">>") always produce integral results. (But see also |
13a2d996 | 2398 | L<Bitwise String Operators>.) However, C<use integer> still has meaning for |
19799a22 GS |
2399 | them. By default, their results are interpreted as unsigned integers, but |
2400 | if C<use integer> is in effect, their results are interpreted | |
2401 | as signed integers. For example, C<~0> usually evaluates to a large | |
0be96356 | 2402 | integral value. However, C<use integer; ~0> is C<-1> on two's-complement |
19799a22 | 2403 | machines. |
68dc0745 | 2404 | |
2405 | =head2 Floating-point Arithmetic | |
d74e8afc | 2406 | X<floating-point> X<floating point> X<float> X<real> |
68dc0745 | 2407 | |
2408 | While C<use integer> provides integer-only arithmetic, there is no | |
19799a22 GS |
2409 | analogous mechanism to provide automatic rounding or truncation to a |
2410 | certain number of decimal places. For rounding to a certain number | |
2411 | of digits, sprintf() or printf() is usually the easiest route. | |
2412 | See L<perlfaq4>. | |
68dc0745 | 2413 | |
5a964f20 TC |
2414 | Floating-point numbers are only approximations to what a mathematician |
2415 | would call real numbers. There are infinitely more reals than floats, | |
2416 | so some corners must be cut. For example: | |
2417 | ||
2418 | printf "%.20g\n", 123456789123456789; | |
2419 | # produces 123456789123456784 | |
2420 | ||
2421 | Testing for exact equality of floating-point equality or inequality is | |
2422 | not a good idea. Here's a (relatively expensive) work-around to compare | |
2423 | whether two floating-point numbers are equal to a particular number of | |
2424 | decimal places. See Knuth, volume II, for a more robust treatment of | |
2425 | this topic. | |
2426 | ||
2427 | sub fp_equal { | |
2428 | my ($X, $Y, $POINTS) = @_; | |
2429 | my ($tX, $tY); | |
2430 | $tX = sprintf("%.${POINTS}g", $X); | |
2431 | $tY = sprintf("%.${POINTS}g", $Y); | |
2432 | return $tX eq $tY; | |
2433 | } | |
2434 | ||
68dc0745 | 2435 | The POSIX module (part of the standard perl distribution) implements |
19799a22 GS |
2436 | ceil(), floor(), and other mathematical and trigonometric functions. |
2437 | The Math::Complex module (part of the standard perl distribution) | |
2438 | defines mathematical functions that work on both the reals and the | |
2439 | imaginary numbers. Math::Complex not as efficient as POSIX, but | |
68dc0745 | 2440 | POSIX can't work with complex numbers. |
2441 | ||
2442 | Rounding in financial applications can have serious implications, and | |
2443 | the rounding method used should be specified precisely. In these | |
2444 | cases, it probably pays not to trust whichever system rounding is | |
2445 | being used by Perl, but to instead implement the rounding function you | |
2446 | need yourself. | |
5a964f20 TC |
2447 | |
2448 | =head2 Bigger Numbers | |
d74e8afc | 2449 | X<number, arbitrary precision> |
5a964f20 TC |
2450 | |
2451 | The standard Math::BigInt and Math::BigFloat modules provide | |
19799a22 | 2452 | variable-precision arithmetic and overloaded operators, although |
cd5c4fce | 2453 | they're currently pretty slow. At the cost of some space and |
19799a22 GS |
2454 | considerable speed, they avoid the normal pitfalls associated with |
2455 | limited-precision representations. | |
5a964f20 TC |
2456 | |
2457 | use Math::BigInt; | |
2458 | $x = Math::BigInt->new('123456789123456789'); | |
2459 | print $x * $x; | |
2460 | ||
2461 | # prints +15241578780673678515622620750190521 | |
19799a22 | 2462 | |
cd5c4fce T |
2463 | There are several modules that let you calculate with (bound only by |
2464 | memory and cpu-time) unlimited or fixed precision. There are also | |
2465 | some non-standard modules that provide faster implementations via | |
2466 | external C libraries. | |
2467 | ||
2468 | Here is a short, but incomplete summary: | |
2469 | ||
2470 | Math::Fraction big, unlimited fractions like 9973 / 12967 | |
2471 | Math::String treat string sequences like numbers | |
2472 | Math::FixedPrecision calculate with a fixed precision | |
2473 | Math::Currency for currency calculations | |
2474 | Bit::Vector manipulate bit vectors fast (uses C) | |
2475 | Math::BigIntFast Bit::Vector wrapper for big numbers | |
2476 | Math::Pari provides access to the Pari C library | |
2477 | Math::BigInteger uses an external C library | |
2478 | Math::Cephes uses external Cephes C library (no big numbers) | |
2479 | Math::Cephes::Fraction fractions via the Cephes library | |
2480 | Math::GMP another one using an external C library | |
2481 | ||
2482 | Choose wisely. | |
16070b82 GS |
2483 | |
2484 | =cut |