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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, | |
19799a22 GS |
8 | listed from highest precedence to lowest. Operators borrowed from |
9 | C keep the same precedence relationship with each other, even where | |
10 | C's precedence is slightly screwy. (This makes learning Perl easier | |
11 | for C folks.) With very few exceptions, these all operate on scalar | |
12 | 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 | ||
19799a22 GS |
67 | the commas on the right of the sort are evaluated before the sort, |
68 | but the commas on the left are evaluated after. In other words, | |
69 | list operators tend to gobble up all arguments that follow, and | |
a0d0e21e | 70 | then act like a simple TERM with regard to the preceding expression. |
19799a22 | 71 | 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 | ||
35f2feb0 | 98 | "C<< -> >>" is an infix dereference operator, just as it is in C |
19799a22 GS |
99 | and C++. If the right side is either a C<[...]>, C<{...}>, or a |
100 | C<(...)> subscript, then the left side must be either a hard or | |
101 | symbolic reference to an array, a hash, or a subroutine respectively. | |
102 | (Or technically speaking, a location capable of holding a hard | |
103 | reference, if it's an array or hash reference being used for | |
104 | assignment.) See L<perlreftut> and L<perlref>. | |
a0d0e21e | 105 | |
19799a22 GS |
106 | Otherwise, the right side is a method name or a simple scalar |
107 | variable containing either the method name or a subroutine reference, | |
108 | and the left side must be either an object (a blessed reference) | |
109 | or a class name (that is, a package name). See L<perlobj>. | |
a0d0e21e | 110 | |
5f05dabc | 111 | =head2 Auto-increment and Auto-decrement |
a0d0e21e LW |
112 | |
113 | "++" and "--" work as in C. That is, if placed before a variable, they | |
114 | increment or decrement the variable before returning the value, and if | |
115 | placed after, increment or decrement the variable after returning the value. | |
116 | ||
54310121 | 117 | The auto-increment operator has a little extra builtin magic to it. If |
a0d0e21e LW |
118 | you increment a variable that is numeric, or that has ever been used in |
119 | a numeric context, you get a normal increment. If, however, the | |
5f05dabc | 120 | variable has been used in only string contexts since it was set, and |
5a964f20 | 121 | has a value that is not the empty string and matches the pattern |
a0d0e21e LW |
122 | C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each |
123 | character within its range, with carry: | |
124 | ||
125 | print ++($foo = '99'); # prints '100' | |
126 | print ++($foo = 'a0'); # prints 'a1' | |
127 | print ++($foo = 'Az'); # prints 'Ba' | |
128 | print ++($foo = 'zz'); # prints 'aaa' | |
129 | ||
5f05dabc | 130 | The auto-decrement operator is not magical. |
a0d0e21e LW |
131 | |
132 | =head2 Exponentiation | |
133 | ||
19799a22 | 134 | Binary "**" is the exponentiation operator. It binds even more |
cb1a09d0 AD |
135 | tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is |
136 | implemented using C's pow(3) function, which actually works on doubles | |
137 | internally.) | |
a0d0e21e LW |
138 | |
139 | =head2 Symbolic Unary Operators | |
140 | ||
5f05dabc | 141 | Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower |
a0d0e21e LW |
142 | precedence version of this. |
143 | ||
144 | Unary "-" performs arithmetic negation if the operand is numeric. If | |
145 | the operand is an identifier, a string consisting of a minus sign | |
146 | concatenated with the identifier is returned. Otherwise, if the string | |
147 | starts with a plus or minus, a string starting with the opposite sign | |
148 | is returned. One effect of these rules is that C<-bareword> is equivalent | |
149 | to C<"-bareword">. | |
150 | ||
972b05a9 JH |
151 | Unary "~" performs bitwise negation, i.e., 1's complement. For |
152 | example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and | |
153 | L<Bitwise String Operators>.) Note that the width of the result is | |
154 | platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 | |
155 | bits wide on a 64-bit platform, so if you are expecting a certain bit | |
156 | width, remember use the & operator to mask off the excess bits. | |
a0d0e21e LW |
157 | |
158 | Unary "+" has no effect whatsoever, even on strings. It is useful | |
159 | syntactically for separating a function name from a parenthesized expression | |
160 | that would otherwise be interpreted as the complete list of function | |
5ba421f6 | 161 | arguments. (See examples above under L<Terms and List Operators (Leftward)>.) |
a0d0e21e | 162 | |
19799a22 GS |
163 | Unary "\" creates a reference to whatever follows it. See L<perlreftut> |
164 | and L<perlref>. Do not confuse this behavior with the behavior of | |
165 | backslash within a string, although both forms do convey the notion | |
166 | of protecting the next thing from interpolation. | |
a0d0e21e LW |
167 | |
168 | =head2 Binding Operators | |
169 | ||
c07a80fd | 170 | Binary "=~" binds a scalar expression to a pattern match. Certain operations |
cb1a09d0 AD |
171 | search or modify the string $_ by default. This operator makes that kind |
172 | of operation work on some other string. The right argument is a search | |
2c268ad5 TP |
173 | pattern, substitution, or transliteration. The left argument is what is |
174 | supposed to be searched, substituted, or transliterated instead of the default | |
f8bab1e9 GS |
175 | $_. When used in scalar context, the return value generally indicates the |
176 | success of the operation. Behavior in list context depends on the particular | |
177 | operator. See L</"Regexp Quote-Like Operators"> for details. | |
178 | ||
179 | If the right argument is an expression rather than a search pattern, | |
2c268ad5 | 180 | substitution, or transliteration, it is interpreted as a search pattern at run |
2decb4fb GS |
181 | time. This can be less efficient than an explicit search, because the |
182 | pattern must be compiled every time the expression is evaluated. | |
a0d0e21e LW |
183 | |
184 | Binary "!~" is just like "=~" except the return value is negated in | |
185 | the logical sense. | |
186 | ||
187 | =head2 Multiplicative Operators | |
188 | ||
189 | Binary "*" multiplies two numbers. | |
190 | ||
191 | Binary "/" divides two numbers. | |
192 | ||
54310121 | 193 | Binary "%" computes the modulus of two numbers. Given integer |
194 | operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is | |
195 | C<$a> minus the largest multiple of C<$b> that is not greater than | |
196 | C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the | |
197 | smallest multiple of C<$b> that is not less than C<$a> (i.e. the | |
6bb4e6d4 | 198 | result will be less than or equal to zero). |
5a964f20 | 199 | Note than when C<use integer> is in scope, "%" give you direct access |
55d729e4 GS |
200 | to the modulus operator as implemented by your C compiler. This |
201 | operator is not as well defined for negative operands, but it will | |
202 | execute faster. | |
203 | ||
62d10b70 GS |
204 | Binary "x" is the repetition operator. In scalar context or if the left |
205 | operand is not enclosed in parentheses, it returns a string consisting | |
206 | of the left operand repeated the number of times specified by the right | |
207 | operand. In list context, if the left operand is enclosed in | |
208 | parentheses, it repeats the list. | |
a0d0e21e LW |
209 | |
210 | print '-' x 80; # print row of dashes | |
211 | ||
212 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over | |
213 | ||
214 | @ones = (1) x 80; # a list of 80 1's | |
215 | @ones = (5) x @ones; # set all elements to 5 | |
216 | ||
217 | ||
218 | =head2 Additive Operators | |
219 | ||
220 | Binary "+" returns the sum of two numbers. | |
221 | ||
222 | Binary "-" returns the difference of two numbers. | |
223 | ||
224 | Binary "." concatenates two strings. | |
225 | ||
226 | =head2 Shift Operators | |
227 | ||
55497cff | 228 | Binary "<<" returns the value of its left argument shifted left by the |
229 | number of bits specified by the right argument. Arguments should be | |
982ce180 | 230 | integers. (See also L<Integer Arithmetic>.) |
a0d0e21e | 231 | |
55497cff | 232 | Binary ">>" returns the value of its left argument shifted right by |
233 | the number of bits specified by the right argument. Arguments should | |
982ce180 | 234 | be integers. (See also L<Integer Arithmetic>.) |
a0d0e21e LW |
235 | |
236 | =head2 Named Unary Operators | |
237 | ||
238 | The various named unary operators are treated as functions with one | |
239 | argument, with optional parentheses. These include the filetest | |
240 | operators, like C<-f>, C<-M>, etc. See L<perlfunc>. | |
241 | ||
242 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
243 | is followed by a left parenthesis as the next token, the operator and | |
244 | arguments within parentheses are taken to be of highest precedence, | |
245 | just like a normal function call. Examples: | |
246 | ||
247 | chdir $foo || die; # (chdir $foo) || die | |
248 | chdir($foo) || die; # (chdir $foo) || die | |
249 | chdir ($foo) || die; # (chdir $foo) || die | |
250 | chdir +($foo) || die; # (chdir $foo) || die | |
251 | ||
252 | but, because * is higher precedence than ||: | |
253 | ||
254 | chdir $foo * 20; # chdir ($foo * 20) | |
255 | chdir($foo) * 20; # (chdir $foo) * 20 | |
256 | chdir ($foo) * 20; # (chdir $foo) * 20 | |
257 | chdir +($foo) * 20; # chdir ($foo * 20) | |
258 | ||
259 | rand 10 * 20; # rand (10 * 20) | |
260 | rand(10) * 20; # (rand 10) * 20 | |
261 | rand (10) * 20; # (rand 10) * 20 | |
262 | rand +(10) * 20; # rand (10 * 20) | |
263 | ||
5ba421f6 | 264 | See also L<"Terms and List Operators (Leftward)">. |
a0d0e21e LW |
265 | |
266 | =head2 Relational Operators | |
267 | ||
35f2feb0 | 268 | Binary "<" returns true if the left argument is numerically less than |
a0d0e21e LW |
269 | the right argument. |
270 | ||
35f2feb0 | 271 | Binary ">" returns true if the left argument is numerically greater |
a0d0e21e LW |
272 | than the right argument. |
273 | ||
35f2feb0 | 274 | Binary "<=" returns true if the left argument is numerically less than |
a0d0e21e LW |
275 | or equal to the right argument. |
276 | ||
35f2feb0 | 277 | Binary ">=" returns true if the left argument is numerically greater |
a0d0e21e LW |
278 | than or equal to the right argument. |
279 | ||
280 | Binary "lt" returns true if the left argument is stringwise less than | |
281 | the right argument. | |
282 | ||
283 | Binary "gt" returns true if the left argument is stringwise greater | |
284 | than the right argument. | |
285 | ||
286 | Binary "le" returns true if the left argument is stringwise less than | |
287 | or equal to the right argument. | |
288 | ||
289 | Binary "ge" returns true if the left argument is stringwise greater | |
290 | than or equal to the right argument. | |
291 | ||
292 | =head2 Equality Operators | |
293 | ||
294 | Binary "==" returns true if the left argument is numerically equal to | |
295 | the right argument. | |
296 | ||
297 | Binary "!=" returns true if the left argument is numerically not equal | |
298 | to the right argument. | |
299 | ||
35f2feb0 | 300 | Binary "<=>" returns -1, 0, or 1 depending on whether the left |
6ee5d4e7 | 301 | argument is numerically less than, equal to, or greater than the right |
302 | argument. | |
a0d0e21e LW |
303 | |
304 | Binary "eq" returns true if the left argument is stringwise equal to | |
305 | the right argument. | |
306 | ||
307 | Binary "ne" returns true if the left argument is stringwise not equal | |
308 | to the right argument. | |
309 | ||
310 | Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise | |
311 | less than, equal to, or greater than the right argument. | |
312 | ||
a034a98d DD |
313 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
314 | by the current locale if C<use locale> is in effect. See L<perllocale>. | |
315 | ||
a0d0e21e LW |
316 | =head2 Bitwise And |
317 | ||
318 | Binary "&" returns its operators ANDed together bit by bit. | |
2c268ad5 | 319 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e LW |
320 | |
321 | =head2 Bitwise Or and Exclusive Or | |
322 | ||
323 | Binary "|" returns its operators ORed together bit by bit. | |
2c268ad5 | 324 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e LW |
325 | |
326 | Binary "^" returns its operators XORed together bit by bit. | |
2c268ad5 | 327 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e LW |
328 | |
329 | =head2 C-style Logical And | |
330 | ||
331 | Binary "&&" performs a short-circuit logical AND operation. That is, | |
332 | if the left operand is false, the right operand is not even evaluated. | |
333 | Scalar or list context propagates down to the right operand if it | |
334 | is evaluated. | |
335 | ||
336 | =head2 C-style Logical Or | |
337 | ||
338 | Binary "||" performs a short-circuit logical OR operation. That is, | |
339 | if the left operand is true, the right operand is not even evaluated. | |
340 | Scalar or list context propagates down to the right operand if it | |
341 | is evaluated. | |
342 | ||
343 | The C<||> and C<&&> operators differ from C's in that, rather than returning | |
344 | 0 or 1, they return the last value evaluated. Thus, a reasonably portable | |
345 | way to find out the home directory (assuming it's not "0") might be: | |
346 | ||
347 | $home = $ENV{'HOME'} || $ENV{'LOGDIR'} || | |
348 | (getpwuid($<))[7] || die "You're homeless!\n"; | |
349 | ||
5a964f20 TC |
350 | In particular, this means that you shouldn't use this |
351 | for selecting between two aggregates for assignment: | |
352 | ||
353 | @a = @b || @c; # this is wrong | |
354 | @a = scalar(@b) || @c; # really meant this | |
355 | @a = @b ? @b : @c; # this works fine, though | |
356 | ||
357 | As more readable alternatives to C<&&> and C<||> when used for | |
358 | control flow, Perl provides C<and> and C<or> operators (see below). | |
359 | The short-circuit behavior is identical. The precedence of "and" and | |
360 | "or" is much lower, however, so that you can safely use them after a | |
361 | list operator without the need for parentheses: | |
a0d0e21e LW |
362 | |
363 | unlink "alpha", "beta", "gamma" | |
364 | or gripe(), next LINE; | |
365 | ||
366 | With the C-style operators that would have been written like this: | |
367 | ||
368 | unlink("alpha", "beta", "gamma") | |
369 | || (gripe(), next LINE); | |
370 | ||
eeb6a2c9 | 371 | Using "or" for assignment is unlikely to do what you want; see below. |
5a964f20 TC |
372 | |
373 | =head2 Range Operators | |
a0d0e21e LW |
374 | |
375 | Binary ".." is the range operator, which is really two different | |
5a964f20 | 376 | operators depending on the context. In list context, it returns an |
2cdbc966 JD |
377 | array of values counting (up by ones) from the left value to the right |
378 | value. If the left value is greater than the right value then it | |
379 | returns the empty array. The range operator is useful for writing | |
380 | C<foreach (1..10)> loops and for doing slice operations on arrays. In | |
381 | the current implementation, no temporary array is created when the | |
382 | range operator is used as the expression in C<foreach> loops, but older | |
383 | versions of Perl might burn a lot of memory when you write something | |
384 | like this: | |
a0d0e21e LW |
385 | |
386 | for (1 .. 1_000_000) { | |
387 | # code | |
54310121 | 388 | } |
a0d0e21e | 389 | |
5a964f20 | 390 | In scalar context, ".." returns a boolean value. The operator is |
a0d0e21e LW |
391 | bistable, like a flip-flop, and emulates the line-range (comma) operator |
392 | of B<sed>, B<awk>, and various editors. Each ".." operator maintains its | |
393 | own boolean state. It is false as long as its left operand is false. | |
394 | Once the left operand is true, the range operator stays true until the | |
395 | right operand is true, I<AFTER> which the range operator becomes false | |
19799a22 | 396 | again. It doesn't become false till the next time the range operator is |
a0d0e21e LW |
397 | evaluated. It can test the right operand and become false on the same |
398 | evaluation it became true (as in B<awk>), but it still returns true once. | |
19799a22 GS |
399 | If you don't want it to test the right operand till the next |
400 | evaluation, as in B<sed>, just use three dots ("...") instead of | |
401 | two. In all other regards, "..." behaves just like ".." does. | |
402 | ||
403 | The right operand is not evaluated while the operator is in the | |
404 | "false" state, and the left operand is not evaluated while the | |
405 | operator is in the "true" state. The precedence is a little lower | |
406 | than || and &&. The value returned is either the empty string for | |
407 | false, or a sequence number (beginning with 1) for true. The | |
408 | sequence number is reset for each range encountered. The final | |
409 | sequence number in a range has the string "E0" appended to it, which | |
410 | doesn't affect its numeric value, but gives you something to search | |
411 | for if you want to exclude the endpoint. You can exclude the | |
412 | beginning point by waiting for the sequence number to be greater | |
413 | than 1. If either operand of scalar ".." is a constant expression, | |
414 | that operand is implicitly compared to the C<$.> variable, the | |
415 | current line number. Examples: | |
a0d0e21e LW |
416 | |
417 | As a scalar operator: | |
418 | ||
419 | if (101 .. 200) { print; } # print 2nd hundred lines | |
420 | next line if (1 .. /^$/); # skip header lines | |
421 | s/^/> / if (/^$/ .. eof()); # quote body | |
422 | ||
5a964f20 TC |
423 | # parse mail messages |
424 | while (<>) { | |
425 | $in_header = 1 .. /^$/; | |
426 | $in_body = /^$/ .. eof(); | |
427 | # do something based on those | |
428 | } continue { | |
429 | close ARGV if eof; # reset $. each file | |
430 | } | |
431 | ||
a0d0e21e LW |
432 | As a list operator: |
433 | ||
434 | for (101 .. 200) { print; } # print $_ 100 times | |
3e3baf6d | 435 | @foo = @foo[0 .. $#foo]; # an expensive no-op |
a0d0e21e LW |
436 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items |
437 | ||
5a964f20 | 438 | The range operator (in list context) makes use of the magical |
5f05dabc | 439 | auto-increment algorithm if the operands are strings. You |
a0d0e21e LW |
440 | can say |
441 | ||
442 | @alphabet = ('A' .. 'Z'); | |
443 | ||
19799a22 | 444 | to get all normal letters of the alphabet, or |
a0d0e21e LW |
445 | |
446 | $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; | |
447 | ||
448 | to get a hexadecimal digit, or | |
449 | ||
450 | @z2 = ('01' .. '31'); print $z2[$mday]; | |
451 | ||
452 | to get dates with leading zeros. If the final value specified is not | |
453 | in the sequence that the magical increment would produce, the sequence | |
454 | goes until the next value would be longer than the final value | |
455 | specified. | |
456 | ||
457 | =head2 Conditional Operator | |
458 | ||
459 | Ternary "?:" is the conditional operator, just as in C. It works much | |
460 | like an if-then-else. If the argument before the ? is true, the | |
461 | argument before the : is returned, otherwise the argument after the : | |
cb1a09d0 AD |
462 | is returned. For example: |
463 | ||
54310121 | 464 | printf "I have %d dog%s.\n", $n, |
cb1a09d0 AD |
465 | ($n == 1) ? '' : "s"; |
466 | ||
467 | Scalar or list context propagates downward into the 2nd | |
54310121 | 468 | or 3rd argument, whichever is selected. |
cb1a09d0 AD |
469 | |
470 | $a = $ok ? $b : $c; # get a scalar | |
471 | @a = $ok ? @b : @c; # get an array | |
472 | $a = $ok ? @b : @c; # oops, that's just a count! | |
473 | ||
474 | The operator may be assigned to if both the 2nd and 3rd arguments are | |
475 | legal lvalues (meaning that you can assign to them): | |
a0d0e21e LW |
476 | |
477 | ($a_or_b ? $a : $b) = $c; | |
478 | ||
5a964f20 TC |
479 | Because this operator produces an assignable result, using assignments |
480 | without parentheses will get you in trouble. For example, this: | |
481 | ||
482 | $a % 2 ? $a += 10 : $a += 2 | |
483 | ||
484 | Really means this: | |
485 | ||
486 | (($a % 2) ? ($a += 10) : $a) += 2 | |
487 | ||
488 | Rather than this: | |
489 | ||
490 | ($a % 2) ? ($a += 10) : ($a += 2) | |
491 | ||
19799a22 GS |
492 | That should probably be written more simply as: |
493 | ||
494 | $a += ($a % 2) ? 10 : 2; | |
495 | ||
4633a7c4 | 496 | =head2 Assignment Operators |
a0d0e21e LW |
497 | |
498 | "=" is the ordinary assignment operator. | |
499 | ||
500 | Assignment operators work as in C. That is, | |
501 | ||
502 | $a += 2; | |
503 | ||
504 | is equivalent to | |
505 | ||
506 | $a = $a + 2; | |
507 | ||
508 | although without duplicating any side effects that dereferencing the lvalue | |
54310121 | 509 | might trigger, such as from tie(). Other assignment operators work similarly. |
510 | The following are recognized: | |
a0d0e21e LW |
511 | |
512 | **= += *= &= <<= &&= | |
513 | -= /= |= >>= ||= | |
514 | .= %= ^= | |
515 | x= | |
516 | ||
19799a22 | 517 | Although these are grouped by family, they all have the precedence |
a0d0e21e LW |
518 | of assignment. |
519 | ||
b350dd2f GS |
520 | Unlike in C, the scalar assignment operator produces a valid lvalue. |
521 | Modifying an assignment is equivalent to doing the assignment and | |
522 | then modifying the variable that was assigned to. This is useful | |
523 | for modifying a copy of something, like this: | |
a0d0e21e LW |
524 | |
525 | ($tmp = $global) =~ tr [A-Z] [a-z]; | |
526 | ||
527 | Likewise, | |
528 | ||
529 | ($a += 2) *= 3; | |
530 | ||
531 | is equivalent to | |
532 | ||
533 | $a += 2; | |
534 | $a *= 3; | |
535 | ||
b350dd2f GS |
536 | Similarly, a list assignment in list context produces the list of |
537 | lvalues assigned to, and a list assignment in scalar context returns | |
538 | the number of elements produced by the expression on the right hand | |
539 | side of the assignment. | |
540 | ||
748a9306 | 541 | =head2 Comma Operator |
a0d0e21e | 542 | |
5a964f20 | 543 | Binary "," is the comma operator. In scalar context it evaluates |
a0d0e21e LW |
544 | its left argument, throws that value away, then evaluates its right |
545 | argument and returns that value. This is just like C's comma operator. | |
546 | ||
5a964f20 | 547 | In list context, it's just the list argument separator, and inserts |
a0d0e21e LW |
548 | both its arguments into the list. |
549 | ||
35f2feb0 | 550 | The => digraph is mostly just a synonym for the comma operator. It's useful for |
cb1a09d0 | 551 | documenting arguments that come in pairs. As of release 5.001, it also forces |
4633a7c4 | 552 | any word to the left of it to be interpreted as a string. |
748a9306 | 553 | |
a0d0e21e LW |
554 | =head2 List Operators (Rightward) |
555 | ||
556 | On the right side of a list operator, it has very low precedence, | |
557 | such that it controls all comma-separated expressions found there. | |
558 | The only operators with lower precedence are the logical operators | |
559 | "and", "or", and "not", which may be used to evaluate calls to list | |
560 | operators without the need for extra parentheses: | |
561 | ||
562 | open HANDLE, "filename" | |
563 | or die "Can't open: $!\n"; | |
564 | ||
5ba421f6 | 565 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
a0d0e21e LW |
566 | |
567 | =head2 Logical Not | |
568 | ||
569 | Unary "not" returns the logical negation of the expression to its right. | |
570 | It's the equivalent of "!" except for the very low precedence. | |
571 | ||
572 | =head2 Logical And | |
573 | ||
574 | Binary "and" returns the logical conjunction of the two surrounding | |
575 | expressions. It's equivalent to && except for the very low | |
5f05dabc | 576 | precedence. This means that it short-circuits: i.e., the right |
a0d0e21e LW |
577 | expression is evaluated only if the left expression is true. |
578 | ||
579 | =head2 Logical or and Exclusive Or | |
580 | ||
581 | Binary "or" returns the logical disjunction of the two surrounding | |
5a964f20 TC |
582 | expressions. It's equivalent to || except for the very low precedence. |
583 | This makes it useful for control flow | |
584 | ||
585 | print FH $data or die "Can't write to FH: $!"; | |
586 | ||
587 | This means that it short-circuits: i.e., the right expression is evaluated | |
588 | only if the left expression is false. Due to its precedence, you should | |
589 | probably avoid using this for assignment, only for control flow. | |
590 | ||
591 | $a = $b or $c; # bug: this is wrong | |
592 | ($a = $b) or $c; # really means this | |
593 | $a = $b || $c; # better written this way | |
594 | ||
19799a22 | 595 | However, when it's a list-context assignment and you're trying to use |
5a964f20 TC |
596 | "||" for control flow, you probably need "or" so that the assignment |
597 | takes higher precedence. | |
598 | ||
599 | @info = stat($file) || die; # oops, scalar sense of stat! | |
600 | @info = stat($file) or die; # better, now @info gets its due | |
601 | ||
19799a22 | 602 | Then again, you could always use parentheses. |
a0d0e21e LW |
603 | |
604 | Binary "xor" returns the exclusive-OR of the two surrounding expressions. | |
605 | It cannot short circuit, of course. | |
606 | ||
607 | =head2 C Operators Missing From Perl | |
608 | ||
609 | Here is what C has that Perl doesn't: | |
610 | ||
611 | =over 8 | |
612 | ||
613 | =item unary & | |
614 | ||
615 | Address-of operator. (But see the "\" operator for taking a reference.) | |
616 | ||
617 | =item unary * | |
618 | ||
54310121 | 619 | Dereference-address operator. (Perl's prefix dereferencing |
a0d0e21e LW |
620 | operators are typed: $, @, %, and &.) |
621 | ||
622 | =item (TYPE) | |
623 | ||
19799a22 | 624 | Type-casting operator. |
a0d0e21e LW |
625 | |
626 | =back | |
627 | ||
5f05dabc | 628 | =head2 Quote and Quote-like Operators |
a0d0e21e LW |
629 | |
630 | While we usually think of quotes as literal values, in Perl they | |
631 | function as operators, providing various kinds of interpolating and | |
632 | pattern matching capabilities. Perl provides customary quote characters | |
633 | for these behaviors, but also provides a way for you to choose your | |
634 | quote character for any of them. In the following table, a C<{}> represents | |
87275199 | 635 | any pair of delimiters you choose. |
a0d0e21e | 636 | |
2c268ad5 TP |
637 | Customary Generic Meaning Interpolates |
638 | '' q{} Literal no | |
639 | "" qq{} Literal yes | |
01ae956f | 640 | `` qx{} Command yes (unless '' is delimiter) |
2c268ad5 | 641 | qw{} Word list no |
f70b4f9c AB |
642 | // m{} Pattern match yes (unless '' is delimiter) |
643 | qr{} Pattern yes (unless '' is delimiter) | |
644 | s{}{} Substitution yes (unless '' is delimiter) | |
2c268ad5 | 645 | tr{}{} Transliteration no (but see below) |
a0d0e21e | 646 | |
87275199 GS |
647 | Non-bracketing delimiters use the same character fore and aft, but the four |
648 | sorts of brackets (round, angle, square, curly) will all nest, which means | |
649 | that | |
650 | ||
651 | q{foo{bar}baz} | |
35f2feb0 | 652 | |
87275199 GS |
653 | is the same as |
654 | ||
655 | 'foo{bar}baz' | |
656 | ||
657 | Note, however, that this does not always work for quoting Perl code: | |
658 | ||
659 | $s = q{ if($a eq "}") ... }; # WRONG | |
660 | ||
661 | is a syntax error. The C<Text::Balanced> module on CPAN is able to do this | |
662 | properly. | |
663 | ||
19799a22 | 664 | There can be whitespace between the operator and the quoting |
fb73857a | 665 | characters, except when C<#> is being used as the quoting character. |
19799a22 GS |
666 | C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the |
667 | operator C<q> followed by a comment. Its argument will be taken | |
668 | from the next line. This allows you to write: | |
fb73857a | 669 | |
670 | s {foo} # Replace foo | |
671 | {bar} # with bar. | |
672 | ||
19799a22 GS |
673 | For constructs that do interpolate, variables beginning with "C<$>" |
674 | or "C<@>" are interpolated, as are the following escape sequences. Within | |
a0ed51b3 | 675 | a transliteration, the first eleven of these sequences may be used. |
a0d0e21e | 676 | |
6ee5d4e7 | 677 | \t tab (HT, TAB) |
5a964f20 | 678 | \n newline (NL) |
6ee5d4e7 | 679 | \r return (CR) |
680 | \f form feed (FF) | |
681 | \b backspace (BS) | |
682 | \a alarm (bell) (BEL) | |
683 | \e escape (ESC) | |
a0ed51b3 LW |
684 | \033 octal char (ESC) |
685 | \x1b hex char (ESC) | |
686 | \x{263a} wide hex char (SMILEY) | |
19799a22 | 687 | \c[ control char (ESC) |
4a2d328f | 688 | \N{name} named char |
2c268ad5 | 689 | |
a0d0e21e LW |
690 | \l lowercase next char |
691 | \u uppercase next char | |
692 | \L lowercase till \E | |
693 | \U uppercase till \E | |
694 | \E end case modification | |
1d2dff63 | 695 | \Q quote non-word characters till \E |
a0d0e21e | 696 | |
a034a98d | 697 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> |
423cee85 | 698 | and C<\U> is taken from the current locale. See L<perllocale>. For |
4a2d328f | 699 | documentation of C<\N{name}>, see L<charnames>. |
a034a98d | 700 | |
5a964f20 TC |
701 | All systems use the virtual C<"\n"> to represent a line terminator, |
702 | called a "newline". There is no such thing as an unvarying, physical | |
19799a22 | 703 | newline character. It is only an illusion that the operating system, |
5a964f20 TC |
704 | device drivers, C libraries, and Perl all conspire to preserve. Not all |
705 | systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example, | |
706 | on a Mac, these are reversed, and on systems without line terminator, | |
707 | printing C<"\n"> may emit no actual data. In general, use C<"\n"> when | |
708 | you mean a "newline" for your system, but use the literal ASCII when you | |
709 | need an exact character. For example, most networking protocols expect | |
710 | and prefer a CR+LF (C<"\012\015"> or C<"\cJ\cM">) for line terminators, | |
711 | and although they often accept just C<"\012">, they seldom tolerate just | |
712 | C<"\015">. If you get in the habit of using C<"\n"> for networking, | |
713 | you may be burned some day. | |
714 | ||
1d2dff63 GS |
715 | You cannot include a literal C<$> or C<@> within a C<\Q> sequence. |
716 | An unescaped C<$> or C<@> interpolates the corresponding variable, | |
717 | while escaping will cause the literal string C<\$> to be inserted. | |
718 | You'll need to write something like C<m/\Quser\E\@\Qhost/>. | |
719 | ||
a0d0e21e LW |
720 | Patterns are subject to an additional level of interpretation as a |
721 | regular expression. This is done as a second pass, after variables are | |
722 | interpolated, so that regular expressions may be incorporated into the | |
723 | pattern from the variables. If this is not what you want, use C<\Q> to | |
724 | interpolate a variable literally. | |
725 | ||
19799a22 GS |
726 | Apart from the behavior described above, Perl does not expand |
727 | multiple levels of interpolation. In particular, contrary to the | |
728 | expectations of shell programmers, back-quotes do I<NOT> interpolate | |
729 | within double quotes, nor do single quotes impede evaluation of | |
730 | variables when used within double quotes. | |
a0d0e21e | 731 | |
5f05dabc | 732 | =head2 Regexp Quote-Like Operators |
cb1a09d0 | 733 | |
5f05dabc | 734 | Here are the quote-like operators that apply to pattern |
cb1a09d0 AD |
735 | matching and related activities. |
736 | ||
a0d0e21e LW |
737 | =over 8 |
738 | ||
739 | =item ?PATTERN? | |
740 | ||
741 | This is just like the C</pattern/> search, except that it matches only | |
742 | once between calls to the reset() operator. This is a useful | |
5f05dabc | 743 | optimization when you want to see only the first occurrence of |
a0d0e21e LW |
744 | something in each file of a set of files, for instance. Only C<??> |
745 | patterns local to the current package are reset. | |
746 | ||
5a964f20 TC |
747 | while (<>) { |
748 | if (?^$?) { | |
749 | # blank line between header and body | |
750 | } | |
751 | } continue { | |
752 | reset if eof; # clear ?? status for next file | |
753 | } | |
754 | ||
19799a22 GS |
755 | This usage is vaguely depreciated, which means it just might possibly |
756 | be removed in some distant future version of Perl, perhaps somewhere | |
757 | around the year 2168. | |
a0d0e21e | 758 | |
fb73857a | 759 | =item m/PATTERN/cgimosx |
a0d0e21e | 760 | |
fb73857a | 761 | =item /PATTERN/cgimosx |
a0d0e21e | 762 | |
5a964f20 | 763 | Searches a string for a pattern match, and in scalar context returns |
19799a22 GS |
764 | true if it succeeds, false if it fails. If no string is specified |
765 | via the C<=~> or C<!~> operator, the $_ string is searched. (The | |
766 | string specified with C<=~> need not be an lvalue--it may be the | |
767 | result of an expression evaluation, but remember the C<=~> binds | |
768 | rather tightly.) See also L<perlre>. See L<perllocale> for | |
769 | discussion of additional considerations that apply when C<use locale> | |
770 | is in effect. | |
a0d0e21e LW |
771 | |
772 | Options are: | |
773 | ||
fb73857a | 774 | c Do not reset search position on a failed match when /g is in effect. |
5f05dabc | 775 | g Match globally, i.e., find all occurrences. |
a0d0e21e LW |
776 | i Do case-insensitive pattern matching. |
777 | m Treat string as multiple lines. | |
5f05dabc | 778 | o Compile pattern only once. |
a0d0e21e LW |
779 | s Treat string as single line. |
780 | x Use extended regular expressions. | |
781 | ||
782 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> | |
01ae956f | 783 | you can use any pair of non-alphanumeric, non-whitespace characters |
19799a22 GS |
784 | as delimiters. This is particularly useful for matching path names |
785 | that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is | |
7bac28a0 | 786 | the delimiter, then the match-only-once rule of C<?PATTERN?> applies. |
19799a22 | 787 | If "'" is the delimiter, no interpolation is performed on the PATTERN. |
a0d0e21e LW |
788 | |
789 | PATTERN may contain variables, which will be interpolated (and the | |
f70b4f9c AB |
790 | pattern recompiled) every time the pattern search is evaluated, except |
791 | for when the delimiter is a single quote. (Note that C<$)> and C<$|> | |
792 | might not be interpolated because they look like end-of-string tests.) | |
793 | If you want such a pattern to be compiled only once, add a C</o> after | |
794 | the trailing delimiter. This avoids expensive run-time recompilations, | |
795 | and is useful when the value you are interpolating won't change over | |
796 | the life of the script. However, mentioning C</o> constitutes a promise | |
797 | that you won't change the variables in the pattern. If you change them, | |
0b8d69e9 | 798 | Perl won't even notice. See also L<"qr//">. |
a0d0e21e | 799 | |
5a964f20 TC |
800 | If the PATTERN evaluates to the empty string, the last |
801 | I<successfully> matched regular expression is used instead. | |
a0d0e21e | 802 | |
19799a22 | 803 | If the C</g> option is not used, C<m//> in list context returns a |
a0d0e21e | 804 | list consisting of the subexpressions matched by the parentheses in the |
f7e33566 GS |
805 | pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are |
806 | also set, and that this differs from Perl 4's behavior.) When there are | |
807 | no parentheses in the pattern, the return value is the list C<(1)> for | |
808 | success. With or without parentheses, an empty list is returned upon | |
809 | failure. | |
a0d0e21e LW |
810 | |
811 | Examples: | |
812 | ||
813 | open(TTY, '/dev/tty'); | |
814 | <TTY> =~ /^y/i && foo(); # do foo if desired | |
815 | ||
816 | if (/Version: *([0-9.]*)/) { $version = $1; } | |
817 | ||
818 | next if m#^/usr/spool/uucp#; | |
819 | ||
820 | # poor man's grep | |
821 | $arg = shift; | |
822 | while (<>) { | |
823 | print if /$arg/o; # compile only once | |
824 | } | |
825 | ||
826 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) | |
827 | ||
828 | This last example splits $foo into the first two words and the | |
5f05dabc | 829 | remainder of the line, and assigns those three fields to $F1, $F2, and |
830 | $Etc. The conditional is true if any variables were assigned, i.e., if | |
a0d0e21e LW |
831 | the pattern matched. |
832 | ||
19799a22 GS |
833 | The C</g> modifier specifies global pattern matching--that is, |
834 | matching as many times as possible within the string. How it behaves | |
835 | depends on the context. In list context, it returns a list of the | |
836 | substrings matched by any capturing parentheses in the regular | |
837 | expression. If there are no parentheses, it returns a list of all | |
838 | the matched strings, as if there were parentheses around the whole | |
839 | pattern. | |
a0d0e21e | 840 | |
7e86de3e | 841 | In scalar context, each execution of C<m//g> finds the next match, |
19799a22 | 842 | returning true if it matches, and false if there is no further match. |
7e86de3e MG |
843 | The position after the last match can be read or set using the pos() |
844 | function; see L<perlfunc/pos>. A failed match normally resets the | |
845 | search position to the beginning of the string, but you can avoid that | |
846 | by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target | |
847 | string also resets the search position. | |
c90c0ff4 | 848 | |
849 | You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a | |
850 | zero-width assertion that matches the exact position where the previous | |
851 | C<m//g>, if any, left off. The C<\G> assertion is not supported without | |
19799a22 GS |
852 | the C</g> modifier. (Currently, without C</g>, C<\G> behaves just like |
853 | C<\A>, but that's accidental and may change in the future.) | |
c90c0ff4 | 854 | |
855 | Examples: | |
a0d0e21e LW |
856 | |
857 | # list context | |
858 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); | |
859 | ||
860 | # scalar context | |
19799a22 GS |
861 | $/ = ""; $* = 1; # $* deprecated in modern perls |
862 | while (defined($paragraph = <>)) { | |
863 | while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { | |
864 | $sentences++; | |
a0d0e21e LW |
865 | } |
866 | } | |
867 | print "$sentences\n"; | |
868 | ||
c90c0ff4 | 869 | # using m//gc with \G |
137443ea | 870 | $_ = "ppooqppqq"; |
44a8e56a | 871 | while ($i++ < 2) { |
872 | print "1: '"; | |
c90c0ff4 | 873 | print $1 while /(o)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 874 | print "2: '"; |
c90c0ff4 | 875 | print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 876 | print "3: '"; |
c90c0ff4 | 877 | print $1 while /(p)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 878 | } |
879 | ||
880 | The last example should print: | |
881 | ||
882 | 1: 'oo', pos=4 | |
137443ea | 883 | 2: 'q', pos=5 |
44a8e56a | 884 | 3: 'pp', pos=7 |
885 | 1: '', pos=7 | |
137443ea | 886 | 2: 'q', pos=8 |
887 | 3: '', pos=8 | |
44a8e56a | 888 | |
c90c0ff4 | 889 | A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can |
e7ea3e70 | 890 | combine several regexps like this to process a string part-by-part, |
c90c0ff4 | 891 | doing different actions depending on which regexp matched. Each |
892 | regexp tries to match where the previous one leaves off. | |
e7ea3e70 | 893 | |
3fe9a6f1 | 894 | $_ = <<'EOL'; |
e7ea3e70 | 895 | $url = new URI::URL "http://www/"; die if $url eq "xXx"; |
3fe9a6f1 | 896 | EOL |
897 | LOOP: | |
e7ea3e70 | 898 | { |
c90c0ff4 | 899 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; |
900 | print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc; | |
901 | print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc; | |
902 | print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc; | |
903 | print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc; | |
904 | print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc; | |
905 | print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc; | |
e7ea3e70 IZ |
906 | print ". That's all!\n"; |
907 | } | |
908 | ||
909 | Here is the output (split into several lines): | |
910 | ||
911 | line-noise lowercase line-noise lowercase UPPERCASE line-noise | |
912 | UPPERCASE line-noise lowercase line-noise lowercase line-noise | |
913 | lowercase lowercase line-noise lowercase lowercase line-noise | |
914 | MiXeD line-noise. That's all! | |
44a8e56a | 915 | |
a0d0e21e LW |
916 | =item q/STRING/ |
917 | ||
918 | =item C<'STRING'> | |
919 | ||
19799a22 | 920 | A single-quoted, literal string. A backslash represents a backslash |
68dc0745 | 921 | unless followed by the delimiter or another backslash, in which case |
922 | the delimiter or backslash is interpolated. | |
a0d0e21e LW |
923 | |
924 | $foo = q!I said, "You said, 'She said it.'"!; | |
925 | $bar = q('This is it.'); | |
68dc0745 | 926 | $baz = '\n'; # a two-character string |
a0d0e21e LW |
927 | |
928 | =item qq/STRING/ | |
929 | ||
930 | =item "STRING" | |
931 | ||
932 | A double-quoted, interpolated string. | |
933 | ||
934 | $_ .= qq | |
935 | (*** The previous line contains the naughty word "$1".\n) | |
19799a22 | 936 | if /\b(tcl|java|python)\b/i; # :-) |
68dc0745 | 937 | $baz = "\n"; # a one-character string |
a0d0e21e | 938 | |
eec2d3df GS |
939 | =item qr/STRING/imosx |
940 | ||
19799a22 GS |
941 | This operators quotes--and compiles--its I<STRING> as a regular |
942 | expression. I<STRING> is interpolated the same way as I<PATTERN> | |
943 | in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation | |
944 | is done. Returns a Perl value which may be used instead of the | |
945 | corresponding C</STRING/imosx> expression. | |
4b6a7270 IZ |
946 | |
947 | For example, | |
948 | ||
949 | $rex = qr/my.STRING/is; | |
950 | s/$rex/foo/; | |
951 | ||
952 | is equivalent to | |
953 | ||
954 | s/my.STRING/foo/is; | |
955 | ||
956 | The result may be used as a subpattern in a match: | |
eec2d3df GS |
957 | |
958 | $re = qr/$pattern/; | |
0a92e3a8 GS |
959 | $string =~ /foo${re}bar/; # can be interpolated in other patterns |
960 | $string =~ $re; # or used standalone | |
4b6a7270 IZ |
961 | $string =~ /$re/; # or this way |
962 | ||
963 | Since Perl may compile the pattern at the moment of execution of qr() | |
19799a22 | 964 | operator, using qr() may have speed advantages in some situations, |
4b6a7270 IZ |
965 | notably if the result of qr() is used standalone: |
966 | ||
967 | sub match { | |
968 | my $patterns = shift; | |
969 | my @compiled = map qr/$_/i, @$patterns; | |
970 | grep { | |
971 | my $success = 0; | |
a7665c5e | 972 | foreach my $pat (@compiled) { |
4b6a7270 IZ |
973 | $success = 1, last if /$pat/; |
974 | } | |
975 | $success; | |
976 | } @_; | |
977 | } | |
978 | ||
19799a22 GS |
979 | Precompilation of the pattern into an internal representation at |
980 | the moment of qr() avoids a need to recompile the pattern every | |
981 | time a match C</$pat/> is attempted. (Perl has many other internal | |
982 | optimizations, but none would be triggered in the above example if | |
983 | we did not use qr() operator.) | |
eec2d3df GS |
984 | |
985 | Options are: | |
986 | ||
987 | i Do case-insensitive pattern matching. | |
988 | m Treat string as multiple lines. | |
989 | o Compile pattern only once. | |
990 | s Treat string as single line. | |
991 | x Use extended regular expressions. | |
992 | ||
0a92e3a8 GS |
993 | See L<perlre> for additional information on valid syntax for STRING, and |
994 | for a detailed look at the semantics of regular expressions. | |
995 | ||
a0d0e21e LW |
996 | =item qx/STRING/ |
997 | ||
998 | =item `STRING` | |
999 | ||
5a964f20 TC |
1000 | A string which is (possibly) interpolated and then executed as a system |
1001 | command with C</bin/sh> or its equivalent. Shell wildcards, pipes, | |
1002 | and redirections will be honored. The collected standard output of the | |
1003 | command is returned; standard error is unaffected. In scalar context, | |
1004 | it comes back as a single (potentially multi-line) string. In list | |
1005 | context, returns a list of lines (however you've defined lines with $/ | |
1006 | or $INPUT_RECORD_SEPARATOR). | |
1007 | ||
1008 | Because backticks do not affect standard error, use shell file descriptor | |
1009 | syntax (assuming the shell supports this) if you care to address this. | |
1010 | To capture a command's STDERR and STDOUT together: | |
a0d0e21e | 1011 | |
5a964f20 TC |
1012 | $output = `cmd 2>&1`; |
1013 | ||
1014 | To capture a command's STDOUT but discard its STDERR: | |
1015 | ||
1016 | $output = `cmd 2>/dev/null`; | |
1017 | ||
1018 | To capture a command's STDERR but discard its STDOUT (ordering is | |
1019 | important here): | |
1020 | ||
1021 | $output = `cmd 2>&1 1>/dev/null`; | |
1022 | ||
1023 | To exchange a command's STDOUT and STDERR in order to capture the STDERR | |
1024 | but leave its STDOUT to come out the old STDERR: | |
1025 | ||
1026 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; | |
1027 | ||
1028 | To read both a command's STDOUT and its STDERR separately, it's easiest | |
1029 | and safest to redirect them separately to files, and then read from those | |
1030 | files when the program is done: | |
1031 | ||
1032 | system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr"); | |
1033 | ||
1034 | Using single-quote as a delimiter protects the command from Perl's | |
1035 | double-quote interpolation, passing it on to the shell instead: | |
1036 | ||
1037 | $perl_info = qx(ps $$); # that's Perl's $$ | |
1038 | $shell_info = qx'ps $$'; # that's the new shell's $$ | |
1039 | ||
19799a22 | 1040 | How that string gets evaluated is entirely subject to the command |
5a964f20 TC |
1041 | interpreter on your system. On most platforms, you will have to protect |
1042 | shell metacharacters if you want them treated literally. This is in | |
1043 | practice difficult to do, as it's unclear how to escape which characters. | |
1044 | See L<perlsec> for a clean and safe example of a manual fork() and exec() | |
1045 | to emulate backticks safely. | |
a0d0e21e | 1046 | |
bb32b41a GS |
1047 | On some platforms (notably DOS-like ones), the shell may not be |
1048 | capable of dealing with multiline commands, so putting newlines in | |
1049 | the string may not get you what you want. You may be able to evaluate | |
1050 | multiple commands in a single line by separating them with the command | |
1051 | separator character, if your shell supports that (e.g. C<;> on many Unix | |
1052 | shells; C<&> on the Windows NT C<cmd> shell). | |
1053 | ||
0f897271 GS |
1054 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
1055 | output before starting the child process, but this may not be supported | |
1056 | on some platforms (see L<perlport>). To be safe, you may need to set | |
1057 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of | |
1058 | C<IO::Handle> on any open handles. | |
1059 | ||
bb32b41a GS |
1060 | Beware that some command shells may place restrictions on the length |
1061 | of the command line. You must ensure your strings don't exceed this | |
1062 | limit after any necessary interpolations. See the platform-specific | |
1063 | release notes for more details about your particular environment. | |
1064 | ||
5a964f20 TC |
1065 | Using this operator can lead to programs that are difficult to port, |
1066 | because the shell commands called vary between systems, and may in | |
1067 | fact not be present at all. As one example, the C<type> command under | |
1068 | the POSIX shell is very different from the C<type> command under DOS. | |
1069 | That doesn't mean you should go out of your way to avoid backticks | |
1070 | when they're the right way to get something done. Perl was made to be | |
1071 | a glue language, and one of the things it glues together is commands. | |
1072 | Just understand what you're getting yourself into. | |
bb32b41a | 1073 | |
dc848c6f | 1074 | See L<"I/O Operators"> for more discussion. |
a0d0e21e LW |
1075 | |
1076 | =item qw/STRING/ | |
1077 | ||
8127e0e3 GS |
1078 | Evaluates to a list of the words extracted out of STRING, using embedded |
1079 | whitespace as the word delimiters. It can be understood as being roughly | |
1080 | equivalent to: | |
a0d0e21e LW |
1081 | |
1082 | split(' ', q/STRING/); | |
1083 | ||
26ef7447 GS |
1084 | the difference being that it generates a real list at compile time. So |
1085 | this expression: | |
1086 | ||
1087 | qw(foo bar baz) | |
1088 | ||
c0c5a66b | 1089 | is semantically equivalent to the list: |
26ef7447 | 1090 | |
c0c5a66b | 1091 | 'foo', 'bar', 'baz' |
5a964f20 | 1092 | |
a0d0e21e LW |
1093 | Some frequently seen examples: |
1094 | ||
1095 | use POSIX qw( setlocale localeconv ) | |
1096 | @EXPORT = qw( foo bar baz ); | |
1097 | ||
19799a22 GS |
1098 | A common mistake is to try to separate the words with comma or to |
1099 | put comments into a multi-line C<qw>-string. For this reason, the | |
9f1b1f2d GS |
1100 | C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable) |
1101 | produces warnings if the STRING contains the "," or the "#" character. | |
7bac28a0 | 1102 | |
a0d0e21e LW |
1103 | =item s/PATTERN/REPLACEMENT/egimosx |
1104 | ||
1105 | Searches a string for a pattern, and if found, replaces that pattern | |
1106 | with the replacement text and returns the number of substitutions | |
e37d713d | 1107 | made. Otherwise it returns false (specifically, the empty string). |
a0d0e21e LW |
1108 | |
1109 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> | |
1110 | variable is searched and modified. (The string specified with C<=~> must | |
5a964f20 | 1111 | be scalar variable, an array element, a hash element, or an assignment |
5f05dabc | 1112 | to one of those, i.e., an lvalue.) |
a0d0e21e | 1113 | |
19799a22 | 1114 | If the delimiter chosen is a single quote, no interpolation is |
a0d0e21e LW |
1115 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the |
1116 | PATTERN contains a $ that looks like a variable rather than an | |
1117 | end-of-string test, the variable will be interpolated into the pattern | |
5f05dabc | 1118 | at run-time. If you want the pattern compiled only once the first time |
a0d0e21e | 1119 | the variable is interpolated, use the C</o> option. If the pattern |
5a964f20 | 1120 | evaluates to the empty string, the last successfully executed regular |
a0d0e21e | 1121 | expression is used instead. See L<perlre> for further explanation on these. |
5a964f20 | 1122 | See L<perllocale> for discussion of additional considerations that apply |
a034a98d | 1123 | when C<use locale> is in effect. |
a0d0e21e LW |
1124 | |
1125 | Options are: | |
1126 | ||
1127 | e Evaluate the right side as an expression. | |
5f05dabc | 1128 | g Replace globally, i.e., all occurrences. |
a0d0e21e LW |
1129 | i Do case-insensitive pattern matching. |
1130 | m Treat string as multiple lines. | |
5f05dabc | 1131 | o Compile pattern only once. |
a0d0e21e LW |
1132 | s Treat string as single line. |
1133 | x Use extended regular expressions. | |
1134 | ||
1135 | Any non-alphanumeric, non-whitespace delimiter may replace the | |
1136 | slashes. If single quotes are used, no interpretation is done on the | |
e37d713d | 1137 | replacement string (the C</e> modifier overrides this, however). Unlike |
54310121 | 1138 | Perl 4, Perl 5 treats backticks as normal delimiters; the replacement |
e37d713d | 1139 | text is not evaluated as a command. If the |
a0d0e21e | 1140 | PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own |
5f05dabc | 1141 | pair of quotes, which may or may not be bracketing quotes, e.g., |
35f2feb0 | 1142 | C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the |
cec88af6 GS |
1143 | replacement portion to be treated as a full-fledged Perl expression |
1144 | and evaluated right then and there. It is, however, syntax checked at | |
1145 | compile-time. A second C<e> modifier will cause the replacement portion | |
1146 | to be C<eval>ed before being run as a Perl expression. | |
a0d0e21e LW |
1147 | |
1148 | Examples: | |
1149 | ||
1150 | s/\bgreen\b/mauve/g; # don't change wintergreen | |
1151 | ||
1152 | $path =~ s|/usr/bin|/usr/local/bin|; | |
1153 | ||
1154 | s/Login: $foo/Login: $bar/; # run-time pattern | |
1155 | ||
5a964f20 | 1156 | ($foo = $bar) =~ s/this/that/; # copy first, then change |
a0d0e21e | 1157 | |
5a964f20 | 1158 | $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count |
a0d0e21e LW |
1159 | |
1160 | $_ = 'abc123xyz'; | |
1161 | s/\d+/$&*2/e; # yields 'abc246xyz' | |
1162 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' | |
1163 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' | |
1164 | ||
1165 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e | |
1166 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e | |
1167 | s/^=(\w+)/&pod($1)/ge; # use function call | |
1168 | ||
5a964f20 TC |
1169 | # expand variables in $_, but dynamics only, using |
1170 | # symbolic dereferencing | |
1171 | s/\$(\w+)/${$1}/g; | |
1172 | ||
cec88af6 GS |
1173 | # Add one to the value of any numbers in the string |
1174 | s/(\d+)/1 + $1/eg; | |
1175 | ||
1176 | # This will expand any embedded scalar variable | |
1177 | # (including lexicals) in $_ : First $1 is interpolated | |
1178 | # to the variable name, and then evaluated | |
a0d0e21e LW |
1179 | s/(\$\w+)/$1/eeg; |
1180 | ||
5a964f20 | 1181 | # Delete (most) C comments. |
a0d0e21e | 1182 | $program =~ s { |
4633a7c4 LW |
1183 | /\* # Match the opening delimiter. |
1184 | .*? # Match a minimal number of characters. | |
1185 | \*/ # Match the closing delimiter. | |
a0d0e21e LW |
1186 | } []gsx; |
1187 | ||
5a964f20 TC |
1188 | s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively |
1189 | ||
1190 | for ($variable) { # trim white space in $variable, cheap | |
1191 | s/^\s+//; | |
1192 | s/\s+$//; | |
1193 | } | |
a0d0e21e LW |
1194 | |
1195 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields | |
1196 | ||
54310121 | 1197 | Note the use of $ instead of \ in the last example. Unlike |
35f2feb0 GS |
1198 | B<sed>, we use the \<I<digit>> form in only the left hand side. |
1199 | Anywhere else it's $<I<digit>>. | |
a0d0e21e | 1200 | |
5f05dabc | 1201 | Occasionally, you can't use just a C</g> to get all the changes |
19799a22 | 1202 | to occur that you might want. Here are two common cases: |
a0d0e21e LW |
1203 | |
1204 | # put commas in the right places in an integer | |
19799a22 | 1205 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; |
a0d0e21e LW |
1206 | |
1207 | # expand tabs to 8-column spacing | |
1208 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; | |
1209 | ||
a0ed51b3 | 1210 | =item tr/SEARCHLIST/REPLACEMENTLIST/cdsUC |
a0d0e21e | 1211 | |
a0ed51b3 | 1212 | =item y/SEARCHLIST/REPLACEMENTLIST/cdsUC |
a0d0e21e | 1213 | |
2c268ad5 | 1214 | Transliterates all occurrences of the characters found in the search list |
a0d0e21e LW |
1215 | with the corresponding character in the replacement list. It returns |
1216 | the number of characters replaced or deleted. If no string is | |
2c268ad5 | 1217 | specified via the =~ or !~ operator, the $_ string is transliterated. (The |
54310121 | 1218 | string specified with =~ must be a scalar variable, an array element, a |
1219 | hash element, or an assignment to one of those, i.e., an lvalue.) | |
8ada0baa | 1220 | |
2c268ad5 TP |
1221 | A character range may be specified with a hyphen, so C<tr/A-J/0-9/> |
1222 | does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. | |
54310121 | 1223 | For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the |
1224 | SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has | |
1225 | its own pair of quotes, which may or may not be bracketing quotes, | |
2c268ad5 | 1226 | e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>. |
a0d0e21e | 1227 | |
8ada0baa JH |
1228 | Note also that the whole range idea is rather unportable between |
1229 | character sets--and even within character sets they may cause results | |
1230 | you probably didn't expect. A sound principle is to use only ranges | |
1231 | that begin from and end at either alphabets of equal case (a-e, A-E), | |
1232 | or digits (0-4). Anything else is unsafe. If in doubt, spell out the | |
1233 | character sets in full. | |
1234 | ||
a0d0e21e LW |
1235 | Options: |
1236 | ||
1237 | c Complement the SEARCHLIST. | |
1238 | d Delete found but unreplaced characters. | |
1239 | s Squash duplicate replaced characters. | |
a0ed51b3 LW |
1240 | U Translate to/from UTF-8. |
1241 | C Translate to/from 8-bit char (octet). | |
a0d0e21e | 1242 | |
19799a22 GS |
1243 | If the C</c> modifier is specified, the SEARCHLIST character set |
1244 | is complemented. If the C</d> modifier is specified, any characters | |
1245 | specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. | |
1246 | (Note that this is slightly more flexible than the behavior of some | |
1247 | B<tr> programs, which delete anything they find in the SEARCHLIST, | |
1248 | period.) If the C</s> modifier is specified, sequences of characters | |
1249 | that were transliterated to the same character are squashed down | |
1250 | to a single instance of the character. | |
a0d0e21e LW |
1251 | |
1252 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted | |
1253 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter | |
1254 | than the SEARCHLIST, the final character is replicated till it is long | |
5a964f20 | 1255 | enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. |
a0d0e21e LW |
1256 | This latter is useful for counting characters in a class or for |
1257 | squashing character sequences in a class. | |
1258 | ||
a0ed51b3 LW |
1259 | The first C</U> or C</C> modifier applies to the left side of the translation. |
1260 | The second one applies to the right side. If present, these modifiers override | |
1261 | the current utf8 state. | |
1262 | ||
a0d0e21e LW |
1263 | Examples: |
1264 | ||
1265 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case | |
1266 | ||
1267 | $cnt = tr/*/*/; # count the stars in $_ | |
1268 | ||
1269 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky | |
1270 | ||
1271 | $cnt = tr/0-9//; # count the digits in $_ | |
1272 | ||
1273 | tr/a-zA-Z//s; # bookkeeper -> bokeper | |
1274 | ||
1275 | ($HOST = $host) =~ tr/a-z/A-Z/; | |
1276 | ||
1277 | tr/a-zA-Z/ /cs; # change non-alphas to single space | |
1278 | ||
1279 | tr [\200-\377] | |
1280 | [\000-\177]; # delete 8th bit | |
1281 | ||
19799a22 GS |
1282 | tr/\0-\xFF//CU; # change Latin-1 to Unicode |
1283 | tr/\0-\x{FF}//UC; # change Unicode to Latin-1 | |
a0ed51b3 | 1284 | |
19799a22 GS |
1285 | If multiple transliterations are given for a character, only the |
1286 | first one is used: | |
748a9306 LW |
1287 | |
1288 | tr/AAA/XYZ/ | |
1289 | ||
2c268ad5 | 1290 | will transliterate any A to X. |
748a9306 | 1291 | |
19799a22 | 1292 | Because the transliteration table is built at compile time, neither |
a0d0e21e | 1293 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote |
19799a22 GS |
1294 | interpolation. That means that if you want to use variables, you |
1295 | must use an eval(): | |
a0d0e21e LW |
1296 | |
1297 | eval "tr/$oldlist/$newlist/"; | |
1298 | die $@ if $@; | |
1299 | ||
1300 | eval "tr/$oldlist/$newlist/, 1" or die $@; | |
1301 | ||
1302 | =back | |
1303 | ||
75e14d17 IZ |
1304 | =head2 Gory details of parsing quoted constructs |
1305 | ||
19799a22 GS |
1306 | When presented with something that might have several different |
1307 | interpretations, Perl uses the B<DWIM> (that's "Do What I Mean") | |
1308 | principle to pick the most probable interpretation. This strategy | |
1309 | is so successful that Perl programmers often do not suspect the | |
1310 | ambivalence of what they write. But from time to time, Perl's | |
1311 | notions differ substantially from what the author honestly meant. | |
1312 | ||
1313 | This section hopes to clarify how Perl handles quoted constructs. | |
1314 | Although the most common reason to learn this is to unravel labyrinthine | |
1315 | regular expressions, because the initial steps of parsing are the | |
1316 | same for all quoting operators, they are all discussed together. | |
1317 | ||
1318 | The most important Perl parsing rule is the first one discussed | |
1319 | below: when processing a quoted construct, Perl first finds the end | |
1320 | of that construct, then interprets its contents. If you understand | |
1321 | this rule, you may skip the rest of this section on the first | |
1322 | reading. The other rules are likely to contradict the user's | |
1323 | expectations much less frequently than this first one. | |
1324 | ||
1325 | Some passes discussed below are performed concurrently, but because | |
1326 | their results are the same, we consider them individually. For different | |
1327 | quoting constructs, Perl performs different numbers of passes, from | |
1328 | one to five, but these passes are always performed in the same order. | |
75e14d17 IZ |
1329 | |
1330 | =over | |
1331 | ||
1332 | =item Finding the end | |
1333 | ||
19799a22 GS |
1334 | The first pass is finding the end of the quoted construct, whether |
1335 | it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF> | |
1336 | construct, a C</> that terminates a C<qq//> construct, a C<]> which | |
35f2feb0 GS |
1337 | terminates C<qq[]> construct, or a C<< > >> which terminates a |
1338 | fileglob started with C<< < >>. | |
75e14d17 | 1339 | |
19799a22 GS |
1340 | When searching for single-character non-pairing delimiters, such |
1341 | as C</>, combinations of C<\\> and C<\/> are skipped. However, | |
1342 | when searching for single-character pairing delimiter like C<[>, | |
1343 | combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested | |
1344 | C<[>, C<]> are skipped as well. When searching for multicharacter | |
1345 | delimiters, nothing is skipped. | |
75e14d17 | 1346 | |
19799a22 GS |
1347 | For constructs with three-part delimiters (C<s///>, C<y///>, and |
1348 | C<tr///>), the search is repeated once more. | |
75e14d17 | 1349 | |
19799a22 GS |
1350 | During this search no attention is paid to the semantics of the construct. |
1351 | Thus: | |
75e14d17 IZ |
1352 | |
1353 | "$hash{"$foo/$bar"}" | |
1354 | ||
2a94b7ce | 1355 | or: |
75e14d17 IZ |
1356 | |
1357 | m/ | |
2a94b7ce | 1358 | bar # NOT a comment, this slash / terminated m//! |
75e14d17 IZ |
1359 | /x |
1360 | ||
19799a22 GS |
1361 | do not form legal quoted expressions. The quoted part ends on the |
1362 | first C<"> and C</>, and the rest happens to be a syntax error. | |
1363 | Because the slash that terminated C<m//> was followed by a C<SPACE>, | |
1364 | the example above is not C<m//x>, but rather C<m//> with no C</x> | |
1365 | modifier. So the embedded C<#> is interpreted as a literal C<#>. | |
75e14d17 IZ |
1366 | |
1367 | =item Removal of backslashes before delimiters | |
1368 | ||
19799a22 GS |
1369 | During the second pass, text between the starting and ending |
1370 | delimiters is copied to a safe location, and the C<\> is removed | |
1371 | from combinations consisting of C<\> and delimiter--or delimiters, | |
1372 | meaning both starting and ending delimiters will should these differ. | |
1373 | This removal does not happen for multi-character delimiters. | |
1374 | Note that the combination C<\\> is left intact, just as it was. | |
75e14d17 | 1375 | |
19799a22 GS |
1376 | Starting from this step no information about the delimiters is |
1377 | used in parsing. | |
75e14d17 IZ |
1378 | |
1379 | =item Interpolation | |
1380 | ||
19799a22 GS |
1381 | The next step is interpolation in the text obtained, which is now |
1382 | delimiter-independent. There are four different cases. | |
75e14d17 IZ |
1383 | |
1384 | =over | |
1385 | ||
1386 | =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///> | |
1387 | ||
1388 | No interpolation is performed. | |
1389 | ||
1390 | =item C<''>, C<q//> | |
1391 | ||
1392 | The only interpolation is removal of C<\> from pairs C<\\>. | |
1393 | ||
35f2feb0 | 1394 | =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >> |
75e14d17 | 1395 | |
19799a22 GS |
1396 | C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are |
1397 | converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar"> | |
1398 | is converted to C<$foo . (quotemeta("baz" . $bar))> internally. | |
1399 | The other combinations are replaced with appropriate expansions. | |
2a94b7ce | 1400 | |
19799a22 GS |
1401 | Let it be stressed that I<whatever falls between C<\Q> and C<\E>> |
1402 | is interpolated in the usual way. Something like C<"\Q\\E"> has | |
1403 | no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the | |
1404 | result is the same as for C<"\\\\E">. As a general rule, backslashes | |
1405 | between C<\Q> and C<\E> may lead to counterintuitive results. So, | |
1406 | C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same | |
1407 | as C<"\\\t"> (since TAB is not alphanumeric). Note also that: | |
2a94b7ce IZ |
1408 | |
1409 | $str = '\t'; | |
1410 | return "\Q$str"; | |
1411 | ||
1412 | may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. | |
1413 | ||
19799a22 | 1414 | Interpolated scalars and arrays are converted internally to the C<join> and |
92d29cee | 1415 | C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes: |
75e14d17 | 1416 | |
19799a22 | 1417 | $foo . " XXX '" . (join $", @arr) . "'"; |
75e14d17 | 1418 | |
19799a22 | 1419 | All operations above are performed simultaneously, left to right. |
75e14d17 | 1420 | |
19799a22 GS |
1421 | Because the result of C<"\Q STRING \E"> has all metacharacters |
1422 | quoted, there is no way to insert a literal C<$> or C<@> inside a | |
1423 | C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became | |
1424 | C<"\\\$">; if not, it is interpreted as the start of an interpolated | |
1425 | scalar. | |
75e14d17 | 1426 | |
19799a22 GS |
1427 | Note also that the interpolation code needs to make a decision on |
1428 | where the interpolated scalar ends. For instance, whether | |
35f2feb0 | 1429 | C<< "a $b -> {c}" >> really means: |
75e14d17 IZ |
1430 | |
1431 | "a " . $b . " -> {c}"; | |
1432 | ||
2a94b7ce | 1433 | or: |
75e14d17 IZ |
1434 | |
1435 | "a " . $b -> {c}; | |
1436 | ||
19799a22 GS |
1437 | Most of the time, the longest possible text that does not include |
1438 | spaces between components and which contains matching braces or | |
1439 | brackets. because the outcome may be determined by voting based | |
1440 | on heuristic estimators, the result is not strictly predictable. | |
1441 | Fortunately, it's usually correct for ambiguous cases. | |
75e14d17 IZ |
1442 | |
1443 | =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, | |
1444 | ||
19799a22 GS |
1445 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation |
1446 | happens (almost) as with C<qq//> constructs, but the substitution | |
1447 | of C<\> followed by RE-special chars (including C<\>) is not | |
1448 | performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and | |
1449 | a C<#>-comment in a C<//x>-regular expression, no processing is | |
1450 | performed whatsoever. This is the first step at which the presence | |
1451 | of the C<//x> modifier is relevant. | |
1452 | ||
1453 | Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not | |
1454 | interpolated, and constructs C<$var[SOMETHING]> are voted (by several | |
1455 | different estimators) to be either an array element or C<$var> | |
1456 | followed by an RE alternative. This is where the notation | |
1457 | C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as | |
1458 | array element C<-9>, not as a regular expression from the variable | |
1459 | C<$arr> followed by a digit, which would be the interpretation of | |
1460 | C</$arr[0-9]/>. Since voting among different estimators may occur, | |
1461 | the result is not predictable. | |
1462 | ||
1463 | It is at this step that C<\1> is begrudgingly converted to C<$1> in | |
1464 | the replacement text of C<s///> to correct the incorrigible | |
1465 | I<sed> hackers who haven't picked up the saner idiom yet. A warning | |
9f1b1f2d GS |
1466 | is emitted if the C<use warnings> pragma or the B<-w> command-line flag |
1467 | (that is, the C<$^W> variable) was set. | |
19799a22 GS |
1468 | |
1469 | The lack of processing of C<\\> creates specific restrictions on | |
1470 | the post-processed text. If the delimiter is C</>, one cannot get | |
1471 | the combination C<\/> into the result of this step. C</> will | |
1472 | finish the regular expression, C<\/> will be stripped to C</> on | |
1473 | the previous step, and C<\\/> will be left as is. Because C</> is | |
1474 | equivalent to C<\/> inside a regular expression, this does not | |
1475 | matter unless the delimiter happens to be character special to the | |
1476 | RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an | |
1477 | alphanumeric char, as in: | |
2a94b7ce IZ |
1478 | |
1479 | m m ^ a \s* b mmx; | |
1480 | ||
19799a22 | 1481 | In the RE above, which is intentionally obfuscated for illustration, the |
2a94b7ce | 1482 | delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the |
19799a22 GS |
1483 | RE is the same as for C<m/ ^ a s* b /mx>). There's more than one |
1484 | reason you're encouraged to restrict your delimiters to non-alphanumeric, | |
1485 | non-whitespace choices. | |
75e14d17 IZ |
1486 | |
1487 | =back | |
1488 | ||
19799a22 | 1489 | This step is the last one for all constructs except regular expressions, |
75e14d17 IZ |
1490 | which are processed further. |
1491 | ||
1492 | =item Interpolation of regular expressions | |
1493 | ||
19799a22 GS |
1494 | Previous steps were performed during the compilation of Perl code, |
1495 | but this one happens at run time--although it may be optimized to | |
1496 | be calculated at compile time if appropriate. After preprocessing | |
1497 | described above, and possibly after evaluation if catenation, | |
1498 | joining, casing translation, or metaquoting are involved, the | |
1499 | resulting I<string> is passed to the RE engine for compilation. | |
1500 | ||
1501 | Whatever happens in the RE engine might be better discussed in L<perlre>, | |
1502 | but for the sake of continuity, we shall do so here. | |
1503 | ||
1504 | This is another step where the presence of the C<//x> modifier is | |
1505 | relevant. The RE engine scans the string from left to right and | |
1506 | converts it to a finite automaton. | |
1507 | ||
1508 | Backslashed characters are either replaced with corresponding | |
1509 | literal strings (as with C<\{>), or else they generate special nodes | |
1510 | in the finite automaton (as with C<\b>). Characters special to the | |
1511 | RE engine (such as C<|>) generate corresponding nodes or groups of | |
1512 | nodes. C<(?#...)> comments are ignored. All the rest is either | |
1513 | converted to literal strings to match, or else is ignored (as is | |
1514 | whitespace and C<#>-style comments if C<//x> is present). | |
1515 | ||
1516 | Parsing of the bracketed character class construct, C<[...]>, is | |
1517 | rather different than the rule used for the rest of the pattern. | |
1518 | The terminator of this construct is found using the same rules as | |
1519 | for finding the terminator of a C<{}>-delimited construct, the only | |
1520 | exception being that C<]> immediately following C<[> is treated as | |
1521 | though preceded by a backslash. Similarly, the terminator of | |
1522 | C<(?{...})> is found using the same rules as for finding the | |
1523 | terminator of a C<{}>-delimited construct. | |
1524 | ||
1525 | It is possible to inspect both the string given to RE engine and the | |
1526 | resulting finite automaton. See the arguments C<debug>/C<debugcolor> | |
1527 | in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line | |
4a4eefd0 | 1528 | switch documented in L<perlrun/"Command Switches">. |
75e14d17 IZ |
1529 | |
1530 | =item Optimization of regular expressions | |
1531 | ||
7522fed5 | 1532 | This step is listed for completeness only. Since it does not change |
75e14d17 | 1533 | semantics, details of this step are not documented and are subject |
19799a22 GS |
1534 | to change without notice. This step is performed over the finite |
1535 | automaton that was generated during the previous pass. | |
2a94b7ce | 1536 | |
19799a22 GS |
1537 | It is at this stage that C<split()> silently optimizes C</^/> to |
1538 | mean C</^/m>. | |
75e14d17 IZ |
1539 | |
1540 | =back | |
1541 | ||
a0d0e21e LW |
1542 | =head2 I/O Operators |
1543 | ||
54310121 | 1544 | There are several I/O operators you should know about. |
fbad3eb5 | 1545 | |
7b8d334a | 1546 | A string enclosed by backticks (grave accents) first undergoes |
19799a22 GS |
1547 | double-quote interpolation. It is then interpreted as an external |
1548 | command, and the output of that command is the value of the | |
1549 | pseudo-literal, j | |
1550 | string consisting of all output is returned. In list context, a | |
1551 | list of values is returned, one per line of output. (You can set | |
1552 | C<$/> to use a different line terminator.) The command is executed | |
a0d0e21e LW |
1553 | each time the pseudo-literal is evaluated. The status value of the |
1554 | command is returned in C<$?> (see L<perlvar> for the interpretation | |
1555 | of C<$?>). Unlike in B<csh>, no translation is done on the return | |
1556 | data--newlines remain newlines. Unlike in any of the shells, single | |
1557 | quotes do not hide variable names in the command from interpretation. | |
19799a22 GS |
1558 | To pass a literal dollar-sign through to the shell you need to hide |
1559 | it with a backslash. The generalized form of backticks is C<qx//>. | |
1560 | (Because backticks always undergo shell expansion as well, see | |
1561 | L<perlsec> for security concerns.) | |
1562 | ||
1563 | In scalar context, evaluating a filehandle in angle brackets yields | |
1564 | the next line from that file (the newline, if any, included), or | |
1565 | C<undef> at end-of-file or on error. When C<$/> is set to C<undef> | |
1566 | (sometimes known as file-slurp mode) and the file is empty, it | |
1567 | returns C<''> the first time, followed by C<undef> subsequently. | |
1568 | ||
1569 | Ordinarily you must assign the returned value to a variable, but | |
1570 | there is one situation where an automatic assignment happens. If | |
1571 | and only if the input symbol is the only thing inside the conditional | |
1572 | of a C<while> statement (even if disguised as a C<for(;;)> loop), | |
1573 | the value is automatically assigned to the global variable $_, | |
1574 | destroying whatever was there previously. (This may seem like an | |
1575 | odd thing to you, but you'll use the construct in almost every Perl | |
1576 | script you write.) The $_ variables is not implicitly localized. | |
1577 | You'll have to put a C<local $_;> before the loop if you want that | |
1578 | to happen. | |
1579 | ||
1580 | The following lines are equivalent: | |
a0d0e21e | 1581 | |
748a9306 | 1582 | while (defined($_ = <STDIN>)) { print; } |
7b8d334a | 1583 | while ($_ = <STDIN>) { print; } |
a0d0e21e LW |
1584 | while (<STDIN>) { print; } |
1585 | for (;<STDIN>;) { print; } | |
748a9306 | 1586 | print while defined($_ = <STDIN>); |
7b8d334a | 1587 | print while ($_ = <STDIN>); |
a0d0e21e LW |
1588 | print while <STDIN>; |
1589 | ||
19799a22 | 1590 | This also behaves similarly, but avoids $_ : |
7b8d334a GS |
1591 | |
1592 | while (my $line = <STDIN>) { print $line } | |
1593 | ||
19799a22 GS |
1594 | In these loop constructs, the assigned value (whether assignment |
1595 | is automatic or explicit) is then tested to see whether it is | |
1596 | defined. The defined test avoids problems where line has a string | |
1597 | value that would be treated as false by Perl, for example a "" or | |
1598 | a "0" with no trailing newline. If you really mean for such values | |
1599 | to terminate the loop, they should be tested for explicitly: | |
7b8d334a GS |
1600 | |
1601 | while (($_ = <STDIN>) ne '0') { ... } | |
1602 | while (<STDIN>) { last unless $_; ... } | |
1603 | ||
35f2feb0 | 1604 | In other boolean contexts, C<< <I<filehandle>> >> without an |
9f1b1f2d GS |
1605 | explicit C<defined> test or comparison elicit a warning if the |
1606 | C<use warnings> pragma or the B<-w> | |
19799a22 | 1607 | command-line switch (the C<$^W> variable) is in effect. |
7b8d334a | 1608 | |
5f05dabc | 1609 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
19799a22 GS |
1610 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except |
1611 | in packages, where they would be interpreted as local identifiers | |
1612 | rather than global.) Additional filehandles may be created with | |
1613 | the open() function, amongst others. See L<perlopentut> and | |
1614 | L<perlfunc/open> for details on this. | |
a0d0e21e | 1615 | |
35f2feb0 | 1616 | If a <FILEHANDLE> is used in a context that is looking for |
19799a22 GS |
1617 | a list, a list comprising all input lines is returned, one line per |
1618 | list element. It's easy to grow to a rather large data space this | |
1619 | way, so use with care. | |
a0d0e21e | 1620 | |
35f2feb0 | 1621 | <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>. |
19799a22 | 1622 | See L<perlfunc/readline>. |
fbad3eb5 | 1623 | |
35f2feb0 GS |
1624 | The null filehandle <> is special: it can be used to emulate the |
1625 | behavior of B<sed> and B<awk>. Input from <> comes either from | |
a0d0e21e | 1626 | standard input, or from each file listed on the command line. Here's |
35f2feb0 | 1627 | how it works: the first time <> is evaluated, the @ARGV array is |
5a964f20 | 1628 | checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened |
a0d0e21e LW |
1629 | gives you standard input. The @ARGV array is then processed as a list |
1630 | of filenames. The loop | |
1631 | ||
1632 | while (<>) { | |
1633 | ... # code for each line | |
1634 | } | |
1635 | ||
1636 | is equivalent to the following Perl-like pseudo code: | |
1637 | ||
3e3baf6d | 1638 | unshift(@ARGV, '-') unless @ARGV; |
a0d0e21e LW |
1639 | while ($ARGV = shift) { |
1640 | open(ARGV, $ARGV); | |
1641 | while (<ARGV>) { | |
1642 | ... # code for each line | |
1643 | } | |
1644 | } | |
1645 | ||
19799a22 GS |
1646 | except that it isn't so cumbersome to say, and will actually work. |
1647 | It really does shift the @ARGV array and put the current filename | |
1648 | into the $ARGV variable. It also uses filehandle I<ARGV> | |
35f2feb0 | 1649 | internally--<> is just a synonym for <ARGV>, which |
19799a22 | 1650 | is magical. (The pseudo code above doesn't work because it treats |
35f2feb0 | 1651 | <ARGV> as non-magical.) |
a0d0e21e | 1652 | |
35f2feb0 | 1653 | You can modify @ARGV before the first <> as long as the array ends up |
a0d0e21e | 1654 | containing the list of filenames you really want. Line numbers (C<$.>) |
19799a22 GS |
1655 | continue as though the input were one big happy file. See the example |
1656 | in L<perlfunc/eof> for how to reset line numbers on each file. | |
5a964f20 TC |
1657 | |
1658 | If you want to set @ARGV to your own list of files, go right ahead. | |
1659 | This sets @ARGV to all plain text files if no @ARGV was given: | |
1660 | ||
1661 | @ARGV = grep { -f && -T } glob('*') unless @ARGV; | |
a0d0e21e | 1662 | |
5a964f20 TC |
1663 | You can even set them to pipe commands. For example, this automatically |
1664 | filters compressed arguments through B<gzip>: | |
1665 | ||
1666 | @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; | |
1667 | ||
1668 | If you want to pass switches into your script, you can use one of the | |
a0d0e21e LW |
1669 | Getopts modules or put a loop on the front like this: |
1670 | ||
1671 | while ($_ = $ARGV[0], /^-/) { | |
1672 | shift; | |
1673 | last if /^--$/; | |
1674 | if (/^-D(.*)/) { $debug = $1 } | |
1675 | if (/^-v/) { $verbose++ } | |
5a964f20 | 1676 | # ... # other switches |
a0d0e21e | 1677 | } |
5a964f20 | 1678 | |
a0d0e21e | 1679 | while (<>) { |
5a964f20 | 1680 | # ... # code for each line |
a0d0e21e LW |
1681 | } |
1682 | ||
35f2feb0 | 1683 | The <> symbol will return C<undef> for end-of-file only once. |
19799a22 GS |
1684 | If you call it again after this, it will assume you are processing another |
1685 | @ARGV list, and if you haven't set @ARGV, will read input from STDIN. | |
a0d0e21e | 1686 | |
19799a22 | 1687 | If angle brackets contain is a simple scalar variable (e.g., |
35f2feb0 | 1688 | <$foo>), then that variable contains the name of the |
19799a22 GS |
1689 | filehandle to input from, or its typeglob, or a reference to the |
1690 | same. For example: | |
cb1a09d0 AD |
1691 | |
1692 | $fh = \*STDIN; | |
1693 | $line = <$fh>; | |
a0d0e21e | 1694 | |
5a964f20 TC |
1695 | If what's within the angle brackets is neither a filehandle nor a simple |
1696 | scalar variable containing a filehandle name, typeglob, or typeglob | |
1697 | reference, it is interpreted as a filename pattern to be globbed, and | |
1698 | either a list of filenames or the next filename in the list is returned, | |
19799a22 | 1699 | depending on context. This distinction is determined on syntactic |
35f2feb0 GS |
1700 | grounds alone. That means C<< <$x> >> is always a readline() from |
1701 | an indirect handle, but C<< <$hash{key}> >> is always a glob(). | |
5a964f20 TC |
1702 | That's because $x is a simple scalar variable, but C<$hash{key}> is |
1703 | not--it's a hash element. | |
1704 | ||
1705 | One level of double-quote interpretation is done first, but you can't | |
35f2feb0 | 1706 | say C<< <$foo> >> because that's an indirect filehandle as explained |
5a964f20 TC |
1707 | in the previous paragraph. (In older versions of Perl, programmers |
1708 | would insert curly brackets to force interpretation as a filename glob: | |
35f2feb0 | 1709 | C<< <${foo}> >>. These days, it's considered cleaner to call the |
5a964f20 | 1710 | internal function directly as C<glob($foo)>, which is probably the right |
19799a22 | 1711 | way to have done it in the first place.) For example: |
a0d0e21e LW |
1712 | |
1713 | while (<*.c>) { | |
1714 | chmod 0644, $_; | |
1715 | } | |
1716 | ||
3a4b19e4 | 1717 | is roughly equivalent to: |
a0d0e21e LW |
1718 | |
1719 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); | |
1720 | while (<FOO>) { | |
1721 | chop; | |
1722 | chmod 0644, $_; | |
1723 | } | |
1724 | ||
3a4b19e4 GS |
1725 | except that the globbing is actually done internally using the standard |
1726 | C<File::Glob> extension. Of course, the shortest way to do the above is: | |
a0d0e21e LW |
1727 | |
1728 | chmod 0644, <*.c>; | |
1729 | ||
19799a22 GS |
1730 | A (file)glob evaluates its (embedded) argument only when it is |
1731 | starting a new list. All values must be read before it will start | |
1732 | over. In list context, this isn't important because you automatically | |
1733 | get them all anyway. However, in scalar context the operator returns | |
1734 | the next value each time it's called, or C | |
1735 | run out. As with filehandle reads, an automatic C<defined> is | |
1736 | generated when the glob occurs in the test part of a C<while>, | |
1737 | because legal glob returns (e.g. a file called F<0>) would otherwise | |
1738 | terminate the loop. Again, C<undef> is returned only once. So if | |
1739 | you're expecting a single value from a glob, it is much better to | |
1740 | say | |
4633a7c4 LW |
1741 | |
1742 | ($file) = <blurch*>; | |
1743 | ||
1744 | than | |
1745 | ||
1746 | $file = <blurch*>; | |
1747 | ||
1748 | because the latter will alternate between returning a filename and | |
19799a22 | 1749 | returning false. |
4633a7c4 LW |
1750 | |
1751 | It you're trying to do variable interpolation, it's definitely better | |
1752 | to use the glob() function, because the older notation can cause people | |
e37d713d | 1753 | to become confused with the indirect filehandle notation. |
4633a7c4 LW |
1754 | |
1755 | @files = glob("$dir/*.[ch]"); | |
1756 | @files = glob($files[$i]); | |
1757 | ||
a0d0e21e LW |
1758 | =head2 Constant Folding |
1759 | ||
1760 | Like C, Perl does a certain amount of expression evaluation at | |
19799a22 | 1761 | compile time whenever it determines that all arguments to an |
a0d0e21e LW |
1762 | operator are static and have no side effects. In particular, string |
1763 | concatenation happens at compile time between literals that don't do | |
19799a22 | 1764 | variable substitution. Backslash interpolation also happens at |
a0d0e21e LW |
1765 | compile time. You can say |
1766 | ||
1767 | 'Now is the time for all' . "\n" . | |
1768 | 'good men to come to.' | |
1769 | ||
54310121 | 1770 | and this all reduces to one string internally. Likewise, if |
a0d0e21e LW |
1771 | you say |
1772 | ||
1773 | foreach $file (@filenames) { | |
5a964f20 | 1774 | if (-s $file > 5 + 100 * 2**16) { } |
54310121 | 1775 | } |
a0d0e21e | 1776 | |
19799a22 GS |
1777 | the compiler will precompute the number which that expression |
1778 | represents so that the interpreter won't have to. | |
a0d0e21e | 1779 | |
2c268ad5 TP |
1780 | =head2 Bitwise String Operators |
1781 | ||
1782 | Bitstrings of any size may be manipulated by the bitwise operators | |
1783 | (C<~ | & ^>). | |
1784 | ||
19799a22 GS |
1785 | If the operands to a binary bitwise op are strings of different |
1786 | sizes, B<|> and B<^> ops act as though the shorter operand had | |
1787 | additional zero bits on the right, while the B<&> op acts as though | |
1788 | the longer operand were truncated to the length of the shorter. | |
1789 | The granularity for such extension or truncation is one or more | |
1790 | bytes. | |
2c268ad5 TP |
1791 | |
1792 | # ASCII-based examples | |
1793 | print "j p \n" ^ " a h"; # prints "JAPH\n" | |
1794 | print "JA" | " ph\n"; # prints "japh\n" | |
1795 | print "japh\nJunk" & '_____'; # prints "JAPH\n"; | |
1796 | print 'p N$' ^ " E<H\n"; # prints "Perl\n"; | |
1797 | ||
19799a22 | 1798 | If you are intending to manipulate bitstrings, be certain that |
2c268ad5 | 1799 | you're supplying bitstrings: If an operand is a number, that will imply |
19799a22 | 1800 | a B<numeric> bitwise operation. You may explicitly show which type of |
2c268ad5 TP |
1801 | operation you intend by using C<""> or C<0+>, as in the examples below. |
1802 | ||
1803 | $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF) | |
1804 | $foo = '150' | 105 ; # yields 255 | |
1805 | $foo = 150 | '105'; # yields 255 | |
1806 | $foo = '150' | '105'; # yields string '155' (under ASCII) | |
1807 | ||
1808 | $baz = 0+$foo & 0+$bar; # both ops explicitly numeric | |
1809 | $biz = "$foo" ^ "$bar"; # both ops explicitly stringy | |
a0d0e21e | 1810 | |
1ae175c8 GS |
1811 | See L<perlfunc/vec> for information on how to manipulate individual bits |
1812 | in a bit vector. | |
1813 | ||
55497cff | 1814 | =head2 Integer Arithmetic |
a0d0e21e | 1815 | |
19799a22 | 1816 | By default, Perl assumes that it must do most of its arithmetic in |
a0d0e21e LW |
1817 | floating point. But by saying |
1818 | ||
1819 | use integer; | |
1820 | ||
1821 | you may tell the compiler that it's okay to use integer operations | |
19799a22 GS |
1822 | (if it feels like it) from here to the end of the enclosing BLOCK. |
1823 | An inner BLOCK may countermand this by saying | |
a0d0e21e LW |
1824 | |
1825 | no integer; | |
1826 | ||
19799a22 GS |
1827 | which lasts until the end of that BLOCK. Note that this doesn't |
1828 | mean everything is only an integer, merely that Perl may use integer | |
1829 | operations if it is so inclined. For example, even under C<use | |
1830 | integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731> | |
1831 | or so. | |
1832 | ||
1833 | Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", | |
1834 | and ">>") always produce integral results. (But see also L<Bitwise | |
1835 | String Operators>.) However, C<use integer> still has meaning for | |
1836 | them. By default, their results are interpreted as unsigned integers, but | |
1837 | if C<use integer> is in effect, their results are interpreted | |
1838 | as signed integers. For example, C<~0> usually evaluates to a large | |
1839 | integral value. However, C<use integer; ~0> is C<-1> on twos-complement | |
1840 | machines. | |
68dc0745 | 1841 | |
1842 | =head2 Floating-point Arithmetic | |
1843 | ||
1844 | While C<use integer> provides integer-only arithmetic, there is no | |
19799a22 GS |
1845 | analogous mechanism to provide automatic rounding or truncation to a |
1846 | certain number of decimal places. For rounding to a certain number | |
1847 | of digits, sprintf() or printf() is usually the easiest route. | |
1848 | See L<perlfaq4>. | |
68dc0745 | 1849 | |
5a964f20 TC |
1850 | Floating-point numbers are only approximations to what a mathematician |
1851 | would call real numbers. There are infinitely more reals than floats, | |
1852 | so some corners must be cut. For example: | |
1853 | ||
1854 | printf "%.20g\n", 123456789123456789; | |
1855 | # produces 123456789123456784 | |
1856 | ||
1857 | Testing for exact equality of floating-point equality or inequality is | |
1858 | not a good idea. Here's a (relatively expensive) work-around to compare | |
1859 | whether two floating-point numbers are equal to a particular number of | |
1860 | decimal places. See Knuth, volume II, for a more robust treatment of | |
1861 | this topic. | |
1862 | ||
1863 | sub fp_equal { | |
1864 | my ($X, $Y, $POINTS) = @_; | |
1865 | my ($tX, $tY); | |
1866 | $tX = sprintf("%.${POINTS}g", $X); | |
1867 | $tY = sprintf("%.${POINTS}g", $Y); | |
1868 | return $tX eq $tY; | |
1869 | } | |
1870 | ||
68dc0745 | 1871 | The POSIX module (part of the standard perl distribution) implements |
19799a22 GS |
1872 | ceil(), floor(), and other mathematical and trigonometric functions. |
1873 | The Math::Complex module (part of the standard perl distribution) | |
1874 | defines mathematical functions that work on both the reals and the | |
1875 | imaginary numbers. Math::Complex not as efficient as POSIX, but | |
68dc0745 | 1876 | POSIX can't work with complex numbers. |
1877 | ||
1878 | Rounding in financial applications can have serious implications, and | |
1879 | the rounding method used should be specified precisely. In these | |
1880 | cases, it probably pays not to trust whichever system rounding is | |
1881 | being used by Perl, but to instead implement the rounding function you | |
1882 | need yourself. | |
5a964f20 TC |
1883 | |
1884 | =head2 Bigger Numbers | |
1885 | ||
1886 | The standard Math::BigInt and Math::BigFloat modules provide | |
19799a22 GS |
1887 | variable-precision arithmetic and overloaded operators, although |
1888 | they're currently pretty slow. At the cost of some space and | |
1889 | considerable speed, they avoid the normal pitfalls associated with | |
1890 | limited-precision representations. | |
5a964f20 TC |
1891 | |
1892 | use Math::BigInt; | |
1893 | $x = Math::BigInt->new('123456789123456789'); | |
1894 | print $x * $x; | |
1895 | ||
1896 | # prints +15241578780673678515622620750190521 | |
19799a22 GS |
1897 | |
1898 | The non-standard modules SSLeay::BN and Math::Pari provide | |
1899 | equivalent functionality (and much more) with a substantial | |
1900 | performance savings. | |
16070b82 GS |
1901 | |
1902 | =cut |