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