4 perlop - Perl operators and precedence
8 =head2 Operator Precedence and Associativity
9 X<operator, precedence> X<precedence> X<associativity>
11 Operator precedence and associativity work in Perl more or less like
12 they do in mathematics.
14 I<Operator precedence> means some operators are evaluated before
15 others. For example, in C<2 + 4 * 5>, the multiplication has higher
16 precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
17 22> and not C<6 * 5 == 30>.
19 I<Operator associativity> defines what happens if a sequence of the
20 same operators is used one after another: whether the evaluator will
21 evaluate the left operations first or the right. For example, in C<8
22 - 4 - 2>, subtraction is left associative so Perl evaluates the
23 expression left to right. C<8 - 4> is evaluated first making the
24 expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
26 Perl operators have the following associativity and precedence,
27 listed from highest precedence to lowest. Operators borrowed from
28 C keep the same precedence relationship with each other, even where
29 C's precedence is slightly screwy. (This makes learning Perl easier
30 for C folks.) With very few exceptions, these all operate on scalar
31 values only, not array values.
33 left terms and list operators (leftward)
37 right ! ~ \ and unary + and -
42 nonassoc named unary operators
43 nonassoc < > <= >= lt gt le ge
44 nonassoc == != <=> eq ne cmp ~~
53 nonassoc list operators (rightward)
58 In the following sections, these operators are covered in precedence order.
60 Many operators can be overloaded for objects. See L<overload>.
62 =head2 Terms and List Operators (Leftward)
63 X<list operator> X<operator, list> X<term>
65 A TERM has the highest precedence in Perl. They include variables,
66 quote and quote-like operators, any expression in parentheses,
67 and any function whose arguments are parenthesized. Actually, there
68 aren't really functions in this sense, just list operators and unary
69 operators behaving as functions because you put parentheses around
70 the arguments. These are all documented in L<perlfunc>.
72 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
73 is followed by a left parenthesis as the next token, the operator and
74 arguments within parentheses are taken to be of highest precedence,
75 just like a normal function call.
77 In the absence of parentheses, the precedence of list operators such as
78 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
79 whether you are looking at the left side or the right side of the operator.
82 @ary = (1, 3, sort 4, 2);
83 print @ary; # prints 1324
85 the commas on the right of the sort are evaluated before the sort,
86 but the commas on the left are evaluated after. In other words,
87 list operators tend to gobble up all arguments that follow, and
88 then act like a simple TERM with regard to the preceding expression.
89 Be careful with parentheses:
91 # These evaluate exit before doing the print:
92 print($foo, exit); # Obviously not what you want.
93 print $foo, exit; # Nor is this.
95 # These do the print before evaluating exit:
96 (print $foo), exit; # This is what you want.
97 print($foo), exit; # Or this.
98 print ($foo), exit; # Or even this.
102 print ($foo & 255) + 1, "\n";
104 probably doesn't do what you expect at first glance. The parentheses
105 enclose the argument list for C<print> which is evaluated (printing
106 the result of C<$foo & 255>). Then one is added to the return value
107 of C<print> (usually 1). The result is something like this:
109 1 + 1, "\n"; # Obviously not what you meant.
111 To do what you meant properly, you must write:
113 print(($foo & 255) + 1, "\n");
115 See L<Named Unary Operators> for more discussion of this.
117 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
118 well as subroutine and method calls, and the anonymous
119 constructors C<[]> and C<{}>.
121 See also L<Quote and Quote-like Operators> toward the end of this section,
122 as well as L</"I/O Operators">.
124 =head2 The Arrow Operator
125 X<arrow> X<dereference> X<< -> >>
127 "C<< -> >>" is an infix dereference operator, just as it is in C
128 and C++. If the right side is either a C<[...]>, C<{...}>, or a
129 C<(...)> subscript, then the left side must be either a hard or
130 symbolic reference to an array, a hash, or a subroutine respectively.
131 (Or technically speaking, a location capable of holding a hard
132 reference, if it's an array or hash reference being used for
133 assignment.) See L<perlreftut> and L<perlref>.
135 Otherwise, the right side is a method name or a simple scalar
136 variable containing either the method name or a subroutine reference,
137 and the left side must be either an object (a blessed reference)
138 or a class name (that is, a package name). See L<perlobj>.
140 =head2 Auto-increment and Auto-decrement
141 X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
143 "++" and "--" work as in C. That is, if placed before a variable,
144 they increment or decrement the variable by one before returning the
145 value, and if placed after, increment or decrement after returning the
149 print $i++; # prints 0
150 print ++$j; # prints 1
152 Note that just as in C, Perl doesn't define B<when> the variable is
153 incremented or decremented. You just know it will be done sometime
154 before or after the value is returned. This also means that modifying
155 a variable twice in the same statement will lead to undefined behavior.
156 Avoid statements like:
161 Perl will not guarantee what the result of the above statements is.
163 The auto-increment operator has a little extra builtin magic to it. If
164 you increment a variable that is numeric, or that has ever been used in
165 a numeric context, you get a normal increment. If, however, the
166 variable has been used in only string contexts since it was set, and
167 has a value that is not the empty string and matches the pattern
168 C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
169 character within its range, with carry:
171 print ++($foo = "99"); # prints "100"
172 print ++($foo = "a0"); # prints "a1"
173 print ++($foo = "Az"); # prints "Ba"
174 print ++($foo = "zz"); # prints "aaa"
176 C<undef> is always treated as numeric, and in particular is changed
177 to C<0> before incrementing (so that a post-increment of an undef value
178 will return C<0> rather than C<undef>).
180 The auto-decrement operator is not magical.
182 =head2 Exponentiation
183 X<**> X<exponentiation> X<power>
185 Binary "**" is the exponentiation operator. It binds even more
186 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
187 implemented using C's pow(3) function, which actually works on doubles
190 =head2 Symbolic Unary Operators
191 X<unary operator> X<operator, unary>
193 Unary "!" performs logical negation, that is, "not". See also C<not> for a lower
194 precedence version of this.
197 Unary "-" performs arithmetic negation if the operand is numeric,
198 including any string that looks like a number. If the operand is
199 an identifier, a string consisting of a minus sign concatenated
200 with the identifier is returned. Otherwise, if the string starts
201 with a plus or minus, a string starting with the opposite sign is
202 returned. One effect of these rules is that -bareword is equivalent
203 to the string "-bareword". If, however, the string begins with a
204 non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
205 the string to a numeric and the arithmetic negation is performed. If the
206 string cannot be cleanly converted to a numeric, Perl will give the warning
207 B<Argument "the string" isn't numeric in negation (-) at ...>.
208 X<-> X<negation, arithmetic>
210 Unary "~" performs bitwise negation, that is, 1's complement. For
211 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
212 L<Bitwise String Operators>.) Note that the width of the result is
213 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
214 bits wide on a 64-bit platform, so if you are expecting a certain bit
215 width, remember to use the "&" operator to mask off the excess bits.
216 X<~> X<negation, binary>
218 When complementing strings, if all characters have ordinal values under
219 256, then their complements will, also. But if they do not, all
220 characters will be in either 32- or 64-bit complements, depending on your
221 architecture. So for example, C<~"\x{3B1}"> is C<"\x{FFFF_FC4E}"> on
222 32-bit machines and C<"\x{FFFF_FFFF_FFFF_FC4E}"> on 64-bit machines.
224 Unary "+" has no effect whatsoever, even on strings. It is useful
225 syntactically for separating a function name from a parenthesized expression
226 that would otherwise be interpreted as the complete list of function
227 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
230 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
231 and L<perlref>. Do not confuse this behavior with the behavior of
232 backslash within a string, although both forms do convey the notion
233 of protecting the next thing from interpolation.
234 X<\> X<reference> X<backslash>
236 =head2 Binding Operators
237 X<binding> X<operator, binding> X<=~> X<!~>
239 Binary "=~" binds a scalar expression to a pattern match. Certain operations
240 search or modify the string $_ by default. This operator makes that kind
241 of operation work on some other string. The right argument is a search
242 pattern, substitution, or transliteration. The left argument is what is
243 supposed to be searched, substituted, or transliterated instead of the default
244 $_. When used in scalar context, the return value generally indicates the
245 success of the operation. The exceptions are substitution (s///)
246 and transliteration (y///) with the C</r> (non-destructive) option,
247 which cause the B<r>eturn value to be the result of the substitution.
248 Behavior in list context depends on the particular operator.
249 See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
250 examples using these operators.
252 If the right argument is an expression rather than a search pattern,
253 substitution, or transliteration, it is interpreted as a search pattern at run
254 time. Note that this means that its contents will be interpolated twice, so
258 is not ok, as the regex engine will end up trying to compile the
259 pattern C<\>, which it will consider a syntax error.
261 Binary "!~" is just like "=~" except the return value is negated in
264 Binary "!~" with a non-destructive substitution (s///r) or transliteration
265 (y///r) is a syntax error.
267 =head2 Multiplicative Operators
268 X<operator, multiplicative>
270 Binary "*" multiplies two numbers.
273 Binary "/" divides two numbers.
276 Binary "%" is the modulo operator, which computes the division
277 remainder of its first argument with respect to its second argument.
279 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
280 C<$a> minus the largest multiple of C<$b> less than or equal to
281 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
282 smallest multiple of C<$b> that is not less than C<$a> (that is, the
283 result will be less than or equal to zero). If the operands
284 C<$a> and C<$b> are floating point values and the absolute value of
285 C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
286 the integer portion of C<$a> and C<$b> will be used in the operation
287 (Note: here C<UV_MAX> means the maximum of the unsigned integer type).
288 If the absolute value of the right operand (C<abs($b)>) is greater than
289 or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
290 C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
291 integer that makes C<$r> have the same sign as the right operand
292 C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
293 and the absolute value less than that of C<$b>.
294 Note that when C<use integer> is in scope, "%" gives you direct access
295 to the modulo operator as implemented by your C compiler. This
296 operator is not as well defined for negative operands, but it will
298 X<%> X<remainder> X<modulo> X<mod>
300 Binary "x" is the repetition operator. In scalar context or if the left
301 operand is not enclosed in parentheses, it returns a string consisting
302 of the left operand repeated the number of times specified by the right
303 operand. In list context, if the left operand is enclosed in
304 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
305 If the right operand is zero or negative, it returns an empty string
306 or an empty list, depending on the context.
309 print '-' x 80; # print row of dashes
311 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
313 @ones = (1) x 80; # a list of 80 1's
314 @ones = (5) x @ones; # set all elements to 5
317 =head2 Additive Operators
318 X<operator, additive>
320 Binary C<+> returns the sum of two numbers.
323 Binary C<-> returns the difference of two numbers.
326 Binary C<.> concatenates two strings.
327 X<string, concatenation> X<concatenation>
328 X<cat> X<concat> X<concatenate> X<.>
330 =head2 Shift Operators
331 X<shift operator> X<operator, shift> X<<< << >>>
332 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
333 X<shl> X<shr> X<shift, right> X<shift, left>
335 Binary C<<< << >>> returns the value of its left argument shifted left by the
336 number of bits specified by the right argument. Arguments should be
337 integers. (See also L<Integer Arithmetic>.)
339 Binary C<<< >> >>> returns the value of its left argument shifted right by
340 the number of bits specified by the right argument. Arguments should
341 be integers. (See also L<Integer Arithmetic>.)
343 Note that both C<<< << >>> and C<<< >> >>> in Perl are implemented directly using
344 C<<< << >>> and C<<< >> >>> in C. If C<use integer> (see L<Integer Arithmetic>) is
345 in force then signed C integers are used, else unsigned C integers are
346 used. Either way, the implementation isn't going to generate results
347 larger than the size of the integer type Perl was built with (32 bits
350 The result of overflowing the range of the integers is undefined
351 because it is undefined also in C. In other words, using 32-bit
352 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
353 of bits is also undefined.
355 If you get tired of being subject to your platform's native integers,
356 the C<use bigint> pragma neatly sidesteps the issue altogether:
358 print 20 << 20; # 20971520
359 print 20 << 40; # 5120 on 32-bit machines,
360 # 21990232555520 on 64-bit machines
362 print 20 << 100; # 25353012004564588029934064107520
364 =head2 Named Unary Operators
365 X<operator, named unary>
367 The various named unary operators are treated as functions with one
368 argument, with optional parentheses.
370 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
371 is followed by a left parenthesis as the next token, the operator and
372 arguments within parentheses are taken to be of highest precedence,
373 just like a normal function call. For example,
374 because named unary operators are higher precedence than C<||>:
376 chdir $foo || die; # (chdir $foo) || die
377 chdir($foo) || die; # (chdir $foo) || die
378 chdir ($foo) || die; # (chdir $foo) || die
379 chdir +($foo) || die; # (chdir $foo) || die
381 but, because * is higher precedence than named operators:
383 chdir $foo * 20; # chdir ($foo * 20)
384 chdir($foo) * 20; # (chdir $foo) * 20
385 chdir ($foo) * 20; # (chdir $foo) * 20
386 chdir +($foo) * 20; # chdir ($foo * 20)
388 rand 10 * 20; # rand (10 * 20)
389 rand(10) * 20; # (rand 10) * 20
390 rand (10) * 20; # (rand 10) * 20
391 rand +(10) * 20; # rand (10 * 20)
393 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
394 treated like named unary operators, but they don't follow this functional
395 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
396 equivalent to C<-f "$file.bak">.
397 X<-X> X<filetest> X<operator, filetest>
399 See also L<"Terms and List Operators (Leftward)">.
401 =head2 Relational Operators
402 X<relational operator> X<operator, relational>
404 Perl operators that return true or false generally return values
405 that can be safely used as numbers. For example, the relational
406 operators in this section and the equality operators in the next
407 one return C<1> for true and a special version of the defined empty
408 string, C<"">, which counts as a zero but is exempt from warnings
409 about improper numeric conversions, just as C<"0 but true"> is.
411 Binary "<" returns true if the left argument is numerically less than
415 Binary ">" returns true if the left argument is numerically greater
416 than the right argument.
419 Binary "<=" returns true if the left argument is numerically less than
420 or equal to the right argument.
423 Binary ">=" returns true if the left argument is numerically greater
424 than or equal to the right argument.
427 Binary "lt" returns true if the left argument is stringwise less than
431 Binary "gt" returns true if the left argument is stringwise greater
432 than the right argument.
435 Binary "le" returns true if the left argument is stringwise less than
436 or equal to the right argument.
439 Binary "ge" returns true if the left argument is stringwise greater
440 than or equal to the right argument.
443 =head2 Equality Operators
444 X<equality> X<equal> X<equals> X<operator, equality>
446 Binary "==" returns true if the left argument is numerically equal to
450 Binary "!=" returns true if the left argument is numerically not equal
451 to the right argument.
454 Binary "<=>" returns -1, 0, or 1 depending on whether the left
455 argument is numerically less than, equal to, or greater than the right
456 argument. If your platform supports NaNs (not-a-numbers) as numeric
457 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
458 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
459 returns true, as does NaN != anything else. If your platform doesn't
460 support NaNs then NaN is just a string with numeric value 0.
461 X<< <=> >> X<spaceship>
463 $ perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
464 $ perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
466 (Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all
469 Binary "eq" returns true if the left argument is stringwise equal to
473 Binary "ne" returns true if the left argument is stringwise not equal
474 to the right argument.
477 Binary "cmp" returns -1, 0, or 1 depending on whether the left
478 argument is stringwise less than, equal to, or greater than the right
482 Binary "~~" does a smartmatch between its arguments. Smart matching
483 is described in the next section.
486 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
487 by the current locale if a legacy C<use locale> (but not
488 C<use locale ':not_characters'>) is in effect. See
489 L<perllocale>. Do not mix these with Unicode, only with legacy binary
490 encodings. The standard L<Unicode::Collate> and
491 L<Unicode::Collate::Locale> modules offer much more powerful solutions to
494 =head2 Smartmatch Operator
496 First available in Perl 5.10.1 (the 5.10.0 version behaved differently),
497 binary C<~~> does a "smartmatch" between its arguments. This is mostly
498 used implicitly in the C<when> construct described in L<perlsyn>, although
499 not all C<when> clauses call the smartmatch operator. Unique among all of
500 Perl's operators, the smartmatch operator can recurse.
502 It is also unique in that all other Perl operators impose a context
503 (usually string or numeric context) on their operands, autoconverting
504 those operands to those imposed contexts. In contrast, smartmatch
505 I<infers> contexts from the actual types of its operands and uses that
506 type information to select a suitable comparison mechanism.
508 The C<~~> operator compares its operands "polymorphically", determining how
509 to compare them according to their actual types (numeric, string, array,
510 hash, etc.) Like the equality operators with which it shares the same
511 precedence, C<~~> returns 1 for true and C<""> for false. It is often best
512 read aloud as "in", "inside of", or "is contained in", because the left
513 operand is often looked for I<inside> the right operand. That makes the
514 order of the operands to the smartmatch operand often opposite that of
515 the regular match operator. In other words, the "smaller" thing is usually
516 placed in the left operand and the larger one in the right.
518 The behavior of a smartmatch depends on what type of things its arguments
519 are, as determined by the following table. The first row of the table
520 whose types apply determines the smartmatch behavior. Because what
521 actually happens is mostly determined by the type of the second operand,
522 the table is sorted on the right operand instead of on the left.
524 Left Right Description and pseudocode
525 ===============================================================
526 Any undef check whether Any is undefined
529 Any Object invoke ~~ overloading on Object, or die
531 Right operand is an ARRAY:
533 Left Right Description and pseudocode
534 ===============================================================
535 ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2]
536 like: (ARRAY1[0] ~~ ARRAY2[0])
537 && (ARRAY1[1] ~~ ARRAY2[1]) && ...
538 HASH ARRAY any ARRAY elements exist as HASH keys
539 like: grep { exists HASH->{$_} } ARRAY
540 Regexp ARRAY any ARRAY elements pattern match Regexp
541 like: grep { /Regexp/ } ARRAY
542 undef ARRAY undef in ARRAY
543 like: grep { !defined } ARRAY
544 Any ARRAY smartmatch each ARRAY element[3]
545 like: grep { Any ~~ $_ } ARRAY
547 Right operand is a HASH:
549 Left Right Description and pseudocode
550 ===============================================================
551 HASH1 HASH2 all same keys in both HASHes
553 grep { exists HASH2->{$_} } keys HASH1
554 ARRAY HASH any ARRAY elements exist as HASH keys
555 like: grep { exists HASH->{$_} } ARRAY
556 Regexp HASH any HASH keys pattern match Regexp
557 like: grep { /Regexp/ } keys HASH
558 undef HASH always false (undef can't be a key)
560 Any HASH HASH key existence
561 like: exists HASH->{Any}
563 Right operand is CODE:
565 Left Right Description and pseudocode
566 ===============================================================
567 ARRAY CODE sub returns true on all ARRAY elements[1]
568 like: !grep { !CODE->($_) } ARRAY
569 HASH CODE sub returns true on all HASH keys[1]
570 like: !grep { !CODE->($_) } keys HASH
571 Any CODE sub passed Any returns true
574 Right operand is a Regexp:
576 Left Right Description and pseudocode
577 ===============================================================
578 ARRAY Regexp any ARRAY elements match Regexp
579 like: grep { /Regexp/ } ARRAY
580 HASH Regexp any HASH keys match Regexp
581 like: grep { /Regexp/ } keys HASH
582 Any Regexp pattern match
583 like: Any =~ /Regexp/
587 Left Right Description and pseudocode
588 ===============================================================
589 Object Any invoke ~~ overloading on Object,
592 Any Num numeric equality
594 Num nummy[4] numeric equality
596 undef Any check whether undefined
598 Any Any string equality
607 Empty hashes or arrays match.
610 That is, each element smartmatches the element of the same index in the other array.[3]
613 If a circular reference is found, fall back to referential equality.
616 Either an actual number, or a string that looks like one.
620 The smartmatch implicitly dereferences any non-blessed hash or array
621 reference, so the C<I<HASH>> and C<I<ARRAY>> entries apply in those cases.
622 For blessed references, the C<I<Object>> entries apply. Smartmatches
623 involving hashes only consider hash keys, never hash values.
625 The "like" code entry is not always an exact rendition. For example, the
626 smartmatch operator short-circuits whenever possible, but C<grep> does
627 not. Also, C<grep> in scalar context returns the number of matches, but
628 C<~~> returns only true or false.
630 Unlike most operators, the smartmatch operator knows to treat C<undef>
634 @array = (1, 2, 3, undef, 4, 5);
635 say "some elements undefined" if undef ~~ @array;
637 Each operand is considered in a modified scalar context, the modification
638 being that array and hash variables are passed by reference to the
639 operator, which implicitly dereferences them. Both elements
640 of each pair are the same:
644 my %hash = (red => 1, blue => 2, green => 3,
645 orange => 4, yellow => 5, purple => 6,
646 black => 7, grey => 8, white => 9);
648 my @array = qw(red blue green);
650 say "some array elements in hash keys" if @array ~~ %hash;
651 say "some array elements in hash keys" if \@array ~~ \%hash;
653 say "red in array" if "red" ~~ @array;
654 say "red in array" if "red" ~~ \@array;
656 say "some keys end in e" if /e$/ ~~ %hash;
657 say "some keys end in e" if /e$/ ~~ \%hash;
659 Two arrays smartmatch if each element in the first array smartmatches
660 (that is, is "in") the corresponding element in the second array,
664 my @little = qw(red blue green);
665 my @bigger = ("red", "blue", [ "orange", "green" ] );
666 if (@little ~~ @bigger) { # true!
667 say "little is contained in bigger";
670 Because the smartmatch operator recurses on nested arrays, this
671 will still report that "red" is in the array.
674 my @array = qw(red blue green);
675 my $nested_array = [[[[[[[ @array ]]]]]]];
676 say "red in array" if "red" ~~ $nested_array;
678 If two arrays smartmatch each other, then they are deep
679 copies of each others' values, as this example reports:
682 my @a = (0, 1, 2, [3, [4, 5], 6], 7);
683 my @b = (0, 1, 2, [3, [4, 5], 6], 7);
685 if (@a ~~ @b && @b ~~ @a) {
686 say "a and b are deep copies of each other";
689 say "a smartmatches in b";
692 say "b smartmatches in a";
695 say "a and b don't smartmatch each other at all";
699 If you were to set C<$b[3] = 4>, then instead of reporting that "a and b
700 are deep copies of each other", it now reports that "b smartmatches in a".
701 That because the corresponding position in C<@a> contains an array that
702 (eventually) has a 4 in it.
704 Smartmatching one hash against another reports whether both contain the
705 same keys, no more and no less. This could be used to see whether two
706 records have the same field names, without caring what values those fields
707 might have. For example:
711 state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
713 my ($class, $init_fields) = @_;
715 die "Must supply (only) name, rank, and serial number"
716 unless $init_fields ~~ $REQUIRED_FIELDS;
721 or, if other non-required fields are allowed, use ARRAY ~~ HASH:
725 state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
727 my ($class, $init_fields) = @_;
729 die "Must supply (at least) name, rank, and serial number"
730 unless [keys %{$init_fields}] ~~ $REQUIRED_FIELDS;
735 The smartmatch operator is most often used as the implicit operator of a
736 C<when> clause. See the section on "Switch Statements" in L<perlsyn>.
738 =head3 Smartmatching of Objects
740 To avoid relying on an object's underlying representation, if the
741 smartmatch's right operand is an object that doesn't overload C<~~>,
742 it raises the exception "C<Smartmatching a non-overloaded object
743 breaks encapsulation>". That's because one has no business digging
744 around to see whether something is "in" an object. These are all
745 illegal on objects without a C<~~> overload:
751 However, you can change the way an object is smartmatched by overloading
752 the C<~~> operator. This is allowed to extend the usual smartmatch semantics.
753 For objects that do have an C<~~> overload, see L<overload>.
755 Using an object as the left operand is allowed, although not very useful.
756 Smartmatching rules take precedence over overloading, so even if the
757 object in the left operand has smartmatch overloading, this will be
758 ignored. A left operand that is a non-overloaded object falls back on a
759 string or numeric comparison of whatever the C<ref> operator returns. That
764 does I<not> invoke the overload method with C<I<X>> as an argument.
765 Instead the above table is consulted as normal, and based on the type of
766 C<I<X>>, overloading may or may not be invoked. For simple strings or
767 numbers, in becomes equivalent to this:
769 $object ~~ $number ref($object) == $number
770 $object ~~ $string ref($object) eq $string
772 For example, this reports that the handle smells IOish
773 (but please don't really do this!):
776 my $fh = IO::Handle->new();
777 if ($fh ~~ /\bIO\b/) {
778 say "handle smells IOish";
781 That's because it treats C<$fh> as a string like
782 C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that.
785 X<operator, bitwise, and> X<bitwise and> X<&>
787 Binary "&" returns its operands ANDed together bit by bit.
788 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
790 Note that "&" has lower priority than relational operators, so for example
791 the parentheses are essential in a test like
793 print "Even\n" if ($x & 1) == 0;
795 =head2 Bitwise Or and Exclusive Or
796 X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
799 Binary "|" returns its operands ORed together bit by bit.
800 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
802 Binary "^" returns its operands XORed together bit by bit.
803 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
805 Note that "|" and "^" have lower priority than relational operators, so
806 for example the brackets are essential in a test like
808 print "false\n" if (8 | 2) != 10;
810 =head2 C-style Logical And
811 X<&&> X<logical and> X<operator, logical, and>
813 Binary "&&" performs a short-circuit logical AND operation. That is,
814 if the left operand is false, the right operand is not even evaluated.
815 Scalar or list context propagates down to the right operand if it
818 =head2 C-style Logical Or
819 X<||> X<operator, logical, or>
821 Binary "||" performs a short-circuit logical OR operation. That is,
822 if the left operand is true, the right operand is not even evaluated.
823 Scalar or list context propagates down to the right operand if it
826 =head2 Logical Defined-Or
827 X<//> X<operator, logical, defined-or>
829 Although it has no direct equivalent in C, Perl's C<//> operator is related
830 to its C-style or. In fact, it's exactly the same as C<||>, except that it
831 tests the left hand side's definedness instead of its truth. Thus,
832 C<< EXPR1 // EXPR2 >> returns the value of C<< EXPR1 >> if it's defined,
833 otherwise, the value of C<< EXPR2 >> is returned. (C<< EXPR1 >> is evaluated
834 in scalar context, C<< EXPR2 >> in the context of C<< // >> itself). Usually,
835 this is the same result as C<< defined(EXPR1) ? EXPR1 : EXPR2 >> (except that
836 the ternary-operator form can be used as a lvalue, while C<< EXPR1 // EXPR2 >>
837 cannot). This is very useful for
838 providing default values for variables. If you actually want to test if
839 at least one of C<$a> and C<$b> is defined, use C<defined($a // $b)>.
841 The C<||>, C<//> and C<&&> operators return the last value evaluated
842 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
843 portable way to find out the home directory might be:
848 // die "You're homeless!\n";
850 In particular, this means that you shouldn't use this
851 for selecting between two aggregates for assignment:
853 @a = @b || @c; # this is wrong
854 @a = scalar(@b) || @c; # really meant this
855 @a = @b ? @b : @c; # this works fine, though
857 As alternatives to C<&&> and C<||> when used for
858 control flow, Perl provides the C<and> and C<or> operators (see below).
859 The short-circuit behavior is identical. The precedence of "and"
860 and "or" is much lower, however, so that you can safely use them after a
861 list operator without the need for parentheses:
863 unlink "alpha", "beta", "gamma"
864 or gripe(), next LINE;
866 With the C-style operators that would have been written like this:
868 unlink("alpha", "beta", "gamma")
869 || (gripe(), next LINE);
871 It would be even more readable to write that this way:
873 unless(unlink("alpha", "beta", "gamma")) {
878 Using "or" for assignment is unlikely to do what you want; see below.
880 =head2 Range Operators
881 X<operator, range> X<range> X<..> X<...>
883 Binary ".." is the range operator, which is really two different
884 operators depending on the context. In list context, it returns a
885 list of values counting (up by ones) from the left value to the right
886 value. If the left value is greater than the right value then it
887 returns the empty list. The range operator is useful for writing
888 C<foreach (1..10)> loops and for doing slice operations on arrays. In
889 the current implementation, no temporary array is created when the
890 range operator is used as the expression in C<foreach> loops, but older
891 versions of Perl might burn a lot of memory when you write something
894 for (1 .. 1_000_000) {
898 The range operator also works on strings, using the magical
899 auto-increment, see below.
901 In scalar context, ".." returns a boolean value. The operator is
902 bistable, like a flip-flop, and emulates the line-range (comma)
903 operator of B<sed>, B<awk>, and various editors. Each ".." operator
904 maintains its own boolean state, even across calls to a subroutine
905 that contains it. It is false as long as its left operand is false.
906 Once the left operand is true, the range operator stays true until the
907 right operand is true, I<AFTER> which the range operator becomes false
908 again. It doesn't become false till the next time the range operator
909 is evaluated. It can test the right operand and become false on the
910 same evaluation it became true (as in B<awk>), but it still returns
911 true once. If you don't want it to test the right operand until the
912 next evaluation, as in B<sed>, just use three dots ("...") instead of
913 two. In all other regards, "..." behaves just like ".." does.
915 The right operand is not evaluated while the operator is in the
916 "false" state, and the left operand is not evaluated while the
917 operator is in the "true" state. The precedence is a little lower
918 than || and &&. The value returned is either the empty string for
919 false, or a sequence number (beginning with 1) for true. The sequence
920 number is reset for each range encountered. The final sequence number
921 in a range has the string "E0" appended to it, which doesn't affect
922 its numeric value, but gives you something to search for if you want
923 to exclude the endpoint. You can exclude the beginning point by
924 waiting for the sequence number to be greater than 1.
926 If either operand of scalar ".." is a constant expression,
927 that operand is considered true if it is equal (C<==>) to the current
928 input line number (the C<$.> variable).
930 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
931 but that is only an issue if you use a floating point expression; when
932 implicitly using C<$.> as described in the previous paragraph, the
933 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
934 is set to a floating point value and you are not reading from a file.
935 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
936 you want in scalar context because each of the operands are evaluated
937 using their integer representation.
941 As a scalar operator:
943 if (101 .. 200) { print; } # print 2nd hundred lines, short for
944 # if ($. == 101 .. $. == 200) { print; }
946 next LINE if (1 .. /^$/); # skip header lines, short for
947 # next LINE if ($. == 1 .. /^$/);
948 # (typically in a loop labeled LINE)
950 s/^/> / if (/^$/ .. eof()); # quote body
952 # parse mail messages
954 $in_header = 1 .. /^$/;
955 $in_body = /^$/ .. eof;
962 close ARGV if eof; # reset $. each file
965 Here's a simple example to illustrate the difference between
966 the two range operators:
979 This program will print only the line containing "Bar". If
980 the range operator is changed to C<...>, it will also print the
983 And now some examples as a list operator:
985 for (101 .. 200) { print } # print $_ 100 times
986 @foo = @foo[0 .. $#foo]; # an expensive no-op
987 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
989 The range operator (in list context) makes use of the magical
990 auto-increment algorithm if the operands are strings. You
993 @alphabet = ("A" .. "Z");
995 to get all normal letters of the English alphabet, or
997 $hexdigit = (0 .. 9, "a" .. "f")[$num & 15];
999 to get a hexadecimal digit, or
1001 @z2 = ("01" .. "31");
1004 to get dates with leading zeros.
1006 If the final value specified is not in the sequence that the magical
1007 increment would produce, the sequence goes until the next value would
1008 be longer than the final value specified.
1010 If the initial value specified isn't part of a magical increment
1011 sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>),
1012 only the initial value will be returned. So the following will only
1015 use charnames "greek";
1016 my @greek_small = ("\N{alpha}" .. "\N{omega}");
1018 To get the 25 traditional lowercase Greek letters, including both sigmas,
1019 you could use this instead:
1021 use charnames "greek";
1022 my @greek_small = map { chr } ( ord("\N{alpha}")
1027 However, because there are I<many> other lowercase Greek characters than
1028 just those, to match lowercase Greek characters in a regular expression,
1029 you would use the pattern C</(?:(?=\p{Greek})\p{Lower})+/>.
1031 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
1032 return two elements in list context.
1034 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
1036 =head2 Conditional Operator
1037 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
1039 Ternary "?:" is the conditional operator, just as in C. It works much
1040 like an if-then-else. If the argument before the ? is true, the
1041 argument before the : is returned, otherwise the argument after the :
1042 is returned. For example:
1044 printf "I have %d dog%s.\n", $n,
1045 ($n == 1) ? "" : "s";
1047 Scalar or list context propagates downward into the 2nd
1048 or 3rd argument, whichever is selected.
1050 $a = $ok ? $b : $c; # get a scalar
1051 @a = $ok ? @b : @c; # get an array
1052 $a = $ok ? @b : @c; # oops, that's just a count!
1054 The operator may be assigned to if both the 2nd and 3rd arguments are
1055 legal lvalues (meaning that you can assign to them):
1057 ($a_or_b ? $a : $b) = $c;
1059 Because this operator produces an assignable result, using assignments
1060 without parentheses will get you in trouble. For example, this:
1062 $a % 2 ? $a += 10 : $a += 2
1066 (($a % 2) ? ($a += 10) : $a) += 2
1070 ($a % 2) ? ($a += 10) : ($a += 2)
1072 That should probably be written more simply as:
1074 $a += ($a % 2) ? 10 : 2;
1076 =head2 Assignment Operators
1077 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
1078 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
1081 "=" is the ordinary assignment operator.
1083 Assignment operators work as in C. That is,
1091 although without duplicating any side effects that dereferencing the lvalue
1092 might trigger, such as from tie(). Other assignment operators work similarly.
1093 The following are recognized:
1095 **= += *= &= <<= &&=
1100 Although these are grouped by family, they all have the precedence
1103 Unlike in C, the scalar assignment operator produces a valid lvalue.
1104 Modifying an assignment is equivalent to doing the assignment and
1105 then modifying the variable that was assigned to. This is useful
1106 for modifying a copy of something, like this:
1108 ($tmp = $global) =~ tr/13579/24680/;
1110 Although as of 5.14, that can be also be accomplished this way:
1113 $tmp = ($global =~ tr/13579/24680/r);
1124 Similarly, a list assignment in list context produces the list of
1125 lvalues assigned to, and a list assignment in scalar context returns
1126 the number of elements produced by the expression on the right hand
1127 side of the assignment.
1129 =head2 Comma Operator
1130 X<comma> X<operator, comma> X<,>
1132 Binary "," is the comma operator. In scalar context it evaluates
1133 its left argument, throws that value away, then evaluates its right
1134 argument and returns that value. This is just like C's comma operator.
1136 In list context, it's just the list argument separator, and inserts
1137 both its arguments into the list. These arguments are also evaluated
1140 The C<< => >> operator is a synonym for the comma except that it causes a
1141 word on its left to be interpreted as a string if it begins with a letter
1142 or underscore and is composed only of letters, digits and underscores.
1143 This includes operands that might otherwise be interpreted as operators,
1144 constants, single number v-strings or function calls. If in doubt about
1145 this behavior, the left operand can be quoted explicitly.
1147 Otherwise, the C<< => >> operator behaves exactly as the comma operator
1148 or list argument separator, according to context.
1152 use constant FOO => "something";
1154 my %h = ( FOO => 23 );
1158 my %h = ("FOO", 23);
1162 my %h = ("something", 23);
1164 The C<< => >> operator is helpful in documenting the correspondence
1165 between keys and values in hashes, and other paired elements in lists.
1167 %hash = ( $key => $value );
1168 login( $username => $password );
1170 The special quoting behavior ignores precedence, and hence may apply to
1171 I<part> of the left operand:
1173 print time.shift => "bbb";
1175 That example prints something like "1314363215shiftbbb", because the
1176 C<< => >> implicitly quotes the C<shift> immediately on its left, ignoring
1177 the fact that C<time.shift> is the entire left operand.
1179 =head2 List Operators (Rightward)
1180 X<operator, list, rightward> X<list operator>
1182 On the right side of a list operator, the comma has very low precedence,
1183 such that it controls all comma-separated expressions found there.
1184 The only operators with lower precedence are the logical operators
1185 "and", "or", and "not", which may be used to evaluate calls to list
1186 operators without the need for parentheses:
1188 open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n";
1190 However, some people find that code harder to read than writing
1191 it with parentheses:
1193 open(HANDLE, "< :utf8", "filename") or die "Can't open: $!\n";
1195 in which case you might as well just use the more customary "||" operator:
1197 open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n";
1199 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
1202 X<operator, logical, not> X<not>
1204 Unary "not" returns the logical negation of the expression to its right.
1205 It's the equivalent of "!" except for the very low precedence.
1208 X<operator, logical, and> X<and>
1210 Binary "and" returns the logical conjunction of the two surrounding
1211 expressions. It's equivalent to C<&&> except for the very low
1212 precedence. This means that it short-circuits: the right
1213 expression is evaluated only if the left expression is true.
1215 =head2 Logical or and Exclusive Or
1216 X<operator, logical, or> X<operator, logical, xor>
1217 X<operator, logical, exclusive or>
1220 Binary "or" returns the logical disjunction of the two surrounding
1221 expressions. It's equivalent to C<||> except for the very low precedence.
1222 This makes it useful for control flow:
1224 print FH $data or die "Can't write to FH: $!";
1226 This means that it short-circuits: the right expression is evaluated
1227 only if the left expression is false. Due to its precedence, you must
1228 be careful to avoid using it as replacement for the C<||> operator.
1229 It usually works out better for flow control than in assignments:
1231 $a = $b or $c; # bug: this is wrong
1232 ($a = $b) or $c; # really means this
1233 $a = $b || $c; # better written this way
1235 However, when it's a list-context assignment and you're trying to use
1236 C<||> for control flow, you probably need "or" so that the assignment
1237 takes higher precedence.
1239 @info = stat($file) || die; # oops, scalar sense of stat!
1240 @info = stat($file) or die; # better, now @info gets its due
1242 Then again, you could always use parentheses.
1244 Binary C<xor> returns the exclusive-OR of the two surrounding expressions.
1245 It cannot short-circuit (of course).
1247 There is no low precedence operator for defined-OR.
1249 =head2 C Operators Missing From Perl
1250 X<operator, missing from perl> X<&> X<*>
1251 X<typecasting> X<(TYPE)>
1253 Here is what C has that Perl doesn't:
1259 Address-of operator. (But see the "\" operator for taking a reference.)
1263 Dereference-address operator. (Perl's prefix dereferencing
1264 operators are typed: $, @, %, and &.)
1268 Type-casting operator.
1272 =head2 Quote and Quote-like Operators
1273 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
1274 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
1275 X<escape sequence> X<escape>
1277 While we usually think of quotes as literal values, in Perl they
1278 function as operators, providing various kinds of interpolating and
1279 pattern matching capabilities. Perl provides customary quote characters
1280 for these behaviors, but also provides a way for you to choose your
1281 quote character for any of them. In the following table, a C<{}> represents
1282 any pair of delimiters you choose.
1284 Customary Generic Meaning Interpolates
1287 `` qx{} Command yes*
1289 // m{} Pattern match yes*
1291 s{}{} Substitution yes*
1292 tr{}{} Transliteration no (but see below)
1293 y{}{} Transliteration no (but see below)
1296 * unless the delimiter is ''.
1298 Non-bracketing delimiters use the same character fore and aft, but the four
1299 sorts of ASCII brackets (round, angle, square, curly) all nest, which means
1308 Note, however, that this does not always work for quoting Perl code:
1310 $s = q{ if($a eq "}") ... }; # WRONG
1312 is a syntax error. The C<Text::Balanced> module (standard as of v5.8,
1313 and from CPAN before then) is able to do this properly.
1315 There can be whitespace between the operator and the quoting
1316 characters, except when C<#> is being used as the quoting character.
1317 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
1318 operator C<q> followed by a comment. Its argument will be taken
1319 from the next line. This allows you to write:
1321 s {foo} # Replace foo
1324 The following escape sequences are available in constructs that interpolate,
1325 and in transliterations:
1326 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
1329 Sequence Note Description
1335 \a alarm (bell) (BEL)
1337 \x{263A} [1,8] hex char (example: SMILEY)
1338 \x1b [2,8] restricted range hex char (example: ESC)
1339 \N{name} [3] named Unicode character or character sequence
1340 \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
1341 \c[ [5] control char (example: chr(27))
1342 \o{23072} [6,8] octal char (example: SMILEY)
1343 \033 [7,8] restricted range octal char (example: ESC)
1349 The result is the character specified by the hexadecimal number between
1350 the braces. See L</[8]> below for details on which character.
1352 Only hexadecimal digits are valid between the braces. If an invalid
1353 character is encountered, a warning will be issued and the invalid
1354 character and all subsequent characters (valid or invalid) within the
1355 braces will be discarded.
1357 If there are no valid digits between the braces, the generated character is
1358 the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
1359 will not cause a warning (currently).
1363 The result is the character specified by the hexadecimal number in the range
1364 0x00 to 0xFF. See L</[8]> below for details on which character.
1366 Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
1367 by fewer than two valid digits, any valid digits will be zero-padded. This
1368 means that C<\x7> will be interpreted as C<\x07>, and a lone <\x> will be
1369 interpreted as C<\x00>. Except at the end of a string, having fewer than
1370 two valid digits will result in a warning. Note that although the warning
1371 says the illegal character is ignored, it is only ignored as part of the
1372 escape and will still be used as the subsequent character in the string.
1375 Original Result Warns?
1383 The result is the Unicode character or character sequence given by I<name>.
1388 C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code
1389 point is I<hexadecimal number>.
1393 The character following C<\c> is mapped to some other character as shown in the
1410 In other words, it's the character whose code point has had 64 xor'd with
1411 its uppercase. C<\c?> is DELETE because C<ord("@") ^ 64> is 127, and
1412 C<\c@> is NULL because the ord of "@" is 64, so xor'ing 64 itself produces 0.
1414 Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the
1415 end of a string, because the backslash would be parsed as escaping the end
1418 On ASCII platforms, the resulting characters from the list above are the
1419 complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
1420 L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these
1421 sequences mean on both ASCII and EBCDIC platforms.
1423 Use of any other character following the "c" besides those listed above is
1424 discouraged, and some are deprecated with the intention of removing
1425 those in a later Perl version. What happens for any of these
1426 other characters currently though, is that the value is derived by xor'ing
1427 with the seventh bit, which is 64.
1429 To get platform independent controls, you can use C<\N{...}>.
1433 The result is the character specified by the octal number between the braces.
1434 See L</[8]> below for details on which character.
1436 If a character that isn't an octal digit is encountered, a warning is raised,
1437 and the value is based on the octal digits before it, discarding it and all
1438 following characters up to the closing brace. It is a fatal error if there are
1439 no octal digits at all.
1443 The result is the character specified by the three-digit octal number in the
1444 range 000 to 777 (but best to not use above 077, see next paragraph). See
1445 L</[8]> below for details on which character.
1447 Some contexts allow 2 or even 1 digit, but any usage without exactly
1448 three digits, the first being a zero, may give unintended results. (For
1449 example, in a regular expression it may be confused with a backreference;
1450 see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
1451 use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to
1452 use this construct only for ordinals C<\077> and below, remembering to pad to
1453 the left with zeros to make three digits. For larger ordinals, either use
1454 C<\o{}>, or convert to something else, such as to hex and use C<\x{}>
1457 Having fewer than 3 digits may lead to a misleading warning message that says
1458 that what follows is ignored. For example, C<"\128"> in the ASCII character set
1459 is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal
1460 digit '8' ignored> will be thrown. If C<"\n8"> is what you want, you can
1461 avoid this warning by padding your octal number with C<0>'s: C<"\0128">.
1465 Several constructs above specify a character by a number. That number
1466 gives the character's position in the character set encoding (indexed from 0).
1467 This is called synonymously its ordinal, code position, or code point. Perl
1468 works on platforms that have a native encoding currently of either ASCII/Latin1
1469 or EBCDIC, each of which allow specification of 256 characters. In general, if
1470 the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
1471 native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
1472 it as a Unicode code point and the result is the corresponding Unicode
1473 character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
1474 decimal, which is less than 256, so the number is interpreted in the native
1475 character set encoding. In ASCII the character in the 80th position (indexed
1476 from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".
1477 C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
1478 as a Unicode code point no matter what the native encoding is. The name of the
1479 character in the 256th position (indexed by 0) in Unicode is
1480 C<LATIN CAPITAL LETTER A WITH MACRON>.
1482 There are a couple of exceptions to the above rule. S<C<\N{U+I<hex number>}>> is
1483 always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even
1484 on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the
1485 number is considered to be in that encoding, and is translated from that into
1486 the platform's native encoding if there is a corresponding native character;
1487 otherwise to Unicode.
1491 B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
1492 the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>. (C<\v>
1493 does have meaning in regular expression patterns in Perl, see L<perlre>.)
1495 The following escape sequences are available in constructs that interpolate,
1496 but not in transliterations.
1497 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> X<\F>
1499 \l lowercase next character only
1500 \u titlecase (not uppercase!) next character only
1501 \L lowercase all characters till \E or end of string
1502 \U uppercase all characters till \E or end of string
1503 \F foldcase all characters till \E or end of string
1504 \Q quote (disable) pattern metacharacters till \E or
1506 \E end either case modification or quoted section
1507 (whichever was last seen)
1509 See L<perlfunc/quotemeta> for the exact definition of characters that
1510 are quoted by C<\Q>.
1512 C<\L>, C<\U>, C<\F>, and C<\Q> can stack, in which case you need one
1513 C<\E> for each. For example:
1515 say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
1516 This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?
1518 If C<use locale> is in effect (but not C<use locale ':not_characters'>),
1519 the case map used by C<\l>, C<\L>,
1520 C<\u>, and C<\U> is taken from the current locale. See L<perllocale>.
1521 If Unicode (for example, C<\N{}> or code points of 0x100 or
1522 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u>, and
1523 C<\U> is as defined by Unicode. That means that case-mapping
1524 a single character can sometimes produce several characters.
1525 Under C<use locale>, C<\F> produces the same results as C<\L>.
1527 All systems use the virtual C<"\n"> to represent a line terminator,
1528 called a "newline". There is no such thing as an unvarying, physical
1529 newline character. It is only an illusion that the operating system,
1530 device drivers, C libraries, and Perl all conspire to preserve. Not all
1531 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1532 on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,
1533 and on systems without line terminator,
1534 printing C<"\n"> might emit no actual data. In general, use C<"\n"> when
1535 you mean a "newline" for your system, but use the literal ASCII when you
1536 need an exact character. For example, most networking protocols expect
1537 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
1538 and although they often accept just C<"\012">, they seldom tolerate just
1539 C<"\015">. If you get in the habit of using C<"\n"> for networking,
1540 you may be burned some day.
1541 X<newline> X<line terminator> X<eol> X<end of line>
1544 For constructs that do interpolate, variables beginning with "C<$>"
1545 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
1546 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1547 But method calls such as C<< $obj->meth >> are not.
1549 Interpolating an array or slice interpolates the elements in order,
1550 separated by the value of C<$">, so is equivalent to interpolating
1551 C<join $", @array>. "Punctuation" arrays such as C<@*> are usually
1552 interpolated only if the name is enclosed in braces C<@{*}>, but the
1553 arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces.
1555 For double-quoted strings, the quoting from C<\Q> is applied after
1556 interpolation and escapes are processed.
1558 "abc\Qfoo\tbar$s\Exyz"
1562 "abc" . quotemeta("foo\tbar$s") . "xyz"
1564 For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
1565 the quoting from C<\Q> is applied after interpolation is processed,
1566 but before escapes are processed. This allows the pattern to match
1567 literally (except for C<$> and C<@>). For example, the following matches:
1571 Because C<$> or C<@> trigger interpolation, you'll need to use something
1572 like C</\Quser\E\@\Qhost/> to match them literally.
1574 Patterns are subject to an additional level of interpretation as a
1575 regular expression. This is done as a second pass, after variables are
1576 interpolated, so that regular expressions may be incorporated into the
1577 pattern from the variables. If this is not what you want, use C<\Q> to
1578 interpolate a variable literally.
1580 Apart from the behavior described above, Perl does not expand
1581 multiple levels of interpolation. In particular, contrary to the
1582 expectations of shell programmers, back-quotes do I<NOT> interpolate
1583 within double quotes, nor do single quotes impede evaluation of
1584 variables when used within double quotes.
1586 =head2 Regexp Quote-Like Operators
1589 Here are the quote-like operators that apply to pattern
1590 matching and related activities.
1594 =item qr/STRING/msixpodual
1595 X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
1597 This operator quotes (and possibly compiles) its I<STRING> as a regular
1598 expression. I<STRING> is interpolated the same way as I<PATTERN>
1599 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1600 is done. Returns a Perl value which may be used instead of the
1601 corresponding C</STRING/msixpodual> expression. The returned value is a
1602 normalized version of the original pattern. It magically differs from
1603 a string containing the same characters: C<ref(qr/x/)> returns "Regexp";
1604 however, dereferencing it is not well defined (you currently get the
1605 normalized version of the original pattern, but this may change).
1610 $rex = qr/my.STRING/is;
1611 print $rex; # prints (?si-xm:my.STRING)
1618 The result may be used as a subpattern in a match:
1621 $string =~ /foo${re}bar/; # can be interpolated in other
1623 $string =~ $re; # or used standalone
1624 $string =~ /$re/; # or this way
1626 Since Perl may compile the pattern at the moment of execution of the qr()
1627 operator, using qr() may have speed advantages in some situations,
1628 notably if the result of qr() is used standalone:
1631 my $patterns = shift;
1632 my @compiled = map qr/$_/i, @$patterns;
1635 foreach my $pat (@compiled) {
1636 $success = 1, last if /$pat/;
1642 Precompilation of the pattern into an internal representation at
1643 the moment of qr() avoids a need to recompile the pattern every
1644 time a match C</$pat/> is attempted. (Perl has many other internal
1645 optimizations, but none would be triggered in the above example if
1646 we did not use qr() operator.)
1648 Options (specified by the following modifiers) are:
1650 m Treat string as multiple lines.
1651 s Treat string as single line. (Make . match a newline)
1652 i Do case-insensitive pattern matching.
1653 x Use extended regular expressions.
1654 p When matching preserve a copy of the matched string so
1655 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be
1657 o Compile pattern only once.
1658 a ASCII-restrict: Use ASCII for \d, \s, \w; specifying two
1659 a's further restricts /i matching so that no ASCII
1660 character will match a non-ASCII one
1663 d Use Unicode or native charset, as in 5.12 and earlier
1665 If a precompiled pattern is embedded in a larger pattern then the effect
1666 of "msixpluad" will be propagated appropriately. The effect the "o"
1667 modifier has is not propagated, being restricted to those patterns
1668 explicitly using it.
1670 The last four modifiers listed above, added in Perl 5.14,
1671 control the character set semantics, but C</a> is the only one you are likely
1672 to want to specify explicitly; the other three are selected
1673 automatically by various pragmas.
1675 See L<perlre> for additional information on valid syntax for STRING, and
1676 for a detailed look at the semantics of regular expressions. In
1677 particular, all modifiers except the largely obsolete C</o> are further
1678 explained in L<perlre/Modifiers>. C</o> is described in the next section.
1680 =item m/PATTERN/msixpodualgc
1681 X<m> X<operator, match>
1682 X<regexp, options> X<regexp> X<regex, options> X<regex>
1683 X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
1685 =item /PATTERN/msixpodualgc
1687 Searches a string for a pattern match, and in scalar context returns
1688 true if it succeeds, false if it fails. If no string is specified
1689 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1690 string specified with C<=~> need not be an lvalue--it may be the
1691 result of an expression evaluation, but remember the C<=~> binds
1692 rather tightly.) See also L<perlre>.
1694 Options are as described in C<qr//> above; in addition, the following match
1695 process modifiers are available:
1697 g Match globally, i.e., find all occurrences.
1698 c Do not reset search position on a failed match when /g is
1701 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1702 you can use any pair of non-whitespace (ASCII) characters
1703 as delimiters. This is particularly useful for matching path names
1704 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1705 the delimiter, then a match-only-once rule applies,
1706 described in C<m?PATTERN?> below.
1707 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1708 When using a character valid in an identifier, whitespace is required
1711 PATTERN may contain variables, which will be interpolated
1712 every time the pattern search is evaluated, except
1713 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1714 C<$|> are not interpolated because they look like end-of-string tests.)
1715 Perl will not recompile the pattern unless an interpolated
1716 variable that it contains changes. You can force Perl to skip the
1717 test and never recompile by adding a C</o> (which stands for "once")
1718 after the trailing delimiter.
1719 Once upon a time, Perl would recompile regular expressions
1720 unnecessarily, and this modifier was useful to tell it not to do so, in the
1721 interests of speed. But now, the only reasons to use C</o> are either:
1727 The variables are thousands of characters long and you know that they
1728 don't change, and you need to wring out the last little bit of speed by
1729 having Perl skip testing for that. (There is a maintenance penalty for
1730 doing this, as mentioning C</o> constitutes a promise that you won't
1731 change the variables in the pattern. If you do change them, Perl won't
1736 you want the pattern to use the initial values of the variables
1737 regardless of whether they change or not. (But there are saner ways
1738 of accomplishing this than using C</o>.)
1742 If the pattern contains embedded code, such as
1745 $code = 'foo(?{ $x })';
1748 then perl will recompile each time, even though the pattern string hasn't
1749 changed, to ensure that the current value of C<$x> is seen each time.
1750 Use C</o> if you want to avoid this.
1754 The bottom line is that using C</o> is almost never a good idea.
1756 =item The empty pattern //
1758 If the PATTERN evaluates to the empty string, the last
1759 I<successfully> matched regular expression is used instead. In this
1760 case, only the C<g> and C<c> flags on the empty pattern are honored;
1761 the other flags are taken from the original pattern. If no match has
1762 previously succeeded, this will (silently) act instead as a genuine
1763 empty pattern (which will always match).
1765 Note that it's possible to confuse Perl into thinking C<//> (the empty
1766 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1767 good about this, but some pathological cases might trigger this, such as
1768 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1769 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1770 will assume you meant defined-or. If you meant the empty regex, just
1771 use parentheses or spaces to disambiguate, or even prefix the empty
1772 regex with an C<m> (so C<//> becomes C<m//>).
1774 =item Matching in list context
1776 If the C</g> option is not used, C<m//> in list context returns a
1777 list consisting of the subexpressions matched by the parentheses in the
1778 pattern, that is, (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1779 also set, and that this differs from Perl 4's behavior.) When there are
1780 no parentheses in the pattern, the return value is the list C<(1)> for
1781 success. With or without parentheses, an empty list is returned upon
1786 open(TTY, "+</dev/tty")
1787 || die "can't access /dev/tty: $!";
1789 <TTY> =~ /^y/i && foo(); # do foo if desired
1791 if (/Version: *([0-9.]*)/) { $version = $1; }
1793 next if m#^/usr/spool/uucp#;
1798 print if /$arg/o; # compile only once (no longer needed!)
1801 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1803 This last example splits $foo into the first two words and the
1804 remainder of the line, and assigns those three fields to $F1, $F2, and
1805 $Etc. The conditional is true if any variables were assigned; that is,
1806 if the pattern matched.
1808 The C</g> modifier specifies global pattern matching--that is,
1809 matching as many times as possible within the string. How it behaves
1810 depends on the context. In list context, it returns a list of the
1811 substrings matched by any capturing parentheses in the regular
1812 expression. If there are no parentheses, it returns a list of all
1813 the matched strings, as if there were parentheses around the whole
1816 In scalar context, each execution of C<m//g> finds the next match,
1817 returning true if it matches, and false if there is no further match.
1818 The position after the last match can be read or set using the C<pos()>
1819 function; see L<perlfunc/pos>. A failed match normally resets the
1820 search position to the beginning of the string, but you can avoid that
1821 by adding the C</c> modifier (for example, C<m//gc>). Modifying the target
1822 string also resets the search position.
1826 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1827 zero-width assertion that matches the exact position where the
1828 previous C<m//g>, if any, left off. Without the C</g> modifier, the
1829 C<\G> assertion still anchors at C<pos()> as it was at the start of
1830 the operation (see L<perlfunc/pos>), but the match is of course only
1831 attempted once. Using C<\G> without C</g> on a target string that has
1832 not previously had a C</g> match applied to it is the same as using
1833 the C<\A> assertion to match the beginning of the string. Note also
1834 that, currently, C<\G> is only properly supported when anchored at the
1835 very beginning of the pattern.
1840 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1844 while ($paragraph = <>) {
1845 while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
1851 Here's another way to check for sentences in a paragraph:
1853 my $sentence_rx = qr{
1854 (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or
1856 \p{Lu} # capital letter
1857 .*? # a bunch of anything
1858 (?<= \S ) # that ends in non-
1860 (?<! \b [DMS]r ) # but isn't a common abbr.
1864 [.?!] # followed by a sentence
1866 (?= $ | \s ) # in front of end-of-string
1870 while (my $paragraph = <>) {
1871 say "NEW PARAGRAPH";
1873 while ($paragraph =~ /($sentence_rx)/g) {
1874 printf "\tgot sentence %d: <%s>\n", ++$count, $1;
1878 Here's how to use C<m//gc> with C<\G>:
1883 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1885 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1887 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1889 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1891 The last example should print:
1901 Notice that the final match matched C<q> instead of C<p>, which a match
1902 without the C<\G> anchor would have done. Also note that the final match
1903 did not update C<pos>. C<pos> is only updated on a C</g> match. If the
1904 final match did indeed match C<p>, it's a good bet that you're running a
1905 very old (pre-5.6.0) version of Perl.
1907 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1908 combine several regexps like this to process a string part-by-part,
1909 doing different actions depending on which regexp matched. Each
1910 regexp tries to match where the previous one leaves off.
1913 $url = URI::URL->new( "http://example.com/" );
1914 die if $url eq "xXx";
1918 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1919 print(" lowercase"), redo LOOP
1920 if /\G\p{Ll}+\b[,.;]?\s*/gc;
1921 print(" UPPERCASE"), redo LOOP
1922 if /\G\p{Lu}+\b[,.;]?\s*/gc;
1923 print(" Capitalized"), redo LOOP
1924 if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
1925 print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
1926 print(" alphanumeric"), redo LOOP
1927 if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
1928 print(" line-noise"), redo LOOP if /\G\W+/gc;
1929 print ". That's all!\n";
1932 Here is the output (split into several lines):
1934 line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
1935 line-noise lowercase line-noise lowercase line-noise lowercase
1936 lowercase line-noise lowercase lowercase line-noise lowercase
1937 lowercase line-noise MiXeD line-noise. That's all!
1939 =item m?PATTERN?msixpodualgc
1940 X<?> X<operator, match-once>
1942 =item ?PATTERN?msixpodualgc
1944 This is just like the C<m/PATTERN/> search, except that it matches
1945 only once between calls to the reset() operator. This is a useful
1946 optimization when you want to see only the first occurrence of
1947 something in each file of a set of files, for instance. Only C<m??>
1948 patterns local to the current package are reset.
1952 # blank line between header and body
1955 reset if eof; # clear m?? status for next file
1958 Another example switched the first "latin1" encoding it finds
1959 to "utf8" in a pod file:
1961 s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
1963 The match-once behavior is controlled by the match delimiter being
1964 C<?>; with any other delimiter this is the normal C<m//> operator.
1966 For historical reasons, the leading C<m> in C<m?PATTERN?> is optional,
1967 but the resulting C<?PATTERN?> syntax is deprecated, will warn on
1968 usage and might be removed from a future stable release of Perl (without
1971 =item s/PATTERN/REPLACEMENT/msixpodualgcer
1972 X<substitute> X<substitution> X<replace> X<regexp, replace>
1973 X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
1975 Searches a string for a pattern, and if found, replaces that pattern
1976 with the replacement text and returns the number of substitutions
1977 made. Otherwise it returns false (specifically, the empty string).
1979 If the C</r> (non-destructive) option is used then it runs the
1980 substitution on a copy of the string and instead of returning the
1981 number of substitutions, it returns the copy whether or not a
1982 substitution occurred. The original string is never changed when
1983 C</r> is used. The copy will always be a plain string, even if the
1984 input is an object or a tied variable.
1986 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1987 variable is searched and modified. Unless the C</r> option is used,
1988 the string specified must be a scalar variable, an array element, a
1989 hash element, or an assignment to one of those; that is, some sort of
1992 If the delimiter chosen is a single quote, no interpolation is
1993 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1994 PATTERN contains a $ that looks like a variable rather than an
1995 end-of-string test, the variable will be interpolated into the pattern
1996 at run-time. If you want the pattern compiled only once the first time
1997 the variable is interpolated, use the C</o> option. If the pattern
1998 evaluates to the empty string, the last successfully executed regular
1999 expression is used instead. See L<perlre> for further explanation on these.
2001 Options are as with m// with the addition of the following replacement
2004 e Evaluate the right side as an expression.
2005 ee Evaluate the right side as a string then eval the
2007 r Return substitution and leave the original string
2010 Any non-whitespace delimiter may replace the slashes. Add space after
2011 the C<s> when using a character allowed in identifiers. If single quotes
2012 are used, no interpretation is done on the replacement string (the C</e>
2013 modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks
2014 as normal delimiters; the replacement text is not evaluated as a command.
2015 If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
2016 its own pair of quotes, which may or may not be bracketing quotes, for example,
2017 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
2018 replacement portion to be treated as a full-fledged Perl expression
2019 and evaluated right then and there. It is, however, syntax checked at
2020 compile-time. A second C<e> modifier will cause the replacement portion
2021 to be C<eval>ed before being run as a Perl expression.
2025 s/\bgreen\b/mauve/g; # don't change wintergreen
2027 $path =~ s|/usr/bin|/usr/local/bin|;
2029 s/Login: $foo/Login: $bar/; # run-time pattern
2031 ($foo = $bar) =~ s/this/that/; # copy first, then
2033 ($foo = "$bar") =~ s/this/that/; # convert to string,
2035 $foo = $bar =~ s/this/that/r; # Same as above using /r
2036 $foo = $bar =~ s/this/that/r
2037 =~ s/that/the other/r; # Chained substitutes
2039 @foo = map { s/this/that/r } @bar # /r is very useful in
2042 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-cnt
2045 s/\d+/$&*2/e; # yields 'abc246xyz'
2046 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
2047 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
2049 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
2050 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
2051 s/^=(\w+)/pod($1)/ge; # use function call
2054 $a = s/abc/def/r; # $a is 'def123xyz' and
2055 # $_ remains 'abc123xyz'.
2057 # expand variables in $_, but dynamics only, using
2058 # symbolic dereferencing
2061 # Add one to the value of any numbers in the string
2064 # Titlecase words in the last 30 characters only
2065 substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;
2067 # This will expand any embedded scalar variable
2068 # (including lexicals) in $_ : First $1 is interpolated
2069 # to the variable name, and then evaluated
2072 # Delete (most) C comments.
2074 /\* # Match the opening delimiter.
2075 .*? # Match a minimal number of characters.
2076 \*/ # Match the closing delimiter.
2079 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_,
2082 for ($variable) { # trim whitespace in $variable,
2088 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
2090 Note the use of $ instead of \ in the last example. Unlike
2091 B<sed>, we use the \<I<digit>> form in only the left hand side.
2092 Anywhere else it's $<I<digit>>.
2094 Occasionally, you can't use just a C</g> to get all the changes
2095 to occur that you might want. Here are two common cases:
2097 # put commas in the right places in an integer
2098 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
2100 # expand tabs to 8-column spacing
2101 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
2105 =head2 Quote-Like Operators
2106 X<operator, quote-like>
2111 X<q> X<quote, single> X<'> X<''>
2115 A single-quoted, literal string. A backslash represents a backslash
2116 unless followed by the delimiter or another backslash, in which case
2117 the delimiter or backslash is interpolated.
2119 $foo = q!I said, "You said, 'She said it.'"!;
2120 $bar = q('This is it.');
2121 $baz = '\n'; # a two-character string
2124 X<qq> X<quote, double> X<"> X<"">
2128 A double-quoted, interpolated string.
2131 (*** The previous line contains the naughty word "$1".\n)
2132 if /\b(tcl|java|python)\b/i; # :-)
2133 $baz = "\n"; # a one-character string
2136 X<qx> X<`> X<``> X<backtick>
2140 A string which is (possibly) interpolated and then executed as a
2141 system command with C</bin/sh> or its equivalent. Shell wildcards,
2142 pipes, and redirections will be honored. The collected standard
2143 output of the command is returned; standard error is unaffected. In
2144 scalar context, it comes back as a single (potentially multi-line)
2145 string, or undef if the command failed. In list context, returns a
2146 list of lines (however you've defined lines with $/ or
2147 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
2149 Because backticks do not affect standard error, use shell file descriptor
2150 syntax (assuming the shell supports this) if you care to address this.
2151 To capture a command's STDERR and STDOUT together:
2153 $output = `cmd 2>&1`;
2155 To capture a command's STDOUT but discard its STDERR:
2157 $output = `cmd 2>/dev/null`;
2159 To capture a command's STDERR but discard its STDOUT (ordering is
2162 $output = `cmd 2>&1 1>/dev/null`;
2164 To exchange a command's STDOUT and STDERR in order to capture the STDERR
2165 but leave its STDOUT to come out the old STDERR:
2167 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
2169 To read both a command's STDOUT and its STDERR separately, it's easiest
2170 to redirect them separately to files, and then read from those files
2171 when the program is done:
2173 system("program args 1>program.stdout 2>program.stderr");
2175 The STDIN filehandle used by the command is inherited from Perl's STDIN.
2178 open(SPLAT, "stuff") || die "can't open stuff: $!";
2179 open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
2180 print STDOUT `sort`;
2182 will print the sorted contents of the file named F<"stuff">.
2184 Using single-quote as a delimiter protects the command from Perl's
2185 double-quote interpolation, passing it on to the shell instead:
2187 $perl_info = qx(ps $$); # that's Perl's $$
2188 $shell_info = qx'ps $$'; # that's the new shell's $$
2190 How that string gets evaluated is entirely subject to the command
2191 interpreter on your system. On most platforms, you will have to protect
2192 shell metacharacters if you want them treated literally. This is in
2193 practice difficult to do, as it's unclear how to escape which characters.
2194 See L<perlsec> for a clean and safe example of a manual fork() and exec()
2195 to emulate backticks safely.
2197 On some platforms (notably DOS-like ones), the shell may not be
2198 capable of dealing with multiline commands, so putting newlines in
2199 the string may not get you what you want. You may be able to evaluate
2200 multiple commands in a single line by separating them with the command
2201 separator character, if your shell supports that (for example, C<;> on
2202 many Unix shells and C<&> on the Windows NT C<cmd> shell).
2204 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2205 output before starting the child process, but this may not be supported
2206 on some platforms (see L<perlport>). To be safe, you may need to set
2207 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2208 C<IO::Handle> on any open handles.
2210 Beware that some command shells may place restrictions on the length
2211 of the command line. You must ensure your strings don't exceed this
2212 limit after any necessary interpolations. See the platform-specific
2213 release notes for more details about your particular environment.
2215 Using this operator can lead to programs that are difficult to port,
2216 because the shell commands called vary between systems, and may in
2217 fact not be present at all. As one example, the C<type> command under
2218 the POSIX shell is very different from the C<type> command under DOS.
2219 That doesn't mean you should go out of your way to avoid backticks
2220 when they're the right way to get something done. Perl was made to be
2221 a glue language, and one of the things it glues together is commands.
2222 Just understand what you're getting yourself into.
2224 See L</"I/O Operators"> for more discussion.
2227 X<qw> X<quote, list> X<quote, words>
2229 Evaluates to a list of the words extracted out of STRING, using embedded
2230 whitespace as the word delimiters. It can be understood as being roughly
2233 split(" ", q/STRING/);
2235 the differences being that it generates a real list at compile time, and
2236 in scalar context it returns the last element in the list. So
2241 is semantically equivalent to the list:
2245 Some frequently seen examples:
2247 use POSIX qw( setlocale localeconv )
2248 @EXPORT = qw( foo bar baz );
2250 A common mistake is to try to separate the words with comma or to
2251 put comments into a multi-line C<qw>-string. For this reason, the
2252 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
2253 produces warnings if the STRING contains the "," or the "#" character.
2255 =item tr/SEARCHLIST/REPLACEMENTLIST/cdsr
2256 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
2258 =item y/SEARCHLIST/REPLACEMENTLIST/cdsr
2260 Transliterates all occurrences of the characters found in the search list
2261 with the corresponding character in the replacement list. It returns
2262 the number of characters replaced or deleted. If no string is
2263 specified via the C<=~> or C<!~> operator, the $_ string is transliterated.
2265 If the C</r> (non-destructive) option is present, a new copy of the string
2266 is made and its characters transliterated, and this copy is returned no
2267 matter whether it was modified or not: the original string is always
2268 left unchanged. The new copy is always a plain string, even if the input
2269 string is an object or a tied variable.
2271 Unless the C</r> option is used, the string specified with C<=~> must be a
2272 scalar variable, an array element, a hash element, or an assignment to one
2273 of those; in other words, an lvalue.
2275 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2276 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
2277 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
2278 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
2279 its own pair of quotes, which may or may not be bracketing quotes;
2280 for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>.
2282 Note that C<tr> does B<not> do regular expression character classes such as
2283 C<\d> or C<\pL>. The C<tr> operator is not equivalent to the tr(1)
2284 utility. If you want to map strings between lower/upper cases, see
2285 L<perlfunc/lc> and L<perlfunc/uc>, and in general consider using the C<s>
2286 operator if you need regular expressions. The C<\U>, C<\u>, C<\L>, and
2287 C<\l> string-interpolation escapes on the right side of a substitution
2288 operator will perform correct case-mappings, but C<tr[a-z][A-Z]> will not
2289 (except sometimes on legacy 7-bit data).
2291 Note also that the whole range idea is rather unportable between
2292 character sets--and even within character sets they may cause results
2293 you probably didn't expect. A sound principle is to use only ranges
2294 that begin from and end at either alphabets of equal case (a-e, A-E),
2295 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
2296 character sets in full.
2300 c Complement the SEARCHLIST.
2301 d Delete found but unreplaced characters.
2302 s Squash duplicate replaced characters.
2303 r Return the modified string and leave the original string
2306 If the C</c> modifier is specified, the SEARCHLIST character set
2307 is complemented. If the C</d> modifier is specified, any characters
2308 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
2309 (Note that this is slightly more flexible than the behavior of some
2310 B<tr> programs, which delete anything they find in the SEARCHLIST,
2311 period.) If the C</s> modifier is specified, sequences of characters
2312 that were transliterated to the same character are squashed down
2313 to a single instance of the character.
2315 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
2316 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
2317 than the SEARCHLIST, the final character is replicated till it is long
2318 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
2319 This latter is useful for counting characters in a class or for
2320 squashing character sequences in a class.
2324 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
2326 $cnt = tr/*/*/; # count the stars in $_
2328 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
2330 $cnt = tr/0-9//; # count the digits in $_
2332 tr/a-zA-Z//s; # bookkeeper -> bokeper
2334 ($HOST = $host) =~ tr/a-z/A-Z/;
2335 $HOST = $host =~ tr/a-z/A-Z/r; # same thing
2337 $HOST = $host =~ tr/a-z/A-Z/r # chained with s///r
2340 tr/a-zA-Z/ /cs; # change non-alphas to single space
2342 @stripped = map tr/a-zA-Z/ /csr, @original;
2346 [\000-\177]; # wickedly delete 8th bit
2348 If multiple transliterations are given for a character, only the
2353 will transliterate any A to X.
2355 Because the transliteration table is built at compile time, neither
2356 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
2357 interpolation. That means that if you want to use variables, you
2360 eval "tr/$oldlist/$newlist/";
2363 eval "tr/$oldlist/$newlist/, 1" or die $@;
2366 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
2368 A line-oriented form of quoting is based on the shell "here-document"
2369 syntax. Following a C<< << >> you specify a string to terminate
2370 the quoted material, and all lines following the current line down to
2371 the terminating string are the value of the item.
2373 The terminating string may be either an identifier (a word), or some
2374 quoted text. An unquoted identifier works like double quotes.
2375 There may not be a space between the C<< << >> and the identifier,
2376 unless the identifier is explicitly quoted. (If you put a space it
2377 will be treated as a null identifier, which is valid, and matches the
2378 first empty line.) The terminating string must appear by itself
2379 (unquoted and with no surrounding whitespace) on the terminating line.
2381 If the terminating string is quoted, the type of quotes used determine
2382 the treatment of the text.
2388 Double quotes indicate that the text will be interpolated using exactly
2389 the same rules as normal double quoted strings.
2392 The price is $Price.
2395 print << "EOF"; # same as above
2396 The price is $Price.
2402 Single quotes indicate the text is to be treated literally with no
2403 interpolation of its content. This is similar to single quoted
2404 strings except that backslashes have no special meaning, with C<\\>
2405 being treated as two backslashes and not one as they would in every
2406 other quoting construct.
2408 Just as in the shell, a backslashed bareword following the C<<< << >>>
2409 means the same thing as a single-quoted string does:
2411 $cost = <<'VISTA'; # hasta la ...
2412 That'll be $10 please, ma'am.
2415 $cost = <<\VISTA; # Same thing!
2416 That'll be $10 please, ma'am.
2419 This is the only form of quoting in perl where there is no need
2420 to worry about escaping content, something that code generators
2421 can and do make good use of.
2425 The content of the here doc is treated just as it would be if the
2426 string were embedded in backticks. Thus the content is interpolated
2427 as though it were double quoted and then executed via the shell, with
2428 the results of the execution returned.
2430 print << `EOC`; # execute command and get results
2436 It is possible to stack multiple here-docs in a row:
2438 print <<"foo", <<"bar"; # you can stack them
2444 myfunc(<< "THIS", 23, <<'THAT');
2451 Just don't forget that you have to put a semicolon on the end
2452 to finish the statement, as Perl doesn't know you're not going to
2460 If you want to remove the line terminator from your here-docs,
2463 chomp($string = <<'END');
2467 If you want your here-docs to be indented with the rest of the code,
2468 you'll need to remove leading whitespace from each line manually:
2470 ($quote = <<'FINIS') =~ s/^\s+//gm;
2471 The Road goes ever on and on,
2472 down from the door where it began.
2475 If you use a here-doc within a delimited construct, such as in C<s///eg>,
2476 the quoted material must come on the lines following the final delimiter.
2491 If the terminating identifier is on the last line of the program, you
2492 must be sure there is a newline after it; otherwise, Perl will give the
2493 warning B<Can't find string terminator "END" anywhere before EOF...>.
2495 Additionally, quoting rules for the end-of-string identifier are
2496 unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
2497 supported in place of C<''> and C<"">, and the only interpolation is for
2498 backslashing the quoting character:
2500 print << "abc\"def";
2504 Finally, quoted strings cannot span multiple lines. The general rule is
2505 that the identifier must be a string literal. Stick with that, and you
2510 =head2 Gory details of parsing quoted constructs
2511 X<quote, gory details>
2513 When presented with something that might have several different
2514 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2515 principle to pick the most probable interpretation. This strategy
2516 is so successful that Perl programmers often do not suspect the
2517 ambivalence of what they write. But from time to time, Perl's
2518 notions differ substantially from what the author honestly meant.
2520 This section hopes to clarify how Perl handles quoted constructs.
2521 Although the most common reason to learn this is to unravel labyrinthine
2522 regular expressions, because the initial steps of parsing are the
2523 same for all quoting operators, they are all discussed together.
2525 The most important Perl parsing rule is the first one discussed
2526 below: when processing a quoted construct, Perl first finds the end
2527 of that construct, then interprets its contents. If you understand
2528 this rule, you may skip the rest of this section on the first
2529 reading. The other rules are likely to contradict the user's
2530 expectations much less frequently than this first one.
2532 Some passes discussed below are performed concurrently, but because
2533 their results are the same, we consider them individually. For different
2534 quoting constructs, Perl performs different numbers of passes, from
2535 one to four, but these passes are always performed in the same order.
2539 =item Finding the end
2541 The first pass is finding the end of the quoted construct, where
2542 the information about the delimiters is used in parsing.
2543 During this search, text between the starting and ending delimiters
2544 is copied to a safe location. The text copied gets delimiter-independent.
2546 If the construct is a here-doc, the ending delimiter is a line
2547 that has a terminating string as the content. Therefore C<<<EOF> is
2548 terminated by C<EOF> immediately followed by C<"\n"> and starting
2549 from the first column of the terminating line.
2550 When searching for the terminating line of a here-doc, nothing
2551 is skipped. In other words, lines after the here-doc syntax
2552 are compared with the terminating string line by line.
2554 For the constructs except here-docs, single characters are used as starting
2555 and ending delimiters. If the starting delimiter is an opening punctuation
2556 (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2557 corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2558 If the starting delimiter is an unpaired character like C</> or a closing
2559 punctuation, the ending delimiter is same as the starting delimiter.
2560 Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
2561 C<qq[]> and C<qq]]> constructs.
2563 When searching for single-character delimiters, escaped delimiters
2564 and C<\\> are skipped. For example, while searching for terminating C</>,
2565 combinations of C<\\> and C<\/> are skipped. If the delimiters are
2566 bracketing, nested pairs are also skipped. For example, while searching
2567 for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
2568 and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2569 However, when backslashes are used as the delimiters (like C<qq\\> and
2570 C<tr\\\>), nothing is skipped.
2571 During the search for the end, backslashes that escape delimiters or
2572 other backslashes are removed (exactly speaking, they are not copied to the
2575 For constructs with three-part delimiters (C<s///>, C<y///>, and
2576 C<tr///>), the search is repeated once more.
2577 If the first delimiter is not an opening punctuation, three delimiters must
2578 be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
2579 terminates the left part and starts the right part at once.
2580 If the left part is delimited by bracketing punctuation (that is C<()>,
2581 C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
2582 delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace
2583 and comments are allowed between both parts, though the comment must follow
2584 at least one whitespace character; otherwise a character expected as the
2585 start of the comment may be regarded as the starting delimiter of the right part.
2587 During this search no attention is paid to the semantics of the construct.
2590 "$hash{"$foo/$bar"}"
2595 bar # NOT a comment, this slash / terminated m//!
2598 do not form legal quoted expressions. The quoted part ends on the
2599 first C<"> and C</>, and the rest happens to be a syntax error.
2600 Because the slash that terminated C<m//> was followed by a C<SPACE>,
2601 the example above is not C<m//x>, but rather C<m//> with no C</x>
2602 modifier. So the embedded C<#> is interpreted as a literal C<#>.
2604 Also no attention is paid to C<\c\> (multichar control char syntax) during
2605 this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
2606 of C<\/>, and the following C</> is not recognized as a delimiter.
2607 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2612 The next step is interpolation in the text obtained, which is now
2613 delimiter-independent. There are multiple cases.
2619 No interpolation is performed.
2620 Note that the combination C<\\> is left intact, since escaped delimiters
2621 are not available for here-docs.
2623 =item C<m''>, the pattern of C<s'''>
2625 No interpolation is performed at this stage.
2626 Any backslashed sequences including C<\\> are treated at the stage
2627 to L</"parsing regular expressions">.
2629 =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
2631 The only interpolation is removal of C<\> from pairs of C<\\>.
2632 Therefore C<-> in C<tr'''> and C<y'''> is treated literally
2633 as a hyphen and no character range is available.
2634 C<\1> in the replacement of C<s'''> does not work as C<$1>.
2636 =item C<tr///>, C<y///>
2638 No variable interpolation occurs. String modifying combinations for
2639 case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2640 The other escape sequences such as C<\200> and C<\t> and backslashed
2641 characters such as C<\\> and C<\-> are converted to appropriate literals.
2642 The character C<-> is treated specially and therefore C<\-> is treated
2645 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
2647 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> (possibly paired with C<\E>) are
2648 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
2649 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
2650 The other escape sequences such as C<\200> and C<\t> and backslashed
2651 characters such as C<\\> and C<\-> are replaced with appropriate
2654 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2655 is interpolated in the usual way. Something like C<"\Q\\E"> has
2656 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
2657 result is the same as for C<"\\\\E">. As a general rule, backslashes
2658 between C<\Q> and C<\E> may lead to counterintuitive results. So,
2659 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2660 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2665 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2667 Interpolated scalars and arrays are converted internally to the C<join> and
2668 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
2670 $foo . " XXX '" . (join $", @arr) . "'";
2672 All operations above are performed simultaneously, left to right.
2674 Because the result of C<"\Q STRING \E"> has all metacharacters
2675 quoted, there is no way to insert a literal C<$> or C<@> inside a
2676 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
2677 C<"\\\$">; if not, it is interpreted as the start of an interpolated
2680 Note also that the interpolation code needs to make a decision on
2681 where the interpolated scalar ends. For instance, whether
2682 C<< "a $b -> {c}" >> really means:
2684 "a " . $b . " -> {c}";
2690 Most of the time, the longest possible text that does not include
2691 spaces between components and which contains matching braces or
2692 brackets. because the outcome may be determined by voting based
2693 on heuristic estimators, the result is not strictly predictable.
2694 Fortunately, it's usually correct for ambiguous cases.
2696 =item the replacement of C<s///>
2698 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> and interpolation
2699 happens as with C<qq//> constructs.
2701 It is at this step that C<\1> is begrudgingly converted to C<$1> in
2702 the replacement text of C<s///>, in order to correct the incorrigible
2703 I<sed> hackers who haven't picked up the saner idiom yet. A warning
2704 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2705 (that is, the C<$^W> variable) was set.
2707 =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2709 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F>, C<\E>,
2710 and interpolation happens (almost) as with C<qq//> constructs.
2712 Processing of C<\N{...}> is also done here, and compiled into an intermediate
2713 form for the regex compiler. (This is because, as mentioned below, the regex
2714 compilation may be done at execution time, and C<\N{...}> is a compile-time
2717 However any other combinations of C<\> followed by a character
2718 are not substituted but only skipped, in order to parse them
2719 as regular expressions at the following step.
2720 As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
2721 treated as an array symbol (for example C<@foo>),
2722 even though the same text in C<qq//> gives interpolation of C<\c@>.
2724 Code blocks such as C<(?{BLOCK})> are handled by temporarily passing control
2725 back to the perl parser, in a similar way that an interpolated array
2726 subscript expression such as C<"foo$array[1+f("[xyz")]bar"> would be.
2728 Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
2729 a C<#>-comment in a C<//x>-regular expression, no processing is
2730 performed whatsoever. This is the first step at which the presence
2731 of the C<//x> modifier is relevant.
2733 Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2734 and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2735 voted (by several different estimators) to be either an array element
2736 or C<$var> followed by an RE alternative. This is where the notation
2737 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2738 array element C<-9>, not as a regular expression from the variable
2739 C<$arr> followed by a digit, which would be the interpretation of
2740 C</$arr[0-9]/>. Since voting among different estimators may occur,
2741 the result is not predictable.
2743 The lack of processing of C<\\> creates specific restrictions on
2744 the post-processed text. If the delimiter is C</>, one cannot get
2745 the combination C<\/> into the result of this step. C</> will
2746 finish the regular expression, C<\/> will be stripped to C</> on
2747 the previous step, and C<\\/> will be left as is. Because C</> is
2748 equivalent to C<\/> inside a regular expression, this does not
2749 matter unless the delimiter happens to be character special to the
2750 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2751 alphanumeric char, as in:
2755 In the RE above, which is intentionally obfuscated for illustration, the
2756 delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
2757 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
2758 reason you're encouraged to restrict your delimiters to non-alphanumeric,
2759 non-whitespace choices.
2763 This step is the last one for all constructs except regular expressions,
2764 which are processed further.
2766 =item parsing regular expressions
2769 Previous steps were performed during the compilation of Perl code,
2770 but this one happens at run time, although it may be optimized to
2771 be calculated at compile time if appropriate. After preprocessing
2772 described above, and possibly after evaluation if concatenation,
2773 joining, casing translation, or metaquoting are involved, the
2774 resulting I<string> is passed to the RE engine for compilation.
2776 Whatever happens in the RE engine might be better discussed in L<perlre>,
2777 but for the sake of continuity, we shall do so here.
2779 This is another step where the presence of the C<//x> modifier is
2780 relevant. The RE engine scans the string from left to right and
2781 converts it to a finite automaton.
2783 Backslashed characters are either replaced with corresponding
2784 literal strings (as with C<\{>), or else they generate special nodes
2785 in the finite automaton (as with C<\b>). Characters special to the
2786 RE engine (such as C<|>) generate corresponding nodes or groups of
2787 nodes. C<(?#...)> comments are ignored. All the rest is either
2788 converted to literal strings to match, or else is ignored (as is
2789 whitespace and C<#>-style comments if C<//x> is present).
2791 Parsing of the bracketed character class construct, C<[...]>, is
2792 rather different than the rule used for the rest of the pattern.
2793 The terminator of this construct is found using the same rules as
2794 for finding the terminator of a C<{}>-delimited construct, the only
2795 exception being that C<]> immediately following C<[> is treated as
2796 though preceded by a backslash.
2798 The terminator of runtime C<(?{...})> is found by temporarily switching
2799 control to the perl parser, which should stop at the point where the
2800 logically balancing terminating C<}> is found.
2802 It is possible to inspect both the string given to RE engine and the
2803 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2804 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
2805 switch documented in L<perlrun/"Command Switches">.
2807 =item Optimization of regular expressions
2808 X<regexp, optimization>
2810 This step is listed for completeness only. Since it does not change
2811 semantics, details of this step are not documented and are subject
2812 to change without notice. This step is performed over the finite
2813 automaton that was generated during the previous pass.
2815 It is at this stage that C<split()> silently optimizes C</^/> to
2820 =head2 I/O Operators
2821 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2824 There are several I/O operators you should know about.
2826 A string enclosed by backticks (grave accents) first undergoes
2827 double-quote interpolation. It is then interpreted as an external
2828 command, and the output of that command is the value of the
2829 backtick string, like in a shell. In scalar context, a single string
2830 consisting of all output is returned. In list context, a list of
2831 values is returned, one per line of output. (You can set C<$/> to use
2832 a different line terminator.) The command is executed each time the
2833 pseudo-literal is evaluated. The status value of the command is
2834 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2835 Unlike in B<csh>, no translation is done on the return data--newlines
2836 remain newlines. Unlike in any of the shells, single quotes do not
2837 hide variable names in the command from interpretation. To pass a
2838 literal dollar-sign through to the shell you need to hide it with a
2839 backslash. The generalized form of backticks is C<qx//>. (Because
2840 backticks always undergo shell expansion as well, see L<perlsec> for
2842 X<qx> X<`> X<``> X<backtick> X<glob>
2844 In scalar context, evaluating a filehandle in angle brackets yields
2845 the next line from that file (the newline, if any, included), or
2846 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2847 (sometimes known as file-slurp mode) and the file is empty, it
2848 returns C<''> the first time, followed by C<undef> subsequently.
2850 Ordinarily you must assign the returned value to a variable, but
2851 there is one situation where an automatic assignment happens. If
2852 and only if the input symbol is the only thing inside the conditional
2853 of a C<while> statement (even if disguised as a C<for(;;)> loop),
2854 the value is automatically assigned to the global variable $_,
2855 destroying whatever was there previously. (This may seem like an
2856 odd thing to you, but you'll use the construct in almost every Perl
2857 script you write.) The $_ variable is not implicitly localized.
2858 You'll have to put a C<local $_;> before the loop if you want that
2861 The following lines are equivalent:
2863 while (defined($_ = <STDIN>)) { print; }
2864 while ($_ = <STDIN>) { print; }
2865 while (<STDIN>) { print; }
2866 for (;<STDIN>;) { print; }
2867 print while defined($_ = <STDIN>);
2868 print while ($_ = <STDIN>);
2869 print while <STDIN>;
2871 This also behaves similarly, but assigns to a lexical variable
2872 instead of to C<$_>:
2874 while (my $line = <STDIN>) { print $line }
2876 In these loop constructs, the assigned value (whether assignment
2877 is automatic or explicit) is then tested to see whether it is
2878 defined. The defined test avoids problems where the line has a string
2879 value that would be treated as false by Perl; for example a "" or
2880 a "0" with no trailing newline. If you really mean for such values
2881 to terminate the loop, they should be tested for explicitly:
2883 while (($_ = <STDIN>) ne '0') { ... }
2884 while (<STDIN>) { last unless $_; ... }
2886 In other boolean contexts, C<< <FILEHANDLE> >> without an
2887 explicit C<defined> test or comparison elicits a warning if the
2888 C<use warnings> pragma or the B<-w>
2889 command-line switch (the C<$^W> variable) is in effect.
2891 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2892 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2893 in packages, where they would be interpreted as local identifiers
2894 rather than global.) Additional filehandles may be created with
2895 the open() function, amongst others. See L<perlopentut> and
2896 L<perlfunc/open> for details on this.
2897 X<stdin> X<stdout> X<sterr>
2899 If a <FILEHANDLE> is used in a context that is looking for
2900 a list, a list comprising all input lines is returned, one line per
2901 list element. It's easy to grow to a rather large data space this
2902 way, so use with care.
2904 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2905 See L<perlfunc/readline>.
2907 The null filehandle <> is special: it can be used to emulate the
2908 behavior of B<sed> and B<awk>, and any other Unix filter program
2909 that takes a list of filenames, doing the same to each line
2910 of input from all of them. Input from <> comes either from
2911 standard input, or from each file listed on the command line. Here's
2912 how it works: the first time <> is evaluated, the @ARGV array is
2913 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2914 gives you standard input. The @ARGV array is then processed as a list
2915 of filenames. The loop
2918 ... # code for each line
2921 is equivalent to the following Perl-like pseudo code:
2923 unshift(@ARGV, '-') unless @ARGV;
2924 while ($ARGV = shift) {
2927 ... # code for each line
2931 except that it isn't so cumbersome to say, and will actually work.
2932 It really does shift the @ARGV array and put the current filename
2933 into the $ARGV variable. It also uses filehandle I<ARGV>
2934 internally. <> is just a synonym for <ARGV>, which
2935 is magical. (The pseudo code above doesn't work because it treats
2936 <ARGV> as non-magical.)
2938 Since the null filehandle uses the two argument form of L<perlfunc/open>
2939 it interprets special characters, so if you have a script like this:
2945 and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2946 pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2947 If you want all items in C<@ARGV> to be interpreted as file names, you
2948 can use the module C<ARGV::readonly> from CPAN.
2950 You can modify @ARGV before the first <> as long as the array ends up
2951 containing the list of filenames you really want. Line numbers (C<$.>)
2952 continue as though the input were one big happy file. See the example
2953 in L<perlfunc/eof> for how to reset line numbers on each file.
2955 If you want to set @ARGV to your own list of files, go right ahead.
2956 This sets @ARGV to all plain text files if no @ARGV was given:
2958 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2960 You can even set them to pipe commands. For example, this automatically
2961 filters compressed arguments through B<gzip>:
2963 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2965 If you want to pass switches into your script, you can use one of the
2966 Getopts modules or put a loop on the front like this:
2968 while ($_ = $ARGV[0], /^-/) {
2971 if (/^-D(.*)/) { $debug = $1 }
2972 if (/^-v/) { $verbose++ }
2973 # ... # other switches
2977 # ... # code for each line
2980 The <> symbol will return C<undef> for end-of-file only once.
2981 If you call it again after this, it will assume you are processing another
2982 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2984 If what the angle brackets contain is a simple scalar variable (for example,
2985 <$foo>), then that variable contains the name of the
2986 filehandle to input from, or its typeglob, or a reference to the
2992 If what's within the angle brackets is neither a filehandle nor a simple
2993 scalar variable containing a filehandle name, typeglob, or typeglob
2994 reference, it is interpreted as a filename pattern to be globbed, and
2995 either a list of filenames or the next filename in the list is returned,
2996 depending on context. This distinction is determined on syntactic
2997 grounds alone. That means C<< <$x> >> is always a readline() from
2998 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2999 That's because $x is a simple scalar variable, but C<$hash{key}> is
3000 not--it's a hash element. Even C<< <$x > >> (note the extra space)
3001 is treated as C<glob("$x ")>, not C<readline($x)>.
3003 One level of double-quote interpretation is done first, but you can't
3004 say C<< <$foo> >> because that's an indirect filehandle as explained
3005 in the previous paragraph. (In older versions of Perl, programmers
3006 would insert curly brackets to force interpretation as a filename glob:
3007 C<< <${foo}> >>. These days, it's considered cleaner to call the
3008 internal function directly as C<glob($foo)>, which is probably the right
3009 way to have done it in the first place.) For example:
3015 is roughly equivalent to:
3017 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
3023 except that the globbing is actually done internally using the standard
3024 C<File::Glob> extension. Of course, the shortest way to do the above is:
3028 A (file)glob evaluates its (embedded) argument only when it is
3029 starting a new list. All values must be read before it will start
3030 over. In list context, this isn't important because you automatically
3031 get them all anyway. However, in scalar context the operator returns
3032 the next value each time it's called, or C<undef> when the list has
3033 run out. As with filehandle reads, an automatic C<defined> is
3034 generated when the glob occurs in the test part of a C<while>,
3035 because legal glob returns (for example,
3036 a file called F<0>) would otherwise
3037 terminate the loop. Again, C<undef> is returned only once. So if
3038 you're expecting a single value from a glob, it is much better to
3041 ($file) = <blurch*>;
3047 because the latter will alternate between returning a filename and
3050 If you're trying to do variable interpolation, it's definitely better
3051 to use the glob() function, because the older notation can cause people
3052 to become confused with the indirect filehandle notation.
3054 @files = glob("$dir/*.[ch]");
3055 @files = glob($files[$i]);
3057 =head2 Constant Folding
3058 X<constant folding> X<folding>
3060 Like C, Perl does a certain amount of expression evaluation at
3061 compile time whenever it determines that all arguments to an
3062 operator are static and have no side effects. In particular, string
3063 concatenation happens at compile time between literals that don't do
3064 variable substitution. Backslash interpolation also happens at
3065 compile time. You can say
3067 'Now is the time for all'
3069 . 'good men to come to.'
3071 and this all reduces to one string internally. Likewise, if
3074 foreach $file (@filenames) {
3075 if (-s $file > 5 + 100 * 2**16) { }
3078 the compiler precomputes the number which that expression
3079 represents so that the interpreter won't have to.
3084 Perl doesn't officially have a no-op operator, but the bare constants
3085 C<0> and C<1> are special-cased not to produce a warning in void
3086 context, so you can for example safely do
3090 =head2 Bitwise String Operators
3091 X<operator, bitwise, string>
3093 Bitstrings of any size may be manipulated by the bitwise operators
3096 If the operands to a binary bitwise op are strings of different
3097 sizes, B<|> and B<^> ops act as though the shorter operand had
3098 additional zero bits on the right, while the B<&> op acts as though
3099 the longer operand were truncated to the length of the shorter.
3100 The granularity for such extension or truncation is one or more
3103 # ASCII-based examples
3104 print "j p \n" ^ " a h"; # prints "JAPH\n"
3105 print "JA" | " ph\n"; # prints "japh\n"
3106 print "japh\nJunk" & '_____'; # prints "JAPH\n";
3107 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
3109 If you are intending to manipulate bitstrings, be certain that
3110 you're supplying bitstrings: If an operand is a number, that will imply
3111 a B<numeric> bitwise operation. You may explicitly show which type of
3112 operation you intend by using C<""> or C<0+>, as in the examples below.
3114 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
3115 $foo = '150' | 105; # yields 255
3116 $foo = 150 | '105'; # yields 255
3117 $foo = '150' | '105'; # yields string '155' (under ASCII)
3119 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
3120 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
3122 See L<perlfunc/vec> for information on how to manipulate individual bits
3125 =head2 Integer Arithmetic
3128 By default, Perl assumes that it must do most of its arithmetic in
3129 floating point. But by saying
3133 you may tell the compiler to use integer operations
3134 (see L<integer> for a detailed explanation) from here to the end of
3135 the enclosing BLOCK. An inner BLOCK may countermand this by saying
3139 which lasts until the end of that BLOCK. Note that this doesn't
3140 mean everything is an integer, merely that Perl will use integer
3141 operations for arithmetic, comparison, and bitwise operators. For
3142 example, even under C<use integer>, if you take the C<sqrt(2)>, you'll
3143 still get C<1.4142135623731> or so.
3145 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
3146 and ">>") always produce integral results. (But see also
3147 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
3148 them. By default, their results are interpreted as unsigned integers, but
3149 if C<use integer> is in effect, their results are interpreted
3150 as signed integers. For example, C<~0> usually evaluates to a large
3151 integral value. However, C<use integer; ~0> is C<-1> on two's-complement
3154 =head2 Floating-point Arithmetic
3156 X<floating-point> X<floating point> X<float> X<real>
3158 While C<use integer> provides integer-only arithmetic, there is no
3159 analogous mechanism to provide automatic rounding or truncation to a
3160 certain number of decimal places. For rounding to a certain number
3161 of digits, sprintf() or printf() is usually the easiest route.
3164 Floating-point numbers are only approximations to what a mathematician
3165 would call real numbers. There are infinitely more reals than floats,
3166 so some corners must be cut. For example:
3168 printf "%.20g\n", 123456789123456789;
3169 # produces 123456789123456784
3171 Testing for exact floating-point equality or inequality is not a
3172 good idea. Here's a (relatively expensive) work-around to compare
3173 whether two floating-point numbers are equal to a particular number of
3174 decimal places. See Knuth, volume II, for a more robust treatment of
3178 my ($X, $Y, $POINTS) = @_;
3180 $tX = sprintf("%.${POINTS}g", $X);
3181 $tY = sprintf("%.${POINTS}g", $Y);
3185 The POSIX module (part of the standard perl distribution) implements
3186 ceil(), floor(), and other mathematical and trigonometric functions.
3187 The Math::Complex module (part of the standard perl distribution)
3188 defines mathematical functions that work on both the reals and the
3189 imaginary numbers. Math::Complex not as efficient as POSIX, but
3190 POSIX can't work with complex numbers.
3192 Rounding in financial applications can have serious implications, and
3193 the rounding method used should be specified precisely. In these
3194 cases, it probably pays not to trust whichever system rounding is
3195 being used by Perl, but to instead implement the rounding function you
3198 =head2 Bigger Numbers
3199 X<number, arbitrary precision>
3201 The standard C<Math::BigInt>, C<Math::BigRat>, and C<Math::BigFloat> modules,
3202 along with the C<bignum>, C<bigint>, and C<bigrat> pragmas, provide
3203 variable-precision arithmetic and overloaded operators, although
3204 they're currently pretty slow. At the cost of some space and
3205 considerable speed, they avoid the normal pitfalls associated with
3206 limited-precision representations.
3209 use bigint; # easy interface to Math::BigInt
3210 $x = 123456789123456789;
3212 +15241578780673678515622620750190521
3220 say "a/b is ", $a/$b;
3221 say "a*b is ", $a*$b;
3225 Several modules let you calculate with (bound only by memory and CPU time)
3226 unlimited or fixed precision. There are also some non-standard modules that
3227 provide faster implementations via external C libraries.
3229 Here is a short, but incomplete summary:
3231 Math::Fraction big, unlimited fractions like 9973 / 12967
3232 Math::String treat string sequences like numbers
3233 Math::FixedPrecision calculate with a fixed precision
3234 Math::Currency for currency calculations
3235 Bit::Vector manipulate bit vectors fast (uses C)
3236 Math::BigIntFast Bit::Vector wrapper for big numbers
3237 Math::Pari provides access to the Pari C library
3238 Math::BigInteger uses an external C library
3239 Math::Cephes uses external Cephes C library (no big numbers)
3240 Math::Cephes::Fraction fractions via the Cephes library
3241 Math::GMP another one using an external C library