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 |
f23102e2 | 56 | left or xor |
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 |
353c6505 | 203 | non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert |
06705523 SP |
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 | $_. When used in scalar context, the return value generally indicates the |
238 | success of the operation. Behavior in list context depends on the particular | |
89d205f2 | 239 | operator. See L</"Regexp Quote-Like Operators"> for details and |
d7782e69 | 240 | L<perlretut> for examples using these operators. |
f8bab1e9 GS |
241 | |
242 | If the right argument is an expression rather than a search pattern, | |
2c268ad5 | 243 | substitution, or transliteration, it is interpreted as a search pattern at run |
89d205f2 YO |
244 | time. Note that this means that its contents will be interpolated twice, so |
245 | ||
246 | '\\' =~ q'\\'; | |
247 | ||
248 | is not ok, as the regex engine will end up trying to compile the | |
249 | pattern C<\>, which it will consider a syntax error. | |
a0d0e21e LW |
250 | |
251 | Binary "!~" is just like "=~" except the return value is negated in | |
252 | the logical sense. | |
253 | ||
254 | =head2 Multiplicative Operators | |
d74e8afc | 255 | X<operator, multiplicative> |
a0d0e21e LW |
256 | |
257 | Binary "*" multiplies two numbers. | |
d74e8afc | 258 | X<*> |
a0d0e21e LW |
259 | |
260 | Binary "/" divides two numbers. | |
d74e8afc | 261 | X</> X<slash> |
a0d0e21e | 262 | |
f7918450 KW |
263 | Binary "%" is the modulo operator, which computes the division |
264 | remainder of its first argument with respect to its second argument. | |
265 | Given integer | |
54310121 | 266 | operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is |
f7918450 | 267 | C<$a> minus the largest multiple of C<$b> less than or equal to |
54310121 | 268 | C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the |
269 | smallest multiple of C<$b> that is not less than C<$a> (i.e. the | |
89b4f0ad | 270 | result will be less than or equal to zero). If the operands |
4848a83b TS |
271 | C<$a> and C<$b> are floating point values and the absolute value of |
272 | C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only | |
273 | the integer portion of C<$a> and C<$b> will be used in the operation | |
274 | (Note: here C<UV_MAX> means the maximum of the unsigned integer type). | |
275 | If the absolute value of the right operand (C<abs($b)>) is greater than | |
276 | or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder | |
277 | C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain | |
f7918450 | 278 | integer that makes C<$r> have the same sign as the right operand |
4848a83b TS |
279 | C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>) |
280 | and the absolute value less than that of C<$b>. | |
0412d526 | 281 | Note that when C<use integer> is in scope, "%" gives you direct access |
f7918450 | 282 | to the modulo operator as implemented by your C compiler. This |
55d729e4 GS |
283 | operator is not as well defined for negative operands, but it will |
284 | execute faster. | |
f7918450 | 285 | X<%> X<remainder> X<modulo> X<mod> |
55d729e4 | 286 | |
62d10b70 GS |
287 | Binary "x" is the repetition operator. In scalar context or if the left |
288 | operand is not enclosed in parentheses, it returns a string consisting | |
289 | of the left operand repeated the number of times specified by the right | |
290 | operand. In list context, if the left operand is enclosed in | |
3585017f YST |
291 | parentheses or is a list formed by C<qw/STRING/>, it repeats the list. |
292 | If the right operand is zero or negative, it returns an empty string | |
293 | or an empty list, depending on the context. | |
d74e8afc | 294 | X<x> |
a0d0e21e LW |
295 | |
296 | print '-' x 80; # print row of dashes | |
297 | ||
298 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over | |
299 | ||
300 | @ones = (1) x 80; # a list of 80 1's | |
301 | @ones = (5) x @ones; # set all elements to 5 | |
302 | ||
303 | ||
304 | =head2 Additive Operators | |
d74e8afc | 305 | X<operator, additive> |
a0d0e21e LW |
306 | |
307 | Binary "+" returns the sum of two numbers. | |
d74e8afc | 308 | X<+> |
a0d0e21e LW |
309 | |
310 | Binary "-" returns the difference of two numbers. | |
d74e8afc | 311 | X<-> |
a0d0e21e LW |
312 | |
313 | Binary "." concatenates two strings. | |
d74e8afc ITB |
314 | X<string, concatenation> X<concatenation> |
315 | X<cat> X<concat> X<concatenate> X<.> | |
a0d0e21e LW |
316 | |
317 | =head2 Shift Operators | |
d74e8afc ITB |
318 | X<shift operator> X<operator, shift> X<<< << >>> |
319 | X<<< >> >>> X<right shift> X<left shift> X<bitwise shift> | |
320 | X<shl> X<shr> X<shift, right> X<shift, left> | |
a0d0e21e | 321 | |
55497cff | 322 | Binary "<<" returns the value of its left argument shifted left by the |
323 | number of bits specified by the right argument. Arguments should be | |
982ce180 | 324 | integers. (See also L<Integer Arithmetic>.) |
a0d0e21e | 325 | |
55497cff | 326 | Binary ">>" returns the value of its left argument shifted right by |
327 | the number of bits specified by the right argument. Arguments should | |
982ce180 | 328 | be integers. (See also L<Integer Arithmetic>.) |
a0d0e21e | 329 | |
b16cf6df JH |
330 | Note that both "<<" and ">>" in Perl are implemented directly using |
331 | "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is | |
332 | in force then signed C integers are used, else unsigned C integers are | |
333 | used. Either way, the implementation isn't going to generate results | |
334 | larger than the size of the integer type Perl was built with (32 bits | |
335 | or 64 bits). | |
336 | ||
337 | The result of overflowing the range of the integers is undefined | |
338 | because it is undefined also in C. In other words, using 32-bit | |
339 | integers, C<< 1 << 32 >> is undefined. Shifting by a negative number | |
340 | of bits is also undefined. | |
341 | ||
a0d0e21e | 342 | =head2 Named Unary Operators |
d74e8afc | 343 | X<operator, named unary> |
a0d0e21e LW |
344 | |
345 | The various named unary operators are treated as functions with one | |
568e6d8b | 346 | argument, with optional parentheses. |
a0d0e21e LW |
347 | |
348 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
349 | is followed by a left parenthesis as the next token, the operator and | |
350 | arguments within parentheses are taken to be of highest precedence, | |
3981b0eb JA |
351 | just like a normal function call. For example, |
352 | because named unary operators are higher precedence than ||: | |
a0d0e21e LW |
353 | |
354 | chdir $foo || die; # (chdir $foo) || die | |
355 | chdir($foo) || die; # (chdir $foo) || die | |
356 | chdir ($foo) || die; # (chdir $foo) || die | |
357 | chdir +($foo) || die; # (chdir $foo) || die | |
358 | ||
3981b0eb | 359 | but, because * is higher precedence than named operators: |
a0d0e21e LW |
360 | |
361 | chdir $foo * 20; # chdir ($foo * 20) | |
362 | chdir($foo) * 20; # (chdir $foo) * 20 | |
363 | chdir ($foo) * 20; # (chdir $foo) * 20 | |
364 | chdir +($foo) * 20; # chdir ($foo * 20) | |
365 | ||
366 | rand 10 * 20; # rand (10 * 20) | |
367 | rand(10) * 20; # (rand 10) * 20 | |
368 | rand (10) * 20; # (rand 10) * 20 | |
369 | rand +(10) * 20; # rand (10 * 20) | |
370 | ||
568e6d8b RGS |
371 | Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are |
372 | treated like named unary operators, but they don't follow this functional | |
373 | parenthesis rule. That means, for example, that C<-f($file).".bak"> is | |
374 | equivalent to C<-f "$file.bak">. | |
d74e8afc | 375 | X<-X> X<filetest> X<operator, filetest> |
568e6d8b | 376 | |
5ba421f6 | 377 | See also L<"Terms and List Operators (Leftward)">. |
a0d0e21e LW |
378 | |
379 | =head2 Relational Operators | |
d74e8afc | 380 | X<relational operator> X<operator, relational> |
a0d0e21e | 381 | |
35f2feb0 | 382 | Binary "<" returns true if the left argument is numerically less than |
a0d0e21e | 383 | the right argument. |
d74e8afc | 384 | X<< < >> |
a0d0e21e | 385 | |
35f2feb0 | 386 | Binary ">" returns true if the left argument is numerically greater |
a0d0e21e | 387 | than the right argument. |
d74e8afc | 388 | X<< > >> |
a0d0e21e | 389 | |
35f2feb0 | 390 | Binary "<=" returns true if the left argument is numerically less than |
a0d0e21e | 391 | or equal to the right argument. |
d74e8afc | 392 | X<< <= >> |
a0d0e21e | 393 | |
35f2feb0 | 394 | Binary ">=" returns true if the left argument is numerically greater |
a0d0e21e | 395 | than or equal to the right argument. |
d74e8afc | 396 | X<< >= >> |
a0d0e21e LW |
397 | |
398 | Binary "lt" returns true if the left argument is stringwise less than | |
399 | the right argument. | |
d74e8afc | 400 | X<< lt >> |
a0d0e21e LW |
401 | |
402 | Binary "gt" returns true if the left argument is stringwise greater | |
403 | than the right argument. | |
d74e8afc | 404 | X<< gt >> |
a0d0e21e LW |
405 | |
406 | Binary "le" returns true if the left argument is stringwise less than | |
407 | or equal to the right argument. | |
d74e8afc | 408 | X<< le >> |
a0d0e21e LW |
409 | |
410 | Binary "ge" returns true if the left argument is stringwise greater | |
411 | than or equal to the right argument. | |
d74e8afc | 412 | X<< ge >> |
a0d0e21e LW |
413 | |
414 | =head2 Equality Operators | |
d74e8afc | 415 | X<equality> X<equal> X<equals> X<operator, equality> |
a0d0e21e LW |
416 | |
417 | Binary "==" returns true if the left argument is numerically equal to | |
418 | the right argument. | |
d74e8afc | 419 | X<==> |
a0d0e21e LW |
420 | |
421 | Binary "!=" returns true if the left argument is numerically not equal | |
422 | to the right argument. | |
d74e8afc | 423 | X<!=> |
a0d0e21e | 424 | |
35f2feb0 | 425 | Binary "<=>" returns -1, 0, or 1 depending on whether the left |
6ee5d4e7 | 426 | argument is numerically less than, equal to, or greater than the right |
d4ad863d | 427 | argument. If your platform supports NaNs (not-a-numbers) as numeric |
7d3a9d88 NC |
428 | values, using them with "<=>" returns undef. NaN is not "<", "==", ">", |
429 | "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN | |
430 | returns true, as does NaN != anything else. If your platform doesn't | |
431 | support NaNs then NaN is just a string with numeric value 0. | |
d74e8afc | 432 | X<< <=> >> X<spaceship> |
7d3a9d88 | 433 | |
2b54f59f YST |
434 | perl -le '$a = "NaN"; print "No NaN support here" if $a == $a' |
435 | perl -le '$a = "NaN"; print "NaN support here" if $a != $a' | |
a0d0e21e LW |
436 | |
437 | Binary "eq" returns true if the left argument is stringwise equal to | |
438 | the right argument. | |
d74e8afc | 439 | X<eq> |
a0d0e21e LW |
440 | |
441 | Binary "ne" returns true if the left argument is stringwise not equal | |
442 | to the right argument. | |
d74e8afc | 443 | X<ne> |
a0d0e21e | 444 | |
d4ad863d JH |
445 | Binary "cmp" returns -1, 0, or 1 depending on whether the left |
446 | argument is stringwise less than, equal to, or greater than the right | |
447 | argument. | |
d74e8afc | 448 | X<cmp> |
a0d0e21e | 449 | |
0d863452 | 450 | Binary "~~" does a smart match between its arguments. Smart matching |
0f7107a0 | 451 | is described in L<perlsyn/"Smart matching in detail">. |
0d863452 RH |
452 | X<~~> |
453 | ||
a034a98d DD |
454 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
455 | by the current locale if C<use locale> is in effect. See L<perllocale>. | |
456 | ||
a0d0e21e | 457 | =head2 Bitwise And |
d74e8afc | 458 | X<operator, bitwise, and> X<bitwise and> X<&> |
a0d0e21e | 459 | |
2cdc098b | 460 | Binary "&" returns its operands ANDed together bit by bit. |
2c268ad5 | 461 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e | 462 | |
2cdc098b MG |
463 | Note that "&" has lower priority than relational operators, so for example |
464 | the brackets are essential in a test like | |
465 | ||
466 | print "Even\n" if ($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 | ||
f23102e2 RGS |
525 | As more readable alternatives to C<&&> and C<||> when used for |
526 | control flow, Perl provides the C<and> and C<or> operators (see below). | |
527 | The short-circuit behavior is identical. The precedence of "and" | |
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 | |
8f0f46f8 | 559 | The range operator also works on strings, using the magical |
560 | auto-increment, see below. | |
54ae734e | 561 | |
5a964f20 | 562 | In scalar context, ".." returns a boolean value. The operator is |
8f0f46f8 | 563 | bistable, like a flip-flop, and emulates the line-range (comma) |
564 | operator of B<sed>, B<awk>, and various editors. Each ".." operator | |
565 | maintains its own boolean state, even across calls to a subroutine | |
566 | that contains it. It is false as long as its left operand is false. | |
a0d0e21e LW |
567 | Once the left operand is true, the range operator stays true until the |
568 | right operand is true, I<AFTER> which the range operator becomes false | |
8f0f46f8 | 569 | again. It doesn't become false till the next time the range operator |
570 | is evaluated. It can test the right operand and become false on the | |
571 | same evaluation it became true (as in B<awk>), but it still returns | |
572 | true once. If you don't want it to test the right operand until the | |
573 | next evaluation, as in B<sed>, just use three dots ("...") instead of | |
19799a22 GS |
574 | two. In all other regards, "..." behaves just like ".." does. |
575 | ||
576 | The right operand is not evaluated while the operator is in the | |
577 | "false" state, and the left operand is not evaluated while the | |
578 | operator is in the "true" state. The precedence is a little lower | |
579 | than || and &&. The value returned is either the empty string for | |
8f0f46f8 | 580 | false, or a sequence number (beginning with 1) for true. The sequence |
581 | number is reset for each range encountered. The final sequence number | |
582 | in a range has the string "E0" appended to it, which doesn't affect | |
583 | its numeric value, but gives you something to search for if you want | |
584 | to exclude the endpoint. You can exclude the beginning point by | |
585 | waiting for the sequence number to be greater than 1. | |
df5f8116 CW |
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 | 604 | if (101 .. 200) { print; } # print 2nd hundred lines, short for |
f343f960 | 605 | # if ($. == 101 .. $. == 200) { print; } |
9f10b797 RGS |
606 | |
607 | next LINE if (1 .. /^$/); # skip header lines, short for | |
f343f960 | 608 | # next LINE 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) { | |
f343f960 | 618 | # do something |
df5f8116 | 619 | } else { # in body |
f343f960 | 620 | # do something else |
df5f8116 | 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 | |
344f2c40 IG |
786 | The C<< => >> operator is a synonym for the comma except that it causes |
787 | its left operand to be interpreted as a string if it begins with a letter | |
788 | or underscore and is composed only of letters, digits and underscores. | |
789 | This includes operands that might otherwise be interpreted as operators, | |
790 | constants, single number v-strings or function calls. If in doubt about | |
791 | this behaviour, the left operand can be quoted explicitly. | |
792 | ||
793 | Otherwise, the C<< => >> operator behaves exactly as the comma operator | |
794 | or list argument separator, according to context. | |
795 | ||
796 | For example: | |
a44e5664 MS |
797 | |
798 | use constant FOO => "something"; | |
799 | ||
800 | my %h = ( FOO => 23 ); | |
801 | ||
802 | is equivalent to: | |
803 | ||
804 | my %h = ("FOO", 23); | |
805 | ||
806 | It is I<NOT>: | |
807 | ||
808 | my %h = ("something", 23); | |
809 | ||
719b43e8 RGS |
810 | The C<< => >> operator is helpful in documenting the correspondence |
811 | between keys and values in hashes, and other paired elements in lists. | |
748a9306 | 812 | |
a44e5664 MS |
813 | %hash = ( $key => $value ); |
814 | login( $username => $password ); | |
815 | ||
678ae90b RGS |
816 | =head2 Yada Yada Operator |
817 | X<...> X<... operator> X<yada yada operator> | |
be25f609 | 818 | |
e8163f9b | 819 | The yada yada operator (noted C<...>) is a placeholder for code. Perl |
820 | parses it without error, but when you try to execute a yada yada, it | |
821 | throws an exception with the text C<Unimplemented>: | |
822 | ||
823 | sub unimplemented { ... } | |
824 | ||
825 | eval { unimplemented() }; | |
826 | if( $@ eq 'Unimplemented' ) { | |
827 | print "I found the yada yada!\n"; | |
828 | } | |
829 | ||
830 | You can only use the yada yada to stand in for a complete statement. | |
831 | These examples of the yada yada work: | |
832 | ||
833 | { ... } | |
834 | ||
835 | sub foo { ... } | |
836 | ||
837 | ...; | |
838 | ||
839 | eval { ... }; | |
840 | ||
841 | sub foo { | |
842 | my( $self ) = shift; | |
843 | ||
844 | ...; | |
845 | } | |
846 | ||
847 | do { my $n; ...; print 'Hurrah!' }; | |
848 | ||
849 | The yada yada cannot stand in for an expression that is part of a | |
850 | larger statement since the C<...> is also the three-dot version of the | |
851 | range operator (see L<Range Operators>). These examples of the yada | |
852 | yada are still syntax errors: | |
853 | ||
854 | print ...; | |
855 | ||
856 | open my($fh), '>', '/dev/passwd' or ...; | |
857 | ||
858 | if( $condition && ... ) { print "Hello\n" }; | |
859 | ||
860 | There are some cases where Perl can't immediately tell the difference | |
861 | between an expression and a statement. For instance, the syntax for a | |
862 | block and an anonymous hash reference constructor look the same unless | |
863 | there's something in the braces that give Perl a hint. The yada yada | |
864 | is a syntax error if Perl doesn't guess that the C<{ ... }> is a | |
865 | block. In that case, it doesn't think the C<...> is the yada yada | |
866 | because it's expecting an expression instead of a statement: | |
867 | ||
868 | my @transformed = map { ... } @input; # syntax error | |
869 | ||
870 | You can use a C<;> inside your block to denote that the C<{ ... }> is | |
871 | a block and not a hash reference constructor. Now the yada yada works: | |
872 | ||
873 | my @transformed = map {; ... } @input; # ; disambiguates | |
874 | ||
875 | my @transformed = map { ...; } @input; # ; disambiguates | |
be25f609 | 876 | |
a0d0e21e | 877 | =head2 List Operators (Rightward) |
d74e8afc | 878 | X<operator, list, rightward> X<list operator> |
a0d0e21e LW |
879 | |
880 | On the right side of a list operator, it has very low precedence, | |
881 | such that it controls all comma-separated expressions found there. | |
882 | The only operators with lower precedence are the logical operators | |
883 | "and", "or", and "not", which may be used to evaluate calls to list | |
884 | operators without the need for extra parentheses: | |
885 | ||
886 | open HANDLE, "filename" | |
887 | or die "Can't open: $!\n"; | |
888 | ||
5ba421f6 | 889 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
a0d0e21e LW |
890 | |
891 | =head2 Logical Not | |
d74e8afc | 892 | X<operator, logical, not> X<not> |
a0d0e21e LW |
893 | |
894 | Unary "not" returns the logical negation of the expression to its right. | |
895 | It's the equivalent of "!" except for the very low precedence. | |
896 | ||
897 | =head2 Logical And | |
d74e8afc | 898 | X<operator, logical, and> X<and> |
a0d0e21e LW |
899 | |
900 | Binary "and" returns the logical conjunction of the two surrounding | |
901 | expressions. It's equivalent to && except for the very low | |
5f05dabc | 902 | precedence. This means that it short-circuits: i.e., the right |
a0d0e21e LW |
903 | expression is evaluated only if the left expression is true. |
904 | ||
c963b151 | 905 | =head2 Logical or, Defined or, and Exclusive Or |
f23102e2 | 906 | X<operator, logical, or> X<operator, logical, xor> |
d74e8afc | 907 | X<operator, logical, defined or> X<operator, logical, exclusive or> |
f23102e2 | 908 | X<or> X<xor> |
a0d0e21e LW |
909 | |
910 | Binary "or" returns the logical disjunction of the two surrounding | |
5a964f20 TC |
911 | expressions. It's equivalent to || except for the very low precedence. |
912 | This makes it useful for control flow | |
913 | ||
914 | print FH $data or die "Can't write to FH: $!"; | |
915 | ||
916 | This means that it short-circuits: i.e., the right expression is evaluated | |
917 | only if the left expression is false. Due to its precedence, you should | |
918 | probably avoid using this for assignment, only for control flow. | |
919 | ||
920 | $a = $b or $c; # bug: this is wrong | |
921 | ($a = $b) or $c; # really means this | |
922 | $a = $b || $c; # better written this way | |
923 | ||
19799a22 | 924 | However, when it's a list-context assignment and you're trying to use |
5a964f20 TC |
925 | "||" for control flow, you probably need "or" so that the assignment |
926 | takes higher precedence. | |
927 | ||
928 | @info = stat($file) || die; # oops, scalar sense of stat! | |
929 | @info = stat($file) or die; # better, now @info gets its due | |
930 | ||
c963b151 BD |
931 | Then again, you could always use parentheses. |
932 | ||
a0d0e21e LW |
933 | Binary "xor" returns the exclusive-OR of the two surrounding expressions. |
934 | It cannot short circuit, of course. | |
935 | ||
936 | =head2 C Operators Missing From Perl | |
d74e8afc ITB |
937 | X<operator, missing from perl> X<&> X<*> |
938 | X<typecasting> X<(TYPE)> | |
a0d0e21e LW |
939 | |
940 | Here is what C has that Perl doesn't: | |
941 | ||
942 | =over 8 | |
943 | ||
944 | =item unary & | |
945 | ||
946 | Address-of operator. (But see the "\" operator for taking a reference.) | |
947 | ||
948 | =item unary * | |
949 | ||
54310121 | 950 | Dereference-address operator. (Perl's prefix dereferencing |
a0d0e21e LW |
951 | operators are typed: $, @, %, and &.) |
952 | ||
953 | =item (TYPE) | |
954 | ||
19799a22 | 955 | Type-casting operator. |
a0d0e21e LW |
956 | |
957 | =back | |
958 | ||
5f05dabc | 959 | =head2 Quote and Quote-like Operators |
89d205f2 | 960 | X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m> |
d74e8afc ITB |
961 | X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>> |
962 | X<escape sequence> X<escape> | |
963 | ||
a0d0e21e LW |
964 | |
965 | While we usually think of quotes as literal values, in Perl they | |
966 | function as operators, providing various kinds of interpolating and | |
967 | pattern matching capabilities. Perl provides customary quote characters | |
968 | for these behaviors, but also provides a way for you to choose your | |
969 | quote character for any of them. In the following table, a C<{}> represents | |
9f10b797 | 970 | any pair of delimiters you choose. |
a0d0e21e | 971 | |
2c268ad5 TP |
972 | Customary Generic Meaning Interpolates |
973 | '' q{} Literal no | |
974 | "" qq{} Literal yes | |
af9219ee | 975 | `` qx{} Command yes* |
2c268ad5 | 976 | qw{} Word list no |
af9219ee MG |
977 | // m{} Pattern match yes* |
978 | qr{} Pattern yes* | |
979 | s{}{} Substitution yes* | |
2c268ad5 | 980 | tr{}{} Transliteration no (but see below) |
7e3b091d | 981 | <<EOF here-doc yes* |
a0d0e21e | 982 | |
af9219ee MG |
983 | * unless the delimiter is ''. |
984 | ||
87275199 GS |
985 | Non-bracketing delimiters use the same character fore and aft, but the four |
986 | sorts of brackets (round, angle, square, curly) will all nest, which means | |
9f10b797 | 987 | that |
87275199 | 988 | |
9f10b797 | 989 | q{foo{bar}baz} |
35f2feb0 | 990 | |
9f10b797 | 991 | is the same as |
87275199 GS |
992 | |
993 | 'foo{bar}baz' | |
994 | ||
995 | Note, however, that this does not always work for quoting Perl code: | |
996 | ||
997 | $s = q{ if($a eq "}") ... }; # WRONG | |
998 | ||
83df6a1d JH |
999 | is a syntax error. The C<Text::Balanced> module (from CPAN, and |
1000 | starting from Perl 5.8 part of the standard distribution) is able | |
1001 | to do this properly. | |
87275199 | 1002 | |
19799a22 | 1003 | There can be whitespace between the operator and the quoting |
fb73857a | 1004 | characters, except when C<#> is being used as the quoting character. |
19799a22 GS |
1005 | C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the |
1006 | operator C<q> followed by a comment. Its argument will be taken | |
1007 | from the next line. This allows you to write: | |
fb73857a | 1008 | |
1009 | s {foo} # Replace foo | |
1010 | {bar} # with bar. | |
1011 | ||
904501ec MG |
1012 | The following escape sequences are available in constructs that interpolate |
1013 | and in transliterations. | |
5691ca5f KW |
1014 | X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}> |
1015 | ||
1016 | Sequence Note Description | |
1017 | \t tab (HT, TAB) | |
1018 | \n newline (NL) | |
1019 | \r return (CR) | |
1020 | \f form feed (FF) | |
1021 | \b backspace (BS) | |
1022 | \a alarm (bell) (BEL) | |
1023 | \e escape (ESC) | |
1024 | \033 octal char (example: ESC) | |
1025 | \x1b hex char (example: ESC) | |
1026 | \x{263a} wide hex char (example: SMILEY) | |
1027 | \c[ [1] control char (example: chr(27)) | |
1028 | \N{name} [2] named Unicode character | |
1029 | \N{U+263D} [3] Unicode character (example: FIRST QUARTER MOON) | |
1030 | ||
1031 | =over 4 | |
1032 | ||
1033 | =item [1] | |
1034 | ||
1035 | The character following C<\c> is mapped to some other character as shown in the | |
1036 | table: | |
1037 | ||
1038 | Sequence Value | |
1039 | \c@ chr(0) | |
1040 | \cA chr(1) | |
1041 | \ca chr(1) | |
1042 | \cB chr(2) | |
1043 | \cb chr(2) | |
1044 | ... | |
1045 | \cZ chr(26) | |
1046 | \cz chr(26) | |
1047 | \c[ chr(27) | |
1048 | \c] chr(29) | |
1049 | \c^ chr(30) | |
1050 | \c? chr(127) | |
1051 | ||
1052 | Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the | |
1053 | end of a string, because the backslash would be parsed as escaping the end | |
1054 | quote. | |
1055 | ||
1056 | On ASCII platforms, the resulting characters from the list above are the | |
1057 | complete set of ASCII controls. This isn't the case on EBCDIC platforms; see | |
1058 | L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these | |
1059 | sequences mean on both ASCII and EBCDIC platforms. | |
1060 | ||
1061 | Use of any other character following the "c" besides those listed above is | |
1062 | prohibited on EBCDIC platforms, and discouraged (and may become deprecated or | |
1063 | forbidden) on ASCII ones. What happens for those other characters currently | |
1064 | though, is that the value is derived by inverting the 7th bit (0x40). | |
1065 | ||
1066 | To get platform independent controls, you can use C<\N{...}>. | |
1067 | ||
1068 | =item [2] | |
1069 | ||
1070 | For documentation of C<\N{name}>, see L<charnames>. | |
1071 | ||
1072 | =item [3] | |
ee9f418e | 1073 | |
e526e8bb KW |
1074 | C<\N{U+I<wide hex char>}> means the Unicode character whose Unicode ordinal |
1075 | number is I<wide hex char>. | |
5691ca5f KW |
1076 | |
1077 | =back | |
4c77eaa2 | 1078 | |
e526e8bb KW |
1079 | B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for |
1080 | the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>. (C<\v> | |
1081 | does have meaning in regular expression patterns in Perl, see L<perlre>.) | |
1082 | ||
1083 | The following escape sequences are available in constructs that interpolate, | |
904501ec | 1084 | but not in transliterations. |
d74e8afc | 1085 | X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> |
904501ec | 1086 | |
a0d0e21e LW |
1087 | \l lowercase next char |
1088 | \u uppercase next char | |
1089 | \L lowercase till \E | |
1090 | \U uppercase till \E | |
1091 | \E end case modification | |
1d2dff63 | 1092 | \Q quote non-word characters till \E |
a0d0e21e | 1093 | |
95cc3e0c JH |
1094 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, |
1095 | C<\u> and C<\U> is taken from the current locale. See L<perllocale>. | |
1096 | If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or | |
1097 | beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and | |
e526e8bb | 1098 | C<\U> is as defined by Unicode. |
a034a98d | 1099 | |
5a964f20 TC |
1100 | All systems use the virtual C<"\n"> to represent a line terminator, |
1101 | called a "newline". There is no such thing as an unvarying, physical | |
19799a22 | 1102 | newline character. It is only an illusion that the operating system, |
5a964f20 TC |
1103 | device drivers, C libraries, and Perl all conspire to preserve. Not all |
1104 | systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example, | |
1105 | on a Mac, these are reversed, and on systems without line terminator, | |
1106 | printing C<"\n"> may emit no actual data. In general, use C<"\n"> when | |
1107 | you mean a "newline" for your system, but use the literal ASCII when you | |
1108 | need an exact character. For example, most networking protocols expect | |
2a380090 | 1109 | and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators, |
5a964f20 TC |
1110 | and although they often accept just C<"\012">, they seldom tolerate just |
1111 | C<"\015">. If you get in the habit of using C<"\n"> for networking, | |
1112 | you may be burned some day. | |
d74e8afc ITB |
1113 | X<newline> X<line terminator> X<eol> X<end of line> |
1114 | X<\n> X<\r> X<\r\n> | |
5a964f20 | 1115 | |
904501ec MG |
1116 | For constructs that do interpolate, variables beginning with "C<$>" |
1117 | or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or | |
ad0f383a A |
1118 | C<< $href->{key}[0] >> are also interpolated, as are array and hash slices. |
1119 | But method calls such as C<< $obj->meth >> are not. | |
af9219ee MG |
1120 | |
1121 | Interpolating an array or slice interpolates the elements in order, | |
1122 | separated by the value of C<$">, so is equivalent to interpolating | |
6deea57f TS |
1123 | C<join $", @array>. "Punctuation" arrays such as C<@*> are only |
1124 | interpolated if the name is enclosed in braces C<@{*}>, but special | |
1125 | arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces. | |
af9219ee | 1126 | |
89d205f2 YO |
1127 | You cannot include a literal C<$> or C<@> within a C<\Q> sequence. |
1128 | An unescaped C<$> or C<@> interpolates the corresponding variable, | |
1d2dff63 | 1129 | while escaping will cause the literal string C<\$> to be inserted. |
89d205f2 | 1130 | You'll need to write something like C<m/\Quser\E\@\Qhost/>. |
1d2dff63 | 1131 | |
a0d0e21e LW |
1132 | Patterns are subject to an additional level of interpretation as a |
1133 | regular expression. This is done as a second pass, after variables are | |
1134 | interpolated, so that regular expressions may be incorporated into the | |
1135 | pattern from the variables. If this is not what you want, use C<\Q> to | |
1136 | interpolate a variable literally. | |
1137 | ||
19799a22 GS |
1138 | Apart from the behavior described above, Perl does not expand |
1139 | multiple levels of interpolation. In particular, contrary to the | |
1140 | expectations of shell programmers, back-quotes do I<NOT> interpolate | |
1141 | within double quotes, nor do single quotes impede evaluation of | |
1142 | variables when used within double quotes. | |
a0d0e21e | 1143 | |
5f05dabc | 1144 | =head2 Regexp Quote-Like Operators |
d74e8afc | 1145 | X<operator, regexp> |
cb1a09d0 | 1146 | |
5f05dabc | 1147 | Here are the quote-like operators that apply to pattern |
cb1a09d0 AD |
1148 | matching and related activities. |
1149 | ||
a0d0e21e LW |
1150 | =over 8 |
1151 | ||
87e95b7f | 1152 | =item qr/STRING/msixpo |
01c6f5f4 | 1153 | X<qr> X</i> X</m> X</o> X</s> X</x> X</p> |
a0d0e21e | 1154 | |
87e95b7f YO |
1155 | This operator quotes (and possibly compiles) its I<STRING> as a regular |
1156 | expression. I<STRING> is interpolated the same way as I<PATTERN> | |
1157 | in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation | |
1158 | is done. Returns a Perl value which may be used instead of the | |
64c5a566 | 1159 | corresponding C</STRING/msixpo> expression. The returned value is a |
85dd5c8b | 1160 | normalized version of the original pattern. It magically differs from |
64c5a566 | 1161 | a string containing the same characters: C<ref(qr/x/)> returns "Regexp", |
85dd5c8b | 1162 | even though dereferencing the result returns undef. |
a0d0e21e | 1163 | |
87e95b7f YO |
1164 | For example, |
1165 | ||
1166 | $rex = qr/my.STRING/is; | |
85dd5c8b | 1167 | print $rex; # prints (?si-xm:my.STRING) |
87e95b7f YO |
1168 | s/$rex/foo/; |
1169 | ||
1170 | is equivalent to | |
1171 | ||
1172 | s/my.STRING/foo/is; | |
1173 | ||
1174 | The result may be used as a subpattern in a match: | |
1175 | ||
1176 | $re = qr/$pattern/; | |
1177 | $string =~ /foo${re}bar/; # can be interpolated in other patterns | |
1178 | $string =~ $re; # or used standalone | |
1179 | $string =~ /$re/; # or this way | |
1180 | ||
1181 | Since Perl may compile the pattern at the moment of execution of qr() | |
1182 | operator, using qr() may have speed advantages in some situations, | |
1183 | notably if the result of qr() is used standalone: | |
1184 | ||
1185 | sub match { | |
1186 | my $patterns = shift; | |
1187 | my @compiled = map qr/$_/i, @$patterns; | |
1188 | grep { | |
1189 | my $success = 0; | |
1190 | foreach my $pat (@compiled) { | |
1191 | $success = 1, last if /$pat/; | |
1192 | } | |
1193 | $success; | |
1194 | } @_; | |
5a964f20 TC |
1195 | } |
1196 | ||
87e95b7f YO |
1197 | Precompilation of the pattern into an internal representation at |
1198 | the moment of qr() avoids a need to recompile the pattern every | |
1199 | time a match C</$pat/> is attempted. (Perl has many other internal | |
1200 | optimizations, but none would be triggered in the above example if | |
1201 | we did not use qr() operator.) | |
1202 | ||
1203 | Options are: | |
1204 | ||
1205 | m Treat string as multiple lines. | |
1206 | s Treat string as single line. (Make . match a newline) | |
1207 | i Do case-insensitive pattern matching. | |
1208 | x Use extended regular expressions. | |
1209 | p When matching preserve a copy of the matched string so | |
1210 | that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined. | |
1211 | o Compile pattern only once. | |
1212 | ||
1213 | If a precompiled pattern is embedded in a larger pattern then the effect | |
1214 | of 'msixp' will be propagated appropriately. The effect of the 'o' | |
1215 | modifier has is not propagated, being restricted to those patterns | |
1216 | explicitly using it. | |
1217 | ||
1218 | See L<perlre> for additional information on valid syntax for STRING, and | |
1219 | for a detailed look at the semantics of regular expressions. | |
a0d0e21e | 1220 | |
87e95b7f | 1221 | =item m/PATTERN/msixpogc |
89d205f2 YO |
1222 | X<m> X<operator, match> |
1223 | X<regexp, options> X<regexp> X<regex, options> X<regex> | |
01c6f5f4 | 1224 | X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> |
a0d0e21e | 1225 | |
87e95b7f | 1226 | =item /PATTERN/msixpogc |
a0d0e21e | 1227 | |
5a964f20 | 1228 | Searches a string for a pattern match, and in scalar context returns |
19799a22 GS |
1229 | true if it succeeds, false if it fails. If no string is specified |
1230 | via the C<=~> or C<!~> operator, the $_ string is searched. (The | |
1231 | string specified with C<=~> need not be an lvalue--it may be the | |
1232 | result of an expression evaluation, but remember the C<=~> binds | |
1233 | rather tightly.) See also L<perlre>. See L<perllocale> for | |
1234 | discussion of additional considerations that apply when C<use locale> | |
1235 | is in effect. | |
a0d0e21e | 1236 | |
01c6f5f4 RGS |
1237 | Options are as described in C<qr//>; in addition, the following match |
1238 | process modifiers are available: | |
a0d0e21e | 1239 | |
cde0cee5 YO |
1240 | g Match globally, i.e., find all occurrences. |
1241 | c Do not reset search position on a failed match when /g is in effect. | |
a0d0e21e LW |
1242 | |
1243 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> | |
ed02a3bf | 1244 | you can use any pair of non-whitespace characters |
19799a22 GS |
1245 | as delimiters. This is particularly useful for matching path names |
1246 | that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is | |
7bac28a0 | 1247 | the delimiter, then the match-only-once rule of C<?PATTERN?> applies. |
19799a22 | 1248 | If "'" is the delimiter, no interpolation is performed on the PATTERN. |
ed02a3bf DN |
1249 | When using a character valid in an identifier, whitespace is required |
1250 | after the C<m>. | |
a0d0e21e LW |
1251 | |
1252 | PATTERN may contain variables, which will be interpolated (and the | |
f70b4f9c | 1253 | pattern recompiled) every time the pattern search is evaluated, except |
1f247705 GS |
1254 | for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and |
1255 | C<$|> are not interpolated because they look like end-of-string tests.) | |
f70b4f9c AB |
1256 | If you want such a pattern to be compiled only once, add a C</o> after |
1257 | the trailing delimiter. This avoids expensive run-time recompilations, | |
1258 | and is useful when the value you are interpolating won't change over | |
1259 | the life of the script. However, mentioning C</o> constitutes a promise | |
1260 | that you won't change the variables in the pattern. If you change them, | |
01c6f5f4 | 1261 | Perl won't even notice. See also L<"qr/STRING/msixpo">. |
a0d0e21e | 1262 | |
e9d89077 DN |
1263 | =item The empty pattern // |
1264 | ||
5a964f20 | 1265 | If the PATTERN evaluates to the empty string, the last |
d65afb4b HS |
1266 | I<successfully> matched regular expression is used instead. In this |
1267 | case, only the C<g> and C<c> flags on the empty pattern is honoured - | |
1268 | the other flags are taken from the original pattern. If no match has | |
1269 | previously succeeded, this will (silently) act instead as a genuine | |
1270 | empty pattern (which will always match). | |
a0d0e21e | 1271 | |
89d205f2 YO |
1272 | Note that it's possible to confuse Perl into thinking C<//> (the empty |
1273 | regex) is really C<//> (the defined-or operator). Perl is usually pretty | |
1274 | good about this, but some pathological cases might trigger this, such as | |
1275 | C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //> | |
1276 | (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl | |
1277 | will assume you meant defined-or. If you meant the empty regex, just | |
1278 | use parentheses or spaces to disambiguate, or even prefix the empty | |
c963b151 BD |
1279 | regex with an C<m> (so C<//> becomes C<m//>). |
1280 | ||
e9d89077 DN |
1281 | =item Matching in list context |
1282 | ||
19799a22 | 1283 | If the C</g> option is not used, C<m//> in list context returns a |
a0d0e21e | 1284 | list consisting of the subexpressions matched by the parentheses in the |
f7e33566 GS |
1285 | pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are |
1286 | also set, and that this differs from Perl 4's behavior.) When there are | |
1287 | no parentheses in the pattern, the return value is the list C<(1)> for | |
1288 | success. With or without parentheses, an empty list is returned upon | |
1289 | failure. | |
a0d0e21e LW |
1290 | |
1291 | Examples: | |
1292 | ||
1293 | open(TTY, '/dev/tty'); | |
1294 | <TTY> =~ /^y/i && foo(); # do foo if desired | |
1295 | ||
1296 | if (/Version: *([0-9.]*)/) { $version = $1; } | |
1297 | ||
1298 | next if m#^/usr/spool/uucp#; | |
1299 | ||
1300 | # poor man's grep | |
1301 | $arg = shift; | |
1302 | while (<>) { | |
1303 | print if /$arg/o; # compile only once | |
1304 | } | |
1305 | ||
1306 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) | |
1307 | ||
1308 | This last example splits $foo into the first two words and the | |
5f05dabc | 1309 | remainder of the line, and assigns those three fields to $F1, $F2, and |
1310 | $Etc. The conditional is true if any variables were assigned, i.e., if | |
a0d0e21e LW |
1311 | the pattern matched. |
1312 | ||
19799a22 GS |
1313 | The C</g> modifier specifies global pattern matching--that is, |
1314 | matching as many times as possible within the string. How it behaves | |
1315 | depends on the context. In list context, it returns a list of the | |
1316 | substrings matched by any capturing parentheses in the regular | |
1317 | expression. If there are no parentheses, it returns a list of all | |
1318 | the matched strings, as if there were parentheses around the whole | |
1319 | pattern. | |
a0d0e21e | 1320 | |
7e86de3e | 1321 | In scalar context, each execution of C<m//g> finds the next match, |
19799a22 | 1322 | returning true if it matches, and false if there is no further match. |
7e86de3e MG |
1323 | The position after the last match can be read or set using the pos() |
1324 | function; see L<perlfunc/pos>. A failed match normally resets the | |
1325 | search position to the beginning of the string, but you can avoid that | |
1326 | by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target | |
1327 | string also resets the search position. | |
c90c0ff4 | 1328 | |
e9d89077 DN |
1329 | =item \G assertion |
1330 | ||
c90c0ff4 | 1331 | You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a |
1332 | zero-width assertion that matches the exact position where the previous | |
5d43e42d DC |
1333 | C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion |
1334 | still anchors at pos(), but the match is of course only attempted once. | |
1335 | Using C<\G> without C</g> on a target string that has not previously had a | |
1336 | C</g> match applied to it is the same as using the C<\A> assertion to match | |
fe4b3f22 RGS |
1337 | the beginning of the string. Note also that, currently, C<\G> is only |
1338 | properly supported when anchored at the very beginning of the pattern. | |
c90c0ff4 | 1339 | |
1340 | Examples: | |
a0d0e21e LW |
1341 | |
1342 | # list context | |
1343 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); | |
1344 | ||
1345 | # scalar context | |
5d43e42d | 1346 | $/ = ""; |
19799a22 GS |
1347 | while (defined($paragraph = <>)) { |
1348 | while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { | |
1349 | $sentences++; | |
a0d0e21e LW |
1350 | } |
1351 | } | |
1352 | print "$sentences\n"; | |
1353 | ||
c90c0ff4 | 1354 | # using m//gc with \G |
137443ea | 1355 | $_ = "ppooqppqq"; |
44a8e56a | 1356 | while ($i++ < 2) { |
1357 | print "1: '"; | |
c90c0ff4 | 1358 | print $1 while /(o)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1359 | print "2: '"; |
c90c0ff4 | 1360 | print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1361 | print "3: '"; |
c90c0ff4 | 1362 | print $1 while /(p)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1363 | } |
5d43e42d | 1364 | print "Final: '$1', pos=",pos,"\n" if /\G(.)/; |
44a8e56a | 1365 | |
1366 | The last example should print: | |
1367 | ||
1368 | 1: 'oo', pos=4 | |
137443ea | 1369 | 2: 'q', pos=5 |
44a8e56a | 1370 | 3: 'pp', pos=7 |
1371 | 1: '', pos=7 | |
137443ea | 1372 | 2: 'q', pos=8 |
1373 | 3: '', pos=8 | |
5d43e42d DC |
1374 | Final: 'q', pos=8 |
1375 | ||
1376 | Notice that the final match matched C<q> instead of C<p>, which a match | |
1377 | without the C<\G> anchor would have done. Also note that the final match | |
ac036724 | 1378 | did not update C<pos>. C<pos> is only updated on a C</g> match. If the |
5d43e42d DC |
1379 | final match did indeed match C<p>, it's a good bet that you're running an |
1380 | older (pre-5.6.0) Perl. | |
44a8e56a | 1381 | |
c90c0ff4 | 1382 | A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can |
e7ea3e70 | 1383 | combine several regexps like this to process a string part-by-part, |
c90c0ff4 | 1384 | doing different actions depending on which regexp matched. Each |
1385 | regexp tries to match where the previous one leaves off. | |
e7ea3e70 | 1386 | |
3fe9a6f1 | 1387 | $_ = <<'EOL'; |
46c3340e | 1388 | $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx"; |
3fe9a6f1 | 1389 | EOL |
1390 | LOOP: | |
e7ea3e70 | 1391 | { |
c90c0ff4 | 1392 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; |
1393 | print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc; | |
1394 | print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc; | |
1395 | print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc; | |
1396 | print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc; | |
1397 | print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc; | |
1398 | print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc; | |
e7ea3e70 IZ |
1399 | print ". That's all!\n"; |
1400 | } | |
1401 | ||
1402 | Here is the output (split into several lines): | |
1403 | ||
1404 | line-noise lowercase line-noise lowercase UPPERCASE line-noise | |
1405 | UPPERCASE line-noise lowercase line-noise lowercase line-noise | |
1406 | lowercase lowercase line-noise lowercase lowercase line-noise | |
1407 | MiXeD line-noise. That's all! | |
44a8e56a | 1408 | |
87e95b7f YO |
1409 | =item ?PATTERN? |
1410 | X<?> | |
1411 | ||
1412 | This is just like the C</pattern/> search, except that it matches only | |
1413 | once between calls to the reset() operator. This is a useful | |
1414 | optimization when you want to see only the first occurrence of | |
1415 | something in each file of a set of files, for instance. Only C<??> | |
1416 | patterns local to the current package are reset. | |
1417 | ||
1418 | while (<>) { | |
1419 | if (?^$?) { | |
1420 | # blank line between header and body | |
1421 | } | |
1422 | } continue { | |
1423 | reset if eof; # clear ?? status for next file | |
1424 | } | |
1425 | ||
1426 | This usage is vaguely deprecated, which means it just might possibly | |
1427 | be removed in some distant future version of Perl, perhaps somewhere | |
1428 | around the year 2168. | |
1429 | ||
1430 | =item s/PATTERN/REPLACEMENT/msixpogce | |
1431 | X<substitute> X<substitution> X<replace> X<regexp, replace> | |
01c6f5f4 | 1432 | X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> |
87e95b7f YO |
1433 | |
1434 | Searches a string for a pattern, and if found, replaces that pattern | |
1435 | with the replacement text and returns the number of substitutions | |
1436 | made. Otherwise it returns false (specifically, the empty string). | |
1437 | ||
1438 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> | |
1439 | variable is searched and modified. (The string specified with C<=~> must | |
1440 | be scalar variable, an array element, a hash element, or an assignment | |
1441 | to one of those, i.e., an lvalue.) | |
1442 | ||
1443 | If the delimiter chosen is a single quote, no interpolation is | |
1444 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the | |
1445 | PATTERN contains a $ that looks like a variable rather than an | |
1446 | end-of-string test, the variable will be interpolated into the pattern | |
1447 | at run-time. If you want the pattern compiled only once the first time | |
1448 | the variable is interpolated, use the C</o> option. If the pattern | |
1449 | evaluates to the empty string, the last successfully executed regular | |
1450 | expression is used instead. See L<perlre> for further explanation on these. | |
1451 | See L<perllocale> for discussion of additional considerations that apply | |
1452 | when C<use locale> is in effect. | |
1453 | ||
1454 | Options are as with m// with the addition of the following replacement | |
1455 | specific options: | |
1456 | ||
1457 | e Evaluate the right side as an expression. | |
1458 | ee Evaluate the right side as a string then eval the result | |
1459 | ||
ed02a3bf DN |
1460 | Any non-whitespace delimiter may replace the slashes. Add space after |
1461 | the C<s> when using a character allowed in identifiers. If single quotes | |
1462 | are used, no interpretation is done on the replacement string (the C</e> | |
1463 | modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks | |
1464 | as normal delimiters; the replacement text is not evaluated as a command. | |
1465 | If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has | |
1466 | its own pair of quotes, which may or may not be bracketing quotes, e.g., | |
87e95b7f YO |
1467 | C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the |
1468 | replacement portion to be treated as a full-fledged Perl expression | |
1469 | and evaluated right then and there. It is, however, syntax checked at | |
1470 | compile-time. A second C<e> modifier will cause the replacement portion | |
1471 | to be C<eval>ed before being run as a Perl expression. | |
1472 | ||
1473 | Examples: | |
1474 | ||
1475 | s/\bgreen\b/mauve/g; # don't change wintergreen | |
1476 | ||
1477 | $path =~ s|/usr/bin|/usr/local/bin|; | |
1478 | ||
1479 | s/Login: $foo/Login: $bar/; # run-time pattern | |
1480 | ||
1481 | ($foo = $bar) =~ s/this/that/; # copy first, then change | |
1482 | ||
1483 | $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count | |
1484 | ||
1485 | $_ = 'abc123xyz'; | |
1486 | s/\d+/$&*2/e; # yields 'abc246xyz' | |
1487 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' | |
1488 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' | |
1489 | ||
1490 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e | |
1491 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e | |
1492 | s/^=(\w+)/pod($1)/ge; # use function call | |
1493 | ||
1494 | # expand variables in $_, but dynamics only, using | |
1495 | # symbolic dereferencing | |
1496 | s/\$(\w+)/${$1}/g; | |
1497 | ||
1498 | # Add one to the value of any numbers in the string | |
1499 | s/(\d+)/1 + $1/eg; | |
1500 | ||
1501 | # This will expand any embedded scalar variable | |
1502 | # (including lexicals) in $_ : First $1 is interpolated | |
1503 | # to the variable name, and then evaluated | |
1504 | s/(\$\w+)/$1/eeg; | |
1505 | ||
1506 | # Delete (most) C comments. | |
1507 | $program =~ s { | |
1508 | /\* # Match the opening delimiter. | |
1509 | .*? # Match a minimal number of characters. | |
1510 | \*/ # Match the closing delimiter. | |
1511 | } []gsx; | |
1512 | ||
1513 | s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively | |
1514 | ||
1515 | for ($variable) { # trim whitespace in $variable, cheap | |
1516 | s/^\s+//; | |
1517 | s/\s+$//; | |
1518 | } | |
1519 | ||
1520 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields | |
1521 | ||
1522 | Note the use of $ instead of \ in the last example. Unlike | |
1523 | B<sed>, we use the \<I<digit>> form in only the left hand side. | |
1524 | Anywhere else it's $<I<digit>>. | |
1525 | ||
1526 | Occasionally, you can't use just a C</g> to get all the changes | |
1527 | to occur that you might want. Here are two common cases: | |
1528 | ||
1529 | # put commas in the right places in an integer | |
1530 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; | |
1531 | ||
1532 | # expand tabs to 8-column spacing | |
1533 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; | |
1534 | ||
1535 | =back | |
1536 | ||
1537 | =head2 Quote-Like Operators | |
1538 | X<operator, quote-like> | |
1539 | ||
01c6f5f4 RGS |
1540 | =over 4 |
1541 | ||
a0d0e21e | 1542 | =item q/STRING/ |
5d44bfff | 1543 | X<q> X<quote, single> X<'> X<''> |
a0d0e21e | 1544 | |
5d44bfff | 1545 | =item 'STRING' |
a0d0e21e | 1546 | |
19799a22 | 1547 | A single-quoted, literal string. A backslash represents a backslash |
68dc0745 | 1548 | unless followed by the delimiter or another backslash, in which case |
1549 | the delimiter or backslash is interpolated. | |
a0d0e21e LW |
1550 | |
1551 | $foo = q!I said, "You said, 'She said it.'"!; | |
1552 | $bar = q('This is it.'); | |
68dc0745 | 1553 | $baz = '\n'; # a two-character string |
a0d0e21e LW |
1554 | |
1555 | =item qq/STRING/ | |
d74e8afc | 1556 | X<qq> X<quote, double> X<"> X<""> |
a0d0e21e LW |
1557 | |
1558 | =item "STRING" | |
1559 | ||
1560 | A double-quoted, interpolated string. | |
1561 | ||
1562 | $_ .= qq | |
1563 | (*** The previous line contains the naughty word "$1".\n) | |
19799a22 | 1564 | if /\b(tcl|java|python)\b/i; # :-) |
68dc0745 | 1565 | $baz = "\n"; # a one-character string |
a0d0e21e LW |
1566 | |
1567 | =item qx/STRING/ | |
d74e8afc | 1568 | X<qx> X<`> X<``> X<backtick> |
a0d0e21e LW |
1569 | |
1570 | =item `STRING` | |
1571 | ||
43dd4d21 JH |
1572 | A string which is (possibly) interpolated and then executed as a |
1573 | system command with C</bin/sh> or its equivalent. Shell wildcards, | |
1574 | pipes, and redirections will be honored. The collected standard | |
1575 | output of the command is returned; standard error is unaffected. In | |
1576 | scalar context, it comes back as a single (potentially multi-line) | |
1577 | string, or undef if the command failed. In list context, returns a | |
1578 | list of lines (however you've defined lines with $/ or | |
1579 | $INPUT_RECORD_SEPARATOR), or an empty list if the command failed. | |
5a964f20 TC |
1580 | |
1581 | Because backticks do not affect standard error, use shell file descriptor | |
1582 | syntax (assuming the shell supports this) if you care to address this. | |
1583 | To capture a command's STDERR and STDOUT together: | |
a0d0e21e | 1584 | |
5a964f20 TC |
1585 | $output = `cmd 2>&1`; |
1586 | ||
1587 | To capture a command's STDOUT but discard its STDERR: | |
1588 | ||
1589 | $output = `cmd 2>/dev/null`; | |
1590 | ||
1591 | To capture a command's STDERR but discard its STDOUT (ordering is | |
1592 | important here): | |
1593 | ||
1594 | $output = `cmd 2>&1 1>/dev/null`; | |
1595 | ||
1596 | To exchange a command's STDOUT and STDERR in order to capture the STDERR | |
1597 | but leave its STDOUT to come out the old STDERR: | |
1598 | ||
1599 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; | |
1600 | ||
1601 | To read both a command's STDOUT and its STDERR separately, it's easiest | |
2359510d SD |
1602 | to redirect them separately to files, and then read from those files |
1603 | when the program is done: | |
5a964f20 | 1604 | |
2359510d | 1605 | system("program args 1>program.stdout 2>program.stderr"); |
5a964f20 | 1606 | |
30398227 SP |
1607 | The STDIN filehandle used by the command is inherited from Perl's STDIN. |
1608 | For example: | |
1609 | ||
1610 | open BLAM, "blam" || die "Can't open: $!"; | |
1611 | open STDIN, "<&BLAM"; | |
1612 | print `sort`; | |
1613 | ||
1614 | will print the sorted contents of the file "blam". | |
1615 | ||
5a964f20 TC |
1616 | Using single-quote as a delimiter protects the command from Perl's |
1617 | double-quote interpolation, passing it on to the shell instead: | |
1618 | ||
1619 | $perl_info = qx(ps $$); # that's Perl's $$ | |
1620 | $shell_info = qx'ps $$'; # that's the new shell's $$ | |
1621 | ||
19799a22 | 1622 | How that string gets evaluated is entirely subject to the command |
5a964f20 TC |
1623 | interpreter on your system. On most platforms, you will have to protect |
1624 | shell metacharacters if you want them treated literally. This is in | |
1625 | practice difficult to do, as it's unclear how to escape which characters. | |
1626 | See L<perlsec> for a clean and safe example of a manual fork() and exec() | |
1627 | to emulate backticks safely. | |
a0d0e21e | 1628 | |
bb32b41a GS |
1629 | On some platforms (notably DOS-like ones), the shell may not be |
1630 | capable of dealing with multiline commands, so putting newlines in | |
1631 | the string may not get you what you want. You may be able to evaluate | |
1632 | multiple commands in a single line by separating them with the command | |
1633 | separator character, if your shell supports that (e.g. C<;> on many Unix | |
1634 | shells; C<&> on the Windows NT C<cmd> shell). | |
1635 | ||
0f897271 GS |
1636 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
1637 | output before starting the child process, but this may not be supported | |
1638 | on some platforms (see L<perlport>). To be safe, you may need to set | |
1639 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of | |
1640 | C<IO::Handle> on any open handles. | |
1641 | ||
bb32b41a GS |
1642 | Beware that some command shells may place restrictions on the length |
1643 | of the command line. You must ensure your strings don't exceed this | |
1644 | limit after any necessary interpolations. See the platform-specific | |
1645 | release notes for more details about your particular environment. | |
1646 | ||
5a964f20 TC |
1647 | Using this operator can lead to programs that are difficult to port, |
1648 | because the shell commands called vary between systems, and may in | |
1649 | fact not be present at all. As one example, the C<type> command under | |
1650 | the POSIX shell is very different from the C<type> command under DOS. | |
1651 | That doesn't mean you should go out of your way to avoid backticks | |
1652 | when they're the right way to get something done. Perl was made to be | |
1653 | a glue language, and one of the things it glues together is commands. | |
1654 | Just understand what you're getting yourself into. | |
bb32b41a | 1655 | |
da87341d | 1656 | See L</"I/O Operators"> for more discussion. |
a0d0e21e | 1657 | |
945c54fd | 1658 | =item qw/STRING/ |
d74e8afc | 1659 | X<qw> X<quote, list> X<quote, words> |
945c54fd JH |
1660 | |
1661 | Evaluates to a list of the words extracted out of STRING, using embedded | |
1662 | whitespace as the word delimiters. It can be understood as being roughly | |
1663 | equivalent to: | |
1664 | ||
1665 | split(' ', q/STRING/); | |
1666 | ||
efb1e162 CW |
1667 | the differences being that it generates a real list at compile time, and |
1668 | in scalar context it returns the last element in the list. So | |
945c54fd JH |
1669 | this expression: |
1670 | ||
1671 | qw(foo bar baz) | |
1672 | ||
1673 | is semantically equivalent to the list: | |
1674 | ||
1675 | 'foo', 'bar', 'baz' | |
1676 | ||
1677 | Some frequently seen examples: | |
1678 | ||
1679 | use POSIX qw( setlocale localeconv ) | |
1680 | @EXPORT = qw( foo bar baz ); | |
1681 | ||
1682 | A common mistake is to try to separate the words with comma or to | |
1683 | put comments into a multi-line C<qw>-string. For this reason, the | |
89d205f2 | 1684 | C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable) |
945c54fd JH |
1685 | produces warnings if the STRING contains the "," or the "#" character. |
1686 | ||
a0d0e21e | 1687 | |
6940069f | 1688 | =item tr/SEARCHLIST/REPLACEMENTLIST/cds |
d74e8afc | 1689 | X<tr> X<y> X<transliterate> X</c> X</d> X</s> |
a0d0e21e | 1690 | |
6940069f | 1691 | =item y/SEARCHLIST/REPLACEMENTLIST/cds |
a0d0e21e | 1692 | |
2c268ad5 | 1693 | Transliterates all occurrences of the characters found in the search list |
a0d0e21e LW |
1694 | with the corresponding character in the replacement list. It returns |
1695 | the number of characters replaced or deleted. If no string is | |
2c268ad5 | 1696 | specified via the =~ or !~ operator, the $_ string is transliterated. (The |
54310121 | 1697 | string specified with =~ must be a scalar variable, an array element, a |
1698 | hash element, or an assignment to one of those, i.e., an lvalue.) | |
8ada0baa | 1699 | |
89d205f2 | 1700 | A character range may be specified with a hyphen, so C<tr/A-J/0-9/> |
2c268ad5 | 1701 | does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. |
54310121 | 1702 | For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the |
1703 | SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has | |
1704 | its own pair of quotes, which may or may not be bracketing quotes, | |
2c268ad5 | 1705 | e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>. |
a0d0e21e | 1706 | |
cc255d5f | 1707 | Note that C<tr> does B<not> do regular expression character classes |
e0c83546 | 1708 | such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to |
cc255d5f JH |
1709 | the tr(1) utility. If you want to map strings between lower/upper |
1710 | cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider | |
1711 | using the C<s> operator if you need regular expressions. | |
1712 | ||
8ada0baa JH |
1713 | Note also that the whole range idea is rather unportable between |
1714 | character sets--and even within character sets they may cause results | |
1715 | you probably didn't expect. A sound principle is to use only ranges | |
1716 | that begin from and end at either alphabets of equal case (a-e, A-E), | |
1717 | or digits (0-4). Anything else is unsafe. If in doubt, spell out the | |
1718 | character sets in full. | |
1719 | ||
a0d0e21e LW |
1720 | Options: |
1721 | ||
1722 | c Complement the SEARCHLIST. | |
1723 | d Delete found but unreplaced characters. | |
1724 | s Squash duplicate replaced characters. | |
1725 | ||
19799a22 GS |
1726 | If the C</c> modifier is specified, the SEARCHLIST character set |
1727 | is complemented. If the C</d> modifier is specified, any characters | |
1728 | specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. | |
1729 | (Note that this is slightly more flexible than the behavior of some | |
1730 | B<tr> programs, which delete anything they find in the SEARCHLIST, | |
1731 | period.) If the C</s> modifier is specified, sequences of characters | |
1732 | that were transliterated to the same character are squashed down | |
1733 | to a single instance of the character. | |
a0d0e21e LW |
1734 | |
1735 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted | |
1736 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter | |
1737 | than the SEARCHLIST, the final character is replicated till it is long | |
5a964f20 | 1738 | enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. |
a0d0e21e LW |
1739 | This latter is useful for counting characters in a class or for |
1740 | squashing character sequences in a class. | |
1741 | ||
1742 | Examples: | |
1743 | ||
1744 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case | |
1745 | ||
1746 | $cnt = tr/*/*/; # count the stars in $_ | |
1747 | ||
1748 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky | |
1749 | ||
1750 | $cnt = tr/0-9//; # count the digits in $_ | |
1751 | ||
1752 | tr/a-zA-Z//s; # bookkeeper -> bokeper | |
1753 | ||
1754 | ($HOST = $host) =~ tr/a-z/A-Z/; | |
1755 | ||
1756 | tr/a-zA-Z/ /cs; # change non-alphas to single space | |
1757 | ||
1758 | tr [\200-\377] | |
1759 | [\000-\177]; # delete 8th bit | |
1760 | ||
19799a22 GS |
1761 | If multiple transliterations are given for a character, only the |
1762 | first one is used: | |
748a9306 LW |
1763 | |
1764 | tr/AAA/XYZ/ | |
1765 | ||
2c268ad5 | 1766 | will transliterate any A to X. |
748a9306 | 1767 | |
19799a22 | 1768 | Because the transliteration table is built at compile time, neither |
a0d0e21e | 1769 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote |
19799a22 GS |
1770 | interpolation. That means that if you want to use variables, you |
1771 | must use an eval(): | |
a0d0e21e LW |
1772 | |
1773 | eval "tr/$oldlist/$newlist/"; | |
1774 | die $@ if $@; | |
1775 | ||
1776 | eval "tr/$oldlist/$newlist/, 1" or die $@; | |
1777 | ||
7e3b091d | 1778 | =item <<EOF |
d74e8afc | 1779 | X<here-doc> X<heredoc> X<here-document> X<<< << >>> |
7e3b091d DA |
1780 | |
1781 | A line-oriented form of quoting is based on the shell "here-document" | |
1782 | syntax. Following a C<< << >> you specify a string to terminate | |
1783 | the quoted material, and all lines following the current line down to | |
89d205f2 YO |
1784 | the terminating string are the value of the item. |
1785 | ||
1786 | The terminating string may be either an identifier (a word), or some | |
1787 | quoted text. An unquoted identifier works like double quotes. | |
1788 | There may not be a space between the C<< << >> and the identifier, | |
1789 | unless the identifier is explicitly quoted. (If you put a space it | |
1790 | will be treated as a null identifier, which is valid, and matches the | |
1791 | first empty line.) The terminating string must appear by itself | |
1792 | (unquoted and with no surrounding whitespace) on the terminating line. | |
1793 | ||
1794 | If the terminating string is quoted, the type of quotes used determine | |
1795 | the treatment of the text. | |
1796 | ||
1797 | =over 4 | |
1798 | ||
1799 | =item Double Quotes | |
1800 | ||
1801 | Double quotes indicate that the text will be interpolated using exactly | |
1802 | the same rules as normal double quoted strings. | |
7e3b091d DA |
1803 | |
1804 | print <<EOF; | |
1805 | The price is $Price. | |
1806 | EOF | |
1807 | ||
1808 | print << "EOF"; # same as above | |
1809 | The price is $Price. | |
1810 | EOF | |
1811 | ||
89d205f2 YO |
1812 | |
1813 | =item Single Quotes | |
1814 | ||
1815 | Single quotes indicate the text is to be treated literally with no | |
1816 | interpolation of its content. This is similar to single quoted | |
1817 | strings except that backslashes have no special meaning, with C<\\> | |
1818 | being treated as two backslashes and not one as they would in every | |
1819 | other quoting construct. | |
1820 | ||
1821 | This is the only form of quoting in perl where there is no need | |
1822 | to worry about escaping content, something that code generators | |
1823 | can and do make good use of. | |
1824 | ||
1825 | =item Backticks | |
1826 | ||
1827 | The content of the here doc is treated just as it would be if the | |
1828 | string were embedded in backticks. Thus the content is interpolated | |
1829 | as though it were double quoted and then executed via the shell, with | |
1830 | the results of the execution returned. | |
1831 | ||
1832 | print << `EOC`; # execute command and get results | |
7e3b091d | 1833 | echo hi there |
7e3b091d DA |
1834 | EOC |
1835 | ||
89d205f2 YO |
1836 | =back |
1837 | ||
1838 | It is possible to stack multiple here-docs in a row: | |
1839 | ||
7e3b091d DA |
1840 | print <<"foo", <<"bar"; # you can stack them |
1841 | I said foo. | |
1842 | foo | |
1843 | I said bar. | |
1844 | bar | |
1845 | ||
1846 | myfunc(<< "THIS", 23, <<'THAT'); | |
1847 | Here's a line | |
1848 | or two. | |
1849 | THIS | |
1850 | and here's another. | |
1851 | THAT | |
1852 | ||
1853 | Just don't forget that you have to put a semicolon on the end | |
1854 | to finish the statement, as Perl doesn't know you're not going to | |
1855 | try to do this: | |
1856 | ||
1857 | print <<ABC | |
1858 | 179231 | |
1859 | ABC | |
1860 | + 20; | |
1861 | ||
872d7e53 TS |
1862 | If you want to remove the line terminator from your here-docs, |
1863 | use C<chomp()>. | |
1864 | ||
1865 | chomp($string = <<'END'); | |
1866 | This is a string. | |
1867 | END | |
1868 | ||
1869 | If you want your here-docs to be indented with the rest of the code, | |
1870 | you'll need to remove leading whitespace from each line manually: | |
7e3b091d DA |
1871 | |
1872 | ($quote = <<'FINIS') =~ s/^\s+//gm; | |
89d205f2 | 1873 | The Road goes ever on and on, |
7e3b091d DA |
1874 | down from the door where it began. |
1875 | FINIS | |
1876 | ||
1877 | If you use a here-doc within a delimited construct, such as in C<s///eg>, | |
1878 | the quoted material must come on the lines following the final delimiter. | |
1879 | So instead of | |
1880 | ||
1881 | s/this/<<E . 'that' | |
1882 | the other | |
1883 | E | |
1884 | . 'more '/eg; | |
1885 | ||
1886 | you have to write | |
1887 | ||
89d205f2 YO |
1888 | s/this/<<E . 'that' |
1889 | . 'more '/eg; | |
1890 | the other | |
1891 | E | |
7e3b091d DA |
1892 | |
1893 | If the terminating identifier is on the last line of the program, you | |
1894 | must be sure there is a newline after it; otherwise, Perl will give the | |
1895 | warning B<Can't find string terminator "END" anywhere before EOF...>. | |
1896 | ||
89d205f2 | 1897 | Additionally, the quoting rules for the end of string identifier are not |
ac036724 | 1898 | related to Perl's quoting rules. C<q()>, C<qq()>, and the like are not |
89d205f2 YO |
1899 | supported in place of C<''> and C<"">, and the only interpolation is for |
1900 | backslashing the quoting character: | |
7e3b091d DA |
1901 | |
1902 | print << "abc\"def"; | |
1903 | testing... | |
1904 | abc"def | |
1905 | ||
1906 | Finally, quoted strings cannot span multiple lines. The general rule is | |
1907 | that the identifier must be a string literal. Stick with that, and you | |
1908 | should be safe. | |
1909 | ||
a0d0e21e LW |
1910 | =back |
1911 | ||
75e14d17 | 1912 | =head2 Gory details of parsing quoted constructs |
d74e8afc | 1913 | X<quote, gory details> |
75e14d17 | 1914 | |
19799a22 GS |
1915 | When presented with something that might have several different |
1916 | interpretations, Perl uses the B<DWIM> (that's "Do What I Mean") | |
1917 | principle to pick the most probable interpretation. This strategy | |
1918 | is so successful that Perl programmers often do not suspect the | |
1919 | ambivalence of what they write. But from time to time, Perl's | |
1920 | notions differ substantially from what the author honestly meant. | |
1921 | ||
1922 | This section hopes to clarify how Perl handles quoted constructs. | |
1923 | Although the most common reason to learn this is to unravel labyrinthine | |
1924 | regular expressions, because the initial steps of parsing are the | |
1925 | same for all quoting operators, they are all discussed together. | |
1926 | ||
1927 | The most important Perl parsing rule is the first one discussed | |
1928 | below: when processing a quoted construct, Perl first finds the end | |
1929 | of that construct, then interprets its contents. If you understand | |
1930 | this rule, you may skip the rest of this section on the first | |
1931 | reading. The other rules are likely to contradict the user's | |
1932 | expectations much less frequently than this first one. | |
1933 | ||
1934 | Some passes discussed below are performed concurrently, but because | |
1935 | their results are the same, we consider them individually. For different | |
1936 | quoting constructs, Perl performs different numbers of passes, from | |
6deea57f | 1937 | one to four, but these passes are always performed in the same order. |
75e14d17 | 1938 | |
13a2d996 | 1939 | =over 4 |
75e14d17 IZ |
1940 | |
1941 | =item Finding the end | |
1942 | ||
6deea57f TS |
1943 | The first pass is finding the end of the quoted construct, where |
1944 | the information about the delimiters is used in parsing. | |
1945 | During this search, text between the starting and ending delimiters | |
1946 | is copied to a safe location. The text copied gets delimiter-independent. | |
1947 | ||
1948 | If the construct is a here-doc, the ending delimiter is a line | |
1949 | that has a terminating string as the content. Therefore C<<<EOF> is | |
1950 | terminated by C<EOF> immediately followed by C<"\n"> and starting | |
1951 | from the first column of the terminating line. | |
1952 | When searching for the terminating line of a here-doc, nothing | |
1953 | is skipped. In other words, lines after the here-doc syntax | |
1954 | are compared with the terminating string line by line. | |
1955 | ||
1956 | For the constructs except here-docs, single characters are used as starting | |
1957 | and ending delimiters. If the starting delimiter is an opening punctuation | |
1958 | (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the | |
1959 | corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>). | |
1960 | If the starting delimiter is an unpaired character like C</> or a closing | |
1961 | punctuation, the ending delimiter is same as the starting delimiter. | |
1962 | Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates | |
1963 | C<qq[]> and C<qq]]> constructs. | |
1964 | ||
1965 | When searching for single-character delimiters, escaped delimiters | |
1966 | and C<\\> are skipped. For example, while searching for terminating C</>, | |
1967 | combinations of C<\\> and C<\/> are skipped. If the delimiters are | |
1968 | bracketing, nested pairs are also skipped. For example, while searching | |
1969 | for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>, | |
1970 | and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well. | |
1971 | However, when backslashes are used as the delimiters (like C<qq\\> and | |
1972 | C<tr\\\>), nothing is skipped. | |
1973 | During the search for the end, backslashes that escape delimiters | |
1974 | are removed (exactly speaking, they are not copied to the safe location). | |
75e14d17 | 1975 | |
19799a22 GS |
1976 | For constructs with three-part delimiters (C<s///>, C<y///>, and |
1977 | C<tr///>), the search is repeated once more. | |
6deea57f TS |
1978 | If the first delimiter is not an opening punctuation, three delimiters must |
1979 | be same such as C<s!!!> and C<tr)))>, in which case the second delimiter | |
1980 | terminates the left part and starts the right part at once. | |
1981 | If the left part is delimited by bracketing punctuations (that is C<()>, | |
1982 | C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of | |
1983 | delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces | |
1984 | and comments are allowed between both parts, though the comment must follow | |
1985 | at least one whitespace; otherwise a character expected as the start of | |
1986 | the comment may be regarded as the starting delimiter of the right part. | |
75e14d17 | 1987 | |
19799a22 GS |
1988 | During this search no attention is paid to the semantics of the construct. |
1989 | Thus: | |
75e14d17 IZ |
1990 | |
1991 | "$hash{"$foo/$bar"}" | |
1992 | ||
2a94b7ce | 1993 | or: |
75e14d17 | 1994 | |
89d205f2 | 1995 | m/ |
2a94b7ce | 1996 | bar # NOT a comment, this slash / terminated m//! |
75e14d17 IZ |
1997 | /x |
1998 | ||
19799a22 GS |
1999 | do not form legal quoted expressions. The quoted part ends on the |
2000 | first C<"> and C</>, and the rest happens to be a syntax error. | |
2001 | Because the slash that terminated C<m//> was followed by a C<SPACE>, | |
2002 | the example above is not C<m//x>, but rather C<m//> with no C</x> | |
2003 | modifier. So the embedded C<#> is interpreted as a literal C<#>. | |
75e14d17 | 2004 | |
89d205f2 YO |
2005 | Also no attention is paid to C<\c\> (multichar control char syntax) during |
2006 | this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part | |
2007 | of C<\/>, and the following C</> is not recognized as a delimiter. | |
0d594e51 TS |
2008 | Instead, use C<\034> or C<\x1c> at the end of quoted constructs. |
2009 | ||
75e14d17 | 2010 | =item Interpolation |
d74e8afc | 2011 | X<interpolation> |
75e14d17 | 2012 | |
19799a22 | 2013 | The next step is interpolation in the text obtained, which is now |
89d205f2 | 2014 | delimiter-independent. There are multiple cases. |
75e14d17 | 2015 | |
13a2d996 | 2016 | =over 4 |
75e14d17 | 2017 | |
89d205f2 | 2018 | =item C<<<'EOF'> |
75e14d17 IZ |
2019 | |
2020 | No interpolation is performed. | |
6deea57f TS |
2021 | Note that the combination C<\\> is left intact, since escaped delimiters |
2022 | are not available for here-docs. | |
75e14d17 | 2023 | |
6deea57f | 2024 | =item C<m''>, the pattern of C<s'''> |
89d205f2 | 2025 | |
6deea57f TS |
2026 | No interpolation is performed at this stage. |
2027 | Any backslashed sequences including C<\\> are treated at the stage | |
2028 | to L</"parsing regular expressions">. | |
89d205f2 | 2029 | |
6deea57f | 2030 | =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''> |
75e14d17 | 2031 | |
89d205f2 | 2032 | The only interpolation is removal of C<\> from pairs of C<\\>. |
6deea57f TS |
2033 | Therefore C<-> in C<tr'''> and C<y'''> is treated literally |
2034 | as a hyphen and no character range is available. | |
2035 | C<\1> in the replacement of C<s'''> does not work as C<$1>. | |
89d205f2 YO |
2036 | |
2037 | =item C<tr///>, C<y///> | |
2038 | ||
6deea57f TS |
2039 | No variable interpolation occurs. String modifying combinations for |
2040 | case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized. | |
2041 | The other escape sequences such as C<\200> and C<\t> and backslashed | |
2042 | characters such as C<\\> and C<\-> are converted to appropriate literals. | |
89d205f2 YO |
2043 | The character C<-> is treated specially and therefore C<\-> is treated |
2044 | as a literal C<->. | |
75e14d17 | 2045 | |
89d205f2 | 2046 | =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF"> |
75e14d17 | 2047 | |
19799a22 GS |
2048 | C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are |
2049 | converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar"> | |
2050 | is converted to C<$foo . (quotemeta("baz" . $bar))> internally. | |
6deea57f TS |
2051 | The other escape sequences such as C<\200> and C<\t> and backslashed |
2052 | characters such as C<\\> and C<\-> are replaced with appropriate | |
2053 | expansions. | |
2a94b7ce | 2054 | |
19799a22 GS |
2055 | Let it be stressed that I<whatever falls between C<\Q> and C<\E>> |
2056 | is interpolated in the usual way. Something like C<"\Q\\E"> has | |
2057 | no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the | |
2058 | result is the same as for C<"\\\\E">. As a general rule, backslashes | |
2059 | between C<\Q> and C<\E> may lead to counterintuitive results. So, | |
2060 | C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same | |
2061 | as C<"\\\t"> (since TAB is not alphanumeric). Note also that: | |
2a94b7ce IZ |
2062 | |
2063 | $str = '\t'; | |
2064 | return "\Q$str"; | |
2065 | ||
2066 | may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. | |
2067 | ||
19799a22 | 2068 | Interpolated scalars and arrays are converted internally to the C<join> and |
92d29cee | 2069 | C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes: |
75e14d17 | 2070 | |
19799a22 | 2071 | $foo . " XXX '" . (join $", @arr) . "'"; |
75e14d17 | 2072 | |
19799a22 | 2073 | All operations above are performed simultaneously, left to right. |
75e14d17 | 2074 | |
19799a22 GS |
2075 | Because the result of C<"\Q STRING \E"> has all metacharacters |
2076 | quoted, there is no way to insert a literal C<$> or C<@> inside a | |
2077 | C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became | |
2078 | C<"\\\$">; if not, it is interpreted as the start of an interpolated | |
2079 | scalar. | |
75e14d17 | 2080 | |
19799a22 | 2081 | Note also that the interpolation code needs to make a decision on |
89d205f2 | 2082 | where the interpolated scalar ends. For instance, whether |
35f2feb0 | 2083 | C<< "a $b -> {c}" >> really means: |
75e14d17 IZ |
2084 | |
2085 | "a " . $b . " -> {c}"; | |
2086 | ||
2a94b7ce | 2087 | or: |
75e14d17 IZ |
2088 | |
2089 | "a " . $b -> {c}; | |
2090 | ||
19799a22 GS |
2091 | Most of the time, the longest possible text that does not include |
2092 | spaces between components and which contains matching braces or | |
2093 | brackets. because the outcome may be determined by voting based | |
2094 | on heuristic estimators, the result is not strictly predictable. | |
2095 | Fortunately, it's usually correct for ambiguous cases. | |
75e14d17 | 2096 | |
6deea57f | 2097 | =item the replacement of C<s///> |
75e14d17 | 2098 | |
19799a22 | 2099 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation |
6deea57f TS |
2100 | happens as with C<qq//> constructs. |
2101 | ||
2102 | It is at this step that C<\1> is begrudgingly converted to C<$1> in | |
2103 | the replacement text of C<s///>, in order to correct the incorrigible | |
2104 | I<sed> hackers who haven't picked up the saner idiom yet. A warning | |
2105 | is emitted if the C<use warnings> pragma or the B<-w> command-line flag | |
2106 | (that is, the C<$^W> variable) was set. | |
2107 | ||
2108 | =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, | |
2109 | ||
cc74c5bd TS |
2110 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>, |
2111 | and interpolation happens (almost) as with C<qq//> constructs. | |
2112 | ||
5d03b57c KW |
2113 | Processing of C<\N{...}> is also done here, and compiled into an intermediate |
2114 | form for the regex compiler. (This is because, as mentioned below, the regex | |
2115 | compilation may be done at execution time, and C<\N{...}> is a compile-time | |
2116 | construct.) | |
2117 | ||
cc74c5bd TS |
2118 | However any other combinations of C<\> followed by a character |
2119 | are not substituted but only skipped, in order to parse them | |
2120 | as regular expressions at the following step. | |
6deea57f | 2121 | As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly |
1749ea0d | 2122 | treated as an array symbol (for example C<@foo>), |
6deea57f | 2123 | even though the same text in C<qq//> gives interpolation of C<\c@>. |
6deea57f TS |
2124 | |
2125 | Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and | |
19799a22 GS |
2126 | a C<#>-comment in a C<//x>-regular expression, no processing is |
2127 | performed whatsoever. This is the first step at which the presence | |
2128 | of the C<//x> modifier is relevant. | |
2129 | ||
1749ea0d TS |
2130 | Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+> |
2131 | and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are | |
2132 | voted (by several different estimators) to be either an array element | |
2133 | or C<$var> followed by an RE alternative. This is where the notation | |
19799a22 GS |
2134 | C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as |
2135 | array element C<-9>, not as a regular expression from the variable | |
2136 | C<$arr> followed by a digit, which would be the interpretation of | |
2137 | C</$arr[0-9]/>. Since voting among different estimators may occur, | |
2138 | the result is not predictable. | |
2139 | ||
19799a22 GS |
2140 | The lack of processing of C<\\> creates specific restrictions on |
2141 | the post-processed text. If the delimiter is C</>, one cannot get | |
2142 | the combination C<\/> into the result of this step. C</> will | |
2143 | finish the regular expression, C<\/> will be stripped to C</> on | |
2144 | the previous step, and C<\\/> will be left as is. Because C</> is | |
2145 | equivalent to C<\/> inside a regular expression, this does not | |
2146 | matter unless the delimiter happens to be character special to the | |
2147 | RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an | |
2148 | alphanumeric char, as in: | |
2a94b7ce IZ |
2149 | |
2150 | m m ^ a \s* b mmx; | |
2151 | ||
19799a22 | 2152 | In the RE above, which is intentionally obfuscated for illustration, the |
6deea57f | 2153 | delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the |
89d205f2 | 2154 | RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one |
19799a22 GS |
2155 | reason you're encouraged to restrict your delimiters to non-alphanumeric, |
2156 | non-whitespace choices. | |
75e14d17 IZ |
2157 | |
2158 | =back | |
2159 | ||
19799a22 | 2160 | This step is the last one for all constructs except regular expressions, |
75e14d17 IZ |
2161 | which are processed further. |
2162 | ||
6deea57f TS |
2163 | =item parsing regular expressions |
2164 | X<regexp, parse> | |
75e14d17 | 2165 | |
19799a22 | 2166 | Previous steps were performed during the compilation of Perl code, |
ac036724 | 2167 | but this one happens at run time, although it may be optimized to |
19799a22 | 2168 | be calculated at compile time if appropriate. After preprocessing |
6deea57f | 2169 | described above, and possibly after evaluation if concatenation, |
19799a22 GS |
2170 | joining, casing translation, or metaquoting are involved, the |
2171 | resulting I<string> is passed to the RE engine for compilation. | |
2172 | ||
2173 | Whatever happens in the RE engine might be better discussed in L<perlre>, | |
2174 | but for the sake of continuity, we shall do so here. | |
2175 | ||
2176 | This is another step where the presence of the C<//x> modifier is | |
2177 | relevant. The RE engine scans the string from left to right and | |
2178 | converts it to a finite automaton. | |
2179 | ||
2180 | Backslashed characters are either replaced with corresponding | |
2181 | literal strings (as with C<\{>), or else they generate special nodes | |
2182 | in the finite automaton (as with C<\b>). Characters special to the | |
2183 | RE engine (such as C<|>) generate corresponding nodes or groups of | |
2184 | nodes. C<(?#...)> comments are ignored. All the rest is either | |
2185 | converted to literal strings to match, or else is ignored (as is | |
2186 | whitespace and C<#>-style comments if C<//x> is present). | |
2187 | ||
2188 | Parsing of the bracketed character class construct, C<[...]>, is | |
2189 | rather different than the rule used for the rest of the pattern. | |
2190 | The terminator of this construct is found using the same rules as | |
2191 | for finding the terminator of a C<{}>-delimited construct, the only | |
2192 | exception being that C<]> immediately following C<[> is treated as | |
2193 | though preceded by a backslash. Similarly, the terminator of | |
2194 | C<(?{...})> is found using the same rules as for finding the | |
2195 | terminator of a C<{}>-delimited construct. | |
2196 | ||
2197 | It is possible to inspect both the string given to RE engine and the | |
2198 | resulting finite automaton. See the arguments C<debug>/C<debugcolor> | |
2199 | in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line | |
4a4eefd0 | 2200 | switch documented in L<perlrun/"Command Switches">. |
75e14d17 IZ |
2201 | |
2202 | =item Optimization of regular expressions | |
d74e8afc | 2203 | X<regexp, optimization> |
75e14d17 | 2204 | |
7522fed5 | 2205 | This step is listed for completeness only. Since it does not change |
75e14d17 | 2206 | semantics, details of this step are not documented and are subject |
19799a22 GS |
2207 | to change without notice. This step is performed over the finite |
2208 | automaton that was generated during the previous pass. | |
2a94b7ce | 2209 | |
19799a22 GS |
2210 | It is at this stage that C<split()> silently optimizes C</^/> to |
2211 | mean C</^/m>. | |
75e14d17 IZ |
2212 | |
2213 | =back | |
2214 | ||
a0d0e21e | 2215 | =head2 I/O Operators |
d74e8afc ITB |
2216 | X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle> |
2217 | X<< <> >> X<@ARGV> | |
a0d0e21e | 2218 | |
54310121 | 2219 | There are several I/O operators you should know about. |
fbad3eb5 | 2220 | |
7b8d334a | 2221 | A string enclosed by backticks (grave accents) first undergoes |
19799a22 GS |
2222 | double-quote interpolation. It is then interpreted as an external |
2223 | command, and the output of that command is the value of the | |
e9c56f9b JH |
2224 | backtick string, like in a shell. In scalar context, a single string |
2225 | consisting of all output is returned. In list context, a list of | |
2226 | values is returned, one per line of output. (You can set C<$/> to use | |
2227 | a different line terminator.) The command is executed each time the | |
2228 | pseudo-literal is evaluated. The status value of the command is | |
2229 | returned in C<$?> (see L<perlvar> for the interpretation of C<$?>). | |
2230 | Unlike in B<csh>, no translation is done on the return data--newlines | |
2231 | remain newlines. Unlike in any of the shells, single quotes do not | |
2232 | hide variable names in the command from interpretation. To pass a | |
2233 | literal dollar-sign through to the shell you need to hide it with a | |
2234 | backslash. The generalized form of backticks is C<qx//>. (Because | |
2235 | backticks always undergo shell expansion as well, see L<perlsec> for | |
2236 | security concerns.) | |
d74e8afc | 2237 | X<qx> X<`> X<``> X<backtick> X<glob> |
19799a22 GS |
2238 | |
2239 | In scalar context, evaluating a filehandle in angle brackets yields | |
2240 | the next line from that file (the newline, if any, included), or | |
2241 | C<undef> at end-of-file or on error. When C<$/> is set to C<undef> | |
2242 | (sometimes known as file-slurp mode) and the file is empty, it | |
2243 | returns C<''> the first time, followed by C<undef> subsequently. | |
2244 | ||
2245 | Ordinarily you must assign the returned value to a variable, but | |
2246 | there is one situation where an automatic assignment happens. If | |
2247 | and only if the input symbol is the only thing inside the conditional | |
2248 | of a C<while> statement (even if disguised as a C<for(;;)> loop), | |
2249 | the value is automatically assigned to the global variable $_, | |
2250 | destroying whatever was there previously. (This may seem like an | |
2251 | odd thing to you, but you'll use the construct in almost every Perl | |
17b829fa | 2252 | script you write.) The $_ variable is not implicitly localized. |
19799a22 GS |
2253 | You'll have to put a C<local $_;> before the loop if you want that |
2254 | to happen. | |
2255 | ||
2256 | The following lines are equivalent: | |
a0d0e21e | 2257 | |
748a9306 | 2258 | while (defined($_ = <STDIN>)) { print; } |
7b8d334a | 2259 | while ($_ = <STDIN>) { print; } |
a0d0e21e LW |
2260 | while (<STDIN>) { print; } |
2261 | for (;<STDIN>;) { print; } | |
748a9306 | 2262 | print while defined($_ = <STDIN>); |
7b8d334a | 2263 | print while ($_ = <STDIN>); |
a0d0e21e LW |
2264 | print while <STDIN>; |
2265 | ||
19799a22 | 2266 | This also behaves similarly, but avoids $_ : |
7b8d334a | 2267 | |
89d205f2 | 2268 | while (my $line = <STDIN>) { print $line } |
7b8d334a | 2269 | |
19799a22 GS |
2270 | In these loop constructs, the assigned value (whether assignment |
2271 | is automatic or explicit) is then tested to see whether it is | |
2272 | defined. The defined test avoids problems where line has a string | |
2273 | value that would be treated as false by Perl, for example a "" or | |
2274 | a "0" with no trailing newline. If you really mean for such values | |
2275 | to terminate the loop, they should be tested for explicitly: | |
7b8d334a GS |
2276 | |
2277 | while (($_ = <STDIN>) ne '0') { ... } | |
2278 | while (<STDIN>) { last unless $_; ... } | |
2279 | ||
5ef4d93e | 2280 | In other boolean contexts, C<< <filehandle> >> without an |
2281 | explicit C<defined> test or comparison elicits a warning if the | |
9f1b1f2d | 2282 | C<use warnings> pragma or the B<-w> |
19799a22 | 2283 | command-line switch (the C<$^W> variable) is in effect. |
7b8d334a | 2284 | |
5f05dabc | 2285 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
19799a22 GS |
2286 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except |
2287 | in packages, where they would be interpreted as local identifiers | |
2288 | rather than global.) Additional filehandles may be created with | |
2289 | the open() function, amongst others. See L<perlopentut> and | |
2290 | L<perlfunc/open> for details on this. | |
d74e8afc | 2291 | X<stdin> X<stdout> X<sterr> |
a0d0e21e | 2292 | |
35f2feb0 | 2293 | If a <FILEHANDLE> is used in a context that is looking for |
19799a22 GS |
2294 | a list, a list comprising all input lines is returned, one line per |
2295 | list element. It's easy to grow to a rather large data space this | |
2296 | way, so use with care. | |
a0d0e21e | 2297 | |
35f2feb0 | 2298 | <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>. |
19799a22 | 2299 | See L<perlfunc/readline>. |
fbad3eb5 | 2300 | |
35f2feb0 GS |
2301 | The null filehandle <> is special: it can be used to emulate the |
2302 | behavior of B<sed> and B<awk>. Input from <> comes either from | |
a0d0e21e | 2303 | standard input, or from each file listed on the command line. Here's |
35f2feb0 | 2304 | how it works: the first time <> is evaluated, the @ARGV array is |
5a964f20 | 2305 | checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened |
a0d0e21e LW |
2306 | gives you standard input. The @ARGV array is then processed as a list |
2307 | of filenames. The loop | |
2308 | ||
2309 | while (<>) { | |
2310 | ... # code for each line | |
2311 | } | |
2312 | ||
2313 | is equivalent to the following Perl-like pseudo code: | |
2314 | ||
3e3baf6d | 2315 | unshift(@ARGV, '-') unless @ARGV; |
a0d0e21e LW |
2316 | while ($ARGV = shift) { |
2317 | open(ARGV, $ARGV); | |
2318 | while (<ARGV>) { | |
2319 | ... # code for each line | |
2320 | } | |
2321 | } | |
2322 | ||
19799a22 GS |
2323 | except that it isn't so cumbersome to say, and will actually work. |
2324 | It really does shift the @ARGV array and put the current filename | |
2325 | into the $ARGV variable. It also uses filehandle I<ARGV> | |
ac036724 | 2326 | internally. <> is just a synonym for <ARGV>, which |
19799a22 | 2327 | is magical. (The pseudo code above doesn't work because it treats |
35f2feb0 | 2328 | <ARGV> as non-magical.) |
a0d0e21e | 2329 | |
48ab5743 ML |
2330 | Since the null filehandle uses the two argument form of L<perlfunc/open> |
2331 | it interprets special characters, so if you have a script like this: | |
2332 | ||
2333 | while (<>) { | |
2334 | print; | |
2335 | } | |
2336 | ||
2337 | and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a | |
2338 | pipe, executes the C<rm> command and reads C<rm>'s output from that pipe. | |
2339 | If you want all items in C<@ARGV> to be interpreted as file names, you | |
2340 | can use the module C<ARGV::readonly> from CPAN. | |
2341 | ||
35f2feb0 | 2342 | You can modify @ARGV before the first <> as long as the array ends up |
a0d0e21e | 2343 | containing the list of filenames you really want. Line numbers (C<$.>) |
19799a22 GS |
2344 | continue as though the input were one big happy file. See the example |
2345 | in L<perlfunc/eof> for how to reset line numbers on each file. | |
5a964f20 | 2346 | |
89d205f2 | 2347 | If you want to set @ARGV to your own list of files, go right ahead. |
5a964f20 TC |
2348 | This sets @ARGV to all plain text files if no @ARGV was given: |
2349 | ||
2350 | @ARGV = grep { -f && -T } glob('*') unless @ARGV; | |
a0d0e21e | 2351 | |
5a964f20 TC |
2352 | You can even set them to pipe commands. For example, this automatically |
2353 | filters compressed arguments through B<gzip>: | |
2354 | ||
2355 | @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; | |
2356 | ||
2357 | If you want to pass switches into your script, you can use one of the | |
a0d0e21e LW |
2358 | Getopts modules or put a loop on the front like this: |
2359 | ||
2360 | while ($_ = $ARGV[0], /^-/) { | |
2361 | shift; | |
2362 | last if /^--$/; | |
2363 | if (/^-D(.*)/) { $debug = $1 } | |
2364 | if (/^-v/) { $verbose++ } | |
5a964f20 | 2365 | # ... # other switches |
a0d0e21e | 2366 | } |
5a964f20 | 2367 | |
a0d0e21e | 2368 | while (<>) { |
5a964f20 | 2369 | # ... # code for each line |
a0d0e21e LW |
2370 | } |
2371 | ||
89d205f2 YO |
2372 | The <> symbol will return C<undef> for end-of-file only once. |
2373 | If you call it again after this, it will assume you are processing another | |
19799a22 | 2374 | @ARGV list, and if you haven't set @ARGV, will read input from STDIN. |
a0d0e21e | 2375 | |
b159ebd3 | 2376 | If what the angle brackets contain is a simple scalar variable (e.g., |
35f2feb0 | 2377 | <$foo>), then that variable contains the name of the |
19799a22 GS |
2378 | filehandle to input from, or its typeglob, or a reference to the |
2379 | same. For example: | |
cb1a09d0 AD |
2380 | |
2381 | $fh = \*STDIN; | |
2382 | $line = <$fh>; | |
a0d0e21e | 2383 | |
5a964f20 TC |
2384 | If what's within the angle brackets is neither a filehandle nor a simple |
2385 | scalar variable containing a filehandle name, typeglob, or typeglob | |
2386 | reference, it is interpreted as a filename pattern to be globbed, and | |
2387 | either a list of filenames or the next filename in the list is returned, | |
19799a22 | 2388 | depending on context. This distinction is determined on syntactic |
35f2feb0 GS |
2389 | grounds alone. That means C<< <$x> >> is always a readline() from |
2390 | an indirect handle, but C<< <$hash{key}> >> is always a glob(). | |
5a964f20 | 2391 | That's because $x is a simple scalar variable, but C<$hash{key}> is |
ef191992 YST |
2392 | not--it's a hash element. Even C<< <$x > >> (note the extra space) |
2393 | is treated as C<glob("$x ")>, not C<readline($x)>. | |
5a964f20 TC |
2394 | |
2395 | One level of double-quote interpretation is done first, but you can't | |
35f2feb0 | 2396 | say C<< <$foo> >> because that's an indirect filehandle as explained |
5a964f20 TC |
2397 | in the previous paragraph. (In older versions of Perl, programmers |
2398 | would insert curly brackets to force interpretation as a filename glob: | |
35f2feb0 | 2399 | C<< <${foo}> >>. These days, it's considered cleaner to call the |
5a964f20 | 2400 | internal function directly as C<glob($foo)>, which is probably the right |
19799a22 | 2401 | way to have done it in the first place.) For example: |
a0d0e21e LW |
2402 | |
2403 | while (<*.c>) { | |
2404 | chmod 0644, $_; | |
2405 | } | |
2406 | ||
3a4b19e4 | 2407 | is roughly equivalent to: |
a0d0e21e LW |
2408 | |
2409 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); | |
2410 | while (<FOO>) { | |
5b3eff12 | 2411 | chomp; |
a0d0e21e LW |
2412 | chmod 0644, $_; |
2413 | } | |
2414 | ||
3a4b19e4 GS |
2415 | except that the globbing is actually done internally using the standard |
2416 | C<File::Glob> extension. Of course, the shortest way to do the above is: | |
a0d0e21e LW |
2417 | |
2418 | chmod 0644, <*.c>; | |
2419 | ||
19799a22 GS |
2420 | A (file)glob evaluates its (embedded) argument only when it is |
2421 | starting a new list. All values must be read before it will start | |
2422 | over. In list context, this isn't important because you automatically | |
2423 | get them all anyway. However, in scalar context the operator returns | |
069e01df | 2424 | the next value each time it's called, or C<undef> when the list has |
19799a22 GS |
2425 | run out. As with filehandle reads, an automatic C<defined> is |
2426 | generated when the glob occurs in the test part of a C<while>, | |
2427 | because legal glob returns (e.g. a file called F<0>) would otherwise | |
2428 | terminate the loop. Again, C<undef> is returned only once. So if | |
2429 | you're expecting a single value from a glob, it is much better to | |
2430 | say | |
4633a7c4 LW |
2431 | |
2432 | ($file) = <blurch*>; | |
2433 | ||
2434 | than | |
2435 | ||
2436 | $file = <blurch*>; | |
2437 | ||
2438 | because the latter will alternate between returning a filename and | |
19799a22 | 2439 | returning false. |
4633a7c4 | 2440 | |
b159ebd3 | 2441 | If you're trying to do variable interpolation, it's definitely better |
4633a7c4 | 2442 | to use the glob() function, because the older notation can cause people |
e37d713d | 2443 | to become confused with the indirect filehandle notation. |
4633a7c4 LW |
2444 | |
2445 | @files = glob("$dir/*.[ch]"); | |
2446 | @files = glob($files[$i]); | |
2447 | ||
a0d0e21e | 2448 | =head2 Constant Folding |
d74e8afc | 2449 | X<constant folding> X<folding> |
a0d0e21e LW |
2450 | |
2451 | Like C, Perl does a certain amount of expression evaluation at | |
19799a22 | 2452 | compile time whenever it determines that all arguments to an |
a0d0e21e LW |
2453 | operator are static and have no side effects. In particular, string |
2454 | concatenation happens at compile time between literals that don't do | |
19799a22 | 2455 | variable substitution. Backslash interpolation also happens at |
a0d0e21e LW |
2456 | compile time. You can say |
2457 | ||
2458 | 'Now is the time for all' . "\n" . | |
2459 | 'good men to come to.' | |
2460 | ||
54310121 | 2461 | and this all reduces to one string internally. Likewise, if |
a0d0e21e LW |
2462 | you say |
2463 | ||
2464 | foreach $file (@filenames) { | |
5a964f20 | 2465 | if (-s $file > 5 + 100 * 2**16) { } |
54310121 | 2466 | } |
a0d0e21e | 2467 | |
19799a22 GS |
2468 | the compiler will precompute the number which that expression |
2469 | represents so that the interpreter won't have to. | |
a0d0e21e | 2470 | |
fd1abbef | 2471 | =head2 No-ops |
d74e8afc | 2472 | X<no-op> X<nop> |
fd1abbef DN |
2473 | |
2474 | Perl doesn't officially have a no-op operator, but the bare constants | |
2475 | C<0> and C<1> are special-cased to not produce a warning in a void | |
2476 | context, so you can for example safely do | |
2477 | ||
2478 | 1 while foo(); | |
2479 | ||
2c268ad5 | 2480 | =head2 Bitwise String Operators |
d74e8afc | 2481 | X<operator, bitwise, string> |
2c268ad5 TP |
2482 | |
2483 | Bitstrings of any size may be manipulated by the bitwise operators | |
2484 | (C<~ | & ^>). | |
2485 | ||
19799a22 GS |
2486 | If the operands to a binary bitwise op are strings of different |
2487 | sizes, B<|> and B<^> ops act as though the shorter operand had | |
2488 | additional zero bits on the right, while the B<&> op acts as though | |
2489 | the longer operand were truncated to the length of the shorter. | |
2490 | The granularity for such extension or truncation is one or more | |
2491 | bytes. | |
2c268ad5 | 2492 | |
89d205f2 | 2493 | # ASCII-based examples |
2c268ad5 TP |
2494 | print "j p \n" ^ " a h"; # prints "JAPH\n" |
2495 | print "JA" | " ph\n"; # prints "japh\n" | |
2496 | print "japh\nJunk" & '_____'; # prints "JAPH\n"; | |
2497 | print 'p N$' ^ " E<H\n"; # prints "Perl\n"; | |
2498 | ||
19799a22 | 2499 | If you are intending to manipulate bitstrings, be certain that |
2c268ad5 | 2500 | you're supplying bitstrings: If an operand is a number, that will imply |
19799a22 | 2501 | a B<numeric> bitwise operation. You may explicitly show which type of |
2c268ad5 TP |
2502 | operation you intend by using C<""> or C<0+>, as in the examples below. |
2503 | ||
4358a253 SS |
2504 | $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF) |
2505 | $foo = '150' | 105; # yields 255 | |
2c268ad5 TP |
2506 | $foo = 150 | '105'; # yields 255 |
2507 | $foo = '150' | '105'; # yields string '155' (under ASCII) | |
2508 | ||
2509 | $baz = 0+$foo & 0+$bar; # both ops explicitly numeric | |
2510 | $biz = "$foo" ^ "$bar"; # both ops explicitly stringy | |
a0d0e21e | 2511 | |
1ae175c8 GS |
2512 | See L<perlfunc/vec> for information on how to manipulate individual bits |
2513 | in a bit vector. | |
2514 | ||
55497cff | 2515 | =head2 Integer Arithmetic |
d74e8afc | 2516 | X<integer> |
a0d0e21e | 2517 | |
19799a22 | 2518 | By default, Perl assumes that it must do most of its arithmetic in |
a0d0e21e LW |
2519 | floating point. But by saying |
2520 | ||
2521 | use integer; | |
2522 | ||
2523 | you may tell the compiler that it's okay to use integer operations | |
19799a22 GS |
2524 | (if it feels like it) from here to the end of the enclosing BLOCK. |
2525 | An inner BLOCK may countermand this by saying | |
a0d0e21e LW |
2526 | |
2527 | no integer; | |
2528 | ||
19799a22 GS |
2529 | which lasts until the end of that BLOCK. Note that this doesn't |
2530 | mean everything is only an integer, merely that Perl may use integer | |
2531 | operations if it is so inclined. For example, even under C<use | |
2532 | integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731> | |
2533 | or so. | |
2534 | ||
2535 | Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", | |
89d205f2 | 2536 | and ">>") always produce integral results. (But see also |
13a2d996 | 2537 | L<Bitwise String Operators>.) However, C<use integer> still has meaning for |
19799a22 GS |
2538 | them. By default, their results are interpreted as unsigned integers, but |
2539 | if C<use integer> is in effect, their results are interpreted | |
2540 | as signed integers. For example, C<~0> usually evaluates to a large | |
0be96356 | 2541 | integral value. However, C<use integer; ~0> is C<-1> on two's-complement |
19799a22 | 2542 | machines. |
68dc0745 | 2543 | |
2544 | =head2 Floating-point Arithmetic | |
d74e8afc | 2545 | X<floating-point> X<floating point> X<float> X<real> |
68dc0745 | 2546 | |
2547 | While C<use integer> provides integer-only arithmetic, there is no | |
19799a22 GS |
2548 | analogous mechanism to provide automatic rounding or truncation to a |
2549 | certain number of decimal places. For rounding to a certain number | |
2550 | of digits, sprintf() or printf() is usually the easiest route. | |
2551 | See L<perlfaq4>. | |
68dc0745 | 2552 | |
5a964f20 TC |
2553 | Floating-point numbers are only approximations to what a mathematician |
2554 | would call real numbers. There are infinitely more reals than floats, | |
2555 | so some corners must be cut. For example: | |
2556 | ||
2557 | printf "%.20g\n", 123456789123456789; | |
2558 | # produces 123456789123456784 | |
2559 | ||
8548cb57 RGS |
2560 | Testing for exact floating-point equality or inequality is not a |
2561 | good idea. Here's a (relatively expensive) work-around to compare | |
5a964f20 TC |
2562 | whether two floating-point numbers are equal to a particular number of |
2563 | decimal places. See Knuth, volume II, for a more robust treatment of | |
2564 | this topic. | |
2565 | ||
2566 | sub fp_equal { | |
2567 | my ($X, $Y, $POINTS) = @_; | |
2568 | my ($tX, $tY); | |
2569 | $tX = sprintf("%.${POINTS}g", $X); | |
2570 | $tY = sprintf("%.${POINTS}g", $Y); | |
2571 | return $tX eq $tY; | |
2572 | } | |
2573 | ||
68dc0745 | 2574 | The POSIX module (part of the standard perl distribution) implements |
19799a22 GS |
2575 | ceil(), floor(), and other mathematical and trigonometric functions. |
2576 | The Math::Complex module (part of the standard perl distribution) | |
2577 | defines mathematical functions that work on both the reals and the | |
2578 | imaginary numbers. Math::Complex not as efficient as POSIX, but | |
68dc0745 | 2579 | POSIX can't work with complex numbers. |
2580 | ||
2581 | Rounding in financial applications can have serious implications, and | |
2582 | the rounding method used should be specified precisely. In these | |
2583 | cases, it probably pays not to trust whichever system rounding is | |
2584 | being used by Perl, but to instead implement the rounding function you | |
2585 | need yourself. | |
5a964f20 TC |
2586 | |
2587 | =head2 Bigger Numbers | |
d74e8afc | 2588 | X<number, arbitrary precision> |
5a964f20 TC |
2589 | |
2590 | The standard Math::BigInt and Math::BigFloat modules provide | |
19799a22 | 2591 | variable-precision arithmetic and overloaded operators, although |
cd5c4fce | 2592 | they're currently pretty slow. At the cost of some space and |
19799a22 GS |
2593 | considerable speed, they avoid the normal pitfalls associated with |
2594 | limited-precision representations. | |
5a964f20 TC |
2595 | |
2596 | use Math::BigInt; | |
2597 | $x = Math::BigInt->new('123456789123456789'); | |
2598 | print $x * $x; | |
2599 | ||
2600 | # prints +15241578780673678515622620750190521 | |
19799a22 | 2601 | |
cd5c4fce T |
2602 | There are several modules that let you calculate with (bound only by |
2603 | memory and cpu-time) unlimited or fixed precision. There are also | |
2604 | some non-standard modules that provide faster implementations via | |
2605 | external C libraries. | |
2606 | ||
2607 | Here is a short, but incomplete summary: | |
2608 | ||
2609 | Math::Fraction big, unlimited fractions like 9973 / 12967 | |
2610 | Math::String treat string sequences like numbers | |
2611 | Math::FixedPrecision calculate with a fixed precision | |
2612 | Math::Currency for currency calculations | |
2613 | Bit::Vector manipulate bit vectors fast (uses C) | |
2614 | Math::BigIntFast Bit::Vector wrapper for big numbers | |
2615 | Math::Pari provides access to the Pari C library | |
2616 | Math::BigInteger uses an external C library | |
2617 | Math::Cephes uses external Cephes C library (no big numbers) | |
2618 | Math::Cephes::Fraction fractions via the Cephes library | |
2619 | Math::GMP another one using an external C library | |
2620 | ||
2621 | Choose wisely. | |
16070b82 GS |
2622 | |
2623 | =cut |