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<$a + $b>
10 is always a numeric addition, and if C<$a> or C<$b> do not contain
11 numbers, an attempt is made to convert them to numbers first.
13 This is in contrast to many other dynamic languages, where the
14 operation is determined by the type of the first argument. It also
15 means that Perl has two versions of some operators, one for numeric
16 and one for string comparison. For example C<$a == $b> compares
17 two numbers for equality, and C<$a eq $b> 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<|> 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 =head2 Auto-increment and Auto-decrement
157 X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
159 "++" and "--" work as in C. That is, if placed before a variable,
160 they increment or decrement the variable by one before returning the
161 value, and if placed after, increment or decrement after returning the
165 print $i++; # prints 0
166 print ++$j; # prints 1
168 Note that just as in C, Perl doesn't define B<when> the variable is
169 incremented or decremented. You just know it will be done sometime
170 before or after the value is returned. This also means that modifying
171 a variable twice in the same statement will lead to undefined behavior.
172 Avoid statements like:
177 Perl will not guarantee what the result of the above statements is.
179 The auto-increment operator has a little extra builtin magic to it. If
180 you increment a variable that is numeric, or that has ever been used in
181 a numeric context, you get a normal increment. If, however, the
182 variable has been used in only string contexts since it was set, and
183 has a value that is not the empty string and matches the pattern
184 C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
185 character within its range, with carry:
187 print ++($foo = "99"); # prints "100"
188 print ++($foo = "a0"); # prints "a1"
189 print ++($foo = "Az"); # prints "Ba"
190 print ++($foo = "zz"); # prints "aaa"
192 C<undef> is always treated as numeric, and in particular is changed
193 to C<0> before incrementing (so that a post-increment of an undef value
194 will return C<0> rather than C<undef>).
196 The auto-decrement operator is not magical.
198 =head2 Exponentiation
199 X<**> X<exponentiation> X<power>
201 Binary "**" is the exponentiation operator. It binds even more
202 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
203 implemented using C's pow(3) function, which actually works on doubles
206 =head2 Symbolic Unary Operators
207 X<unary operator> X<operator, unary>
209 Unary "!" performs logical negation, that is, "not". See also C<not> for a lower
210 precedence version of this.
213 Unary "-" performs arithmetic negation if the operand is numeric,
214 including any string that looks like a number. If the operand is
215 an identifier, a string consisting of a minus sign concatenated
216 with the identifier is returned. Otherwise, if the string starts
217 with a plus or minus, a string starting with the opposite sign is
218 returned. One effect of these rules is that -bareword is equivalent
219 to the string "-bareword". If, however, the string begins with a
220 non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
221 the string to a numeric and the arithmetic negation is performed. If the
222 string cannot be cleanly converted to a numeric, Perl will give the warning
223 B<Argument "the string" isn't numeric in negation (-) at ...>.
224 X<-> X<negation, arithmetic>
226 Unary "~" performs bitwise negation, that is, 1's complement. For
227 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
228 L<Bitwise String Operators>.) Note that the width of the result is
229 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
230 bits wide on a 64-bit platform, so if you are expecting a certain bit
231 width, remember to use the "&" operator to mask off the excess bits.
232 X<~> X<negation, binary>
234 When complementing strings, if all characters have ordinal values under
235 256, then their complements will, also. But if they do not, all
236 characters will be in either 32- or 64-bit complements, depending on your
237 architecture. So for example, C<~"\x{3B1}"> is C<"\x{FFFF_FC4E}"> on
238 32-bit machines and C<"\x{FFFF_FFFF_FFFF_FC4E}"> on 64-bit machines.
240 Unary "+" has no effect whatsoever, even on strings. It is useful
241 syntactically for separating a function name from a parenthesized expression
242 that would otherwise be interpreted as the complete list of function
243 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
246 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
247 and L<perlref>. Do not confuse this behavior with the behavior of
248 backslash within a string, although both forms do convey the notion
249 of protecting the next thing from interpolation.
250 X<\> X<reference> X<backslash>
252 =head2 Binding Operators
253 X<binding> X<operator, binding> X<=~> X<!~>
255 Binary "=~" binds a scalar expression to a pattern match. Certain operations
256 search or modify the string $_ by default. This operator makes that kind
257 of operation work on some other string. The right argument is a search
258 pattern, substitution, or transliteration. The left argument is what is
259 supposed to be searched, substituted, or transliterated instead of the default
260 $_. When used in scalar context, the return value generally indicates the
261 success of the operation. The exceptions are substitution (s///)
262 and transliteration (y///) with the C</r> (non-destructive) option,
263 which cause the B<r>eturn value to be the result of the substitution.
264 Behavior in list context depends on the particular operator.
265 See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
266 examples using these operators.
268 If the right argument is an expression rather than a search pattern,
269 substitution, or transliteration, it is interpreted as a search pattern at run
270 time. Note that this means that its contents will be interpolated twice, so
274 is not ok, as the regex engine will end up trying to compile the
275 pattern C<\>, which it will consider a syntax error.
277 Binary "!~" is just like "=~" except the return value is negated in
280 Binary "!~" with a non-destructive substitution (s///r) or transliteration
281 (y///r) is a syntax error.
283 =head2 Multiplicative Operators
284 X<operator, multiplicative>
286 Binary "*" multiplies two numbers.
289 Binary "/" divides two numbers.
292 Binary "%" is the modulo operator, which computes the division
293 remainder of its first argument with respect to its second argument.
295 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
296 C<$a> minus the largest multiple of C<$b> less than or equal to
297 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
298 smallest multiple of C<$b> that is not less than C<$a> (that is, the
299 result will be less than or equal to zero). If the operands
300 C<$a> and C<$b> are floating point values and the absolute value of
301 C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
302 the integer portion of C<$a> and C<$b> will be used in the operation
303 (Note: here C<UV_MAX> means the maximum of the unsigned integer type).
304 If the absolute value of the right operand (C<abs($b)>) is greater than
305 or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
306 C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
307 integer that makes C<$r> have the same sign as the right operand
308 C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
309 and the absolute value less than that of C<$b>.
310 Note that when C<use integer> is in scope, "%" gives you direct access
311 to the modulo operator as implemented by your C compiler. This
312 operator is not as well defined for negative operands, but it will
314 X<%> X<remainder> X<modulo> X<mod>
316 Binary "x" is the repetition operator. In scalar context or if the left
317 operand is not enclosed in parentheses, it returns a string consisting
318 of the left operand repeated the number of times specified by the right
319 operand. In list context, if the left operand is enclosed in
320 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
321 If the right operand is zero or negative, it returns an empty string
322 or an empty list, depending on the context.
325 print '-' x 80; # print row of dashes
327 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
329 @ones = (1) x 80; # a list of 80 1's
330 @ones = (5) x @ones; # set all elements to 5
333 =head2 Additive Operators
334 X<operator, additive>
336 Binary C<+> returns the sum of two numbers.
339 Binary C<-> returns the difference of two numbers.
342 Binary C<.> concatenates two strings.
343 X<string, concatenation> X<concatenation>
344 X<cat> X<concat> X<concatenate> X<.>
346 =head2 Shift Operators
347 X<shift operator> X<operator, shift> X<<< << >>>
348 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
349 X<shl> X<shr> X<shift, right> X<shift, left>
351 Binary C<<< << >>> returns the value of its left argument shifted left by the
352 number of bits specified by the right argument. Arguments should be
353 integers. (See also L<Integer Arithmetic>.)
355 Binary C<<< >> >>> returns the value of its left argument shifted right by
356 the number of bits specified by the right argument. Arguments should
357 be integers. (See also L<Integer Arithmetic>.)
359 Note that both C<<< << >>> and C<<< >> >>> in Perl are implemented directly using
360 C<<< << >>> and C<<< >> >>> in C. If C<use integer> (see L<Integer Arithmetic>) is
361 in force then signed C integers are used, else unsigned C integers are
362 used. Either way, the implementation isn't going to generate results
363 larger than the size of the integer type Perl was built with (32 bits
366 The result of overflowing the range of the integers is undefined
367 because it is undefined also in C. In other words, using 32-bit
368 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
369 of bits is also undefined.
371 If you get tired of being subject to your platform's native integers,
372 the C<use bigint> pragma neatly sidesteps the issue altogether:
374 print 20 << 20; # 20971520
375 print 20 << 40; # 5120 on 32-bit machines,
376 # 21990232555520 on 64-bit machines
378 print 20 << 100; # 25353012004564588029934064107520
380 =head2 Named Unary Operators
381 X<operator, named unary>
383 The various named unary operators are treated as functions with one
384 argument, with optional parentheses.
386 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
387 is followed by a left parenthesis as the next token, the operator and
388 arguments within parentheses are taken to be of highest precedence,
389 just like a normal function call. For example,
390 because named unary operators are higher precedence than C<||>:
392 chdir $foo || die; # (chdir $foo) || die
393 chdir($foo) || die; # (chdir $foo) || die
394 chdir ($foo) || die; # (chdir $foo) || die
395 chdir +($foo) || die; # (chdir $foo) || die
397 but, because * is higher precedence than named operators:
399 chdir $foo * 20; # chdir ($foo * 20)
400 chdir($foo) * 20; # (chdir $foo) * 20
401 chdir ($foo) * 20; # (chdir $foo) * 20
402 chdir +($foo) * 20; # chdir ($foo * 20)
404 rand 10 * 20; # rand (10 * 20)
405 rand(10) * 20; # (rand 10) * 20
406 rand (10) * 20; # (rand 10) * 20
407 rand +(10) * 20; # rand (10 * 20)
409 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
410 treated like named unary operators, but they don't follow this functional
411 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
412 equivalent to C<-f "$file.bak">.
413 X<-X> X<filetest> X<operator, filetest>
415 See also L<"Terms and List Operators (Leftward)">.
417 =head2 Relational Operators
418 X<relational operator> X<operator, relational>
420 Perl operators that return true or false generally return values
421 that can be safely used as numbers. For example, the relational
422 operators in this section and the equality operators in the next
423 one return C<1> for true and a special version of the defined empty
424 string, C<"">, which counts as a zero but is exempt from warnings
425 about improper numeric conversions, just as C<"0 but true"> is.
427 Binary "<" returns true if the left argument is numerically less than
431 Binary ">" returns true if the left argument is numerically greater
432 than the right argument.
435 Binary "<=" returns true if the left argument is numerically less than
436 or equal to the right argument.
439 Binary ">=" returns true if the left argument is numerically greater
440 than or equal to the right argument.
443 Binary "lt" returns true if the left argument is stringwise less than
447 Binary "gt" returns true if the left argument is stringwise greater
448 than the right argument.
451 Binary "le" returns true if the left argument is stringwise less than
452 or equal to the right argument.
455 Binary "ge" returns true if the left argument is stringwise greater
456 than or equal to the right argument.
459 =head2 Equality Operators
460 X<equality> X<equal> X<equals> X<operator, equality>
462 Binary "==" returns true if the left argument is numerically equal to
466 Binary "!=" returns true if the left argument is numerically not equal
467 to the right argument.
470 Binary "<=>" returns -1, 0, or 1 depending on whether the left
471 argument is numerically less than, equal to, or greater than the right
472 argument. If your platform supports NaNs (not-a-numbers) as numeric
473 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
474 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
475 returns true, as does NaN != anything else. If your platform doesn't
476 support NaNs then NaN is just a string with numeric value 0.
477 X<< <=> >> X<spaceship>
479 $ perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
480 $ perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
482 (Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all
485 Binary "eq" returns true if the left argument is stringwise equal to
489 Binary "ne" returns true if the left argument is stringwise not equal
490 to the right argument.
493 Binary "cmp" returns -1, 0, or 1 depending on whether the left
494 argument is stringwise less than, equal to, or greater than the right
498 Binary "~~" does a smartmatch between its arguments. Smart matching
499 is described in the next section.
502 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
503 by the current locale if a legacy C<use locale> (but not
504 C<use locale ':not_characters'>) is in effect. See
505 L<perllocale>. Do not mix these with Unicode, only with legacy binary
506 encodings. The standard L<Unicode::Collate> and
507 L<Unicode::Collate::Locale> modules offer much more powerful solutions to
510 =head2 Smartmatch Operator
512 First available in Perl 5.10.1 (the 5.10.0 version behaved differently),
513 binary C<~~> does a "smartmatch" between its arguments. This is mostly
514 used implicitly in the C<when> construct described in L<perlsyn>, although
515 not all C<when> clauses call the smartmatch operator. Unique among all of
516 Perl's operators, the smartmatch operator can recurse.
518 It is also unique in that all other Perl operators impose a context
519 (usually string or numeric context) on their operands, autoconverting
520 those operands to those imposed contexts. In contrast, smartmatch
521 I<infers> contexts from the actual types of its operands and uses that
522 type information to select a suitable comparison mechanism.
524 The C<~~> operator compares its operands "polymorphically", determining how
525 to compare them according to their actual types (numeric, string, array,
526 hash, etc.) Like the equality operators with which it shares the same
527 precedence, C<~~> returns 1 for true and C<""> for false. It is often best
528 read aloud as "in", "inside of", or "is contained in", because the left
529 operand is often looked for I<inside> the right operand. That makes the
530 order of the operands to the smartmatch operand often opposite that of
531 the regular match operator. In other words, the "smaller" thing is usually
532 placed in the left operand and the larger one in the right.
534 The behavior of a smartmatch depends on what type of things its arguments
535 are, as determined by the following table. The first row of the table
536 whose types apply determines the smartmatch behavior. Because what
537 actually happens is mostly determined by the type of the second operand,
538 the table is sorted on the right operand instead of on the left.
540 Left Right Description and pseudocode
541 ===============================================================
542 Any undef check whether Any is undefined
545 Any Object invoke ~~ overloading on Object, or die
547 Right operand is an ARRAY:
549 Left Right Description and pseudocode
550 ===============================================================
551 ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2]
552 like: (ARRAY1[0] ~~ ARRAY2[0])
553 && (ARRAY1[1] ~~ ARRAY2[1]) && ...
554 HASH ARRAY any ARRAY elements exist as HASH keys
555 like: grep { exists HASH->{$_} } ARRAY
556 Regexp ARRAY any ARRAY elements pattern match Regexp
557 like: grep { /Regexp/ } ARRAY
558 undef ARRAY undef in ARRAY
559 like: grep { !defined } ARRAY
560 Any ARRAY smartmatch each ARRAY element[3]
561 like: grep { Any ~~ $_ } ARRAY
563 Right operand is a HASH:
565 Left Right Description and pseudocode
566 ===============================================================
567 HASH1 HASH2 all same keys in both HASHes
569 grep { exists HASH2->{$_} } keys HASH1
570 ARRAY HASH any ARRAY elements exist as HASH keys
571 like: grep { exists HASH->{$_} } ARRAY
572 Regexp HASH any HASH keys pattern match Regexp
573 like: grep { /Regexp/ } keys HASH
574 undef HASH always false (undef can't be a key)
576 Any HASH HASH key existence
577 like: exists HASH->{Any}
579 Right operand is CODE:
581 Left Right Description and pseudocode
582 ===============================================================
583 ARRAY CODE sub returns true on all ARRAY elements[1]
584 like: !grep { !CODE->($_) } ARRAY
585 HASH CODE sub returns true on all HASH keys[1]
586 like: !grep { !CODE->($_) } keys HASH
587 Any CODE sub passed Any returns true
590 Right operand is a Regexp:
592 Left Right Description and pseudocode
593 ===============================================================
594 ARRAY Regexp any ARRAY elements match Regexp
595 like: grep { /Regexp/ } ARRAY
596 HASH Regexp any HASH keys match Regexp
597 like: grep { /Regexp/ } keys HASH
598 Any Regexp pattern match
599 like: Any =~ /Regexp/
603 Left Right Description and pseudocode
604 ===============================================================
605 Object Any invoke ~~ overloading on Object,
608 Any Num numeric equality
610 Num nummy[4] numeric equality
612 undef Any check whether undefined
614 Any Any string equality
623 Empty hashes or arrays match.
626 That is, each element smartmatches the element of the same index in the other array.[3]
629 If a circular reference is found, fall back to referential equality.
632 Either an actual number, or a string that looks like one.
636 The smartmatch implicitly dereferences any non-blessed hash or array
637 reference, so the C<I<HASH>> and C<I<ARRAY>> entries apply in those cases.
638 For blessed references, the C<I<Object>> entries apply. Smartmatches
639 involving hashes only consider hash keys, never hash values.
641 The "like" code entry is not always an exact rendition. For example, the
642 smartmatch operator short-circuits whenever possible, but C<grep> does
643 not. Also, C<grep> in scalar context returns the number of matches, but
644 C<~~> returns only true or false.
646 Unlike most operators, the smartmatch operator knows to treat C<undef>
650 @array = (1, 2, 3, undef, 4, 5);
651 say "some elements undefined" if undef ~~ @array;
653 Each operand is considered in a modified scalar context, the modification
654 being that array and hash variables are passed by reference to the
655 operator, which implicitly dereferences them. Both elements
656 of each pair are the same:
660 my %hash = (red => 1, blue => 2, green => 3,
661 orange => 4, yellow => 5, purple => 6,
662 black => 7, grey => 8, white => 9);
664 my @array = qw(red blue green);
666 say "some array elements in hash keys" if @array ~~ %hash;
667 say "some array elements in hash keys" if \@array ~~ \%hash;
669 say "red in array" if "red" ~~ @array;
670 say "red in array" if "red" ~~ \@array;
672 say "some keys end in e" if /e$/ ~~ %hash;
673 say "some keys end in e" if /e$/ ~~ \%hash;
675 Two arrays smartmatch if each element in the first array smartmatches
676 (that is, is "in") the corresponding element in the second array,
680 my @little = qw(red blue green);
681 my @bigger = ("red", "blue", [ "orange", "green" ] );
682 if (@little ~~ @bigger) { # true!
683 say "little is contained in bigger";
686 Because the smartmatch operator recurses on nested arrays, this
687 will still report that "red" is in the array.
690 my @array = qw(red blue green);
691 my $nested_array = [[[[[[[ @array ]]]]]]];
692 say "red in array" if "red" ~~ $nested_array;
694 If two arrays smartmatch each other, then they are deep
695 copies of each others' values, as this example reports:
698 my @a = (0, 1, 2, [3, [4, 5], 6], 7);
699 my @b = (0, 1, 2, [3, [4, 5], 6], 7);
701 if (@a ~~ @b && @b ~~ @a) {
702 say "a and b are deep copies of each other";
705 say "a smartmatches in b";
708 say "b smartmatches in a";
711 say "a and b don't smartmatch each other at all";
715 If you were to set C<$b[3] = 4>, then instead of reporting that "a and b
716 are deep copies of each other", it now reports that "b smartmatches in a".
717 That because the corresponding position in C<@a> contains an array that
718 (eventually) has a 4 in it.
720 Smartmatching one hash against another reports whether both contain the
721 same keys, no more and no less. This could be used to see whether two
722 records have the same field names, without caring what values those fields
723 might have. For example:
727 state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
729 my ($class, $init_fields) = @_;
731 die "Must supply (only) name, rank, and serial number"
732 unless $init_fields ~~ $REQUIRED_FIELDS;
737 or, if other non-required fields are allowed, use ARRAY ~~ HASH:
741 state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
743 my ($class, $init_fields) = @_;
745 die "Must supply (at least) name, rank, and serial number"
746 unless [keys %{$init_fields}] ~~ $REQUIRED_FIELDS;
751 The smartmatch operator is most often used as the implicit operator of a
752 C<when> clause. See the section on "Switch Statements" in L<perlsyn>.
754 =head3 Smartmatching of Objects
756 To avoid relying on an object's underlying representation, if the
757 smartmatch's right operand is an object that doesn't overload C<~~>,
758 it raises the exception "C<Smartmatching a non-overloaded object
759 breaks encapsulation>". That's because one has no business digging
760 around to see whether something is "in" an object. These are all
761 illegal on objects without a C<~~> overload:
767 However, you can change the way an object is smartmatched by overloading
768 the C<~~> operator. This is allowed to extend the usual smartmatch semantics.
769 For objects that do have an C<~~> overload, see L<overload>.
771 Using an object as the left operand is allowed, although not very useful.
772 Smartmatching rules take precedence over overloading, so even if the
773 object in the left operand has smartmatch overloading, this will be
774 ignored. A left operand that is a non-overloaded object falls back on a
775 string or numeric comparison of whatever the C<ref> operator returns. That
780 does I<not> invoke the overload method with C<I<X>> as an argument.
781 Instead the above table is consulted as normal, and based on the type of
782 C<I<X>>, overloading may or may not be invoked. For simple strings or
783 numbers, in becomes equivalent to this:
785 $object ~~ $number ref($object) == $number
786 $object ~~ $string ref($object) eq $string
788 For example, this reports that the handle smells IOish
789 (but please don't really do this!):
792 my $fh = IO::Handle->new();
793 if ($fh ~~ /\bIO\b/) {
794 say "handle smells IOish";
797 That's because it treats C<$fh> as a string like
798 C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that.
801 X<operator, bitwise, and> X<bitwise and> X<&>
803 Binary "&" returns its operands ANDed together bit by bit.
804 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
806 Note that "&" has lower priority than relational operators, so for example
807 the parentheses are essential in a test like
809 print "Even\n" if ($x & 1) == 0;
811 =head2 Bitwise Or and Exclusive Or
812 X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
815 Binary "|" returns its operands ORed together bit by bit.
816 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
818 Binary "^" returns its operands XORed together bit by bit.
819 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
821 Note that "|" and "^" have lower priority than relational operators, so
822 for example the brackets are essential in a test like
824 print "false\n" if (8 | 2) != 10;
826 =head2 C-style Logical And
827 X<&&> X<logical and> X<operator, logical, and>
829 Binary "&&" performs a short-circuit logical AND operation. That is,
830 if the left operand is false, the right operand is not even evaluated.
831 Scalar or list context propagates down to the right operand if it
834 =head2 C-style Logical Or
835 X<||> X<operator, logical, or>
837 Binary "||" performs a short-circuit logical OR operation. That is,
838 if the left operand is true, the right operand is not even evaluated.
839 Scalar or list context propagates down to the right operand if it
842 =head2 Logical Defined-Or
843 X<//> X<operator, logical, defined-or>
845 Although it has no direct equivalent in C, Perl's C<//> operator is related
846 to its C-style or. In fact, it's exactly the same as C<||>, except that it
847 tests the left hand side's definedness instead of its truth. Thus,
848 C<< EXPR1 // EXPR2 >> returns the value of C<< EXPR1 >> if it's defined,
849 otherwise, the value of C<< EXPR2 >> is returned. (C<< EXPR1 >> is evaluated
850 in scalar context, C<< EXPR2 >> in the context of C<< // >> itself). Usually,
851 this is the same result as C<< defined(EXPR1) ? EXPR1 : EXPR2 >> (except that
852 the ternary-operator form can be used as a lvalue, while C<< EXPR1 // EXPR2 >>
853 cannot). This is very useful for
854 providing default values for variables. If you actually want to test if
855 at least one of C<$a> and C<$b> is defined, use C<defined($a // $b)>.
857 The C<||>, C<//> and C<&&> operators return the last value evaluated
858 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
859 portable way to find out the home directory might be:
864 // die "You're homeless!\n";
866 In particular, this means that you shouldn't use this
867 for selecting between two aggregates for assignment:
869 @a = @b || @c; # this is wrong
870 @a = scalar(@b) || @c; # really meant this
871 @a = @b ? @b : @c; # this works fine, though
873 As alternatives to C<&&> and C<||> when used for
874 control flow, Perl provides the C<and> and C<or> operators (see below).
875 The short-circuit behavior is identical. The precedence of "and"
876 and "or" is much lower, however, so that you can safely use them after a
877 list operator without the need for parentheses:
879 unlink "alpha", "beta", "gamma"
880 or gripe(), next LINE;
882 With the C-style operators that would have been written like this:
884 unlink("alpha", "beta", "gamma")
885 || (gripe(), next LINE);
887 It would be even more readable to write that this way:
889 unless(unlink("alpha", "beta", "gamma")) {
894 Using "or" for assignment is unlikely to do what you want; see below.
896 =head2 Range Operators
897 X<operator, range> X<range> X<..> X<...>
899 Binary ".." is the range operator, which is really two different
900 operators depending on the context. In list context, it returns a
901 list of values counting (up by ones) from the left value to the right
902 value. If the left value is greater than the right value then it
903 returns the empty list. The range operator is useful for writing
904 C<foreach (1..10)> loops and for doing slice operations on arrays. In
905 the current implementation, no temporary array is created when the
906 range operator is used as the expression in C<foreach> loops, but older
907 versions of Perl might burn a lot of memory when you write something
910 for (1 .. 1_000_000) {
914 The range operator also works on strings, using the magical
915 auto-increment, see below.
917 In scalar context, ".." returns a boolean value. The operator is
918 bistable, like a flip-flop, and emulates the line-range (comma)
919 operator of B<sed>, B<awk>, and various editors. Each ".." operator
920 maintains its own boolean state, even across calls to a subroutine
921 that contains it. It is false as long as its left operand is false.
922 Once the left operand is true, the range operator stays true until the
923 right operand is true, I<AFTER> which the range operator becomes false
924 again. It doesn't become false till the next time the range operator
925 is evaluated. It can test the right operand and become false on the
926 same evaluation it became true (as in B<awk>), but it still returns
927 true once. If you don't want it to test the right operand until the
928 next evaluation, as in B<sed>, just use three dots ("...") instead of
929 two. In all other regards, "..." behaves just like ".." does.
931 The right operand is not evaluated while the operator is in the
932 "false" state, and the left operand is not evaluated while the
933 operator is in the "true" state. The precedence is a little lower
934 than || and &&. The value returned is either the empty string for
935 false, or a sequence number (beginning with 1) for true. The sequence
936 number is reset for each range encountered. The final sequence number
937 in a range has the string "E0" appended to it, which doesn't affect
938 its numeric value, but gives you something to search for if you want
939 to exclude the endpoint. You can exclude the beginning point by
940 waiting for the sequence number to be greater than 1.
942 If either operand of scalar ".." is a constant expression,
943 that operand is considered true if it is equal (C<==>) to the current
944 input line number (the C<$.> variable).
946 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
947 but that is only an issue if you use a floating point expression; when
948 implicitly using C<$.> as described in the previous paragraph, the
949 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
950 is set to a floating point value and you are not reading from a file.
951 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
952 you want in scalar context because each of the operands are evaluated
953 using their integer representation.
957 As a scalar operator:
959 if (101 .. 200) { print; } # print 2nd hundred lines, short for
960 # if ($. == 101 .. $. == 200) { print; }
962 next LINE if (1 .. /^$/); # skip header lines, short for
963 # next LINE if ($. == 1 .. /^$/);
964 # (typically in a loop labeled LINE)
966 s/^/> / if (/^$/ .. eof()); # quote body
968 # parse mail messages
970 $in_header = 1 .. /^$/;
971 $in_body = /^$/ .. eof;
978 close ARGV if eof; # reset $. each file
981 Here's a simple example to illustrate the difference between
982 the two range operators:
995 This program will print only the line containing "Bar". If
996 the range operator is changed to C<...>, it will also print the
999 And now some examples as a list operator:
1001 for (101 .. 200) { print } # print $_ 100 times
1002 @foo = @foo[0 .. $#foo]; # an expensive no-op
1003 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
1005 The range operator (in list context) makes use of the magical
1006 auto-increment algorithm if the operands are strings. You
1009 @alphabet = ("A" .. "Z");
1011 to get all normal letters of the English alphabet, or
1013 $hexdigit = (0 .. 9, "a" .. "f")[$num & 15];
1015 to get a hexadecimal digit, or
1017 @z2 = ("01" .. "31");
1020 to get dates with leading zeros.
1022 If the final value specified is not in the sequence that the magical
1023 increment would produce, the sequence goes until the next value would
1024 be longer than the final value specified.
1026 If the initial value specified isn't part of a magical increment
1027 sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>),
1028 only the initial value will be returned. So the following will only
1031 use charnames "greek";
1032 my @greek_small = ("\N{alpha}" .. "\N{omega}");
1034 To get the 25 traditional lowercase Greek letters, including both sigmas,
1035 you could use this instead:
1037 use charnames "greek";
1038 my @greek_small = map { chr } ( ord("\N{alpha}")
1043 However, because there are I<many> other lowercase Greek characters than
1044 just those, to match lowercase Greek characters in a regular expression,
1045 you would use the pattern C</(?:(?=\p{Greek})\p{Lower})+/>.
1047 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
1048 return two elements in list context.
1050 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
1052 =head2 Conditional Operator
1053 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
1055 Ternary "?:" is the conditional operator, just as in C. It works much
1056 like an if-then-else. If the argument before the ? is true, the
1057 argument before the : is returned, otherwise the argument after the :
1058 is returned. For example:
1060 printf "I have %d dog%s.\n", $n,
1061 ($n == 1) ? "" : "s";
1063 Scalar or list context propagates downward into the 2nd
1064 or 3rd argument, whichever is selected.
1066 $a = $ok ? $b : $c; # get a scalar
1067 @a = $ok ? @b : @c; # get an array
1068 $a = $ok ? @b : @c; # oops, that's just a count!
1070 The operator may be assigned to if both the 2nd and 3rd arguments are
1071 legal lvalues (meaning that you can assign to them):
1073 ($a_or_b ? $a : $b) = $c;
1075 Because this operator produces an assignable result, using assignments
1076 without parentheses will get you in trouble. For example, this:
1078 $a % 2 ? $a += 10 : $a += 2
1082 (($a % 2) ? ($a += 10) : $a) += 2
1086 ($a % 2) ? ($a += 10) : ($a += 2)
1088 That should probably be written more simply as:
1090 $a += ($a % 2) ? 10 : 2;
1092 =head2 Assignment Operators
1093 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
1094 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
1097 "=" is the ordinary assignment operator.
1099 Assignment operators work as in C. That is,
1107 although without duplicating any side effects that dereferencing the lvalue
1108 might trigger, such as from tie(). Other assignment operators work similarly.
1109 The following are recognized:
1111 **= += *= &= <<= &&=
1116 Although these are grouped by family, they all have the precedence
1119 Unlike in C, the scalar assignment operator produces a valid lvalue.
1120 Modifying an assignment is equivalent to doing the assignment and
1121 then modifying the variable that was assigned to. This is useful
1122 for modifying a copy of something, like this:
1124 ($tmp = $global) =~ tr/13579/24680/;
1126 Although as of 5.14, that can be also be accomplished this way:
1129 $tmp = ($global =~ tr/13579/24680/r);
1140 Similarly, a list assignment in list context produces the list of
1141 lvalues assigned to, and a list assignment in scalar context returns
1142 the number of elements produced by the expression on the right hand
1143 side of the assignment.
1145 =head2 Comma Operator
1146 X<comma> X<operator, comma> X<,>
1148 Binary "," is the comma operator. In scalar context it evaluates
1149 its left argument, throws that value away, then evaluates its right
1150 argument and returns that value. This is just like C's comma operator.
1152 In list context, it's just the list argument separator, and inserts
1153 both its arguments into the list. These arguments are also evaluated
1156 The C<< => >> operator is a synonym for the comma except that it causes a
1157 word on its left to be interpreted as a string if it begins with a letter
1158 or underscore and is composed only of letters, digits and underscores.
1159 This includes operands that might otherwise be interpreted as operators,
1160 constants, single number v-strings or function calls. If in doubt about
1161 this behavior, the left operand can be quoted explicitly.
1163 Otherwise, the C<< => >> operator behaves exactly as the comma operator
1164 or list argument separator, according to context.
1168 use constant FOO => "something";
1170 my %h = ( FOO => 23 );
1174 my %h = ("FOO", 23);
1178 my %h = ("something", 23);
1180 The C<< => >> operator is helpful in documenting the correspondence
1181 between keys and values in hashes, and other paired elements in lists.
1183 %hash = ( $key => $value );
1184 login( $username => $password );
1186 The special quoting behavior ignores precedence, and hence may apply to
1187 I<part> of the left operand:
1189 print time.shift => "bbb";
1191 That example prints something like "1314363215shiftbbb", because the
1192 C<< => >> implicitly quotes the C<shift> immediately on its left, ignoring
1193 the fact that C<time.shift> is the entire left operand.
1195 =head2 List Operators (Rightward)
1196 X<operator, list, rightward> X<list operator>
1198 On the right side of a list operator, the comma has very low precedence,
1199 such that it controls all comma-separated expressions found there.
1200 The only operators with lower precedence are the logical operators
1201 "and", "or", and "not", which may be used to evaluate calls to list
1202 operators without the need for parentheses:
1204 open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n";
1206 However, some people find that code harder to read than writing
1207 it with parentheses:
1209 open(HANDLE, "< :utf8", "filename") or die "Can't open: $!\n";
1211 in which case you might as well just use the more customary "||" operator:
1213 open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n";
1215 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
1218 X<operator, logical, not> X<not>
1220 Unary "not" returns the logical negation of the expression to its right.
1221 It's the equivalent of "!" except for the very low precedence.
1224 X<operator, logical, and> X<and>
1226 Binary "and" returns the logical conjunction of the two surrounding
1227 expressions. It's equivalent to C<&&> except for the very low
1228 precedence. This means that it short-circuits: the right
1229 expression is evaluated only if the left expression is true.
1231 =head2 Logical or and Exclusive Or
1232 X<operator, logical, or> X<operator, logical, xor>
1233 X<operator, logical, exclusive or>
1236 Binary "or" returns the logical disjunction of the two surrounding
1237 expressions. It's equivalent to C<||> except for the very low precedence.
1238 This makes it useful for control flow:
1240 print FH $data or die "Can't write to FH: $!";
1242 This means that it short-circuits: the right expression is evaluated
1243 only if the left expression is false. Due to its precedence, you must
1244 be careful to avoid using it as replacement for the C<||> operator.
1245 It usually works out better for flow control than in assignments:
1247 $a = $b or $c; # bug: this is wrong
1248 ($a = $b) or $c; # really means this
1249 $a = $b || $c; # better written this way
1251 However, when it's a list-context assignment and you're trying to use
1252 C<||> for control flow, you probably need "or" so that the assignment
1253 takes higher precedence.
1255 @info = stat($file) || die; # oops, scalar sense of stat!
1256 @info = stat($file) or die; # better, now @info gets its due
1258 Then again, you could always use parentheses.
1260 Binary C<xor> returns the exclusive-OR of the two surrounding expressions.
1261 It cannot short-circuit (of course).
1263 There is no low precedence operator for defined-OR.
1265 =head2 C Operators Missing From Perl
1266 X<operator, missing from perl> X<&> X<*>
1267 X<typecasting> X<(TYPE)>
1269 Here is what C has that Perl doesn't:
1275 Address-of operator. (But see the "\" operator for taking a reference.)
1279 Dereference-address operator. (Perl's prefix dereferencing
1280 operators are typed: $, @, %, and &.)
1284 Type-casting operator.
1288 =head2 Quote and Quote-like Operators
1289 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
1290 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
1291 X<escape sequence> X<escape>
1293 While we usually think of quotes as literal values, in Perl they
1294 function as operators, providing various kinds of interpolating and
1295 pattern matching capabilities. Perl provides customary quote characters
1296 for these behaviors, but also provides a way for you to choose your
1297 quote character for any of them. In the following table, a C<{}> represents
1298 any pair of delimiters you choose.
1300 Customary Generic Meaning Interpolates
1303 `` qx{} Command yes*
1305 // m{} Pattern match yes*
1307 s{}{} Substitution yes*
1308 tr{}{} Transliteration no (but see below)
1309 y{}{} Transliteration no (but see below)
1312 * unless the delimiter is ''.
1314 Non-bracketing delimiters use the same character fore and aft, but the four
1315 sorts of ASCII brackets (round, angle, square, curly) all nest, which means
1324 Note, however, that this does not always work for quoting Perl code:
1326 $s = q{ if($a eq "}") ... }; # WRONG
1328 is a syntax error. The C<Text::Balanced> module (standard as of v5.8,
1329 and from CPAN before then) is able to do this properly.
1331 There can be whitespace between the operator and the quoting
1332 characters, except when C<#> is being used as the quoting character.
1333 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
1334 operator C<q> followed by a comment. Its argument will be taken
1335 from the next line. This allows you to write:
1337 s {foo} # Replace foo
1340 The following escape sequences are available in constructs that interpolate,
1341 and in transliterations:
1342 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
1345 Sequence Note Description
1351 \a alarm (bell) (BEL)
1353 \x{263A} [1,8] hex char (example: SMILEY)
1354 \x1b [2,8] restricted range hex char (example: ESC)
1355 \N{name} [3] named Unicode character or character sequence
1356 \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
1357 \c[ [5] control char (example: chr(27))
1358 \o{23072} [6,8] octal char (example: SMILEY)
1359 \033 [7,8] restricted range octal char (example: ESC)
1365 The result is the character specified by the hexadecimal number between
1366 the braces. See L</[8]> below for details on which character.
1368 Only hexadecimal digits are valid between the braces. If an invalid
1369 character is encountered, a warning will be issued and the invalid
1370 character and all subsequent characters (valid or invalid) within the
1371 braces will be discarded.
1373 If there are no valid digits between the braces, the generated character is
1374 the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
1375 will not cause a warning (currently).
1379 The result is the character specified by the hexadecimal number in the range
1380 0x00 to 0xFF. See L</[8]> below for details on which character.
1382 Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
1383 by fewer than two valid digits, any valid digits will be zero-padded. This
1384 means that C<\x7> will be interpreted as C<\x07>, and a lone <\x> will be
1385 interpreted as C<\x00>. Except at the end of a string, having fewer than
1386 two valid digits will result in a warning. Note that although the warning
1387 says the illegal character is ignored, it is only ignored as part of the
1388 escape and will still be used as the subsequent character in the string.
1391 Original Result Warns?
1399 The result is the Unicode character or character sequence given by I<name>.
1404 C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code
1405 point is I<hexadecimal number>.
1409 The character following C<\c> is mapped to some other character as shown in the
1426 In other words, it's the character whose code point has had 64 xor'd with
1427 its uppercase. C<\c?> is DELETE because C<ord("?") ^ 64> is 127, and
1428 C<\c@> is NULL because the ord of "@" is 64, so xor'ing 64 itself produces 0.
1430 Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the
1431 end of a string, because the backslash would be parsed as escaping the end
1434 On ASCII platforms, the resulting characters from the list above are the
1435 complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
1436 L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these
1437 sequences mean on both ASCII and EBCDIC platforms.
1439 Use of any other character following the "c" besides those listed above is
1440 discouraged, and some are deprecated with the intention of removing
1441 those in a later Perl version. What happens for any of these
1442 other characters currently though, is that the value is derived by xor'ing
1443 with the seventh bit, which is 64.
1445 To get platform independent controls, you can use C<\N{...}>.
1449 The result is the character specified by the octal number between the braces.
1450 See L</[8]> below for details on which character.
1452 If a character that isn't an octal digit is encountered, a warning is raised,
1453 and the value is based on the octal digits before it, discarding it and all
1454 following characters up to the closing brace. It is a fatal error if there are
1455 no octal digits at all.
1459 The result is the character specified by the three-digit octal number in the
1460 range 000 to 777 (but best to not use above 077, see next paragraph). See
1461 L</[8]> below for details on which character.
1463 Some contexts allow 2 or even 1 digit, but any usage without exactly
1464 three digits, the first being a zero, may give unintended results. (For
1465 example, in a regular expression it may be confused with a backreference;
1466 see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
1467 use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to
1468 use this construct only for ordinals C<\077> and below, remembering to pad to
1469 the left with zeros to make three digits. For larger ordinals, either use
1470 C<\o{}>, or convert to something else, such as to hex and use C<\x{}>
1473 Having fewer than 3 digits may lead to a misleading warning message that says
1474 that what follows is ignored. For example, C<"\128"> in the ASCII character set
1475 is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal
1476 digit '8' ignored> will be thrown. If C<"\n8"> is what you want, you can
1477 avoid this warning by padding your octal number with C<0>'s: C<"\0128">.
1481 Several constructs above specify a character by a number. That number
1482 gives the character's position in the character set encoding (indexed from 0).
1483 This is called synonymously its ordinal, code position, or code point. Perl
1484 works on platforms that have a native encoding currently of either ASCII/Latin1
1485 or EBCDIC, each of which allow specification of 256 characters. In general, if
1486 the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
1487 native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
1488 it as a Unicode code point and the result is the corresponding Unicode
1489 character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
1490 decimal, which is less than 256, so the number is interpreted in the native
1491 character set encoding. In ASCII the character in the 80th position (indexed
1492 from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".
1493 C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
1494 as a Unicode code point no matter what the native encoding is. The name of the
1495 character in the 256th position (indexed by 0) in Unicode is
1496 C<LATIN CAPITAL LETTER A WITH MACRON>.
1498 There are a couple of exceptions to the above rule. S<C<\N{U+I<hex number>}>> is
1499 always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even
1500 on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the
1501 number is considered to be in that encoding, and is translated from that into
1502 the platform's native encoding if there is a corresponding native character;
1503 otherwise to Unicode.
1507 B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
1508 the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may
1511 does have meaning in regular expression patterns in Perl, see L<perlre>.)
1513 The following escape sequences are available in constructs that interpolate,
1514 but not in transliterations.
1515 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> X<\F>
1517 \l lowercase next character only
1518 \u titlecase (not uppercase!) next character only
1519 \L lowercase all characters till \E or end of string
1520 \U uppercase all characters till \E or end of string
1521 \F foldcase all characters till \E or end of string
1522 \Q quote (disable) pattern metacharacters till \E or
1524 \E end either case modification or quoted section
1525 (whichever was last seen)
1527 See L<perlfunc/quotemeta> for the exact definition of characters that
1528 are quoted by C<\Q>.
1530 C<\L>, C<\U>, C<\F>, and C<\Q> can stack, in which case you need one
1531 C<\E> for each. For example:
1533 say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
1534 This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?
1536 If C<use locale> is in effect (but not C<use locale ':not_characters'>),
1537 the case map used by C<\l>, C<\L>,
1538 C<\u>, and C<\U> is taken from the current locale. See L<perllocale>.
1539 If Unicode (for example, C<\N{}> or code points of 0x100 or
1540 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u>, and
1541 C<\U> is as defined by Unicode. That means that case-mapping
1542 a single character can sometimes produce several characters.
1543 Under C<use locale>, C<\F> produces the same results as C<\L>.
1545 All systems use the virtual C<"\n"> to represent a line terminator,
1546 called a "newline". There is no such thing as an unvarying, physical
1547 newline character. It is only an illusion that the operating system,
1548 device drivers, C libraries, and Perl all conspire to preserve. Not all
1549 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1550 on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,
1551 and on systems without line terminator,
1552 printing C<"\n"> might emit no actual data. In general, use C<"\n"> when
1553 you mean a "newline" for your system, but use the literal ASCII when you
1554 need an exact character. For example, most networking protocols expect
1555 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
1556 and although they often accept just C<"\012">, they seldom tolerate just
1557 C<"\015">. If you get in the habit of using C<"\n"> for networking,
1558 you may be burned some day.
1559 X<newline> X<line terminator> X<eol> X<end of line>
1562 For constructs that do interpolate, variables beginning with "C<$>"
1563 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
1564 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1565 But method calls such as C<< $obj->meth >> are not.
1567 Interpolating an array or slice interpolates the elements in order,
1568 separated by the value of C<$">, so is equivalent to interpolating
1569 C<join $", @array>. "Punctuation" arrays such as C<@*> are usually
1570 interpolated only if the name is enclosed in braces C<@{*}>, but the
1571 arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces.
1573 For double-quoted strings, the quoting from C<\Q> is applied after
1574 interpolation and escapes are processed.
1576 "abc\Qfoo\tbar$s\Exyz"
1580 "abc" . quotemeta("foo\tbar$s") . "xyz"
1582 For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
1583 the quoting from C<\Q> is applied after interpolation is processed,
1584 but before escapes are processed. This allows the pattern to match
1585 literally (except for C<$> and C<@>). For example, the following matches:
1589 Because C<$> or C<@> trigger interpolation, you'll need to use something
1590 like C</\Quser\E\@\Qhost/> to match them literally.
1592 Patterns are subject to an additional level of interpretation as a
1593 regular expression. This is done as a second pass, after variables are
1594 interpolated, so that regular expressions may be incorporated into the
1595 pattern from the variables. If this is not what you want, use C<\Q> to
1596 interpolate a variable literally.
1598 Apart from the behavior described above, Perl does not expand
1599 multiple levels of interpolation. In particular, contrary to the
1600 expectations of shell programmers, back-quotes do I<NOT> interpolate
1601 within double quotes, nor do single quotes impede evaluation of
1602 variables when used within double quotes.
1604 =head2 Regexp Quote-Like Operators
1607 Here are the quote-like operators that apply to pattern
1608 matching and related activities.
1612 =item qr/STRING/msixpodual
1613 X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
1615 This operator quotes (and possibly compiles) its I<STRING> as a regular
1616 expression. I<STRING> is interpolated the same way as I<PATTERN>
1617 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1618 is done. Returns a Perl value which may be used instead of the
1619 corresponding C</STRING/msixpodual> expression. The returned value is a
1620 normalized version of the original pattern. It magically differs from
1621 a string containing the same characters: C<ref(qr/x/)> returns "Regexp";
1622 however, dereferencing it is not well defined (you currently get the
1623 normalized version of the original pattern, but this may change).
1628 $rex = qr/my.STRING/is;
1629 print $rex; # prints (?si-xm:my.STRING)
1636 The result may be used as a subpattern in a match:
1639 $string =~ /foo${re}bar/; # can be interpolated in other
1641 $string =~ $re; # or used standalone
1642 $string =~ /$re/; # or this way
1644 Since Perl may compile the pattern at the moment of execution of the qr()
1645 operator, using qr() may have speed advantages in some situations,
1646 notably if the result of qr() is used standalone:
1649 my $patterns = shift;
1650 my @compiled = map qr/$_/i, @$patterns;
1653 foreach my $pat (@compiled) {
1654 $success = 1, last if /$pat/;
1660 Precompilation of the pattern into an internal representation at
1661 the moment of qr() avoids a need to recompile the pattern every
1662 time a match C</$pat/> is attempted. (Perl has many other internal
1663 optimizations, but none would be triggered in the above example if
1664 we did not use qr() operator.)
1666 Options (specified by the following modifiers) are:
1668 m Treat string as multiple lines.
1669 s Treat string as single line. (Make . match a newline)
1670 i Do case-insensitive pattern matching.
1671 x Use extended regular expressions.
1672 p When matching preserve a copy of the matched string so
1673 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be
1675 o Compile pattern only once.
1676 a ASCII-restrict: Use ASCII for \d, \s, \w; specifying two
1677 a's further restricts /i matching so that no ASCII
1678 character will match a non-ASCII one.
1680 u Use Unicode rules.
1681 d Use Unicode or native charset, as in 5.12 and earlier.
1683 If a precompiled pattern is embedded in a larger pattern then the effect
1684 of "msixpluad" will be propagated appropriately. The effect the "o"
1685 modifier has is not propagated, being restricted to those patterns
1686 explicitly using it.
1688 The last four modifiers listed above, added in Perl 5.14,
1689 control the character set semantics, but C</a> is the only one you are likely
1690 to want to specify explicitly; the other three are selected
1691 automatically by various pragmas.
1693 See L<perlre> for additional information on valid syntax for STRING, and
1694 for a detailed look at the semantics of regular expressions. In
1695 particular, all modifiers except the largely obsolete C</o> are further
1696 explained in L<perlre/Modifiers>. C</o> is described in the next section.
1698 =item m/PATTERN/msixpodualgc
1699 X<m> X<operator, match>
1700 X<regexp, options> X<regexp> X<regex, options> X<regex>
1701 X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
1703 =item /PATTERN/msixpodualgc
1705 Searches a string for a pattern match, and in scalar context returns
1706 true if it succeeds, false if it fails. If no string is specified
1707 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1708 string specified with C<=~> need not be an lvalue--it may be the
1709 result of an expression evaluation, but remember the C<=~> binds
1710 rather tightly.) See also L<perlre>.
1712 Options are as described in C<qr//> above; in addition, the following match
1713 process modifiers are available:
1715 g Match globally, i.e., find all occurrences.
1716 c Do not reset search position on a failed match when /g is
1719 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1720 you can use any pair of non-whitespace (ASCII) characters
1721 as delimiters. This is particularly useful for matching path names
1722 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1723 the delimiter, then a match-only-once rule applies,
1724 described in C<m?PATTERN?> below. If "'" (single quote) is the delimiter,
1725 no interpolation is performed on the PATTERN.
1726 When using a character valid in an identifier, whitespace is required
1729 PATTERN may contain variables, which will be interpolated
1730 every time the pattern search is evaluated, except
1731 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1732 C<$|> are not interpolated because they look like end-of-string tests.)
1733 Perl will not recompile the pattern unless an interpolated
1734 variable that it contains changes. You can force Perl to skip the
1735 test and never recompile by adding a C</o> (which stands for "once")
1736 after the trailing delimiter.
1737 Once upon a time, Perl would recompile regular expressions
1738 unnecessarily, and this modifier was useful to tell it not to do so, in the
1739 interests of speed. But now, the only reasons to use C</o> are either:
1745 The variables are thousands of characters long and you know that they
1746 don't change, and you need to wring out the last little bit of speed by
1747 having Perl skip testing for that. (There is a maintenance penalty for
1748 doing this, as mentioning C</o> constitutes a promise that you won't
1749 change the variables in the pattern. If you do change them, Perl won't
1754 you want the pattern to use the initial values of the variables
1755 regardless of whether they change or not. (But there are saner ways
1756 of accomplishing this than using C</o>.)
1760 If the pattern contains embedded code, such as
1763 $code = 'foo(?{ $x })';
1766 then perl will recompile each time, even though the pattern string hasn't
1767 changed, to ensure that the current value of C<$x> is seen each time.
1768 Use C</o> if you want to avoid this.
1772 The bottom line is that using C</o> is almost never a good idea.
1774 =item The empty pattern //
1776 If the PATTERN evaluates to the empty string, the last
1777 I<successfully> matched regular expression is used instead. In this
1778 case, only the C<g> and C<c> flags on the empty pattern are honored;
1779 the other flags are taken from the original pattern. If no match has
1780 previously succeeded, this will (silently) act instead as a genuine
1781 empty pattern (which will always match).
1783 Note that it's possible to confuse Perl into thinking C<//> (the empty
1784 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1785 good about this, but some pathological cases might trigger this, such as
1786 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1787 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1788 will assume you meant defined-or. If you meant the empty regex, just
1789 use parentheses or spaces to disambiguate, or even prefix the empty
1790 regex with an C<m> (so C<//> becomes C<m//>).
1792 =item Matching in list context
1794 If the C</g> option is not used, C<m//> in list context returns a
1795 list consisting of the subexpressions matched by the parentheses in the
1796 pattern, that is, (C<$1>, C<$2>, C<$3>...) (Note that here C<$1> etc. are
1797 also set). When there are no parentheses in the pattern, the return
1798 value is the list C<(1)> for success.
1799 With or without parentheses, an empty list is returned upon failure.
1803 open(TTY, "+</dev/tty")
1804 || die "can't access /dev/tty: $!";
1806 <TTY> =~ /^y/i && foo(); # do foo if desired
1808 if (/Version: *([0-9.]*)/) { $version = $1; }
1810 next if m#^/usr/spool/uucp#;
1815 print if /$arg/o; # compile only once (no longer needed!)
1818 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1820 This last example splits $foo into the first two words and the
1821 remainder of the line, and assigns those three fields to $F1, $F2, and
1822 $Etc. The conditional is true if any variables were assigned; that is,
1823 if the pattern matched.
1825 The C</g> modifier specifies global pattern matching--that is,
1826 matching as many times as possible within the string. How it behaves
1827 depends on the context. In list context, it returns a list of the
1828 substrings matched by any capturing parentheses in the regular
1829 expression. If there are no parentheses, it returns a list of all
1830 the matched strings, as if there were parentheses around the whole
1833 In scalar context, each execution of C<m//g> finds the next match,
1834 returning true if it matches, and false if there is no further match.
1835 The position after the last match can be read or set using the C<pos()>
1836 function; see L<perlfunc/pos>. A failed match normally resets the
1837 search position to the beginning of the string, but you can avoid that
1838 by adding the C</c> modifier (for example, C<m//gc>). Modifying the target
1839 string also resets the search position.
1843 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1844 zero-width assertion that matches the exact position where the
1845 previous C<m//g>, if any, left off. Without the C</g> modifier, the
1846 C<\G> assertion still anchors at C<pos()> as it was at the start of
1847 the operation (see L<perlfunc/pos>), but the match is of course only
1848 attempted once. Using C<\G> without C</g> on a target string that has
1849 not previously had a C</g> match applied to it is the same as using
1850 the C<\A> assertion to match the beginning of the string. Note also
1851 that, currently, C<\G> is only properly supported when anchored at the
1852 very beginning of the pattern.
1857 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1861 while ($paragraph = <>) {
1862 while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
1868 Here's another way to check for sentences in a paragraph:
1870 my $sentence_rx = qr{
1871 (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or
1873 \p{Lu} # capital letter
1874 .*? # a bunch of anything
1875 (?<= \S ) # that ends in non-
1877 (?<! \b [DMS]r ) # but isn't a common abbr.
1881 [.?!] # followed by a sentence
1883 (?= $ | \s ) # in front of end-of-string
1887 while (my $paragraph = <>) {
1888 say "NEW PARAGRAPH";
1890 while ($paragraph =~ /($sentence_rx)/g) {
1891 printf "\tgot sentence %d: <%s>\n", ++$count, $1;
1895 Here's how to use C<m//gc> with C<\G>:
1900 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1902 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1904 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1906 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1908 The last example should print:
1918 Notice that the final match matched C<q> instead of C<p>, which a match
1919 without the C<\G> anchor would have done. Also note that the final match
1920 did not update C<pos>. C<pos> is only updated on a C</g> match. If the
1921 final match did indeed match C<p>, it's a good bet that you're running a
1922 very old (pre-5.6.0) version of Perl.
1924 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1925 combine several regexps like this to process a string part-by-part,
1926 doing different actions depending on which regexp matched. Each
1927 regexp tries to match where the previous one leaves off.
1930 $url = URI::URL->new( "http://example.com/" );
1931 die if $url eq "xXx";
1935 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1936 print(" lowercase"), redo LOOP
1937 if /\G\p{Ll}+\b[,.;]?\s*/gc;
1938 print(" UPPERCASE"), redo LOOP
1939 if /\G\p{Lu}+\b[,.;]?\s*/gc;
1940 print(" Capitalized"), redo LOOP
1941 if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
1942 print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
1943 print(" alphanumeric"), redo LOOP
1944 if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
1945 print(" line-noise"), redo LOOP if /\G\W+/gc;
1946 print ". That's all!\n";
1949 Here is the output (split into several lines):
1951 line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
1952 line-noise lowercase line-noise lowercase line-noise lowercase
1953 lowercase line-noise lowercase lowercase line-noise lowercase
1954 lowercase line-noise MiXeD line-noise. That's all!
1956 =item m?PATTERN?msixpodualgc
1957 X<?> X<operator, match-once>
1959 =item ?PATTERN?msixpodualgc
1961 This is just like the C<m/PATTERN/> search, except that it matches
1962 only once between calls to the reset() operator. This is a useful
1963 optimization when you want to see only the first occurrence of
1964 something in each file of a set of files, for instance. Only C<m??>
1965 patterns local to the current package are reset.
1969 # blank line between header and body
1972 reset if eof; # clear m?? status for next file
1975 Another example switched the first "latin1" encoding it finds
1976 to "utf8" in a pod file:
1978 s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
1980 The match-once behavior is controlled by the match delimiter being
1981 C<?>; with any other delimiter this is the normal C<m//> operator.
1983 For historical reasons, the leading C<m> in C<m?PATTERN?> is optional,
1984 but the resulting C<?PATTERN?> syntax is deprecated, will warn on
1985 usage and might be removed from a future stable release of Perl (without
1988 =item s/PATTERN/REPLACEMENT/msixpodualgcer
1989 X<substitute> X<substitution> X<replace> X<regexp, replace>
1990 X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
1992 Searches a string for a pattern, and if found, replaces that pattern
1993 with the replacement text and returns the number of substitutions
1994 made. Otherwise it returns false (specifically, the empty string).
1996 If the C</r> (non-destructive) option is used then it runs the
1997 substitution on a copy of the string and instead of returning the
1998 number of substitutions, it returns the copy whether or not a
1999 substitution occurred. The original string is never changed when
2000 C</r> is used. The copy will always be a plain string, even if the
2001 input is an object or a tied variable.
2003 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
2004 variable is searched and modified. Unless the C</r> option is used,
2005 the string specified must be a scalar variable, an array element, a
2006 hash element, or an assignment to one of those; that is, some sort of
2009 If the delimiter chosen is a single quote, no interpolation is
2010 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
2011 PATTERN contains a $ that looks like a variable rather than an
2012 end-of-string test, the variable will be interpolated into the pattern
2013 at run-time. If you want the pattern compiled only once the first time
2014 the variable is interpolated, use the C</o> option. If the pattern
2015 evaluates to the empty string, the last successfully executed regular
2016 expression is used instead. See L<perlre> for further explanation on these.
2018 Options are as with m// with the addition of the following replacement
2021 e Evaluate the right side as an expression.
2022 ee Evaluate the right side as a string then eval the
2024 r Return substitution and leave the original string
2027 Any non-whitespace delimiter may replace the slashes. Add space after
2028 the C<s> when using a character allowed in identifiers. If single quotes
2029 are used, no interpretation is done on the replacement string (the C</e>
2030 modifier overrides this, however). Note that Perl treats backticks
2031 as normal delimiters; the replacement text is not evaluated as a command.
2032 If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
2033 its own pair of quotes, which may or may not be bracketing quotes, for example,
2034 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
2035 replacement portion to be treated as a full-fledged Perl expression
2036 and evaluated right then and there. It is, however, syntax checked at
2037 compile-time. A second C<e> modifier will cause the replacement portion
2038 to be C<eval>ed before being run as a Perl expression.
2042 s/\bgreen\b/mauve/g; # don't change wintergreen
2044 $path =~ s|/usr/bin|/usr/local/bin|;
2046 s/Login: $foo/Login: $bar/; # run-time pattern
2048 ($foo = $bar) =~ s/this/that/; # copy first, then
2050 ($foo = "$bar") =~ s/this/that/; # convert to string,
2052 $foo = $bar =~ s/this/that/r; # Same as above using /r
2053 $foo = $bar =~ s/this/that/r
2054 =~ s/that/the other/r; # Chained substitutes
2056 @foo = map { s/this/that/r } @bar # /r is very useful in
2059 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-cnt
2062 s/\d+/$&*2/e; # yields 'abc246xyz'
2063 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
2064 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
2066 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
2067 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
2068 s/^=(\w+)/pod($1)/ge; # use function call
2071 $a = s/abc/def/r; # $a is 'def123xyz' and
2072 # $_ remains 'abc123xyz'.
2074 # expand variables in $_, but dynamics only, using
2075 # symbolic dereferencing
2078 # Add one to the value of any numbers in the string
2081 # Titlecase words in the last 30 characters only
2082 substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;
2084 # This will expand any embedded scalar variable
2085 # (including lexicals) in $_ : First $1 is interpolated
2086 # to the variable name, and then evaluated
2089 # Delete (most) C comments.
2091 /\* # Match the opening delimiter.
2092 .*? # Match a minimal number of characters.
2093 \*/ # Match the closing delimiter.
2096 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_,
2099 for ($variable) { # trim whitespace in $variable,
2105 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
2107 Note the use of $ instead of \ in the last example. Unlike
2108 B<sed>, we use the \<I<digit>> form in only the left hand side.
2109 Anywhere else it's $<I<digit>>.
2111 Occasionally, you can't use just a C</g> to get all the changes
2112 to occur that you might want. Here are two common cases:
2114 # put commas in the right places in an integer
2115 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
2117 # expand tabs to 8-column spacing
2118 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
2122 =head2 Quote-Like Operators
2123 X<operator, quote-like>
2128 X<q> X<quote, single> X<'> X<''>
2132 A single-quoted, literal string. A backslash represents a backslash
2133 unless followed by the delimiter or another backslash, in which case
2134 the delimiter or backslash is interpolated.
2136 $foo = q!I said, "You said, 'She said it.'"!;
2137 $bar = q('This is it.');
2138 $baz = '\n'; # a two-character string
2141 X<qq> X<quote, double> X<"> X<"">
2145 A double-quoted, interpolated string.
2148 (*** The previous line contains the naughty word "$1".\n)
2149 if /\b(tcl|java|python)\b/i; # :-)
2150 $baz = "\n"; # a one-character string
2153 X<qx> X<`> X<``> X<backtick>
2157 A string which is (possibly) interpolated and then executed as a
2158 system command with F</bin/sh> or its equivalent. Shell wildcards,
2159 pipes, and redirections will be honored. The collected standard
2160 output of the command is returned; standard error is unaffected. In
2161 scalar context, it comes back as a single (potentially multi-line)
2162 string, or undef if the command failed. In list context, returns a
2163 list of lines (however you've defined lines with $/ or
2164 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
2166 Because backticks do not affect standard error, use shell file descriptor
2167 syntax (assuming the shell supports this) if you care to address this.
2168 To capture a command's STDERR and STDOUT together:
2170 $output = `cmd 2>&1`;
2172 To capture a command's STDOUT but discard its STDERR:
2174 $output = `cmd 2>/dev/null`;
2176 To capture a command's STDERR but discard its STDOUT (ordering is
2179 $output = `cmd 2>&1 1>/dev/null`;
2181 To exchange a command's STDOUT and STDERR in order to capture the STDERR
2182 but leave its STDOUT to come out the old STDERR:
2184 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
2186 To read both a command's STDOUT and its STDERR separately, it's easiest
2187 to redirect them separately to files, and then read from those files
2188 when the program is done:
2190 system("program args 1>program.stdout 2>program.stderr");
2192 The STDIN filehandle used by the command is inherited from Perl's STDIN.
2195 open(SPLAT, "stuff") || die "can't open stuff: $!";
2196 open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
2197 print STDOUT `sort`;
2199 will print the sorted contents of the file named F<"stuff">.
2201 Using single-quote as a delimiter protects the command from Perl's
2202 double-quote interpolation, passing it on to the shell instead:
2204 $perl_info = qx(ps $$); # that's Perl's $$
2205 $shell_info = qx'ps $$'; # that's the new shell's $$
2207 How that string gets evaluated is entirely subject to the command
2208 interpreter on your system. On most platforms, you will have to protect
2209 shell metacharacters if you want them treated literally. This is in
2210 practice difficult to do, as it's unclear how to escape which characters.
2211 See L<perlsec> for a clean and safe example of a manual fork() and exec()
2212 to emulate backticks safely.
2214 On some platforms (notably DOS-like ones), the shell may not be
2215 capable of dealing with multiline commands, so putting newlines in
2216 the string may not get you what you want. You may be able to evaluate
2217 multiple commands in a single line by separating them with the command
2218 separator character, if your shell supports that (for example, C<;> on
2219 many Unix shells and C<&> on the Windows NT C<cmd> shell).
2221 Perl will attempt to flush all files opened for
2222 output before starting the child process, but this may not be supported
2223 on some platforms (see L<perlport>). To be safe, you may need to set
2224 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2225 C<IO::Handle> on any open handles.
2227 Beware that some command shells may place restrictions on the length
2228 of the command line. You must ensure your strings don't exceed this
2229 limit after any necessary interpolations. See the platform-specific
2230 release notes for more details about your particular environment.
2232 Using this operator can lead to programs that are difficult to port,
2233 because the shell commands called vary between systems, and may in
2234 fact not be present at all. As one example, the C<type> command under
2235 the POSIX shell is very different from the C<type> command under DOS.
2236 That doesn't mean you should go out of your way to avoid backticks
2237 when they're the right way to get something done. Perl was made to be
2238 a glue language, and one of the things it glues together is commands.
2239 Just understand what you're getting yourself into.
2241 See L</"I/O Operators"> for more discussion.
2244 X<qw> X<quote, list> X<quote, words>
2246 Evaluates to a list of the words extracted out of STRING, using embedded
2247 whitespace as the word delimiters. It can be understood as being roughly
2250 split(" ", q/STRING/);
2252 the differences being that it generates a real list at compile time, and
2253 in scalar context it returns the last element in the list. So
2258 is semantically equivalent to the list:
2262 Some frequently seen examples:
2264 use POSIX qw( setlocale localeconv )
2265 @EXPORT = qw( foo bar baz );
2267 A common mistake is to try to separate the words with comma or to
2268 put comments into a multi-line C<qw>-string. For this reason, the
2269 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
2270 produces warnings if the STRING contains the "," or the "#" character.
2272 =item tr/SEARCHLIST/REPLACEMENTLIST/cdsr
2273 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
2275 =item y/SEARCHLIST/REPLACEMENTLIST/cdsr
2277 Transliterates all occurrences of the characters found in the search list
2278 with the corresponding character in the replacement list. It returns
2279 the number of characters replaced or deleted. If no string is
2280 specified via the C<=~> or C<!~> operator, the $_ string is transliterated.
2282 If the C</r> (non-destructive) option is present, a new copy of the string
2283 is made and its characters transliterated, and this copy is returned no
2284 matter whether it was modified or not: the original string is always
2285 left unchanged. The new copy is always a plain string, even if the input
2286 string is an object or a tied variable.
2288 Unless the C</r> option is used, the string specified with C<=~> must be a
2289 scalar variable, an array element, a hash element, or an assignment to one
2290 of those; in other words, an lvalue.
2292 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2293 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
2294 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
2295 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
2296 its own pair of quotes, which may or may not be bracketing quotes;
2297 for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>.
2299 Note that C<tr> does B<not> do regular expression character classes such as
2300 C<\d> or C<\pL>. The C<tr> operator is not equivalent to the tr(1)
2301 utility. If you want to map strings between lower/upper cases, see
2302 L<perlfunc/lc> and L<perlfunc/uc>, and in general consider using the C<s>
2303 operator if you need regular expressions. The C<\U>, C<\u>, C<\L>, and
2304 C<\l> string-interpolation escapes on the right side of a substitution
2305 operator will perform correct case-mappings, but C<tr[a-z][A-Z]> will not
2306 (except sometimes on legacy 7-bit data).
2308 Note also that the whole range idea is rather unportable between
2309 character sets--and even within character sets they may cause results
2310 you probably didn't expect. A sound principle is to use only ranges
2311 that begin from and end at either alphabets of equal case (a-e, A-E),
2312 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
2313 character sets in full.
2317 c Complement the SEARCHLIST.
2318 d Delete found but unreplaced characters.
2319 s Squash duplicate replaced characters.
2320 r Return the modified string and leave the original string
2323 If the C</c> modifier is specified, the SEARCHLIST character set
2324 is complemented. If the C</d> modifier is specified, any characters
2325 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
2326 (Note that this is slightly more flexible than the behavior of some
2327 B<tr> programs, which delete anything they find in the SEARCHLIST,
2328 period.) If the C</s> modifier is specified, sequences of characters
2329 that were transliterated to the same character are squashed down
2330 to a single instance of the character.
2332 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
2333 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
2334 than the SEARCHLIST, the final character is replicated till it is long
2335 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
2336 This latter is useful for counting characters in a class or for
2337 squashing character sequences in a class.
2341 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
2343 $cnt = tr/*/*/; # count the stars in $_
2345 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
2347 $cnt = tr/0-9//; # count the digits in $_
2349 tr/a-zA-Z//s; # bookkeeper -> bokeper
2351 ($HOST = $host) =~ tr/a-z/A-Z/;
2352 $HOST = $host =~ tr/a-z/A-Z/r; # same thing
2354 $HOST = $host =~ tr/a-z/A-Z/r # chained with s///r
2357 tr/a-zA-Z/ /cs; # change non-alphas to single space
2359 @stripped = map tr/a-zA-Z/ /csr, @original;
2363 [\000-\177]; # wickedly delete 8th bit
2365 If multiple transliterations are given for a character, only the
2370 will transliterate any A to X.
2372 Because the transliteration table is built at compile time, neither
2373 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
2374 interpolation. That means that if you want to use variables, you
2377 eval "tr/$oldlist/$newlist/";
2380 eval "tr/$oldlist/$newlist/, 1" or die $@;
2383 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
2385 A line-oriented form of quoting is based on the shell "here-document"
2386 syntax. Following a C<< << >> you specify a string to terminate
2387 the quoted material, and all lines following the current line down to
2388 the terminating string are the value of the item.
2390 The terminating string may be either an identifier (a word), or some
2391 quoted text. An unquoted identifier works like double quotes.
2392 There may not be a space between the C<< << >> and the identifier,
2393 unless the identifier is explicitly quoted. (If you put a space it
2394 will be treated as a null identifier, which is valid, and matches the
2395 first empty line.) The terminating string must appear by itself
2396 (unquoted and with no surrounding whitespace) on the terminating line.
2398 If the terminating string is quoted, the type of quotes used determine
2399 the treatment of the text.
2405 Double quotes indicate that the text will be interpolated using exactly
2406 the same rules as normal double quoted strings.
2409 The price is $Price.
2412 print << "EOF"; # same as above
2413 The price is $Price.
2419 Single quotes indicate the text is to be treated literally with no
2420 interpolation of its content. This is similar to single quoted
2421 strings except that backslashes have no special meaning, with C<\\>
2422 being treated as two backslashes and not one as they would in every
2423 other quoting construct.
2425 Just as in the shell, a backslashed bareword following the C<<< << >>>
2426 means the same thing as a single-quoted string does:
2428 $cost = <<'VISTA'; # hasta la ...
2429 That'll be $10 please, ma'am.
2432 $cost = <<\VISTA; # Same thing!
2433 That'll be $10 please, ma'am.
2436 This is the only form of quoting in perl where there is no need
2437 to worry about escaping content, something that code generators
2438 can and do make good use of.
2442 The content of the here doc is treated just as it would be if the
2443 string were embedded in backticks. Thus the content is interpolated
2444 as though it were double quoted and then executed via the shell, with
2445 the results of the execution returned.
2447 print << `EOC`; # execute command and get results
2453 It is possible to stack multiple here-docs in a row:
2455 print <<"foo", <<"bar"; # you can stack them
2461 myfunc(<< "THIS", 23, <<'THAT');
2468 Just don't forget that you have to put a semicolon on the end
2469 to finish the statement, as Perl doesn't know you're not going to
2477 If you want to remove the line terminator from your here-docs,
2480 chomp($string = <<'END');
2484 If you want your here-docs to be indented with the rest of the code,
2485 you'll need to remove leading whitespace from each line manually:
2487 ($quote = <<'FINIS') =~ s/^\s+//gm;
2488 The Road goes ever on and on,
2489 down from the door where it began.
2492 If you use a here-doc within a delimited construct, such as in C<s///eg>,
2493 the quoted material must still come on the line following the
2494 C<<< <<FOO >>> marker, which means it may be inside the delimited
2502 It works this way as of Perl 5.18. Historically, it was inconsistent, and
2503 you would have to write
2510 outside of string evals.
2512 Additionally, quoting rules for the end-of-string identifier are
2513 unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
2514 supported in place of C<''> and C<"">, and the only interpolation is for
2515 backslashing the quoting character:
2517 print << "abc\"def";
2521 Finally, quoted strings cannot span multiple lines. The general rule is
2522 that the identifier must be a string literal. Stick with that, and you
2527 =head2 Gory details of parsing quoted constructs
2528 X<quote, gory details>
2530 When presented with something that might have several different
2531 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2532 principle to pick the most probable interpretation. This strategy
2533 is so successful that Perl programmers often do not suspect the
2534 ambivalence of what they write. But from time to time, Perl's
2535 notions differ substantially from what the author honestly meant.
2537 This section hopes to clarify how Perl handles quoted constructs.
2538 Although the most common reason to learn this is to unravel labyrinthine
2539 regular expressions, because the initial steps of parsing are the
2540 same for all quoting operators, they are all discussed together.
2542 The most important Perl parsing rule is the first one discussed
2543 below: when processing a quoted construct, Perl first finds the end
2544 of that construct, then interprets its contents. If you understand
2545 this rule, you may skip the rest of this section on the first
2546 reading. The other rules are likely to contradict the user's
2547 expectations much less frequently than this first one.
2549 Some passes discussed below are performed concurrently, but because
2550 their results are the same, we consider them individually. For different
2551 quoting constructs, Perl performs different numbers of passes, from
2552 one to four, but these passes are always performed in the same order.
2556 =item Finding the end
2558 The first pass is finding the end of the quoted construct, where
2559 the information about the delimiters is used in parsing.
2560 During this search, text between the starting and ending delimiters
2561 is copied to a safe location. The text copied gets delimiter-independent.
2563 If the construct is a here-doc, the ending delimiter is a line
2564 that has a terminating string as the content. Therefore C<<<EOF> is
2565 terminated by C<EOF> immediately followed by C<"\n"> and starting
2566 from the first column of the terminating line.
2567 When searching for the terminating line of a here-doc, nothing
2568 is skipped. In other words, lines after the here-doc syntax
2569 are compared with the terminating string line by line.
2571 For the constructs except here-docs, single characters are used as starting
2572 and ending delimiters. If the starting delimiter is an opening punctuation
2573 (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2574 corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2575 If the starting delimiter is an unpaired character like C</> or a closing
2576 punctuation, the ending delimiter is same as the starting delimiter.
2577 Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
2578 C<qq[]> and C<qq]]> constructs.
2580 When searching for single-character delimiters, escaped delimiters
2581 and C<\\> are skipped. For example, while searching for terminating C</>,
2582 combinations of C<\\> and C<\/> are skipped. If the delimiters are
2583 bracketing, nested pairs are also skipped. For example, while searching
2584 for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
2585 and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2586 However, when backslashes are used as the delimiters (like C<qq\\> and
2587 C<tr\\\>), nothing is skipped.
2588 During the search for the end, backslashes that escape delimiters or
2589 other backslashes are removed (exactly speaking, they are not copied to the
2592 For constructs with three-part delimiters (C<s///>, C<y///>, and
2593 C<tr///>), the search is repeated once more.
2594 If the first delimiter is not an opening punctuation, three delimiters must
2595 be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
2596 terminates the left part and starts the right part at once.
2597 If the left part is delimited by bracketing punctuation (that is C<()>,
2598 C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
2599 delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace
2600 and comments are allowed between both parts, though the comment must follow
2601 at least one whitespace character; otherwise a character expected as the
2602 start of the comment may be regarded as the starting delimiter of the right part.
2604 During this search no attention is paid to the semantics of the construct.
2607 "$hash{"$foo/$bar"}"
2612 bar # NOT a comment, this slash / terminated m//!
2615 do not form legal quoted expressions. The quoted part ends on the
2616 first C<"> and C</>, and the rest happens to be a syntax error.
2617 Because the slash that terminated C<m//> was followed by a C<SPACE>,
2618 the example above is not C<m//x>, but rather C<m//> with no C</x>
2619 modifier. So the embedded C<#> is interpreted as a literal C<#>.
2621 Also no attention is paid to C<\c\> (multichar control char syntax) during
2622 this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
2623 of C<\/>, and the following C</> is not recognized as a delimiter.
2624 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2629 The next step is interpolation in the text obtained, which is now
2630 delimiter-independent. There are multiple cases.
2636 No interpolation is performed.
2637 Note that the combination C<\\> is left intact, since escaped delimiters
2638 are not available for here-docs.
2640 =item C<m''>, the pattern of C<s'''>
2642 No interpolation is performed at this stage.
2643 Any backslashed sequences including C<\\> are treated at the stage
2644 to L</"parsing regular expressions">.
2646 =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
2648 The only interpolation is removal of C<\> from pairs of C<\\>.
2649 Therefore C<-> in C<tr'''> and C<y'''> is treated literally
2650 as a hyphen and no character range is available.
2651 C<\1> in the replacement of C<s'''> does not work as C<$1>.
2653 =item C<tr///>, C<y///>
2655 No variable interpolation occurs. String modifying combinations for
2656 case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2657 The other escape sequences such as C<\200> and C<\t> and backslashed
2658 characters such as C<\\> and C<\-> are converted to appropriate literals.
2659 The character C<-> is treated specially and therefore C<\-> is treated
2662 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
2664 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> (possibly paired with C<\E>) are
2665 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
2666 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
2667 The other escape sequences such as C<\200> and C<\t> and backslashed
2668 characters such as C<\\> and C<\-> are replaced with appropriate
2671 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2672 is interpolated in the usual way. Something like C<"\Q\\E"> has
2673 no C<\E> inside. Instead, it has C<\Q>, C<\\>, and C<E>, so the
2674 result is the same as for C<"\\\\E">. As a general rule, backslashes
2675 between C<\Q> and C<\E> may lead to counterintuitive results. So,
2676 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2677 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2682 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2684 Interpolated scalars and arrays are converted internally to the C<join> and
2685 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
2687 $foo . " XXX '" . (join $", @arr) . "'";
2689 All operations above are performed simultaneously, left to right.
2691 Because the result of C<"\Q STRING \E"> has all metacharacters
2692 quoted, there is no way to insert a literal C<$> or C<@> inside a
2693 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
2694 C<"\\\$">; if not, it is interpreted as the start of an interpolated
2697 Note also that the interpolation code needs to make a decision on
2698 where the interpolated scalar ends. For instance, whether
2699 C<< "a $b -> {c}" >> really means:
2701 "a " . $b . " -> {c}";
2707 Most of the time, the longest possible text that does not include
2708 spaces between components and which contains matching braces or
2709 brackets. because the outcome may be determined by voting based
2710 on heuristic estimators, the result is not strictly predictable.
2711 Fortunately, it's usually correct for ambiguous cases.
2713 =item the replacement of C<s///>
2715 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> and interpolation
2716 happens as with C<qq//> constructs.
2718 It is at this step that C<\1> is begrudgingly converted to C<$1> in
2719 the replacement text of C<s///>, in order to correct the incorrigible
2720 I<sed> hackers who haven't picked up the saner idiom yet. A warning
2721 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2722 (that is, the C<$^W> variable) was set.
2724 =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2726 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F>, C<\E>,
2727 and interpolation happens (almost) as with C<qq//> constructs.
2729 Processing of C<\N{...}> is also done here, and compiled into an intermediate
2730 form for the regex compiler. (This is because, as mentioned below, the regex
2731 compilation may be done at execution time, and C<\N{...}> is a compile-time
2734 However any other combinations of C<\> followed by a character
2735 are not substituted but only skipped, in order to parse them
2736 as regular expressions at the following step.
2737 As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
2738 treated as an array symbol (for example C<@foo>),
2739 even though the same text in C<qq//> gives interpolation of C<\c@>.
2741 Code blocks such as C<(?{BLOCK})> are handled by temporarily passing control
2742 back to the perl parser, in a similar way that an interpolated array
2743 subscript expression such as C<"foo$array[1+f("[xyz")]bar"> would be.
2745 Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
2746 a C<#>-comment in a C<//x>-regular expression, no processing is
2747 performed whatsoever. This is the first step at which the presence
2748 of the C<//x> modifier is relevant.
2750 Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2751 and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2752 voted (by several different estimators) to be either an array element
2753 or C<$var> followed by an RE alternative. This is where the notation
2754 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2755 array element C<-9>, not as a regular expression from the variable
2756 C<$arr> followed by a digit, which would be the interpretation of
2757 C</$arr[0-9]/>. Since voting among different estimators may occur,
2758 the result is not predictable.
2760 The lack of processing of C<\\> creates specific restrictions on
2761 the post-processed text. If the delimiter is C</>, one cannot get
2762 the combination C<\/> into the result of this step. C</> will
2763 finish the regular expression, C<\/> will be stripped to C</> on
2764 the previous step, and C<\\/> will be left as is. Because C</> is
2765 equivalent to C<\/> inside a regular expression, this does not
2766 matter unless the delimiter happens to be character special to the
2767 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2768 alphanumeric char, as in:
2772 In the RE above, which is intentionally obfuscated for illustration, the
2773 delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
2774 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
2775 reason you're encouraged to restrict your delimiters to non-alphanumeric,
2776 non-whitespace choices.
2780 This step is the last one for all constructs except regular expressions,
2781 which are processed further.
2783 =item parsing regular expressions
2786 Previous steps were performed during the compilation of Perl code,
2787 but this one happens at run time, although it may be optimized to
2788 be calculated at compile time if appropriate. After preprocessing
2789 described above, and possibly after evaluation if concatenation,
2790 joining, casing translation, or metaquoting are involved, the
2791 resulting I<string> is passed to the RE engine for compilation.
2793 Whatever happens in the RE engine might be better discussed in L<perlre>,
2794 but for the sake of continuity, we shall do so here.
2796 This is another step where the presence of the C<//x> modifier is
2797 relevant. The RE engine scans the string from left to right and
2798 converts it to a finite automaton.
2800 Backslashed characters are either replaced with corresponding
2801 literal strings (as with C<\{>), or else they generate special nodes
2802 in the finite automaton (as with C<\b>). Characters special to the
2803 RE engine (such as C<|>) generate corresponding nodes or groups of
2804 nodes. C<(?#...)> comments are ignored. All the rest is either
2805 converted to literal strings to match, or else is ignored (as is
2806 whitespace and C<#>-style comments if C<//x> is present).
2808 Parsing of the bracketed character class construct, C<[...]>, is
2809 rather different than the rule used for the rest of the pattern.
2810 The terminator of this construct is found using the same rules as
2811 for finding the terminator of a C<{}>-delimited construct, the only
2812 exception being that C<]> immediately following C<[> is treated as
2813 though preceded by a backslash.
2815 The terminator of runtime C<(?{...})> is found by temporarily switching
2816 control to the perl parser, which should stop at the point where the
2817 logically balancing terminating C<}> is found.
2819 It is possible to inspect both the string given to RE engine and the
2820 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2821 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
2822 switch documented in L<perlrun/"Command Switches">.
2824 =item Optimization of regular expressions
2825 X<regexp, optimization>
2827 This step is listed for completeness only. Since it does not change
2828 semantics, details of this step are not documented and are subject
2829 to change without notice. This step is performed over the finite
2830 automaton that was generated during the previous pass.
2832 It is at this stage that C<split()> silently optimizes C</^/> to
2837 =head2 I/O Operators
2838 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2841 There are several I/O operators you should know about.
2843 A string enclosed by backticks (grave accents) first undergoes
2844 double-quote interpolation. It is then interpreted as an external
2845 command, and the output of that command is the value of the
2846 backtick string, like in a shell. In scalar context, a single string
2847 consisting of all output is returned. In list context, a list of
2848 values is returned, one per line of output. (You can set C<$/> to use
2849 a different line terminator.) The command is executed each time the
2850 pseudo-literal is evaluated. The status value of the command is
2851 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2852 Unlike in B<csh>, no translation is done on the return data--newlines
2853 remain newlines. Unlike in any of the shells, single quotes do not
2854 hide variable names in the command from interpretation. To pass a
2855 literal dollar-sign through to the shell you need to hide it with a
2856 backslash. The generalized form of backticks is C<qx//>. (Because
2857 backticks always undergo shell expansion as well, see L<perlsec> for
2859 X<qx> X<`> X<``> X<backtick> X<glob>
2861 In scalar context, evaluating a filehandle in angle brackets yields
2862 the next line from that file (the newline, if any, included), or
2863 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2864 (sometimes known as file-slurp mode) and the file is empty, it
2865 returns C<''> the first time, followed by C<undef> subsequently.
2867 Ordinarily you must assign the returned value to a variable, but
2868 there is one situation where an automatic assignment happens. If
2869 and only if the input symbol is the only thing inside the conditional
2870 of a C<while> statement (even if disguised as a C<for(;;)> loop),
2871 the value is automatically assigned to the global variable $_,
2872 destroying whatever was there previously. (This may seem like an
2873 odd thing to you, but you'll use the construct in almost every Perl
2874 script you write.) The $_ variable is not implicitly localized.
2875 You'll have to put a C<local $_;> before the loop if you want that
2878 The following lines are equivalent:
2880 while (defined($_ = <STDIN>)) { print; }
2881 while ($_ = <STDIN>) { print; }
2882 while (<STDIN>) { print; }
2883 for (;<STDIN>;) { print; }
2884 print while defined($_ = <STDIN>);
2885 print while ($_ = <STDIN>);
2886 print while <STDIN>;
2888 This also behaves similarly, but assigns to a lexical variable
2889 instead of to C<$_>:
2891 while (my $line = <STDIN>) { print $line }
2893 In these loop constructs, the assigned value (whether assignment
2894 is automatic or explicit) is then tested to see whether it is
2895 defined. The defined test avoids problems where the line has a string
2896 value that would be treated as false by Perl; for example a "" or
2897 a "0" with no trailing newline. If you really mean for such values
2898 to terminate the loop, they should be tested for explicitly:
2900 while (($_ = <STDIN>) ne '0') { ... }
2901 while (<STDIN>) { last unless $_; ... }
2903 In other boolean contexts, C<< <FILEHANDLE> >> without an
2904 explicit C<defined> test or comparison elicits a warning if the
2905 C<use warnings> pragma or the B<-w>
2906 command-line switch (the C<$^W> variable) is in effect.
2908 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2909 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2910 in packages, where they would be interpreted as local identifiers
2911 rather than global.) Additional filehandles may be created with
2912 the open() function, amongst others. See L<perlopentut> and
2913 L<perlfunc/open> for details on this.
2914 X<stdin> X<stdout> X<sterr>
2916 If a <FILEHANDLE> is used in a context that is looking for
2917 a list, a list comprising all input lines is returned, one line per
2918 list element. It's easy to grow to a rather large data space this
2919 way, so use with care.
2921 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2922 See L<perlfunc/readline>.
2924 The null filehandle <> is special: it can be used to emulate the
2925 behavior of B<sed> and B<awk>, and any other Unix filter program
2926 that takes a list of filenames, doing the same to each line
2927 of input from all of them. Input from <> comes either from
2928 standard input, or from each file listed on the command line. Here's
2929 how it works: the first time <> is evaluated, the @ARGV array is
2930 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2931 gives you standard input. The @ARGV array is then processed as a list
2932 of filenames. The loop
2935 ... # code for each line
2938 is equivalent to the following Perl-like pseudo code:
2940 unshift(@ARGV, '-') unless @ARGV;
2941 while ($ARGV = shift) {
2944 ... # code for each line
2948 except that it isn't so cumbersome to say, and will actually work.
2949 It really does shift the @ARGV array and put the current filename
2950 into the $ARGV variable. It also uses filehandle I<ARGV>
2951 internally. <> is just a synonym for <ARGV>, which
2952 is magical. (The pseudo code above doesn't work because it treats
2953 <ARGV> as non-magical.)
2955 Since the null filehandle uses the two argument form of L<perlfunc/open>
2956 it interprets special characters, so if you have a script like this:
2962 and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2963 pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2964 If you want all items in C<@ARGV> to be interpreted as file names, you
2965 can use the module C<ARGV::readonly> from CPAN.
2967 You can modify @ARGV before the first <> as long as the array ends up
2968 containing the list of filenames you really want. Line numbers (C<$.>)
2969 continue as though the input were one big happy file. See the example
2970 in L<perlfunc/eof> for how to reset line numbers on each file.
2972 If you want to set @ARGV to your own list of files, go right ahead.
2973 This sets @ARGV to all plain text files if no @ARGV was given:
2975 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2977 You can even set them to pipe commands. For example, this automatically
2978 filters compressed arguments through B<gzip>:
2980 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2982 If you want to pass switches into your script, you can use one of the
2983 Getopts modules or put a loop on the front like this:
2985 while ($_ = $ARGV[0], /^-/) {
2988 if (/^-D(.*)/) { $debug = $1 }
2989 if (/^-v/) { $verbose++ }
2990 # ... # other switches
2994 # ... # code for each line
2997 The <> symbol will return C<undef> for end-of-file only once.
2998 If you call it again after this, it will assume you are processing another
2999 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
3001 If what the angle brackets contain is a simple scalar variable (for example,
3002 <$foo>), then that variable contains the name of the
3003 filehandle to input from, or its typeglob, or a reference to the
3009 If what's within the angle brackets is neither a filehandle nor a simple
3010 scalar variable containing a filehandle name, typeglob, or typeglob
3011 reference, it is interpreted as a filename pattern to be globbed, and
3012 either a list of filenames or the next filename in the list is returned,
3013 depending on context. This distinction is determined on syntactic
3014 grounds alone. That means C<< <$x> >> is always a readline() from
3015 an indirect handle, but C<< <$hash{key}> >> is always a glob().
3016 That's because $x is a simple scalar variable, but C<$hash{key}> is
3017 not--it's a hash element. Even C<< <$x > >> (note the extra space)
3018 is treated as C<glob("$x ")>, not C<readline($x)>.
3020 One level of double-quote interpretation is done first, but you can't
3021 say C<< <$foo> >> because that's an indirect filehandle as explained
3022 in the previous paragraph. (In older versions of Perl, programmers
3023 would insert curly brackets to force interpretation as a filename glob:
3024 C<< <${foo}> >>. These days, it's considered cleaner to call the
3025 internal function directly as C<glob($foo)>, which is probably the right
3026 way to have done it in the first place.) For example:
3032 is roughly equivalent to:
3034 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
3040 except that the globbing is actually done internally using the standard
3041 C<File::Glob> extension. Of course, the shortest way to do the above is:
3045 A (file)glob evaluates its (embedded) argument only when it is
3046 starting a new list. All values must be read before it will start
3047 over. In list context, this isn't important because you automatically
3048 get them all anyway. However, in scalar context the operator returns
3049 the next value each time it's called, or C<undef> when the list has
3050 run out. As with filehandle reads, an automatic C<defined> is
3051 generated when the glob occurs in the test part of a C<while>,
3052 because legal glob returns (for example,
3053 a file called F<0>) would otherwise
3054 terminate the loop. Again, C<undef> is returned only once. So if
3055 you're expecting a single value from a glob, it is much better to
3058 ($file) = <blurch*>;
3064 because the latter will alternate between returning a filename and
3067 If you're trying to do variable interpolation, it's definitely better
3068 to use the glob() function, because the older notation can cause people
3069 to become confused with the indirect filehandle notation.
3071 @files = glob("$dir/*.[ch]");
3072 @files = glob($files[$i]);
3074 =head2 Constant Folding
3075 X<constant folding> X<folding>
3077 Like C, Perl does a certain amount of expression evaluation at
3078 compile time whenever it determines that all arguments to an
3079 operator are static and have no side effects. In particular, string
3080 concatenation happens at compile time between literals that don't do
3081 variable substitution. Backslash interpolation also happens at
3082 compile time. You can say
3084 'Now is the time for all'
3086 . 'good men to come to.'
3088 and this all reduces to one string internally. Likewise, if
3091 foreach $file (@filenames) {
3092 if (-s $file > 5 + 100 * 2**16) { }
3095 the compiler precomputes the number which that expression
3096 represents so that the interpreter won't have to.
3101 Perl doesn't officially have a no-op operator, but the bare constants
3102 C<0> and C<1> are special-cased not to produce a warning in void
3103 context, so you can for example safely do
3107 =head2 Bitwise String Operators
3108 X<operator, bitwise, string>
3110 Bitstrings of any size may be manipulated by the bitwise operators
3113 If the operands to a binary bitwise op are strings of different
3114 sizes, B<|> and B<^> ops act as though the shorter operand had
3115 additional zero bits on the right, while the B<&> op acts as though
3116 the longer operand were truncated to the length of the shorter.
3117 The granularity for such extension or truncation is one or more
3120 # ASCII-based examples
3121 print "j p \n" ^ " a h"; # prints "JAPH\n"
3122 print "JA" | " ph\n"; # prints "japh\n"
3123 print "japh\nJunk" & '_____'; # prints "JAPH\n";
3124 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
3126 If you are intending to manipulate bitstrings, be certain that
3127 you're supplying bitstrings: If an operand is a number, that will imply
3128 a B<numeric> bitwise operation. You may explicitly show which type of
3129 operation you intend by using C<""> or C<0+>, as in the examples below.
3131 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
3132 $foo = '150' | 105; # yields 255
3133 $foo = 150 | '105'; # yields 255
3134 $foo = '150' | '105'; # yields string '155' (under ASCII)
3136 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
3137 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
3139 See L<perlfunc/vec> for information on how to manipulate individual bits
3142 =head2 Integer Arithmetic
3145 By default, Perl assumes that it must do most of its arithmetic in
3146 floating point. But by saying
3150 you may tell the compiler to use integer operations
3151 (see L<integer> for a detailed explanation) from here to the end of
3152 the enclosing BLOCK. An inner BLOCK may countermand this by saying
3156 which lasts until the end of that BLOCK. Note that this doesn't
3157 mean everything is an integer, merely that Perl will use integer
3158 operations for arithmetic, comparison, and bitwise operators. For
3159 example, even under C<use integer>, if you take the C<sqrt(2)>, you'll
3160 still get C<1.4142135623731> or so.
3162 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
3163 and ">>") always produce integral results. (But see also
3164 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
3165 them. By default, their results are interpreted as unsigned integers, but
3166 if C<use integer> is in effect, their results are interpreted
3167 as signed integers. For example, C<~0> usually evaluates to a large
3168 integral value. However, C<use integer; ~0> is C<-1> on two's-complement
3171 =head2 Floating-point Arithmetic
3173 X<floating-point> X<floating point> X<float> X<real>
3175 While C<use integer> provides integer-only arithmetic, there is no
3176 analogous mechanism to provide automatic rounding or truncation to a
3177 certain number of decimal places. For rounding to a certain number
3178 of digits, sprintf() or printf() is usually the easiest route.
3181 Floating-point numbers are only approximations to what a mathematician
3182 would call real numbers. There are infinitely more reals than floats,
3183 so some corners must be cut. For example:
3185 printf "%.20g\n", 123456789123456789;
3186 # produces 123456789123456784
3188 Testing for exact floating-point equality or inequality is not a
3189 good idea. Here's a (relatively expensive) work-around to compare
3190 whether two floating-point numbers are equal to a particular number of
3191 decimal places. See Knuth, volume II, for a more robust treatment of
3195 my ($X, $Y, $POINTS) = @_;
3197 $tX = sprintf("%.${POINTS}g", $X);
3198 $tY = sprintf("%.${POINTS}g", $Y);
3202 The POSIX module (part of the standard perl distribution) implements
3203 ceil(), floor(), and other mathematical and trigonometric functions.
3204 The Math::Complex module (part of the standard perl distribution)
3205 defines mathematical functions that work on both the reals and the
3206 imaginary numbers. Math::Complex not as efficient as POSIX, but
3207 POSIX can't work with complex numbers.
3209 Rounding in financial applications can have serious implications, and
3210 the rounding method used should be specified precisely. In these
3211 cases, it probably pays not to trust whichever system rounding is
3212 being used by Perl, but to instead implement the rounding function you
3215 =head2 Bigger Numbers
3216 X<number, arbitrary precision>
3218 The standard C<Math::BigInt>, C<Math::BigRat>, and C<Math::BigFloat> modules,
3219 along with the C<bignum>, C<bigint>, and C<bigrat> pragmas, provide
3220 variable-precision arithmetic and overloaded operators, although
3221 they're currently pretty slow. At the cost of some space and
3222 considerable speed, they avoid the normal pitfalls associated with
3223 limited-precision representations.
3226 use bigint; # easy interface to Math::BigInt
3227 $x = 123456789123456789;
3229 +15241578780673678515622620750190521
3237 say "a/b is ", $a/$b;
3238 say "a*b is ", $a*$b;
3242 Several modules let you calculate with (bound only by memory and CPU time)
3243 unlimited or fixed precision. There are also some non-standard modules that
3244 provide faster implementations via external C libraries.
3246 Here is a short, but incomplete summary:
3248 Math::String treat string sequences like numbers
3249 Math::FixedPrecision calculate with a fixed precision
3250 Math::Currency for currency calculations
3251 Bit::Vector manipulate bit vectors fast (uses C)
3252 Math::BigIntFast Bit::Vector wrapper for big numbers
3253 Math::Pari provides access to the Pari C library
3254 Math::Cephes uses the external Cephes C library (no
3256 Math::Cephes::Fraction fractions via the Cephes library
3257 Math::GMP another one using an external C library
3258 Math::GMPz an alternative interface to libgmp's big ints
3259 Math::GMPq an interface to libgmp's fraction numbers
3260 Math::GMPf an interface to libgmp's floating point numbers