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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 | ||
5a964f20 TC |
41 | Many operators can be overloaded for objects. See L<overload>. |
42 | ||
cb1a09d0 | 43 | =head1 DESCRIPTION |
a0d0e21e LW |
44 | |
45 | =head2 Terms and List Operators (Leftward) | |
46 | ||
62c18ce2 | 47 | A TERM has the highest precedence in Perl. They include variables, |
5f05dabc | 48 | quote and quote-like operators, any expression in parentheses, |
a0d0e21e LW |
49 | and any function whose arguments are parenthesized. Actually, there |
50 | aren't really functions in this sense, just list operators and unary | |
51 | operators behaving as functions because you put parentheses around | |
52 | the arguments. These are all documented in L<perlfunc>. | |
53 | ||
54 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
55 | is followed by a left parenthesis as the next token, the operator and | |
56 | arguments within parentheses are taken to be of highest precedence, | |
57 | just like a normal function call. | |
58 | ||
59 | In the absence of parentheses, the precedence of list operators such as | |
60 | C<print>, C<sort>, or C<chmod> is either very high or very low depending on | |
54310121 | 61 | whether you are looking at the left side or the right side of the operator. |
a0d0e21e LW |
62 | For example, in |
63 | ||
64 | @ary = (1, 3, sort 4, 2); | |
65 | print @ary; # prints 1324 | |
66 | ||
67 | the commas on the right of the sort are evaluated before the sort, but | |
68 | the commas on the left are evaluated after. In other words, list | |
69 | operators tend to gobble up all the arguments that follow them, and | |
70 | then act like a simple TERM with regard to the preceding expression. | |
5f05dabc | 71 | Note that you have to be careful with parentheses: |
a0d0e21e LW |
72 | |
73 | # These evaluate exit before doing the print: | |
74 | print($foo, exit); # Obviously not what you want. | |
75 | print $foo, exit; # Nor is this. | |
76 | ||
77 | # These do the print before evaluating exit: | |
78 | (print $foo), exit; # This is what you want. | |
79 | print($foo), exit; # Or this. | |
80 | print ($foo), exit; # Or even this. | |
81 | ||
82 | Also note that | |
83 | ||
84 | print ($foo & 255) + 1, "\n"; | |
85 | ||
54310121 | 86 | probably doesn't do what you expect at first glance. See |
a0d0e21e LW |
87 | L<Named Unary Operators> for more discussion of this. |
88 | ||
89 | Also parsed as terms are the C<do {}> and C<eval {}> constructs, as | |
54310121 | 90 | well as subroutine and method calls, and the anonymous |
a0d0e21e LW |
91 | constructors C<[]> and C<{}>. |
92 | ||
2ae324a7 | 93 | See also L<Quote and Quote-like Operators> toward the end of this section, |
c07a80fd | 94 | as well as L<"I/O Operators">. |
a0d0e21e LW |
95 | |
96 | =head2 The Arrow Operator | |
97 | ||
98 | Just as in C and C++, "C<-E<gt>>" is an infix dereference operator. If the | |
99 | right side is either a C<[...]> or C<{...}> subscript, then the left side | |
100 | must be either a hard or symbolic reference to an array or hash (or | |
101 | a location capable of holding a hard reference, if it's an lvalue (assignable)). | |
102 | See L<perlref>. | |
103 | ||
104 | Otherwise, the right side is a method name or a simple scalar variable | |
105 | containing the method name, and the left side must either be an object | |
106 | (a blessed reference) or a class name (that is, a package name). | |
107 | See L<perlobj>. | |
108 | ||
5f05dabc | 109 | =head2 Auto-increment and Auto-decrement |
a0d0e21e LW |
110 | |
111 | "++" and "--" work as in C. That is, if placed before a variable, they | |
112 | increment or decrement the variable before returning the value, and if | |
113 | placed after, increment or decrement the variable after returning the value. | |
114 | ||
54310121 | 115 | The auto-increment operator has a little extra builtin magic to it. If |
a0d0e21e LW |
116 | you increment a variable that is numeric, or that has ever been used in |
117 | a numeric context, you get a normal increment. If, however, the | |
5f05dabc | 118 | variable has been used in only string contexts since it was set, and |
5a964f20 | 119 | has a value that is not the empty string and matches the pattern |
a0d0e21e LW |
120 | C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each |
121 | character within its range, with carry: | |
122 | ||
123 | print ++($foo = '99'); # prints '100' | |
124 | print ++($foo = 'a0'); # prints 'a1' | |
125 | print ++($foo = 'Az'); # prints 'Ba' | |
126 | print ++($foo = 'zz'); # prints 'aaa' | |
127 | ||
5f05dabc | 128 | The auto-decrement operator is not magical. |
a0d0e21e LW |
129 | |
130 | =head2 Exponentiation | |
131 | ||
132 | Binary "**" is the exponentiation operator. Note that it binds even more | |
cb1a09d0 AD |
133 | tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is |
134 | implemented using C's pow(3) function, which actually works on doubles | |
135 | internally.) | |
a0d0e21e LW |
136 | |
137 | =head2 Symbolic Unary Operators | |
138 | ||
5f05dabc | 139 | Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower |
a0d0e21e LW |
140 | precedence version of this. |
141 | ||
142 | Unary "-" performs arithmetic negation if the operand is numeric. If | |
143 | the operand is an identifier, a string consisting of a minus sign | |
144 | concatenated with the identifier is returned. Otherwise, if the string | |
145 | starts with a plus or minus, a string starting with the opposite sign | |
146 | is returned. One effect of these rules is that C<-bareword> is equivalent | |
147 | to C<"-bareword">. | |
148 | ||
5a964f20 TC |
149 | Unary "~" performs bitwise negation, i.e., 1's complement. For example, |
150 | C<0666 &~ 027> is 0640. (See also L<Integer Arithmetic> and L<Bitwise | |
151 | String Operators>.) | |
a0d0e21e LW |
152 | |
153 | Unary "+" has no effect whatsoever, even on strings. It is useful | |
154 | syntactically for separating a function name from a parenthesized expression | |
155 | that would otherwise be interpreted as the complete list of function | |
5ba421f6 | 156 | arguments. (See examples above under L<Terms and List Operators (Leftward)>.) |
a0d0e21e LW |
157 | |
158 | Unary "\" creates a reference to whatever follows it. See L<perlref>. | |
159 | Do not confuse this behavior with the behavior of backslash within a | |
160 | string, although both forms do convey the notion of protecting the next | |
161 | thing from interpretation. | |
162 | ||
163 | =head2 Binding Operators | |
164 | ||
c07a80fd | 165 | Binary "=~" binds a scalar expression to a pattern match. Certain operations |
cb1a09d0 AD |
166 | search or modify the string $_ by default. This operator makes that kind |
167 | of operation work on some other string. The right argument is a search | |
2c268ad5 TP |
168 | pattern, substitution, or transliteration. The left argument is what is |
169 | supposed to be searched, substituted, or transliterated instead of the default | |
cb1a09d0 AD |
170 | $_. The return value indicates the success of the operation. (If the |
171 | right argument is an expression rather than a search pattern, | |
2c268ad5 | 172 | substitution, or transliteration, it is interpreted as a search pattern at run |
aa689395 | 173 | time. This can be is less efficient than an explicit search, because the |
174 | pattern must be compiled every time the expression is evaluated. | |
a0d0e21e LW |
175 | |
176 | Binary "!~" is just like "=~" except the return value is negated in | |
177 | the logical sense. | |
178 | ||
179 | =head2 Multiplicative Operators | |
180 | ||
181 | Binary "*" multiplies two numbers. | |
182 | ||
183 | Binary "/" divides two numbers. | |
184 | ||
54310121 | 185 | Binary "%" computes the modulus of two numbers. Given integer |
186 | operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is | |
187 | C<$a> minus the largest multiple of C<$b> that is not greater than | |
188 | C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the | |
189 | smallest multiple of C<$b> that is not less than C<$a> (i.e. the | |
6bb4e6d4 | 190 | result will be less than or equal to zero). |
5a964f20 | 191 | Note than when C<use integer> is in scope, "%" give you direct access |
55d729e4 GS |
192 | to the modulus operator as implemented by your C compiler. This |
193 | operator is not as well defined for negative operands, but it will | |
194 | execute faster. | |
195 | ||
5a964f20 | 196 | Binary "x" is the repetition operator. In scalar context, it |
a0d0e21e | 197 | returns a string consisting of the left operand repeated the number of |
5a964f20 | 198 | times specified by the right operand. In list context, if the left |
5f05dabc | 199 | operand is a list in parentheses, it repeats the list. |
a0d0e21e LW |
200 | |
201 | print '-' x 80; # print row of dashes | |
202 | ||
203 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over | |
204 | ||
205 | @ones = (1) x 80; # a list of 80 1's | |
206 | @ones = (5) x @ones; # set all elements to 5 | |
207 | ||
208 | ||
209 | =head2 Additive Operators | |
210 | ||
211 | Binary "+" returns the sum of two numbers. | |
212 | ||
213 | Binary "-" returns the difference of two numbers. | |
214 | ||
215 | Binary "." concatenates two strings. | |
216 | ||
217 | =head2 Shift Operators | |
218 | ||
55497cff | 219 | Binary "<<" returns the value of its left argument shifted left by the |
220 | number of bits specified by the right argument. Arguments should be | |
221 | integers. (See also L<Integer Arithmetic>.) | |
a0d0e21e | 222 | |
55497cff | 223 | Binary ">>" returns the value of its left argument shifted right by |
224 | the number of bits specified by the right argument. Arguments should | |
225 | be integers. (See also L<Integer Arithmetic>.) | |
a0d0e21e LW |
226 | |
227 | =head2 Named Unary Operators | |
228 | ||
229 | The various named unary operators are treated as functions with one | |
230 | argument, with optional parentheses. These include the filetest | |
231 | operators, like C<-f>, C<-M>, etc. See L<perlfunc>. | |
232 | ||
233 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
234 | is followed by a left parenthesis as the next token, the operator and | |
235 | arguments within parentheses are taken to be of highest precedence, | |
236 | just like a normal function call. Examples: | |
237 | ||
238 | chdir $foo || die; # (chdir $foo) || die | |
239 | chdir($foo) || die; # (chdir $foo) || die | |
240 | chdir ($foo) || die; # (chdir $foo) || die | |
241 | chdir +($foo) || die; # (chdir $foo) || die | |
242 | ||
243 | but, because * is higher precedence than ||: | |
244 | ||
245 | chdir $foo * 20; # chdir ($foo * 20) | |
246 | chdir($foo) * 20; # (chdir $foo) * 20 | |
247 | chdir ($foo) * 20; # (chdir $foo) * 20 | |
248 | chdir +($foo) * 20; # chdir ($foo * 20) | |
249 | ||
250 | rand 10 * 20; # rand (10 * 20) | |
251 | rand(10) * 20; # (rand 10) * 20 | |
252 | rand (10) * 20; # (rand 10) * 20 | |
253 | rand +(10) * 20; # rand (10 * 20) | |
254 | ||
5ba421f6 | 255 | See also L<"Terms and List Operators (Leftward)">. |
a0d0e21e LW |
256 | |
257 | =head2 Relational Operators | |
258 | ||
6ee5d4e7 | 259 | Binary "E<lt>" returns true if the left argument is numerically less than |
a0d0e21e LW |
260 | the right argument. |
261 | ||
6ee5d4e7 | 262 | Binary "E<gt>" returns true if the left argument is numerically greater |
a0d0e21e LW |
263 | than the right argument. |
264 | ||
6ee5d4e7 | 265 | Binary "E<lt>=" returns true if the left argument is numerically less than |
a0d0e21e LW |
266 | or equal to the right argument. |
267 | ||
6ee5d4e7 | 268 | Binary "E<gt>=" returns true if the left argument is numerically greater |
a0d0e21e LW |
269 | than or equal to the right argument. |
270 | ||
271 | Binary "lt" returns true if the left argument is stringwise less than | |
272 | the right argument. | |
273 | ||
274 | Binary "gt" returns true if the left argument is stringwise greater | |
275 | than the right argument. | |
276 | ||
277 | Binary "le" returns true if the left argument is stringwise less than | |
278 | or equal to the right argument. | |
279 | ||
280 | Binary "ge" returns true if the left argument is stringwise greater | |
281 | than or equal to the right argument. | |
282 | ||
283 | =head2 Equality Operators | |
284 | ||
285 | Binary "==" returns true if the left argument is numerically equal to | |
286 | the right argument. | |
287 | ||
288 | Binary "!=" returns true if the left argument is numerically not equal | |
289 | to the right argument. | |
290 | ||
6ee5d4e7 | 291 | Binary "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left |
292 | argument is numerically less than, equal to, or greater than the right | |
293 | argument. | |
a0d0e21e LW |
294 | |
295 | Binary "eq" returns true if the left argument is stringwise equal to | |
296 | the right argument. | |
297 | ||
298 | Binary "ne" returns true if the left argument is stringwise not equal | |
299 | to the right argument. | |
300 | ||
301 | Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise | |
302 | less than, equal to, or greater than the right argument. | |
303 | ||
a034a98d DD |
304 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
305 | by the current locale if C<use locale> is in effect. See L<perllocale>. | |
306 | ||
a0d0e21e LW |
307 | =head2 Bitwise And |
308 | ||
309 | Binary "&" returns its operators ANDed together bit by bit. | |
2c268ad5 | 310 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e LW |
311 | |
312 | =head2 Bitwise Or and Exclusive Or | |
313 | ||
314 | Binary "|" returns its operators ORed together bit by bit. | |
2c268ad5 | 315 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e LW |
316 | |
317 | Binary "^" returns its operators XORed together bit by bit. | |
2c268ad5 | 318 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e LW |
319 | |
320 | =head2 C-style Logical And | |
321 | ||
322 | Binary "&&" performs a short-circuit logical AND operation. That is, | |
323 | if the left operand is false, 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 | =head2 C-style Logical Or | |
328 | ||
329 | Binary "||" performs a short-circuit logical OR operation. That is, | |
330 | if the left operand is true, the right operand is not even evaluated. | |
331 | Scalar or list context propagates down to the right operand if it | |
332 | is evaluated. | |
333 | ||
334 | The C<||> and C<&&> operators differ from C's in that, rather than returning | |
335 | 0 or 1, they return the last value evaluated. Thus, a reasonably portable | |
336 | way to find out the home directory (assuming it's not "0") might be: | |
337 | ||
338 | $home = $ENV{'HOME'} || $ENV{'LOGDIR'} || | |
339 | (getpwuid($<))[7] || die "You're homeless!\n"; | |
340 | ||
5a964f20 TC |
341 | In particular, this means that you shouldn't use this |
342 | for selecting between two aggregates for assignment: | |
343 | ||
344 | @a = @b || @c; # this is wrong | |
345 | @a = scalar(@b) || @c; # really meant this | |
346 | @a = @b ? @b : @c; # this works fine, though | |
347 | ||
348 | As more readable alternatives to C<&&> and C<||> when used for | |
349 | control flow, Perl provides C<and> and C<or> operators (see below). | |
350 | The short-circuit behavior is identical. The precedence of "and" and | |
351 | "or" is much lower, however, so that you can safely use them after a | |
352 | list operator without the need for parentheses: | |
a0d0e21e LW |
353 | |
354 | unlink "alpha", "beta", "gamma" | |
355 | or gripe(), next LINE; | |
356 | ||
357 | With the C-style operators that would have been written like this: | |
358 | ||
359 | unlink("alpha", "beta", "gamma") | |
360 | || (gripe(), next LINE); | |
361 | ||
5a964f20 TC |
362 | Use "or" for assignment is unlikely to do what you want; see below. |
363 | ||
364 | =head2 Range Operators | |
a0d0e21e LW |
365 | |
366 | Binary ".." is the range operator, which is really two different | |
5a964f20 | 367 | operators depending on the context. In list context, it returns an |
a0d0e21e | 368 | array of values counting (by ones) from the left value to the right |
89ea2908 GA |
369 | value. This is useful for writing C<foreach (1..10)> loops and for |
370 | doing slice operations on arrays. In the current implementation, no | |
371 | temporary array is created when the range operator is used as the | |
372 | expression in C<foreach> loops, but older versions of Perl might burn | |
373 | a lot of memory when you write something like this: | |
a0d0e21e LW |
374 | |
375 | for (1 .. 1_000_000) { | |
376 | # code | |
54310121 | 377 | } |
a0d0e21e | 378 | |
5a964f20 | 379 | In scalar context, ".." returns a boolean value. The operator is |
a0d0e21e LW |
380 | bistable, like a flip-flop, and emulates the line-range (comma) operator |
381 | of B<sed>, B<awk>, and various editors. Each ".." operator maintains its | |
382 | own boolean state. It is false as long as its left operand is false. | |
383 | Once the left operand is true, the range operator stays true until the | |
384 | right operand is true, I<AFTER> which the range operator becomes false | |
385 | again. (It doesn't become false till the next time the range operator is | |
386 | evaluated. It can test the right operand and become false on the same | |
387 | evaluation it became true (as in B<awk>), but it still returns true once. | |
388 | If you don't want it to test the right operand till the next evaluation | |
389 | (as in B<sed>), use three dots ("...") instead of two.) The right | |
390 | operand is not evaluated while the operator is in the "false" state, and | |
391 | the left operand is not evaluated while the operator is in the "true" | |
392 | state. The precedence is a little lower than || and &&. The value | |
5a964f20 | 393 | returned is either the empty string for false, or a sequence number |
a0d0e21e LW |
394 | (beginning with 1) for true. The sequence number is reset for each range |
395 | encountered. The final sequence number in a range has the string "E0" | |
396 | appended to it, which doesn't affect its numeric value, but gives you | |
397 | something to search for if you want to exclude the endpoint. You can | |
398 | exclude the beginning point by waiting for the sequence number to be | |
0a528a35 | 399 | greater than 1. If either operand of scalar ".." is a constant expression, |
a0d0e21e LW |
400 | that operand is implicitly compared to the C<$.> variable, the current |
401 | line number. Examples: | |
402 | ||
403 | As a scalar operator: | |
404 | ||
405 | if (101 .. 200) { print; } # print 2nd hundred lines | |
406 | next line if (1 .. /^$/); # skip header lines | |
407 | s/^/> / if (/^$/ .. eof()); # quote body | |
408 | ||
5a964f20 TC |
409 | # parse mail messages |
410 | while (<>) { | |
411 | $in_header = 1 .. /^$/; | |
412 | $in_body = /^$/ .. eof(); | |
413 | # do something based on those | |
414 | } continue { | |
415 | close ARGV if eof; # reset $. each file | |
416 | } | |
417 | ||
a0d0e21e LW |
418 | As a list operator: |
419 | ||
420 | for (101 .. 200) { print; } # print $_ 100 times | |
3e3baf6d | 421 | @foo = @foo[0 .. $#foo]; # an expensive no-op |
a0d0e21e LW |
422 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items |
423 | ||
5a964f20 | 424 | The range operator (in list context) makes use of the magical |
5f05dabc | 425 | auto-increment algorithm if the operands are strings. You |
a0d0e21e LW |
426 | can say |
427 | ||
428 | @alphabet = ('A' .. 'Z'); | |
429 | ||
430 | to get all the letters of the alphabet, or | |
431 | ||
432 | $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; | |
433 | ||
434 | to get a hexadecimal digit, or | |
435 | ||
436 | @z2 = ('01' .. '31'); print $z2[$mday]; | |
437 | ||
438 | to get dates with leading zeros. If the final value specified is not | |
439 | in the sequence that the magical increment would produce, the sequence | |
440 | goes until the next value would be longer than the final value | |
441 | specified. | |
442 | ||
443 | =head2 Conditional Operator | |
444 | ||
445 | Ternary "?:" is the conditional operator, just as in C. It works much | |
446 | like an if-then-else. If the argument before the ? is true, the | |
447 | argument before the : is returned, otherwise the argument after the : | |
cb1a09d0 AD |
448 | is returned. For example: |
449 | ||
54310121 | 450 | printf "I have %d dog%s.\n", $n, |
cb1a09d0 AD |
451 | ($n == 1) ? '' : "s"; |
452 | ||
453 | Scalar or list context propagates downward into the 2nd | |
54310121 | 454 | or 3rd argument, whichever is selected. |
cb1a09d0 AD |
455 | |
456 | $a = $ok ? $b : $c; # get a scalar | |
457 | @a = $ok ? @b : @c; # get an array | |
458 | $a = $ok ? @b : @c; # oops, that's just a count! | |
459 | ||
460 | The operator may be assigned to if both the 2nd and 3rd arguments are | |
461 | legal lvalues (meaning that you can assign to them): | |
a0d0e21e LW |
462 | |
463 | ($a_or_b ? $a : $b) = $c; | |
464 | ||
cb1a09d0 | 465 | This is not necessarily guaranteed to contribute to the readability of your program. |
a0d0e21e | 466 | |
5a964f20 TC |
467 | Because this operator produces an assignable result, using assignments |
468 | without parentheses will get you in trouble. For example, this: | |
469 | ||
470 | $a % 2 ? $a += 10 : $a += 2 | |
471 | ||
472 | Really means this: | |
473 | ||
474 | (($a % 2) ? ($a += 10) : $a) += 2 | |
475 | ||
476 | Rather than this: | |
477 | ||
478 | ($a % 2) ? ($a += 10) : ($a += 2) | |
479 | ||
4633a7c4 | 480 | =head2 Assignment Operators |
a0d0e21e LW |
481 | |
482 | "=" is the ordinary assignment operator. | |
483 | ||
484 | Assignment operators work as in C. That is, | |
485 | ||
486 | $a += 2; | |
487 | ||
488 | is equivalent to | |
489 | ||
490 | $a = $a + 2; | |
491 | ||
492 | although without duplicating any side effects that dereferencing the lvalue | |
54310121 | 493 | might trigger, such as from tie(). Other assignment operators work similarly. |
494 | The following are recognized: | |
a0d0e21e LW |
495 | |
496 | **= += *= &= <<= &&= | |
497 | -= /= |= >>= ||= | |
498 | .= %= ^= | |
499 | x= | |
500 | ||
501 | Note that while these are grouped by family, they all have the precedence | |
502 | of assignment. | |
503 | ||
504 | Unlike in C, the assignment operator produces a valid lvalue. Modifying | |
505 | an assignment is equivalent to doing the assignment and then modifying | |
506 | the variable that was assigned to. This is useful for modifying | |
507 | a copy of something, like this: | |
508 | ||
509 | ($tmp = $global) =~ tr [A-Z] [a-z]; | |
510 | ||
511 | Likewise, | |
512 | ||
513 | ($a += 2) *= 3; | |
514 | ||
515 | is equivalent to | |
516 | ||
517 | $a += 2; | |
518 | $a *= 3; | |
519 | ||
748a9306 | 520 | =head2 Comma Operator |
a0d0e21e | 521 | |
5a964f20 | 522 | Binary "," is the comma operator. In scalar context it evaluates |
a0d0e21e LW |
523 | its left argument, throws that value away, then evaluates its right |
524 | argument and returns that value. This is just like C's comma operator. | |
525 | ||
5a964f20 | 526 | In list context, it's just the list argument separator, and inserts |
a0d0e21e LW |
527 | both its arguments into the list. |
528 | ||
6ee5d4e7 | 529 | The =E<gt> digraph is mostly just a synonym for the comma operator. It's useful for |
cb1a09d0 | 530 | documenting arguments that come in pairs. As of release 5.001, it also forces |
4633a7c4 | 531 | any word to the left of it to be interpreted as a string. |
748a9306 | 532 | |
a0d0e21e LW |
533 | =head2 List Operators (Rightward) |
534 | ||
535 | On the right side of a list operator, it has very low precedence, | |
536 | such that it controls all comma-separated expressions found there. | |
537 | The only operators with lower precedence are the logical operators | |
538 | "and", "or", and "not", which may be used to evaluate calls to list | |
539 | operators without the need for extra parentheses: | |
540 | ||
541 | open HANDLE, "filename" | |
542 | or die "Can't open: $!\n"; | |
543 | ||
5ba421f6 | 544 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
a0d0e21e LW |
545 | |
546 | =head2 Logical Not | |
547 | ||
548 | Unary "not" returns the logical negation of the expression to its right. | |
549 | It's the equivalent of "!" except for the very low precedence. | |
550 | ||
551 | =head2 Logical And | |
552 | ||
553 | Binary "and" returns the logical conjunction of the two surrounding | |
554 | expressions. It's equivalent to && except for the very low | |
5f05dabc | 555 | precedence. This means that it short-circuits: i.e., the right |
a0d0e21e LW |
556 | expression is evaluated only if the left expression is true. |
557 | ||
558 | =head2 Logical or and Exclusive Or | |
559 | ||
560 | Binary "or" returns the logical disjunction of the two surrounding | |
5a964f20 TC |
561 | expressions. It's equivalent to || except for the very low precedence. |
562 | This makes it useful for control flow | |
563 | ||
564 | print FH $data or die "Can't write to FH: $!"; | |
565 | ||
566 | This means that it short-circuits: i.e., the right expression is evaluated | |
567 | only if the left expression is false. Due to its precedence, you should | |
568 | probably avoid using this for assignment, only for control flow. | |
569 | ||
570 | $a = $b or $c; # bug: this is wrong | |
571 | ($a = $b) or $c; # really means this | |
572 | $a = $b || $c; # better written this way | |
573 | ||
574 | However, when it's a list context assignment and you're trying to use | |
575 | "||" for control flow, you probably need "or" so that the assignment | |
576 | takes higher precedence. | |
577 | ||
578 | @info = stat($file) || die; # oops, scalar sense of stat! | |
579 | @info = stat($file) or die; # better, now @info gets its due | |
580 | ||
581 | Then again, you could always use parentheses. | |
a0d0e21e LW |
582 | |
583 | Binary "xor" returns the exclusive-OR of the two surrounding expressions. | |
584 | It cannot short circuit, of course. | |
585 | ||
586 | =head2 C Operators Missing From Perl | |
587 | ||
588 | Here is what C has that Perl doesn't: | |
589 | ||
590 | =over 8 | |
591 | ||
592 | =item unary & | |
593 | ||
594 | Address-of operator. (But see the "\" operator for taking a reference.) | |
595 | ||
596 | =item unary * | |
597 | ||
54310121 | 598 | Dereference-address operator. (Perl's prefix dereferencing |
a0d0e21e LW |
599 | operators are typed: $, @, %, and &.) |
600 | ||
601 | =item (TYPE) | |
602 | ||
54310121 | 603 | Type casting operator. |
a0d0e21e LW |
604 | |
605 | =back | |
606 | ||
5f05dabc | 607 | =head2 Quote and Quote-like Operators |
a0d0e21e LW |
608 | |
609 | While we usually think of quotes as literal values, in Perl they | |
610 | function as operators, providing various kinds of interpolating and | |
611 | pattern matching capabilities. Perl provides customary quote characters | |
612 | for these behaviors, but also provides a way for you to choose your | |
613 | quote character for any of them. In the following table, a C<{}> represents | |
614 | any pair of delimiters you choose. Non-bracketing delimiters use | |
54310121 | 615 | the same character fore and aft, but the 4 sorts of brackets |
a0d0e21e LW |
616 | (round, angle, square, curly) will all nest. |
617 | ||
2c268ad5 TP |
618 | Customary Generic Meaning Interpolates |
619 | '' q{} Literal no | |
620 | "" qq{} Literal yes | |
01ae956f | 621 | `` qx{} Command yes (unless '' is delimiter) |
2c268ad5 | 622 | qw{} Word list no |
f70b4f9c AB |
623 | // m{} Pattern match yes (unless '' is delimiter) |
624 | qr{} Pattern yes (unless '' is delimiter) | |
625 | s{}{} Substitution yes (unless '' is delimiter) | |
2c268ad5 | 626 | tr{}{} Transliteration no (but see below) |
a0d0e21e | 627 | |
fb73857a | 628 | Note that there can be whitespace between the operator and the quoting |
629 | characters, except when C<#> is being used as the quoting character. | |
a3cb178b | 630 | C<q#foo#> is parsed as being the string C<foo>, while C<q #foo#> is the |
fb73857a | 631 | operator C<q> followed by a comment. Its argument will be taken from the |
632 | next line. This allows you to write: | |
633 | ||
634 | s {foo} # Replace foo | |
635 | {bar} # with bar. | |
636 | ||
2c268ad5 TP |
637 | For constructs that do interpolation, variables beginning with "C<$>" |
638 | or "C<@>" are interpolated, as are the following sequences. Within | |
a0ed51b3 | 639 | a transliteration, the first eleven of these sequences may be used. |
a0d0e21e | 640 | |
6ee5d4e7 | 641 | \t tab (HT, TAB) |
5a964f20 | 642 | \n newline (NL) |
6ee5d4e7 | 643 | \r return (CR) |
644 | \f form feed (FF) | |
645 | \b backspace (BS) | |
646 | \a alarm (bell) (BEL) | |
647 | \e escape (ESC) | |
a0ed51b3 LW |
648 | \033 octal char (ESC) |
649 | \x1b hex char (ESC) | |
650 | \x{263a} wide hex char (SMILEY) | |
a0d0e21e | 651 | \c[ control char |
2c268ad5 | 652 | |
a0d0e21e LW |
653 | \l lowercase next char |
654 | \u uppercase next char | |
655 | \L lowercase till \E | |
656 | \U uppercase till \E | |
657 | \E end case modification | |
1d2dff63 | 658 | \Q quote non-word characters till \E |
a0d0e21e | 659 | |
a034a98d | 660 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> |
7b8d334a | 661 | and C<\U> is taken from the current locale. See L<perllocale>. |
a034a98d | 662 | |
5a964f20 TC |
663 | All systems use the virtual C<"\n"> to represent a line terminator, |
664 | called a "newline". There is no such thing as an unvarying, physical | |
665 | newline character. It is an illusion that the operating system, | |
666 | device drivers, C libraries, and Perl all conspire to preserve. Not all | |
667 | systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example, | |
668 | on a Mac, these are reversed, and on systems without line terminator, | |
669 | printing C<"\n"> may emit no actual data. In general, use C<"\n"> when | |
670 | you mean a "newline" for your system, but use the literal ASCII when you | |
671 | need an exact character. For example, most networking protocols expect | |
672 | and prefer a CR+LF (C<"\012\015"> or C<"\cJ\cM">) for line terminators, | |
673 | and although they often accept just C<"\012">, they seldom tolerate just | |
674 | C<"\015">. If you get in the habit of using C<"\n"> for networking, | |
675 | you may be burned some day. | |
676 | ||
1d2dff63 GS |
677 | You cannot include a literal C<$> or C<@> within a C<\Q> sequence. |
678 | An unescaped C<$> or C<@> interpolates the corresponding variable, | |
679 | while escaping will cause the literal string C<\$> to be inserted. | |
680 | You'll need to write something like C<m/\Quser\E\@\Qhost/>. | |
681 | ||
a0d0e21e LW |
682 | Patterns are subject to an additional level of interpretation as a |
683 | regular expression. This is done as a second pass, after variables are | |
684 | interpolated, so that regular expressions may be incorporated into the | |
685 | pattern from the variables. If this is not what you want, use C<\Q> to | |
686 | interpolate a variable literally. | |
687 | ||
688 | Apart from the above, there are no multiple levels of interpolation. In | |
5f05dabc | 689 | particular, contrary to the expectations of shell programmers, back-quotes |
a0d0e21e LW |
690 | do I<NOT> interpolate within double quotes, nor do single quotes impede |
691 | evaluation of variables when used within double quotes. | |
692 | ||
5f05dabc | 693 | =head2 Regexp Quote-Like Operators |
cb1a09d0 | 694 | |
5f05dabc | 695 | Here are the quote-like operators that apply to pattern |
cb1a09d0 AD |
696 | matching and related activities. |
697 | ||
75e14d17 IZ |
698 | Most of this section is related to use of regular expressions from Perl. |
699 | Such a use may be considered from two points of view: Perl handles a | |
700 | a string and a "pattern" to RE (regular expression) engine to match, | |
701 | RE engine finds (or does not find) the match, and Perl uses the findings | |
702 | of RE engine for its operation, possibly asking the engine for other matches. | |
703 | ||
704 | RE engine has no idea what Perl is going to do with what it finds, | |
705 | similarly, the rest of Perl has no idea what a particular regular expression | |
706 | means to RE engine. This creates a clean separation, and in this section | |
707 | we discuss matching from Perl point of view only. The other point of | |
708 | view may be found in L<perlre>. | |
709 | ||
a0d0e21e LW |
710 | =over 8 |
711 | ||
712 | =item ?PATTERN? | |
713 | ||
714 | This is just like the C</pattern/> search, except that it matches only | |
715 | once between calls to the reset() operator. This is a useful | |
5f05dabc | 716 | optimization when you want to see only the first occurrence of |
a0d0e21e LW |
717 | something in each file of a set of files, for instance. Only C<??> |
718 | patterns local to the current package are reset. | |
719 | ||
5a964f20 TC |
720 | while (<>) { |
721 | if (?^$?) { | |
722 | # blank line between header and body | |
723 | } | |
724 | } continue { | |
725 | reset if eof; # clear ?? status for next file | |
726 | } | |
727 | ||
a0d0e21e LW |
728 | This usage is vaguely deprecated, and may be removed in some future |
729 | version of Perl. | |
730 | ||
fb73857a | 731 | =item m/PATTERN/cgimosx |
a0d0e21e | 732 | |
fb73857a | 733 | =item /PATTERN/cgimosx |
a0d0e21e | 734 | |
5a964f20 | 735 | Searches a string for a pattern match, and in scalar context returns |
a0d0e21e LW |
736 | true (1) or false (''). If no string is specified via the C<=~> or |
737 | C<!~> operator, the $_ string is searched. (The string specified with | |
738 | C<=~> need not be an lvalue--it may be the result of an expression | |
739 | evaluation, but remember the C<=~> binds rather tightly.) See also | |
740 | L<perlre>. | |
5a964f20 | 741 | See L<perllocale> for discussion of additional considerations that apply |
a034a98d | 742 | when C<use locale> is in effect. |
a0d0e21e LW |
743 | |
744 | Options are: | |
745 | ||
fb73857a | 746 | c Do not reset search position on a failed match when /g is in effect. |
5f05dabc | 747 | g Match globally, i.e., find all occurrences. |
a0d0e21e LW |
748 | i Do case-insensitive pattern matching. |
749 | m Treat string as multiple lines. | |
5f05dabc | 750 | o Compile pattern only once. |
a0d0e21e LW |
751 | s Treat string as single line. |
752 | x Use extended regular expressions. | |
753 | ||
754 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> | |
01ae956f | 755 | you can use any pair of non-alphanumeric, non-whitespace characters |
f70b4f9c AB |
756 | as delimiters. This is particularly useful for matching Unix path names |
757 | that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is | |
7bac28a0 | 758 | the delimiter, then the match-only-once rule of C<?PATTERN?> applies. |
f70b4f9c AB |
759 | If "'" is the delimiter, no variable interpolation is performed on the |
760 | PATTERN. | |
a0d0e21e LW |
761 | |
762 | PATTERN may contain variables, which will be interpolated (and the | |
f70b4f9c AB |
763 | pattern recompiled) every time the pattern search is evaluated, except |
764 | for when the delimiter is a single quote. (Note that C<$)> and C<$|> | |
765 | might not be interpolated because they look like end-of-string tests.) | |
766 | If you want such a pattern to be compiled only once, add a C</o> after | |
767 | the trailing delimiter. This avoids expensive run-time recompilations, | |
768 | and is useful when the value you are interpolating won't change over | |
769 | the life of the script. However, mentioning C</o> constitutes a promise | |
770 | that you won't change the variables in the pattern. If you change them, | |
771 | Perl won't even notice. | |
a0d0e21e | 772 | |
5a964f20 TC |
773 | If the PATTERN evaluates to the empty string, the last |
774 | I<successfully> matched regular expression is used instead. | |
a0d0e21e | 775 | |
a2008d6d | 776 | If the C</g> option is not used, C<m//> in a list context returns a |
a0d0e21e | 777 | list consisting of the subexpressions matched by the parentheses in the |
f7e33566 GS |
778 | pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are |
779 | also set, and that this differs from Perl 4's behavior.) When there are | |
780 | no parentheses in the pattern, the return value is the list C<(1)> for | |
781 | success. With or without parentheses, an empty list is returned upon | |
782 | failure. | |
a0d0e21e LW |
783 | |
784 | Examples: | |
785 | ||
786 | open(TTY, '/dev/tty'); | |
787 | <TTY> =~ /^y/i && foo(); # do foo if desired | |
788 | ||
789 | if (/Version: *([0-9.]*)/) { $version = $1; } | |
790 | ||
791 | next if m#^/usr/spool/uucp#; | |
792 | ||
793 | # poor man's grep | |
794 | $arg = shift; | |
795 | while (<>) { | |
796 | print if /$arg/o; # compile only once | |
797 | } | |
798 | ||
799 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) | |
800 | ||
801 | This last example splits $foo into the first two words and the | |
5f05dabc | 802 | remainder of the line, and assigns those three fields to $F1, $F2, and |
803 | $Etc. The conditional is true if any variables were assigned, i.e., if | |
a0d0e21e LW |
804 | the pattern matched. |
805 | ||
806 | The C</g> modifier specifies global pattern matching--that is, matching | |
807 | as many times as possible within the string. How it behaves depends on | |
5a964f20 | 808 | the context. In list context, it returns a list of all the |
a0d0e21e LW |
809 | substrings matched by all the parentheses in the regular expression. |
810 | If there are no parentheses, it returns a list of all the matched | |
811 | strings, as if there were parentheses around the whole pattern. | |
812 | ||
7e86de3e MG |
813 | In scalar context, each execution of C<m//g> finds the next match, |
814 | returning TRUE if it matches, and FALSE if there is no further match. | |
815 | The position after the last match can be read or set using the pos() | |
816 | function; see L<perlfunc/pos>. A failed match normally resets the | |
817 | search position to the beginning of the string, but you can avoid that | |
818 | by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target | |
819 | string also resets the search position. | |
c90c0ff4 | 820 | |
821 | You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a | |
822 | zero-width assertion that matches the exact position where the previous | |
823 | C<m//g>, if any, left off. The C<\G> assertion is not supported without | |
824 | the C</g> modifier; currently, without C</g>, C<\G> behaves just like | |
825 | C<\A>, but that's accidental and may change in the future. | |
826 | ||
827 | Examples: | |
a0d0e21e LW |
828 | |
829 | # list context | |
830 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); | |
831 | ||
832 | # scalar context | |
5f05dabc | 833 | $/ = ""; $* = 1; # $* deprecated in modern perls |
54310121 | 834 | while (defined($paragraph = <>)) { |
a0d0e21e LW |
835 | while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { |
836 | $sentences++; | |
837 | } | |
838 | } | |
839 | print "$sentences\n"; | |
840 | ||
c90c0ff4 | 841 | # using m//gc with \G |
137443ea | 842 | $_ = "ppooqppqq"; |
44a8e56a | 843 | while ($i++ < 2) { |
844 | print "1: '"; | |
c90c0ff4 | 845 | print $1 while /(o)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 846 | print "2: '"; |
c90c0ff4 | 847 | print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 848 | print "3: '"; |
c90c0ff4 | 849 | print $1 while /(p)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 850 | } |
851 | ||
852 | The last example should print: | |
853 | ||
854 | 1: 'oo', pos=4 | |
137443ea | 855 | 2: 'q', pos=5 |
44a8e56a | 856 | 3: 'pp', pos=7 |
857 | 1: '', pos=7 | |
137443ea | 858 | 2: 'q', pos=8 |
859 | 3: '', pos=8 | |
44a8e56a | 860 | |
c90c0ff4 | 861 | A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can |
e7ea3e70 | 862 | combine several regexps like this to process a string part-by-part, |
c90c0ff4 | 863 | doing different actions depending on which regexp matched. Each |
864 | regexp tries to match where the previous one leaves off. | |
e7ea3e70 | 865 | |
3fe9a6f1 | 866 | $_ = <<'EOL'; |
e7ea3e70 | 867 | $url = new URI::URL "http://www/"; die if $url eq "xXx"; |
3fe9a6f1 | 868 | EOL |
869 | LOOP: | |
e7ea3e70 | 870 | { |
c90c0ff4 | 871 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; |
872 | print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc; | |
873 | print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc; | |
874 | print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc; | |
875 | print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc; | |
876 | print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc; | |
877 | print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc; | |
e7ea3e70 IZ |
878 | print ". That's all!\n"; |
879 | } | |
880 | ||
881 | Here is the output (split into several lines): | |
882 | ||
883 | line-noise lowercase line-noise lowercase UPPERCASE line-noise | |
884 | UPPERCASE line-noise lowercase line-noise lowercase line-noise | |
885 | lowercase lowercase line-noise lowercase lowercase line-noise | |
886 | MiXeD line-noise. That's all! | |
44a8e56a | 887 | |
a0d0e21e LW |
888 | =item q/STRING/ |
889 | ||
890 | =item C<'STRING'> | |
891 | ||
68dc0745 | 892 | A single-quoted, literal string. A backslash represents a backslash |
893 | unless followed by the delimiter or another backslash, in which case | |
894 | the delimiter or backslash is interpolated. | |
a0d0e21e LW |
895 | |
896 | $foo = q!I said, "You said, 'She said it.'"!; | |
897 | $bar = q('This is it.'); | |
68dc0745 | 898 | $baz = '\n'; # a two-character string |
a0d0e21e LW |
899 | |
900 | =item qq/STRING/ | |
901 | ||
902 | =item "STRING" | |
903 | ||
904 | A double-quoted, interpolated string. | |
905 | ||
906 | $_ .= qq | |
907 | (*** The previous line contains the naughty word "$1".\n) | |
908 | if /(tcl|rexx|python)/; # :-) | |
68dc0745 | 909 | $baz = "\n"; # a one-character string |
a0d0e21e | 910 | |
eec2d3df GS |
911 | =item qr/STRING/imosx |
912 | ||
4b6a7270 | 913 | Quote-as-a-regular-expression operator. I<STRING> is interpolated the |
f70b4f9c AB |
914 | same way as I<PATTERN> in C<m/PATTERN/>. If "'" is used as the |
915 | delimiter, no variable interpolation is done. Returns a Perl value | |
916 | which may be used instead of the corresponding C</STRING/imosx> expression. | |
4b6a7270 IZ |
917 | |
918 | For example, | |
919 | ||
920 | $rex = qr/my.STRING/is; | |
921 | s/$rex/foo/; | |
922 | ||
923 | is equivalent to | |
924 | ||
925 | s/my.STRING/foo/is; | |
926 | ||
927 | The result may be used as a subpattern in a match: | |
eec2d3df GS |
928 | |
929 | $re = qr/$pattern/; | |
0a92e3a8 GS |
930 | $string =~ /foo${re}bar/; # can be interpolated in other patterns |
931 | $string =~ $re; # or used standalone | |
4b6a7270 IZ |
932 | $string =~ /$re/; # or this way |
933 | ||
934 | Since Perl may compile the pattern at the moment of execution of qr() | |
935 | operator, using qr() may have speed advantages in I<some> situations, | |
936 | notably if the result of qr() is used standalone: | |
937 | ||
938 | sub match { | |
939 | my $patterns = shift; | |
940 | my @compiled = map qr/$_/i, @$patterns; | |
941 | grep { | |
942 | my $success = 0; | |
943 | foreach my $pat @compiled { | |
944 | $success = 1, last if /$pat/; | |
945 | } | |
946 | $success; | |
947 | } @_; | |
948 | } | |
949 | ||
950 | Precompilation of the pattern into an internal representation at the | |
951 | moment of qr() avoids a need to recompile the pattern every time a | |
952 | match C</$pat/> is attempted. (Note that Perl has many other | |
953 | internal optimizations, but none would be triggered in the above | |
954 | example if we did not use qr() operator.) | |
eec2d3df GS |
955 | |
956 | Options are: | |
957 | ||
958 | i Do case-insensitive pattern matching. | |
959 | m Treat string as multiple lines. | |
960 | o Compile pattern only once. | |
961 | s Treat string as single line. | |
962 | x Use extended regular expressions. | |
963 | ||
0a92e3a8 GS |
964 | See L<perlre> for additional information on valid syntax for STRING, and |
965 | for a detailed look at the semantics of regular expressions. | |
966 | ||
a0d0e21e LW |
967 | =item qx/STRING/ |
968 | ||
969 | =item `STRING` | |
970 | ||
5a964f20 TC |
971 | A string which is (possibly) interpolated and then executed as a system |
972 | command with C</bin/sh> or its equivalent. Shell wildcards, pipes, | |
973 | and redirections will be honored. The collected standard output of the | |
974 | command is returned; standard error is unaffected. In scalar context, | |
975 | it comes back as a single (potentially multi-line) string. In list | |
976 | context, returns a list of lines (however you've defined lines with $/ | |
977 | or $INPUT_RECORD_SEPARATOR). | |
978 | ||
979 | Because backticks do not affect standard error, use shell file descriptor | |
980 | syntax (assuming the shell supports this) if you care to address this. | |
981 | To capture a command's STDERR and STDOUT together: | |
a0d0e21e | 982 | |
5a964f20 TC |
983 | $output = `cmd 2>&1`; |
984 | ||
985 | To capture a command's STDOUT but discard its STDERR: | |
986 | ||
987 | $output = `cmd 2>/dev/null`; | |
988 | ||
989 | To capture a command's STDERR but discard its STDOUT (ordering is | |
990 | important here): | |
991 | ||
992 | $output = `cmd 2>&1 1>/dev/null`; | |
993 | ||
994 | To exchange a command's STDOUT and STDERR in order to capture the STDERR | |
995 | but leave its STDOUT to come out the old STDERR: | |
996 | ||
997 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; | |
998 | ||
999 | To read both a command's STDOUT and its STDERR separately, it's easiest | |
1000 | and safest to redirect them separately to files, and then read from those | |
1001 | files when the program is done: | |
1002 | ||
1003 | system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr"); | |
1004 | ||
1005 | Using single-quote as a delimiter protects the command from Perl's | |
1006 | double-quote interpolation, passing it on to the shell instead: | |
1007 | ||
1008 | $perl_info = qx(ps $$); # that's Perl's $$ | |
1009 | $shell_info = qx'ps $$'; # that's the new shell's $$ | |
1010 | ||
1011 | Note that how the string gets evaluated is entirely subject to the command | |
1012 | interpreter on your system. On most platforms, you will have to protect | |
1013 | shell metacharacters if you want them treated literally. This is in | |
1014 | practice difficult to do, as it's unclear how to escape which characters. | |
1015 | See L<perlsec> for a clean and safe example of a manual fork() and exec() | |
1016 | to emulate backticks safely. | |
a0d0e21e | 1017 | |
bb32b41a GS |
1018 | On some platforms (notably DOS-like ones), the shell may not be |
1019 | capable of dealing with multiline commands, so putting newlines in | |
1020 | the string may not get you what you want. You may be able to evaluate | |
1021 | multiple commands in a single line by separating them with the command | |
1022 | separator character, if your shell supports that (e.g. C<;> on many Unix | |
1023 | shells; C<&> on the Windows NT C<cmd> shell). | |
1024 | ||
1025 | Beware that some command shells may place restrictions on the length | |
1026 | of the command line. You must ensure your strings don't exceed this | |
1027 | limit after any necessary interpolations. See the platform-specific | |
1028 | release notes for more details about your particular environment. | |
1029 | ||
5a964f20 TC |
1030 | Using this operator can lead to programs that are difficult to port, |
1031 | because the shell commands called vary between systems, and may in | |
1032 | fact not be present at all. As one example, the C<type> command under | |
1033 | the POSIX shell is very different from the C<type> command under DOS. | |
1034 | That doesn't mean you should go out of your way to avoid backticks | |
1035 | when they're the right way to get something done. Perl was made to be | |
1036 | a glue language, and one of the things it glues together is commands. | |
1037 | Just understand what you're getting yourself into. | |
bb32b41a | 1038 | |
dc848c6f | 1039 | See L<"I/O Operators"> for more discussion. |
a0d0e21e LW |
1040 | |
1041 | =item qw/STRING/ | |
1042 | ||
8127e0e3 GS |
1043 | Evaluates to a list of the words extracted out of STRING, using embedded |
1044 | whitespace as the word delimiters. It can be understood as being roughly | |
1045 | equivalent to: | |
a0d0e21e LW |
1046 | |
1047 | split(' ', q/STRING/); | |
1048 | ||
26ef7447 GS |
1049 | the difference being that it generates a real list at compile time. So |
1050 | this expression: | |
1051 | ||
1052 | qw(foo bar baz) | |
1053 | ||
1054 | is exactly equivalent to the list: | |
1055 | ||
1056 | ('foo', 'bar', 'baz') | |
5a964f20 | 1057 | |
a0d0e21e LW |
1058 | Some frequently seen examples: |
1059 | ||
1060 | use POSIX qw( setlocale localeconv ) | |
1061 | @EXPORT = qw( foo bar baz ); | |
1062 | ||
7bac28a0 | 1063 | A common mistake is to try to separate the words with comma or to put |
5a964f20 | 1064 | comments into a multi-line C<qw>-string. For this reason the C<-w> |
7bac28a0 | 1065 | switch produce warnings if the STRING contains the "," or the "#" |
1066 | character. | |
1067 | ||
a0d0e21e LW |
1068 | =item s/PATTERN/REPLACEMENT/egimosx |
1069 | ||
1070 | Searches a string for a pattern, and if found, replaces that pattern | |
1071 | with the replacement text and returns the number of substitutions | |
e37d713d | 1072 | made. Otherwise it returns false (specifically, the empty string). |
a0d0e21e LW |
1073 | |
1074 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> | |
1075 | variable is searched and modified. (The string specified with C<=~> must | |
5a964f20 | 1076 | be scalar variable, an array element, a hash element, or an assignment |
5f05dabc | 1077 | to one of those, i.e., an lvalue.) |
a0d0e21e | 1078 | |
f70b4f9c | 1079 | If the delimiter chosen is a single quote, no variable interpolation is |
a0d0e21e LW |
1080 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the |
1081 | PATTERN contains a $ that looks like a variable rather than an | |
1082 | end-of-string test, the variable will be interpolated into the pattern | |
5f05dabc | 1083 | at run-time. If you want the pattern compiled only once the first time |
a0d0e21e | 1084 | the variable is interpolated, use the C</o> option. If the pattern |
5a964f20 | 1085 | evaluates to the empty string, the last successfully executed regular |
a0d0e21e | 1086 | expression is used instead. See L<perlre> for further explanation on these. |
5a964f20 | 1087 | See L<perllocale> for discussion of additional considerations that apply |
a034a98d | 1088 | when C<use locale> is in effect. |
a0d0e21e LW |
1089 | |
1090 | Options are: | |
1091 | ||
1092 | e Evaluate the right side as an expression. | |
5f05dabc | 1093 | g Replace globally, i.e., all occurrences. |
a0d0e21e LW |
1094 | i Do case-insensitive pattern matching. |
1095 | m Treat string as multiple lines. | |
5f05dabc | 1096 | o Compile pattern only once. |
a0d0e21e LW |
1097 | s Treat string as single line. |
1098 | x Use extended regular expressions. | |
1099 | ||
1100 | Any non-alphanumeric, non-whitespace delimiter may replace the | |
1101 | slashes. If single quotes are used, no interpretation is done on the | |
e37d713d | 1102 | replacement string (the C</e> modifier overrides this, however). Unlike |
54310121 | 1103 | Perl 4, Perl 5 treats backticks as normal delimiters; the replacement |
e37d713d | 1104 | text is not evaluated as a command. If the |
a0d0e21e | 1105 | PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own |
5f05dabc | 1106 | pair of quotes, which may or may not be bracketing quotes, e.g., |
a0d0e21e | 1107 | C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the |
7b8d334a | 1108 | replacement portion to be interpreted as a full-fledged Perl expression |
a0d0e21e LW |
1109 | and eval()ed right then and there. It is, however, syntax checked at |
1110 | compile-time. | |
1111 | ||
1112 | Examples: | |
1113 | ||
1114 | s/\bgreen\b/mauve/g; # don't change wintergreen | |
1115 | ||
1116 | $path =~ s|/usr/bin|/usr/local/bin|; | |
1117 | ||
1118 | s/Login: $foo/Login: $bar/; # run-time pattern | |
1119 | ||
5a964f20 | 1120 | ($foo = $bar) =~ s/this/that/; # copy first, then change |
a0d0e21e | 1121 | |
5a964f20 | 1122 | $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count |
a0d0e21e LW |
1123 | |
1124 | $_ = 'abc123xyz'; | |
1125 | s/\d+/$&*2/e; # yields 'abc246xyz' | |
1126 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' | |
1127 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' | |
1128 | ||
1129 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e | |
1130 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e | |
1131 | s/^=(\w+)/&pod($1)/ge; # use function call | |
1132 | ||
5a964f20 TC |
1133 | # expand variables in $_, but dynamics only, using |
1134 | # symbolic dereferencing | |
1135 | s/\$(\w+)/${$1}/g; | |
1136 | ||
a0d0e21e | 1137 | # /e's can even nest; this will expand |
5a964f20 | 1138 | # any embedded scalar variable (including lexicals) in $_ |
a0d0e21e LW |
1139 | s/(\$\w+)/$1/eeg; |
1140 | ||
5a964f20 | 1141 | # Delete (most) C comments. |
a0d0e21e | 1142 | $program =~ s { |
4633a7c4 LW |
1143 | /\* # Match the opening delimiter. |
1144 | .*? # Match a minimal number of characters. | |
1145 | \*/ # Match the closing delimiter. | |
a0d0e21e LW |
1146 | } []gsx; |
1147 | ||
5a964f20 TC |
1148 | s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively |
1149 | ||
1150 | for ($variable) { # trim white space in $variable, cheap | |
1151 | s/^\s+//; | |
1152 | s/\s+$//; | |
1153 | } | |
a0d0e21e LW |
1154 | |
1155 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields | |
1156 | ||
54310121 | 1157 | Note the use of $ instead of \ in the last example. Unlike |
5f05dabc | 1158 | B<sed>, we use the \E<lt>I<digit>E<gt> form in only the left hand side. |
6ee5d4e7 | 1159 | Anywhere else it's $E<lt>I<digit>E<gt>. |
a0d0e21e | 1160 | |
5f05dabc | 1161 | Occasionally, you can't use just a C</g> to get all the changes |
a0d0e21e LW |
1162 | to occur. Here are two common cases: |
1163 | ||
1164 | # put commas in the right places in an integer | |
1165 | 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4 | |
1166 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5 | |
1167 | ||
1168 | # expand tabs to 8-column spacing | |
1169 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; | |
1170 | ||
1171 | ||
a0ed51b3 | 1172 | =item tr/SEARCHLIST/REPLACEMENTLIST/cdsUC |
a0d0e21e | 1173 | |
a0ed51b3 | 1174 | =item y/SEARCHLIST/REPLACEMENTLIST/cdsUC |
a0d0e21e | 1175 | |
2c268ad5 | 1176 | Transliterates all occurrences of the characters found in the search list |
a0d0e21e LW |
1177 | with the corresponding character in the replacement list. It returns |
1178 | the number of characters replaced or deleted. If no string is | |
2c268ad5 | 1179 | specified via the =~ or !~ operator, the $_ string is transliterated. (The |
54310121 | 1180 | string specified with =~ must be a scalar variable, an array element, a |
1181 | hash element, or an assignment to one of those, i.e., an lvalue.) | |
8ada0baa | 1182 | |
2c268ad5 TP |
1183 | A character range may be specified with a hyphen, so C<tr/A-J/0-9/> |
1184 | does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. | |
54310121 | 1185 | For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the |
1186 | SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has | |
1187 | its own pair of quotes, which may or may not be bracketing quotes, | |
2c268ad5 | 1188 | e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>. |
a0d0e21e | 1189 | |
8ada0baa JH |
1190 | Note also that the whole range idea is rather unportable between |
1191 | character sets--and even within character sets they may cause results | |
1192 | you probably didn't expect. A sound principle is to use only ranges | |
1193 | that begin from and end at either alphabets of equal case (a-e, A-E), | |
1194 | or digits (0-4). Anything else is unsafe. If in doubt, spell out the | |
1195 | character sets in full. | |
1196 | ||
a0d0e21e LW |
1197 | Options: |
1198 | ||
1199 | c Complement the SEARCHLIST. | |
1200 | d Delete found but unreplaced characters. | |
1201 | s Squash duplicate replaced characters. | |
a0ed51b3 LW |
1202 | U Translate to/from UTF-8. |
1203 | C Translate to/from 8-bit char (octet). | |
a0d0e21e LW |
1204 | |
1205 | If the C</c> modifier is specified, the SEARCHLIST character set is | |
1206 | complemented. If the C</d> modifier is specified, any characters specified | |
1207 | by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note | |
1208 | that this is slightly more flexible than the behavior of some B<tr> | |
1209 | programs, which delete anything they find in the SEARCHLIST, period.) | |
1210 | If the C</s> modifier is specified, sequences of characters that were | |
2c268ad5 | 1211 | transliterated to the same character are squashed down to a single instance of the |
a0d0e21e LW |
1212 | character. |
1213 | ||
1214 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted | |
1215 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter | |
1216 | than the SEARCHLIST, the final character is replicated till it is long | |
5a964f20 | 1217 | enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. |
a0d0e21e LW |
1218 | This latter is useful for counting characters in a class or for |
1219 | squashing character sequences in a class. | |
1220 | ||
a0ed51b3 LW |
1221 | The first C</U> or C</C> modifier applies to the left side of the translation. |
1222 | The second one applies to the right side. If present, these modifiers override | |
1223 | the current utf8 state. | |
1224 | ||
a0d0e21e LW |
1225 | Examples: |
1226 | ||
1227 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case | |
1228 | ||
1229 | $cnt = tr/*/*/; # count the stars in $_ | |
1230 | ||
1231 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky | |
1232 | ||
1233 | $cnt = tr/0-9//; # count the digits in $_ | |
1234 | ||
1235 | tr/a-zA-Z//s; # bookkeeper -> bokeper | |
1236 | ||
1237 | ($HOST = $host) =~ tr/a-z/A-Z/; | |
1238 | ||
1239 | tr/a-zA-Z/ /cs; # change non-alphas to single space | |
1240 | ||
1241 | tr [\200-\377] | |
1242 | [\000-\177]; # delete 8th bit | |
1243 | ||
a0ed51b3 LW |
1244 | tr/\0-\xFF//CU; # translate Latin-1 to Unicode |
1245 | tr/\0-\x{FF}//UC; # translate Unicode to Latin-1 | |
1246 | ||
2c268ad5 | 1247 | If multiple transliterations are given for a character, only the first one is used: |
748a9306 LW |
1248 | |
1249 | tr/AAA/XYZ/ | |
1250 | ||
2c268ad5 | 1251 | will transliterate any A to X. |
748a9306 | 1252 | |
2c268ad5 | 1253 | Note that because the transliteration table is built at compile time, neither |
a0d0e21e LW |
1254 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote |
1255 | interpolation. That means that if you want to use variables, you must use | |
1256 | an eval(): | |
1257 | ||
1258 | eval "tr/$oldlist/$newlist/"; | |
1259 | die $@ if $@; | |
1260 | ||
1261 | eval "tr/$oldlist/$newlist/, 1" or die $@; | |
1262 | ||
1263 | =back | |
1264 | ||
75e14d17 IZ |
1265 | =head2 Gory details of parsing quoted constructs |
1266 | ||
1267 | When presented with something which may have several different | |
1268 | interpretations, Perl uses the principle B<DWIM> (expanded to Do What I Mean | |
1269 | - not what I wrote) to pick up the most probable interpretation of the | |
1270 | source. This strategy is so successful that Perl users usually do not | |
1271 | suspect ambivalence of what they write. However, time to time Perl's ideas | |
1272 | differ from what the author meant. | |
1273 | ||
1274 | The target of this section is to clarify the Perl's way of interpreting | |
1275 | quoted constructs. The most frequent reason one may have to want to know the | |
1276 | details discussed in this section is hairy regular expressions. However, the | |
1277 | first steps of parsing are the same for all Perl quoting operators, so here | |
1278 | they are discussed together. | |
1279 | ||
2a94b7ce IZ |
1280 | The most important detail of Perl parsing rules is the first one |
1281 | discussed below; when processing a quoted construct, Perl I<first> | |
1282 | finds the end of the construct, then it interprets the contents of the | |
1283 | construct. If you understand this rule, you may skip the rest of this | |
1284 | section on the first reading. The other rules would | |
1285 | contradict user's expectations much less frequently than the first one. | |
1286 | ||
75e14d17 IZ |
1287 | Some of the passes discussed below are performed concurrently, but as |
1288 | far as results are the same, we consider them one-by-one. For different | |
1289 | quoting constructs Perl performs different number of passes, from | |
1290 | one to five, but they are always performed in the same order. | |
1291 | ||
1292 | =over | |
1293 | ||
1294 | =item Finding the end | |
1295 | ||
2a94b7ce IZ |
1296 | First pass is finding the end of the quoted construct, be it |
1297 | a multichar delimiter | |
75e14d17 | 1298 | C<"\nEOF\n"> of C<<<EOF> construct, C</> which terminates C<qq/> construct, |
7522fed5 | 1299 | C<]> which terminates C<qq[> construct, or C<E<gt>> which terminates a |
75e14d17 IZ |
1300 | fileglob started with C<<>. |
1301 | ||
2a94b7ce | 1302 | When searching for one-char non-matching delimiter, such as C</>, combinations |
75e14d17 IZ |
1303 | C<\\> and C<\/> are skipped. When searching for one-char matching delimiter, |
1304 | such as C<]>, combinations C<\\>, C<\]> and C<\[> are skipped, and | |
2a94b7ce IZ |
1305 | nested C<[>, C<]> are skipped as well. When searching for multichar delimiter |
1306 | no skipping is performed. | |
75e14d17 | 1307 | |
2a94b7ce IZ |
1308 | For constructs with 3-part delimiters (C<s///> etc.) the search is |
1309 | repeated once more. | |
75e14d17 | 1310 | |
2a94b7ce IZ |
1311 | During this search no attention is paid to the semantic of the construct, |
1312 | thus: | |
75e14d17 IZ |
1313 | |
1314 | "$hash{"$foo/$bar"}" | |
1315 | ||
2a94b7ce | 1316 | or: |
75e14d17 IZ |
1317 | |
1318 | m/ | |
2a94b7ce | 1319 | bar # NOT a comment, this slash / terminated m//! |
75e14d17 IZ |
1320 | /x |
1321 | ||
2a94b7ce IZ |
1322 | do not form legal quoted expressions, the quoted part ends on the first C<"> |
1323 | and C</>, and the rest happens to be a syntax error. Note that since the slash | |
1324 | which terminated C<m//> was followed by a C<SPACE>, the above is not C<m//x>, | |
1325 | but rather C<m//> with no 'x' switch. So the embedded C<#> is interpreted | |
1326 | as a literal C<#>. | |
75e14d17 IZ |
1327 | |
1328 | =item Removal of backslashes before delimiters | |
1329 | ||
1330 | During the second pass the text between the starting delimiter and | |
1331 | the ending delimiter is copied to a safe location, and the C<\> is | |
1332 | removed from combinations consisting of C<\> and delimiter(s) (both starting | |
1333 | and ending delimiter if they differ). | |
1334 | ||
1335 | The removal does not happen for multi-char delimiters. | |
1336 | ||
1337 | Note that the combination C<\\> is left as it was! | |
1338 | ||
1339 | Starting from this step no information about the delimiter(s) is used in the | |
1340 | parsing. | |
1341 | ||
1342 | =item Interpolation | |
1343 | ||
1344 | Next step is interpolation in the obtained delimiter-independent text. | |
7522fed5 | 1345 | There are four different cases. |
75e14d17 IZ |
1346 | |
1347 | =over | |
1348 | ||
1349 | =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///> | |
1350 | ||
1351 | No interpolation is performed. | |
1352 | ||
1353 | =item C<''>, C<q//> | |
1354 | ||
1355 | The only interpolation is removal of C<\> from pairs C<\\>. | |
1356 | ||
1357 | =item C<"">, C<``>, C<qq//>, C<qx//>, C<<file*globE<gt>> | |
1358 | ||
1359 | C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are converted | |
2a94b7ce | 1360 | to corresponding Perl constructs, thus C<"$foo\Qbaz$bar"> is converted to : |
75e14d17 IZ |
1361 | |
1362 | $foo . (quotemeta("baz" . $bar)); | |
1363 | ||
1364 | Other combinations of C<\> with following chars are substituted with | |
2a94b7ce IZ |
1365 | appropriate expansions. |
1366 | ||
1367 | Let it be stressed that I<whatever is between C<\Q> and C<\E>> is interpolated | |
1368 | in the usual way. Say, C<"\Q\\E"> has no C<\E> inside: it has C<\Q>, C<\\>, | |
1369 | and C<E>, thus the result is the same as for C<"\\\\E">. Generally speaking, | |
1370 | having backslashes between C<\Q> and C<\E> may lead to counterintuitive | |
1371 | results. So, C<"\Q\t\E"> is converted to: | |
1372 | ||
1373 | quotemeta("\t") | |
1374 | ||
1375 | which is the same as C<"\\\t"> (since TAB is not alphanumerical). Note also | |
1376 | that: | |
1377 | ||
1378 | $str = '\t'; | |
1379 | return "\Q$str"; | |
1380 | ||
1381 | may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. | |
1382 | ||
1383 | Interpolated scalars and arrays are internally converted to the C<join> and | |
1384 | C<.> Perl operations, thus C<"$foo >>> '@arr'"> becomes: | |
75e14d17 | 1385 | |
2a94b7ce | 1386 | $foo . " >>> '" . (join $", @arr) . "'"; |
75e14d17 | 1387 | |
2a94b7ce | 1388 | All the operations in the above are performed simultaneously left-to-right. |
75e14d17 | 1389 | |
2a94b7ce IZ |
1390 | Since the result of "\Q STRING \E" has all the metacharacters quoted |
1391 | there is no way to insert a literal C<$> or C<@> inside a C<\Q\E> pair: if | |
1392 | protected by C<\> C<$> will be quoted to became "\\\$", if not, it is | |
75e14d17 IZ |
1393 | interpreted as starting an interpolated scalar. |
1394 | ||
2a94b7ce IZ |
1395 | Note also that the interpolating code needs to make a decision on where the |
1396 | interpolated scalar ends. For instance, whether C<"a $b -E<gt> {c}"> means: | |
75e14d17 IZ |
1397 | |
1398 | "a " . $b . " -> {c}"; | |
1399 | ||
2a94b7ce | 1400 | or: |
75e14d17 IZ |
1401 | |
1402 | "a " . $b -> {c}; | |
1403 | ||
2a94b7ce IZ |
1404 | I<Most of the time> the decision is to take the longest possible text which |
1405 | does not include spaces between components and contains matching | |
1406 | braces/brackets. Since the outcome may be determined by I<voting> based | |
1407 | on heuristic estimators, the result I<is not strictly predictable>, but | |
1408 | is usually correct for the ambiguous cases. | |
75e14d17 IZ |
1409 | |
1410 | =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, | |
1411 | ||
1412 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> and interpolation happens | |
7522fed5 | 1413 | (almost) as with C<qq//> constructs, but I<the substitution of C<\> followed by |
2a94b7ce IZ |
1414 | RE-special chars (including C<\>) is not performed>! Moreover, |
1415 | inside C<(?{BLOCK})>, C<(?# comment )>, and C<#>-comment of | |
1416 | C<//x>-regular expressions no processing is performed at all. | |
1417 | This is the first step where presence of the C<//x> switch is relevant. | |
75e14d17 | 1418 | |
7522fed5 | 1419 | Interpolation has several quirks: C<$|>, C<$(> and C<$)> are not interpolated, and |
75e14d17 | 1420 | constructs C<$var[SOMETHING]> are I<voted> (by several different estimators) |
7522fed5 | 1421 | to be an array element or C<$var> followed by a RE alternative. This is |
75e14d17 | 1422 | the place where the notation C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> |
7522fed5 A |
1423 | is interpreted as an array element C<-9>, not as a regular expression from |
1424 | variable C<$arr> followed by a digit, which is the interpretation of | |
2a94b7ce IZ |
1425 | C</$arr[0-9]/>. Since voting among different estimators may be performed, |
1426 | the result I<is not predictable>. | |
1427 | ||
1428 | It is on this step that C<\1> is converted to C<$1> in the replacement | |
1429 | text of C<s///>. | |
75e14d17 | 1430 | |
7522fed5 A |
1431 | Note that absence of processing of C<\\> creates specific restrictions on the |
1432 | post-processed text: if the delimiter is C</>, one cannot get the combination | |
75e14d17 IZ |
1433 | C<\/> into the result of this step: C</> will finish the regular expression, |
1434 | C<\/> will be stripped to C</> on the previous step, and C<\\/> will be left | |
1435 | as is. Since C</> is equivalent to C<\/> inside a regular expression, this | |
2a94b7ce IZ |
1436 | does not matter unless the delimiter is a special character for the RE engine, |
1437 | as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>, or an alphanumeric char, as in: | |
1438 | ||
1439 | m m ^ a \s* b mmx; | |
1440 | ||
1441 | In the above RE, which is intentionally obfuscated for illustration, the | |
1442 | delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the | |
1443 | RE is the same as for C<m/ ^ a s* b /mx>). | |
75e14d17 IZ |
1444 | |
1445 | =back | |
1446 | ||
1447 | This step is the last one for all the constructs except regular expressions, | |
1448 | which are processed further. | |
1449 | ||
1450 | =item Interpolation of regular expressions | |
1451 | ||
1452 | All the previous steps were performed during the compilation of Perl code, | |
1453 | this one happens in run time (though it may be optimized to be calculated | |
1454 | at compile time if appropriate). After all the preprocessing performed | |
1455 | above (and possibly after evaluation if catenation, joining, up/down-casing | |
7522fed5 | 1456 | and C<quotemeta()>ing are involved) the resulting I<string> is passed to RE |
75e14d17 IZ |
1457 | engine for compilation. |
1458 | ||
1459 | Whatever happens in the RE engine is better be discussed in L<perlre>, | |
1460 | but for the sake of continuity let us do it here. | |
1461 | ||
2a94b7ce | 1462 | This is another step where presence of the C<//x> switch is relevant. |
7522fed5 | 1463 | The RE engine scans the string left-to-right, and converts it to a finite |
75e14d17 IZ |
1464 | automaton. |
1465 | ||
1466 | Backslashed chars are either substituted by corresponding literal | |
2a94b7ce IZ |
1467 | strings (as with C<\{>), or generate special nodes of the finite automaton |
1468 | (as with C<\b>). Characters which are special to the RE engine (such as | |
1469 | C<|>) generate corresponding nodes or groups of nodes. C<(?#...)> | |
75e14d17 IZ |
1470 | comments are ignored. All the rest is either converted to literal strings |
1471 | to match, or is ignored (as is whitespace and C<#>-style comments if | |
1472 | C<//x> is present). | |
1473 | ||
1474 | Note that the parsing of the construct C<[...]> is performed using | |
2a94b7ce IZ |
1475 | rather different rules than for the rest of the regular expression. |
1476 | The terminator of this construct is found using the same rules as for | |
1477 | finding a terminator of a C<{}>-delimited construct, the only exception | |
1478 | being that C<]> immediately following C<[> is considered as if preceded | |
1479 | by a backslash. Similarly, the terminator of C<(?{...})> is found using | |
1480 | the same rules as for finding a terminator of a C<{}>-delimited construct. | |
1481 | ||
1482 | It is possible to inspect both the string given to RE engine, and the | |
1483 | resulting finite automaton. See arguments C<debug>/C<debugcolor> | |
1484 | of C<use L<re>> directive, and/or B<-Dr> option of Perl in | |
1485 | L<perlrun/Switches>. | |
75e14d17 IZ |
1486 | |
1487 | =item Optimization of regular expressions | |
1488 | ||
7522fed5 | 1489 | This step is listed for completeness only. Since it does not change |
75e14d17 | 1490 | semantics, details of this step are not documented and are subject |
2a94b7ce IZ |
1491 | to change. This step is performed over the finite automaton generated |
1492 | during the previous pass. | |
1493 | ||
1494 | However, in older versions of Perl C<L<split>> used to silently | |
1495 | optimize C</^/> to mean C</^/m>. This behaviour, though present | |
1496 | in current versions of Perl, may be deprecated in future. | |
75e14d17 IZ |
1497 | |
1498 | =back | |
1499 | ||
a0d0e21e LW |
1500 | =head2 I/O Operators |
1501 | ||
54310121 | 1502 | There are several I/O operators you should know about. |
fbad3eb5 | 1503 | |
7b8d334a | 1504 | A string enclosed by backticks (grave accents) first undergoes |
a0d0e21e LW |
1505 | variable substitution just like a double quoted string. It is then |
1506 | interpreted as a command, and the output of that command is the value | |
5a964f20 TC |
1507 | of the pseudo-literal, like in a shell. In scalar context, a single |
1508 | string consisting of all the output is returned. In list context, | |
a0d0e21e LW |
1509 | a list of values is returned, one for each line of output. (You can |
1510 | set C<$/> to use a different line terminator.) The command is executed | |
1511 | each time the pseudo-literal is evaluated. The status value of the | |
1512 | command is returned in C<$?> (see L<perlvar> for the interpretation | |
1513 | of C<$?>). Unlike in B<csh>, no translation is done on the return | |
1514 | data--newlines remain newlines. Unlike in any of the shells, single | |
1515 | quotes do not hide variable names in the command from interpretation. | |
1516 | To pass a $ through to the shell you need to hide it with a backslash. | |
54310121 | 1517 | The generalized form of backticks is C<qx//>. (Because backticks |
1518 | always undergo shell expansion as well, see L<perlsec> for | |
cb1a09d0 | 1519 | security concerns.) |
a0d0e21e | 1520 | |
fbad3eb5 GS |
1521 | In a scalar context, evaluating a filehandle in angle brackets yields the |
1522 | next line from that file (newline, if any, included), or C<undef> at | |
1523 | end-of-file. When C<$/> is set to C<undef> (i.e. file slurp mode), | |
1524 | it returns C<''> the first time, followed by C<undef> subsequently. | |
1525 | ||
1526 | Ordinarily you must assign the returned value to a variable, but there is one | |
aa689395 | 1527 | situation where an automatic assignment happens. I<If and ONLY if> the |
1528 | input symbol is the only thing inside the conditional of a C<while> or | |
1529 | C<for(;;)> loop, the value is automatically assigned to the variable | |
7b8d334a | 1530 | C<$_>. In these loop constructs, the assigned value (whether assignment |
5a964f20 | 1531 | is automatic or explicit) is then tested to see if it is defined. |
7b8d334a GS |
1532 | The defined test avoids problems where line has a string value |
1533 | that would be treated as false by perl e.g. "" or "0" with no trailing | |
1534 | newline. (This may seem like an odd thing to you, but you'll use the | |
1535 | construct in almost every Perl script you write.) Anyway, the following | |
1536 | lines are equivalent to each other: | |
a0d0e21e | 1537 | |
748a9306 | 1538 | while (defined($_ = <STDIN>)) { print; } |
7b8d334a | 1539 | while ($_ = <STDIN>) { print; } |
a0d0e21e LW |
1540 | while (<STDIN>) { print; } |
1541 | for (;<STDIN>;) { print; } | |
748a9306 | 1542 | print while defined($_ = <STDIN>); |
7b8d334a | 1543 | print while ($_ = <STDIN>); |
a0d0e21e LW |
1544 | print while <STDIN>; |
1545 | ||
7b8d334a GS |
1546 | and this also behaves similarly, but avoids the use of $_ : |
1547 | ||
1548 | while (my $line = <STDIN>) { print $line } | |
1549 | ||
1550 | If you really mean such values to terminate the loop they should be | |
5a964f20 | 1551 | tested for explicitly: |
7b8d334a GS |
1552 | |
1553 | while (($_ = <STDIN>) ne '0') { ... } | |
1554 | while (<STDIN>) { last unless $_; ... } | |
1555 | ||
5a964f20 | 1556 | In other boolean contexts, C<E<lt>I<filehandle>E<gt>> without explicit C<defined> |
7b8d334a GS |
1557 | test or comparison will solicit a warning if C<-w> is in effect. |
1558 | ||
5f05dabc | 1559 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
1560 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in | |
a0d0e21e LW |
1561 | packages, where they would be interpreted as local identifiers rather |
1562 | than global.) Additional filehandles may be created with the open() | |
fbad3eb5 | 1563 | function. See L<perlfunc/open> for details on this. |
a0d0e21e | 1564 | |
6ee5d4e7 | 1565 | If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a |
a0d0e21e LW |
1566 | list consisting of all the input lines is returned, one line per list |
1567 | element. It's easy to make a I<LARGE> data space this way, so use with | |
1568 | care. | |
1569 | ||
fbad3eb5 GS |
1570 | E<lt>FILEHANDLEE<gt> may also be spelt readline(FILEHANDLE). See |
1571 | L<perlfunc/readline>. | |
1572 | ||
d28ebecd | 1573 | The null filehandle E<lt>E<gt> is special and can be used to emulate the |
1574 | behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from | |
a0d0e21e | 1575 | standard input, or from each file listed on the command line. Here's |
d28ebecd | 1576 | how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is |
5a964f20 | 1577 | checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened |
a0d0e21e LW |
1578 | gives you standard input. The @ARGV array is then processed as a list |
1579 | of filenames. The loop | |
1580 | ||
1581 | while (<>) { | |
1582 | ... # code for each line | |
1583 | } | |
1584 | ||
1585 | is equivalent to the following Perl-like pseudo code: | |
1586 | ||
3e3baf6d | 1587 | unshift(@ARGV, '-') unless @ARGV; |
a0d0e21e LW |
1588 | while ($ARGV = shift) { |
1589 | open(ARGV, $ARGV); | |
1590 | while (<ARGV>) { | |
1591 | ... # code for each line | |
1592 | } | |
1593 | } | |
1594 | ||
1595 | except that it isn't so cumbersome to say, and will actually work. It | |
1596 | really does shift array @ARGV and put the current filename into variable | |
5f05dabc | 1597 | $ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a |
1598 | synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above | |
1599 | doesn't work because it treats E<lt>ARGVE<gt> as non-magical.) | |
a0d0e21e | 1600 | |
d28ebecd | 1601 | You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up |
a0d0e21e LW |
1602 | containing the list of filenames you really want. Line numbers (C<$.>) |
1603 | continue as if the input were one big happy file. (But see example | |
5a964f20 TC |
1604 | under C<eof> for how to reset line numbers on each file.) |
1605 | ||
1606 | If you want to set @ARGV to your own list of files, go right ahead. | |
1607 | This sets @ARGV to all plain text files if no @ARGV was given: | |
1608 | ||
1609 | @ARGV = grep { -f && -T } glob('*') unless @ARGV; | |
a0d0e21e | 1610 | |
5a964f20 TC |
1611 | You can even set them to pipe commands. For example, this automatically |
1612 | filters compressed arguments through B<gzip>: | |
1613 | ||
1614 | @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; | |
1615 | ||
1616 | If you want to pass switches into your script, you can use one of the | |
a0d0e21e LW |
1617 | Getopts modules or put a loop on the front like this: |
1618 | ||
1619 | while ($_ = $ARGV[0], /^-/) { | |
1620 | shift; | |
1621 | last if /^--$/; | |
1622 | if (/^-D(.*)/) { $debug = $1 } | |
1623 | if (/^-v/) { $verbose++ } | |
5a964f20 | 1624 | # ... # other switches |
a0d0e21e | 1625 | } |
5a964f20 | 1626 | |
a0d0e21e | 1627 | while (<>) { |
5a964f20 | 1628 | # ... # code for each line |
a0d0e21e LW |
1629 | } |
1630 | ||
7b8d334a GS |
1631 | The E<lt>E<gt> symbol will return C<undef> for end-of-file only once. |
1632 | If you call it again after this it will assume you are processing another | |
1633 | @ARGV list, and if you haven't set @ARGV, will input from STDIN. | |
a0d0e21e LW |
1634 | |
1635 | If the string inside the angle brackets is a reference to a scalar | |
5f05dabc | 1636 | variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the |
5a964f20 | 1637 | filehandle to input from, or its typeglob, or a reference to the same. For example: |
cb1a09d0 AD |
1638 | |
1639 | $fh = \*STDIN; | |
1640 | $line = <$fh>; | |
a0d0e21e | 1641 | |
5a964f20 TC |
1642 | If what's within the angle brackets is neither a filehandle nor a simple |
1643 | scalar variable containing a filehandle name, typeglob, or typeglob | |
1644 | reference, it is interpreted as a filename pattern to be globbed, and | |
1645 | either a list of filenames or the next filename in the list is returned, | |
1646 | depending on context. This distinction is determined on syntactic | |
1647 | grounds alone. That means C<E<lt>$xE<gt>> is always a readline from | |
1648 | an indirect handle, but C<E<lt>$hash{key}E<gt>> is always a glob. | |
1649 | That's because $x is a simple scalar variable, but C<$hash{key}> is | |
1650 | not--it's a hash element. | |
1651 | ||
1652 | One level of double-quote interpretation is done first, but you can't | |
1653 | say C<E<lt>$fooE<gt>> because that's an indirect filehandle as explained | |
1654 | in the previous paragraph. (In older versions of Perl, programmers | |
1655 | would insert curly brackets to force interpretation as a filename glob: | |
1656 | C<E<lt>${foo}E<gt>>. These days, it's considered cleaner to call the | |
1657 | internal function directly as C<glob($foo)>, which is probably the right | |
1658 | way to have done it in the first place.) Example: | |
a0d0e21e LW |
1659 | |
1660 | while (<*.c>) { | |
1661 | chmod 0644, $_; | |
1662 | } | |
1663 | ||
1664 | is equivalent to | |
1665 | ||
1666 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); | |
1667 | while (<FOO>) { | |
1668 | chop; | |
1669 | chmod 0644, $_; | |
1670 | } | |
1671 | ||
1672 | In fact, it's currently implemented that way. (Which means it will not | |
1673 | work on filenames with spaces in them unless you have csh(1) on your | |
1674 | machine.) Of course, the shortest way to do the above is: | |
1675 | ||
1676 | chmod 0644, <*.c>; | |
1677 | ||
1678 | Because globbing invokes a shell, it's often faster to call readdir() yourself | |
5f05dabc | 1679 | and do your own grep() on the filenames. Furthermore, due to its current |
54310121 | 1680 | implementation of using a shell, the glob() routine may get "Arg list too |
a0d0e21e LW |
1681 | long" errors (unless you've installed tcsh(1L) as F</bin/csh>). |
1682 | ||
5f05dabc | 1683 | A glob evaluates its (embedded) argument only when it is starting a new |
4633a7c4 LW |
1684 | list. All values must be read before it will start over. In a list |
1685 | context this isn't important, because you automatically get them all | |
5a964f20 | 1686 | anyway. In scalar context, however, the operator returns the next value |
7b8d334a GS |
1687 | each time it is called, or a C<undef> value if you've just run out. As |
1688 | for filehandles an automatic C<defined> is generated when the glob | |
1689 | occurs in the test part of a C<while> or C<for> - because legal glob returns | |
1690 | (e.g. a file called F<0>) would otherwise terminate the loop. | |
1691 | Again, C<undef> is returned only once. So if you're expecting a single value | |
1692 | from a glob, it is much better to say | |
4633a7c4 LW |
1693 | |
1694 | ($file) = <blurch*>; | |
1695 | ||
1696 | than | |
1697 | ||
1698 | $file = <blurch*>; | |
1699 | ||
1700 | because the latter will alternate between returning a filename and | |
54310121 | 1701 | returning FALSE. |
4633a7c4 LW |
1702 | |
1703 | It you're trying to do variable interpolation, it's definitely better | |
1704 | to use the glob() function, because the older notation can cause people | |
e37d713d | 1705 | to become confused with the indirect filehandle notation. |
4633a7c4 LW |
1706 | |
1707 | @files = glob("$dir/*.[ch]"); | |
1708 | @files = glob($files[$i]); | |
1709 | ||
a0d0e21e LW |
1710 | =head2 Constant Folding |
1711 | ||
1712 | Like C, Perl does a certain amount of expression evaluation at | |
5a964f20 | 1713 | compile time, whenever it determines that all arguments to an |
a0d0e21e LW |
1714 | operator are static and have no side effects. In particular, string |
1715 | concatenation happens at compile time between literals that don't do | |
1716 | variable substitution. Backslash interpretation also happens at | |
1717 | compile time. You can say | |
1718 | ||
1719 | 'Now is the time for all' . "\n" . | |
1720 | 'good men to come to.' | |
1721 | ||
54310121 | 1722 | and this all reduces to one string internally. Likewise, if |
a0d0e21e LW |
1723 | you say |
1724 | ||
1725 | foreach $file (@filenames) { | |
5a964f20 | 1726 | if (-s $file > 5 + 100 * 2**16) { } |
54310121 | 1727 | } |
a0d0e21e | 1728 | |
54310121 | 1729 | the compiler will precompute the number that |
a0d0e21e LW |
1730 | expression represents so that the interpreter |
1731 | won't have to. | |
1732 | ||
2c268ad5 TP |
1733 | =head2 Bitwise String Operators |
1734 | ||
1735 | Bitstrings of any size may be manipulated by the bitwise operators | |
1736 | (C<~ | & ^>). | |
1737 | ||
1738 | If the operands to a binary bitwise op are strings of different sizes, | |
1ae175c8 GS |
1739 | B<|> and B<^> ops will act as if the shorter operand had additional |
1740 | zero bits on the right, while the B<&> op will act as if the longer | |
1741 | operand were truncated to the length of the shorter. Note that the | |
1742 | granularity for such extension or truncation is one or more I<bytes>. | |
2c268ad5 TP |
1743 | |
1744 | # ASCII-based examples | |
1745 | print "j p \n" ^ " a h"; # prints "JAPH\n" | |
1746 | print "JA" | " ph\n"; # prints "japh\n" | |
1747 | print "japh\nJunk" & '_____'; # prints "JAPH\n"; | |
1748 | print 'p N$' ^ " E<H\n"; # prints "Perl\n"; | |
1749 | ||
1750 | If you are intending to manipulate bitstrings, you should be certain that | |
1751 | you're supplying bitstrings: If an operand is a number, that will imply | |
1752 | a B<numeric> bitwise operation. You may explicitly show which type of | |
1753 | operation you intend by using C<""> or C<0+>, as in the examples below. | |
1754 | ||
1755 | $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF) | |
1756 | $foo = '150' | 105 ; # yields 255 | |
1757 | $foo = 150 | '105'; # yields 255 | |
1758 | $foo = '150' | '105'; # yields string '155' (under ASCII) | |
1759 | ||
1760 | $baz = 0+$foo & 0+$bar; # both ops explicitly numeric | |
1761 | $biz = "$foo" ^ "$bar"; # both ops explicitly stringy | |
a0d0e21e | 1762 | |
1ae175c8 GS |
1763 | See L<perlfunc/vec> for information on how to manipulate individual bits |
1764 | in a bit vector. | |
1765 | ||
55497cff | 1766 | =head2 Integer Arithmetic |
a0d0e21e LW |
1767 | |
1768 | By default Perl assumes that it must do most of its arithmetic in | |
1769 | floating point. But by saying | |
1770 | ||
1771 | use integer; | |
1772 | ||
1773 | you may tell the compiler that it's okay to use integer operations | |
1774 | from here to the end of the enclosing BLOCK. An inner BLOCK may | |
54310121 | 1775 | countermand this by saying |
a0d0e21e LW |
1776 | |
1777 | no integer; | |
1778 | ||
1779 | which lasts until the end of that BLOCK. | |
1780 | ||
55497cff | 1781 | The bitwise operators ("&", "|", "^", "~", "<<", and ">>") always |
2c268ad5 TP |
1782 | produce integral results. (But see also L<Bitwise String Operators>.) |
1783 | However, C<use integer> still has meaning | |
55497cff | 1784 | for them. By default, their results are interpreted as unsigned |
1785 | integers. However, if C<use integer> is in effect, their results are | |
5f05dabc | 1786 | interpreted as signed integers. For example, C<~0> usually evaluates |
5a964f20 | 1787 | to a large integral value. However, C<use integer; ~0> is -1 on twos-complement machines. |
68dc0745 | 1788 | |
1789 | =head2 Floating-point Arithmetic | |
1790 | ||
1791 | While C<use integer> provides integer-only arithmetic, there is no | |
1792 | similar ways to provide rounding or truncation at a certain number of | |
1793 | decimal places. For rounding to a certain number of digits, sprintf() | |
1794 | or printf() is usually the easiest route. | |
1795 | ||
5a964f20 TC |
1796 | Floating-point numbers are only approximations to what a mathematician |
1797 | would call real numbers. There are infinitely more reals than floats, | |
1798 | so some corners must be cut. For example: | |
1799 | ||
1800 | printf "%.20g\n", 123456789123456789; | |
1801 | # produces 123456789123456784 | |
1802 | ||
1803 | Testing for exact equality of floating-point equality or inequality is | |
1804 | not a good idea. Here's a (relatively expensive) work-around to compare | |
1805 | whether two floating-point numbers are equal to a particular number of | |
1806 | decimal places. See Knuth, volume II, for a more robust treatment of | |
1807 | this topic. | |
1808 | ||
1809 | sub fp_equal { | |
1810 | my ($X, $Y, $POINTS) = @_; | |
1811 | my ($tX, $tY); | |
1812 | $tX = sprintf("%.${POINTS}g", $X); | |
1813 | $tY = sprintf("%.${POINTS}g", $Y); | |
1814 | return $tX eq $tY; | |
1815 | } | |
1816 | ||
68dc0745 | 1817 | The POSIX module (part of the standard perl distribution) implements |
1818 | ceil(), floor(), and a number of other mathematical and trigonometric | |
1819 | functions. The Math::Complex module (part of the standard perl | |
1820 | distribution) defines a number of mathematical functions that can also | |
1821 | work on real numbers. Math::Complex not as efficient as POSIX, but | |
1822 | POSIX can't work with complex numbers. | |
1823 | ||
1824 | Rounding in financial applications can have serious implications, and | |
1825 | the rounding method used should be specified precisely. In these | |
1826 | cases, it probably pays not to trust whichever system rounding is | |
1827 | being used by Perl, but to instead implement the rounding function you | |
1828 | need yourself. | |
5a964f20 TC |
1829 | |
1830 | =head2 Bigger Numbers | |
1831 | ||
1832 | The standard Math::BigInt and Math::BigFloat modules provide | |
1833 | variable precision arithmetic and overloaded operators. | |
1834 | At the cost of some space and considerable speed, they | |
1835 | avoid the normal pitfalls associated with limited-precision | |
1836 | representations. | |
1837 | ||
1838 | use Math::BigInt; | |
1839 | $x = Math::BigInt->new('123456789123456789'); | |
1840 | print $x * $x; | |
1841 | ||
1842 | # prints +15241578780673678515622620750190521 |