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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 |
9c0670e1 | 122 | C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each |
a0d0e21e LW |
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). |
f3798619 | 199 | Note than when C<use integer> is in scope, "%" gives 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 | 235 | |
b16cf6df JH |
236 | Note that both "<<" and ">>" in Perl are implemented directly using |
237 | "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is | |
238 | in force then signed C integers are used, else unsigned C integers are | |
239 | used. Either way, the implementation isn't going to generate results | |
240 | larger than the size of the integer type Perl was built with (32 bits | |
241 | or 64 bits). | |
242 | ||
243 | The result of overflowing the range of the integers is undefined | |
244 | because it is undefined also in C. In other words, using 32-bit | |
245 | integers, C<< 1 << 32 >> is undefined. Shifting by a negative number | |
246 | of bits is also undefined. | |
247 | ||
a0d0e21e LW |
248 | =head2 Named Unary Operators |
249 | ||
250 | The various named unary operators are treated as functions with one | |
251 | argument, with optional parentheses. These include the filetest | |
252 | operators, like C<-f>, C<-M>, etc. See L<perlfunc>. | |
253 | ||
254 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
255 | is followed by a left parenthesis as the next token, the operator and | |
256 | arguments within parentheses are taken to be of highest precedence, | |
3981b0eb JA |
257 | just like a normal function call. For example, |
258 | because named unary operators are higher precedence than ||: | |
a0d0e21e LW |
259 | |
260 | chdir $foo || die; # (chdir $foo) || die | |
261 | chdir($foo) || die; # (chdir $foo) || die | |
262 | chdir ($foo) || die; # (chdir $foo) || die | |
263 | chdir +($foo) || die; # (chdir $foo) || die | |
264 | ||
3981b0eb | 265 | but, because * is higher precedence than named operators: |
a0d0e21e LW |
266 | |
267 | chdir $foo * 20; # chdir ($foo * 20) | |
268 | chdir($foo) * 20; # (chdir $foo) * 20 | |
269 | chdir ($foo) * 20; # (chdir $foo) * 20 | |
270 | chdir +($foo) * 20; # chdir ($foo * 20) | |
271 | ||
272 | rand 10 * 20; # rand (10 * 20) | |
273 | rand(10) * 20; # (rand 10) * 20 | |
274 | rand (10) * 20; # (rand 10) * 20 | |
275 | rand +(10) * 20; # rand (10 * 20) | |
276 | ||
5ba421f6 | 277 | See also L<"Terms and List Operators (Leftward)">. |
a0d0e21e LW |
278 | |
279 | =head2 Relational Operators | |
280 | ||
35f2feb0 | 281 | Binary "<" returns true if the left argument is numerically less than |
a0d0e21e LW |
282 | the right argument. |
283 | ||
35f2feb0 | 284 | Binary ">" returns true if the left argument is numerically greater |
a0d0e21e LW |
285 | than the right argument. |
286 | ||
35f2feb0 | 287 | Binary "<=" returns true if the left argument is numerically less than |
a0d0e21e LW |
288 | or equal to the right argument. |
289 | ||
35f2feb0 | 290 | Binary ">=" returns true if the left argument is numerically greater |
a0d0e21e LW |
291 | than or equal to the right argument. |
292 | ||
293 | Binary "lt" returns true if the left argument is stringwise less than | |
294 | the right argument. | |
295 | ||
296 | Binary "gt" returns true if the left argument is stringwise greater | |
297 | than the right argument. | |
298 | ||
299 | Binary "le" returns true if the left argument is stringwise less than | |
300 | or equal to the right argument. | |
301 | ||
302 | Binary "ge" returns true if the left argument is stringwise greater | |
303 | than or equal to the right argument. | |
304 | ||
305 | =head2 Equality Operators | |
306 | ||
307 | Binary "==" returns true if the left argument is numerically equal to | |
308 | the right argument. | |
309 | ||
310 | Binary "!=" returns true if the left argument is numerically not equal | |
311 | to the right argument. | |
312 | ||
35f2feb0 | 313 | Binary "<=>" returns -1, 0, or 1 depending on whether the left |
6ee5d4e7 | 314 | argument is numerically less than, equal to, or greater than the right |
d4ad863d | 315 | argument. If your platform supports NaNs (not-a-numbers) as numeric |
7d3a9d88 NC |
316 | values, using them with "<=>" returns undef. NaN is not "<", "==", ">", |
317 | "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN | |
318 | returns true, as does NaN != anything else. If your platform doesn't | |
319 | support NaNs then NaN is just a string with numeric value 0. | |
320 | ||
321 | perl -le '$a = NaN; print "No NaN support here" if $a == $a' | |
322 | perl -le '$a = NaN; print "NaN support here" if $a != $a' | |
a0d0e21e LW |
323 | |
324 | Binary "eq" returns true if the left argument is stringwise equal to | |
325 | the right argument. | |
326 | ||
327 | Binary "ne" returns true if the left argument is stringwise not equal | |
328 | to the right argument. | |
329 | ||
d4ad863d JH |
330 | Binary "cmp" returns -1, 0, or 1 depending on whether the left |
331 | argument is stringwise less than, equal to, or greater than the right | |
332 | argument. | |
a0d0e21e | 333 | |
a034a98d DD |
334 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
335 | by the current locale if C<use locale> is in effect. See L<perllocale>. | |
336 | ||
a0d0e21e LW |
337 | =head2 Bitwise And |
338 | ||
339 | Binary "&" returns its operators ANDed together bit by bit. | |
2c268ad5 | 340 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e LW |
341 | |
342 | =head2 Bitwise Or and Exclusive Or | |
343 | ||
344 | Binary "|" returns its operators ORed together bit by bit. | |
2c268ad5 | 345 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e LW |
346 | |
347 | Binary "^" returns its operators XORed together bit by bit. | |
2c268ad5 | 348 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e LW |
349 | |
350 | =head2 C-style Logical And | |
351 | ||
352 | Binary "&&" performs a short-circuit logical AND operation. That is, | |
353 | if the left operand is false, the right operand is not even evaluated. | |
354 | Scalar or list context propagates down to the right operand if it | |
355 | is evaluated. | |
356 | ||
357 | =head2 C-style Logical Or | |
358 | ||
359 | Binary "||" performs a short-circuit logical OR operation. That is, | |
360 | if the left operand is true, the right operand is not even evaluated. | |
361 | Scalar or list context propagates down to the right operand if it | |
362 | is evaluated. | |
363 | ||
364 | The C<||> and C<&&> operators differ from C's in that, rather than returning | |
365 | 0 or 1, they return the last value evaluated. Thus, a reasonably portable | |
366 | way to find out the home directory (assuming it's not "0") might be: | |
367 | ||
368 | $home = $ENV{'HOME'} || $ENV{'LOGDIR'} || | |
369 | (getpwuid($<))[7] || die "You're homeless!\n"; | |
370 | ||
5a964f20 TC |
371 | In particular, this means that you shouldn't use this |
372 | for selecting between two aggregates for assignment: | |
373 | ||
374 | @a = @b || @c; # this is wrong | |
375 | @a = scalar(@b) || @c; # really meant this | |
376 | @a = @b ? @b : @c; # this works fine, though | |
377 | ||
378 | As more readable alternatives to C<&&> and C<||> when used for | |
379 | control flow, Perl provides C<and> and C<or> operators (see below). | |
380 | The short-circuit behavior is identical. The precedence of "and" and | |
381 | "or" is much lower, however, so that you can safely use them after a | |
382 | list operator without the need for parentheses: | |
a0d0e21e LW |
383 | |
384 | unlink "alpha", "beta", "gamma" | |
385 | or gripe(), next LINE; | |
386 | ||
387 | With the C-style operators that would have been written like this: | |
388 | ||
389 | unlink("alpha", "beta", "gamma") | |
390 | || (gripe(), next LINE); | |
391 | ||
eeb6a2c9 | 392 | Using "or" for assignment is unlikely to do what you want; see below. |
5a964f20 TC |
393 | |
394 | =head2 Range Operators | |
a0d0e21e LW |
395 | |
396 | Binary ".." is the range operator, which is really two different | |
5a964f20 | 397 | operators depending on the context. In list context, it returns an |
2cdbc966 JD |
398 | array of values counting (up by ones) from the left value to the right |
399 | value. If the left value is greater than the right value then it | |
400 | returns the empty array. The range operator is useful for writing | |
401 | C<foreach (1..10)> loops and for doing slice operations on arrays. In | |
402 | the current implementation, no temporary array is created when the | |
403 | range operator is used as the expression in C<foreach> loops, but older | |
404 | versions of Perl might burn a lot of memory when you write something | |
405 | like this: | |
a0d0e21e LW |
406 | |
407 | for (1 .. 1_000_000) { | |
408 | # code | |
54310121 | 409 | } |
a0d0e21e | 410 | |
5a964f20 | 411 | In scalar context, ".." returns a boolean value. The operator is |
a0d0e21e LW |
412 | bistable, like a flip-flop, and emulates the line-range (comma) operator |
413 | of B<sed>, B<awk>, and various editors. Each ".." operator maintains its | |
414 | own boolean state. It is false as long as its left operand is false. | |
415 | Once the left operand is true, the range operator stays true until the | |
416 | right operand is true, I<AFTER> which the range operator becomes false | |
19799a22 | 417 | again. It doesn't become false till the next time the range operator is |
a0d0e21e LW |
418 | evaluated. It can test the right operand and become false on the same |
419 | evaluation it became true (as in B<awk>), but it still returns true once. | |
19799a22 GS |
420 | If you don't want it to test the right operand till the next |
421 | evaluation, as in B<sed>, just use three dots ("...") instead of | |
422 | two. In all other regards, "..." behaves just like ".." does. | |
423 | ||
424 | The right operand is not evaluated while the operator is in the | |
425 | "false" state, and the left operand is not evaluated while the | |
426 | operator is in the "true" state. The precedence is a little lower | |
427 | than || and &&. The value returned is either the empty string for | |
428 | false, or a sequence number (beginning with 1) for true. The | |
429 | sequence number is reset for each range encountered. The final | |
430 | sequence number in a range has the string "E0" appended to it, which | |
431 | doesn't affect its numeric value, but gives you something to search | |
432 | for if you want to exclude the endpoint. You can exclude the | |
433 | beginning point by waiting for the sequence number to be greater | |
434 | than 1. If either operand of scalar ".." is a constant expression, | |
435 | that operand is implicitly compared to the C<$.> variable, the | |
436 | current line number. Examples: | |
a0d0e21e LW |
437 | |
438 | As a scalar operator: | |
439 | ||
440 | if (101 .. 200) { print; } # print 2nd hundred lines | |
441 | next line if (1 .. /^$/); # skip header lines | |
442 | s/^/> / if (/^$/ .. eof()); # quote body | |
443 | ||
5a964f20 TC |
444 | # parse mail messages |
445 | while (<>) { | |
446 | $in_header = 1 .. /^$/; | |
447 | $in_body = /^$/ .. eof(); | |
448 | # do something based on those | |
449 | } continue { | |
450 | close ARGV if eof; # reset $. each file | |
451 | } | |
452 | ||
a0d0e21e LW |
453 | As a list operator: |
454 | ||
455 | for (101 .. 200) { print; } # print $_ 100 times | |
3e3baf6d | 456 | @foo = @foo[0 .. $#foo]; # an expensive no-op |
a0d0e21e LW |
457 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items |
458 | ||
5a964f20 | 459 | The range operator (in list context) makes use of the magical |
5f05dabc | 460 | auto-increment algorithm if the operands are strings. You |
a0d0e21e LW |
461 | can say |
462 | ||
463 | @alphabet = ('A' .. 'Z'); | |
464 | ||
19799a22 | 465 | to get all normal letters of the alphabet, or |
a0d0e21e LW |
466 | |
467 | $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; | |
468 | ||
469 | to get a hexadecimal digit, or | |
470 | ||
471 | @z2 = ('01' .. '31'); print $z2[$mday]; | |
472 | ||
473 | to get dates with leading zeros. If the final value specified is not | |
474 | in the sequence that the magical increment would produce, the sequence | |
475 | goes until the next value would be longer than the final value | |
476 | specified. | |
477 | ||
478 | =head2 Conditional Operator | |
479 | ||
480 | Ternary "?:" is the conditional operator, just as in C. It works much | |
481 | like an if-then-else. If the argument before the ? is true, the | |
482 | argument before the : is returned, otherwise the argument after the : | |
cb1a09d0 AD |
483 | is returned. For example: |
484 | ||
54310121 | 485 | printf "I have %d dog%s.\n", $n, |
cb1a09d0 AD |
486 | ($n == 1) ? '' : "s"; |
487 | ||
488 | Scalar or list context propagates downward into the 2nd | |
54310121 | 489 | or 3rd argument, whichever is selected. |
cb1a09d0 AD |
490 | |
491 | $a = $ok ? $b : $c; # get a scalar | |
492 | @a = $ok ? @b : @c; # get an array | |
493 | $a = $ok ? @b : @c; # oops, that's just a count! | |
494 | ||
495 | The operator may be assigned to if both the 2nd and 3rd arguments are | |
496 | legal lvalues (meaning that you can assign to them): | |
a0d0e21e LW |
497 | |
498 | ($a_or_b ? $a : $b) = $c; | |
499 | ||
5a964f20 TC |
500 | Because this operator produces an assignable result, using assignments |
501 | without parentheses will get you in trouble. For example, this: | |
502 | ||
503 | $a % 2 ? $a += 10 : $a += 2 | |
504 | ||
505 | Really means this: | |
506 | ||
507 | (($a % 2) ? ($a += 10) : $a) += 2 | |
508 | ||
509 | Rather than this: | |
510 | ||
511 | ($a % 2) ? ($a += 10) : ($a += 2) | |
512 | ||
19799a22 GS |
513 | That should probably be written more simply as: |
514 | ||
515 | $a += ($a % 2) ? 10 : 2; | |
516 | ||
4633a7c4 | 517 | =head2 Assignment Operators |
a0d0e21e LW |
518 | |
519 | "=" is the ordinary assignment operator. | |
520 | ||
521 | Assignment operators work as in C. That is, | |
522 | ||
523 | $a += 2; | |
524 | ||
525 | is equivalent to | |
526 | ||
527 | $a = $a + 2; | |
528 | ||
529 | although without duplicating any side effects that dereferencing the lvalue | |
54310121 | 530 | might trigger, such as from tie(). Other assignment operators work similarly. |
531 | The following are recognized: | |
a0d0e21e LW |
532 | |
533 | **= += *= &= <<= &&= | |
534 | -= /= |= >>= ||= | |
535 | .= %= ^= | |
536 | x= | |
537 | ||
19799a22 | 538 | Although these are grouped by family, they all have the precedence |
a0d0e21e LW |
539 | of assignment. |
540 | ||
b350dd2f GS |
541 | Unlike in C, the scalar assignment operator produces a valid lvalue. |
542 | Modifying an assignment is equivalent to doing the assignment and | |
543 | then modifying the variable that was assigned to. This is useful | |
544 | for modifying a copy of something, like this: | |
a0d0e21e LW |
545 | |
546 | ($tmp = $global) =~ tr [A-Z] [a-z]; | |
547 | ||
548 | Likewise, | |
549 | ||
550 | ($a += 2) *= 3; | |
551 | ||
552 | is equivalent to | |
553 | ||
554 | $a += 2; | |
555 | $a *= 3; | |
556 | ||
b350dd2f GS |
557 | Similarly, a list assignment in list context produces the list of |
558 | lvalues assigned to, and a list assignment in scalar context returns | |
559 | the number of elements produced by the expression on the right hand | |
560 | side of the assignment. | |
561 | ||
748a9306 | 562 | =head2 Comma Operator |
a0d0e21e | 563 | |
5a964f20 | 564 | Binary "," is the comma operator. In scalar context it evaluates |
a0d0e21e LW |
565 | its left argument, throws that value away, then evaluates its right |
566 | argument and returns that value. This is just like C's comma operator. | |
567 | ||
5a964f20 | 568 | In list context, it's just the list argument separator, and inserts |
a0d0e21e LW |
569 | both its arguments into the list. |
570 | ||
35f2feb0 | 571 | The => digraph is mostly just a synonym for the comma operator. It's useful for |
cb1a09d0 | 572 | documenting arguments that come in pairs. As of release 5.001, it also forces |
4633a7c4 | 573 | any word to the left of it to be interpreted as a string. |
748a9306 | 574 | |
a0d0e21e LW |
575 | =head2 List Operators (Rightward) |
576 | ||
577 | On the right side of a list operator, it has very low precedence, | |
578 | such that it controls all comma-separated expressions found there. | |
579 | The only operators with lower precedence are the logical operators | |
580 | "and", "or", and "not", which may be used to evaluate calls to list | |
581 | operators without the need for extra parentheses: | |
582 | ||
583 | open HANDLE, "filename" | |
584 | or die "Can't open: $!\n"; | |
585 | ||
5ba421f6 | 586 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
a0d0e21e LW |
587 | |
588 | =head2 Logical Not | |
589 | ||
590 | Unary "not" returns the logical negation of the expression to its right. | |
591 | It's the equivalent of "!" except for the very low precedence. | |
592 | ||
593 | =head2 Logical And | |
594 | ||
595 | Binary "and" returns the logical conjunction of the two surrounding | |
596 | expressions. It's equivalent to && except for the very low | |
5f05dabc | 597 | precedence. This means that it short-circuits: i.e., the right |
a0d0e21e LW |
598 | expression is evaluated only if the left expression is true. |
599 | ||
600 | =head2 Logical or and Exclusive Or | |
601 | ||
602 | Binary "or" returns the logical disjunction of the two surrounding | |
5a964f20 TC |
603 | expressions. It's equivalent to || except for the very low precedence. |
604 | This makes it useful for control flow | |
605 | ||
606 | print FH $data or die "Can't write to FH: $!"; | |
607 | ||
608 | This means that it short-circuits: i.e., the right expression is evaluated | |
609 | only if the left expression is false. Due to its precedence, you should | |
610 | probably avoid using this for assignment, only for control flow. | |
611 | ||
612 | $a = $b or $c; # bug: this is wrong | |
613 | ($a = $b) or $c; # really means this | |
614 | $a = $b || $c; # better written this way | |
615 | ||
19799a22 | 616 | However, when it's a list-context assignment and you're trying to use |
5a964f20 TC |
617 | "||" for control flow, you probably need "or" so that the assignment |
618 | takes higher precedence. | |
619 | ||
620 | @info = stat($file) || die; # oops, scalar sense of stat! | |
621 | @info = stat($file) or die; # better, now @info gets its due | |
622 | ||
19799a22 | 623 | Then again, you could always use parentheses. |
a0d0e21e LW |
624 | |
625 | Binary "xor" returns the exclusive-OR of the two surrounding expressions. | |
626 | It cannot short circuit, of course. | |
627 | ||
628 | =head2 C Operators Missing From Perl | |
629 | ||
630 | Here is what C has that Perl doesn't: | |
631 | ||
632 | =over 8 | |
633 | ||
634 | =item unary & | |
635 | ||
636 | Address-of operator. (But see the "\" operator for taking a reference.) | |
637 | ||
638 | =item unary * | |
639 | ||
54310121 | 640 | Dereference-address operator. (Perl's prefix dereferencing |
a0d0e21e LW |
641 | operators are typed: $, @, %, and &.) |
642 | ||
643 | =item (TYPE) | |
644 | ||
19799a22 | 645 | Type-casting operator. |
a0d0e21e LW |
646 | |
647 | =back | |
648 | ||
5f05dabc | 649 | =head2 Quote and Quote-like Operators |
a0d0e21e LW |
650 | |
651 | While we usually think of quotes as literal values, in Perl they | |
652 | function as operators, providing various kinds of interpolating and | |
653 | pattern matching capabilities. Perl provides customary quote characters | |
654 | for these behaviors, but also provides a way for you to choose your | |
655 | quote character for any of them. In the following table, a C<{}> represents | |
87275199 | 656 | any pair of delimiters you choose. |
a0d0e21e | 657 | |
2c268ad5 TP |
658 | Customary Generic Meaning Interpolates |
659 | '' q{} Literal no | |
660 | "" qq{} Literal yes | |
af9219ee | 661 | `` qx{} Command yes* |
2c268ad5 | 662 | qw{} Word list no |
af9219ee MG |
663 | // m{} Pattern match yes* |
664 | qr{} Pattern yes* | |
665 | s{}{} Substitution yes* | |
2c268ad5 | 666 | tr{}{} Transliteration no (but see below) |
7e3b091d | 667 | <<EOF here-doc yes* |
a0d0e21e | 668 | |
af9219ee MG |
669 | * unless the delimiter is ''. |
670 | ||
87275199 GS |
671 | Non-bracketing delimiters use the same character fore and aft, but the four |
672 | sorts of brackets (round, angle, square, curly) will all nest, which means | |
673 | that | |
674 | ||
675 | q{foo{bar}baz} | |
35f2feb0 | 676 | |
87275199 GS |
677 | is the same as |
678 | ||
679 | 'foo{bar}baz' | |
680 | ||
681 | Note, however, that this does not always work for quoting Perl code: | |
682 | ||
683 | $s = q{ if($a eq "}") ... }; # WRONG | |
684 | ||
83df6a1d JH |
685 | is a syntax error. The C<Text::Balanced> module (from CPAN, and |
686 | starting from Perl 5.8 part of the standard distribution) is able | |
687 | to do this properly. | |
87275199 | 688 | |
19799a22 | 689 | There can be whitespace between the operator and the quoting |
fb73857a | 690 | characters, except when C<#> is being used as the quoting character. |
19799a22 GS |
691 | C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the |
692 | operator C<q> followed by a comment. Its argument will be taken | |
693 | from the next line. This allows you to write: | |
fb73857a | 694 | |
695 | s {foo} # Replace foo | |
696 | {bar} # with bar. | |
697 | ||
904501ec MG |
698 | The following escape sequences are available in constructs that interpolate |
699 | and in transliterations. | |
a0d0e21e | 700 | |
6ee5d4e7 | 701 | \t tab (HT, TAB) |
5a964f20 | 702 | \n newline (NL) |
6ee5d4e7 | 703 | \r return (CR) |
704 | \f form feed (FF) | |
705 | \b backspace (BS) | |
706 | \a alarm (bell) (BEL) | |
707 | \e escape (ESC) | |
a0ed51b3 LW |
708 | \033 octal char (ESC) |
709 | \x1b hex char (ESC) | |
710 | \x{263a} wide hex char (SMILEY) | |
19799a22 | 711 | \c[ control char (ESC) |
4a2d328f | 712 | \N{name} named char |
2c268ad5 | 713 | |
904501ec MG |
714 | The following escape sequences are available in constructs that interpolate |
715 | but not in transliterations. | |
716 | ||
a0d0e21e LW |
717 | \l lowercase next char |
718 | \u uppercase next char | |
719 | \L lowercase till \E | |
720 | \U uppercase till \E | |
721 | \E end case modification | |
1d2dff63 | 722 | \Q quote non-word characters till \E |
a0d0e21e | 723 | |
a034a98d | 724 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> |
423cee85 | 725 | and C<\U> is taken from the current locale. See L<perllocale>. For |
4a2d328f | 726 | documentation of C<\N{name}>, see L<charnames>. |
a034a98d | 727 | |
5a964f20 TC |
728 | All systems use the virtual C<"\n"> to represent a line terminator, |
729 | called a "newline". There is no such thing as an unvarying, physical | |
19799a22 | 730 | newline character. It is only an illusion that the operating system, |
5a964f20 TC |
731 | device drivers, C libraries, and Perl all conspire to preserve. Not all |
732 | systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example, | |
733 | on a Mac, these are reversed, and on systems without line terminator, | |
734 | printing C<"\n"> may emit no actual data. In general, use C<"\n"> when | |
735 | you mean a "newline" for your system, but use the literal ASCII when you | |
736 | need an exact character. For example, most networking protocols expect | |
2a380090 | 737 | and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators, |
5a964f20 TC |
738 | and although they often accept just C<"\012">, they seldom tolerate just |
739 | C<"\015">. If you get in the habit of using C<"\n"> for networking, | |
740 | you may be burned some day. | |
741 | ||
904501ec MG |
742 | For constructs that do interpolate, variables beginning with "C<$>" |
743 | or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or | |
744 | C<$href->{key}[0]> are also interpolated, as are array and hash slices. | |
745 | But method calls such as C<$obj->meth> are not. | |
af9219ee MG |
746 | |
747 | Interpolating an array or slice interpolates the elements in order, | |
748 | separated by the value of C<$">, so is equivalent to interpolating | |
904501ec MG |
749 | C<join $", @array>. "Punctuation" arrays such as C<@+> are only |
750 | interpolated if the name is enclosed in braces C<@{+}>. | |
af9219ee | 751 | |
1d2dff63 GS |
752 | You cannot include a literal C<$> or C<@> within a C<\Q> sequence. |
753 | An unescaped C<$> or C<@> interpolates the corresponding variable, | |
754 | while escaping will cause the literal string C<\$> to be inserted. | |
755 | You'll need to write something like C<m/\Quser\E\@\Qhost/>. | |
756 | ||
a0d0e21e LW |
757 | Patterns are subject to an additional level of interpretation as a |
758 | regular expression. This is done as a second pass, after variables are | |
759 | interpolated, so that regular expressions may be incorporated into the | |
760 | pattern from the variables. If this is not what you want, use C<\Q> to | |
761 | interpolate a variable literally. | |
762 | ||
19799a22 GS |
763 | Apart from the behavior described above, Perl does not expand |
764 | multiple levels of interpolation. In particular, contrary to the | |
765 | expectations of shell programmers, back-quotes do I<NOT> interpolate | |
766 | within double quotes, nor do single quotes impede evaluation of | |
767 | variables when used within double quotes. | |
a0d0e21e | 768 | |
5f05dabc | 769 | =head2 Regexp Quote-Like Operators |
cb1a09d0 | 770 | |
5f05dabc | 771 | Here are the quote-like operators that apply to pattern |
cb1a09d0 AD |
772 | matching and related activities. |
773 | ||
a0d0e21e LW |
774 | =over 8 |
775 | ||
776 | =item ?PATTERN? | |
777 | ||
778 | This is just like the C</pattern/> search, except that it matches only | |
779 | once between calls to the reset() operator. This is a useful | |
5f05dabc | 780 | optimization when you want to see only the first occurrence of |
a0d0e21e LW |
781 | something in each file of a set of files, for instance. Only C<??> |
782 | patterns local to the current package are reset. | |
783 | ||
5a964f20 TC |
784 | while (<>) { |
785 | if (?^$?) { | |
786 | # blank line between header and body | |
787 | } | |
788 | } continue { | |
789 | reset if eof; # clear ?? status for next file | |
790 | } | |
791 | ||
483b4840 | 792 | This usage is vaguely deprecated, which means it just might possibly |
19799a22 GS |
793 | be removed in some distant future version of Perl, perhaps somewhere |
794 | around the year 2168. | |
a0d0e21e | 795 | |
fb73857a | 796 | =item m/PATTERN/cgimosx |
a0d0e21e | 797 | |
fb73857a | 798 | =item /PATTERN/cgimosx |
a0d0e21e | 799 | |
5a964f20 | 800 | Searches a string for a pattern match, and in scalar context returns |
19799a22 GS |
801 | true if it succeeds, false if it fails. If no string is specified |
802 | via the C<=~> or C<!~> operator, the $_ string is searched. (The | |
803 | string specified with C<=~> need not be an lvalue--it may be the | |
804 | result of an expression evaluation, but remember the C<=~> binds | |
805 | rather tightly.) See also L<perlre>. See L<perllocale> for | |
806 | discussion of additional considerations that apply when C<use locale> | |
807 | is in effect. | |
a0d0e21e LW |
808 | |
809 | Options are: | |
810 | ||
fb73857a | 811 | c Do not reset search position on a failed match when /g is in effect. |
5f05dabc | 812 | g Match globally, i.e., find all occurrences. |
a0d0e21e LW |
813 | i Do case-insensitive pattern matching. |
814 | m Treat string as multiple lines. | |
5f05dabc | 815 | o Compile pattern only once. |
a0d0e21e LW |
816 | s Treat string as single line. |
817 | x Use extended regular expressions. | |
818 | ||
819 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> | |
01ae956f | 820 | you can use any pair of non-alphanumeric, non-whitespace characters |
19799a22 GS |
821 | as delimiters. This is particularly useful for matching path names |
822 | that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is | |
7bac28a0 | 823 | the delimiter, then the match-only-once rule of C<?PATTERN?> applies. |
19799a22 | 824 | If "'" is the delimiter, no interpolation is performed on the PATTERN. |
a0d0e21e LW |
825 | |
826 | PATTERN may contain variables, which will be interpolated (and the | |
f70b4f9c | 827 | pattern recompiled) every time the pattern search is evaluated, except |
1f247705 GS |
828 | for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and |
829 | C<$|> are not interpolated because they look like end-of-string tests.) | |
f70b4f9c AB |
830 | If you want such a pattern to be compiled only once, add a C</o> after |
831 | the trailing delimiter. This avoids expensive run-time recompilations, | |
832 | and is useful when the value you are interpolating won't change over | |
833 | the life of the script. However, mentioning C</o> constitutes a promise | |
834 | that you won't change the variables in the pattern. If you change them, | |
13a2d996 | 835 | Perl won't even notice. See also L<"qr/STRING/imosx">. |
a0d0e21e | 836 | |
5a964f20 TC |
837 | If the PATTERN evaluates to the empty string, the last |
838 | I<successfully> matched regular expression is used instead. | |
a0d0e21e | 839 | |
19799a22 | 840 | If the C</g> option is not used, C<m//> in list context returns a |
a0d0e21e | 841 | list consisting of the subexpressions matched by the parentheses in the |
f7e33566 GS |
842 | pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are |
843 | also set, and that this differs from Perl 4's behavior.) When there are | |
844 | no parentheses in the pattern, the return value is the list C<(1)> for | |
845 | success. With or without parentheses, an empty list is returned upon | |
846 | failure. | |
a0d0e21e LW |
847 | |
848 | Examples: | |
849 | ||
850 | open(TTY, '/dev/tty'); | |
851 | <TTY> =~ /^y/i && foo(); # do foo if desired | |
852 | ||
853 | if (/Version: *([0-9.]*)/) { $version = $1; } | |
854 | ||
855 | next if m#^/usr/spool/uucp#; | |
856 | ||
857 | # poor man's grep | |
858 | $arg = shift; | |
859 | while (<>) { | |
860 | print if /$arg/o; # compile only once | |
861 | } | |
862 | ||
863 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) | |
864 | ||
865 | This last example splits $foo into the first two words and the | |
5f05dabc | 866 | remainder of the line, and assigns those three fields to $F1, $F2, and |
867 | $Etc. The conditional is true if any variables were assigned, i.e., if | |
a0d0e21e LW |
868 | the pattern matched. |
869 | ||
19799a22 GS |
870 | The C</g> modifier specifies global pattern matching--that is, |
871 | matching as many times as possible within the string. How it behaves | |
872 | depends on the context. In list context, it returns a list of the | |
873 | substrings matched by any capturing parentheses in the regular | |
874 | expression. If there are no parentheses, it returns a list of all | |
875 | the matched strings, as if there were parentheses around the whole | |
876 | pattern. | |
a0d0e21e | 877 | |
7e86de3e | 878 | In scalar context, each execution of C<m//g> finds the next match, |
19799a22 | 879 | returning true if it matches, and false if there is no further match. |
7e86de3e MG |
880 | The position after the last match can be read or set using the pos() |
881 | function; see L<perlfunc/pos>. A failed match normally resets the | |
882 | search position to the beginning of the string, but you can avoid that | |
883 | by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target | |
884 | string also resets the search position. | |
c90c0ff4 | 885 | |
886 | You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a | |
887 | zero-width assertion that matches the exact position where the previous | |
5d43e42d DC |
888 | C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion |
889 | still anchors at pos(), but the match is of course only attempted once. | |
890 | Using C<\G> without C</g> on a target string that has not previously had a | |
891 | C</g> match applied to it is the same as using the C<\A> assertion to match | |
892 | the beginning of the string. | |
c90c0ff4 | 893 | |
894 | Examples: | |
a0d0e21e LW |
895 | |
896 | # list context | |
897 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); | |
898 | ||
899 | # scalar context | |
5d43e42d | 900 | $/ = ""; |
19799a22 GS |
901 | while (defined($paragraph = <>)) { |
902 | while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { | |
903 | $sentences++; | |
a0d0e21e LW |
904 | } |
905 | } | |
906 | print "$sentences\n"; | |
907 | ||
c90c0ff4 | 908 | # using m//gc with \G |
137443ea | 909 | $_ = "ppooqppqq"; |
44a8e56a | 910 | while ($i++ < 2) { |
911 | print "1: '"; | |
c90c0ff4 | 912 | print $1 while /(o)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 913 | print "2: '"; |
c90c0ff4 | 914 | print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 915 | print "3: '"; |
c90c0ff4 | 916 | print $1 while /(p)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 917 | } |
5d43e42d | 918 | print "Final: '$1', pos=",pos,"\n" if /\G(.)/; |
44a8e56a | 919 | |
920 | The last example should print: | |
921 | ||
922 | 1: 'oo', pos=4 | |
137443ea | 923 | 2: 'q', pos=5 |
44a8e56a | 924 | 3: 'pp', pos=7 |
925 | 1: '', pos=7 | |
137443ea | 926 | 2: 'q', pos=8 |
927 | 3: '', pos=8 | |
5d43e42d DC |
928 | Final: 'q', pos=8 |
929 | ||
930 | Notice that the final match matched C<q> instead of C<p>, which a match | |
931 | without the C<\G> anchor would have done. Also note that the final match | |
932 | did not update C<pos> -- C<pos> is only updated on a C</g> match. If the | |
933 | final match did indeed match C<p>, it's a good bet that you're running an | |
934 | older (pre-5.6.0) Perl. | |
44a8e56a | 935 | |
c90c0ff4 | 936 | A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can |
e7ea3e70 | 937 | combine several regexps like this to process a string part-by-part, |
c90c0ff4 | 938 | doing different actions depending on which regexp matched. Each |
939 | regexp tries to match where the previous one leaves off. | |
e7ea3e70 | 940 | |
3fe9a6f1 | 941 | $_ = <<'EOL'; |
e7ea3e70 | 942 | $url = new URI::URL "http://www/"; die if $url eq "xXx"; |
3fe9a6f1 | 943 | EOL |
944 | LOOP: | |
e7ea3e70 | 945 | { |
c90c0ff4 | 946 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; |
947 | print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc; | |
948 | print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc; | |
949 | print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc; | |
950 | print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc; | |
951 | print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc; | |
952 | print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc; | |
e7ea3e70 IZ |
953 | print ". That's all!\n"; |
954 | } | |
955 | ||
956 | Here is the output (split into several lines): | |
957 | ||
958 | line-noise lowercase line-noise lowercase UPPERCASE line-noise | |
959 | UPPERCASE line-noise lowercase line-noise lowercase line-noise | |
960 | lowercase lowercase line-noise lowercase lowercase line-noise | |
961 | MiXeD line-noise. That's all! | |
44a8e56a | 962 | |
a0d0e21e LW |
963 | =item q/STRING/ |
964 | ||
965 | =item C<'STRING'> | |
966 | ||
19799a22 | 967 | A single-quoted, literal string. A backslash represents a backslash |
68dc0745 | 968 | unless followed by the delimiter or another backslash, in which case |
969 | the delimiter or backslash is interpolated. | |
a0d0e21e LW |
970 | |
971 | $foo = q!I said, "You said, 'She said it.'"!; | |
972 | $bar = q('This is it.'); | |
68dc0745 | 973 | $baz = '\n'; # a two-character string |
a0d0e21e LW |
974 | |
975 | =item qq/STRING/ | |
976 | ||
977 | =item "STRING" | |
978 | ||
979 | A double-quoted, interpolated string. | |
980 | ||
981 | $_ .= qq | |
982 | (*** The previous line contains the naughty word "$1".\n) | |
19799a22 | 983 | if /\b(tcl|java|python)\b/i; # :-) |
68dc0745 | 984 | $baz = "\n"; # a one-character string |
a0d0e21e | 985 | |
eec2d3df GS |
986 | =item qr/STRING/imosx |
987 | ||
322edccd | 988 | This operator quotes (and possibly compiles) its I<STRING> as a regular |
19799a22 GS |
989 | expression. I<STRING> is interpolated the same way as I<PATTERN> |
990 | in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation | |
991 | is done. Returns a Perl value which may be used instead of the | |
992 | corresponding C</STRING/imosx> expression. | |
4b6a7270 IZ |
993 | |
994 | For example, | |
995 | ||
996 | $rex = qr/my.STRING/is; | |
997 | s/$rex/foo/; | |
998 | ||
999 | is equivalent to | |
1000 | ||
1001 | s/my.STRING/foo/is; | |
1002 | ||
1003 | The result may be used as a subpattern in a match: | |
eec2d3df GS |
1004 | |
1005 | $re = qr/$pattern/; | |
0a92e3a8 GS |
1006 | $string =~ /foo${re}bar/; # can be interpolated in other patterns |
1007 | $string =~ $re; # or used standalone | |
4b6a7270 IZ |
1008 | $string =~ /$re/; # or this way |
1009 | ||
1010 | Since Perl may compile the pattern at the moment of execution of qr() | |
19799a22 | 1011 | operator, using qr() may have speed advantages in some situations, |
4b6a7270 IZ |
1012 | notably if the result of qr() is used standalone: |
1013 | ||
1014 | sub match { | |
1015 | my $patterns = shift; | |
1016 | my @compiled = map qr/$_/i, @$patterns; | |
1017 | grep { | |
1018 | my $success = 0; | |
a7665c5e | 1019 | foreach my $pat (@compiled) { |
4b6a7270 IZ |
1020 | $success = 1, last if /$pat/; |
1021 | } | |
1022 | $success; | |
1023 | } @_; | |
1024 | } | |
1025 | ||
19799a22 GS |
1026 | Precompilation of the pattern into an internal representation at |
1027 | the moment of qr() avoids a need to recompile the pattern every | |
1028 | time a match C</$pat/> is attempted. (Perl has many other internal | |
1029 | optimizations, but none would be triggered in the above example if | |
1030 | we did not use qr() operator.) | |
eec2d3df GS |
1031 | |
1032 | Options are: | |
1033 | ||
1034 | i Do case-insensitive pattern matching. | |
1035 | m Treat string as multiple lines. | |
1036 | o Compile pattern only once. | |
1037 | s Treat string as single line. | |
1038 | x Use extended regular expressions. | |
1039 | ||
0a92e3a8 GS |
1040 | See L<perlre> for additional information on valid syntax for STRING, and |
1041 | for a detailed look at the semantics of regular expressions. | |
1042 | ||
a0d0e21e LW |
1043 | =item qx/STRING/ |
1044 | ||
1045 | =item `STRING` | |
1046 | ||
43dd4d21 JH |
1047 | A string which is (possibly) interpolated and then executed as a |
1048 | system command with C</bin/sh> or its equivalent. Shell wildcards, | |
1049 | pipes, and redirections will be honored. The collected standard | |
1050 | output of the command is returned; standard error is unaffected. In | |
1051 | scalar context, it comes back as a single (potentially multi-line) | |
1052 | string, or undef if the command failed. In list context, returns a | |
1053 | list of lines (however you've defined lines with $/ or | |
1054 | $INPUT_RECORD_SEPARATOR), or an empty list if the command failed. | |
5a964f20 TC |
1055 | |
1056 | Because backticks do not affect standard error, use shell file descriptor | |
1057 | syntax (assuming the shell supports this) if you care to address this. | |
1058 | To capture a command's STDERR and STDOUT together: | |
a0d0e21e | 1059 | |
5a964f20 TC |
1060 | $output = `cmd 2>&1`; |
1061 | ||
1062 | To capture a command's STDOUT but discard its STDERR: | |
1063 | ||
1064 | $output = `cmd 2>/dev/null`; | |
1065 | ||
1066 | To capture a command's STDERR but discard its STDOUT (ordering is | |
1067 | important here): | |
1068 | ||
1069 | $output = `cmd 2>&1 1>/dev/null`; | |
1070 | ||
1071 | To exchange a command's STDOUT and STDERR in order to capture the STDERR | |
1072 | but leave its STDOUT to come out the old STDERR: | |
1073 | ||
1074 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; | |
1075 | ||
1076 | To read both a command's STDOUT and its STDERR separately, it's easiest | |
1077 | and safest to redirect them separately to files, and then read from those | |
1078 | files when the program is done: | |
1079 | ||
1080 | system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr"); | |
1081 | ||
1082 | Using single-quote as a delimiter protects the command from Perl's | |
1083 | double-quote interpolation, passing it on to the shell instead: | |
1084 | ||
1085 | $perl_info = qx(ps $$); # that's Perl's $$ | |
1086 | $shell_info = qx'ps $$'; # that's the new shell's $$ | |
1087 | ||
19799a22 | 1088 | How that string gets evaluated is entirely subject to the command |
5a964f20 TC |
1089 | interpreter on your system. On most platforms, you will have to protect |
1090 | shell metacharacters if you want them treated literally. This is in | |
1091 | practice difficult to do, as it's unclear how to escape which characters. | |
1092 | See L<perlsec> for a clean and safe example of a manual fork() and exec() | |
1093 | to emulate backticks safely. | |
a0d0e21e | 1094 | |
bb32b41a GS |
1095 | On some platforms (notably DOS-like ones), the shell may not be |
1096 | capable of dealing with multiline commands, so putting newlines in | |
1097 | the string may not get you what you want. You may be able to evaluate | |
1098 | multiple commands in a single line by separating them with the command | |
1099 | separator character, if your shell supports that (e.g. C<;> on many Unix | |
1100 | shells; C<&> on the Windows NT C<cmd> shell). | |
1101 | ||
0f897271 GS |
1102 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
1103 | output before starting the child process, but this may not be supported | |
1104 | on some platforms (see L<perlport>). To be safe, you may need to set | |
1105 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of | |
1106 | C<IO::Handle> on any open handles. | |
1107 | ||
bb32b41a GS |
1108 | Beware that some command shells may place restrictions on the length |
1109 | of the command line. You must ensure your strings don't exceed this | |
1110 | limit after any necessary interpolations. See the platform-specific | |
1111 | release notes for more details about your particular environment. | |
1112 | ||
5a964f20 TC |
1113 | Using this operator can lead to programs that are difficult to port, |
1114 | because the shell commands called vary between systems, and may in | |
1115 | fact not be present at all. As one example, the C<type> command under | |
1116 | the POSIX shell is very different from the C<type> command under DOS. | |
1117 | That doesn't mean you should go out of your way to avoid backticks | |
1118 | when they're the right way to get something done. Perl was made to be | |
1119 | a glue language, and one of the things it glues together is commands. | |
1120 | Just understand what you're getting yourself into. | |
bb32b41a | 1121 | |
dc848c6f | 1122 | See L<"I/O Operators"> for more discussion. |
a0d0e21e | 1123 | |
945c54fd JH |
1124 | =item qw/STRING/ |
1125 | ||
1126 | Evaluates to a list of the words extracted out of STRING, using embedded | |
1127 | whitespace as the word delimiters. It can be understood as being roughly | |
1128 | equivalent to: | |
1129 | ||
1130 | split(' ', q/STRING/); | |
1131 | ||
1132 | the difference being that it generates a real list at compile time. So | |
1133 | this expression: | |
1134 | ||
1135 | qw(foo bar baz) | |
1136 | ||
1137 | is semantically equivalent to the list: | |
1138 | ||
1139 | 'foo', 'bar', 'baz' | |
1140 | ||
1141 | Some frequently seen examples: | |
1142 | ||
1143 | use POSIX qw( setlocale localeconv ) | |
1144 | @EXPORT = qw( foo bar baz ); | |
1145 | ||
1146 | A common mistake is to try to separate the words with comma or to | |
1147 | put comments into a multi-line C<qw>-string. For this reason, the | |
1148 | C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable) | |
1149 | produces warnings if the STRING contains the "," or the "#" character. | |
1150 | ||
a0d0e21e LW |
1151 | =item s/PATTERN/REPLACEMENT/egimosx |
1152 | ||
1153 | Searches a string for a pattern, and if found, replaces that pattern | |
1154 | with the replacement text and returns the number of substitutions | |
e37d713d | 1155 | made. Otherwise it returns false (specifically, the empty string). |
a0d0e21e LW |
1156 | |
1157 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> | |
1158 | variable is searched and modified. (The string specified with C<=~> must | |
5a964f20 | 1159 | be scalar variable, an array element, a hash element, or an assignment |
5f05dabc | 1160 | to one of those, i.e., an lvalue.) |
a0d0e21e | 1161 | |
19799a22 | 1162 | If the delimiter chosen is a single quote, no interpolation is |
a0d0e21e LW |
1163 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the |
1164 | PATTERN contains a $ that looks like a variable rather than an | |
1165 | end-of-string test, the variable will be interpolated into the pattern | |
5f05dabc | 1166 | at run-time. If you want the pattern compiled only once the first time |
a0d0e21e | 1167 | the variable is interpolated, use the C</o> option. If the pattern |
5a964f20 | 1168 | evaluates to the empty string, the last successfully executed regular |
a0d0e21e | 1169 | expression is used instead. See L<perlre> for further explanation on these. |
5a964f20 | 1170 | See L<perllocale> for discussion of additional considerations that apply |
a034a98d | 1171 | when C<use locale> is in effect. |
a0d0e21e LW |
1172 | |
1173 | Options are: | |
1174 | ||
1175 | e Evaluate the right side as an expression. | |
5f05dabc | 1176 | g Replace globally, i.e., all occurrences. |
a0d0e21e LW |
1177 | i Do case-insensitive pattern matching. |
1178 | m Treat string as multiple lines. | |
5f05dabc | 1179 | o Compile pattern only once. |
a0d0e21e LW |
1180 | s Treat string as single line. |
1181 | x Use extended regular expressions. | |
1182 | ||
1183 | Any non-alphanumeric, non-whitespace delimiter may replace the | |
1184 | slashes. If single quotes are used, no interpretation is done on the | |
e37d713d | 1185 | replacement string (the C</e> modifier overrides this, however). Unlike |
54310121 | 1186 | Perl 4, Perl 5 treats backticks as normal delimiters; the replacement |
e37d713d | 1187 | text is not evaluated as a command. If the |
a0d0e21e | 1188 | PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own |
5f05dabc | 1189 | pair of quotes, which may or may not be bracketing quotes, e.g., |
35f2feb0 | 1190 | C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the |
cec88af6 GS |
1191 | replacement portion to be treated as a full-fledged Perl expression |
1192 | and evaluated right then and there. It is, however, syntax checked at | |
1193 | compile-time. A second C<e> modifier will cause the replacement portion | |
1194 | to be C<eval>ed before being run as a Perl expression. | |
a0d0e21e LW |
1195 | |
1196 | Examples: | |
1197 | ||
1198 | s/\bgreen\b/mauve/g; # don't change wintergreen | |
1199 | ||
1200 | $path =~ s|/usr/bin|/usr/local/bin|; | |
1201 | ||
1202 | s/Login: $foo/Login: $bar/; # run-time pattern | |
1203 | ||
5a964f20 | 1204 | ($foo = $bar) =~ s/this/that/; # copy first, then change |
a0d0e21e | 1205 | |
5a964f20 | 1206 | $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count |
a0d0e21e LW |
1207 | |
1208 | $_ = 'abc123xyz'; | |
1209 | s/\d+/$&*2/e; # yields 'abc246xyz' | |
1210 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' | |
1211 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' | |
1212 | ||
1213 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e | |
1214 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e | |
1215 | s/^=(\w+)/&pod($1)/ge; # use function call | |
1216 | ||
5a964f20 TC |
1217 | # expand variables in $_, but dynamics only, using |
1218 | # symbolic dereferencing | |
1219 | s/\$(\w+)/${$1}/g; | |
1220 | ||
cec88af6 GS |
1221 | # Add one to the value of any numbers in the string |
1222 | s/(\d+)/1 + $1/eg; | |
1223 | ||
1224 | # This will expand any embedded scalar variable | |
1225 | # (including lexicals) in $_ : First $1 is interpolated | |
1226 | # to the variable name, and then evaluated | |
a0d0e21e LW |
1227 | s/(\$\w+)/$1/eeg; |
1228 | ||
5a964f20 | 1229 | # Delete (most) C comments. |
a0d0e21e | 1230 | $program =~ s { |
4633a7c4 LW |
1231 | /\* # Match the opening delimiter. |
1232 | .*? # Match a minimal number of characters. | |
1233 | \*/ # Match the closing delimiter. | |
a0d0e21e LW |
1234 | } []gsx; |
1235 | ||
5a964f20 TC |
1236 | s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively |
1237 | ||
1238 | for ($variable) { # trim white space in $variable, cheap | |
1239 | s/^\s+//; | |
1240 | s/\s+$//; | |
1241 | } | |
a0d0e21e LW |
1242 | |
1243 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields | |
1244 | ||
54310121 | 1245 | Note the use of $ instead of \ in the last example. Unlike |
35f2feb0 GS |
1246 | B<sed>, we use the \<I<digit>> form in only the left hand side. |
1247 | Anywhere else it's $<I<digit>>. | |
a0d0e21e | 1248 | |
5f05dabc | 1249 | Occasionally, you can't use just a C</g> to get all the changes |
19799a22 | 1250 | to occur that you might want. Here are two common cases: |
a0d0e21e LW |
1251 | |
1252 | # put commas in the right places in an integer | |
19799a22 | 1253 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; |
a0d0e21e LW |
1254 | |
1255 | # expand tabs to 8-column spacing | |
1256 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; | |
1257 | ||
6940069f | 1258 | =item tr/SEARCHLIST/REPLACEMENTLIST/cds |
a0d0e21e | 1259 | |
6940069f | 1260 | =item y/SEARCHLIST/REPLACEMENTLIST/cds |
a0d0e21e | 1261 | |
2c268ad5 | 1262 | Transliterates all occurrences of the characters found in the search list |
a0d0e21e LW |
1263 | with the corresponding character in the replacement list. It returns |
1264 | the number of characters replaced or deleted. If no string is | |
2c268ad5 | 1265 | specified via the =~ or !~ operator, the $_ string is transliterated. (The |
54310121 | 1266 | string specified with =~ must be a scalar variable, an array element, a |
1267 | hash element, or an assignment to one of those, i.e., an lvalue.) | |
8ada0baa | 1268 | |
2c268ad5 TP |
1269 | A character range may be specified with a hyphen, so C<tr/A-J/0-9/> |
1270 | does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. | |
54310121 | 1271 | For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the |
1272 | SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has | |
1273 | its own pair of quotes, which may or may not be bracketing quotes, | |
2c268ad5 | 1274 | e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>. |
a0d0e21e | 1275 | |
cc255d5f JH |
1276 | Note that C<tr> does B<not> do regular expression character classes |
1277 | such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to | |
1278 | the tr(1) utility. If you want to map strings between lower/upper | |
1279 | cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider | |
1280 | using the C<s> operator if you need regular expressions. | |
1281 | ||
8ada0baa JH |
1282 | Note also that the whole range idea is rather unportable between |
1283 | character sets--and even within character sets they may cause results | |
1284 | you probably didn't expect. A sound principle is to use only ranges | |
1285 | that begin from and end at either alphabets of equal case (a-e, A-E), | |
1286 | or digits (0-4). Anything else is unsafe. If in doubt, spell out the | |
1287 | character sets in full. | |
1288 | ||
a0d0e21e LW |
1289 | Options: |
1290 | ||
1291 | c Complement the SEARCHLIST. | |
1292 | d Delete found but unreplaced characters. | |
1293 | s Squash duplicate replaced characters. | |
1294 | ||
19799a22 GS |
1295 | If the C</c> modifier is specified, the SEARCHLIST character set |
1296 | is complemented. If the C</d> modifier is specified, any characters | |
1297 | specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. | |
1298 | (Note that this is slightly more flexible than the behavior of some | |
1299 | B<tr> programs, which delete anything they find in the SEARCHLIST, | |
1300 | period.) If the C</s> modifier is specified, sequences of characters | |
1301 | that were transliterated to the same character are squashed down | |
1302 | to a single instance of the character. | |
a0d0e21e LW |
1303 | |
1304 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted | |
1305 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter | |
1306 | than the SEARCHLIST, the final character is replicated till it is long | |
5a964f20 | 1307 | enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. |
a0d0e21e LW |
1308 | This latter is useful for counting characters in a class or for |
1309 | squashing character sequences in a class. | |
1310 | ||
1311 | Examples: | |
1312 | ||
1313 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case | |
1314 | ||
1315 | $cnt = tr/*/*/; # count the stars in $_ | |
1316 | ||
1317 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky | |
1318 | ||
1319 | $cnt = tr/0-9//; # count the digits in $_ | |
1320 | ||
1321 | tr/a-zA-Z//s; # bookkeeper -> bokeper | |
1322 | ||
1323 | ($HOST = $host) =~ tr/a-z/A-Z/; | |
1324 | ||
1325 | tr/a-zA-Z/ /cs; # change non-alphas to single space | |
1326 | ||
1327 | tr [\200-\377] | |
1328 | [\000-\177]; # delete 8th bit | |
1329 | ||
19799a22 GS |
1330 | If multiple transliterations are given for a character, only the |
1331 | first one is used: | |
748a9306 LW |
1332 | |
1333 | tr/AAA/XYZ/ | |
1334 | ||
2c268ad5 | 1335 | will transliterate any A to X. |
748a9306 | 1336 | |
19799a22 | 1337 | Because the transliteration table is built at compile time, neither |
a0d0e21e | 1338 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote |
19799a22 GS |
1339 | interpolation. That means that if you want to use variables, you |
1340 | must use an eval(): | |
a0d0e21e LW |
1341 | |
1342 | eval "tr/$oldlist/$newlist/"; | |
1343 | die $@ if $@; | |
1344 | ||
1345 | eval "tr/$oldlist/$newlist/, 1" or die $@; | |
1346 | ||
7e3b091d DA |
1347 | =item <<EOF |
1348 | ||
1349 | A line-oriented form of quoting is based on the shell "here-document" | |
1350 | syntax. Following a C<< << >> you specify a string to terminate | |
1351 | the quoted material, and all lines following the current line down to | |
1352 | the terminating string are the value of the item. The terminating | |
1353 | string may be either an identifier (a word), or some quoted text. If | |
1354 | quoted, the type of quotes you use determines the treatment of the | |
1355 | text, just as in regular quoting. An unquoted identifier works like | |
1356 | double quotes. There must be no space between the C<< << >> and | |
1357 | the identifier, unless the identifier is quoted. (If you put a space it | |
1358 | will be treated as a null identifier, which is valid, and matches the first | |
1359 | empty line.) The terminating string must appear by itself (unquoted and | |
1360 | with no surrounding whitespace) on the terminating line. | |
1361 | ||
1362 | print <<EOF; | |
1363 | The price is $Price. | |
1364 | EOF | |
1365 | ||
1366 | print << "EOF"; # same as above | |
1367 | The price is $Price. | |
1368 | EOF | |
1369 | ||
1370 | print << `EOC`; # execute commands | |
1371 | echo hi there | |
1372 | echo lo there | |
1373 | EOC | |
1374 | ||
1375 | print <<"foo", <<"bar"; # you can stack them | |
1376 | I said foo. | |
1377 | foo | |
1378 | I said bar. | |
1379 | bar | |
1380 | ||
1381 | myfunc(<< "THIS", 23, <<'THAT'); | |
1382 | Here's a line | |
1383 | or two. | |
1384 | THIS | |
1385 | and here's another. | |
1386 | THAT | |
1387 | ||
1388 | Just don't forget that you have to put a semicolon on the end | |
1389 | to finish the statement, as Perl doesn't know you're not going to | |
1390 | try to do this: | |
1391 | ||
1392 | print <<ABC | |
1393 | 179231 | |
1394 | ABC | |
1395 | + 20; | |
1396 | ||
1397 | If you want your here-docs to be indented with the | |
1398 | rest of the code, you'll need to remove leading whitespace | |
1399 | from each line manually: | |
1400 | ||
1401 | ($quote = <<'FINIS') =~ s/^\s+//gm; | |
1402 | The Road goes ever on and on, | |
1403 | down from the door where it began. | |
1404 | FINIS | |
1405 | ||
1406 | If you use a here-doc within a delimited construct, such as in C<s///eg>, | |
1407 | the quoted material must come on the lines following the final delimiter. | |
1408 | So instead of | |
1409 | ||
1410 | s/this/<<E . 'that' | |
1411 | the other | |
1412 | E | |
1413 | . 'more '/eg; | |
1414 | ||
1415 | you have to write | |
1416 | ||
1417 | s/this/<<E . 'that' | |
1418 | . 'more '/eg; | |
1419 | the other | |
1420 | E | |
1421 | ||
1422 | If the terminating identifier is on the last line of the program, you | |
1423 | must be sure there is a newline after it; otherwise, Perl will give the | |
1424 | warning B<Can't find string terminator "END" anywhere before EOF...>. | |
1425 | ||
1426 | Additionally, the quoting rules for the identifier are not related to | |
1427 | Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported | |
1428 | in place of C<''> and C<"">, and the only interpolation is for backslashing | |
1429 | the quoting character: | |
1430 | ||
1431 | print << "abc\"def"; | |
1432 | testing... | |
1433 | abc"def | |
1434 | ||
1435 | Finally, quoted strings cannot span multiple lines. The general rule is | |
1436 | that the identifier must be a string literal. Stick with that, and you | |
1437 | should be safe. | |
1438 | ||
a0d0e21e LW |
1439 | =back |
1440 | ||
75e14d17 IZ |
1441 | =head2 Gory details of parsing quoted constructs |
1442 | ||
19799a22 GS |
1443 | When presented with something that might have several different |
1444 | interpretations, Perl uses the B<DWIM> (that's "Do What I Mean") | |
1445 | principle to pick the most probable interpretation. This strategy | |
1446 | is so successful that Perl programmers often do not suspect the | |
1447 | ambivalence of what they write. But from time to time, Perl's | |
1448 | notions differ substantially from what the author honestly meant. | |
1449 | ||
1450 | This section hopes to clarify how Perl handles quoted constructs. | |
1451 | Although the most common reason to learn this is to unravel labyrinthine | |
1452 | regular expressions, because the initial steps of parsing are the | |
1453 | same for all quoting operators, they are all discussed together. | |
1454 | ||
1455 | The most important Perl parsing rule is the first one discussed | |
1456 | below: when processing a quoted construct, Perl first finds the end | |
1457 | of that construct, then interprets its contents. If you understand | |
1458 | this rule, you may skip the rest of this section on the first | |
1459 | reading. The other rules are likely to contradict the user's | |
1460 | expectations much less frequently than this first one. | |
1461 | ||
1462 | Some passes discussed below are performed concurrently, but because | |
1463 | their results are the same, we consider them individually. For different | |
1464 | quoting constructs, Perl performs different numbers of passes, from | |
1465 | one to five, but these passes are always performed in the same order. | |
75e14d17 | 1466 | |
13a2d996 | 1467 | =over 4 |
75e14d17 IZ |
1468 | |
1469 | =item Finding the end | |
1470 | ||
19799a22 GS |
1471 | The first pass is finding the end of the quoted construct, whether |
1472 | it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF> | |
1473 | construct, a C</> that terminates a C<qq//> construct, a C<]> which | |
35f2feb0 GS |
1474 | terminates C<qq[]> construct, or a C<< > >> which terminates a |
1475 | fileglob started with C<< < >>. | |
75e14d17 | 1476 | |
19799a22 GS |
1477 | When searching for single-character non-pairing delimiters, such |
1478 | as C</>, combinations of C<\\> and C<\/> are skipped. However, | |
1479 | when searching for single-character pairing delimiter like C<[>, | |
1480 | combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested | |
1481 | C<[>, C<]> are skipped as well. When searching for multicharacter | |
1482 | delimiters, nothing is skipped. | |
75e14d17 | 1483 | |
19799a22 GS |
1484 | For constructs with three-part delimiters (C<s///>, C<y///>, and |
1485 | C<tr///>), the search is repeated once more. | |
75e14d17 | 1486 | |
19799a22 GS |
1487 | During this search no attention is paid to the semantics of the construct. |
1488 | Thus: | |
75e14d17 IZ |
1489 | |
1490 | "$hash{"$foo/$bar"}" | |
1491 | ||
2a94b7ce | 1492 | or: |
75e14d17 IZ |
1493 | |
1494 | m/ | |
2a94b7ce | 1495 | bar # NOT a comment, this slash / terminated m//! |
75e14d17 IZ |
1496 | /x |
1497 | ||
19799a22 GS |
1498 | do not form legal quoted expressions. The quoted part ends on the |
1499 | first C<"> and C</>, and the rest happens to be a syntax error. | |
1500 | Because the slash that terminated C<m//> was followed by a C<SPACE>, | |
1501 | the example above is not C<m//x>, but rather C<m//> with no C</x> | |
1502 | modifier. So the embedded C<#> is interpreted as a literal C<#>. | |
75e14d17 IZ |
1503 | |
1504 | =item Removal of backslashes before delimiters | |
1505 | ||
19799a22 GS |
1506 | During the second pass, text between the starting and ending |
1507 | delimiters is copied to a safe location, and the C<\> is removed | |
1508 | from combinations consisting of C<\> and delimiter--or delimiters, | |
1509 | meaning both starting and ending delimiters will should these differ. | |
1510 | This removal does not happen for multi-character delimiters. | |
1511 | Note that the combination C<\\> is left intact, just as it was. | |
75e14d17 | 1512 | |
19799a22 GS |
1513 | Starting from this step no information about the delimiters is |
1514 | used in parsing. | |
75e14d17 IZ |
1515 | |
1516 | =item Interpolation | |
1517 | ||
19799a22 GS |
1518 | The next step is interpolation in the text obtained, which is now |
1519 | delimiter-independent. There are four different cases. | |
75e14d17 | 1520 | |
13a2d996 | 1521 | =over 4 |
75e14d17 IZ |
1522 | |
1523 | =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///> | |
1524 | ||
1525 | No interpolation is performed. | |
1526 | ||
1527 | =item C<''>, C<q//> | |
1528 | ||
1529 | The only interpolation is removal of C<\> from pairs C<\\>. | |
1530 | ||
35f2feb0 | 1531 | =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >> |
75e14d17 | 1532 | |
19799a22 GS |
1533 | C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are |
1534 | converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar"> | |
1535 | is converted to C<$foo . (quotemeta("baz" . $bar))> internally. | |
1536 | The other combinations are replaced with appropriate expansions. | |
2a94b7ce | 1537 | |
19799a22 GS |
1538 | Let it be stressed that I<whatever falls between C<\Q> and C<\E>> |
1539 | is interpolated in the usual way. Something like C<"\Q\\E"> has | |
1540 | no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the | |
1541 | result is the same as for C<"\\\\E">. As a general rule, backslashes | |
1542 | between C<\Q> and C<\E> may lead to counterintuitive results. So, | |
1543 | C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same | |
1544 | as C<"\\\t"> (since TAB is not alphanumeric). Note also that: | |
2a94b7ce IZ |
1545 | |
1546 | $str = '\t'; | |
1547 | return "\Q$str"; | |
1548 | ||
1549 | may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. | |
1550 | ||
19799a22 | 1551 | Interpolated scalars and arrays are converted internally to the C<join> and |
92d29cee | 1552 | C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes: |
75e14d17 | 1553 | |
19799a22 | 1554 | $foo . " XXX '" . (join $", @arr) . "'"; |
75e14d17 | 1555 | |
19799a22 | 1556 | All operations above are performed simultaneously, left to right. |
75e14d17 | 1557 | |
19799a22 GS |
1558 | Because the result of C<"\Q STRING \E"> has all metacharacters |
1559 | quoted, there is no way to insert a literal C<$> or C<@> inside a | |
1560 | C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became | |
1561 | C<"\\\$">; if not, it is interpreted as the start of an interpolated | |
1562 | scalar. | |
75e14d17 | 1563 | |
19799a22 GS |
1564 | Note also that the interpolation code needs to make a decision on |
1565 | where the interpolated scalar ends. For instance, whether | |
35f2feb0 | 1566 | C<< "a $b -> {c}" >> really means: |
75e14d17 IZ |
1567 | |
1568 | "a " . $b . " -> {c}"; | |
1569 | ||
2a94b7ce | 1570 | or: |
75e14d17 IZ |
1571 | |
1572 | "a " . $b -> {c}; | |
1573 | ||
19799a22 GS |
1574 | Most of the time, the longest possible text that does not include |
1575 | spaces between components and which contains matching braces or | |
1576 | brackets. because the outcome may be determined by voting based | |
1577 | on heuristic estimators, the result is not strictly predictable. | |
1578 | Fortunately, it's usually correct for ambiguous cases. | |
75e14d17 IZ |
1579 | |
1580 | =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, | |
1581 | ||
19799a22 GS |
1582 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation |
1583 | happens (almost) as with C<qq//> constructs, but the substitution | |
1584 | of C<\> followed by RE-special chars (including C<\>) is not | |
1585 | performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and | |
1586 | a C<#>-comment in a C<//x>-regular expression, no processing is | |
1587 | performed whatsoever. This is the first step at which the presence | |
1588 | of the C<//x> modifier is relevant. | |
1589 | ||
1590 | Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not | |
1591 | interpolated, and constructs C<$var[SOMETHING]> are voted (by several | |
1592 | different estimators) to be either an array element or C<$var> | |
1593 | followed by an RE alternative. This is where the notation | |
1594 | C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as | |
1595 | array element C<-9>, not as a regular expression from the variable | |
1596 | C<$arr> followed by a digit, which would be the interpretation of | |
1597 | C</$arr[0-9]/>. Since voting among different estimators may occur, | |
1598 | the result is not predictable. | |
1599 | ||
1600 | It is at this step that C<\1> is begrudgingly converted to C<$1> in | |
1601 | the replacement text of C<s///> to correct the incorrigible | |
1602 | I<sed> hackers who haven't picked up the saner idiom yet. A warning | |
9f1b1f2d GS |
1603 | is emitted if the C<use warnings> pragma or the B<-w> command-line flag |
1604 | (that is, the C<$^W> variable) was set. | |
19799a22 GS |
1605 | |
1606 | The lack of processing of C<\\> creates specific restrictions on | |
1607 | the post-processed text. If the delimiter is C</>, one cannot get | |
1608 | the combination C<\/> into the result of this step. C</> will | |
1609 | finish the regular expression, C<\/> will be stripped to C</> on | |
1610 | the previous step, and C<\\/> will be left as is. Because C</> is | |
1611 | equivalent to C<\/> inside a regular expression, this does not | |
1612 | matter unless the delimiter happens to be character special to the | |
1613 | RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an | |
1614 | alphanumeric char, as in: | |
2a94b7ce IZ |
1615 | |
1616 | m m ^ a \s* b mmx; | |
1617 | ||
19799a22 | 1618 | In the RE above, which is intentionally obfuscated for illustration, the |
2a94b7ce | 1619 | delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the |
19799a22 GS |
1620 | RE is the same as for C<m/ ^ a s* b /mx>). There's more than one |
1621 | reason you're encouraged to restrict your delimiters to non-alphanumeric, | |
1622 | non-whitespace choices. | |
75e14d17 IZ |
1623 | |
1624 | =back | |
1625 | ||
19799a22 | 1626 | This step is the last one for all constructs except regular expressions, |
75e14d17 IZ |
1627 | which are processed further. |
1628 | ||
1629 | =item Interpolation of regular expressions | |
1630 | ||
19799a22 GS |
1631 | Previous steps were performed during the compilation of Perl code, |
1632 | but this one happens at run time--although it may be optimized to | |
1633 | be calculated at compile time if appropriate. After preprocessing | |
1634 | described above, and possibly after evaluation if catenation, | |
1635 | joining, casing translation, or metaquoting are involved, the | |
1636 | resulting I<string> is passed to the RE engine for compilation. | |
1637 | ||
1638 | Whatever happens in the RE engine might be better discussed in L<perlre>, | |
1639 | but for the sake of continuity, we shall do so here. | |
1640 | ||
1641 | This is another step where the presence of the C<//x> modifier is | |
1642 | relevant. The RE engine scans the string from left to right and | |
1643 | converts it to a finite automaton. | |
1644 | ||
1645 | Backslashed characters are either replaced with corresponding | |
1646 | literal strings (as with C<\{>), or else they generate special nodes | |
1647 | in the finite automaton (as with C<\b>). Characters special to the | |
1648 | RE engine (such as C<|>) generate corresponding nodes or groups of | |
1649 | nodes. C<(?#...)> comments are ignored. All the rest is either | |
1650 | converted to literal strings to match, or else is ignored (as is | |
1651 | whitespace and C<#>-style comments if C<//x> is present). | |
1652 | ||
1653 | Parsing of the bracketed character class construct, C<[...]>, is | |
1654 | rather different than the rule used for the rest of the pattern. | |
1655 | The terminator of this construct is found using the same rules as | |
1656 | for finding the terminator of a C<{}>-delimited construct, the only | |
1657 | exception being that C<]> immediately following C<[> is treated as | |
1658 | though preceded by a backslash. Similarly, the terminator of | |
1659 | C<(?{...})> is found using the same rules as for finding the | |
1660 | terminator of a C<{}>-delimited construct. | |
1661 | ||
1662 | It is possible to inspect both the string given to RE engine and the | |
1663 | resulting finite automaton. See the arguments C<debug>/C<debugcolor> | |
1664 | in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line | |
4a4eefd0 | 1665 | switch documented in L<perlrun/"Command Switches">. |
75e14d17 IZ |
1666 | |
1667 | =item Optimization of regular expressions | |
1668 | ||
7522fed5 | 1669 | This step is listed for completeness only. Since it does not change |
75e14d17 | 1670 | semantics, details of this step are not documented and are subject |
19799a22 GS |
1671 | to change without notice. This step is performed over the finite |
1672 | automaton that was generated during the previous pass. | |
2a94b7ce | 1673 | |
19799a22 GS |
1674 | It is at this stage that C<split()> silently optimizes C</^/> to |
1675 | mean C</^/m>. | |
75e14d17 IZ |
1676 | |
1677 | =back | |
1678 | ||
a0d0e21e LW |
1679 | =head2 I/O Operators |
1680 | ||
54310121 | 1681 | There are several I/O operators you should know about. |
fbad3eb5 | 1682 | |
7b8d334a | 1683 | A string enclosed by backticks (grave accents) first undergoes |
19799a22 GS |
1684 | double-quote interpolation. It is then interpreted as an external |
1685 | command, and the output of that command is the value of the | |
e9c56f9b JH |
1686 | backtick string, like in a shell. In scalar context, a single string |
1687 | consisting of all output is returned. In list context, a list of | |
1688 | values is returned, one per line of output. (You can set C<$/> to use | |
1689 | a different line terminator.) The command is executed each time the | |
1690 | pseudo-literal is evaluated. The status value of the command is | |
1691 | returned in C<$?> (see L<perlvar> for the interpretation of C<$?>). | |
1692 | Unlike in B<csh>, no translation is done on the return data--newlines | |
1693 | remain newlines. Unlike in any of the shells, single quotes do not | |
1694 | hide variable names in the command from interpretation. To pass a | |
1695 | literal dollar-sign through to the shell you need to hide it with a | |
1696 | backslash. The generalized form of backticks is C<qx//>. (Because | |
1697 | backticks always undergo shell expansion as well, see L<perlsec> for | |
1698 | security concerns.) | |
19799a22 GS |
1699 | |
1700 | In scalar context, evaluating a filehandle in angle brackets yields | |
1701 | the next line from that file (the newline, if any, included), or | |
1702 | C<undef> at end-of-file or on error. When C<$/> is set to C<undef> | |
1703 | (sometimes known as file-slurp mode) and the file is empty, it | |
1704 | returns C<''> the first time, followed by C<undef> subsequently. | |
1705 | ||
1706 | Ordinarily you must assign the returned value to a variable, but | |
1707 | there is one situation where an automatic assignment happens. If | |
1708 | and only if the input symbol is the only thing inside the conditional | |
1709 | of a C<while> statement (even if disguised as a C<for(;;)> loop), | |
1710 | the value is automatically assigned to the global variable $_, | |
1711 | destroying whatever was there previously. (This may seem like an | |
1712 | odd thing to you, but you'll use the construct in almost every Perl | |
17b829fa | 1713 | script you write.) The $_ variable is not implicitly localized. |
19799a22 GS |
1714 | You'll have to put a C<local $_;> before the loop if you want that |
1715 | to happen. | |
1716 | ||
1717 | The following lines are equivalent: | |
a0d0e21e | 1718 | |
748a9306 | 1719 | while (defined($_ = <STDIN>)) { print; } |
7b8d334a | 1720 | while ($_ = <STDIN>) { print; } |
a0d0e21e LW |
1721 | while (<STDIN>) { print; } |
1722 | for (;<STDIN>;) { print; } | |
748a9306 | 1723 | print while defined($_ = <STDIN>); |
7b8d334a | 1724 | print while ($_ = <STDIN>); |
a0d0e21e LW |
1725 | print while <STDIN>; |
1726 | ||
19799a22 | 1727 | This also behaves similarly, but avoids $_ : |
7b8d334a GS |
1728 | |
1729 | while (my $line = <STDIN>) { print $line } | |
1730 | ||
19799a22 GS |
1731 | In these loop constructs, the assigned value (whether assignment |
1732 | is automatic or explicit) is then tested to see whether it is | |
1733 | defined. The defined test avoids problems where line has a string | |
1734 | value that would be treated as false by Perl, for example a "" or | |
1735 | a "0" with no trailing newline. If you really mean for such values | |
1736 | to terminate the loop, they should be tested for explicitly: | |
7b8d334a GS |
1737 | |
1738 | while (($_ = <STDIN>) ne '0') { ... } | |
1739 | while (<STDIN>) { last unless $_; ... } | |
1740 | ||
35f2feb0 | 1741 | In other boolean contexts, C<< <I<filehandle>> >> without an |
9f1b1f2d GS |
1742 | explicit C<defined> test or comparison elicit a warning if the |
1743 | C<use warnings> pragma or the B<-w> | |
19799a22 | 1744 | command-line switch (the C<$^W> variable) is in effect. |
7b8d334a | 1745 | |
5f05dabc | 1746 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
19799a22 GS |
1747 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except |
1748 | in packages, where they would be interpreted as local identifiers | |
1749 | rather than global.) Additional filehandles may be created with | |
1750 | the open() function, amongst others. See L<perlopentut> and | |
1751 | L<perlfunc/open> for details on this. | |
a0d0e21e | 1752 | |
35f2feb0 | 1753 | If a <FILEHANDLE> is used in a context that is looking for |
19799a22 GS |
1754 | a list, a list comprising all input lines is returned, one line per |
1755 | list element. It's easy to grow to a rather large data space this | |
1756 | way, so use with care. | |
a0d0e21e | 1757 | |
35f2feb0 | 1758 | <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>. |
19799a22 | 1759 | See L<perlfunc/readline>. |
fbad3eb5 | 1760 | |
35f2feb0 GS |
1761 | The null filehandle <> is special: it can be used to emulate the |
1762 | behavior of B<sed> and B<awk>. Input from <> comes either from | |
a0d0e21e | 1763 | standard input, or from each file listed on the command line. Here's |
35f2feb0 | 1764 | how it works: the first time <> is evaluated, the @ARGV array is |
5a964f20 | 1765 | checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened |
a0d0e21e LW |
1766 | gives you standard input. The @ARGV array is then processed as a list |
1767 | of filenames. The loop | |
1768 | ||
1769 | while (<>) { | |
1770 | ... # code for each line | |
1771 | } | |
1772 | ||
1773 | is equivalent to the following Perl-like pseudo code: | |
1774 | ||
3e3baf6d | 1775 | unshift(@ARGV, '-') unless @ARGV; |
a0d0e21e LW |
1776 | while ($ARGV = shift) { |
1777 | open(ARGV, $ARGV); | |
1778 | while (<ARGV>) { | |
1779 | ... # code for each line | |
1780 | } | |
1781 | } | |
1782 | ||
19799a22 GS |
1783 | except that it isn't so cumbersome to say, and will actually work. |
1784 | It really does shift the @ARGV array and put the current filename | |
1785 | into the $ARGV variable. It also uses filehandle I<ARGV> | |
35f2feb0 | 1786 | internally--<> is just a synonym for <ARGV>, which |
19799a22 | 1787 | is magical. (The pseudo code above doesn't work because it treats |
35f2feb0 | 1788 | <ARGV> as non-magical.) |
a0d0e21e | 1789 | |
35f2feb0 | 1790 | You can modify @ARGV before the first <> as long as the array ends up |
a0d0e21e | 1791 | containing the list of filenames you really want. Line numbers (C<$.>) |
19799a22 GS |
1792 | continue as though the input were one big happy file. See the example |
1793 | in L<perlfunc/eof> for how to reset line numbers on each file. | |
5a964f20 TC |
1794 | |
1795 | If you want to set @ARGV to your own list of files, go right ahead. | |
1796 | This sets @ARGV to all plain text files if no @ARGV was given: | |
1797 | ||
1798 | @ARGV = grep { -f && -T } glob('*') unless @ARGV; | |
a0d0e21e | 1799 | |
5a964f20 TC |
1800 | You can even set them to pipe commands. For example, this automatically |
1801 | filters compressed arguments through B<gzip>: | |
1802 | ||
1803 | @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; | |
1804 | ||
1805 | If you want to pass switches into your script, you can use one of the | |
a0d0e21e LW |
1806 | Getopts modules or put a loop on the front like this: |
1807 | ||
1808 | while ($_ = $ARGV[0], /^-/) { | |
1809 | shift; | |
1810 | last if /^--$/; | |
1811 | if (/^-D(.*)/) { $debug = $1 } | |
1812 | if (/^-v/) { $verbose++ } | |
5a964f20 | 1813 | # ... # other switches |
a0d0e21e | 1814 | } |
5a964f20 | 1815 | |
a0d0e21e | 1816 | while (<>) { |
5a964f20 | 1817 | # ... # code for each line |
a0d0e21e LW |
1818 | } |
1819 | ||
35f2feb0 | 1820 | The <> symbol will return C<undef> for end-of-file only once. |
19799a22 GS |
1821 | If you call it again after this, it will assume you are processing another |
1822 | @ARGV list, and if you haven't set @ARGV, will read input from STDIN. | |
a0d0e21e | 1823 | |
b159ebd3 | 1824 | If what the angle brackets contain is a simple scalar variable (e.g., |
35f2feb0 | 1825 | <$foo>), then that variable contains the name of the |
19799a22 GS |
1826 | filehandle to input from, or its typeglob, or a reference to the |
1827 | same. For example: | |
cb1a09d0 AD |
1828 | |
1829 | $fh = \*STDIN; | |
1830 | $line = <$fh>; | |
a0d0e21e | 1831 | |
5a964f20 TC |
1832 | If what's within the angle brackets is neither a filehandle nor a simple |
1833 | scalar variable containing a filehandle name, typeglob, or typeglob | |
1834 | reference, it is interpreted as a filename pattern to be globbed, and | |
1835 | either a list of filenames or the next filename in the list is returned, | |
19799a22 | 1836 | depending on context. This distinction is determined on syntactic |
35f2feb0 GS |
1837 | grounds alone. That means C<< <$x> >> is always a readline() from |
1838 | an indirect handle, but C<< <$hash{key}> >> is always a glob(). | |
5a964f20 TC |
1839 | That's because $x is a simple scalar variable, but C<$hash{key}> is |
1840 | not--it's a hash element. | |
1841 | ||
1842 | One level of double-quote interpretation is done first, but you can't | |
35f2feb0 | 1843 | say C<< <$foo> >> because that's an indirect filehandle as explained |
5a964f20 TC |
1844 | in the previous paragraph. (In older versions of Perl, programmers |
1845 | would insert curly brackets to force interpretation as a filename glob: | |
35f2feb0 | 1846 | C<< <${foo}> >>. These days, it's considered cleaner to call the |
5a964f20 | 1847 | internal function directly as C<glob($foo)>, which is probably the right |
19799a22 | 1848 | way to have done it in the first place.) For example: |
a0d0e21e LW |
1849 | |
1850 | while (<*.c>) { | |
1851 | chmod 0644, $_; | |
1852 | } | |
1853 | ||
3a4b19e4 | 1854 | is roughly equivalent to: |
a0d0e21e LW |
1855 | |
1856 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); | |
1857 | while (<FOO>) { | |
5b3eff12 | 1858 | chomp; |
a0d0e21e LW |
1859 | chmod 0644, $_; |
1860 | } | |
1861 | ||
3a4b19e4 GS |
1862 | except that the globbing is actually done internally using the standard |
1863 | C<File::Glob> extension. Of course, the shortest way to do the above is: | |
a0d0e21e LW |
1864 | |
1865 | chmod 0644, <*.c>; | |
1866 | ||
19799a22 GS |
1867 | A (file)glob evaluates its (embedded) argument only when it is |
1868 | starting a new list. All values must be read before it will start | |
1869 | over. In list context, this isn't important because you automatically | |
1870 | get them all anyway. However, in scalar context the operator returns | |
069e01df | 1871 | the next value each time it's called, or C<undef> when the list has |
19799a22 GS |
1872 | run out. As with filehandle reads, an automatic C<defined> is |
1873 | generated when the glob occurs in the test part of a C<while>, | |
1874 | because legal glob returns (e.g. a file called F<0>) would otherwise | |
1875 | terminate the loop. Again, C<undef> is returned only once. So if | |
1876 | you're expecting a single value from a glob, it is much better to | |
1877 | say | |
4633a7c4 LW |
1878 | |
1879 | ($file) = <blurch*>; | |
1880 | ||
1881 | than | |
1882 | ||
1883 | $file = <blurch*>; | |
1884 | ||
1885 | because the latter will alternate between returning a filename and | |
19799a22 | 1886 | returning false. |
4633a7c4 | 1887 | |
b159ebd3 | 1888 | If you're trying to do variable interpolation, it's definitely better |
4633a7c4 | 1889 | to use the glob() function, because the older notation can cause people |
e37d713d | 1890 | to become confused with the indirect filehandle notation. |
4633a7c4 LW |
1891 | |
1892 | @files = glob("$dir/*.[ch]"); | |
1893 | @files = glob($files[$i]); | |
1894 | ||
a0d0e21e LW |
1895 | =head2 Constant Folding |
1896 | ||
1897 | Like C, Perl does a certain amount of expression evaluation at | |
19799a22 | 1898 | compile time whenever it determines that all arguments to an |
a0d0e21e LW |
1899 | operator are static and have no side effects. In particular, string |
1900 | concatenation happens at compile time between literals that don't do | |
19799a22 | 1901 | variable substitution. Backslash interpolation also happens at |
a0d0e21e LW |
1902 | compile time. You can say |
1903 | ||
1904 | 'Now is the time for all' . "\n" . | |
1905 | 'good men to come to.' | |
1906 | ||
54310121 | 1907 | and this all reduces to one string internally. Likewise, if |
a0d0e21e LW |
1908 | you say |
1909 | ||
1910 | foreach $file (@filenames) { | |
5a964f20 | 1911 | if (-s $file > 5 + 100 * 2**16) { } |
54310121 | 1912 | } |
a0d0e21e | 1913 | |
19799a22 GS |
1914 | the compiler will precompute the number which that expression |
1915 | represents so that the interpreter won't have to. | |
a0d0e21e | 1916 | |
2c268ad5 TP |
1917 | =head2 Bitwise String Operators |
1918 | ||
1919 | Bitstrings of any size may be manipulated by the bitwise operators | |
1920 | (C<~ | & ^>). | |
1921 | ||
19799a22 GS |
1922 | If the operands to a binary bitwise op are strings of different |
1923 | sizes, B<|> and B<^> ops act as though the shorter operand had | |
1924 | additional zero bits on the right, while the B<&> op acts as though | |
1925 | the longer operand were truncated to the length of the shorter. | |
1926 | The granularity for such extension or truncation is one or more | |
1927 | bytes. | |
2c268ad5 TP |
1928 | |
1929 | # ASCII-based examples | |
1930 | print "j p \n" ^ " a h"; # prints "JAPH\n" | |
1931 | print "JA" | " ph\n"; # prints "japh\n" | |
1932 | print "japh\nJunk" & '_____'; # prints "JAPH\n"; | |
1933 | print 'p N$' ^ " E<H\n"; # prints "Perl\n"; | |
1934 | ||
19799a22 | 1935 | If you are intending to manipulate bitstrings, be certain that |
2c268ad5 | 1936 | you're supplying bitstrings: If an operand is a number, that will imply |
19799a22 | 1937 | a B<numeric> bitwise operation. You may explicitly show which type of |
2c268ad5 TP |
1938 | operation you intend by using C<""> or C<0+>, as in the examples below. |
1939 | ||
1940 | $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF) | |
1941 | $foo = '150' | 105 ; # yields 255 | |
1942 | $foo = 150 | '105'; # yields 255 | |
1943 | $foo = '150' | '105'; # yields string '155' (under ASCII) | |
1944 | ||
1945 | $baz = 0+$foo & 0+$bar; # both ops explicitly numeric | |
1946 | $biz = "$foo" ^ "$bar"; # both ops explicitly stringy | |
a0d0e21e | 1947 | |
1ae175c8 GS |
1948 | See L<perlfunc/vec> for information on how to manipulate individual bits |
1949 | in a bit vector. | |
1950 | ||
55497cff | 1951 | =head2 Integer Arithmetic |
a0d0e21e | 1952 | |
19799a22 | 1953 | By default, Perl assumes that it must do most of its arithmetic in |
a0d0e21e LW |
1954 | floating point. But by saying |
1955 | ||
1956 | use integer; | |
1957 | ||
1958 | you may tell the compiler that it's okay to use integer operations | |
19799a22 GS |
1959 | (if it feels like it) from here to the end of the enclosing BLOCK. |
1960 | An inner BLOCK may countermand this by saying | |
a0d0e21e LW |
1961 | |
1962 | no integer; | |
1963 | ||
19799a22 GS |
1964 | which lasts until the end of that BLOCK. Note that this doesn't |
1965 | mean everything is only an integer, merely that Perl may use integer | |
1966 | operations if it is so inclined. For example, even under C<use | |
1967 | integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731> | |
1968 | or so. | |
1969 | ||
1970 | Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", | |
13a2d996 SP |
1971 | and ">>") always produce integral results. (But see also |
1972 | L<Bitwise String Operators>.) However, C<use integer> still has meaning for | |
19799a22 GS |
1973 | them. By default, their results are interpreted as unsigned integers, but |
1974 | if C<use integer> is in effect, their results are interpreted | |
1975 | as signed integers. For example, C<~0> usually evaluates to a large | |
1976 | integral value. However, C<use integer; ~0> is C<-1> on twos-complement | |
1977 | machines. | |
68dc0745 | 1978 | |
1979 | =head2 Floating-point Arithmetic | |
1980 | ||
1981 | While C<use integer> provides integer-only arithmetic, there is no | |
19799a22 GS |
1982 | analogous mechanism to provide automatic rounding or truncation to a |
1983 | certain number of decimal places. For rounding to a certain number | |
1984 | of digits, sprintf() or printf() is usually the easiest route. | |
1985 | See L<perlfaq4>. | |
68dc0745 | 1986 | |
5a964f20 TC |
1987 | Floating-point numbers are only approximations to what a mathematician |
1988 | would call real numbers. There are infinitely more reals than floats, | |
1989 | so some corners must be cut. For example: | |
1990 | ||
1991 | printf "%.20g\n", 123456789123456789; | |
1992 | # produces 123456789123456784 | |
1993 | ||
1994 | Testing for exact equality of floating-point equality or inequality is | |
1995 | not a good idea. Here's a (relatively expensive) work-around to compare | |
1996 | whether two floating-point numbers are equal to a particular number of | |
1997 | decimal places. See Knuth, volume II, for a more robust treatment of | |
1998 | this topic. | |
1999 | ||
2000 | sub fp_equal { | |
2001 | my ($X, $Y, $POINTS) = @_; | |
2002 | my ($tX, $tY); | |
2003 | $tX = sprintf("%.${POINTS}g", $X); | |
2004 | $tY = sprintf("%.${POINTS}g", $Y); | |
2005 | return $tX eq $tY; | |
2006 | } | |
2007 | ||
68dc0745 | 2008 | The POSIX module (part of the standard perl distribution) implements |
19799a22 GS |
2009 | ceil(), floor(), and other mathematical and trigonometric functions. |
2010 | The Math::Complex module (part of the standard perl distribution) | |
2011 | defines mathematical functions that work on both the reals and the | |
2012 | imaginary numbers. Math::Complex not as efficient as POSIX, but | |
68dc0745 | 2013 | POSIX can't work with complex numbers. |
2014 | ||
2015 | Rounding in financial applications can have serious implications, and | |
2016 | the rounding method used should be specified precisely. In these | |
2017 | cases, it probably pays not to trust whichever system rounding is | |
2018 | being used by Perl, but to instead implement the rounding function you | |
2019 | need yourself. | |
5a964f20 TC |
2020 | |
2021 | =head2 Bigger Numbers | |
2022 | ||
2023 | The standard Math::BigInt and Math::BigFloat modules provide | |
19799a22 | 2024 | variable-precision arithmetic and overloaded operators, although |
cd5c4fce | 2025 | they're currently pretty slow. At the cost of some space and |
19799a22 GS |
2026 | considerable speed, they avoid the normal pitfalls associated with |
2027 | limited-precision representations. | |
5a964f20 TC |
2028 | |
2029 | use Math::BigInt; | |
2030 | $x = Math::BigInt->new('123456789123456789'); | |
2031 | print $x * $x; | |
2032 | ||
2033 | # prints +15241578780673678515622620750190521 | |
19799a22 | 2034 | |
cd5c4fce T |
2035 | There are several modules that let you calculate with (bound only by |
2036 | memory and cpu-time) unlimited or fixed precision. There are also | |
2037 | some non-standard modules that provide faster implementations via | |
2038 | external C libraries. | |
2039 | ||
2040 | Here is a short, but incomplete summary: | |
2041 | ||
2042 | Math::Fraction big, unlimited fractions like 9973 / 12967 | |
2043 | Math::String treat string sequences like numbers | |
2044 | Math::FixedPrecision calculate with a fixed precision | |
2045 | Math::Currency for currency calculations | |
2046 | Bit::Vector manipulate bit vectors fast (uses C) | |
2047 | Math::BigIntFast Bit::Vector wrapper for big numbers | |
2048 | Math::Pari provides access to the Pari C library | |
2049 | Math::BigInteger uses an external C library | |
2050 | Math::Cephes uses external Cephes C library (no big numbers) | |
2051 | Math::Cephes::Fraction fractions via the Cephes library | |
2052 | Math::GMP another one using an external C library | |
2053 | ||
2054 | Choose wisely. | |
16070b82 GS |
2055 | |
2056 | =cut |