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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 | |
54310121 | 11 | Perl easier for C folks.) With very few exceptions, these all |
c07a80fd | 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 - | |
54310121 | 19 | left =~ !~ |
a0d0e21e LW |
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 || | |
137443ea | 30 | nonassoc .. ... |
a0d0e21e LW |
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 | ||
54310121 | 45 | A TERM has the highest precedence in Perl. They 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 | |
54310121 | 59 | whether you are looking at the left side or the right side of the operator. |
a0d0e21e LW |
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 | ||
54310121 | 84 | probably doesn't do what you expect at first glance. See |
a0d0e21e LW |
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 | |
54310121 | 88 | well as subroutine and method calls, and the anonymous |
a0d0e21e LW |
89 | constructors C<[]> and C<{}>. |
90 | ||
2ae324a7 | 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 | ||
54310121 | 113 | The auto-increment operator has a little extra builtin 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 |
a0d0e21e LW |
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 | $_. The return value indicates the success of the operation. (If the | |
168 | right argument is an expression rather than a search pattern, | |
169 | substitution, or translation, it is interpreted as a search pattern at run | |
aa689395 | 170 | time. This can be is less efficient than an explicit search, because the |
171 | pattern must be compiled every time the expression is evaluated. | |
a0d0e21e LW |
172 | |
173 | Binary "!~" is just like "=~" except the return value is negated in | |
174 | the logical sense. | |
175 | ||
176 | =head2 Multiplicative Operators | |
177 | ||
178 | Binary "*" multiplies two numbers. | |
179 | ||
180 | Binary "/" divides two numbers. | |
181 | ||
54310121 | 182 | Binary "%" computes the modulus of two numbers. Given integer |
183 | operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is | |
184 | C<$a> minus the largest multiple of C<$b> that is not greater than | |
185 | C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the | |
186 | smallest multiple of C<$b> that is not less than C<$a> (i.e. the | |
187 | result will be less than or equal to zero). | |
a0d0e21e LW |
188 | |
189 | Binary "x" is the repetition operator. In a scalar context, it | |
190 | returns a string consisting of the left operand repeated the number of | |
191 | times specified by the right operand. In a list context, if the left | |
5f05dabc | 192 | operand is a list in parentheses, it repeats the list. |
a0d0e21e LW |
193 | |
194 | print '-' x 80; # print row of dashes | |
195 | ||
196 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over | |
197 | ||
198 | @ones = (1) x 80; # a list of 80 1's | |
199 | @ones = (5) x @ones; # set all elements to 5 | |
200 | ||
201 | ||
202 | =head2 Additive Operators | |
203 | ||
204 | Binary "+" returns the sum of two numbers. | |
205 | ||
206 | Binary "-" returns the difference of two numbers. | |
207 | ||
208 | Binary "." concatenates two strings. | |
209 | ||
210 | =head2 Shift Operators | |
211 | ||
55497cff | 212 | Binary "<<" returns the value of its left argument shifted left by the |
213 | number of bits specified by the right argument. Arguments should be | |
214 | integers. (See also L<Integer Arithmetic>.) | |
a0d0e21e | 215 | |
55497cff | 216 | Binary ">>" returns the value of its left argument shifted right by |
217 | the number of bits specified by the right argument. Arguments should | |
218 | be integers. (See also L<Integer Arithmetic>.) | |
a0d0e21e LW |
219 | |
220 | =head2 Named Unary Operators | |
221 | ||
222 | The various named unary operators are treated as functions with one | |
223 | argument, with optional parentheses. These include the filetest | |
224 | operators, like C<-f>, C<-M>, etc. See L<perlfunc>. | |
225 | ||
226 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
227 | is followed by a left parenthesis as the next token, the operator and | |
228 | arguments within parentheses are taken to be of highest precedence, | |
229 | just like a normal function call. Examples: | |
230 | ||
231 | chdir $foo || die; # (chdir $foo) || die | |
232 | chdir($foo) || die; # (chdir $foo) || die | |
233 | chdir ($foo) || die; # (chdir $foo) || die | |
234 | chdir +($foo) || die; # (chdir $foo) || die | |
235 | ||
236 | but, because * is higher precedence than ||: | |
237 | ||
238 | chdir $foo * 20; # chdir ($foo * 20) | |
239 | chdir($foo) * 20; # (chdir $foo) * 20 | |
240 | chdir ($foo) * 20; # (chdir $foo) * 20 | |
241 | chdir +($foo) * 20; # chdir ($foo * 20) | |
242 | ||
243 | rand 10 * 20; # rand (10 * 20) | |
244 | rand(10) * 20; # (rand 10) * 20 | |
245 | rand (10) * 20; # (rand 10) * 20 | |
246 | rand +(10) * 20; # rand (10 * 20) | |
247 | ||
5ba421f6 | 248 | See also L<"Terms and List Operators (Leftward)">. |
a0d0e21e LW |
249 | |
250 | =head2 Relational Operators | |
251 | ||
6ee5d4e7 | 252 | Binary "E<lt>" returns true if the left argument is numerically less than |
a0d0e21e LW |
253 | the right argument. |
254 | ||
6ee5d4e7 | 255 | Binary "E<gt>" returns true if the left argument is numerically greater |
a0d0e21e LW |
256 | than the right argument. |
257 | ||
6ee5d4e7 | 258 | Binary "E<lt>=" returns true if the left argument is numerically less than |
a0d0e21e LW |
259 | or equal to the right argument. |
260 | ||
6ee5d4e7 | 261 | Binary "E<gt>=" returns true if the left argument is numerically greater |
a0d0e21e LW |
262 | than or equal to the right argument. |
263 | ||
264 | Binary "lt" returns true if the left argument is stringwise less than | |
265 | the right argument. | |
266 | ||
267 | Binary "gt" returns true if the left argument is stringwise greater | |
268 | than the right argument. | |
269 | ||
270 | Binary "le" returns true if the left argument is stringwise less than | |
271 | or equal to the right argument. | |
272 | ||
273 | Binary "ge" returns true if the left argument is stringwise greater | |
274 | than or equal to the right argument. | |
275 | ||
276 | =head2 Equality Operators | |
277 | ||
278 | Binary "==" returns true if the left argument is numerically equal to | |
279 | the right argument. | |
280 | ||
281 | Binary "!=" returns true if the left argument is numerically not equal | |
282 | to the right argument. | |
283 | ||
6ee5d4e7 | 284 | Binary "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left |
285 | argument is numerically less than, equal to, or greater than the right | |
286 | argument. | |
a0d0e21e LW |
287 | |
288 | Binary "eq" returns true if the left argument is stringwise equal to | |
289 | the right argument. | |
290 | ||
291 | Binary "ne" returns true if the left argument is stringwise not equal | |
292 | to the right argument. | |
293 | ||
294 | Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise | |
295 | less than, equal to, or greater than the right argument. | |
296 | ||
a034a98d DD |
297 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
298 | by the current locale if C<use locale> is in effect. See L<perllocale>. | |
299 | ||
a0d0e21e LW |
300 | =head2 Bitwise And |
301 | ||
302 | Binary "&" returns its operators ANDed together bit by bit. | |
55497cff | 303 | (See also L<Integer Arithmetic>.) |
a0d0e21e LW |
304 | |
305 | =head2 Bitwise Or and Exclusive Or | |
306 | ||
307 | Binary "|" returns its operators ORed together bit by bit. | |
55497cff | 308 | (See also L<Integer Arithmetic>.) |
a0d0e21e LW |
309 | |
310 | Binary "^" returns its operators XORed together bit by bit. | |
55497cff | 311 | (See also L<Integer Arithmetic>.) |
a0d0e21e LW |
312 | |
313 | =head2 C-style Logical And | |
314 | ||
315 | Binary "&&" performs a short-circuit logical AND operation. That is, | |
316 | if the left operand is false, the right operand is not even evaluated. | |
317 | Scalar or list context propagates down to the right operand if it | |
318 | is evaluated. | |
319 | ||
320 | =head2 C-style Logical Or | |
321 | ||
322 | Binary "||" performs a short-circuit logical OR operation. That is, | |
323 | if the left operand is true, the right operand is not even evaluated. | |
324 | Scalar or list context propagates down to the right operand if it | |
325 | is evaluated. | |
326 | ||
327 | The C<||> and C<&&> operators differ from C's in that, rather than returning | |
328 | 0 or 1, they return the last value evaluated. Thus, a reasonably portable | |
329 | way to find out the home directory (assuming it's not "0") might be: | |
330 | ||
331 | $home = $ENV{'HOME'} || $ENV{'LOGDIR'} || | |
332 | (getpwuid($<))[7] || die "You're homeless!\n"; | |
333 | ||
334 | As more readable alternatives to C<&&> and C<||>, Perl provides "and" and | |
335 | "or" operators (see below). The short-circuit behavior is identical. The | |
336 | precedence of "and" and "or" is much lower, however, so that you can | |
337 | safely use them after a list operator without the need for | |
338 | parentheses: | |
339 | ||
340 | unlink "alpha", "beta", "gamma" | |
341 | or gripe(), next LINE; | |
342 | ||
343 | With the C-style operators that would have been written like this: | |
344 | ||
345 | unlink("alpha", "beta", "gamma") | |
346 | || (gripe(), next LINE); | |
347 | ||
348 | =head2 Range Operator | |
349 | ||
350 | Binary ".." is the range operator, which is really two different | |
351 | operators depending on the context. In a list context, it returns an | |
352 | array of values counting (by ones) from the left value to the right | |
353 | value. This is useful for writing C<for (1..10)> loops and for doing | |
354 | slice operations on arrays. Be aware that under the current implementation, | |
54310121 | 355 | a temporary array is created, so you'll burn a lot of memory if you |
a0d0e21e LW |
356 | write something like this: |
357 | ||
358 | for (1 .. 1_000_000) { | |
359 | # code | |
54310121 | 360 | } |
a0d0e21e LW |
361 | |
362 | In a scalar context, ".." returns a boolean value. The operator is | |
363 | bistable, like a flip-flop, and emulates the line-range (comma) operator | |
364 | of B<sed>, B<awk>, and various editors. Each ".." operator maintains its | |
365 | own boolean state. It is false as long as its left operand is false. | |
366 | Once the left operand is true, the range operator stays true until the | |
367 | right operand is true, I<AFTER> which the range operator becomes false | |
368 | again. (It doesn't become false till the next time the range operator is | |
369 | evaluated. It can test the right operand and become false on the same | |
370 | evaluation it became true (as in B<awk>), but it still returns true once. | |
371 | If you don't want it to test the right operand till the next evaluation | |
372 | (as in B<sed>), use three dots ("...") instead of two.) The right | |
373 | operand is not evaluated while the operator is in the "false" state, and | |
374 | the left operand is not evaluated while the operator is in the "true" | |
375 | state. The precedence is a little lower than || and &&. The value | |
376 | returned is either the null string for false, or a sequence number | |
377 | (beginning with 1) for true. The sequence number is reset for each range | |
378 | encountered. The final sequence number in a range has the string "E0" | |
379 | appended to it, which doesn't affect its numeric value, but gives you | |
380 | something to search for if you want to exclude the endpoint. You can | |
381 | exclude the beginning point by waiting for the sequence number to be | |
382 | greater than 1. If either operand of scalar ".." is a numeric literal, | |
383 | that operand is implicitly compared to the C<$.> variable, the current | |
384 | line number. Examples: | |
385 | ||
386 | As a scalar operator: | |
387 | ||
388 | if (101 .. 200) { print; } # print 2nd hundred lines | |
389 | next line if (1 .. /^$/); # skip header lines | |
390 | s/^/> / if (/^$/ .. eof()); # quote body | |
391 | ||
392 | As a list operator: | |
393 | ||
394 | for (101 .. 200) { print; } # print $_ 100 times | |
3e3baf6d | 395 | @foo = @foo[0 .. $#foo]; # an expensive no-op |
a0d0e21e LW |
396 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items |
397 | ||
398 | The range operator (in a list context) makes use of the magical | |
5f05dabc | 399 | auto-increment algorithm if the operands are strings. You |
a0d0e21e LW |
400 | can say |
401 | ||
402 | @alphabet = ('A' .. 'Z'); | |
403 | ||
404 | to get all the letters of the alphabet, or | |
405 | ||
406 | $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; | |
407 | ||
408 | to get a hexadecimal digit, or | |
409 | ||
410 | @z2 = ('01' .. '31'); print $z2[$mday]; | |
411 | ||
412 | to get dates with leading zeros. If the final value specified is not | |
413 | in the sequence that the magical increment would produce, the sequence | |
414 | goes until the next value would be longer than the final value | |
415 | specified. | |
416 | ||
417 | =head2 Conditional Operator | |
418 | ||
419 | Ternary "?:" is the conditional operator, just as in C. It works much | |
420 | like an if-then-else. If the argument before the ? is true, the | |
421 | argument before the : is returned, otherwise the argument after the : | |
cb1a09d0 AD |
422 | is returned. For example: |
423 | ||
54310121 | 424 | printf "I have %d dog%s.\n", $n, |
cb1a09d0 AD |
425 | ($n == 1) ? '' : "s"; |
426 | ||
427 | Scalar or list context propagates downward into the 2nd | |
54310121 | 428 | or 3rd argument, whichever is selected. |
cb1a09d0 AD |
429 | |
430 | $a = $ok ? $b : $c; # get a scalar | |
431 | @a = $ok ? @b : @c; # get an array | |
432 | $a = $ok ? @b : @c; # oops, that's just a count! | |
433 | ||
434 | The operator may be assigned to if both the 2nd and 3rd arguments are | |
435 | legal lvalues (meaning that you can assign to them): | |
a0d0e21e LW |
436 | |
437 | ($a_or_b ? $a : $b) = $c; | |
438 | ||
cb1a09d0 | 439 | This is not necessarily guaranteed to contribute to the readability of your program. |
a0d0e21e | 440 | |
4633a7c4 | 441 | =head2 Assignment Operators |
a0d0e21e LW |
442 | |
443 | "=" is the ordinary assignment operator. | |
444 | ||
445 | Assignment operators work as in C. That is, | |
446 | ||
447 | $a += 2; | |
448 | ||
449 | is equivalent to | |
450 | ||
451 | $a = $a + 2; | |
452 | ||
453 | although without duplicating any side effects that dereferencing the lvalue | |
54310121 | 454 | might trigger, such as from tie(). Other assignment operators work similarly. |
455 | The following are recognized: | |
a0d0e21e LW |
456 | |
457 | **= += *= &= <<= &&= | |
458 | -= /= |= >>= ||= | |
459 | .= %= ^= | |
460 | x= | |
461 | ||
462 | Note that while these are grouped by family, they all have the precedence | |
463 | of assignment. | |
464 | ||
465 | Unlike in C, the assignment operator produces a valid lvalue. Modifying | |
466 | an assignment is equivalent to doing the assignment and then modifying | |
467 | the variable that was assigned to. This is useful for modifying | |
468 | a copy of something, like this: | |
469 | ||
470 | ($tmp = $global) =~ tr [A-Z] [a-z]; | |
471 | ||
472 | Likewise, | |
473 | ||
474 | ($a += 2) *= 3; | |
475 | ||
476 | is equivalent to | |
477 | ||
478 | $a += 2; | |
479 | $a *= 3; | |
480 | ||
748a9306 | 481 | =head2 Comma Operator |
a0d0e21e LW |
482 | |
483 | Binary "," is the comma operator. In a scalar context it evaluates | |
484 | its left argument, throws that value away, then evaluates its right | |
485 | argument and returns that value. This is just like C's comma operator. | |
486 | ||
487 | In a list context, it's just the list argument separator, and inserts | |
488 | both its arguments into the list. | |
489 | ||
6ee5d4e7 | 490 | The =E<gt> digraph is mostly just a synonym for the comma operator. It's useful for |
cb1a09d0 | 491 | documenting arguments that come in pairs. As of release 5.001, it also forces |
4633a7c4 | 492 | any word to the left of it to be interpreted as a string. |
748a9306 | 493 | |
a0d0e21e LW |
494 | =head2 List Operators (Rightward) |
495 | ||
496 | On the right side of a list operator, it has very low precedence, | |
497 | such that it controls all comma-separated expressions found there. | |
498 | The only operators with lower precedence are the logical operators | |
499 | "and", "or", and "not", which may be used to evaluate calls to list | |
500 | operators without the need for extra parentheses: | |
501 | ||
502 | open HANDLE, "filename" | |
503 | or die "Can't open: $!\n"; | |
504 | ||
5ba421f6 | 505 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
a0d0e21e LW |
506 | |
507 | =head2 Logical Not | |
508 | ||
509 | Unary "not" returns the logical negation of the expression to its right. | |
510 | It's the equivalent of "!" except for the very low precedence. | |
511 | ||
512 | =head2 Logical And | |
513 | ||
514 | Binary "and" returns the logical conjunction of the two surrounding | |
515 | expressions. It's equivalent to && except for the very low | |
5f05dabc | 516 | precedence. This means that it short-circuits: i.e., the right |
a0d0e21e LW |
517 | expression is evaluated only if the left expression is true. |
518 | ||
519 | =head2 Logical or and Exclusive Or | |
520 | ||
521 | Binary "or" returns the logical disjunction of the two surrounding | |
522 | expressions. It's equivalent to || except for the very low | |
5f05dabc | 523 | precedence. This means that it short-circuits: i.e., the right |
a0d0e21e LW |
524 | expression is evaluated only if the left expression is false. |
525 | ||
526 | Binary "xor" returns the exclusive-OR of the two surrounding expressions. | |
527 | It cannot short circuit, of course. | |
528 | ||
529 | =head2 C Operators Missing From Perl | |
530 | ||
531 | Here is what C has that Perl doesn't: | |
532 | ||
533 | =over 8 | |
534 | ||
535 | =item unary & | |
536 | ||
537 | Address-of operator. (But see the "\" operator for taking a reference.) | |
538 | ||
539 | =item unary * | |
540 | ||
54310121 | 541 | Dereference-address operator. (Perl's prefix dereferencing |
a0d0e21e LW |
542 | operators are typed: $, @, %, and &.) |
543 | ||
544 | =item (TYPE) | |
545 | ||
54310121 | 546 | Type casting operator. |
a0d0e21e LW |
547 | |
548 | =back | |
549 | ||
5f05dabc | 550 | =head2 Quote and Quote-like Operators |
a0d0e21e LW |
551 | |
552 | While we usually think of quotes as literal values, in Perl they | |
553 | function as operators, providing various kinds of interpolating and | |
554 | pattern matching capabilities. Perl provides customary quote characters | |
555 | for these behaviors, but also provides a way for you to choose your | |
556 | quote character for any of them. In the following table, a C<{}> represents | |
557 | any pair of delimiters you choose. Non-bracketing delimiters use | |
54310121 | 558 | the same character fore and aft, but the 4 sorts of brackets |
a0d0e21e LW |
559 | (round, angle, square, curly) will all nest. |
560 | ||
561 | Customary Generic Meaning Interpolates | |
562 | '' q{} Literal no | |
563 | "" qq{} Literal yes | |
564 | `` qx{} Command yes | |
565 | qw{} Word list no | |
566 | // m{} Pattern match yes | |
567 | s{}{} Substitution yes | |
568 | tr{}{} Translation no | |
569 | ||
fb73857a | 570 | Note that there can be whitespace between the operator and the quoting |
571 | characters, except when C<#> is being used as the quoting character. | |
572 | C<q#foo#> is parsed as being the string C<foo>, which C<q #foo#> is the | |
573 | operator C<q> followed by a comment. Its argument will be taken from the | |
574 | next line. This allows you to write: | |
575 | ||
576 | s {foo} # Replace foo | |
577 | {bar} # with bar. | |
578 | ||
cb1a09d0 | 579 | For constructs that do interpolation, variables beginning with "C<$>" or "C<@>" |
a0d0e21e LW |
580 | are interpolated, as are the following sequences: |
581 | ||
6ee5d4e7 | 582 | \t tab (HT, TAB) |
583 | \n newline (LF, NL) | |
584 | \r return (CR) | |
585 | \f form feed (FF) | |
586 | \b backspace (BS) | |
587 | \a alarm (bell) (BEL) | |
588 | \e escape (ESC) | |
a0d0e21e LW |
589 | \033 octal char |
590 | \x1b hex char | |
591 | \c[ control char | |
592 | \l lowercase next char | |
593 | \u uppercase next char | |
594 | \L lowercase till \E | |
595 | \U uppercase till \E | |
596 | \E end case modification | |
597 | \Q quote regexp metacharacters till \E | |
598 | ||
a034a98d DD |
599 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> |
600 | and <\U> is taken from the current locale. See L<perllocale>. | |
601 | ||
a0d0e21e LW |
602 | Patterns are subject to an additional level of interpretation as a |
603 | regular expression. This is done as a second pass, after variables are | |
604 | interpolated, so that regular expressions may be incorporated into the | |
605 | pattern from the variables. If this is not what you want, use C<\Q> to | |
606 | interpolate a variable literally. | |
607 | ||
608 | Apart from the above, there are no multiple levels of interpolation. In | |
5f05dabc | 609 | particular, contrary to the expectations of shell programmers, back-quotes |
a0d0e21e LW |
610 | do I<NOT> interpolate within double quotes, nor do single quotes impede |
611 | evaluation of variables when used within double quotes. | |
612 | ||
5f05dabc | 613 | =head2 Regexp Quote-Like Operators |
cb1a09d0 | 614 | |
5f05dabc | 615 | Here are the quote-like operators that apply to pattern |
cb1a09d0 AD |
616 | matching and related activities. |
617 | ||
a0d0e21e LW |
618 | =over 8 |
619 | ||
620 | =item ?PATTERN? | |
621 | ||
622 | This is just like the C</pattern/> search, except that it matches only | |
623 | once between calls to the reset() operator. This is a useful | |
5f05dabc | 624 | optimization when you want to see only the first occurrence of |
a0d0e21e LW |
625 | something in each file of a set of files, for instance. Only C<??> |
626 | patterns local to the current package are reset. | |
627 | ||
628 | This usage is vaguely deprecated, and may be removed in some future | |
629 | version of Perl. | |
630 | ||
fb73857a | 631 | =item m/PATTERN/cgimosx |
a0d0e21e | 632 | |
fb73857a | 633 | =item /PATTERN/cgimosx |
a0d0e21e LW |
634 | |
635 | Searches a string for a pattern match, and in a scalar context returns | |
636 | true (1) or false (''). If no string is specified via the C<=~> or | |
637 | C<!~> operator, the $_ string is searched. (The string specified with | |
638 | C<=~> need not be an lvalue--it may be the result of an expression | |
639 | evaluation, but remember the C<=~> binds rather tightly.) See also | |
640 | L<perlre>. | |
a034a98d DD |
641 | See L<perllocale> for discussion of additional considerations which apply |
642 | when C<use locale> is in effect. | |
a0d0e21e LW |
643 | |
644 | Options are: | |
645 | ||
fb73857a | 646 | c Do not reset search position on a failed match when /g is in effect. |
5f05dabc | 647 | g Match globally, i.e., find all occurrences. |
a0d0e21e LW |
648 | i Do case-insensitive pattern matching. |
649 | m Treat string as multiple lines. | |
5f05dabc | 650 | o Compile pattern only once. |
a0d0e21e LW |
651 | s Treat string as single line. |
652 | x Use extended regular expressions. | |
653 | ||
654 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> | |
655 | you can use any pair of non-alphanumeric, non-whitespace characters as | |
656 | delimiters. This is particularly useful for matching Unix path names | |
7bac28a0 | 657 | that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is |
658 | the delimiter, then the match-only-once rule of C<?PATTERN?> applies. | |
a0d0e21e LW |
659 | |
660 | PATTERN may contain variables, which will be interpolated (and the | |
661 | pattern recompiled) every time the pattern search is evaluated. (Note | |
662 | that C<$)> and C<$|> might not be interpolated because they look like | |
663 | end-of-string tests.) If you want such a pattern to be compiled only | |
664 | once, add a C</o> after the trailing delimiter. This avoids expensive | |
665 | run-time recompilations, and is useful when the value you are | |
666 | interpolating won't change over the life of the script. However, mentioning | |
667 | C</o> constitutes a promise that you won't change the variables in the pattern. | |
668 | If you change them, Perl won't even notice. | |
669 | ||
4633a7c4 LW |
670 | If the PATTERN evaluates to a null string, the last |
671 | successfully executed regular expression is used instead. | |
a0d0e21e LW |
672 | |
673 | If used in a context that requires a list value, a pattern match returns a | |
674 | list consisting of the subexpressions matched by the parentheses in the | |
5f05dabc | 675 | pattern, i.e., (C<$1>, $2, $3...). (Note that here $1 etc. are also set, and |
a0d0e21e LW |
676 | that this differs from Perl 4's behavior.) If the match fails, a null |
677 | array is returned. If the match succeeds, but there were no parentheses, | |
678 | a list value of (1) is returned. | |
679 | ||
680 | Examples: | |
681 | ||
682 | open(TTY, '/dev/tty'); | |
683 | <TTY> =~ /^y/i && foo(); # do foo if desired | |
684 | ||
685 | if (/Version: *([0-9.]*)/) { $version = $1; } | |
686 | ||
687 | next if m#^/usr/spool/uucp#; | |
688 | ||
689 | # poor man's grep | |
690 | $arg = shift; | |
691 | while (<>) { | |
692 | print if /$arg/o; # compile only once | |
693 | } | |
694 | ||
695 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) | |
696 | ||
697 | This last example splits $foo into the first two words and the | |
5f05dabc | 698 | remainder of the line, and assigns those three fields to $F1, $F2, and |
699 | $Etc. The conditional is true if any variables were assigned, i.e., if | |
a0d0e21e LW |
700 | the pattern matched. |
701 | ||
702 | The C</g> modifier specifies global pattern matching--that is, matching | |
703 | as many times as possible within the string. How it behaves depends on | |
704 | the context. In a list context, it returns a list of all the | |
705 | substrings matched by all the parentheses in the regular expression. | |
706 | If there are no parentheses, it returns a list of all the matched | |
707 | strings, as if there were parentheses around the whole pattern. | |
708 | ||
709 | In a scalar context, C<m//g> iterates through the string, returning TRUE | |
c90c0ff4 | 710 | each time it matches, and FALSE when it eventually runs out of matches. |
711 | (In other words, it remembers where it left off last time and restarts | |
712 | the search at that point. You can actually find the current match | |
713 | position of a string or set it using the pos() function; see | |
714 | L<perlfunc/pos>.) A failed match normally resets the search position to | |
90248788 | 715 | the beginning of the string, but you can avoid that by adding the C</c> |
c90c0ff4 | 716 | modifier (e.g. C<m//gc>). Modifying the target string also resets the |
717 | search position. | |
718 | ||
719 | You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a | |
720 | zero-width assertion that matches the exact position where the previous | |
721 | C<m//g>, if any, left off. The C<\G> assertion is not supported without | |
722 | the C</g> modifier; currently, without C</g>, C<\G> behaves just like | |
723 | C<\A>, but that's accidental and may change in the future. | |
724 | ||
725 | Examples: | |
a0d0e21e LW |
726 | |
727 | # list context | |
728 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); | |
729 | ||
730 | # scalar context | |
5f05dabc | 731 | $/ = ""; $* = 1; # $* deprecated in modern perls |
54310121 | 732 | while (defined($paragraph = <>)) { |
a0d0e21e LW |
733 | while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { |
734 | $sentences++; | |
735 | } | |
736 | } | |
737 | print "$sentences\n"; | |
738 | ||
c90c0ff4 | 739 | # using m//gc with \G |
137443ea | 740 | $_ = "ppooqppqq"; |
44a8e56a | 741 | while ($i++ < 2) { |
742 | print "1: '"; | |
c90c0ff4 | 743 | print $1 while /(o)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 744 | print "2: '"; |
c90c0ff4 | 745 | print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 746 | print "3: '"; |
c90c0ff4 | 747 | print $1 while /(p)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 748 | } |
749 | ||
750 | The last example should print: | |
751 | ||
752 | 1: 'oo', pos=4 | |
137443ea | 753 | 2: 'q', pos=5 |
44a8e56a | 754 | 3: 'pp', pos=7 |
755 | 1: '', pos=7 | |
137443ea | 756 | 2: 'q', pos=8 |
757 | 3: '', pos=8 | |
44a8e56a | 758 | |
c90c0ff4 | 759 | A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can |
e7ea3e70 | 760 | combine several regexps like this to process a string part-by-part, |
c90c0ff4 | 761 | doing different actions depending on which regexp matched. Each |
762 | regexp tries to match where the previous one leaves off. | |
e7ea3e70 | 763 | |
3fe9a6f1 | 764 | $_ = <<'EOL'; |
e7ea3e70 | 765 | $url = new URI::URL "http://www/"; die if $url eq "xXx"; |
3fe9a6f1 | 766 | EOL |
767 | LOOP: | |
e7ea3e70 | 768 | { |
c90c0ff4 | 769 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; |
770 | print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc; | |
771 | print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc; | |
772 | print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc; | |
773 | print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc; | |
774 | print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc; | |
775 | print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc; | |
e7ea3e70 IZ |
776 | print ". That's all!\n"; |
777 | } | |
778 | ||
779 | Here is the output (split into several lines): | |
780 | ||
781 | line-noise lowercase line-noise lowercase UPPERCASE line-noise | |
782 | UPPERCASE line-noise lowercase line-noise lowercase line-noise | |
783 | lowercase lowercase line-noise lowercase lowercase line-noise | |
784 | MiXeD line-noise. That's all! | |
44a8e56a | 785 | |
a0d0e21e LW |
786 | =item q/STRING/ |
787 | ||
788 | =item C<'STRING'> | |
789 | ||
68dc0745 | 790 | A single-quoted, literal string. A backslash represents a backslash |
791 | unless followed by the delimiter or another backslash, in which case | |
792 | the delimiter or backslash is interpolated. | |
a0d0e21e LW |
793 | |
794 | $foo = q!I said, "You said, 'She said it.'"!; | |
795 | $bar = q('This is it.'); | |
68dc0745 | 796 | $baz = '\n'; # a two-character string |
a0d0e21e LW |
797 | |
798 | =item qq/STRING/ | |
799 | ||
800 | =item "STRING" | |
801 | ||
802 | A double-quoted, interpolated string. | |
803 | ||
804 | $_ .= qq | |
805 | (*** The previous line contains the naughty word "$1".\n) | |
806 | if /(tcl|rexx|python)/; # :-) | |
68dc0745 | 807 | $baz = "\n"; # a one-character string |
a0d0e21e LW |
808 | |
809 | =item qx/STRING/ | |
810 | ||
811 | =item `STRING` | |
812 | ||
813 | A string which is interpolated and then executed as a system command. | |
814 | The collected standard output of the command is returned. In scalar | |
4a6725af | 815 | context, it comes back as a single (potentially multi-line) string. |
a0d0e21e LW |
816 | In list context, returns a list of lines (however you've defined lines |
817 | with $/ or $INPUT_RECORD_SEPARATOR). | |
818 | ||
819 | $today = qx{ date }; | |
820 | ||
bb32b41a GS |
821 | Note that how the string gets evaluated is entirely subject to the |
822 | command interpreter on your system. On most platforms, you will have | |
823 | to protect shell metacharacters if you want them treated literally. | |
824 | On some platforms (notably DOS-like ones), the shell may not be | |
825 | capable of dealing with multiline commands, so putting newlines in | |
826 | the string may not get you what you want. You may be able to evaluate | |
827 | multiple commands in a single line by separating them with the command | |
828 | separator character, if your shell supports that (e.g. C<;> on many Unix | |
829 | shells; C<&> on the Windows NT C<cmd> shell). | |
830 | ||
831 | Beware that some command shells may place restrictions on the length | |
832 | of the command line. You must ensure your strings don't exceed this | |
833 | limit after any necessary interpolations. See the platform-specific | |
834 | release notes for more details about your particular environment. | |
835 | ||
836 | Also realize that using this operator frequently leads to unportable | |
837 | programs. | |
838 | ||
dc848c6f | 839 | See L<"I/O Operators"> for more discussion. |
a0d0e21e LW |
840 | |
841 | =item qw/STRING/ | |
842 | ||
843 | Returns a list of the words extracted out of STRING, using embedded | |
844 | whitespace as the word delimiters. It is exactly equivalent to | |
845 | ||
846 | split(' ', q/STRING/); | |
847 | ||
848 | Some frequently seen examples: | |
849 | ||
850 | use POSIX qw( setlocale localeconv ) | |
851 | @EXPORT = qw( foo bar baz ); | |
852 | ||
7bac28a0 | 853 | A common mistake is to try to separate the words with comma or to put |
854 | comments into a multi-line qw-string. For this reason the C<-w> | |
855 | switch produce warnings if the STRING contains the "," or the "#" | |
856 | character. | |
857 | ||
a0d0e21e LW |
858 | =item s/PATTERN/REPLACEMENT/egimosx |
859 | ||
860 | Searches a string for a pattern, and if found, replaces that pattern | |
861 | with the replacement text and returns the number of substitutions | |
e37d713d | 862 | made. Otherwise it returns false (specifically, the empty string). |
a0d0e21e LW |
863 | |
864 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> | |
865 | variable is searched and modified. (The string specified with C<=~> must | |
866 | be a scalar variable, an array element, a hash element, or an assignment | |
5f05dabc | 867 | to one of those, i.e., an lvalue.) |
a0d0e21e LW |
868 | |
869 | If the delimiter chosen is single quote, no variable interpolation is | |
870 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the | |
871 | PATTERN contains a $ that looks like a variable rather than an | |
872 | end-of-string test, the variable will be interpolated into the pattern | |
5f05dabc | 873 | at run-time. If you want the pattern compiled only once the first time |
a0d0e21e | 874 | the variable is interpolated, use the C</o> option. If the pattern |
4633a7c4 | 875 | evaluates to a null string, the last successfully executed regular |
a0d0e21e | 876 | expression is used instead. See L<perlre> for further explanation on these. |
a034a98d DD |
877 | See L<perllocale> for discussion of additional considerations which apply |
878 | when C<use locale> is in effect. | |
a0d0e21e LW |
879 | |
880 | Options are: | |
881 | ||
882 | e Evaluate the right side as an expression. | |
5f05dabc | 883 | g Replace globally, i.e., all occurrences. |
a0d0e21e LW |
884 | i Do case-insensitive pattern matching. |
885 | m Treat string as multiple lines. | |
5f05dabc | 886 | o Compile pattern only once. |
a0d0e21e LW |
887 | s Treat string as single line. |
888 | x Use extended regular expressions. | |
889 | ||
890 | Any non-alphanumeric, non-whitespace delimiter may replace the | |
891 | slashes. If single quotes are used, no interpretation is done on the | |
e37d713d | 892 | replacement string (the C</e> modifier overrides this, however). Unlike |
54310121 | 893 | Perl 4, Perl 5 treats backticks as normal delimiters; the replacement |
e37d713d | 894 | text is not evaluated as a command. If the |
a0d0e21e | 895 | PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own |
5f05dabc | 896 | pair of quotes, which may or may not be bracketing quotes, e.g., |
a0d0e21e LW |
897 | C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the |
898 | replacement portion to be interpreter as a full-fledged Perl expression | |
899 | and eval()ed right then and there. It is, however, syntax checked at | |
900 | compile-time. | |
901 | ||
902 | Examples: | |
903 | ||
904 | s/\bgreen\b/mauve/g; # don't change wintergreen | |
905 | ||
906 | $path =~ s|/usr/bin|/usr/local/bin|; | |
907 | ||
908 | s/Login: $foo/Login: $bar/; # run-time pattern | |
909 | ||
910 | ($foo = $bar) =~ s/this/that/; | |
911 | ||
912 | $count = ($paragraph =~ s/Mister\b/Mr./g); | |
913 | ||
914 | $_ = 'abc123xyz'; | |
915 | s/\d+/$&*2/e; # yields 'abc246xyz' | |
916 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' | |
917 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' | |
918 | ||
919 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e | |
920 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e | |
921 | s/^=(\w+)/&pod($1)/ge; # use function call | |
922 | ||
923 | # /e's can even nest; this will expand | |
924 | # simple embedded variables in $_ | |
925 | s/(\$\w+)/$1/eeg; | |
926 | ||
927 | # Delete C comments. | |
928 | $program =~ s { | |
4633a7c4 LW |
929 | /\* # Match the opening delimiter. |
930 | .*? # Match a minimal number of characters. | |
931 | \*/ # Match the closing delimiter. | |
a0d0e21e LW |
932 | } []gsx; |
933 | ||
934 | s/^\s*(.*?)\s*$/$1/; # trim white space | |
935 | ||
936 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields | |
937 | ||
54310121 | 938 | Note the use of $ instead of \ in the last example. Unlike |
5f05dabc | 939 | B<sed>, we use the \E<lt>I<digit>E<gt> form in only the left hand side. |
6ee5d4e7 | 940 | Anywhere else it's $E<lt>I<digit>E<gt>. |
a0d0e21e | 941 | |
5f05dabc | 942 | Occasionally, you can't use just a C</g> to get all the changes |
a0d0e21e LW |
943 | to occur. Here are two common cases: |
944 | ||
945 | # put commas in the right places in an integer | |
946 | 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4 | |
947 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5 | |
948 | ||
949 | # expand tabs to 8-column spacing | |
950 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; | |
951 | ||
952 | ||
953 | =item tr/SEARCHLIST/REPLACEMENTLIST/cds | |
954 | ||
955 | =item y/SEARCHLIST/REPLACEMENTLIST/cds | |
956 | ||
957 | Translates all occurrences of the characters found in the search list | |
958 | with the corresponding character in the replacement list. It returns | |
959 | the number of characters replaced or deleted. If no string is | |
960 | specified via the =~ or !~ operator, the $_ string is translated. (The | |
54310121 | 961 | string specified with =~ must be a scalar variable, an array element, a |
962 | hash element, or an assignment to one of those, i.e., an lvalue.) | |
963 | For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the | |
964 | SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has | |
965 | its own pair of quotes, which may or may not be bracketing quotes, | |
966 | e.g., C<tr[A-Z][a-z]> or C<tr(+-*/)/ABCD/>. | |
a0d0e21e LW |
967 | |
968 | Options: | |
969 | ||
970 | c Complement the SEARCHLIST. | |
971 | d Delete found but unreplaced characters. | |
972 | s Squash duplicate replaced characters. | |
973 | ||
974 | If the C</c> modifier is specified, the SEARCHLIST character set is | |
975 | complemented. If the C</d> modifier is specified, any characters specified | |
976 | by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note | |
977 | that this is slightly more flexible than the behavior of some B<tr> | |
978 | programs, which delete anything they find in the SEARCHLIST, period.) | |
979 | If the C</s> modifier is specified, sequences of characters that were | |
980 | translated to the same character are squashed down to a single instance of the | |
981 | character. | |
982 | ||
983 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted | |
984 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter | |
985 | than the SEARCHLIST, the final character is replicated till it is long | |
986 | enough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated. | |
987 | This latter is useful for counting characters in a class or for | |
988 | squashing character sequences in a class. | |
989 | ||
990 | Examples: | |
991 | ||
992 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case | |
993 | ||
994 | $cnt = tr/*/*/; # count the stars in $_ | |
995 | ||
996 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky | |
997 | ||
998 | $cnt = tr/0-9//; # count the digits in $_ | |
999 | ||
1000 | tr/a-zA-Z//s; # bookkeeper -> bokeper | |
1001 | ||
1002 | ($HOST = $host) =~ tr/a-z/A-Z/; | |
1003 | ||
1004 | tr/a-zA-Z/ /cs; # change non-alphas to single space | |
1005 | ||
1006 | tr [\200-\377] | |
1007 | [\000-\177]; # delete 8th bit | |
1008 | ||
748a9306 LW |
1009 | If multiple translations are given for a character, only the first one is used: |
1010 | ||
1011 | tr/AAA/XYZ/ | |
1012 | ||
1013 | will translate any A to X. | |
1014 | ||
a0d0e21e LW |
1015 | Note that because the translation table is built at compile time, neither |
1016 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote | |
1017 | interpolation. That means that if you want to use variables, you must use | |
1018 | an eval(): | |
1019 | ||
1020 | eval "tr/$oldlist/$newlist/"; | |
1021 | die $@ if $@; | |
1022 | ||
1023 | eval "tr/$oldlist/$newlist/, 1" or die $@; | |
1024 | ||
1025 | =back | |
1026 | ||
1027 | =head2 I/O Operators | |
1028 | ||
54310121 | 1029 | There are several I/O operators you should know about. |
1030 | A string is enclosed by backticks (grave accents) first undergoes | |
a0d0e21e LW |
1031 | variable substitution just like a double quoted string. It is then |
1032 | interpreted as a command, and the output of that command is the value | |
1033 | of the pseudo-literal, like in a shell. In a scalar context, a single | |
1034 | string consisting of all the output is returned. In a list context, | |
1035 | a list of values is returned, one for each line of output. (You can | |
1036 | set C<$/> to use a different line terminator.) The command is executed | |
1037 | each time the pseudo-literal is evaluated. The status value of the | |
1038 | command is returned in C<$?> (see L<perlvar> for the interpretation | |
1039 | of C<$?>). Unlike in B<csh>, no translation is done on the return | |
1040 | data--newlines remain newlines. Unlike in any of the shells, single | |
1041 | quotes do not hide variable names in the command from interpretation. | |
1042 | To pass a $ through to the shell you need to hide it with a backslash. | |
54310121 | 1043 | The generalized form of backticks is C<qx//>. (Because backticks |
1044 | always undergo shell expansion as well, see L<perlsec> for | |
cb1a09d0 | 1045 | security concerns.) |
a0d0e21e LW |
1046 | |
1047 | Evaluating a filehandle in angle brackets yields the next line from | |
aa689395 | 1048 | that file (newline, if any, included), or C<undef> at end of file. |
1049 | Ordinarily you must assign that value to a variable, but there is one | |
1050 | situation where an automatic assignment happens. I<If and ONLY if> the | |
1051 | input symbol is the only thing inside the conditional of a C<while> or | |
1052 | C<for(;;)> loop, the value is automatically assigned to the variable | |
1053 | C<$_>. The assigned value is then tested to see if it is defined. | |
1054 | (This may seem like an odd thing to you, but you'll use the construct | |
1055 | in almost every Perl script you write.) Anyway, the following lines | |
1056 | are equivalent to each other: | |
a0d0e21e | 1057 | |
748a9306 | 1058 | while (defined($_ = <STDIN>)) { print; } |
a0d0e21e LW |
1059 | while (<STDIN>) { print; } |
1060 | for (;<STDIN>;) { print; } | |
748a9306 | 1061 | print while defined($_ = <STDIN>); |
a0d0e21e LW |
1062 | print while <STDIN>; |
1063 | ||
5f05dabc | 1064 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
1065 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in | |
a0d0e21e LW |
1066 | packages, where they would be interpreted as local identifiers rather |
1067 | than global.) Additional filehandles may be created with the open() | |
cb1a09d0 | 1068 | function. See L<perlfunc/open()> for details on this. |
a0d0e21e | 1069 | |
6ee5d4e7 | 1070 | If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a |
a0d0e21e LW |
1071 | list consisting of all the input lines is returned, one line per list |
1072 | element. It's easy to make a I<LARGE> data space this way, so use with | |
1073 | care. | |
1074 | ||
d28ebecd | 1075 | The null filehandle E<lt>E<gt> is special and can be used to emulate the |
1076 | behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from | |
a0d0e21e | 1077 | standard input, or from each file listed on the command line. Here's |
d28ebecd | 1078 | how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is |
a0d0e21e LW |
1079 | checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened |
1080 | gives you standard input. The @ARGV array is then processed as a list | |
1081 | of filenames. The loop | |
1082 | ||
1083 | while (<>) { | |
1084 | ... # code for each line | |
1085 | } | |
1086 | ||
1087 | is equivalent to the following Perl-like pseudo code: | |
1088 | ||
3e3baf6d | 1089 | unshift(@ARGV, '-') unless @ARGV; |
a0d0e21e LW |
1090 | while ($ARGV = shift) { |
1091 | open(ARGV, $ARGV); | |
1092 | while (<ARGV>) { | |
1093 | ... # code for each line | |
1094 | } | |
1095 | } | |
1096 | ||
1097 | except that it isn't so cumbersome to say, and will actually work. It | |
1098 | really does shift array @ARGV and put the current filename into variable | |
5f05dabc | 1099 | $ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a |
1100 | synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above | |
1101 | doesn't work because it treats E<lt>ARGVE<gt> as non-magical.) | |
a0d0e21e | 1102 | |
d28ebecd | 1103 | You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up |
a0d0e21e LW |
1104 | containing the list of filenames you really want. Line numbers (C<$.>) |
1105 | continue as if the input were one big happy file. (But see example | |
1106 | under eof() for how to reset line numbers on each file.) | |
1107 | ||
1108 | If you want to set @ARGV to your own list of files, go right ahead. If | |
54310121 | 1109 | you want to pass switches into your script, you can use one of the |
a0d0e21e LW |
1110 | Getopts modules or put a loop on the front like this: |
1111 | ||
1112 | while ($_ = $ARGV[0], /^-/) { | |
1113 | shift; | |
1114 | last if /^--$/; | |
1115 | if (/^-D(.*)/) { $debug = $1 } | |
1116 | if (/^-v/) { $verbose++ } | |
1117 | ... # other switches | |
1118 | } | |
1119 | while (<>) { | |
1120 | ... # code for each line | |
1121 | } | |
1122 | ||
d28ebecd | 1123 | The E<lt>E<gt> symbol will return FALSE only once. If you call it again after |
a0d0e21e LW |
1124 | this it will assume you are processing another @ARGV list, and if you |
1125 | haven't set @ARGV, will input from STDIN. | |
1126 | ||
1127 | If the string inside the angle brackets is a reference to a scalar | |
5f05dabc | 1128 | variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the |
cb1a09d0 AD |
1129 | filehandle to input from, or a reference to the same. For example: |
1130 | ||
1131 | $fh = \*STDIN; | |
1132 | $line = <$fh>; | |
a0d0e21e | 1133 | |
cb1a09d0 AD |
1134 | If the string inside angle brackets is not a filehandle or a scalar |
1135 | variable containing a filehandle name or reference, then it is interpreted | |
4633a7c4 LW |
1136 | as a filename pattern to be globbed, and either a list of filenames or the |
1137 | next filename in the list is returned, depending on context. One level of | |
1138 | $ interpretation is done first, but you can't say C<E<lt>$fooE<gt>> | |
1139 | because that's an indirect filehandle as explained in the previous | |
6ee5d4e7 | 1140 | paragraph. (In older versions of Perl, programmers would insert curly |
4633a7c4 | 1141 | brackets to force interpretation as a filename glob: C<E<lt>${foo}E<gt>>. |
d28ebecd | 1142 | These days, it's considered cleaner to call the internal function directly |
4633a7c4 LW |
1143 | as C<glob($foo)>, which is probably the right way to have done it in the |
1144 | first place.) Example: | |
a0d0e21e LW |
1145 | |
1146 | while (<*.c>) { | |
1147 | chmod 0644, $_; | |
1148 | } | |
1149 | ||
1150 | is equivalent to | |
1151 | ||
1152 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); | |
1153 | while (<FOO>) { | |
1154 | chop; | |
1155 | chmod 0644, $_; | |
1156 | } | |
1157 | ||
1158 | In fact, it's currently implemented that way. (Which means it will not | |
1159 | work on filenames with spaces in them unless you have csh(1) on your | |
1160 | machine.) Of course, the shortest way to do the above is: | |
1161 | ||
1162 | chmod 0644, <*.c>; | |
1163 | ||
1164 | Because globbing invokes a shell, it's often faster to call readdir() yourself | |
5f05dabc | 1165 | and do your own grep() on the filenames. Furthermore, due to its current |
54310121 | 1166 | implementation of using a shell, the glob() routine may get "Arg list too |
a0d0e21e LW |
1167 | long" errors (unless you've installed tcsh(1L) as F</bin/csh>). |
1168 | ||
5f05dabc | 1169 | A glob evaluates its (embedded) argument only when it is starting a new |
4633a7c4 LW |
1170 | list. All values must be read before it will start over. In a list |
1171 | context this isn't important, because you automatically get them all | |
1172 | anyway. In a scalar context, however, the operator returns the next value | |
1173 | each time it is called, or a FALSE value if you've just run out. Again, | |
1174 | FALSE is returned only once. So if you're expecting a single value from | |
1175 | a glob, it is much better to say | |
1176 | ||
1177 | ($file) = <blurch*>; | |
1178 | ||
1179 | than | |
1180 | ||
1181 | $file = <blurch*>; | |
1182 | ||
1183 | because the latter will alternate between returning a filename and | |
54310121 | 1184 | returning FALSE. |
4633a7c4 LW |
1185 | |
1186 | It you're trying to do variable interpolation, it's definitely better | |
1187 | to use the glob() function, because the older notation can cause people | |
e37d713d | 1188 | to become confused with the indirect filehandle notation. |
4633a7c4 LW |
1189 | |
1190 | @files = glob("$dir/*.[ch]"); | |
1191 | @files = glob($files[$i]); | |
1192 | ||
a0d0e21e LW |
1193 | =head2 Constant Folding |
1194 | ||
1195 | Like C, Perl does a certain amount of expression evaluation at | |
1196 | compile time, whenever it determines that all of the arguments to an | |
1197 | operator are static and have no side effects. In particular, string | |
1198 | concatenation happens at compile time between literals that don't do | |
1199 | variable substitution. Backslash interpretation also happens at | |
1200 | compile time. You can say | |
1201 | ||
1202 | 'Now is the time for all' . "\n" . | |
1203 | 'good men to come to.' | |
1204 | ||
54310121 | 1205 | and this all reduces to one string internally. Likewise, if |
a0d0e21e LW |
1206 | you say |
1207 | ||
1208 | foreach $file (@filenames) { | |
1209 | if (-s $file > 5 + 100 * 2**16) { ... } | |
54310121 | 1210 | } |
a0d0e21e | 1211 | |
54310121 | 1212 | the compiler will precompute the number that |
a0d0e21e LW |
1213 | expression represents so that the interpreter |
1214 | won't have to. | |
1215 | ||
1216 | ||
55497cff | 1217 | =head2 Integer Arithmetic |
a0d0e21e LW |
1218 | |
1219 | By default Perl assumes that it must do most of its arithmetic in | |
1220 | floating point. But by saying | |
1221 | ||
1222 | use integer; | |
1223 | ||
1224 | you may tell the compiler that it's okay to use integer operations | |
1225 | from here to the end of the enclosing BLOCK. An inner BLOCK may | |
54310121 | 1226 | countermand this by saying |
a0d0e21e LW |
1227 | |
1228 | no integer; | |
1229 | ||
1230 | which lasts until the end of that BLOCK. | |
1231 | ||
55497cff | 1232 | The bitwise operators ("&", "|", "^", "~", "<<", and ">>") always |
1233 | produce integral results. However, C<use integer> still has meaning | |
1234 | for them. By default, their results are interpreted as unsigned | |
1235 | integers. However, if C<use integer> is in effect, their results are | |
5f05dabc | 1236 | interpreted as signed integers. For example, C<~0> usually evaluates |
55497cff | 1237 | to a large integral value. However, C<use integer; ~0> is -1. |
68dc0745 | 1238 | |
1239 | =head2 Floating-point Arithmetic | |
1240 | ||
1241 | While C<use integer> provides integer-only arithmetic, there is no | |
1242 | similar ways to provide rounding or truncation at a certain number of | |
1243 | decimal places. For rounding to a certain number of digits, sprintf() | |
1244 | or printf() is usually the easiest route. | |
1245 | ||
1246 | The POSIX module (part of the standard perl distribution) implements | |
1247 | ceil(), floor(), and a number of other mathematical and trigonometric | |
1248 | functions. The Math::Complex module (part of the standard perl | |
1249 | distribution) defines a number of mathematical functions that can also | |
1250 | work on real numbers. Math::Complex not as efficient as POSIX, but | |
1251 | POSIX can't work with complex numbers. | |
1252 | ||
1253 | Rounding in financial applications can have serious implications, and | |
1254 | the rounding method used should be specified precisely. In these | |
1255 | cases, it probably pays not to trust whichever system rounding is | |
1256 | being used by Perl, but to instead implement the rounding function you | |
1257 | need yourself. |