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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perlop - Perl operators and precedence | |
4 | ||
5 | =head1 SYNOPSIS | |
6 | ||
7 | Perl operators have the following associativity and precedence, | |
8 | listed from highest precedence to lowest. Note that all operators | |
9 | borrowed from C keep the same precedence relationship with each other, | |
10 | even where C's precedence is slightly screwy. (This makes learning | |
c07a80fd | 11 | Perl easier for C folks.) With very few exceptions, these all |
12 | operate on scalar values only, not array values. | |
a0d0e21e LW |
13 | |
14 | left terms and list operators (leftward) | |
15 | left -> | |
16 | nonassoc ++ -- | |
17 | right ** | |
18 | right ! ~ \ and unary + and - | |
19 | left =~ !~ | |
20 | left * / % x | |
21 | left + - . | |
22 | left << >> | |
23 | nonassoc named unary operators | |
24 | nonassoc < > <= >= lt gt le ge | |
25 | nonassoc == != <=> eq ne cmp | |
26 | left & | |
27 | left | ^ | |
28 | left && | |
29 | left || | |
30 | nonassoc .. | |
31 | right ?: | |
32 | right = += -= *= etc. | |
33 | left , => | |
34 | nonassoc list operators (rightward) | |
a5f75d66 | 35 | right not |
a0d0e21e LW |
36 | left and |
37 | left or xor | |
38 | ||
39 | In the following sections, these operators are covered in precedence order. | |
40 | ||
cb1a09d0 | 41 | =head1 DESCRIPTION |
a0d0e21e LW |
42 | |
43 | =head2 Terms and List Operators (Leftward) | |
44 | ||
45 | Any TERM is of highest precedence of Perl. These includes variables, | |
5f05dabc | 46 | quote and quote-like operators, any expression in parentheses, |
a0d0e21e LW |
47 | and any function whose arguments are parenthesized. Actually, there |
48 | aren't really functions in this sense, just list operators and unary | |
49 | operators behaving as functions because you put parentheses around | |
50 | the arguments. These are all documented in L<perlfunc>. | |
51 | ||
52 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
53 | is followed by a left parenthesis as the next token, the operator and | |
54 | arguments within parentheses are taken to be of highest precedence, | |
55 | just like a normal function call. | |
56 | ||
57 | In the absence of parentheses, the precedence of list operators such as | |
58 | C<print>, C<sort>, or C<chmod> is either very high or very low depending on | |
59 | whether you look at the left side of operator or the right side of it. | |
60 | For example, in | |
61 | ||
62 | @ary = (1, 3, sort 4, 2); | |
63 | print @ary; # prints 1324 | |
64 | ||
65 | the commas on the right of the sort are evaluated before the sort, but | |
66 | the commas on the left are evaluated after. In other words, list | |
67 | operators tend to gobble up all the arguments that follow them, and | |
68 | then act like a simple TERM with regard to the preceding expression. | |
5f05dabc | 69 | Note that you have to be careful with parentheses: |
a0d0e21e LW |
70 | |
71 | # These evaluate exit before doing the print: | |
72 | print($foo, exit); # Obviously not what you want. | |
73 | print $foo, exit; # Nor is this. | |
74 | ||
75 | # These do the print before evaluating exit: | |
76 | (print $foo), exit; # This is what you want. | |
77 | print($foo), exit; # Or this. | |
78 | print ($foo), exit; # Or even this. | |
79 | ||
80 | Also note that | |
81 | ||
82 | print ($foo & 255) + 1, "\n"; | |
83 | ||
84 | probably doesn't do what you expect at first glance. See | |
85 | L<Named Unary Operators> for more discussion of this. | |
86 | ||
87 | Also parsed as terms are the C<do {}> and C<eval {}> constructs, as | |
88 | well as subroutine and method calls, and the anonymous | |
89 | constructors C<[]> and C<{}>. | |
90 | ||
5f05dabc | 91 | See also L<Quote and Quote-Like Operators> toward the end of this section, |
c07a80fd | 92 | as well as L<"I/O Operators">. |
a0d0e21e LW |
93 | |
94 | =head2 The Arrow Operator | |
95 | ||
96 | Just as in C and C++, "C<-E<gt>>" is an infix dereference operator. If the | |
97 | right side is either a C<[...]> or C<{...}> subscript, then the left side | |
98 | must be either a hard or symbolic reference to an array or hash (or | |
99 | a location capable of holding a hard reference, if it's an lvalue (assignable)). | |
100 | See L<perlref>. | |
101 | ||
102 | Otherwise, the right side is a method name or a simple scalar variable | |
103 | containing the method name, and the left side must either be an object | |
104 | (a blessed reference) or a class name (that is, a package name). | |
105 | See L<perlobj>. | |
106 | ||
5f05dabc | 107 | =head2 Auto-increment and Auto-decrement |
a0d0e21e LW |
108 | |
109 | "++" and "--" work as in C. That is, if placed before a variable, they | |
110 | increment or decrement the variable before returning the value, and if | |
111 | placed after, increment or decrement the variable after returning the value. | |
112 | ||
5f05dabc | 113 | The auto-increment operator has a little extra built-in magic to it. If |
a0d0e21e LW |
114 | you increment a variable that is numeric, or that has ever been used in |
115 | a numeric context, you get a normal increment. If, however, the | |
5f05dabc | 116 | variable has been used in only string contexts since it was set, and |
a0d0e21e LW |
117 | has a value that is not null and matches the pattern |
118 | C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each | |
119 | character within its range, with carry: | |
120 | ||
121 | print ++($foo = '99'); # prints '100' | |
122 | print ++($foo = 'a0'); # prints 'a1' | |
123 | print ++($foo = 'Az'); # prints 'Ba' | |
124 | print ++($foo = 'zz'); # prints 'aaa' | |
125 | ||
5f05dabc | 126 | The auto-decrement operator is not magical. |
a0d0e21e LW |
127 | |
128 | =head2 Exponentiation | |
129 | ||
130 | Binary "**" is the exponentiation operator. Note that it binds even more | |
cb1a09d0 AD |
131 | tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is |
132 | implemented using C's pow(3) function, which actually works on doubles | |
133 | internally.) | |
a0d0e21e LW |
134 | |
135 | =head2 Symbolic Unary Operators | |
136 | ||
5f05dabc | 137 | Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower |
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138 | precedence version of this. |
139 | ||
140 | Unary "-" performs arithmetic negation if the operand is numeric. If | |
141 | the operand is an identifier, a string consisting of a minus sign | |
142 | concatenated with the identifier is returned. Otherwise, if the string | |
143 | starts with a plus or minus, a string starting with the opposite sign | |
144 | is returned. One effect of these rules is that C<-bareword> is equivalent | |
145 | to C<"-bareword">. | |
146 | ||
5f05dabc | 147 | Unary "~" performs bitwise negation, i.e., 1's complement. |
55497cff | 148 | (See also L<Integer Arithmetic>.) |
a0d0e21e LW |
149 | |
150 | Unary "+" has no effect whatsoever, even on strings. It is useful | |
151 | syntactically for separating a function name from a parenthesized expression | |
152 | that would otherwise be interpreted as the complete list of function | |
5ba421f6 | 153 | arguments. (See examples above under L<Terms and List Operators (Leftward)>.) |
a0d0e21e LW |
154 | |
155 | Unary "\" creates a reference to whatever follows it. See L<perlref>. | |
156 | Do not confuse this behavior with the behavior of backslash within a | |
157 | string, although both forms do convey the notion of protecting the next | |
158 | thing from interpretation. | |
159 | ||
160 | =head2 Binding Operators | |
161 | ||
c07a80fd | 162 | Binary "=~" binds a scalar expression to a pattern match. Certain operations |
cb1a09d0 AD |
163 | search or modify the string $_ by default. This operator makes that kind |
164 | of operation work on some other string. The right argument is a search | |
165 | pattern, substitution, or translation. The left argument is what is | |
166 | supposed to be searched, substituted, or translated instead of the default | |
167 | $_. The return value indicates the success of the operation. (If the | |
168 | right argument is an expression rather than a search pattern, | |
169 | substitution, or translation, it is interpreted as a search pattern at run | |
5f05dabc | 170 | time. This is less efficient than an explicit search, because the pattern |
cb1a09d0 AD |
171 | must be compiled every time the expression is evaluated--unless you've |
172 | used C</o>.) | |
a0d0e21e LW |
173 | |
174 | Binary "!~" is just like "=~" except the return value is negated in | |
175 | the logical sense. | |
176 | ||
177 | =head2 Multiplicative Operators | |
178 | ||
179 | Binary "*" multiplies two numbers. | |
180 | ||
181 | Binary "/" divides two numbers. | |
182 | ||
183 | Binary "%" computes the modulus of the two numbers. | |
184 | ||
185 | Binary "x" is the repetition operator. In a scalar context, it | |
186 | returns a string consisting of the left operand repeated the number of | |
187 | times specified by the right operand. In a list context, if the left | |
5f05dabc | 188 | operand is a list in parentheses, it repeats the list. |
a0d0e21e LW |
189 | |
190 | print '-' x 80; # print row of dashes | |
191 | ||
192 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over | |
193 | ||
194 | @ones = (1) x 80; # a list of 80 1's | |
195 | @ones = (5) x @ones; # set all elements to 5 | |
196 | ||
197 | ||
198 | =head2 Additive Operators | |
199 | ||
200 | Binary "+" returns the sum of two numbers. | |
201 | ||
202 | Binary "-" returns the difference of two numbers. | |
203 | ||
204 | Binary "." concatenates two strings. | |
205 | ||
206 | =head2 Shift Operators | |
207 | ||
55497cff | 208 | Binary "<<" returns the value of its left argument shifted left by the |
209 | number of bits specified by the right argument. Arguments should be | |
210 | integers. (See also L<Integer Arithmetic>.) | |
a0d0e21e | 211 | |
55497cff | 212 | Binary ">>" returns the value of its left argument shifted right by |
213 | the number of bits specified by the right argument. Arguments should | |
214 | be integers. (See also L<Integer Arithmetic>.) | |
a0d0e21e LW |
215 | |
216 | =head2 Named Unary Operators | |
217 | ||
218 | The various named unary operators are treated as functions with one | |
219 | argument, with optional parentheses. These include the filetest | |
220 | operators, like C<-f>, C<-M>, etc. See L<perlfunc>. | |
221 | ||
222 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
223 | is followed by a left parenthesis as the next token, the operator and | |
224 | arguments within parentheses are taken to be of highest precedence, | |
225 | just like a normal function call. Examples: | |
226 | ||
227 | chdir $foo || die; # (chdir $foo) || die | |
228 | chdir($foo) || die; # (chdir $foo) || die | |
229 | chdir ($foo) || die; # (chdir $foo) || die | |
230 | chdir +($foo) || die; # (chdir $foo) || die | |
231 | ||
232 | but, because * is higher precedence than ||: | |
233 | ||
234 | chdir $foo * 20; # chdir ($foo * 20) | |
235 | chdir($foo) * 20; # (chdir $foo) * 20 | |
236 | chdir ($foo) * 20; # (chdir $foo) * 20 | |
237 | chdir +($foo) * 20; # chdir ($foo * 20) | |
238 | ||
239 | rand 10 * 20; # rand (10 * 20) | |
240 | rand(10) * 20; # (rand 10) * 20 | |
241 | rand (10) * 20; # (rand 10) * 20 | |
242 | rand +(10) * 20; # rand (10 * 20) | |
243 | ||
5ba421f6 | 244 | See also L<"Terms and List Operators (Leftward)">. |
a0d0e21e LW |
245 | |
246 | =head2 Relational Operators | |
247 | ||
6ee5d4e7 | 248 | Binary "E<lt>" returns true if the left argument is numerically less than |
a0d0e21e LW |
249 | the right argument. |
250 | ||
6ee5d4e7 | 251 | Binary "E<gt>" returns true if the left argument is numerically greater |
a0d0e21e LW |
252 | than the right argument. |
253 | ||
6ee5d4e7 | 254 | Binary "E<lt>=" returns true if the left argument is numerically less than |
a0d0e21e LW |
255 | or equal to the right argument. |
256 | ||
6ee5d4e7 | 257 | Binary "E<gt>=" returns true if the left argument is numerically greater |
a0d0e21e LW |
258 | than or equal to the right argument. |
259 | ||
260 | Binary "lt" returns true if the left argument is stringwise less than | |
261 | the right argument. | |
262 | ||
263 | Binary "gt" returns true if the left argument is stringwise greater | |
264 | than the right argument. | |
265 | ||
266 | Binary "le" returns true if the left argument is stringwise less than | |
267 | or equal to the right argument. | |
268 | ||
269 | Binary "ge" returns true if the left argument is stringwise greater | |
270 | than or equal to the right argument. | |
271 | ||
272 | =head2 Equality Operators | |
273 | ||
274 | Binary "==" returns true if the left argument is numerically equal to | |
275 | the right argument. | |
276 | ||
277 | Binary "!=" returns true if the left argument is numerically not equal | |
278 | to the right argument. | |
279 | ||
6ee5d4e7 | 280 | Binary "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left |
281 | argument is numerically less than, equal to, or greater than the right | |
282 | argument. | |
a0d0e21e LW |
283 | |
284 | Binary "eq" returns true if the left argument is stringwise equal to | |
285 | the right argument. | |
286 | ||
287 | Binary "ne" returns true if the left argument is stringwise not equal | |
288 | to the right argument. | |
289 | ||
290 | Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise | |
291 | less than, equal to, or greater than the right argument. | |
292 | ||
a034a98d DD |
293 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
294 | by the current locale if C<use locale> is in effect. See L<perllocale>. | |
295 | ||
a0d0e21e LW |
296 | =head2 Bitwise And |
297 | ||
298 | Binary "&" returns its operators ANDed together bit by bit. | |
55497cff | 299 | (See also L<Integer Arithmetic>.) |
a0d0e21e LW |
300 | |
301 | =head2 Bitwise Or and Exclusive Or | |
302 | ||
303 | Binary "|" returns its operators ORed together bit by bit. | |
55497cff | 304 | (See also L<Integer Arithmetic>.) |
a0d0e21e LW |
305 | |
306 | Binary "^" returns its operators XORed together bit by bit. | |
55497cff | 307 | (See also L<Integer Arithmetic>.) |
a0d0e21e LW |
308 | |
309 | =head2 C-style Logical And | |
310 | ||
311 | Binary "&&" performs a short-circuit logical AND operation. That is, | |
312 | if the left operand is false, the right operand is not even evaluated. | |
313 | Scalar or list context propagates down to the right operand if it | |
314 | is evaluated. | |
315 | ||
316 | =head2 C-style Logical Or | |
317 | ||
318 | Binary "||" performs a short-circuit logical OR operation. That is, | |
319 | if the left operand is true, the right operand is not even evaluated. | |
320 | Scalar or list context propagates down to the right operand if it | |
321 | is evaluated. | |
322 | ||
323 | The C<||> and C<&&> operators differ from C's in that, rather than returning | |
324 | 0 or 1, they return the last value evaluated. Thus, a reasonably portable | |
325 | way to find out the home directory (assuming it's not "0") might be: | |
326 | ||
327 | $home = $ENV{'HOME'} || $ENV{'LOGDIR'} || | |
328 | (getpwuid($<))[7] || die "You're homeless!\n"; | |
329 | ||
330 | As more readable alternatives to C<&&> and C<||>, Perl provides "and" and | |
331 | "or" operators (see below). The short-circuit behavior is identical. The | |
332 | precedence of "and" and "or" is much lower, however, so that you can | |
333 | safely use them after a list operator without the need for | |
334 | parentheses: | |
335 | ||
336 | unlink "alpha", "beta", "gamma" | |
337 | or gripe(), next LINE; | |
338 | ||
339 | With the C-style operators that would have been written like this: | |
340 | ||
341 | unlink("alpha", "beta", "gamma") | |
342 | || (gripe(), next LINE); | |
343 | ||
344 | =head2 Range Operator | |
345 | ||
346 | Binary ".." is the range operator, which is really two different | |
347 | operators depending on the context. In a list context, it returns an | |
348 | array of values counting (by ones) from the left value to the right | |
349 | value. This is useful for writing C<for (1..10)> loops and for doing | |
350 | slice operations on arrays. Be aware that under the current implementation, | |
351 | a temporary array is created, so you'll burn a lot of memory if you | |
352 | write something like this: | |
353 | ||
354 | for (1 .. 1_000_000) { | |
355 | # code | |
356 | } | |
357 | ||
358 | In a scalar context, ".." returns a boolean value. The operator is | |
359 | bistable, like a flip-flop, and emulates the line-range (comma) operator | |
360 | of B<sed>, B<awk>, and various editors. Each ".." operator maintains its | |
361 | own boolean state. It is false as long as its left operand is false. | |
362 | Once the left operand is true, the range operator stays true until the | |
363 | right operand is true, I<AFTER> which the range operator becomes false | |
364 | again. (It doesn't become false till the next time the range operator is | |
365 | evaluated. It can test the right operand and become false on the same | |
366 | evaluation it became true (as in B<awk>), but it still returns true once. | |
367 | If you don't want it to test the right operand till the next evaluation | |
368 | (as in B<sed>), use three dots ("...") instead of two.) The right | |
369 | operand is not evaluated while the operator is in the "false" state, and | |
370 | the left operand is not evaluated while the operator is in the "true" | |
371 | state. The precedence is a little lower than || and &&. The value | |
372 | returned is either the null string for false, or a sequence number | |
373 | (beginning with 1) for true. The sequence number is reset for each range | |
374 | encountered. The final sequence number in a range has the string "E0" | |
375 | appended to it, which doesn't affect its numeric value, but gives you | |
376 | something to search for if you want to exclude the endpoint. You can | |
377 | exclude the beginning point by waiting for the sequence number to be | |
378 | greater than 1. If either operand of scalar ".." is a numeric literal, | |
379 | that operand is implicitly compared to the C<$.> variable, the current | |
380 | line number. Examples: | |
381 | ||
382 | As a scalar operator: | |
383 | ||
384 | if (101 .. 200) { print; } # print 2nd hundred lines | |
385 | next line if (1 .. /^$/); # skip header lines | |
386 | s/^/> / if (/^$/ .. eof()); # quote body | |
387 | ||
388 | As a list operator: | |
389 | ||
390 | for (101 .. 200) { print; } # print $_ 100 times | |
391 | @foo = @foo[$[ .. $#foo]; # an expensive no-op | |
392 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items | |
393 | ||
394 | The range operator (in a list context) makes use of the magical | |
5f05dabc | 395 | auto-increment algorithm if the operands are strings. You |
a0d0e21e LW |
396 | can say |
397 | ||
398 | @alphabet = ('A' .. 'Z'); | |
399 | ||
400 | to get all the letters of the alphabet, or | |
401 | ||
402 | $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; | |
403 | ||
404 | to get a hexadecimal digit, or | |
405 | ||
406 | @z2 = ('01' .. '31'); print $z2[$mday]; | |
407 | ||
408 | to get dates with leading zeros. If the final value specified is not | |
409 | in the sequence that the magical increment would produce, the sequence | |
410 | goes until the next value would be longer than the final value | |
411 | specified. | |
412 | ||
413 | =head2 Conditional Operator | |
414 | ||
415 | Ternary "?:" is the conditional operator, just as in C. It works much | |
416 | like an if-then-else. If the argument before the ? is true, the | |
417 | argument before the : is returned, otherwise the argument after the : | |
cb1a09d0 AD |
418 | is returned. For example: |
419 | ||
420 | printf "I have %d dog%s.\n", $n, | |
421 | ($n == 1) ? '' : "s"; | |
422 | ||
423 | Scalar or list context propagates downward into the 2nd | |
424 | or 3rd argument, whichever is selected. | |
425 | ||
426 | $a = $ok ? $b : $c; # get a scalar | |
427 | @a = $ok ? @b : @c; # get an array | |
428 | $a = $ok ? @b : @c; # oops, that's just a count! | |
429 | ||
430 | The operator may be assigned to if both the 2nd and 3rd arguments are | |
431 | legal lvalues (meaning that you can assign to them): | |
a0d0e21e LW |
432 | |
433 | ($a_or_b ? $a : $b) = $c; | |
434 | ||
cb1a09d0 | 435 | This is not necessarily guaranteed to contribute to the readability of your program. |
a0d0e21e | 436 | |
4633a7c4 | 437 | =head2 Assignment Operators |
a0d0e21e LW |
438 | |
439 | "=" is the ordinary assignment operator. | |
440 | ||
441 | Assignment operators work as in C. That is, | |
442 | ||
443 | $a += 2; | |
444 | ||
445 | is equivalent to | |
446 | ||
447 | $a = $a + 2; | |
448 | ||
449 | although without duplicating any side effects that dereferencing the lvalue | |
450 | might trigger, such as from tie(). Other assignment operators work similarly. | |
451 | The following are recognized: | |
452 | ||
453 | **= += *= &= <<= &&= | |
454 | -= /= |= >>= ||= | |
455 | .= %= ^= | |
456 | x= | |
457 | ||
458 | Note that while these are grouped by family, they all have the precedence | |
459 | of assignment. | |
460 | ||
461 | Unlike in C, the assignment operator produces a valid lvalue. Modifying | |
462 | an assignment is equivalent to doing the assignment and then modifying | |
463 | the variable that was assigned to. This is useful for modifying | |
464 | a copy of something, like this: | |
465 | ||
466 | ($tmp = $global) =~ tr [A-Z] [a-z]; | |
467 | ||
468 | Likewise, | |
469 | ||
470 | ($a += 2) *= 3; | |
471 | ||
472 | is equivalent to | |
473 | ||
474 | $a += 2; | |
475 | $a *= 3; | |
476 | ||
748a9306 | 477 | =head2 Comma Operator |
a0d0e21e LW |
478 | |
479 | Binary "," is the comma operator. In a scalar context it evaluates | |
480 | its left argument, throws that value away, then evaluates its right | |
481 | argument and returns that value. This is just like C's comma operator. | |
482 | ||
483 | In a list context, it's just the list argument separator, and inserts | |
484 | both its arguments into the list. | |
485 | ||
6ee5d4e7 | 486 | The =E<gt> digraph is mostly just a synonym for the comma operator. It's useful for |
cb1a09d0 | 487 | documenting arguments that come in pairs. As of release 5.001, it also forces |
4633a7c4 | 488 | any word to the left of it to be interpreted as a string. |
748a9306 | 489 | |
a0d0e21e LW |
490 | =head2 List Operators (Rightward) |
491 | ||
492 | On the right side of a list operator, it has very low precedence, | |
493 | such that it controls all comma-separated expressions found there. | |
494 | The only operators with lower precedence are the logical operators | |
495 | "and", "or", and "not", which may be used to evaluate calls to list | |
496 | operators without the need for extra parentheses: | |
497 | ||
498 | open HANDLE, "filename" | |
499 | or die "Can't open: $!\n"; | |
500 | ||
5ba421f6 | 501 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
a0d0e21e LW |
502 | |
503 | =head2 Logical Not | |
504 | ||
505 | Unary "not" returns the logical negation of the expression to its right. | |
506 | It's the equivalent of "!" except for the very low precedence. | |
507 | ||
508 | =head2 Logical And | |
509 | ||
510 | Binary "and" returns the logical conjunction of the two surrounding | |
511 | expressions. It's equivalent to && except for the very low | |
5f05dabc | 512 | precedence. This means that it short-circuits: i.e., the right |
a0d0e21e LW |
513 | expression is evaluated only if the left expression is true. |
514 | ||
515 | =head2 Logical or and Exclusive Or | |
516 | ||
517 | Binary "or" returns the logical disjunction of the two surrounding | |
518 | expressions. It's equivalent to || except for the very low | |
5f05dabc | 519 | precedence. This means that it short-circuits: i.e., the right |
a0d0e21e LW |
520 | expression is evaluated only if the left expression is false. |
521 | ||
522 | Binary "xor" returns the exclusive-OR of the two surrounding expressions. | |
523 | It cannot short circuit, of course. | |
524 | ||
525 | =head2 C Operators Missing From Perl | |
526 | ||
527 | Here is what C has that Perl doesn't: | |
528 | ||
529 | =over 8 | |
530 | ||
531 | =item unary & | |
532 | ||
533 | Address-of operator. (But see the "\" operator for taking a reference.) | |
534 | ||
535 | =item unary * | |
536 | ||
537 | Dereference-address operator. (Perl's prefix dereferencing | |
538 | operators are typed: $, @, %, and &.) | |
539 | ||
540 | =item (TYPE) | |
541 | ||
542 | Type casting operator. | |
543 | ||
544 | =back | |
545 | ||
5f05dabc | 546 | =head2 Quote and Quote-like Operators |
a0d0e21e LW |
547 | |
548 | While we usually think of quotes as literal values, in Perl they | |
549 | function as operators, providing various kinds of interpolating and | |
550 | pattern matching capabilities. Perl provides customary quote characters | |
551 | for these behaviors, but also provides a way for you to choose your | |
552 | quote character for any of them. In the following table, a C<{}> represents | |
553 | any pair of delimiters you choose. Non-bracketing delimiters use | |
554 | the same character fore and aft, but the 4 sorts of brackets | |
555 | (round, angle, square, curly) will all nest. | |
556 | ||
557 | Customary Generic Meaning Interpolates | |
558 | '' q{} Literal no | |
559 | "" qq{} Literal yes | |
560 | `` qx{} Command yes | |
561 | qw{} Word list no | |
562 | // m{} Pattern match yes | |
563 | s{}{} Substitution yes | |
564 | tr{}{} Translation no | |
565 | ||
cb1a09d0 | 566 | For constructs that do interpolation, variables beginning with "C<$>" or "C<@>" |
a0d0e21e LW |
567 | are interpolated, as are the following sequences: |
568 | ||
6ee5d4e7 | 569 | \t tab (HT, TAB) |
570 | \n newline (LF, NL) | |
571 | \r return (CR) | |
572 | \f form feed (FF) | |
573 | \b backspace (BS) | |
574 | \a alarm (bell) (BEL) | |
575 | \e escape (ESC) | |
a0d0e21e LW |
576 | \033 octal char |
577 | \x1b hex char | |
578 | \c[ control char | |
579 | \l lowercase next char | |
580 | \u uppercase next char | |
581 | \L lowercase till \E | |
582 | \U uppercase till \E | |
583 | \E end case modification | |
584 | \Q quote regexp metacharacters till \E | |
585 | ||
a034a98d DD |
586 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> |
587 | and <\U> is taken from the current locale. See L<perllocale>. | |
588 | ||
a0d0e21e LW |
589 | Patterns are subject to an additional level of interpretation as a |
590 | regular expression. This is done as a second pass, after variables are | |
591 | interpolated, so that regular expressions may be incorporated into the | |
592 | pattern from the variables. If this is not what you want, use C<\Q> to | |
593 | interpolate a variable literally. | |
594 | ||
595 | Apart from the above, there are no multiple levels of interpolation. In | |
5f05dabc | 596 | particular, contrary to the expectations of shell programmers, back-quotes |
a0d0e21e LW |
597 | do I<NOT> interpolate within double quotes, nor do single quotes impede |
598 | evaluation of variables when used within double quotes. | |
599 | ||
5f05dabc | 600 | =head2 Regexp Quote-Like Operators |
cb1a09d0 | 601 | |
5f05dabc | 602 | Here are the quote-like operators that apply to pattern |
cb1a09d0 AD |
603 | matching and related activities. |
604 | ||
a0d0e21e LW |
605 | =over 8 |
606 | ||
607 | =item ?PATTERN? | |
608 | ||
609 | This is just like the C</pattern/> search, except that it matches only | |
610 | once between calls to the reset() operator. This is a useful | |
5f05dabc | 611 | optimization when you want to see only the first occurrence of |
a0d0e21e LW |
612 | something in each file of a set of files, for instance. Only C<??> |
613 | patterns local to the current package are reset. | |
614 | ||
615 | This usage is vaguely deprecated, and may be removed in some future | |
616 | version of Perl. | |
617 | ||
618 | =item m/PATTERN/gimosx | |
619 | ||
620 | =item /PATTERN/gimosx | |
621 | ||
622 | Searches a string for a pattern match, and in a scalar context returns | |
623 | true (1) or false (''). If no string is specified via the C<=~> or | |
624 | C<!~> operator, the $_ string is searched. (The string specified with | |
625 | C<=~> need not be an lvalue--it may be the result of an expression | |
626 | evaluation, but remember the C<=~> binds rather tightly.) See also | |
627 | L<perlre>. | |
a034a98d DD |
628 | See L<perllocale> for discussion of additional considerations which apply |
629 | when C<use locale> is in effect. | |
a0d0e21e LW |
630 | |
631 | Options are: | |
632 | ||
5f05dabc | 633 | g Match globally, i.e., find all occurrences. |
a0d0e21e LW |
634 | i Do case-insensitive pattern matching. |
635 | m Treat string as multiple lines. | |
5f05dabc | 636 | o Compile pattern only once. |
a0d0e21e LW |
637 | s Treat string as single line. |
638 | x Use extended regular expressions. | |
639 | ||
640 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> | |
641 | you can use any pair of non-alphanumeric, non-whitespace characters as | |
642 | delimiters. This is particularly useful for matching Unix path names | |
643 | that contain "/", to avoid LTS (leaning toothpick syndrome). | |
644 | ||
645 | PATTERN may contain variables, which will be interpolated (and the | |
646 | pattern recompiled) every time the pattern search is evaluated. (Note | |
647 | that C<$)> and C<$|> might not be interpolated because they look like | |
648 | end-of-string tests.) If you want such a pattern to be compiled only | |
649 | once, add a C</o> after the trailing delimiter. This avoids expensive | |
650 | run-time recompilations, and is useful when the value you are | |
651 | interpolating won't change over the life of the script. However, mentioning | |
652 | C</o> constitutes a promise that you won't change the variables in the pattern. | |
653 | If you change them, Perl won't even notice. | |
654 | ||
4633a7c4 LW |
655 | If the PATTERN evaluates to a null string, the last |
656 | successfully executed regular expression is used instead. | |
a0d0e21e LW |
657 | |
658 | If used in a context that requires a list value, a pattern match returns a | |
659 | list consisting of the subexpressions matched by the parentheses in the | |
5f05dabc | 660 | pattern, i.e., (C<$1>, $2, $3...). (Note that here $1 etc. are also set, and |
a0d0e21e LW |
661 | that this differs from Perl 4's behavior.) If the match fails, a null |
662 | array is returned. If the match succeeds, but there were no parentheses, | |
663 | a list value of (1) is returned. | |
664 | ||
665 | Examples: | |
666 | ||
667 | open(TTY, '/dev/tty'); | |
668 | <TTY> =~ /^y/i && foo(); # do foo if desired | |
669 | ||
670 | if (/Version: *([0-9.]*)/) { $version = $1; } | |
671 | ||
672 | next if m#^/usr/spool/uucp#; | |
673 | ||
674 | # poor man's grep | |
675 | $arg = shift; | |
676 | while (<>) { | |
677 | print if /$arg/o; # compile only once | |
678 | } | |
679 | ||
680 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) | |
681 | ||
682 | This last example splits $foo into the first two words and the | |
5f05dabc | 683 | remainder of the line, and assigns those three fields to $F1, $F2, and |
684 | $Etc. The conditional is true if any variables were assigned, i.e., if | |
a0d0e21e LW |
685 | the pattern matched. |
686 | ||
687 | The C</g> modifier specifies global pattern matching--that is, matching | |
688 | as many times as possible within the string. How it behaves depends on | |
689 | the context. In a list context, it returns a list of all the | |
690 | substrings matched by all the parentheses in the regular expression. | |
691 | If there are no parentheses, it returns a list of all the matched | |
692 | strings, as if there were parentheses around the whole pattern. | |
693 | ||
694 | In a scalar context, C<m//g> iterates through the string, returning TRUE | |
695 | each time it matches, and FALSE when it eventually runs out of | |
696 | matches. (In other words, it remembers where it left off last time and | |
697 | restarts the search at that point. You can actually find the current | |
44a8e56a | 698 | match position of a string or set it using the pos() function--see |
699 | L<perlfunc/pos>.) Note that you can use this feature to stack C<m//g> | |
700 | matches or intermix C<m//g> matches with C<m/\G.../>. | |
701 | ||
a0d0e21e LW |
702 | If you modify the string in any way, the match position is reset to the |
703 | beginning. Examples: | |
704 | ||
705 | # list context | |
706 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); | |
707 | ||
708 | # scalar context | |
5f05dabc | 709 | $/ = ""; $* = 1; # $* deprecated in modern perls |
a0d0e21e LW |
710 | while ($paragraph = <>) { |
711 | while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { | |
712 | $sentences++; | |
713 | } | |
714 | } | |
715 | print "$sentences\n"; | |
716 | ||
44a8e56a | 717 | # using m//g with \G |
718 | $_ = "ppooqppq"; | |
719 | while ($i++ < 2) { | |
720 | print "1: '"; | |
721 | print $1 while /(o)/g; print "', pos=", pos, "\n"; | |
722 | print "2: '"; | |
723 | print $1 if /\G(q)/; print "', pos=", pos, "\n"; | |
724 | print "3: '"; | |
725 | print $1 while /(p)/g; print "', pos=", pos, "\n"; | |
726 | } | |
727 | ||
728 | The last example should print: | |
729 | ||
730 | 1: 'oo', pos=4 | |
731 | 2: 'q', pos=4 | |
732 | 3: 'pp', pos=7 | |
733 | 1: '', pos=7 | |
734 | 2: 'q', pos=7 | |
735 | 3: '', pos=7 | |
736 | ||
737 | Note how C<m//g> matches change the value reported by C<pos()>, but the | |
738 | non-global match doesn't. | |
739 | ||
e7ea3e70 IZ |
740 | A useful idiom for C<lex>-like scanners is C</\G.../g>. You can |
741 | combine several regexps like this to process a string part-by-part, | |
742 | doing different actions depending on which regexp matched. The next | |
743 | regexp would step in at the place the previous one left off. | |
744 | ||
745 | $_ = <<'EOL'; | |
746 | $url = new URI::URL "http://www/"; die if $url eq "xXx"; | |
747 | EOL | |
748 | LOOP: | |
749 | { | |
750 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/g; | |
751 | print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/g; | |
752 | print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/g; | |
753 | print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/g; | |
754 | print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/g; | |
755 | print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/g; | |
756 | print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/g; | |
757 | print ". That's all!\n"; | |
758 | } | |
759 | ||
760 | Here is the output (split into several lines): | |
761 | ||
762 | line-noise lowercase line-noise lowercase UPPERCASE line-noise | |
763 | UPPERCASE line-noise lowercase line-noise lowercase line-noise | |
764 | lowercase lowercase line-noise lowercase lowercase line-noise | |
765 | MiXeD line-noise. That's all! | |
44a8e56a | 766 | |
a0d0e21e LW |
767 | =item q/STRING/ |
768 | ||
769 | =item C<'STRING'> | |
770 | ||
771 | A single-quoted, literal string. Backslashes are ignored, unless | |
772 | followed by the delimiter or another backslash, in which case the | |
773 | delimiter or backslash is interpolated. | |
774 | ||
775 | $foo = q!I said, "You said, 'She said it.'"!; | |
776 | $bar = q('This is it.'); | |
777 | ||
778 | =item qq/STRING/ | |
779 | ||
780 | =item "STRING" | |
781 | ||
782 | A double-quoted, interpolated string. | |
783 | ||
784 | $_ .= qq | |
785 | (*** The previous line contains the naughty word "$1".\n) | |
786 | if /(tcl|rexx|python)/; # :-) | |
787 | ||
788 | =item qx/STRING/ | |
789 | ||
790 | =item `STRING` | |
791 | ||
792 | A string which is interpolated and then executed as a system command. | |
793 | The collected standard output of the command is returned. In scalar | |
794 | context, it comes back as a single (potentially multi-line) string. | |
795 | In list context, returns a list of lines (however you've defined lines | |
796 | with $/ or $INPUT_RECORD_SEPARATOR). | |
797 | ||
798 | $today = qx{ date }; | |
799 | ||
800 | See L<I/O Operators> for more discussion. | |
801 | ||
802 | =item qw/STRING/ | |
803 | ||
804 | Returns a list of the words extracted out of STRING, using embedded | |
805 | whitespace as the word delimiters. It is exactly equivalent to | |
806 | ||
807 | split(' ', q/STRING/); | |
808 | ||
809 | Some frequently seen examples: | |
810 | ||
811 | use POSIX qw( setlocale localeconv ) | |
812 | @EXPORT = qw( foo bar baz ); | |
813 | ||
814 | =item s/PATTERN/REPLACEMENT/egimosx | |
815 | ||
816 | Searches a string for a pattern, and if found, replaces that pattern | |
817 | with the replacement text and returns the number of substitutions | |
e37d713d | 818 | made. Otherwise it returns false (specifically, the empty string). |
a0d0e21e LW |
819 | |
820 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> | |
821 | variable is searched and modified. (The string specified with C<=~> must | |
822 | be a scalar variable, an array element, a hash element, or an assignment | |
5f05dabc | 823 | to one of those, i.e., an lvalue.) |
a0d0e21e LW |
824 | |
825 | If the delimiter chosen is single quote, no variable interpolation is | |
826 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the | |
827 | PATTERN contains a $ that looks like a variable rather than an | |
828 | end-of-string test, the variable will be interpolated into the pattern | |
5f05dabc | 829 | at run-time. If you want the pattern compiled only once the first time |
a0d0e21e | 830 | the variable is interpolated, use the C</o> option. If the pattern |
4633a7c4 | 831 | evaluates to a null string, the last successfully executed regular |
a0d0e21e | 832 | expression is used instead. See L<perlre> for further explanation on these. |
a034a98d DD |
833 | See L<perllocale> for discussion of additional considerations which apply |
834 | when C<use locale> is in effect. | |
a0d0e21e LW |
835 | |
836 | Options are: | |
837 | ||
838 | e Evaluate the right side as an expression. | |
5f05dabc | 839 | g Replace globally, i.e., all occurrences. |
a0d0e21e LW |
840 | i Do case-insensitive pattern matching. |
841 | m Treat string as multiple lines. | |
5f05dabc | 842 | o Compile pattern only once. |
a0d0e21e LW |
843 | s Treat string as single line. |
844 | x Use extended regular expressions. | |
845 | ||
846 | Any non-alphanumeric, non-whitespace delimiter may replace the | |
847 | slashes. If single quotes are used, no interpretation is done on the | |
e37d713d | 848 | replacement string (the C</e> modifier overrides this, however). Unlike |
5f05dabc | 849 | Perl 4, Perl 5 treats back-ticks as normal delimiters; the replacement |
e37d713d | 850 | text is not evaluated as a command. If the |
a0d0e21e | 851 | PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own |
5f05dabc | 852 | pair of quotes, which may or may not be bracketing quotes, e.g., |
a0d0e21e LW |
853 | C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the |
854 | replacement portion to be interpreter as a full-fledged Perl expression | |
855 | and eval()ed right then and there. It is, however, syntax checked at | |
856 | compile-time. | |
857 | ||
858 | Examples: | |
859 | ||
860 | s/\bgreen\b/mauve/g; # don't change wintergreen | |
861 | ||
862 | $path =~ s|/usr/bin|/usr/local/bin|; | |
863 | ||
864 | s/Login: $foo/Login: $bar/; # run-time pattern | |
865 | ||
866 | ($foo = $bar) =~ s/this/that/; | |
867 | ||
868 | $count = ($paragraph =~ s/Mister\b/Mr./g); | |
869 | ||
870 | $_ = 'abc123xyz'; | |
871 | s/\d+/$&*2/e; # yields 'abc246xyz' | |
872 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' | |
873 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' | |
874 | ||
875 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e | |
876 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e | |
877 | s/^=(\w+)/&pod($1)/ge; # use function call | |
878 | ||
879 | # /e's can even nest; this will expand | |
880 | # simple embedded variables in $_ | |
881 | s/(\$\w+)/$1/eeg; | |
882 | ||
883 | # Delete C comments. | |
884 | $program =~ s { | |
4633a7c4 LW |
885 | /\* # Match the opening delimiter. |
886 | .*? # Match a minimal number of characters. | |
887 | \*/ # Match the closing delimiter. | |
a0d0e21e LW |
888 | } []gsx; |
889 | ||
890 | s/^\s*(.*?)\s*$/$1/; # trim white space | |
891 | ||
892 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields | |
893 | ||
894 | Note the use of $ instead of \ in the last example. Unlike | |
5f05dabc | 895 | B<sed>, we use the \E<lt>I<digit>E<gt> form in only the left hand side. |
6ee5d4e7 | 896 | Anywhere else it's $E<lt>I<digit>E<gt>. |
a0d0e21e | 897 | |
5f05dabc | 898 | Occasionally, you can't use just a C</g> to get all the changes |
a0d0e21e LW |
899 | to occur. Here are two common cases: |
900 | ||
901 | # put commas in the right places in an integer | |
902 | 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4 | |
903 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5 | |
904 | ||
905 | # expand tabs to 8-column spacing | |
906 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; | |
907 | ||
908 | ||
909 | =item tr/SEARCHLIST/REPLACEMENTLIST/cds | |
910 | ||
911 | =item y/SEARCHLIST/REPLACEMENTLIST/cds | |
912 | ||
913 | Translates all occurrences of the characters found in the search list | |
914 | with the corresponding character in the replacement list. It returns | |
915 | the number of characters replaced or deleted. If no string is | |
916 | specified via the =~ or !~ operator, the $_ string is translated. (The | |
917 | string specified with =~ must be a scalar variable, an array element, | |
5f05dabc | 918 | or an assignment to one of those, i.e., an lvalue.) For B<sed> devotees, |
a0d0e21e LW |
919 | C<y> is provided as a synonym for C<tr>. If the SEARCHLIST is |
920 | delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of | |
5f05dabc | 921 | quotes, which may or may not be bracketing quotes, e.g., C<tr[A-Z][a-z]> |
a0d0e21e LW |
922 | or C<tr(+-*/)/ABCD/>. |
923 | ||
924 | Options: | |
925 | ||
926 | c Complement the SEARCHLIST. | |
927 | d Delete found but unreplaced characters. | |
928 | s Squash duplicate replaced characters. | |
929 | ||
930 | If the C</c> modifier is specified, the SEARCHLIST character set is | |
931 | complemented. If the C</d> modifier is specified, any characters specified | |
932 | by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note | |
933 | that this is slightly more flexible than the behavior of some B<tr> | |
934 | programs, which delete anything they find in the SEARCHLIST, period.) | |
935 | If the C</s> modifier is specified, sequences of characters that were | |
936 | translated to the same character are squashed down to a single instance of the | |
937 | character. | |
938 | ||
939 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted | |
940 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter | |
941 | than the SEARCHLIST, the final character is replicated till it is long | |
942 | enough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated. | |
943 | This latter is useful for counting characters in a class or for | |
944 | squashing character sequences in a class. | |
945 | ||
946 | Examples: | |
947 | ||
948 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case | |
949 | ||
950 | $cnt = tr/*/*/; # count the stars in $_ | |
951 | ||
952 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky | |
953 | ||
954 | $cnt = tr/0-9//; # count the digits in $_ | |
955 | ||
956 | tr/a-zA-Z//s; # bookkeeper -> bokeper | |
957 | ||
958 | ($HOST = $host) =~ tr/a-z/A-Z/; | |
959 | ||
960 | tr/a-zA-Z/ /cs; # change non-alphas to single space | |
961 | ||
962 | tr [\200-\377] | |
963 | [\000-\177]; # delete 8th bit | |
964 | ||
748a9306 LW |
965 | If multiple translations are given for a character, only the first one is used: |
966 | ||
967 | tr/AAA/XYZ/ | |
968 | ||
969 | will translate any A to X. | |
970 | ||
a0d0e21e LW |
971 | Note that because the translation table is built at compile time, neither |
972 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote | |
973 | interpolation. That means that if you want to use variables, you must use | |
974 | an eval(): | |
975 | ||
976 | eval "tr/$oldlist/$newlist/"; | |
977 | die $@ if $@; | |
978 | ||
979 | eval "tr/$oldlist/$newlist/, 1" or die $@; | |
980 | ||
981 | =back | |
982 | ||
983 | =head2 I/O Operators | |
984 | ||
985 | There are several I/O operators you should know about. | |
5f05dabc | 986 | A string is enclosed by back-ticks (grave accents) first undergoes |
a0d0e21e LW |
987 | variable substitution just like a double quoted string. It is then |
988 | interpreted as a command, and the output of that command is the value | |
989 | of the pseudo-literal, like in a shell. In a scalar context, a single | |
990 | string consisting of all the output is returned. In a list context, | |
991 | a list of values is returned, one for each line of output. (You can | |
992 | set C<$/> to use a different line terminator.) The command is executed | |
993 | each time the pseudo-literal is evaluated. The status value of the | |
994 | command is returned in C<$?> (see L<perlvar> for the interpretation | |
995 | of C<$?>). Unlike in B<csh>, no translation is done on the return | |
996 | data--newlines remain newlines. Unlike in any of the shells, single | |
997 | quotes do not hide variable names in the command from interpretation. | |
998 | To pass a $ through to the shell you need to hide it with a backslash. | |
5f05dabc | 999 | The generalized form of back-ticks is C<qx//>. (Because back-ticks |
cb1a09d0 AD |
1000 | always undergo shell expansion as well, see L<perlsec> for |
1001 | security concerns.) | |
a0d0e21e LW |
1002 | |
1003 | Evaluating a filehandle in angle brackets yields the next line from | |
748a9306 LW |
1004 | that file (newline included, so it's never false until end of file, at |
1005 | which time an undefined value is returned). Ordinarily you must assign | |
1006 | that value to a variable, but there is one situation where an automatic | |
a0d0e21e LW |
1007 | assignment happens. I<If and ONLY if> the input symbol is the only |
1008 | thing inside the conditional of a C<while> loop, the value is | |
748a9306 LW |
1009 | automatically assigned to the variable C<$_>. The assigned value is |
1010 | then tested to see if it is defined. (This may seem like an odd thing | |
1011 | to you, but you'll use the construct in almost every Perl script you | |
1012 | write.) Anyway, the following lines are equivalent to each other: | |
a0d0e21e | 1013 | |
748a9306 | 1014 | while (defined($_ = <STDIN>)) { print; } |
a0d0e21e LW |
1015 | while (<STDIN>) { print; } |
1016 | for (;<STDIN>;) { print; } | |
748a9306 | 1017 | print while defined($_ = <STDIN>); |
a0d0e21e LW |
1018 | print while <STDIN>; |
1019 | ||
5f05dabc | 1020 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
1021 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in | |
a0d0e21e LW |
1022 | packages, where they would be interpreted as local identifiers rather |
1023 | than global.) Additional filehandles may be created with the open() | |
cb1a09d0 | 1024 | function. See L<perlfunc/open()> for details on this. |
a0d0e21e | 1025 | |
6ee5d4e7 | 1026 | If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a |
a0d0e21e LW |
1027 | list consisting of all the input lines is returned, one line per list |
1028 | element. It's easy to make a I<LARGE> data space this way, so use with | |
1029 | care. | |
1030 | ||
d28ebecd | 1031 | The null filehandle E<lt>E<gt> is special and can be used to emulate the |
1032 | behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from | |
a0d0e21e | 1033 | standard input, or from each file listed on the command line. Here's |
d28ebecd | 1034 | how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is |
a0d0e21e LW |
1035 | checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened |
1036 | gives you standard input. The @ARGV array is then processed as a list | |
1037 | of filenames. The loop | |
1038 | ||
1039 | while (<>) { | |
1040 | ... # code for each line | |
1041 | } | |
1042 | ||
1043 | is equivalent to the following Perl-like pseudo code: | |
1044 | ||
1045 | unshift(@ARGV, '-') if $#ARGV < $[; | |
1046 | while ($ARGV = shift) { | |
1047 | open(ARGV, $ARGV); | |
1048 | while (<ARGV>) { | |
1049 | ... # code for each line | |
1050 | } | |
1051 | } | |
1052 | ||
1053 | except that it isn't so cumbersome to say, and will actually work. It | |
1054 | really does shift array @ARGV and put the current filename into variable | |
5f05dabc | 1055 | $ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a |
1056 | synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above | |
1057 | doesn't work because it treats E<lt>ARGVE<gt> as non-magical.) | |
a0d0e21e | 1058 | |
d28ebecd | 1059 | You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up |
a0d0e21e LW |
1060 | containing the list of filenames you really want. Line numbers (C<$.>) |
1061 | continue as if the input were one big happy file. (But see example | |
1062 | under eof() for how to reset line numbers on each file.) | |
1063 | ||
1064 | If you want to set @ARGV to your own list of files, go right ahead. If | |
1065 | you want to pass switches into your script, you can use one of the | |
1066 | Getopts modules or put a loop on the front like this: | |
1067 | ||
1068 | while ($_ = $ARGV[0], /^-/) { | |
1069 | shift; | |
1070 | last if /^--$/; | |
1071 | if (/^-D(.*)/) { $debug = $1 } | |
1072 | if (/^-v/) { $verbose++ } | |
1073 | ... # other switches | |
1074 | } | |
1075 | while (<>) { | |
1076 | ... # code for each line | |
1077 | } | |
1078 | ||
d28ebecd | 1079 | The E<lt>E<gt> symbol will return FALSE only once. If you call it again after |
a0d0e21e LW |
1080 | this it will assume you are processing another @ARGV list, and if you |
1081 | haven't set @ARGV, will input from STDIN. | |
1082 | ||
1083 | If the string inside the angle brackets is a reference to a scalar | |
5f05dabc | 1084 | variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the |
cb1a09d0 AD |
1085 | filehandle to input from, or a reference to the same. For example: |
1086 | ||
1087 | $fh = \*STDIN; | |
1088 | $line = <$fh>; | |
a0d0e21e | 1089 | |
cb1a09d0 AD |
1090 | If the string inside angle brackets is not a filehandle or a scalar |
1091 | variable containing a filehandle name or reference, then it is interpreted | |
4633a7c4 LW |
1092 | as a filename pattern to be globbed, and either a list of filenames or the |
1093 | next filename in the list is returned, depending on context. One level of | |
1094 | $ interpretation is done first, but you can't say C<E<lt>$fooE<gt>> | |
1095 | because that's an indirect filehandle as explained in the previous | |
6ee5d4e7 | 1096 | paragraph. (In older versions of Perl, programmers would insert curly |
4633a7c4 | 1097 | brackets to force interpretation as a filename glob: C<E<lt>${foo}E<gt>>. |
d28ebecd | 1098 | These days, it's considered cleaner to call the internal function directly |
4633a7c4 LW |
1099 | as C<glob($foo)>, which is probably the right way to have done it in the |
1100 | first place.) Example: | |
a0d0e21e LW |
1101 | |
1102 | while (<*.c>) { | |
1103 | chmod 0644, $_; | |
1104 | } | |
1105 | ||
1106 | is equivalent to | |
1107 | ||
1108 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); | |
1109 | while (<FOO>) { | |
1110 | chop; | |
1111 | chmod 0644, $_; | |
1112 | } | |
1113 | ||
1114 | In fact, it's currently implemented that way. (Which means it will not | |
1115 | work on filenames with spaces in them unless you have csh(1) on your | |
1116 | machine.) Of course, the shortest way to do the above is: | |
1117 | ||
1118 | chmod 0644, <*.c>; | |
1119 | ||
1120 | Because globbing invokes a shell, it's often faster to call readdir() yourself | |
5f05dabc | 1121 | and do your own grep() on the filenames. Furthermore, due to its current |
a0d0e21e LW |
1122 | implementation of using a shell, the glob() routine may get "Arg list too |
1123 | long" errors (unless you've installed tcsh(1L) as F</bin/csh>). | |
1124 | ||
5f05dabc | 1125 | A glob evaluates its (embedded) argument only when it is starting a new |
4633a7c4 LW |
1126 | list. All values must be read before it will start over. In a list |
1127 | context this isn't important, because you automatically get them all | |
1128 | anyway. In a scalar context, however, the operator returns the next value | |
1129 | each time it is called, or a FALSE value if you've just run out. Again, | |
1130 | FALSE is returned only once. So if you're expecting a single value from | |
1131 | a glob, it is much better to say | |
1132 | ||
1133 | ($file) = <blurch*>; | |
1134 | ||
1135 | than | |
1136 | ||
1137 | $file = <blurch*>; | |
1138 | ||
1139 | because the latter will alternate between returning a filename and | |
1140 | returning FALSE. | |
1141 | ||
1142 | It you're trying to do variable interpolation, it's definitely better | |
1143 | to use the glob() function, because the older notation can cause people | |
e37d713d | 1144 | to become confused with the indirect filehandle notation. |
4633a7c4 LW |
1145 | |
1146 | @files = glob("$dir/*.[ch]"); | |
1147 | @files = glob($files[$i]); | |
1148 | ||
a0d0e21e LW |
1149 | =head2 Constant Folding |
1150 | ||
1151 | Like C, Perl does a certain amount of expression evaluation at | |
1152 | compile time, whenever it determines that all of the arguments to an | |
1153 | operator are static and have no side effects. In particular, string | |
1154 | concatenation happens at compile time between literals that don't do | |
1155 | variable substitution. Backslash interpretation also happens at | |
1156 | compile time. You can say | |
1157 | ||
1158 | 'Now is the time for all' . "\n" . | |
1159 | 'good men to come to.' | |
1160 | ||
1161 | and this all reduces to one string internally. Likewise, if | |
1162 | you say | |
1163 | ||
1164 | foreach $file (@filenames) { | |
1165 | if (-s $file > 5 + 100 * 2**16) { ... } | |
1166 | } | |
1167 | ||
1168 | the compiler will pre-compute the number that | |
1169 | expression represents so that the interpreter | |
1170 | won't have to. | |
1171 | ||
1172 | ||
55497cff | 1173 | =head2 Integer Arithmetic |
a0d0e21e LW |
1174 | |
1175 | By default Perl assumes that it must do most of its arithmetic in | |
1176 | floating point. But by saying | |
1177 | ||
1178 | use integer; | |
1179 | ||
1180 | you may tell the compiler that it's okay to use integer operations | |
1181 | from here to the end of the enclosing BLOCK. An inner BLOCK may | |
1182 | countermand this by saying | |
1183 | ||
1184 | no integer; | |
1185 | ||
1186 | which lasts until the end of that BLOCK. | |
1187 | ||
55497cff | 1188 | The bitwise operators ("&", "|", "^", "~", "<<", and ">>") always |
1189 | produce integral results. However, C<use integer> still has meaning | |
1190 | for them. By default, their results are interpreted as unsigned | |
1191 | integers. However, if C<use integer> is in effect, their results are | |
5f05dabc | 1192 | interpreted as signed integers. For example, C<~0> usually evaluates |
55497cff | 1193 | to a large integral value. However, C<use integer; ~0> is -1. |