4 perlop - Perl operators and precedence
8 In Perl, the operator determines what operation is performed,
9 independent of the type of the operands. For example C<$x + $y>
10 is always a numeric addition, and if C<$x> or C<$y> do not contain
11 numbers, an attempt is made to convert them to numbers first.
13 This is in contrast to many other dynamic languages, where the
14 operation is determined by the type of the first argument. It also
15 means that Perl has two versions of some operators, one for numeric
16 and one for string comparison. For example C<$x == $y> compares
17 two numbers for equality, and C<$x eq $y> compares two strings.
19 There are a few exceptions though: C<x> can be either string
20 repetition or list repetition, depending on the type of the left
21 operand, and C<&>, C<|>, C<^> and C<~> can be either string or numeric bit
24 =head2 Operator Precedence and Associativity
25 X<operator, precedence> X<precedence> X<associativity>
27 Operator precedence and associativity work in Perl more or less like
28 they do in mathematics.
30 I<Operator precedence> means some operators are evaluated before
31 others. For example, in C<2 + 4 * 5>, the multiplication has higher
32 precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
33 22> and not C<6 * 5 == 30>.
35 I<Operator associativity> defines what happens if a sequence of the
36 same operators is used one after another: whether the evaluator will
37 evaluate the left operations first or the right. For example, in C<8
38 - 4 - 2>, subtraction is left associative so Perl evaluates the
39 expression left to right. C<8 - 4> is evaluated first making the
40 expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
42 Perl operators have the following associativity and precedence,
43 listed from highest precedence to lowest. Operators borrowed from
44 C keep the same precedence relationship with each other, even where
45 C's precedence is slightly screwy. (This makes learning Perl easier
46 for C folks.) With very few exceptions, these all operate on scalar
47 values only, not array values.
49 left terms and list operators (leftward)
53 right ! ~ \ and unary + and -
58 nonassoc named unary operators
59 nonassoc < > <= >= lt gt le ge
60 nonassoc == != <=> eq ne cmp ~~
67 right = += -= *= etc. goto last next redo dump
69 nonassoc list operators (rightward)
74 In the following sections, these operators are covered in precedence order.
76 Many operators can be overloaded for objects. See L<overload>.
78 =head2 Terms and List Operators (Leftward)
79 X<list operator> X<operator, list> X<term>
81 A TERM has the highest precedence in Perl. They include variables,
82 quote and quote-like operators, any expression in parentheses,
83 and any function whose arguments are parenthesized. Actually, there
84 aren't really functions in this sense, just list operators and unary
85 operators behaving as functions because you put parentheses around
86 the arguments. These are all documented in L<perlfunc>.
88 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
89 is followed by a left parenthesis as the next token, the operator and
90 arguments within parentheses are taken to be of highest precedence,
91 just like a normal function call.
93 In the absence of parentheses, the precedence of list operators such as
94 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
95 whether you are looking at the left side or the right side of the operator.
98 @ary = (1, 3, sort 4, 2);
99 print @ary; # prints 1324
101 the commas on the right of the sort are evaluated before the sort,
102 but the commas on the left are evaluated after. In other words,
103 list operators tend to gobble up all arguments that follow, and
104 then act like a simple TERM with regard to the preceding expression.
105 Be careful with parentheses:
107 # These evaluate exit before doing the print:
108 print($foo, exit); # Obviously not what you want.
109 print $foo, exit; # Nor is this.
111 # These do the print before evaluating exit:
112 (print $foo), exit; # This is what you want.
113 print($foo), exit; # Or this.
114 print ($foo), exit; # Or even this.
118 print ($foo & 255) + 1, "\n";
120 probably doesn't do what you expect at first glance. The parentheses
121 enclose the argument list for C<print> which is evaluated (printing
122 the result of C<$foo & 255>). Then one is added to the return value
123 of C<print> (usually 1). The result is something like this:
125 1 + 1, "\n"; # Obviously not what you meant.
127 To do what you meant properly, you must write:
129 print(($foo & 255) + 1, "\n");
131 See L<Named Unary Operators> for more discussion of this.
133 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
134 well as subroutine and method calls, and the anonymous
135 constructors C<[]> and C<{}>.
137 See also L<Quote and Quote-like Operators> toward the end of this section,
138 as well as L</"I/O Operators">.
140 =head2 The Arrow Operator
141 X<arrow> X<dereference> X<< -> >>
143 "C<< -> >>" is an infix dereference operator, just as it is in C
144 and C++. If the right side is either a C<[...]>, C<{...}>, or a
145 C<(...)> subscript, then the left side must be either a hard or
146 symbolic reference to an array, a hash, or a subroutine respectively.
147 (Or technically speaking, a location capable of holding a hard
148 reference, if it's an array or hash reference being used for
149 assignment.) See L<perlreftut> and L<perlref>.
151 Otherwise, the right side is a method name or a simple scalar
152 variable containing either the method name or a subroutine reference,
153 and the left side must be either an object (a blessed reference)
154 or a class name (that is, a package name). See L<perlobj>.
156 The dereferencing cases (as opposed to method-calling cases) are
157 somewhat extended by the experimental C<postderef> feature. For the
158 details of that feature, consult L<perlref/Postfix Dereference Syntax>.
160 =head2 Auto-increment and Auto-decrement
161 X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
163 "++" and "--" work as in C. That is, if placed before a variable,
164 they increment or decrement the variable by one before returning the
165 value, and if placed after, increment or decrement after returning the
169 print $i++; # prints 0
170 print ++$j; # prints 1
172 Note that just as in C, Perl doesn't define B<when> the variable is
173 incremented or decremented. You just know it will be done sometime
174 before or after the value is returned. This also means that modifying
175 a variable twice in the same statement will lead to undefined behavior.
176 Avoid statements like:
181 Perl will not guarantee what the result of the above statements is.
183 The auto-increment operator has a little extra builtin magic to it. If
184 you increment a variable that is numeric, or that has ever been used in
185 a numeric context, you get a normal increment. If, however, the
186 variable has been used in only string contexts since it was set, and
187 has a value that is not the empty string and matches the pattern
188 C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
189 character within its range, with carry:
191 print ++($foo = "99"); # prints "100"
192 print ++($foo = "a0"); # prints "a1"
193 print ++($foo = "Az"); # prints "Ba"
194 print ++($foo = "zz"); # prints "aaa"
196 C<undef> is always treated as numeric, and in particular is changed
197 to C<0> before incrementing (so that a post-increment of an undef value
198 will return C<0> rather than C<undef>).
200 The auto-decrement operator is not magical.
202 =head2 Exponentiation
203 X<**> X<exponentiation> X<power>
205 Binary "**" is the exponentiation operator. It binds even more
206 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
207 implemented using C's pow(3) function, which actually works on doubles
210 Note that certain exponentiation expressions are ill-defined:
211 these include C<0**0>, C<1**Inf>, and C<Inf**0>. Do not expect
212 any particular results from these special cases, the results
213 are platform-dependent.
215 =head2 Symbolic Unary Operators
216 X<unary operator> X<operator, unary>
218 Unary "!" performs logical negation, that is, "not". See also C<not> for a lower
219 precedence version of this.
222 Unary "-" performs arithmetic negation if the operand is numeric,
223 including any string that looks like a number. If the operand is
224 an identifier, a string consisting of a minus sign concatenated
225 with the identifier is returned. Otherwise, if the string starts
226 with a plus or minus, a string starting with the opposite sign is
227 returned. One effect of these rules is that -bareword is equivalent
228 to the string "-bareword". If, however, the string begins with a
229 non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
230 the string to a numeric and the arithmetic negation is performed. If the
231 string cannot be cleanly converted to a numeric, Perl will give the warning
232 B<Argument "the string" isn't numeric in negation (-) at ...>.
233 X<-> X<negation, arithmetic>
235 Unary "~" performs bitwise negation, that is, 1's complement. For
236 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
237 L<Bitwise String Operators>.) Note that the width of the result is
238 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
239 bits wide on a 64-bit platform, so if you are expecting a certain bit
240 width, remember to use the "&" operator to mask off the excess bits.
241 X<~> X<negation, binary>
243 When complementing strings, if all characters have ordinal values under
244 256, then their complements will, also. But if they do not, all
245 characters will be in either 32- or 64-bit complements, depending on your
246 architecture. So for example, C<~"\x{3B1}"> is C<"\x{FFFF_FC4E}"> on
247 32-bit machines and C<"\x{FFFF_FFFF_FFFF_FC4E}"> on 64-bit machines.
249 Unary "+" has no effect whatsoever, even on strings. It is useful
250 syntactically for separating a function name from a parenthesized expression
251 that would otherwise be interpreted as the complete list of function
252 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
255 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
256 and L<perlref>. Do not confuse this behavior with the behavior of
257 backslash within a string, although both forms do convey the notion
258 of protecting the next thing from interpolation.
259 X<\> X<reference> X<backslash>
261 =head2 Binding Operators
262 X<binding> X<operator, binding> X<=~> X<!~>
264 Binary "=~" binds a scalar expression to a pattern match. Certain operations
265 search or modify the string $_ by default. This operator makes that kind
266 of operation work on some other string. The right argument is a search
267 pattern, substitution, or transliteration. The left argument is what is
268 supposed to be searched, substituted, or transliterated instead of the default
269 $_. When used in scalar context, the return value generally indicates the
270 success of the operation. The exceptions are substitution (s///)
271 and transliteration (y///) with the C</r> (non-destructive) option,
272 which cause the B<r>eturn value to be the result of the substitution.
273 Behavior in list context depends on the particular operator.
274 See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
275 examples using these operators.
277 If the right argument is an expression rather than a search pattern,
278 substitution, or transliteration, it is interpreted as a search pattern at run
279 time. Note that this means that its
280 contents will be interpolated twice, so
284 is not ok, as the regex engine will end up trying to compile the
285 pattern C<\>, which it will consider a syntax error.
287 Binary "!~" is just like "=~" except the return value is negated in
290 Binary "!~" with a non-destructive substitution (s///r) or transliteration
291 (y///r) is a syntax error.
293 =head2 Multiplicative Operators
294 X<operator, multiplicative>
296 Binary "*" multiplies two numbers.
299 Binary "/" divides two numbers.
302 Binary "%" is the modulo operator, which computes the division
303 remainder of its first argument with respect to its second argument.
305 operands C<$m> and C<$n>: If C<$n> is positive, then C<$m % $n> is
306 C<$m> minus the largest multiple of C<$n> less than or equal to
307 C<$m>. If C<$n> is negative, then C<$m % $n> is C<$m> minus the
308 smallest multiple of C<$n> that is not less than C<$m> (that is, the
309 result will be less than or equal to zero). If the operands
310 C<$m> and C<$n> are floating point values and the absolute value of
311 C<$n> (that is C<abs($n)>) is less than C<(UV_MAX + 1)>, only
312 the integer portion of C<$m> and C<$n> will be used in the operation
313 (Note: here C<UV_MAX> means the maximum of the unsigned integer type).
314 If the absolute value of the right operand (C<abs($n)>) is greater than
315 or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
316 C<$r> in the equation C<($r = $m - $i*$n)> where C<$i> is a certain
317 integer that makes C<$r> have the same sign as the right operand
318 C<$n> (B<not> as the left operand C<$m> like C function C<fmod()>)
319 and the absolute value less than that of C<$n>.
320 Note that when C<use integer> is in scope, "%" gives you direct access
321 to the modulo operator as implemented by your C compiler. This
322 operator is not as well defined for negative operands, but it will
324 X<%> X<remainder> X<modulo> X<mod>
326 Binary "x" is the repetition operator. In scalar context or if the left
327 operand is not enclosed in parentheses, it returns a string consisting
328 of the left operand repeated the number of times specified by the right
329 operand. In list context, if the left operand is enclosed in
330 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
331 If the right operand is zero or negative (raising a warning on
332 negative), it returns an empty string
333 or an empty list, depending on the context.
336 print '-' x 80; # print row of dashes
338 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
340 @ones = (1) x 80; # a list of 80 1's
341 @ones = (5) x @ones; # set all elements to 5
344 =head2 Additive Operators
345 X<operator, additive>
347 Binary C<+> returns the sum of two numbers.
350 Binary C<-> returns the difference of two numbers.
353 Binary C<.> concatenates two strings.
354 X<string, concatenation> X<concatenation>
355 X<cat> X<concat> X<concatenate> X<.>
357 =head2 Shift Operators
358 X<shift operator> X<operator, shift> X<<< << >>>
359 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
360 X<shl> X<shr> X<shift, right> X<shift, left>
362 Binary C<<< << >>> returns the value of its left argument shifted left by the
363 number of bits specified by the right argument. Arguments should be
364 integers. (See also L<Integer Arithmetic>.)
366 Binary C<<< >> >>> returns the value of its left argument shifted right by
367 the number of bits specified by the right argument. Arguments should
368 be integers. (See also L<Integer Arithmetic>.)
370 Note that both C<<< << >>> and C<<< >> >>> in Perl are implemented directly using
371 C<<< << >>> and C<<< >> >>> in C. If C<use integer> (see L<Integer Arithmetic>) is
372 in force then signed C integers are used, else unsigned C integers are
373 used. Either way, the implementation isn't going to generate results
374 larger than the size of the integer type Perl was built with (32 bits
377 The result of overflowing the range of the integers is undefined
378 because it is undefined also in C. In other words, using 32-bit
379 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
380 of bits is also undefined.
382 If you get tired of being subject to your platform's native integers,
383 the C<use bigint> pragma neatly sidesteps the issue altogether:
385 print 20 << 20; # 20971520
386 print 20 << 40; # 5120 on 32-bit machines,
387 # 21990232555520 on 64-bit machines
389 print 20 << 100; # 25353012004564588029934064107520
391 =head2 Named Unary Operators
392 X<operator, named unary>
394 The various named unary operators are treated as functions with one
395 argument, with optional parentheses.
397 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
398 is followed by a left parenthesis as the next token, the operator and
399 arguments within parentheses are taken to be of highest precedence,
400 just like a normal function call. For example,
401 because named unary operators are higher precedence than C<||>:
403 chdir $foo || die; # (chdir $foo) || die
404 chdir($foo) || die; # (chdir $foo) || die
405 chdir ($foo) || die; # (chdir $foo) || die
406 chdir +($foo) || die; # (chdir $foo) || die
408 but, because * is higher precedence than named operators:
410 chdir $foo * 20; # chdir ($foo * 20)
411 chdir($foo) * 20; # (chdir $foo) * 20
412 chdir ($foo) * 20; # (chdir $foo) * 20
413 chdir +($foo) * 20; # chdir ($foo * 20)
415 rand 10 * 20; # rand (10 * 20)
416 rand(10) * 20; # (rand 10) * 20
417 rand (10) * 20; # (rand 10) * 20
418 rand +(10) * 20; # rand (10 * 20)
420 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
421 treated like named unary operators, but they don't follow this functional
422 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
423 equivalent to C<-f "$file.bak">.
424 X<-X> X<filetest> X<operator, filetest>
426 See also L<"Terms and List Operators (Leftward)">.
428 =head2 Relational Operators
429 X<relational operator> X<operator, relational>
431 Perl operators that return true or false generally return values
432 that can be safely used as numbers. For example, the relational
433 operators in this section and the equality operators in the next
434 one return C<1> for true and a special version of the defined empty
435 string, C<"">, which counts as a zero but is exempt from warnings
436 about improper numeric conversions, just as C<"0 but true"> is.
438 Binary "<" returns true if the left argument is numerically less than
442 Binary ">" returns true if the left argument is numerically greater
443 than the right argument.
446 Binary "<=" returns true if the left argument is numerically less than
447 or equal to the right argument.
450 Binary ">=" returns true if the left argument is numerically greater
451 than or equal to the right argument.
454 Binary "lt" returns true if the left argument is stringwise less than
458 Binary "gt" returns true if the left argument is stringwise greater
459 than the right argument.
462 Binary "le" returns true if the left argument is stringwise less than
463 or equal to the right argument.
466 Binary "ge" returns true if the left argument is stringwise greater
467 than or equal to the right argument.
470 =head2 Equality Operators
471 X<equality> X<equal> X<equals> X<operator, equality>
473 Binary "==" returns true if the left argument is numerically equal to
477 Binary "!=" returns true if the left argument is numerically not equal
478 to the right argument.
481 Binary "<=>" returns -1, 0, or 1 depending on whether the left
482 argument is numerically less than, equal to, or greater than the right
483 argument. If your platform supports NaNs (not-a-numbers) as numeric
484 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
485 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
486 returns true, as does NaN != anything else. If your platform doesn't
487 support NaNs then NaN is just a string with numeric value 0.
488 X<< <=> >> X<spaceship>
490 $ perl -le '$x = "NaN"; print "No NaN support here" if $x == $x'
491 $ perl -le '$x = "NaN"; print "NaN support here" if $x != $x'
493 (Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all
496 Binary "eq" returns true if the left argument is stringwise equal to
500 Binary "ne" returns true if the left argument is stringwise not equal
501 to the right argument.
504 Binary "cmp" returns -1, 0, or 1 depending on whether the left
505 argument is stringwise less than, equal to, or greater than the right
509 Binary "~~" does a smartmatch between its arguments. Smart matching
510 is described in the next section.
513 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
514 by the current locale if a legacy C<use locale> (but not
515 C<use locale ':not_characters'>) is in effect. See
516 L<perllocale>. Do not mix these with Unicode, only with legacy binary
517 encodings. The standard L<Unicode::Collate> and
518 L<Unicode::Collate::Locale> modules offer much more powerful solutions to
521 =head2 Smartmatch Operator
523 First available in Perl 5.10.1 (the 5.10.0 version behaved differently),
524 binary C<~~> does a "smartmatch" between its arguments. This is mostly
525 used implicitly in the C<when> construct described in L<perlsyn>, although
526 not all C<when> clauses call the smartmatch operator. Unique among all of
527 Perl's operators, the smartmatch operator can recurse. The smartmatch
528 operator is L<experimental|perlpolicy/experimental> and its behavior is
531 It is also unique in that all other Perl operators impose a context
532 (usually string or numeric context) on their operands, autoconverting
533 those operands to those imposed contexts. In contrast, smartmatch
534 I<infers> contexts from the actual types of its operands and uses that
535 type information to select a suitable comparison mechanism.
537 The C<~~> operator compares its operands "polymorphically", determining how
538 to compare them according to their actual types (numeric, string, array,
539 hash, etc.) Like the equality operators with which it shares the same
540 precedence, C<~~> returns 1 for true and C<""> for false. It is often best
541 read aloud as "in", "inside of", or "is contained in", because the left
542 operand is often looked for I<inside> the right operand. That makes the
543 order of the operands to the smartmatch operand often opposite that of
544 the regular match operator. In other words, the "smaller" thing is usually
545 placed in the left operand and the larger one in the right.
547 The behavior of a smartmatch depends on what type of things its arguments
548 are, as determined by the following table. The first row of the table
549 whose types apply determines the smartmatch behavior. Because what
550 actually happens is mostly determined by the type of the second operand,
551 the table is sorted on the right operand instead of on the left.
553 Left Right Description and pseudocode
554 ===============================================================
555 Any undef check whether Any is undefined
558 Any Object invoke ~~ overloading on Object, or die
560 Right operand is an ARRAY:
562 Left Right Description and pseudocode
563 ===============================================================
564 ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2]
565 like: (ARRAY1[0] ~~ ARRAY2[0])
566 && (ARRAY1[1] ~~ ARRAY2[1]) && ...
567 HASH ARRAY any ARRAY elements exist as HASH keys
568 like: grep { exists HASH->{$_} } ARRAY
569 Regexp ARRAY any ARRAY elements pattern match Regexp
570 like: grep { /Regexp/ } ARRAY
571 undef ARRAY undef in ARRAY
572 like: grep { !defined } ARRAY
573 Any ARRAY smartmatch each ARRAY element[3]
574 like: grep { Any ~~ $_ } ARRAY
576 Right operand is a HASH:
578 Left Right Description and pseudocode
579 ===============================================================
580 HASH1 HASH2 all same keys in both HASHes
582 grep { exists HASH2->{$_} } keys HASH1
583 ARRAY HASH any ARRAY elements exist as HASH keys
584 like: grep { exists HASH->{$_} } ARRAY
585 Regexp HASH any HASH keys pattern match Regexp
586 like: grep { /Regexp/ } keys HASH
587 undef HASH always false (undef can't be a key)
589 Any HASH HASH key existence
590 like: exists HASH->{Any}
592 Right operand is CODE:
594 Left Right Description and pseudocode
595 ===============================================================
596 ARRAY CODE sub returns true on all ARRAY elements[1]
597 like: !grep { !CODE->($_) } ARRAY
598 HASH CODE sub returns true on all HASH keys[1]
599 like: !grep { !CODE->($_) } keys HASH
600 Any CODE sub passed Any returns true
603 Right operand is a Regexp:
605 Left Right Description and pseudocode
606 ===============================================================
607 ARRAY Regexp any ARRAY elements match Regexp
608 like: grep { /Regexp/ } ARRAY
609 HASH Regexp any HASH keys match Regexp
610 like: grep { /Regexp/ } keys HASH
611 Any Regexp pattern match
612 like: Any =~ /Regexp/
616 Left Right Description and pseudocode
617 ===============================================================
618 Object Any invoke ~~ overloading on Object,
621 Any Num numeric equality
623 Num nummy[4] numeric equality
625 undef Any check whether undefined
627 Any Any string equality
636 Empty hashes or arrays match.
639 That is, each element smartmatches the element of the same index in the other array.[3]
642 If a circular reference is found, fall back to referential equality.
645 Either an actual number, or a string that looks like one.
649 The smartmatch implicitly dereferences any non-blessed hash or array
650 reference, so the C<I<HASH>> and C<I<ARRAY>> entries apply in those cases.
651 For blessed references, the C<I<Object>> entries apply. Smartmatches
652 involving hashes only consider hash keys, never hash values.
654 The "like" code entry is not always an exact rendition. For example, the
655 smartmatch operator short-circuits whenever possible, but C<grep> does
656 not. Also, C<grep> in scalar context returns the number of matches, but
657 C<~~> returns only true or false.
659 Unlike most operators, the smartmatch operator knows to treat C<undef>
663 @array = (1, 2, 3, undef, 4, 5);
664 say "some elements undefined" if undef ~~ @array;
666 Each operand is considered in a modified scalar context, the modification
667 being that array and hash variables are passed by reference to the
668 operator, which implicitly dereferences them. Both elements
669 of each pair are the same:
673 my %hash = (red => 1, blue => 2, green => 3,
674 orange => 4, yellow => 5, purple => 6,
675 black => 7, grey => 8, white => 9);
677 my @array = qw(red blue green);
679 say "some array elements in hash keys" if @array ~~ %hash;
680 say "some array elements in hash keys" if \@array ~~ \%hash;
682 say "red in array" if "red" ~~ @array;
683 say "red in array" if "red" ~~ \@array;
685 say "some keys end in e" if /e$/ ~~ %hash;
686 say "some keys end in e" if /e$/ ~~ \%hash;
688 Two arrays smartmatch if each element in the first array smartmatches
689 (that is, is "in") the corresponding element in the second array,
693 my @little = qw(red blue green);
694 my @bigger = ("red", "blue", [ "orange", "green" ] );
695 if (@little ~~ @bigger) { # true!
696 say "little is contained in bigger";
699 Because the smartmatch operator recurses on nested arrays, this
700 will still report that "red" is in the array.
703 my @array = qw(red blue green);
704 my $nested_array = [[[[[[[ @array ]]]]]]];
705 say "red in array" if "red" ~~ $nested_array;
707 If two arrays smartmatch each other, then they are deep
708 copies of each others' values, as this example reports:
711 my @a = (0, 1, 2, [3, [4, 5], 6], 7);
712 my @b = (0, 1, 2, [3, [4, 5], 6], 7);
714 if (@a ~~ @b && @b ~~ @a) {
715 say "a and b are deep copies of each other";
718 say "a smartmatches in b";
721 say "b smartmatches in a";
724 say "a and b don't smartmatch each other at all";
728 If you were to set C<$b[3] = 4>, then instead of reporting that "a and b
729 are deep copies of each other", it now reports that "b smartmatches in a".
730 That because the corresponding position in C<@a> contains an array that
731 (eventually) has a 4 in it.
733 Smartmatching one hash against another reports whether both contain the
734 same keys, no more and no less. This could be used to see whether two
735 records have the same field names, without caring what values those fields
736 might have. For example:
740 state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
742 my ($class, $init_fields) = @_;
744 die "Must supply (only) name, rank, and serial number"
745 unless $init_fields ~~ $REQUIRED_FIELDS;
750 or, if other non-required fields are allowed, use ARRAY ~~ HASH:
754 state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
756 my ($class, $init_fields) = @_;
758 die "Must supply (at least) name, rank, and serial number"
759 unless [keys %{$init_fields}] ~~ $REQUIRED_FIELDS;
764 The smartmatch operator is most often used as the implicit operator of a
765 C<when> clause. See the section on "Switch Statements" in L<perlsyn>.
767 =head3 Smartmatching of Objects
769 To avoid relying on an object's underlying representation, if the
770 smartmatch's right operand is an object that doesn't overload C<~~>,
771 it raises the exception "C<Smartmatching a non-overloaded object
772 breaks encapsulation>". That's because one has no business digging
773 around to see whether something is "in" an object. These are all
774 illegal on objects without a C<~~> overload:
780 However, you can change the way an object is smartmatched by overloading
781 the C<~~> operator. This is allowed to
782 extend the usual smartmatch semantics.
783 For objects that do have an C<~~> overload, see L<overload>.
785 Using an object as the left operand is allowed, although not very useful.
786 Smartmatching rules take precedence over overloading, so even if the
787 object in the left operand has smartmatch overloading, this will be
788 ignored. A left operand that is a non-overloaded object falls back on a
789 string or numeric comparison of whatever the C<ref> operator returns. That
794 does I<not> invoke the overload method with C<I<X>> as an argument.
795 Instead the above table is consulted as normal, and based on the type of
796 C<I<X>>, overloading may or may not be invoked. For simple strings or
797 numbers, in becomes equivalent to this:
799 $object ~~ $number ref($object) == $number
800 $object ~~ $string ref($object) eq $string
802 For example, this reports that the handle smells IOish
803 (but please don't really do this!):
806 my $fh = IO::Handle->new();
807 if ($fh ~~ /\bIO\b/) {
808 say "handle smells IOish";
811 That's because it treats C<$fh> as a string like
812 C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that.
815 X<operator, bitwise, and> X<bitwise and> X<&>
817 Binary "&" returns its operands ANDed together bit by bit. Although no
818 warning is currently raised, the result is not well defined when this operation
819 is performed on operands that aren't either numbers (see
820 L<Integer Arithmetic>) or bitstrings (see L<Bitwise String Operators>).
822 Note that "&" has lower priority than relational operators, so for example
823 the parentheses are essential in a test like
825 print "Even\n" if ($x & 1) == 0;
827 =head2 Bitwise Or and Exclusive Or
828 X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
831 Binary "|" returns its operands ORed together bit by bit.
833 Binary "^" returns its operands XORed together bit by bit.
835 Although no warning is currently raised, the results are not well
836 defined when these operations are performed on operands that aren't either
837 numbers (see L<Integer Arithmetic>) or bitstrings (see L<Bitwise String
840 Note that "|" and "^" have lower priority than relational operators, so
841 for example the brackets are essential in a test like
843 print "false\n" if (8 | 2) != 10;
845 =head2 C-style Logical And
846 X<&&> X<logical and> X<operator, logical, and>
848 Binary "&&" performs a short-circuit logical AND operation. That is,
849 if the left operand is false, the right operand is not even evaluated.
850 Scalar or list context propagates down to the right operand if it
853 =head2 C-style Logical Or
854 X<||> X<operator, logical, or>
856 Binary "||" performs a short-circuit logical OR operation. That is,
857 if the left operand is true, the right operand is not even evaluated.
858 Scalar or list context propagates down to the right operand if it
861 =head2 Logical Defined-Or
862 X<//> X<operator, logical, defined-or>
864 Although it has no direct equivalent in C, Perl's C<//> operator is related
865 to its C-style or. In fact, it's exactly the same as C<||>, except that it
866 tests the left hand side's definedness instead of its truth. Thus,
867 C<< EXPR1 // EXPR2 >> returns the value of C<< EXPR1 >> if it's defined,
868 otherwise, the value of C<< EXPR2 >> is returned.
869 (C<< EXPR1 >> is evaluated in scalar context, C<< EXPR2 >>
870 in the context of C<< // >> itself). Usually,
871 this is the same result as C<< defined(EXPR1) ? EXPR1 : EXPR2 >> (except that
872 the ternary-operator form can be used as a lvalue, while C<< EXPR1 // EXPR2 >>
873 cannot). This is very useful for
874 providing default values for variables. If you actually want to test if
875 at least one of C<$x> and C<$y> is defined, use C<defined($x // $y)>.
877 The C<||>, C<//> and C<&&> operators return the last value evaluated
878 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
879 portable way to find out the home directory might be:
884 // die "You're homeless!\n";
886 In particular, this means that you shouldn't use this
887 for selecting between two aggregates for assignment:
889 @a = @b || @c; # this is wrong
890 @a = scalar(@b) || @c; # really meant this
891 @a = @b ? @b : @c; # this works fine, though
893 As alternatives to C<&&> and C<||> when used for
894 control flow, Perl provides the C<and> and C<or> operators (see below).
895 The short-circuit behavior is identical. The precedence of "and"
896 and "or" is much lower, however, so that you can safely use them after a
897 list operator without the need for parentheses:
899 unlink "alpha", "beta", "gamma"
900 or gripe(), next LINE;
902 With the C-style operators that would have been written like this:
904 unlink("alpha", "beta", "gamma")
905 || (gripe(), next LINE);
907 It would be even more readable to write that this way:
909 unless(unlink("alpha", "beta", "gamma")) {
914 Using "or" for assignment is unlikely to do what you want; see below.
916 =head2 Range Operators
917 X<operator, range> X<range> X<..> X<...>
919 Binary ".." is the range operator, which is really two different
920 operators depending on the context. In list context, it returns a
921 list of values counting (up by ones) from the left value to the right
922 value. If the left value is greater than the right value then it
923 returns the empty list. The range operator is useful for writing
924 C<foreach (1..10)> loops and for doing slice operations on arrays. In
925 the current implementation, no temporary array is created when the
926 range operator is used as the expression in C<foreach> loops, but older
927 versions of Perl might burn a lot of memory when you write something
930 for (1 .. 1_000_000) {
934 The range operator also works on strings, using the magical
935 auto-increment, see below.
937 In scalar context, ".." returns a boolean value. The operator is
938 bistable, like a flip-flop, and emulates the line-range (comma)
939 operator of B<sed>, B<awk>, and various editors. Each ".." operator
940 maintains its own boolean state, even across calls to a subroutine
941 that contains it. It is false as long as its left operand is false.
942 Once the left operand is true, the range operator stays true until the
943 right operand is true, I<AFTER> which the range operator becomes false
944 again. It doesn't become false till the next time the range operator
945 is evaluated. It can test the right operand and become false on the
946 same evaluation it became true (as in B<awk>), but it still returns
947 true once. If you don't want it to test the right operand until the
948 next evaluation, as in B<sed>, just use three dots ("...") instead of
949 two. In all other regards, "..." behaves just like ".." does.
951 The right operand is not evaluated while the operator is in the
952 "false" state, and the left operand is not evaluated while the
953 operator is in the "true" state. The precedence is a little lower
954 than || and &&. The value returned is either the empty string for
955 false, or a sequence number (beginning with 1) for true. The sequence
956 number is reset for each range encountered. The final sequence number
957 in a range has the string "E0" appended to it, which doesn't affect
958 its numeric value, but gives you something to search for if you want
959 to exclude the endpoint. You can exclude the beginning point by
960 waiting for the sequence number to be greater than 1.
962 If either operand of scalar ".." is a constant expression,
963 that operand is considered true if it is equal (C<==>) to the current
964 input line number (the C<$.> variable).
966 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
967 but that is only an issue if you use a floating point expression; when
968 implicitly using C<$.> as described in the previous paragraph, the
969 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
970 is set to a floating point value and you are not reading from a file.
971 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
972 you want in scalar context because each of the operands are evaluated
973 using their integer representation.
977 As a scalar operator:
979 if (101 .. 200) { print; } # print 2nd hundred lines, short for
980 # if ($. == 101 .. $. == 200) { print; }
982 next LINE if (1 .. /^$/); # skip header lines, short for
983 # next LINE if ($. == 1 .. /^$/);
984 # (typically in a loop labeled LINE)
986 s/^/> / if (/^$/ .. eof()); # quote body
988 # parse mail messages
990 $in_header = 1 .. /^$/;
991 $in_body = /^$/ .. eof;
998 close ARGV if eof; # reset $. each file
1001 Here's a simple example to illustrate the difference between
1002 the two range operators:
1015 This program will print only the line containing "Bar". If
1016 the range operator is changed to C<...>, it will also print the
1019 And now some examples as a list operator:
1021 for (101 .. 200) { print } # print $_ 100 times
1022 @foo = @foo[0 .. $#foo]; # an expensive no-op
1023 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
1025 The range operator (in list context) makes use of the magical
1026 auto-increment algorithm if the operands are strings. You
1029 @alphabet = ("A" .. "Z");
1031 to get all normal letters of the English alphabet, or
1033 $hexdigit = (0 .. 9, "a" .. "f")[$num & 15];
1035 to get a hexadecimal digit, or
1037 @z2 = ("01" .. "31");
1040 to get dates with leading zeros.
1042 If the final value specified is not in the sequence that the magical
1043 increment would produce, the sequence goes until the next value would
1044 be longer than the final value specified.
1046 If the initial value specified isn't part of a magical increment
1047 sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>),
1048 only the initial value will be returned. So the following will only
1051 use charnames "greek";
1052 my @greek_small = ("\N{alpha}" .. "\N{omega}");
1054 To get the 25 traditional lowercase Greek letters, including both sigmas,
1055 you could use this instead:
1057 use charnames "greek";
1058 my @greek_small = map { chr } ( ord("\N{alpha}")
1063 However, because there are I<many> other lowercase Greek characters than
1064 just those, to match lowercase Greek characters in a regular expression,
1065 you could use the pattern C</(?:(?=\p{Greek})\p{Lower})+/> (or the
1066 L<experimental feature|perlrecharclass/Extended Bracketed Character
1067 Classes> C<S</(?[ \p{Greek} & \p{Lower} ])+/>>).
1069 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
1070 return two elements in list context.
1072 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
1074 =head2 Conditional Operator
1075 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
1077 Ternary "?:" is the conditional operator, just as in C. It works much
1078 like an if-then-else. If the argument before the ? is true, the
1079 argument before the : is returned, otherwise the argument after the :
1080 is returned. For example:
1082 printf "I have %d dog%s.\n", $n,
1083 ($n == 1) ? "" : "s";
1085 Scalar or list context propagates downward into the 2nd
1086 or 3rd argument, whichever is selected.
1088 $x = $ok ? $y : $z; # get a scalar
1089 @x = $ok ? @y : @z; # get an array
1090 $x = $ok ? @y : @z; # oops, that's just a count!
1092 The operator may be assigned to if both the 2nd and 3rd arguments are
1093 legal lvalues (meaning that you can assign to them):
1095 ($x_or_y ? $x : $y) = $z;
1097 Because this operator produces an assignable result, using assignments
1098 without parentheses will get you in trouble. For example, this:
1100 $x % 2 ? $x += 10 : $x += 2
1104 (($x % 2) ? ($x += 10) : $x) += 2
1108 ($x % 2) ? ($x += 10) : ($x += 2)
1110 That should probably be written more simply as:
1112 $x += ($x % 2) ? 10 : 2;
1114 =head2 Assignment Operators
1115 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
1116 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
1119 "=" is the ordinary assignment operator.
1121 Assignment operators work as in C. That is,
1129 although without duplicating any side effects that dereferencing the lvalue
1130 might trigger, such as from tie(). Other assignment operators work similarly.
1131 The following are recognized:
1133 **= += *= &= <<= &&=
1138 Although these are grouped by family, they all have the precedence
1139 of assignment. These combined assignment operators can only operate on
1140 scalars, whereas the ordinary assignment operator can assign to arrays,
1141 hashes, lists and even references. (See L<"Context"|perldata/Context>
1142 and L<perldata/List value constructors>, and L<perlref/Assigning to
1145 Unlike in C, the scalar assignment operator produces a valid lvalue.
1146 Modifying an assignment is equivalent to doing the assignment and
1147 then modifying the variable that was assigned to. This is useful
1148 for modifying a copy of something, like this:
1150 ($tmp = $global) =~ tr/13579/24680/;
1152 Although as of 5.14, that can be also be accomplished this way:
1155 $tmp = ($global =~ tr/13579/24680/r);
1166 Similarly, a list assignment in list context produces the list of
1167 lvalues assigned to, and a list assignment in scalar context returns
1168 the number of elements produced by the expression on the right hand
1169 side of the assignment.
1171 =head2 Comma Operator
1172 X<comma> X<operator, comma> X<,>
1174 Binary "," is the comma operator. In scalar context it evaluates
1175 its left argument, throws that value away, then evaluates its right
1176 argument and returns that value. This is just like C's comma operator.
1178 In list context, it's just the list argument separator, and inserts
1179 both its arguments into the list. These arguments are also evaluated
1182 The C<< => >> operator is a synonym for the comma except that it causes a
1183 word on its left to be interpreted as a string if it begins with a letter
1184 or underscore and is composed only of letters, digits and underscores.
1185 This includes operands that might otherwise be interpreted as operators,
1186 constants, single number v-strings or function calls. If in doubt about
1187 this behavior, the left operand can be quoted explicitly.
1189 Otherwise, the C<< => >> operator behaves exactly as the comma operator
1190 or list argument separator, according to context.
1194 use constant FOO => "something";
1196 my %h = ( FOO => 23 );
1200 my %h = ("FOO", 23);
1204 my %h = ("something", 23);
1206 The C<< => >> operator is helpful in documenting the correspondence
1207 between keys and values in hashes, and other paired elements in lists.
1209 %hash = ( $key => $value );
1210 login( $username => $password );
1212 The special quoting behavior ignores precedence, and hence may apply to
1213 I<part> of the left operand:
1215 print time.shift => "bbb";
1217 That example prints something like "1314363215shiftbbb", because the
1218 C<< => >> implicitly quotes the C<shift> immediately on its left, ignoring
1219 the fact that C<time.shift> is the entire left operand.
1221 =head2 List Operators (Rightward)
1222 X<operator, list, rightward> X<list operator>
1224 On the right side of a list operator, the comma has very low precedence,
1225 such that it controls all comma-separated expressions found there.
1226 The only operators with lower precedence are the logical operators
1227 "and", "or", and "not", which may be used to evaluate calls to list
1228 operators without the need for parentheses:
1230 open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n";
1232 However, some people find that code harder to read than writing
1233 it with parentheses:
1235 open(HANDLE, "< :utf8", "filename") or die "Can't open: $!\n";
1237 in which case you might as well just use the more customary "||" operator:
1239 open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n";
1241 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
1244 X<operator, logical, not> X<not>
1246 Unary "not" returns the logical negation of the expression to its right.
1247 It's the equivalent of "!" except for the very low precedence.
1250 X<operator, logical, and> X<and>
1252 Binary "and" returns the logical conjunction of the two surrounding
1253 expressions. It's equivalent to C<&&> except for the very low
1254 precedence. This means that it short-circuits: the right
1255 expression is evaluated only if the left expression is true.
1257 =head2 Logical or and Exclusive Or
1258 X<operator, logical, or> X<operator, logical, xor>
1259 X<operator, logical, exclusive or>
1262 Binary "or" returns the logical disjunction of the two surrounding
1263 expressions. It's equivalent to C<||> except for the very low precedence.
1264 This makes it useful for control flow:
1266 print FH $data or die "Can't write to FH: $!";
1268 This means that it short-circuits: the right expression is evaluated
1269 only if the left expression is false. Due to its precedence, you must
1270 be careful to avoid using it as replacement for the C<||> operator.
1271 It usually works out better for flow control than in assignments:
1273 $x = $y or $z; # bug: this is wrong
1274 ($x = $y) or $z; # really means this
1275 $x = $y || $z; # better written this way
1277 However, when it's a list-context assignment and you're trying to use
1278 C<||> for control flow, you probably need "or" so that the assignment
1279 takes higher precedence.
1281 @info = stat($file) || die; # oops, scalar sense of stat!
1282 @info = stat($file) or die; # better, now @info gets its due
1284 Then again, you could always use parentheses.
1286 Binary C<xor> returns the exclusive-OR of the two surrounding expressions.
1287 It cannot short-circuit (of course).
1289 There is no low precedence operator for defined-OR.
1291 =head2 C Operators Missing From Perl
1292 X<operator, missing from perl> X<&> X<*>
1293 X<typecasting> X<(TYPE)>
1295 Here is what C has that Perl doesn't:
1301 Address-of operator. (But see the "\" operator for taking a reference.)
1305 Dereference-address operator. (Perl's prefix dereferencing
1306 operators are typed: $, @, %, and &.)
1310 Type-casting operator.
1314 =head2 Quote and Quote-like Operators
1315 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
1316 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
1317 X<escape sequence> X<escape>
1319 While we usually think of quotes as literal values, in Perl they
1320 function as operators, providing various kinds of interpolating and
1321 pattern matching capabilities. Perl provides customary quote characters
1322 for these behaviors, but also provides a way for you to choose your
1323 quote character for any of them. In the following table, a C<{}> represents
1324 any pair of delimiters you choose.
1326 Customary Generic Meaning Interpolates
1329 `` qx{} Command yes*
1331 // m{} Pattern match yes*
1333 s{}{} Substitution yes*
1334 tr{}{} Transliteration no (but see below)
1335 y{}{} Transliteration no (but see below)
1338 * unless the delimiter is ''.
1340 Non-bracketing delimiters use the same character fore and aft, but the four
1341 sorts of ASCII brackets (round, angle, square, curly) all nest, which means
1350 Note, however, that this does not always work for quoting Perl code:
1352 $s = q{ if($x eq "}") ... }; # WRONG
1354 is a syntax error. The C<Text::Balanced> module (standard as of v5.8,
1355 and from CPAN before then) is able to do this properly.
1357 There can be whitespace between the operator and the quoting
1358 characters, except when C<#> is being used as the quoting character.
1359 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
1360 operator C<q> followed by a comment. Its argument will be taken
1361 from the next line. This allows you to write:
1363 s {foo} # Replace foo
1366 The following escape sequences are available in constructs that interpolate,
1367 and in transliterations:
1368 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
1371 Sequence Note Description
1377 \a alarm (bell) (BEL)
1379 \x{263A} [1,8] hex char (example: SMILEY)
1380 \x1b [2,8] restricted range hex char (example: ESC)
1381 \N{name} [3] named Unicode character or character sequence
1382 \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
1383 \c[ [5] control char (example: chr(27))
1384 \o{23072} [6,8] octal char (example: SMILEY)
1385 \033 [7,8] restricted range octal char (example: ESC)
1391 The result is the character specified by the hexadecimal number between
1392 the braces. See L</[8]> below for details on which character.
1394 Only hexadecimal digits are valid between the braces. If an invalid
1395 character is encountered, a warning will be issued and the invalid
1396 character and all subsequent characters (valid or invalid) within the
1397 braces will be discarded.
1399 If there are no valid digits between the braces, the generated character is
1400 the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
1401 will not cause a warning (currently).
1405 The result is the character specified by the hexadecimal number in the range
1406 0x00 to 0xFF. See L</[8]> below for details on which character.
1408 Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
1409 by fewer than two valid digits, any valid digits will be zero-padded. This
1410 means that C<\x7> will be interpreted as C<\x07>, and a lone <\x> will be
1411 interpreted as C<\x00>. Except at the end of a string, having fewer than
1412 two valid digits will result in a warning. Note that although the warning
1413 says the illegal character is ignored, it is only ignored as part of the
1414 escape and will still be used as the subsequent character in the string.
1417 Original Result Warns?
1425 The result is the Unicode character or character sequence given by I<name>.
1430 C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code
1431 point is I<hexadecimal number>.
1435 The character following C<\c> is mapped to some other character as shown in the
1451 \c? chr(127) # (on ASCII platforms)
1453 In other words, it's the character whose code point has had 64 xor'd with
1454 its uppercase. C<\c?> is DELETE on ASCII platforms because
1455 S<C<ord("?") ^ 64>> is 127, and
1456 C<\c@> is NULL because the ord of "@" is 64, so xor'ing 64 itself produces 0.
1458 Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the
1459 end of a string, because the backslash would be parsed as escaping the end
1462 On ASCII platforms, the resulting characters from the list above are the
1463 complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
1464 L<perlebcdic/OPERATOR DIFFERENCES> for a full discussion of the
1465 differences between these for ASCII versus EBCDIC platforms.
1467 Use of any other character following the C<"c"> besides those listed above is
1468 discouraged, and as of Perl v5.20, the only characters actually allowed
1469 are the printable ASCII ones, minus the left brace C<"{">. What happens
1470 for any of the allowed other characters is that the value is derived by
1471 xor'ing with the seventh bit, which is 64, and a warning raised if
1472 enabled. Using the non-allowed characters generates a fatal error.
1474 To get platform independent controls, you can use C<\N{...}>.
1478 The result is the character specified by the octal number between the braces.
1479 See L</[8]> below for details on which character.
1481 If a character that isn't an octal digit is encountered, a warning is raised,
1482 and the value is based on the octal digits before it, discarding it and all
1483 following characters up to the closing brace. It is a fatal error if there are
1484 no octal digits at all.
1488 The result is the character specified by the three-digit octal number in the
1489 range 000 to 777 (but best to not use above 077, see next paragraph). See
1490 L</[8]> below for details on which character.
1492 Some contexts allow 2 or even 1 digit, but any usage without exactly
1493 three digits, the first being a zero, may give unintended results. (For
1494 example, in a regular expression it may be confused with a backreference;
1495 see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
1496 use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to
1497 use this construct only for ordinals C<\077> and below, remembering to pad to
1498 the left with zeros to make three digits. For larger ordinals, either use
1499 C<\o{}>, or convert to something else, such as to hex and use C<\x{}>
1504 Several constructs above specify a character by a number. That number
1505 gives the character's position in the character set encoding (indexed from 0).
1506 This is called synonymously its ordinal, code position, or code point. Perl
1507 works on platforms that have a native encoding currently of either ASCII/Latin1
1508 or EBCDIC, each of which allow specification of 256 characters. In general, if
1509 the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
1510 native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
1511 it as a Unicode code point and the result is the corresponding Unicode
1512 character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
1513 decimal, which is less than 256, so the number is interpreted in the native
1514 character set encoding. In ASCII the character in the 80th position (indexed
1515 from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".
1516 C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
1517 as a Unicode code point no matter what the native encoding is. The name of the
1518 character in the 256th position (indexed by 0) in Unicode is
1519 C<LATIN CAPITAL LETTER A WITH MACRON>.
1521 There are a couple of exceptions to the above rule. S<C<\N{U+I<hex number>}>> is
1522 always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even
1523 on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the
1524 number is considered to be in that encoding, and is translated from that into
1525 the platform's native encoding if there is a corresponding native character;
1526 otherwise to Unicode.
1530 B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
1531 the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may
1534 does have meaning in regular expression patterns in Perl, see L<perlre>.)
1536 The following escape sequences are available in constructs that interpolate,
1537 but not in transliterations.
1538 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> X<\F>
1540 \l lowercase next character only
1541 \u titlecase (not uppercase!) next character only
1542 \L lowercase all characters till \E or end of string
1543 \U uppercase all characters till \E or end of string
1544 \F foldcase all characters till \E or end of string
1545 \Q quote (disable) pattern metacharacters till \E or
1547 \E end either case modification or quoted section
1548 (whichever was last seen)
1550 See L<perlfunc/quotemeta> for the exact definition of characters that
1551 are quoted by C<\Q>.
1553 C<\L>, C<\U>, C<\F>, and C<\Q> can stack, in which case you need one
1554 C<\E> for each. For example:
1556 say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
1557 This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?
1559 If C<use locale> is in effect (but not C<use locale ':not_characters'>),
1560 the case map used by C<\l>, C<\L>,
1561 C<\u>, and C<\U> is taken from the current locale. See L<perllocale>.
1562 If Unicode (for example, C<\N{}> or code points of 0x100 or
1563 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u>, and
1564 C<\U> is as defined by Unicode. That means that case-mapping
1565 a single character can sometimes produce several characters.
1566 Under C<use locale>, C<\F> produces the same results as C<\L>
1567 for all locales but a UTF-8 one, where it instead uses the Unicode
1570 All systems use the virtual C<"\n"> to represent a line terminator,
1571 called a "newline". There is no such thing as an unvarying, physical
1572 newline character. It is only an illusion that the operating system,
1573 device drivers, C libraries, and Perl all conspire to preserve. Not all
1574 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1575 on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,
1576 and on systems without line terminator,
1577 printing C<"\n"> might emit no actual data. In general, use C<"\n"> when
1578 you mean a "newline" for your system, but use the literal ASCII when you
1579 need an exact character. For example, most networking protocols expect
1580 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
1581 and although they often accept just C<"\012">, they seldom tolerate just
1582 C<"\015">. If you get in the habit of using C<"\n"> for networking,
1583 you may be burned some day.
1584 X<newline> X<line terminator> X<eol> X<end of line>
1587 For constructs that do interpolate, variables beginning with "C<$>"
1588 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
1589 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1590 But method calls such as C<< $obj->meth >> are not.
1592 Interpolating an array or slice interpolates the elements in order,
1593 separated by the value of C<$">, so is equivalent to interpolating
1594 C<join $", @array>. "Punctuation" arrays such as C<@*> are usually
1595 interpolated only if the name is enclosed in braces C<@{*}>, but the
1596 arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces.
1598 For double-quoted strings, the quoting from C<\Q> is applied after
1599 interpolation and escapes are processed.
1601 "abc\Qfoo\tbar$s\Exyz"
1605 "abc" . quotemeta("foo\tbar$s") . "xyz"
1607 For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
1608 the quoting from C<\Q> is applied after interpolation is processed,
1609 but before escapes are processed. This allows the pattern to match
1610 literally (except for C<$> and C<@>). For example, the following matches:
1614 Because C<$> or C<@> trigger interpolation, you'll need to use something
1615 like C</\Quser\E\@\Qhost/> to match them literally.
1617 Patterns are subject to an additional level of interpretation as a
1618 regular expression. This is done as a second pass, after variables are
1619 interpolated, so that regular expressions may be incorporated into the
1620 pattern from the variables. If this is not what you want, use C<\Q> to
1621 interpolate a variable literally.
1623 Apart from the behavior described above, Perl does not expand
1624 multiple levels of interpolation. In particular, contrary to the
1625 expectations of shell programmers, back-quotes do I<NOT> interpolate
1626 within double quotes, nor do single quotes impede evaluation of
1627 variables when used within double quotes.
1629 =head2 Regexp Quote-Like Operators
1632 Here are the quote-like operators that apply to pattern
1633 matching and related activities.
1637 =item qr/STRING/msixpodualn
1638 X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
1640 This operator quotes (and possibly compiles) its I<STRING> as a regular
1641 expression. I<STRING> is interpolated the same way as I<PATTERN>
1642 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1643 is done. Returns a Perl value which may be used instead of the
1644 corresponding C</STRING/msixpodualn> expression. The returned value is a
1645 normalized version of the original pattern. It magically differs from
1646 a string containing the same characters: C<ref(qr/x/)> returns "Regexp";
1647 however, dereferencing it is not well defined (you currently get the
1648 normalized version of the original pattern, but this may change).
1653 $rex = qr/my.STRING/is;
1654 print $rex; # prints (?si-xm:my.STRING)
1661 The result may be used as a subpattern in a match:
1664 $string =~ /foo${re}bar/; # can be interpolated in other
1666 $string =~ $re; # or used standalone
1667 $string =~ /$re/; # or this way
1669 Since Perl may compile the pattern at the moment of execution of the qr()
1670 operator, using qr() may have speed advantages in some situations,
1671 notably if the result of qr() is used standalone:
1674 my $patterns = shift;
1675 my @compiled = map qr/$_/i, @$patterns;
1678 foreach my $pat (@compiled) {
1679 $success = 1, last if /$pat/;
1685 Precompilation of the pattern into an internal representation at
1686 the moment of qr() avoids a need to recompile the pattern every
1687 time a match C</$pat/> is attempted. (Perl has many other internal
1688 optimizations, but none would be triggered in the above example if
1689 we did not use qr() operator.)
1691 Options (specified by the following modifiers) are:
1693 m Treat string as multiple lines.
1694 s Treat string as single line. (Make . match a newline)
1695 i Do case-insensitive pattern matching.
1696 x Use extended regular expressions.
1697 p When matching preserve a copy of the matched string so
1698 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be
1700 o Compile pattern only once.
1701 a ASCII-restrict: Use ASCII for \d, \s, \w; specifying two
1702 a's further restricts /i matching so that no ASCII
1703 character will match a non-ASCII one.
1705 u Use Unicode rules.
1706 d Use Unicode or native charset, as in 5.12 and earlier.
1707 n Non-capture mode. Don't let () fill in $1, $2, etc...
1709 If a precompiled pattern is embedded in a larger pattern then the effect
1710 of "msixpluadn" will be propagated appropriately. The effect the "o"
1711 modifier has is not propagated, being restricted to those patterns
1712 explicitly using it.
1714 The last four modifiers listed above, added in Perl 5.14,
1715 control the character set rules, but C</a> is the only one you are likely
1716 to want to specify explicitly; the other three are selected
1717 automatically by various pragmas.
1719 See L<perlre> for additional information on valid syntax for STRING, and
1720 for a detailed look at the semantics of regular expressions. In
1721 particular, all modifiers except the largely obsolete C</o> are further
1722 explained in L<perlre/Modifiers>. C</o> is described in the next section.
1724 =item m/PATTERN/msixpodualngc
1725 X<m> X<operator, match>
1726 X<regexp, options> X<regexp> X<regex, options> X<regex>
1727 X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
1729 =item /PATTERN/msixpodualngc
1731 Searches a string for a pattern match, and in scalar context returns
1732 true if it succeeds, false if it fails. If no string is specified
1733 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1734 string specified with C<=~> need not be an lvalue--it may be the
1735 result of an expression evaluation, but remember the C<=~> binds
1736 rather tightly.) See also L<perlre>.
1738 Options are as described in C<qr//> above; in addition, the following match
1739 process modifiers are available:
1741 g Match globally, i.e., find all occurrences.
1742 c Do not reset search position on a failed match when /g is
1745 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1746 you can use any pair of non-whitespace (ASCII) characters
1747 as delimiters. This is particularly useful for matching path names
1748 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1749 the delimiter, then a match-only-once rule applies,
1750 described in C<m?PATTERN?> below. If "'" (single quote) is the delimiter,
1751 no interpolation is performed on the PATTERN.
1752 When using a character valid in an identifier, whitespace is required
1755 PATTERN may contain variables, which will be interpolated
1756 every time the pattern search is evaluated, except
1757 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1758 C<$|> are not interpolated because they look like end-of-string tests.)
1759 Perl will not recompile the pattern unless an interpolated
1760 variable that it contains changes. You can force Perl to skip the
1761 test and never recompile by adding a C</o> (which stands for "once")
1762 after the trailing delimiter.
1763 Once upon a time, Perl would recompile regular expressions
1764 unnecessarily, and this modifier was useful to tell it not to do so, in the
1765 interests of speed. But now, the only reasons to use C</o> are one of:
1771 The variables are thousands of characters long and you know that they
1772 don't change, and you need to wring out the last little bit of speed by
1773 having Perl skip testing for that. (There is a maintenance penalty for
1774 doing this, as mentioning C</o> constitutes a promise that you won't
1775 change the variables in the pattern. If you do change them, Perl won't
1780 you want the pattern to use the initial values of the variables
1781 regardless of whether they change or not. (But there are saner ways
1782 of accomplishing this than using C</o>.)
1786 If the pattern contains embedded code, such as
1789 $code = 'foo(?{ $x })';
1792 then perl will recompile each time, even though the pattern string hasn't
1793 changed, to ensure that the current value of C<$x> is seen each time.
1794 Use C</o> if you want to avoid this.
1798 The bottom line is that using C</o> is almost never a good idea.
1800 =item The empty pattern //
1802 If the PATTERN evaluates to the empty string, the last
1803 I<successfully> matched regular expression is used instead. In this
1804 case, only the C<g> and C<c> flags on the empty pattern are honored;
1805 the other flags are taken from the original pattern. If no match has
1806 previously succeeded, this will (silently) act instead as a genuine
1807 empty pattern (which will always match).
1809 Note that it's possible to confuse Perl into thinking C<//> (the empty
1810 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1811 good about this, but some pathological cases might trigger this, such as
1812 C<$x///> (is that C<($x) / (//)> or C<$x // />?) and C<print $fh //>
1813 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1814 will assume you meant defined-or. If you meant the empty regex, just
1815 use parentheses or spaces to disambiguate, or even prefix the empty
1816 regex with an C<m> (so C<//> becomes C<m//>).
1818 =item Matching in list context
1820 If the C</g> option is not used, C<m//> in list context returns a
1821 list consisting of the subexpressions matched by the parentheses in the
1822 pattern, that is, (C<$1>, C<$2>, C<$3>...) (Note that here C<$1> etc. are
1823 also set). When there are no parentheses in the pattern, the return
1824 value is the list C<(1)> for success.
1825 With or without parentheses, an empty list is returned upon failure.
1829 open(TTY, "+</dev/tty")
1830 || die "can't access /dev/tty: $!";
1832 <TTY> =~ /^y/i && foo(); # do foo if desired
1834 if (/Version: *([0-9.]*)/) { $version = $1; }
1836 next if m#^/usr/spool/uucp#;
1841 print if /$arg/o; # compile only once (no longer needed!)
1844 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1846 This last example splits $foo into the first two words and the
1847 remainder of the line, and assigns those three fields to $F1, $F2, and
1848 $Etc. The conditional is true if any variables were assigned; that is,
1849 if the pattern matched.
1851 The C</g> modifier specifies global pattern matching--that is,
1852 matching as many times as possible within the string. How it behaves
1853 depends on the context. In list context, it returns a list of the
1854 substrings matched by any capturing parentheses in the regular
1855 expression. If there are no parentheses, it returns a list of all
1856 the matched strings, as if there were parentheses around the whole
1859 In scalar context, each execution of C<m//g> finds the next match,
1860 returning true if it matches, and false if there is no further match.
1861 The position after the last match can be read or set using the C<pos()>
1862 function; see L<perlfunc/pos>. A failed match normally resets the
1863 search position to the beginning of the string, but you can avoid that
1864 by adding the C</c> modifier (for example, C<m//gc>). Modifying the target
1865 string also resets the search position.
1869 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1870 zero-width assertion that matches the exact position where the
1871 previous C<m//g>, if any, left off. Without the C</g> modifier, the
1872 C<\G> assertion still anchors at C<pos()> as it was at the start of
1873 the operation (see L<perlfunc/pos>), but the match is of course only
1874 attempted once. Using C<\G> without C</g> on a target string that has
1875 not previously had a C</g> match applied to it is the same as using
1876 the C<\A> assertion to match the beginning of the string. Note also
1877 that, currently, C<\G> is only properly supported when anchored at the
1878 very beginning of the pattern.
1883 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1887 while ($paragraph = <>) {
1888 while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
1894 Here's another way to check for sentences in a paragraph:
1896 my $sentence_rx = qr{
1897 (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or
1899 \p{Lu} # capital letter
1900 .*? # a bunch of anything
1901 (?<= \S ) # that ends in non-
1903 (?<! \b [DMS]r ) # but isn't a common abbr.
1907 [.?!] # followed by a sentence
1909 (?= $ | \s ) # in front of end-of-string
1913 while (my $paragraph = <>) {
1914 say "NEW PARAGRAPH";
1916 while ($paragraph =~ /($sentence_rx)/g) {
1917 printf "\tgot sentence %d: <%s>\n", ++$count, $1;
1921 Here's how to use C<m//gc> with C<\G>:
1926 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1928 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1930 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1932 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1934 The last example should print:
1944 Notice that the final match matched C<q> instead of C<p>, which a match
1945 without the C<\G> anchor would have done. Also note that the final match
1946 did not update C<pos>. C<pos> is only updated on a C</g> match. If the
1947 final match did indeed match C<p>, it's a good bet that you're running a
1948 very old (pre-5.6.0) version of Perl.
1950 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1951 combine several regexps like this to process a string part-by-part,
1952 doing different actions depending on which regexp matched. Each
1953 regexp tries to match where the previous one leaves off.
1956 $url = URI::URL->new( "http://example.com/" );
1957 die if $url eq "xXx";
1961 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1962 print(" lowercase"), redo LOOP
1963 if /\G\p{Ll}+\b[,.;]?\s*/gc;
1964 print(" UPPERCASE"), redo LOOP
1965 if /\G\p{Lu}+\b[,.;]?\s*/gc;
1966 print(" Capitalized"), redo LOOP
1967 if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
1968 print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
1969 print(" alphanumeric"), redo LOOP
1970 if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
1971 print(" line-noise"), redo LOOP if /\G\W+/gc;
1972 print ". That's all!\n";
1975 Here is the output (split into several lines):
1977 line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
1978 line-noise lowercase line-noise lowercase line-noise lowercase
1979 lowercase line-noise lowercase lowercase line-noise lowercase
1980 lowercase line-noise MiXeD line-noise. That's all!
1982 =item m?PATTERN?msixpodualngc
1983 X<?> X<operator, match-once>
1985 =item ?PATTERN?msixpodualngc
1987 This is just like the C<m/PATTERN/> search, except that it matches
1988 only once between calls to the reset() operator. This is a useful
1989 optimization when you want to see only the first occurrence of
1990 something in each file of a set of files, for instance. Only C<m??>
1991 patterns local to the current package are reset.
1995 # blank line between header and body
1998 reset if eof; # clear m?? status for next file
2001 Another example switched the first "latin1" encoding it finds
2002 to "utf8" in a pod file:
2004 s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
2006 The match-once behavior is controlled by the match delimiter being
2007 C<?>; with any other delimiter this is the normal C<m//> operator.
2009 In the past, the leading C<m> in C<m?PATTERN?> was optional, but omitting it
2010 would produce a deprecation warning. As of v5.22.0, omitting it produces a
2011 syntax error. If you encounter this construct in older code, you can just add
2014 =item s/PATTERN/REPLACEMENT/msixpodualngcer
2015 X<substitute> X<substitution> X<replace> X<regexp, replace>
2016 X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
2018 Searches a string for a pattern, and if found, replaces that pattern
2019 with the replacement text and returns the number of substitutions
2020 made. Otherwise it returns false (specifically, the empty string).
2022 If the C</r> (non-destructive) option is used then it runs the
2023 substitution on a copy of the string and instead of returning the
2024 number of substitutions, it returns the copy whether or not a
2025 substitution occurred. The original string is never changed when
2026 C</r> is used. The copy will always be a plain string, even if the
2027 input is an object or a tied variable.
2029 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
2030 variable is searched and modified. Unless the C</r> option is used,
2031 the string specified must be a scalar variable, an array element, a
2032 hash element, or an assignment to one of those; that is, some sort of
2035 If the delimiter chosen is a single quote, no interpolation is
2036 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
2037 PATTERN contains a $ that looks like a variable rather than an
2038 end-of-string test, the variable will be interpolated into the pattern
2039 at run-time. If you want the pattern compiled only once the first time
2040 the variable is interpolated, use the C</o> option. If the pattern
2041 evaluates to the empty string, the last successfully executed regular
2042 expression is used instead. See L<perlre> for further explanation on these.
2044 Options are as with m// with the addition of the following replacement
2047 e Evaluate the right side as an expression.
2048 ee Evaluate the right side as a string then eval the
2050 r Return substitution and leave the original string
2053 Any non-whitespace delimiter may replace the slashes. Add space after
2054 the C<s> when using a character allowed in identifiers. If single quotes
2055 are used, no interpretation is done on the replacement string (the C</e>
2056 modifier overrides this, however). Note that Perl treats backticks
2057 as normal delimiters; the replacement text is not evaluated as a command.
2058 If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
2059 its own pair of quotes, which may or may not be bracketing quotes, for example,
2060 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
2061 replacement portion to be treated as a full-fledged Perl expression
2062 and evaluated right then and there. It is, however, syntax checked at
2063 compile-time. A second C<e> modifier will cause the replacement portion
2064 to be C<eval>ed before being run as a Perl expression.
2068 s/\bgreen\b/mauve/g; # don't change wintergreen
2070 $path =~ s|/usr/bin|/usr/local/bin|;
2072 s/Login: $foo/Login: $bar/; # run-time pattern
2074 ($foo = $bar) =~ s/this/that/; # copy first, then
2076 ($foo = "$bar") =~ s/this/that/; # convert to string,
2078 $foo = $bar =~ s/this/that/r; # Same as above using /r
2079 $foo = $bar =~ s/this/that/r
2080 =~ s/that/the other/r; # Chained substitutes
2082 @foo = map { s/this/that/r } @bar # /r is very useful in
2085 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-cnt
2088 s/\d+/$&*2/e; # yields 'abc246xyz'
2089 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
2090 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
2092 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
2093 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
2094 s/^=(\w+)/pod($1)/ge; # use function call
2097 $x = s/abc/def/r; # $x is 'def123xyz' and
2098 # $_ remains 'abc123xyz'.
2100 # expand variables in $_, but dynamics only, using
2101 # symbolic dereferencing
2104 # Add one to the value of any numbers in the string
2107 # Titlecase words in the last 30 characters only
2108 substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;
2110 # This will expand any embedded scalar variable
2111 # (including lexicals) in $_ : First $1 is interpolated
2112 # to the variable name, and then evaluated
2115 # Delete (most) C comments.
2117 /\* # Match the opening delimiter.
2118 .*? # Match a minimal number of characters.
2119 \*/ # Match the closing delimiter.
2122 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_,
2125 for ($variable) { # trim whitespace in $variable,
2131 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
2133 Note the use of $ instead of \ in the last example. Unlike
2134 B<sed>, we use the \<I<digit>> form in only the left hand side.
2135 Anywhere else it's $<I<digit>>.
2137 Occasionally, you can't use just a C</g> to get all the changes
2138 to occur that you might want. Here are two common cases:
2140 # put commas in the right places in an integer
2141 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
2143 # expand tabs to 8-column spacing
2144 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
2148 =head2 Quote-Like Operators
2149 X<operator, quote-like>
2154 X<q> X<quote, single> X<'> X<''>
2158 A single-quoted, literal string. A backslash represents a backslash
2159 unless followed by the delimiter or another backslash, in which case
2160 the delimiter or backslash is interpolated.
2162 $foo = q!I said, "You said, 'She said it.'"!;
2163 $bar = q('This is it.');
2164 $baz = '\n'; # a two-character string
2167 X<qq> X<quote, double> X<"> X<"">
2171 A double-quoted, interpolated string.
2174 (*** The previous line contains the naughty word "$1".\n)
2175 if /\b(tcl|java|python)\b/i; # :-)
2176 $baz = "\n"; # a one-character string
2179 X<qx> X<`> X<``> X<backtick>
2183 A string which is (possibly) interpolated and then executed as a
2184 system command with F</bin/sh> or its equivalent. Shell wildcards,
2185 pipes, and redirections will be honored. The collected standard
2186 output of the command is returned; standard error is unaffected. In
2187 scalar context, it comes back as a single (potentially multi-line)
2188 string, or undef if the command failed. In list context, returns a
2189 list of lines (however you've defined lines with $/ or
2190 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
2192 Because backticks do not affect standard error, use shell file descriptor
2193 syntax (assuming the shell supports this) if you care to address this.
2194 To capture a command's STDERR and STDOUT together:
2196 $output = `cmd 2>&1`;
2198 To capture a command's STDOUT but discard its STDERR:
2200 $output = `cmd 2>/dev/null`;
2202 To capture a command's STDERR but discard its STDOUT (ordering is
2205 $output = `cmd 2>&1 1>/dev/null`;
2207 To exchange a command's STDOUT and STDERR in order to capture the STDERR
2208 but leave its STDOUT to come out the old STDERR:
2210 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
2212 To read both a command's STDOUT and its STDERR separately, it's easiest
2213 to redirect them separately to files, and then read from those files
2214 when the program is done:
2216 system("program args 1>program.stdout 2>program.stderr");
2218 The STDIN filehandle used by the command is inherited from Perl's STDIN.
2221 open(SPLAT, "stuff") || die "can't open stuff: $!";
2222 open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
2223 print STDOUT `sort`;
2225 will print the sorted contents of the file named F<"stuff">.
2227 Using single-quote as a delimiter protects the command from Perl's
2228 double-quote interpolation, passing it on to the shell instead:
2230 $perl_info = qx(ps $$); # that's Perl's $$
2231 $shell_info = qx'ps $$'; # that's the new shell's $$
2233 How that string gets evaluated is entirely subject to the command
2234 interpreter on your system. On most platforms, you will have to protect
2235 shell metacharacters if you want them treated literally. This is in
2236 practice difficult to do, as it's unclear how to escape which characters.
2237 See L<perlsec> for a clean and safe example of a manual fork() and exec()
2238 to emulate backticks safely.
2240 On some platforms (notably DOS-like ones), the shell may not be
2241 capable of dealing with multiline commands, so putting newlines in
2242 the string may not get you what you want. You may be able to evaluate
2243 multiple commands in a single line by separating them with the command
2244 separator character, if your shell supports that (for example, C<;> on
2245 many Unix shells and C<&> on the Windows NT C<cmd> shell).
2247 Perl will attempt to flush all files opened for
2248 output before starting the child process, but this may not be supported
2249 on some platforms (see L<perlport>). To be safe, you may need to set
2250 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2251 C<IO::Handle> on any open handles.
2253 Beware that some command shells may place restrictions on the length
2254 of the command line. You must ensure your strings don't exceed this
2255 limit after any necessary interpolations. See the platform-specific
2256 release notes for more details about your particular environment.
2258 Using this operator can lead to programs that are difficult to port,
2259 because the shell commands called vary between systems, and may in
2260 fact not be present at all. As one example, the C<type> command under
2261 the POSIX shell is very different from the C<type> command under DOS.
2262 That doesn't mean you should go out of your way to avoid backticks
2263 when they're the right way to get something done. Perl was made to be
2264 a glue language, and one of the things it glues together is commands.
2265 Just understand what you're getting yourself into.
2267 See L</"I/O Operators"> for more discussion.
2270 X<qw> X<quote, list> X<quote, words>
2272 Evaluates to a list of the words extracted out of STRING, using embedded
2273 whitespace as the word delimiters. It can be understood as being roughly
2276 split(" ", q/STRING/);
2278 the differences being that it generates a real list at compile time, and
2279 in scalar context it returns the last element in the list. So
2284 is semantically equivalent to the list:
2288 Some frequently seen examples:
2290 use POSIX qw( setlocale localeconv )
2291 @EXPORT = qw( foo bar baz );
2293 A common mistake is to try to separate the words with comma or to
2294 put comments into a multi-line C<qw>-string. For this reason, the
2295 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
2296 produces warnings if the STRING contains the "," or the "#" character.
2298 =item tr/SEARCHLIST/REPLACEMENTLIST/cdsr
2299 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
2301 =item y/SEARCHLIST/REPLACEMENTLIST/cdsr
2303 Transliterates all occurrences of the characters found in the search list
2304 with the corresponding character in the replacement list. It returns
2305 the number of characters replaced or deleted. If no string is
2306 specified via the C<=~> or C<!~> operator, the $_ string is transliterated.
2308 If the C</r> (non-destructive) option is present, a new copy of the string
2309 is made and its characters transliterated, and this copy is returned no
2310 matter whether it was modified or not: the original string is always
2311 left unchanged. The new copy is always a plain string, even if the input
2312 string is an object or a tied variable.
2314 Unless the C</r> option is used, the string specified with C<=~> must be a
2315 scalar variable, an array element, a hash element, or an assignment to one
2316 of those; in other words, an lvalue.
2318 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2319 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
2320 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
2321 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
2322 its own pair of quotes, which may or may not be bracketing quotes;
2323 for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>.
2325 Note that C<tr> does B<not> do regular expression character classes such as
2326 C<\d> or C<\pL>. The C<tr> operator is not equivalent to the tr(1)
2327 utility. If you want to map strings between lower/upper cases, see
2328 L<perlfunc/lc> and L<perlfunc/uc>, and in general consider using the C<s>
2329 operator if you need regular expressions. The C<\U>, C<\u>, C<\L>, and
2330 C<\l> string-interpolation escapes on the right side of a substitution
2331 operator will perform correct case-mappings, but C<tr[a-z][A-Z]> will not
2332 (except sometimes on legacy 7-bit data).
2334 Note also that the whole range idea is rather unportable between
2335 character sets--and even within character sets they may cause results
2336 you probably didn't expect. A sound principle is to use only ranges
2337 that begin from and end at either alphabets of equal case (a-e, A-E),
2338 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
2339 character sets in full.
2343 c Complement the SEARCHLIST.
2344 d Delete found but unreplaced characters.
2345 s Squash duplicate replaced characters.
2346 r Return the modified string and leave the original string
2349 If the C</c> modifier is specified, the SEARCHLIST character set
2350 is complemented. If the C</d> modifier is specified, any characters
2351 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
2352 (Note that this is slightly more flexible than the behavior of some
2353 B<tr> programs, which delete anything they find in the SEARCHLIST,
2354 period.) If the C</s> modifier is specified, sequences of characters
2355 that were transliterated to the same character are squashed down
2356 to a single instance of the character.
2358 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
2359 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
2360 than the SEARCHLIST, the final character is replicated till it is long
2361 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
2362 This latter is useful for counting characters in a class or for
2363 squashing character sequences in a class.
2367 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
2369 $cnt = tr/*/*/; # count the stars in $_
2371 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
2373 $cnt = tr/0-9//; # count the digits in $_
2375 tr/a-zA-Z//s; # bookkeeper -> bokeper
2377 ($HOST = $host) =~ tr/a-z/A-Z/;
2378 $HOST = $host =~ tr/a-z/A-Z/r; # same thing
2380 $HOST = $host =~ tr/a-z/A-Z/r # chained with s///r
2383 tr/a-zA-Z/ /cs; # change non-alphas to single space
2385 @stripped = map tr/a-zA-Z/ /csr, @original;
2389 [\000-\177]; # wickedly delete 8th bit
2391 If multiple transliterations are given for a character, only the
2396 will transliterate any A to X.
2398 Because the transliteration table is built at compile time, neither
2399 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
2400 interpolation. That means that if you want to use variables, you
2403 eval "tr/$oldlist/$newlist/";
2406 eval "tr/$oldlist/$newlist/, 1" or die $@;
2409 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
2411 A line-oriented form of quoting is based on the shell "here-document"
2412 syntax. Following a C<< << >> you specify a string to terminate
2413 the quoted material, and all lines following the current line down to
2414 the terminating string are the value of the item.
2416 The terminating string may be either an identifier (a word), or some
2417 quoted text. An unquoted identifier works like double quotes.
2418 There may not be a space between the C<< << >> and the identifier,
2419 unless the identifier is explicitly quoted. (If you put a space it
2420 will be treated as a null identifier, which is valid, and matches the
2421 first empty line.) The terminating string must appear by itself
2422 (unquoted and with no surrounding whitespace) on the terminating line.
2424 If the terminating string is quoted, the type of quotes used determine
2425 the treatment of the text.
2431 Double quotes indicate that the text will be interpolated using exactly
2432 the same rules as normal double quoted strings.
2435 The price is $Price.
2438 print << "EOF"; # same as above
2439 The price is $Price.
2445 Single quotes indicate the text is to be treated literally with no
2446 interpolation of its content. This is similar to single quoted
2447 strings except that backslashes have no special meaning, with C<\\>
2448 being treated as two backslashes and not one as they would in every
2449 other quoting construct.
2451 Just as in the shell, a backslashed bareword following the C<<< << >>>
2452 means the same thing as a single-quoted string does:
2454 $cost = <<'VISTA'; # hasta la ...
2455 That'll be $10 please, ma'am.
2458 $cost = <<\VISTA; # Same thing!
2459 That'll be $10 please, ma'am.
2462 This is the only form of quoting in perl where there is no need
2463 to worry about escaping content, something that code generators
2464 can and do make good use of.
2468 The content of the here doc is treated just as it would be if the
2469 string were embedded in backticks. Thus the content is interpolated
2470 as though it were double quoted and then executed via the shell, with
2471 the results of the execution returned.
2473 print << `EOC`; # execute command and get results
2479 It is possible to stack multiple here-docs in a row:
2481 print <<"foo", <<"bar"; # you can stack them
2487 myfunc(<< "THIS", 23, <<'THAT');
2494 Just don't forget that you have to put a semicolon on the end
2495 to finish the statement, as Perl doesn't know you're not going to
2503 If you want to remove the line terminator from your here-docs,
2506 chomp($string = <<'END');
2510 If you want your here-docs to be indented with the rest of the code,
2511 you'll need to remove leading whitespace from each line manually:
2513 ($quote = <<'FINIS') =~ s/^\s+//gm;
2514 The Road goes ever on and on,
2515 down from the door where it began.
2518 If you use a here-doc within a delimited construct, such as in C<s///eg>,
2519 the quoted material must still come on the line following the
2520 C<<< <<FOO >>> marker, which means it may be inside the delimited
2528 It works this way as of Perl 5.18. Historically, it was inconsistent, and
2529 you would have to write
2536 outside of string evals.
2538 Additionally, quoting rules for the end-of-string identifier are
2539 unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
2540 supported in place of C<''> and C<"">, and the only interpolation is for
2541 backslashing the quoting character:
2543 print << "abc\"def";
2547 Finally, quoted strings cannot span multiple lines. The general rule is
2548 that the identifier must be a string literal. Stick with that, and you
2553 =head2 Gory details of parsing quoted constructs
2554 X<quote, gory details>
2556 When presented with something that might have several different
2557 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2558 principle to pick the most probable interpretation. This strategy
2559 is so successful that Perl programmers often do not suspect the
2560 ambivalence of what they write. But from time to time, Perl's
2561 notions differ substantially from what the author honestly meant.
2563 This section hopes to clarify how Perl handles quoted constructs.
2564 Although the most common reason to learn this is to unravel labyrinthine
2565 regular expressions, because the initial steps of parsing are the
2566 same for all quoting operators, they are all discussed together.
2568 The most important Perl parsing rule is the first one discussed
2569 below: when processing a quoted construct, Perl first finds the end
2570 of that construct, then interprets its contents. If you understand
2571 this rule, you may skip the rest of this section on the first
2572 reading. The other rules are likely to contradict the user's
2573 expectations much less frequently than this first one.
2575 Some passes discussed below are performed concurrently, but because
2576 their results are the same, we consider them individually. For different
2577 quoting constructs, Perl performs different numbers of passes, from
2578 one to four, but these passes are always performed in the same order.
2582 =item Finding the end
2584 The first pass is finding the end of the quoted construct, where
2585 the information about the delimiters is used in parsing.
2586 During this search, text between the starting and ending delimiters
2587 is copied to a safe location. The text copied gets delimiter-independent.
2589 If the construct is a here-doc, the ending delimiter is a line
2590 that has a terminating string as the content. Therefore C<<<EOF> is
2591 terminated by C<EOF> immediately followed by C<"\n"> and starting
2592 from the first column of the terminating line.
2593 When searching for the terminating line of a here-doc, nothing
2594 is skipped. In other words, lines after the here-doc syntax
2595 are compared with the terminating string line by line.
2597 For the constructs except here-docs, single characters are used as starting
2598 and ending delimiters. If the starting delimiter is an opening punctuation
2599 (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2600 corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2601 If the starting delimiter is an unpaired character like C</> or a closing
2602 punctuation, the ending delimiter is same as the starting delimiter.
2603 Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
2604 both C<qq[]> and C<qq]]> constructs.
2606 When searching for single-character delimiters, escaped delimiters
2607 and C<\\> are skipped. For example, while searching for terminating C</>,
2608 combinations of C<\\> and C<\/> are skipped. If the delimiters are
2609 bracketing, nested pairs are also skipped. For example, while searching
2610 for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
2611 and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2612 However, when backslashes are used as the delimiters (like C<qq\\> and
2613 C<tr\\\>), nothing is skipped.
2614 During the search for the end, backslashes that escape delimiters or
2615 other backslashes are removed (exactly speaking, they are not copied to the
2618 For constructs with three-part delimiters (C<s///>, C<y///>, and
2619 C<tr///>), the search is repeated once more.
2620 If the first delimiter is not an opening punctuation, the three delimiters must
2621 be the same, such as C<s!!!> and C<tr)))>,
2622 in which case the second delimiter
2623 terminates the left part and starts the right part at once.
2624 If the left part is delimited by bracketing punctuation (that is C<()>,
2625 C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
2626 delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace
2627 and comments are allowed between the two parts, though the comment must follow
2628 at least one whitespace character; otherwise a character expected as the
2629 start of the comment may be regarded as the starting delimiter of the right part.
2631 During this search no attention is paid to the semantics of the construct.
2634 "$hash{"$foo/$bar"}"
2639 bar # NOT a comment, this slash / terminated m//!
2642 do not form legal quoted expressions. The quoted part ends on the
2643 first C<"> and C</>, and the rest happens to be a syntax error.
2644 Because the slash that terminated C<m//> was followed by a C<SPACE>,
2645 the example above is not C<m//x>, but rather C<m//> with no C</x>
2646 modifier. So the embedded C<#> is interpreted as a literal C<#>.
2648 Also no attention is paid to C<\c\> (multichar control char syntax) during
2649 this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
2650 of C<\/>, and the following C</> is not recognized as a delimiter.
2651 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2656 The next step is interpolation in the text obtained, which is now
2657 delimiter-independent. There are multiple cases.
2663 No interpolation is performed.
2664 Note that the combination C<\\> is left intact, since escaped delimiters
2665 are not available for here-docs.
2667 =item C<m''>, the pattern of C<s'''>
2669 No interpolation is performed at this stage.
2670 Any backslashed sequences including C<\\> are treated at the stage
2671 to L</"parsing regular expressions">.
2673 =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
2675 The only interpolation is removal of C<\> from pairs of C<\\>.
2676 Therefore C<-> in C<tr'''> and C<y'''> is treated literally
2677 as a hyphen and no character range is available.
2678 C<\1> in the replacement of C<s'''> does not work as C<$1>.
2680 =item C<tr///>, C<y///>
2682 No variable interpolation occurs. String modifying combinations for
2683 case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2684 The other escape sequences such as C<\200> and C<\t> and backslashed
2685 characters such as C<\\> and C<\-> are converted to appropriate literals.
2686 The character C<-> is treated specially and therefore C<\-> is treated
2689 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
2691 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> (possibly paired with C<\E>) are
2692 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
2693 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
2694 The other escape sequences such as C<\200> and C<\t> and backslashed
2695 characters such as C<\\> and C<\-> are replaced with appropriate
2698 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2699 is interpolated in the usual way. Something like C<"\Q\\E"> has
2700 no C<\E> inside. Instead, it has C<\Q>, C<\\>, and C<E>, so the
2701 result is the same as for C<"\\\\E">. As a general rule, backslashes
2702 between C<\Q> and C<\E> may lead to counterintuitive results. So,
2703 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2704 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2709 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2711 Interpolated scalars and arrays are converted internally to the C<join> and
2712 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
2714 $foo . " XXX '" . (join $", @arr) . "'";
2716 All operations above are performed simultaneously, left to right.
2718 Because the result of C<"\Q STRING \E"> has all metacharacters
2719 quoted, there is no way to insert a literal C<$> or C<@> inside a
2720 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
2721 C<"\\\$">; if not, it is interpreted as the start of an interpolated
2724 Note also that the interpolation code needs to make a decision on
2725 where the interpolated scalar ends. For instance, whether
2726 C<< "a $x -> {c}" >> really means:
2728 "a " . $x . " -> {c}";
2734 Most of the time, the longest possible text that does not include
2735 spaces between components and which contains matching braces or
2736 brackets. because the outcome may be determined by voting based
2737 on heuristic estimators, the result is not strictly predictable.
2738 Fortunately, it's usually correct for ambiguous cases.
2740 =item the replacement of C<s///>
2742 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> and interpolation
2743 happens as with C<qq//> constructs.
2745 It is at this step that C<\1> is begrudgingly converted to C<$1> in
2746 the replacement text of C<s///>, in order to correct the incorrigible
2747 I<sed> hackers who haven't picked up the saner idiom yet. A warning
2748 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2749 (that is, the C<$^W> variable) was set.
2751 =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2753 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F>, C<\E>,
2754 and interpolation happens (almost) as with C<qq//> constructs.
2756 Processing of C<\N{...}> is also done here, and compiled into an intermediate
2757 form for the regex compiler. (This is because, as mentioned below, the regex
2758 compilation may be done at execution time, and C<\N{...}> is a compile-time
2761 However any other combinations of C<\> followed by a character
2762 are not substituted but only skipped, in order to parse them
2763 as regular expressions at the following step.
2764 As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
2765 treated as an array symbol (for example C<@foo>),
2766 even though the same text in C<qq//> gives interpolation of C<\c@>.
2768 Code blocks such as C<(?{BLOCK})> are handled by temporarily passing control
2769 back to the perl parser, in a similar way that an interpolated array
2770 subscript expression such as C<"foo$array[1+f("[xyz")]bar"> would be.
2772 Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
2773 a C<#>-comment in a C<//x>-regular expression, no processing is
2774 performed whatsoever. This is the first step at which the presence
2775 of the C<//x> modifier is relevant.
2777 Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2778 and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2779 voted (by several different estimators) to be either an array element
2780 or C<$var> followed by an RE alternative. This is where the notation
2781 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2782 array element C<-9>, not as a regular expression from the variable
2783 C<$arr> followed by a digit, which would be the interpretation of
2784 C</$arr[0-9]/>. Since voting among different estimators may occur,
2785 the result is not predictable.
2787 The lack of processing of C<\\> creates specific restrictions on
2788 the post-processed text. If the delimiter is C</>, one cannot get
2789 the combination C<\/> into the result of this step. C</> will
2790 finish the regular expression, C<\/> will be stripped to C</> on
2791 the previous step, and C<\\/> will be left as is. Because C</> is
2792 equivalent to C<\/> inside a regular expression, this does not
2793 matter unless the delimiter happens to be character special to the
2794 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2795 alphanumeric char, as in:
2799 In the RE above, which is intentionally obfuscated for illustration, the
2800 delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
2801 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
2802 reason you're encouraged to restrict your delimiters to non-alphanumeric,
2803 non-whitespace choices.
2807 This step is the last one for all constructs except regular expressions,
2808 which are processed further.
2810 =item parsing regular expressions
2813 Previous steps were performed during the compilation of Perl code,
2814 but this one happens at run time, although it may be optimized to
2815 be calculated at compile time if appropriate. After preprocessing
2816 described above, and possibly after evaluation if concatenation,
2817 joining, casing translation, or metaquoting are involved, the
2818 resulting I<string> is passed to the RE engine for compilation.
2820 Whatever happens in the RE engine might be better discussed in L<perlre>,
2821 but for the sake of continuity, we shall do so here.
2823 This is another step where the presence of the C<//x> modifier is
2824 relevant. The RE engine scans the string from left to right and
2825 converts it to a finite automaton.
2827 Backslashed characters are either replaced with corresponding
2828 literal strings (as with C<\{>), or else they generate special nodes
2829 in the finite automaton (as with C<\b>). Characters special to the
2830 RE engine (such as C<|>) generate corresponding nodes or groups of
2831 nodes. C<(?#...)> comments are ignored. All the rest is either
2832 converted to literal strings to match, or else is ignored (as is
2833 whitespace and C<#>-style comments if C<//x> is present).
2835 Parsing of the bracketed character class construct, C<[...]>, is
2836 rather different than the rule used for the rest of the pattern.
2837 The terminator of this construct is found using the same rules as
2838 for finding the terminator of a C<{}>-delimited construct, the only
2839 exception being that C<]> immediately following C<[> is treated as
2840 though preceded by a backslash.
2842 The terminator of runtime C<(?{...})> is found by temporarily switching
2843 control to the perl parser, which should stop at the point where the
2844 logically balancing terminating C<}> is found.
2846 It is possible to inspect both the string given to RE engine and the
2847 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2848 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
2849 switch documented in L<perlrun/"Command Switches">.
2851 =item Optimization of regular expressions
2852 X<regexp, optimization>
2854 This step is listed for completeness only. Since it does not change
2855 semantics, details of this step are not documented and are subject
2856 to change without notice. This step is performed over the finite
2857 automaton that was generated during the previous pass.
2859 It is at this stage that C<split()> silently optimizes C</^/> to
2864 =head2 I/O Operators
2865 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2866 X<< <> >> X<< <<>> >> X<@ARGV>
2868 There are several I/O operators you should know about.
2870 A string enclosed by backticks (grave accents) first undergoes
2871 double-quote interpolation. It is then interpreted as an external
2872 command, and the output of that command is the value of the
2873 backtick string, like in a shell. In scalar context, a single string
2874 consisting of all output is returned. In list context, a list of
2875 values is returned, one per line of output. (You can set C<$/> to use
2876 a different line terminator.) The command is executed each time the
2877 pseudo-literal is evaluated. The status value of the command is
2878 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2879 Unlike in B<csh>, no translation is done on the return data--newlines
2880 remain newlines. Unlike in any of the shells, single quotes do not
2881 hide variable names in the command from interpretation. To pass a
2882 literal dollar-sign through to the shell you need to hide it with a
2883 backslash. The generalized form of backticks is C<qx//>. (Because
2884 backticks always undergo shell expansion as well, see L<perlsec> for
2886 X<qx> X<`> X<``> X<backtick> X<glob>
2888 In scalar context, evaluating a filehandle in angle brackets yields
2889 the next line from that file (the newline, if any, included), or
2890 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2891 (sometimes known as file-slurp mode) and the file is empty, it
2892 returns C<''> the first time, followed by C<undef> subsequently.
2894 Ordinarily you must assign the returned value to a variable, but
2895 there is one situation where an automatic assignment happens. If
2896 and only if the input symbol is the only thing inside the conditional
2897 of a C<while> statement (even if disguised as a C<for(;;)> loop),
2898 the value is automatically assigned to the global variable $_,
2899 destroying whatever was there previously. (This may seem like an
2900 odd thing to you, but you'll use the construct in almost every Perl
2901 script you write.) The $_ variable is not implicitly localized.
2902 You'll have to put a C<local $_;> before the loop if you want that
2905 The following lines are equivalent:
2907 while (defined($_ = <STDIN>)) { print; }
2908 while ($_ = <STDIN>) { print; }
2909 while (<STDIN>) { print; }
2910 for (;<STDIN>;) { print; }
2911 print while defined($_ = <STDIN>);
2912 print while ($_ = <STDIN>);
2913 print while <STDIN>;
2915 This also behaves similarly, but assigns to a lexical variable
2916 instead of to C<$_>:
2918 while (my $line = <STDIN>) { print $line }
2920 In these loop constructs, the assigned value (whether assignment
2921 is automatic or explicit) is then tested to see whether it is
2922 defined. The defined test avoids problems where the line has a string
2923 value that would be treated as false by Perl; for example a "" or
2924 a "0" with no trailing newline. If you really mean for such values
2925 to terminate the loop, they should be tested for explicitly:
2927 while (($_ = <STDIN>) ne '0') { ... }
2928 while (<STDIN>) { last unless $_; ... }
2930 In other boolean contexts, C<< <FILEHANDLE> >> without an
2931 explicit C<defined> test or comparison elicits a warning if the
2932 C<use warnings> pragma or the B<-w>
2933 command-line switch (the C<$^W> variable) is in effect.
2935 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2936 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2937 in packages, where they would be interpreted as local identifiers
2938 rather than global.) Additional filehandles may be created with
2939 the open() function, amongst others. See L<perlopentut> and
2940 L<perlfunc/open> for details on this.
2941 X<stdin> X<stdout> X<sterr>
2943 If a <FILEHANDLE> is used in a context that is looking for
2944 a list, a list comprising all input lines is returned, one line per
2945 list element. It's easy to grow to a rather large data space this
2946 way, so use with care.
2948 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2949 See L<perlfunc/readline>.
2951 The null filehandle <> is special: it can be used to emulate the
2952 behavior of B<sed> and B<awk>, and any other Unix filter program
2953 that takes a list of filenames, doing the same to each line
2954 of input from all of them. Input from <> comes either from
2955 standard input, or from each file listed on the command line. Here's
2956 how it works: the first time <> is evaluated, the @ARGV array is
2957 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2958 gives you standard input. The @ARGV array is then processed as a list
2959 of filenames. The loop
2962 ... # code for each line
2965 is equivalent to the following Perl-like pseudo code:
2967 unshift(@ARGV, '-') unless @ARGV;
2968 while ($ARGV = shift) {
2971 ... # code for each line
2975 except that it isn't so cumbersome to say, and will actually work.
2976 It really does shift the @ARGV array and put the current filename
2977 into the $ARGV variable. It also uses filehandle I<ARGV>
2978 internally. <> is just a synonym for <ARGV>, which
2979 is magical. (The pseudo code above doesn't work because it treats
2980 <ARGV> as non-magical.)
2982 Since the null filehandle uses the two argument form of L<perlfunc/open>
2983 it interprets special characters, so if you have a script like this:
2989 and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2990 pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2991 If you want all items in C<@ARGV> to be interpreted as file names, you
2992 can use the module C<ARGV::readonly> from CPAN, or use the double bracket:
2998 Using double angle brackets inside of a while causes the open to use the
2999 three argument form (with the second argument being C<< < >>), so all
3000 arguments in ARGV are treated as literal filenames (including "-").
3001 (Note that for convenience, if you use C<< <<>> >> and if @ARGV is
3002 empty, it will still read from the standard input.)
3004 You can modify @ARGV before the first <> as long as the array ends up
3005 containing the list of filenames you really want. Line numbers (C<$.>)
3006 continue as though the input were one big happy file. See the example
3007 in L<perlfunc/eof> for how to reset line numbers on each file.
3009 If you want to set @ARGV to your own list of files, go right ahead.
3010 This sets @ARGV to all plain text files if no @ARGV was given:
3012 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
3014 You can even set them to pipe commands. For example, this automatically
3015 filters compressed arguments through B<gzip>:
3017 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
3019 If you want to pass switches into your script, you can use one of the
3020 Getopts modules or put a loop on the front like this:
3022 while ($_ = $ARGV[0], /^-/) {
3025 if (/^-D(.*)/) { $debug = $1 }
3026 if (/^-v/) { $verbose++ }
3027 # ... # other switches
3031 # ... # code for each line
3034 The <> symbol will return C<undef> for end-of-file only once.
3035 If you call it again after this, it will assume you are processing another
3036 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
3038 If what the angle brackets contain is a simple scalar variable (for example,
3039 <$foo>), then that variable contains the name of the
3040 filehandle to input from, or its typeglob, or a reference to the
3046 If what's within the angle brackets is neither a filehandle nor a simple
3047 scalar variable containing a filehandle name, typeglob, or typeglob
3048 reference, it is interpreted as a filename pattern to be globbed, and
3049 either a list of filenames or the next filename in the list is returned,
3050 depending on context. This distinction is determined on syntactic
3051 grounds alone. That means C<< <$x> >> is always a readline() from
3052 an indirect handle, but C<< <$hash{key}> >> is always a glob().
3053 That's because $x is a simple scalar variable, but C<$hash{key}> is
3054 not--it's a hash element. Even C<< <$x > >> (note the extra space)
3055 is treated as C<glob("$x ")>, not C<readline($x)>.
3057 One level of double-quote interpretation is done first, but you can't
3058 say C<< <$foo> >> because that's an indirect filehandle as explained
3059 in the previous paragraph. (In older versions of Perl, programmers
3060 would insert curly brackets to force interpretation as a filename glob:
3061 C<< <${foo}> >>. These days, it's considered cleaner to call the
3062 internal function directly as C<glob($foo)>, which is probably the right
3063 way to have done it in the first place.) For example:
3069 is roughly equivalent to:
3071 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
3077 except that the globbing is actually done internally using the standard
3078 C<File::Glob> extension. Of course, the shortest way to do the above is:
3082 A (file)glob evaluates its (embedded) argument only when it is
3083 starting a new list. All values must be read before it will start
3084 over. In list context, this isn't important because you automatically
3085 get them all anyway. However, in scalar context the operator returns
3086 the next value each time it's called, or C<undef> when the list has
3087 run out. As with filehandle reads, an automatic C<defined> is
3088 generated when the glob occurs in the test part of a C<while>,
3089 because legal glob returns (for example,
3090 a file called F<0>) would otherwise
3091 terminate the loop. Again, C<undef> is returned only once. So if
3092 you're expecting a single value from a glob, it is much better to
3095 ($file) = <blurch*>;
3101 because the latter will alternate between returning a filename and
3104 If you're trying to do variable interpolation, it's definitely better
3105 to use the glob() function, because the older notation can cause people
3106 to become confused with the indirect filehandle notation.
3108 @files = glob("$dir/*.[ch]");
3109 @files = glob($files[$i]);
3111 =head2 Constant Folding
3112 X<constant folding> X<folding>
3114 Like C, Perl does a certain amount of expression evaluation at
3115 compile time whenever it determines that all arguments to an
3116 operator are static and have no side effects. In particular, string
3117 concatenation happens at compile time between literals that don't do
3118 variable substitution. Backslash interpolation also happens at
3119 compile time. You can say
3121 'Now is the time for all'
3123 . 'good men to come to.'
3125 and this all reduces to one string internally. Likewise, if
3128 foreach $file (@filenames) {
3129 if (-s $file > 5 + 100 * 2**16) { }
3132 the compiler precomputes the number which that expression
3133 represents so that the interpreter won't have to.
3138 Perl doesn't officially have a no-op operator, but the bare constants
3139 C<0> and C<1> are special-cased not to produce a warning in void
3140 context, so you can for example safely do
3144 =head2 Bitwise String Operators
3145 X<operator, bitwise, string>
3147 Bitstrings of any size may be manipulated by the bitwise operators
3150 If the operands to a binary bitwise op are strings of different
3151 sizes, B<|> and B<^> ops act as though the shorter operand had
3152 additional zero bits on the right, while the B<&> op acts as though
3153 the longer operand were truncated to the length of the shorter.
3154 The granularity for such extension or truncation is one or more
3157 # ASCII-based examples
3158 print "j p \n" ^ " a h"; # prints "JAPH\n"
3159 print "JA" | " ph\n"; # prints "japh\n"
3160 print "japh\nJunk" & '_____'; # prints "JAPH\n";
3161 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
3163 If you are intending to manipulate bitstrings, be certain that
3164 you're supplying bitstrings: If an operand is a number, that will imply
3165 a B<numeric> bitwise operation. You may explicitly show which type of
3166 operation you intend by using C<""> or C<0+>, as in the examples below.
3168 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
3169 $foo = '150' | 105; # yields 255
3170 $foo = 150 | '105'; # yields 255
3171 $foo = '150' | '105'; # yields string '155' (under ASCII)
3173 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
3174 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
3176 See L<perlfunc/vec> for information on how to manipulate individual bits
3179 =head2 Integer Arithmetic
3182 By default, Perl assumes that it must do most of its arithmetic in
3183 floating point. But by saying
3187 you may tell the compiler to use integer operations
3188 (see L<integer> for a detailed explanation) from here to the end of
3189 the enclosing BLOCK. An inner BLOCK may countermand this by saying
3193 which lasts until the end of that BLOCK. Note that this doesn't
3194 mean everything is an integer, merely that Perl will use integer
3195 operations for arithmetic, comparison, and bitwise operators. For
3196 example, even under C<use integer>, if you take the C<sqrt(2)>, you'll
3197 still get C<1.4142135623731> or so.
3199 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
3200 and ">>") always produce integral results. (But see also
3201 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
3202 them. By default, their results are interpreted as unsigned integers, but
3203 if C<use integer> is in effect, their results are interpreted
3204 as signed integers. For example, C<~0> usually evaluates to a large
3205 integral value. However, C<use integer; ~0> is C<-1> on two's-complement
3208 =head2 Floating-point Arithmetic
3210 X<floating-point> X<floating point> X<float> X<real>
3212 While C<use integer> provides integer-only arithmetic, there is no
3213 analogous mechanism to provide automatic rounding or truncation to a
3214 certain number of decimal places. For rounding to a certain number
3215 of digits, sprintf() or printf() is usually the easiest route.
3218 Floating-point numbers are only approximations to what a mathematician
3219 would call real numbers. There are infinitely more reals than floats,
3220 so some corners must be cut. For example:
3222 printf "%.20g\n", 123456789123456789;
3223 # produces 123456789123456784
3225 Testing for exact floating-point equality or inequality is not a
3226 good idea. Here's a (relatively expensive) work-around to compare
3227 whether two floating-point numbers are equal to a particular number of
3228 decimal places. See Knuth, volume II, for a more robust treatment of
3232 my ($X, $Y, $POINTS) = @_;
3234 $tX = sprintf("%.${POINTS}g", $X);
3235 $tY = sprintf("%.${POINTS}g", $Y);
3239 The POSIX module (part of the standard perl distribution) implements
3240 ceil(), floor(), and other mathematical and trigonometric functions.
3241 The Math::Complex module (part of the standard perl distribution)
3242 defines mathematical functions that work on both the reals and the
3243 imaginary numbers. Math::Complex not as efficient as POSIX, but
3244 POSIX can't work with complex numbers.
3246 Rounding in financial applications can have serious implications, and
3247 the rounding method used should be specified precisely. In these
3248 cases, it probably pays not to trust whichever system rounding is
3249 being used by Perl, but to instead implement the rounding function you
3252 =head2 Bigger Numbers
3253 X<number, arbitrary precision>
3255 The standard C<Math::BigInt>, C<Math::BigRat>, and C<Math::BigFloat> modules,
3256 along with the C<bignum>, C<bigint>, and C<bigrat> pragmas, provide
3257 variable-precision arithmetic and overloaded operators, although
3258 they're currently pretty slow. At the cost of some space and
3259 considerable speed, they avoid the normal pitfalls associated with
3260 limited-precision representations.
3263 use bigint; # easy interface to Math::BigInt
3264 $x = 123456789123456789;
3266 +15241578780673678515622620750190521
3274 say "x/y is ", $x/$y;
3275 say "x*y is ", $x*$y;
3279 Several modules let you calculate with (bound only by memory and CPU time)
3280 unlimited or fixed precision. There
3281 are also some non-standard modules that
3282 provide faster implementations via external C libraries.
3284 Here is a short, but incomplete summary:
3286 Math::String treat string sequences like numbers
3287 Math::FixedPrecision calculate with a fixed precision
3288 Math::Currency for currency calculations
3289 Bit::Vector manipulate bit vectors fast (uses C)
3290 Math::BigIntFast Bit::Vector wrapper for big numbers
3291 Math::Pari provides access to the Pari C library
3292 Math::Cephes uses the external Cephes C library (no
3294 Math::Cephes::Fraction fractions via the Cephes library
3295 Math::GMP another one using an external C library
3296 Math::GMPz an alternative interface to libgmp's big ints
3297 Math::GMPq an interface to libgmp's fraction numbers
3298 Math::GMPf an interface to libgmp's floating point numbers