4 perlop - Perl operators and precedence
8 =head2 Operator Precedence and Associativity
9 X<operator, precedence> X<precedence> X<associativity>
11 Operator precedence and associativity work in Perl more or less like
12 they do in mathematics.
14 I<Operator precedence> means some operators are evaluated before
15 others. For example, in C<2 + 4 * 5>, the multiplication has higher
16 precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
17 22> and not C<6 * 5 == 30>.
19 I<Operator associativity> defines what happens if a sequence of the
20 same operators is used one after another: whether the evaluator will
21 evaluate the left operations first or the right. For example, in C<8
22 - 4 - 2>, subtraction is left associative so Perl evaluates the
23 expression left to right. C<8 - 4> is evaluated first making the
24 expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
26 Perl operators have the following associativity and precedence,
27 listed from highest precedence to lowest. Operators borrowed from
28 C keep the same precedence relationship with each other, even where
29 C's precedence is slightly screwy. (This makes learning Perl easier
30 for C folks.) With very few exceptions, these all operate on scalar
31 values only, not array values.
33 left terms and list operators (leftward)
37 right ! ~ \ and unary + and -
42 nonassoc named unary operators
43 nonassoc < > <= >= lt gt le ge
44 nonassoc == != <=> eq ne cmp ~~
51 right = += -= *= etc. goto last next redo dump
53 nonassoc list operators (rightward)
58 In the following sections, these operators are covered in precedence order.
60 Many operators can be overloaded for objects. See L<overload>.
62 =head2 Terms and List Operators (Leftward)
63 X<list operator> X<operator, list> X<term>
65 A TERM has the highest precedence in Perl. They include variables,
66 quote and quote-like operators, any expression in parentheses,
67 and any function whose arguments are parenthesized. Actually, there
68 aren't really functions in this sense, just list operators and unary
69 operators behaving as functions because you put parentheses around
70 the arguments. These are all documented in L<perlfunc>.
72 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
73 is followed by a left parenthesis as the next token, the operator and
74 arguments within parentheses are taken to be of highest precedence,
75 just like a normal function call.
77 In the absence of parentheses, the precedence of list operators such as
78 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
79 whether you are looking at the left side or the right side of the operator.
82 @ary = (1, 3, sort 4, 2);
83 print @ary; # prints 1324
85 the commas on the right of the sort are evaluated before the sort,
86 but the commas on the left are evaluated after. In other words,
87 list operators tend to gobble up all arguments that follow, and
88 then act like a simple TERM with regard to the preceding expression.
89 Be careful with parentheses:
91 # These evaluate exit before doing the print:
92 print($foo, exit); # Obviously not what you want.
93 print $foo, exit; # Nor is this.
95 # These do the print before evaluating exit:
96 (print $foo), exit; # This is what you want.
97 print($foo), exit; # Or this.
98 print ($foo), exit; # Or even this.
102 print ($foo & 255) + 1, "\n";
104 probably doesn't do what you expect at first glance. The parentheses
105 enclose the argument list for C<print> which is evaluated (printing
106 the result of C<$foo & 255>). Then one is added to the return value
107 of C<print> (usually 1). The result is something like this:
109 1 + 1, "\n"; # Obviously not what you meant.
111 To do what you meant properly, you must write:
113 print(($foo & 255) + 1, "\n");
115 See L<Named Unary Operators> for more discussion of this.
117 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
118 well as subroutine and method calls, and the anonymous
119 constructors C<[]> and C<{}>.
121 See also L<Quote and Quote-like Operators> toward the end of this section,
122 as well as L</"I/O Operators">.
124 =head2 The Arrow Operator
125 X<arrow> X<dereference> X<< -> >>
127 "C<< -> >>" is an infix dereference operator, just as it is in C
128 and C++. If the right side is either a C<[...]>, C<{...}>, or a
129 C<(...)> subscript, then the left side must be either a hard or
130 symbolic reference to an array, a hash, or a subroutine respectively.
131 (Or technically speaking, a location capable of holding a hard
132 reference, if it's an array or hash reference being used for
133 assignment.) See L<perlreftut> and L<perlref>.
135 Otherwise, the right side is a method name or a simple scalar
136 variable containing either the method name or a subroutine reference,
137 and the left side must be either an object (a blessed reference)
138 or a class name (that is, a package name). See L<perlobj>.
140 =head2 Auto-increment and Auto-decrement
141 X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
143 "++" and "--" work as in C. That is, if placed before a variable,
144 they increment or decrement the variable by one before returning the
145 value, and if placed after, increment or decrement after returning the
149 print $i++; # prints 0
150 print ++$j; # prints 1
152 Note that just as in C, Perl doesn't define B<when> the variable is
153 incremented or decremented. You just know it will be done sometime
154 before or after the value is returned. This also means that modifying
155 a variable twice in the same statement will lead to undefined behavior.
156 Avoid statements like:
161 Perl will not guarantee what the result of the above statements is.
163 The auto-increment operator has a little extra builtin magic to it. If
164 you increment a variable that is numeric, or that has ever been used in
165 a numeric context, you get a normal increment. If, however, the
166 variable has been used in only string contexts since it was set, and
167 has a value that is not the empty string and matches the pattern
168 C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
169 character within its range, with carry:
171 print ++($foo = "99"); # prints "100"
172 print ++($foo = "a0"); # prints "a1"
173 print ++($foo = "Az"); # prints "Ba"
174 print ++($foo = "zz"); # prints "aaa"
176 C<undef> is always treated as numeric, and in particular is changed
177 to C<0> before incrementing (so that a post-increment of an undef value
178 will return C<0> rather than C<undef>).
180 The auto-decrement operator is not magical.
182 =head2 Exponentiation
183 X<**> X<exponentiation> X<power>
185 Binary "**" is the exponentiation operator. It binds even more
186 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
187 implemented using C's pow(3) function, which actually works on doubles
190 =head2 Symbolic Unary Operators
191 X<unary operator> X<operator, unary>
193 Unary "!" performs logical negation, that is, "not". See also C<not> for a lower
194 precedence version of this.
197 Unary "-" performs arithmetic negation if the operand is numeric,
198 including any string that looks like a number. If the operand is
199 an identifier, a string consisting of a minus sign concatenated
200 with the identifier is returned. Otherwise, if the string starts
201 with a plus or minus, a string starting with the opposite sign is
202 returned. One effect of these rules is that -bareword is equivalent
203 to the string "-bareword". If, however, the string begins with a
204 non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
205 the string to a numeric and the arithmetic negation is performed. If the
206 string cannot be cleanly converted to a numeric, Perl will give the warning
207 B<Argument "the string" isn't numeric in negation (-) at ...>.
208 X<-> X<negation, arithmetic>
210 Unary "~" performs bitwise negation, that is, 1's complement. For
211 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
212 L<Bitwise String Operators>.) Note that the width of the result is
213 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
214 bits wide on a 64-bit platform, so if you are expecting a certain bit
215 width, remember to use the "&" operator to mask off the excess bits.
216 X<~> X<negation, binary>
218 When complementing strings, if all characters have ordinal values under
219 256, then their complements will, also. But if they do not, all
220 characters will be in either 32- or 64-bit complements, depending on your
221 architecture. So for example, C<~"\x{3B1}"> is C<"\x{FFFF_FC4E}"> on
222 32-bit machines and C<"\x{FFFF_FFFF_FFFF_FC4E}"> on 64-bit machines.
224 Unary "+" has no effect whatsoever, even on strings. It is useful
225 syntactically for separating a function name from a parenthesized expression
226 that would otherwise be interpreted as the complete list of function
227 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
230 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
231 and L<perlref>. Do not confuse this behavior with the behavior of
232 backslash within a string, although both forms do convey the notion
233 of protecting the next thing from interpolation.
234 X<\> X<reference> X<backslash>
236 =head2 Binding Operators
237 X<binding> X<operator, binding> X<=~> X<!~>
239 Binary "=~" binds a scalar expression to a pattern match. Certain operations
240 search or modify the string $_ by default. This operator makes that kind
241 of operation work on some other string. The right argument is a search
242 pattern, substitution, or transliteration. The left argument is what is
243 supposed to be searched, substituted, or transliterated instead of the default
244 $_. When used in scalar context, the return value generally indicates the
245 success of the operation. The exceptions are substitution (s///)
246 and transliteration (y///) with the C</r> (non-destructive) option,
247 which cause the B<r>eturn value to be the result of the substitution.
248 Behavior in list context depends on the particular operator.
249 See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
250 examples using these operators.
252 If the right argument is an expression rather than a search pattern,
253 substitution, or transliteration, it is interpreted as a search pattern at run
254 time. Note that this means that its contents will be interpolated twice, so
258 is not ok, as the regex engine will end up trying to compile the
259 pattern C<\>, which it will consider a syntax error.
261 Binary "!~" is just like "=~" except the return value is negated in
264 Binary "!~" with a non-destructive substitution (s///r) or transliteration
265 (y///r) is a syntax error.
267 =head2 Multiplicative Operators
268 X<operator, multiplicative>
270 Binary "*" multiplies two numbers.
273 Binary "/" divides two numbers.
276 Binary "%" is the modulo operator, which computes the division
277 remainder of its first argument with respect to its second argument.
279 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
280 C<$a> minus the largest multiple of C<$b> less than or equal to
281 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
282 smallest multiple of C<$b> that is not less than C<$a> (that is, the
283 result will be less than or equal to zero). If the operands
284 C<$a> and C<$b> are floating point values and the absolute value of
285 C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
286 the integer portion of C<$a> and C<$b> will be used in the operation
287 (Note: here C<UV_MAX> means the maximum of the unsigned integer type).
288 If the absolute value of the right operand (C<abs($b)>) is greater than
289 or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
290 C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
291 integer that makes C<$r> have the same sign as the right operand
292 C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
293 and the absolute value less than that of C<$b>.
294 Note that when C<use integer> is in scope, "%" gives you direct access
295 to the modulo operator as implemented by your C compiler. This
296 operator is not as well defined for negative operands, but it will
298 X<%> X<remainder> X<modulo> X<mod>
300 Binary "x" is the repetition operator. In scalar context or if the left
301 operand is not enclosed in parentheses, it returns a string consisting
302 of the left operand repeated the number of times specified by the right
303 operand. In list context, if the left operand is enclosed in
304 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
305 If the right operand is zero or negative, it returns an empty string
306 or an empty list, depending on the context.
309 print '-' x 80; # print row of dashes
311 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
313 @ones = (1) x 80; # a list of 80 1's
314 @ones = (5) x @ones; # set all elements to 5
317 =head2 Additive Operators
318 X<operator, additive>
320 Binary C<+> returns the sum of two numbers.
323 Binary C<-> returns the difference of two numbers.
326 Binary C<.> concatenates two strings.
327 X<string, concatenation> X<concatenation>
328 X<cat> X<concat> X<concatenate> X<.>
330 =head2 Shift Operators
331 X<shift operator> X<operator, shift> X<<< << >>>
332 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
333 X<shl> X<shr> X<shift, right> X<shift, left>
335 Binary C<<< << >>> returns the value of its left argument shifted left by the
336 number of bits specified by the right argument. Arguments should be
337 integers. (See also L<Integer Arithmetic>.)
339 Binary C<<< >> >>> returns the value of its left argument shifted right by
340 the number of bits specified by the right argument. Arguments should
341 be integers. (See also L<Integer Arithmetic>.)
343 Note that both C<<< << >>> and C<<< >> >>> in Perl are implemented directly using
344 C<<< << >>> and C<<< >> >>> in C. If C<use integer> (see L<Integer Arithmetic>) is
345 in force then signed C integers are used, else unsigned C integers are
346 used. Either way, the implementation isn't going to generate results
347 larger than the size of the integer type Perl was built with (32 bits
350 The result of overflowing the range of the integers is undefined
351 because it is undefined also in C. In other words, using 32-bit
352 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
353 of bits is also undefined.
355 If you get tired of being subject to your platform's native integers,
356 the C<use bigint> pragma neatly sidesteps the issue altogether:
358 print 20 << 20; # 20971520
359 print 20 << 40; # 5120 on 32-bit machines,
360 # 21990232555520 on 64-bit machines
362 print 20 << 100; # 25353012004564588029934064107520
364 =head2 Named Unary Operators
365 X<operator, named unary>
367 The various named unary operators are treated as functions with one
368 argument, with optional parentheses.
370 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
371 is followed by a left parenthesis as the next token, the operator and
372 arguments within parentheses are taken to be of highest precedence,
373 just like a normal function call. For example,
374 because named unary operators are higher precedence than C<||>:
376 chdir $foo || die; # (chdir $foo) || die
377 chdir($foo) || die; # (chdir $foo) || die
378 chdir ($foo) || die; # (chdir $foo) || die
379 chdir +($foo) || die; # (chdir $foo) || die
381 but, because * is higher precedence than named operators:
383 chdir $foo * 20; # chdir ($foo * 20)
384 chdir($foo) * 20; # (chdir $foo) * 20
385 chdir ($foo) * 20; # (chdir $foo) * 20
386 chdir +($foo) * 20; # chdir ($foo * 20)
388 rand 10 * 20; # rand (10 * 20)
389 rand(10) * 20; # (rand 10) * 20
390 rand (10) * 20; # (rand 10) * 20
391 rand +(10) * 20; # rand (10 * 20)
393 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
394 treated like named unary operators, but they don't follow this functional
395 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
396 equivalent to C<-f "$file.bak">.
397 X<-X> X<filetest> X<operator, filetest>
399 See also L<"Terms and List Operators (Leftward)">.
401 =head2 Relational Operators
402 X<relational operator> X<operator, relational>
404 Perl operators that return true or false generally return values
405 that can be safely used as numbers. For example, the relational
406 operators in this section and the equality operators in the next
407 one return C<1> for true and a special version of the defined empty
408 string, C<"">, which counts as a zero but is exempt from warnings
409 about improper numeric conversions, just as C<"0 but true"> is.
411 Binary "<" returns true if the left argument is numerically less than
415 Binary ">" returns true if the left argument is numerically greater
416 than the right argument.
419 Binary "<=" returns true if the left argument is numerically less than
420 or equal to the right argument.
423 Binary ">=" returns true if the left argument is numerically greater
424 than or equal to the right argument.
427 Binary "lt" returns true if the left argument is stringwise less than
431 Binary "gt" returns true if the left argument is stringwise greater
432 than the right argument.
435 Binary "le" returns true if the left argument is stringwise less than
436 or equal to the right argument.
439 Binary "ge" returns true if the left argument is stringwise greater
440 than or equal to the right argument.
443 =head2 Equality Operators
444 X<equality> X<equal> X<equals> X<operator, equality>
446 Binary "==" returns true if the left argument is numerically equal to
450 Binary "!=" returns true if the left argument is numerically not equal
451 to the right argument.
454 Binary "<=>" returns -1, 0, or 1 depending on whether the left
455 argument is numerically less than, equal to, or greater than the right
456 argument. If your platform supports NaNs (not-a-numbers) as numeric
457 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
458 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
459 returns true, as does NaN != anything else. If your platform doesn't
460 support NaNs then NaN is just a string with numeric value 0.
461 X<< <=> >> X<spaceship>
463 $ perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
464 $ perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
466 (Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all
469 Binary "eq" returns true if the left argument is stringwise equal to
473 Binary "ne" returns true if the left argument is stringwise not equal
474 to the right argument.
477 Binary "cmp" returns -1, 0, or 1 depending on whether the left
478 argument is stringwise less than, equal to, or greater than the right
482 Binary "~~" does a smartmatch between its arguments. Smart matching
483 is described in the next section.
486 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
487 by the current locale if a legacy C<use locale> (but not
488 C<use locale ':not_characters'>) is in effect. See
489 L<perllocale>. Do not mix these with Unicode, only with legacy binary
490 encodings. The standard L<Unicode::Collate> and
491 L<Unicode::Collate::Locale> modules offer much more powerful solutions to
494 =head2 Smartmatch Operator
496 First available in Perl 5.10.1 (the 5.10.0 version behaved differently),
497 binary C<~~> does a "smartmatch" between its arguments. This is mostly
498 used implicitly in the C<when> construct described in L<perlsyn>, although
499 not all C<when> clauses call the smartmatch operator. Unique among all of
500 Perl's operators, the smartmatch operator can recurse.
502 It is also unique in that all other Perl operators impose a context
503 (usually string or numeric context) on their operands, autoconverting
504 those operands to those imposed contexts. In contrast, smartmatch
505 I<infers> contexts from the actual types of its operands and uses that
506 type information to select a suitable comparison mechanism.
508 The C<~~> operator compares its operands "polymorphically", determining how
509 to compare them according to their actual types (numeric, string, array,
510 hash, etc.) Like the equality operators with which it shares the same
511 precedence, C<~~> returns 1 for true and C<""> for false. It is often best
512 read aloud as "in", "inside of", or "is contained in", because the left
513 operand is often looked for I<inside> the right operand. That makes the
514 order of the operands to the smartmatch operand often opposite that of
515 the regular match operator. In other words, the "smaller" thing is usually
516 placed in the left operand and the larger one in the right.
518 The behavior of a smartmatch depends on what type of things its arguments
519 are, as determined by the following table. The first row of the table
520 whose types apply determines the smartmatch behavior. Because what
521 actually happens is mostly determined by the type of the second operand,
522 the table is sorted on the right operand instead of on the left.
524 Left Right Description and pseudocode
525 ===============================================================
526 Any undef check whether Any is undefined
529 Any Object invoke ~~ overloading on Object, or die
531 Right operand is an ARRAY:
533 Left Right Description and pseudocode
534 ===============================================================
535 ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2]
536 like: (ARRAY1[0] ~~ ARRAY2[0])
537 && (ARRAY1[1] ~~ ARRAY2[1]) && ...
538 HASH ARRAY any ARRAY elements exist as HASH keys
539 like: grep { exists HASH->{$_} } ARRAY
540 Regexp ARRAY any ARRAY elements pattern match Regexp
541 like: grep { /Regexp/ } ARRAY
542 undef ARRAY undef in ARRAY
543 like: grep { !defined } ARRAY
544 Any ARRAY smartmatch each ARRAY element[3]
545 like: grep { Any ~~ $_ } ARRAY
547 Right operand is a HASH:
549 Left Right Description and pseudocode
550 ===============================================================
551 HASH1 HASH2 all same keys in both HASHes
553 grep { exists HASH2->{$_} } keys HASH1
554 ARRAY HASH any ARRAY elements exist as HASH keys
555 like: grep { exists HASH->{$_} } ARRAY
556 Regexp HASH any HASH keys pattern match Regexp
557 like: grep { /Regexp/ } keys HASH
558 undef HASH always false (undef can't be a key)
560 Any HASH HASH key existence
561 like: exists HASH->{Any}
563 Right operand is CODE:
565 Left Right Description and pseudocode
566 ===============================================================
567 ARRAY CODE sub returns true on all ARRAY elements[1]
568 like: !grep { !CODE->($_) } ARRAY
569 HASH CODE sub returns true on all HASH keys[1]
570 like: !grep { !CODE->($_) } keys HASH
571 Any CODE sub passed Any returns true
574 Right operand is a Regexp:
576 Left Right Description and pseudocode
577 ===============================================================
578 ARRAY Regexp any ARRAY elements match Regexp
579 like: grep { /Regexp/ } ARRAY
580 HASH Regexp any HASH keys match Regexp
581 like: grep { /Regexp/ } keys HASH
582 Any Regexp pattern match
583 like: Any =~ /Regexp/
587 Left Right Description and pseudocode
588 ===============================================================
589 Object Any invoke ~~ overloading on Object,
592 Any Num numeric equality
594 Num nummy[4] numeric equality
596 undef Any check whether undefined
598 Any Any string equality
607 Empty hashes or arrays match.
610 That is, each element smartmatches the element of the same index in the other array.[3]
613 If a circular reference is found, fall back to referential equality.
616 Either an actual number, or a string that looks like one.
620 The smartmatch implicitly dereferences any non-blessed hash or array
621 reference, so the C<I<HASH>> and C<I<ARRAY>> entries apply in those cases.
622 For blessed references, the C<I<Object>> entries apply. Smartmatches
623 involving hashes only consider hash keys, never hash values.
625 The "like" code entry is not always an exact rendition. For example, the
626 smartmatch operator short-circuits whenever possible, but C<grep> does
627 not. Also, C<grep> in scalar context returns the number of matches, but
628 C<~~> returns only true or false.
630 Unlike most operators, the smartmatch operator knows to treat C<undef>
634 @array = (1, 2, 3, undef, 4, 5);
635 say "some elements undefined" if undef ~~ @array;
637 Each operand is considered in a modified scalar context, the modification
638 being that array and hash variables are passed by reference to the
639 operator, which implicitly dereferences them. Both elements
640 of each pair are the same:
644 my %hash = (red => 1, blue => 2, green => 3,
645 orange => 4, yellow => 5, purple => 6,
646 black => 7, grey => 8, white => 9);
648 my @array = qw(red blue green);
650 say "some array elements in hash keys" if @array ~~ %hash;
651 say "some array elements in hash keys" if \@array ~~ \%hash;
653 say "red in array" if "red" ~~ @array;
654 say "red in array" if "red" ~~ \@array;
656 say "some keys end in e" if /e$/ ~~ %hash;
657 say "some keys end in e" if /e$/ ~~ \%hash;
659 Two arrays smartmatch if each element in the first array smartmatches
660 (that is, is "in") the corresponding element in the second array,
664 my @little = qw(red blue green);
665 my @bigger = ("red", "blue", [ "orange", "green" ] );
666 if (@little ~~ @bigger) { # true!
667 say "little is contained in bigger";
670 Because the smartmatch operator recurses on nested arrays, this
671 will still report that "red" is in the array.
674 my @array = qw(red blue green);
675 my $nested_array = [[[[[[[ @array ]]]]]]];
676 say "red in array" if "red" ~~ $nested_array;
678 If two arrays smartmatch each other, then they are deep
679 copies of each others' values, as this example reports:
682 my @a = (0, 1, 2, [3, [4, 5], 6], 7);
683 my @b = (0, 1, 2, [3, [4, 5], 6], 7);
685 if (@a ~~ @b && @b ~~ @a) {
686 say "a and b are deep copies of each other";
689 say "a smartmatches in b";
692 say "b smartmatches in a";
695 say "a and b don't smartmatch each other at all";
699 If you were to set C<$b[3] = 4>, then instead of reporting that "a and b
700 are deep copies of each other", it now reports that "b smartmatches in a".
701 That because the corresponding position in C<@a> contains an array that
702 (eventually) has a 4 in it.
704 Smartmatching one hash against another reports whether both contain the
705 same keys, no more and no less. This could be used to see whether two
706 records have the same field names, without caring what values those fields
707 might have. For example:
711 state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
713 my ($class, $init_fields) = @_;
715 die "Must supply (only) name, rank, and serial number"
716 unless $init_fields ~~ $REQUIRED_FIELDS;
721 or, if other non-required fields are allowed, use ARRAY ~~ HASH:
725 state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
727 my ($class, $init_fields) = @_;
729 die "Must supply (at least) name, rank, and serial number"
730 unless [keys %{$init_fields}] ~~ $REQUIRED_FIELDS;
735 The smartmatch operator is most often used as the implicit operator of a
736 C<when> clause. See the section on "Switch Statements" in L<perlsyn>.
738 =head3 Smartmatching of Objects
740 To avoid relying on an object's underlying representation, if the
741 smartmatch's right operand is an object that doesn't overload C<~~>,
742 it raises the exception "C<Smartmatching a non-overloaded object
743 breaks encapsulation>". That's because one has no business digging
744 around to see whether something is "in" an object. These are all
745 illegal on objects without a C<~~> overload:
751 However, you can change the way an object is smartmatched by overloading
752 the C<~~> operator. This is allowed to extend the usual smartmatch semantics.
753 For objects that do have an C<~~> overload, see L<overload>.
755 Using an object as the left operand is allowed, although not very useful.
756 Smartmatching rules take precedence over overloading, so even if the
757 object in the left operand has smartmatch overloading, this will be
758 ignored. A left operand that is a non-overloaded object falls back on a
759 string or numeric comparison of whatever the C<ref> operator returns. That
764 does I<not> invoke the overload method with C<I<X>> as an argument.
765 Instead the above table is consulted as normal, and based on the type of
766 C<I<X>>, overloading may or may not be invoked. For simple strings or
767 numbers, in becomes equivalent to this:
769 $object ~~ $number ref($object) == $number
770 $object ~~ $string ref($object) eq $string
772 For example, this reports that the handle smells IOish
773 (but please don't really do this!):
776 my $fh = IO::Handle->new();
777 if ($fh ~~ /\bIO\b/) {
778 say "handle smells IOish";
781 That's because it treats C<$fh> as a string like
782 C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that.
785 X<operator, bitwise, and> X<bitwise and> X<&>
787 Binary "&" returns its operands ANDed together bit by bit.
788 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
790 Note that "&" has lower priority than relational operators, so for example
791 the parentheses are essential in a test like
793 print "Even\n" if ($x & 1) == 0;
795 =head2 Bitwise Or and Exclusive Or
796 X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
799 Binary "|" returns its operands ORed together bit by bit.
800 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
802 Binary "^" returns its operands XORed together bit by bit.
803 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
805 Note that "|" and "^" have lower priority than relational operators, so
806 for example the brackets are essential in a test like
808 print "false\n" if (8 | 2) != 10;
810 =head2 C-style Logical And
811 X<&&> X<logical and> X<operator, logical, and>
813 Binary "&&" performs a short-circuit logical AND operation. That is,
814 if the left operand is false, the right operand is not even evaluated.
815 Scalar or list context propagates down to the right operand if it
818 =head2 C-style Logical Or
819 X<||> X<operator, logical, or>
821 Binary "||" performs a short-circuit logical OR operation. That is,
822 if the left operand is true, the right operand is not even evaluated.
823 Scalar or list context propagates down to the right operand if it
826 =head2 Logical Defined-Or
827 X<//> X<operator, logical, defined-or>
829 Although it has no direct equivalent in C, Perl's C<//> operator is related
830 to its C-style or. In fact, it's exactly the same as C<||>, except that it
831 tests the left hand side's definedness instead of its truth. Thus,
832 C<< EXPR1 // EXPR2 >> returns the value of C<< EXPR1 >> if it's defined,
833 otherwise, the value of C<< EXPR2 >> is returned. (C<< EXPR1 >> is evaluated
834 in scalar context, C<< EXPR2 >> in the context of C<< // >> itself). Usually,
835 this is the same result as C<< defined(EXPR1) ? EXPR1 : EXPR2 >> (except that
836 the ternary-operator form can be used as a lvalue, while C<< EXPR1 // EXPR2 >>
837 cannot). This is very useful for
838 providing default values for variables. If you actually want to test if
839 at least one of C<$a> and C<$b> is defined, use C<defined($a // $b)>.
841 The C<||>, C<//> and C<&&> operators return the last value evaluated
842 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
843 portable way to find out the home directory might be:
848 // die "You're homeless!\n";
850 In particular, this means that you shouldn't use this
851 for selecting between two aggregates for assignment:
853 @a = @b || @c; # this is wrong
854 @a = scalar(@b) || @c; # really meant this
855 @a = @b ? @b : @c; # this works fine, though
857 As alternatives to C<&&> and C<||> when used for
858 control flow, Perl provides the C<and> and C<or> operators (see below).
859 The short-circuit behavior is identical. The precedence of "and"
860 and "or" is much lower, however, so that you can safely use them after a
861 list operator without the need for parentheses:
863 unlink "alpha", "beta", "gamma"
864 or gripe(), next LINE;
866 With the C-style operators that would have been written like this:
868 unlink("alpha", "beta", "gamma")
869 || (gripe(), next LINE);
871 It would be even more readable to write that this way:
873 unless(unlink("alpha", "beta", "gamma")) {
878 Using "or" for assignment is unlikely to do what you want; see below.
880 =head2 Range Operators
881 X<operator, range> X<range> X<..> X<...>
883 Binary ".." is the range operator, which is really two different
884 operators depending on the context. In list context, it returns a
885 list of values counting (up by ones) from the left value to the right
886 value. If the left value is greater than the right value then it
887 returns the empty list. The range operator is useful for writing
888 C<foreach (1..10)> loops and for doing slice operations on arrays. In
889 the current implementation, no temporary array is created when the
890 range operator is used as the expression in C<foreach> loops, but older
891 versions of Perl might burn a lot of memory when you write something
894 for (1 .. 1_000_000) {
898 The range operator also works on strings, using the magical
899 auto-increment, see below.
901 In scalar context, ".." returns a boolean value. The operator is
902 bistable, like a flip-flop, and emulates the line-range (comma)
903 operator of B<sed>, B<awk>, and various editors. Each ".." operator
904 maintains its own boolean state, even across calls to a subroutine
905 that contains it. It is false as long as its left operand is false.
906 Once the left operand is true, the range operator stays true until the
907 right operand is true, I<AFTER> which the range operator becomes false
908 again. It doesn't become false till the next time the range operator
909 is evaluated. It can test the right operand and become false on the
910 same evaluation it became true (as in B<awk>), but it still returns
911 true once. If you don't want it to test the right operand until the
912 next evaluation, as in B<sed>, just use three dots ("...") instead of
913 two. In all other regards, "..." behaves just like ".." does.
915 The right operand is not evaluated while the operator is in the
916 "false" state, and the left operand is not evaluated while the
917 operator is in the "true" state. The precedence is a little lower
918 than || and &&. The value returned is either the empty string for
919 false, or a sequence number (beginning with 1) for true. The sequence
920 number is reset for each range encountered. The final sequence number
921 in a range has the string "E0" appended to it, which doesn't affect
922 its numeric value, but gives you something to search for if you want
923 to exclude the endpoint. You can exclude the beginning point by
924 waiting for the sequence number to be greater than 1.
926 If either operand of scalar ".." is a constant expression,
927 that operand is considered true if it is equal (C<==>) to the current
928 input line number (the C<$.> variable).
930 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
931 but that is only an issue if you use a floating point expression; when
932 implicitly using C<$.> as described in the previous paragraph, the
933 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
934 is set to a floating point value and you are not reading from a file.
935 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
936 you want in scalar context because each of the operands are evaluated
937 using their integer representation.
941 As a scalar operator:
943 if (101 .. 200) { print; } # print 2nd hundred lines, short for
944 # if ($. == 101 .. $. == 200) { print; }
946 next LINE if (1 .. /^$/); # skip header lines, short for
947 # next LINE if ($. == 1 .. /^$/);
948 # (typically in a loop labeled LINE)
950 s/^/> / if (/^$/ .. eof()); # quote body
952 # parse mail messages
954 $in_header = 1 .. /^$/;
955 $in_body = /^$/ .. eof;
962 close ARGV if eof; # reset $. each file
965 Here's a simple example to illustrate the difference between
966 the two range operators:
979 This program will print only the line containing "Bar". If
980 the range operator is changed to C<...>, it will also print the
983 And now some examples as a list operator:
985 for (101 .. 200) { print } # print $_ 100 times
986 @foo = @foo[0 .. $#foo]; # an expensive no-op
987 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
989 The range operator (in list context) makes use of the magical
990 auto-increment algorithm if the operands are strings. You
993 @alphabet = ("A" .. "Z");
995 to get all normal letters of the English alphabet, or
997 $hexdigit = (0 .. 9, "a" .. "f")[$num & 15];
999 to get a hexadecimal digit, or
1001 @z2 = ("01" .. "31");
1004 to get dates with leading zeros.
1006 If the final value specified is not in the sequence that the magical
1007 increment would produce, the sequence goes until the next value would
1008 be longer than the final value specified.
1010 If the initial value specified isn't part of a magical increment
1011 sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>),
1012 only the initial value will be returned. So the following will only
1015 use charnames "greek";
1016 my @greek_small = ("\N{alpha}" .. "\N{omega}");
1018 To get the 25 traditional lowercase Greek letters, including both sigmas,
1019 you could use this instead:
1021 use charnames "greek";
1022 my @greek_small = map { chr } ( ord("\N{alpha}")
1027 However, because there are I<many> other lowercase Greek characters than
1028 just those, to match lowercase Greek characters in a regular expression,
1029 you would use the pattern C</(?:(?=\p{Greek})\p{Lower})+/>.
1031 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
1032 return two elements in list context.
1034 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
1036 =head2 Conditional Operator
1037 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
1039 Ternary "?:" is the conditional operator, just as in C. It works much
1040 like an if-then-else. If the argument before the ? is true, the
1041 argument before the : is returned, otherwise the argument after the :
1042 is returned. For example:
1044 printf "I have %d dog%s.\n", $n,
1045 ($n == 1) ? "" : "s";
1047 Scalar or list context propagates downward into the 2nd
1048 or 3rd argument, whichever is selected.
1050 $a = $ok ? $b : $c; # get a scalar
1051 @a = $ok ? @b : @c; # get an array
1052 $a = $ok ? @b : @c; # oops, that's just a count!
1054 The operator may be assigned to if both the 2nd and 3rd arguments are
1055 legal lvalues (meaning that you can assign to them):
1057 ($a_or_b ? $a : $b) = $c;
1059 Because this operator produces an assignable result, using assignments
1060 without parentheses will get you in trouble. For example, this:
1062 $a % 2 ? $a += 10 : $a += 2
1066 (($a % 2) ? ($a += 10) : $a) += 2
1070 ($a % 2) ? ($a += 10) : ($a += 2)
1072 That should probably be written more simply as:
1074 $a += ($a % 2) ? 10 : 2;
1076 =head2 Assignment Operators
1077 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
1078 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
1081 "=" is the ordinary assignment operator.
1083 Assignment operators work as in C. That is,
1091 although without duplicating any side effects that dereferencing the lvalue
1092 might trigger, such as from tie(). Other assignment operators work similarly.
1093 The following are recognized:
1095 **= += *= &= <<= &&=
1100 Although these are grouped by family, they all have the precedence
1103 Unlike in C, the scalar assignment operator produces a valid lvalue.
1104 Modifying an assignment is equivalent to doing the assignment and
1105 then modifying the variable that was assigned to. This is useful
1106 for modifying a copy of something, like this:
1108 ($tmp = $global) =~ tr/13579/24680/;
1110 Although as of 5.14, that can be also be accomplished this way:
1113 $tmp = ($global =~ tr/13579/24680/r);
1124 Similarly, a list assignment in list context produces the list of
1125 lvalues assigned to, and a list assignment in scalar context returns
1126 the number of elements produced by the expression on the right hand
1127 side of the assignment.
1129 =head2 Comma Operator
1130 X<comma> X<operator, comma> X<,>
1132 Binary "," is the comma operator. In scalar context it evaluates
1133 its left argument, throws that value away, then evaluates its right
1134 argument and returns that value. This is just like C's comma operator.
1136 In list context, it's just the list argument separator, and inserts
1137 both its arguments into the list. These arguments are also evaluated
1140 The C<< => >> operator is a synonym for the comma except that it causes a
1141 word on its left to be interpreted as a string if it begins with a letter
1142 or underscore and is composed only of letters, digits and underscores.
1143 This includes operands that might otherwise be interpreted as operators,
1144 constants, single number v-strings or function calls. If in doubt about
1145 this behavior, the left operand can be quoted explicitly.
1147 Otherwise, the C<< => >> operator behaves exactly as the comma operator
1148 or list argument separator, according to context.
1152 use constant FOO => "something";
1154 my %h = ( FOO => 23 );
1158 my %h = ("FOO", 23);
1162 my %h = ("something", 23);
1164 The C<< => >> operator is helpful in documenting the correspondence
1165 between keys and values in hashes, and other paired elements in lists.
1167 %hash = ( $key => $value );
1168 login( $username => $password );
1170 The special quoting behavior ignores precedence, and hence may apply to
1171 I<part> of the left operand:
1173 print time.shift => "bbb";
1175 That example prints something like "1314363215shiftbbb", because the
1176 C<< => >> implicitly quotes the C<shift> immediately on its left, ignoring
1177 the fact that C<time.shift> is the entire left operand.
1179 =head2 List Operators (Rightward)
1180 X<operator, list, rightward> X<list operator>
1182 On the right side of a list operator, the comma has very low precedence,
1183 such that it controls all comma-separated expressions found there.
1184 The only operators with lower precedence are the logical operators
1185 "and", "or", and "not", which may be used to evaluate calls to list
1186 operators without the need for parentheses:
1188 open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n";
1190 However, some people find that code harder to read than writing
1191 it with parentheses:
1193 open(HANDLE, "< :utf8", "filename") or die "Can't open: $!\n";
1195 in which case you might as well just use the more customary "||" operator:
1197 open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n";
1199 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
1202 X<operator, logical, not> X<not>
1204 Unary "not" returns the logical negation of the expression to its right.
1205 It's the equivalent of "!" except for the very low precedence.
1208 X<operator, logical, and> X<and>
1210 Binary "and" returns the logical conjunction of the two surrounding
1211 expressions. It's equivalent to C<&&> except for the very low
1212 precedence. This means that it short-circuits: the right
1213 expression is evaluated only if the left expression is true.
1215 =head2 Logical or and Exclusive Or
1216 X<operator, logical, or> X<operator, logical, xor>
1217 X<operator, logical, exclusive or>
1220 Binary "or" returns the logical disjunction of the two surrounding
1221 expressions. It's equivalent to C<||> except for the very low precedence.
1222 This makes it useful for control flow:
1224 print FH $data or die "Can't write to FH: $!";
1226 This means that it short-circuits: the right expression is evaluated
1227 only if the left expression is false. Due to its precedence, you must
1228 be careful to avoid using it as replacement for the C<||> operator.
1229 It usually works out better for flow control than in assignments:
1231 $a = $b or $c; # bug: this is wrong
1232 ($a = $b) or $c; # really means this
1233 $a = $b || $c; # better written this way
1235 However, when it's a list-context assignment and you're trying to use
1236 C<||> for control flow, you probably need "or" so that the assignment
1237 takes higher precedence.
1239 @info = stat($file) || die; # oops, scalar sense of stat!
1240 @info = stat($file) or die; # better, now @info gets its due
1242 Then again, you could always use parentheses.
1244 Binary C<xor> returns the exclusive-OR of the two surrounding expressions.
1245 It cannot short-circuit (of course).
1247 There is no low precedence operator for defined-OR.
1249 =head2 C Operators Missing From Perl
1250 X<operator, missing from perl> X<&> X<*>
1251 X<typecasting> X<(TYPE)>
1253 Here is what C has that Perl doesn't:
1259 Address-of operator. (But see the "\" operator for taking a reference.)
1263 Dereference-address operator. (Perl's prefix dereferencing
1264 operators are typed: $, @, %, and &.)
1268 Type-casting operator.
1272 =head2 Quote and Quote-like Operators
1273 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
1274 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
1275 X<escape sequence> X<escape>
1277 While we usually think of quotes as literal values, in Perl they
1278 function as operators, providing various kinds of interpolating and
1279 pattern matching capabilities. Perl provides customary quote characters
1280 for these behaviors, but also provides a way for you to choose your
1281 quote character for any of them. In the following table, a C<{}> represents
1282 any pair of delimiters you choose.
1284 Customary Generic Meaning Interpolates
1287 `` qx{} Command yes*
1289 // m{} Pattern match yes*
1291 s{}{} Substitution yes*
1292 tr{}{} Transliteration no (but see below)
1293 y{}{} Transliteration no (but see below)
1296 * unless the delimiter is ''.
1298 Non-bracketing delimiters use the same character fore and aft, but the four
1299 sorts of ASCII brackets (round, angle, square, curly) all nest, which means
1308 Note, however, that this does not always work for quoting Perl code:
1310 $s = q{ if($a eq "}") ... }; # WRONG
1312 is a syntax error. The C<Text::Balanced> module (standard as of v5.8,
1313 and from CPAN before then) is able to do this properly.
1315 There can be whitespace between the operator and the quoting
1316 characters, except when C<#> is being used as the quoting character.
1317 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
1318 operator C<q> followed by a comment. Its argument will be taken
1319 from the next line. This allows you to write:
1321 s {foo} # Replace foo
1324 The following escape sequences are available in constructs that interpolate,
1325 and in transliterations:
1326 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
1329 Sequence Note Description
1335 \a alarm (bell) (BEL)
1337 \x{263A} [1,8] hex char (example: SMILEY)
1338 \x1b [2,8] restricted range hex char (example: ESC)
1339 \N{name} [3] named Unicode character or character sequence
1340 \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
1341 \c[ [5] control char (example: chr(27))
1342 \o{23072} [6,8] octal char (example: SMILEY)
1343 \033 [7,8] restricted range octal char (example: ESC)
1349 The result is the character specified by the hexadecimal number between
1350 the braces. See L</[8]> below for details on which character.
1352 Only hexadecimal digits are valid between the braces. If an invalid
1353 character is encountered, a warning will be issued and the invalid
1354 character and all subsequent characters (valid or invalid) within the
1355 braces will be discarded.
1357 If there are no valid digits between the braces, the generated character is
1358 the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
1359 will not cause a warning (currently).
1363 The result is the character specified by the hexadecimal number in the range
1364 0x00 to 0xFF. See L</[8]> below for details on which character.
1366 Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
1367 by fewer than two valid digits, any valid digits will be zero-padded. This
1368 means that C<\x7> will be interpreted as C<\x07>, and a lone <\x> will be
1369 interpreted as C<\x00>. Except at the end of a string, having fewer than
1370 two valid digits will result in a warning. Note that although the warning
1371 says the illegal character is ignored, it is only ignored as part of the
1372 escape and will still be used as the subsequent character in the string.
1375 Original Result Warns?
1383 The result is the Unicode character or character sequence given by I<name>.
1388 C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code
1389 point is I<hexadecimal number>.
1393 The character following C<\c> is mapped to some other character as shown in the
1410 In other words, it's the character whose code point has had 64 xor'd with
1411 its uppercase. C<\c?> is DELETE because C<ord("?") ^ 64> is 127, and
1412 C<\c@> is NULL because the ord of "@" is 64, so xor'ing 64 itself produces 0.
1414 Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the
1415 end of a string, because the backslash would be parsed as escaping the end
1418 On ASCII platforms, the resulting characters from the list above are the
1419 complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
1420 L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these
1421 sequences mean on both ASCII and EBCDIC platforms.
1423 Use of any other character following the "c" besides those listed above is
1424 discouraged, and some are deprecated with the intention of removing
1425 those in a later Perl version. What happens for any of these
1426 other characters currently though, is that the value is derived by xor'ing
1427 with the seventh bit, which is 64.
1429 To get platform independent controls, you can use C<\N{...}>.
1433 The result is the character specified by the octal number between the braces.
1434 See L</[8]> below for details on which character.
1436 If a character that isn't an octal digit is encountered, a warning is raised,
1437 and the value is based on the octal digits before it, discarding it and all
1438 following characters up to the closing brace. It is a fatal error if there are
1439 no octal digits at all.
1443 The result is the character specified by the three-digit octal number in the
1444 range 000 to 777 (but best to not use above 077, see next paragraph). See
1445 L</[8]> below for details on which character.
1447 Some contexts allow 2 or even 1 digit, but any usage without exactly
1448 three digits, the first being a zero, may give unintended results. (For
1449 example, in a regular expression it may be confused with a backreference;
1450 see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
1451 use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to
1452 use this construct only for ordinals C<\077> and below, remembering to pad to
1453 the left with zeros to make three digits. For larger ordinals, either use
1454 C<\o{}>, or convert to something else, such as to hex and use C<\x{}>
1457 Having fewer than 3 digits may lead to a misleading warning message that says
1458 that what follows is ignored. For example, C<"\128"> in the ASCII character set
1459 is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal
1460 digit '8' ignored> will be thrown. If C<"\n8"> is what you want, you can
1461 avoid this warning by padding your octal number with C<0>'s: C<"\0128">.
1465 Several constructs above specify a character by a number. That number
1466 gives the character's position in the character set encoding (indexed from 0).
1467 This is called synonymously its ordinal, code position, or code point. Perl
1468 works on platforms that have a native encoding currently of either ASCII/Latin1
1469 or EBCDIC, each of which allow specification of 256 characters. In general, if
1470 the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
1471 native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
1472 it as a Unicode code point and the result is the corresponding Unicode
1473 character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
1474 decimal, which is less than 256, so the number is interpreted in the native
1475 character set encoding. In ASCII the character in the 80th position (indexed
1476 from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".
1477 C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
1478 as a Unicode code point no matter what the native encoding is. The name of the
1479 character in the 256th position (indexed by 0) in Unicode is
1480 C<LATIN CAPITAL LETTER A WITH MACRON>.
1482 There are a couple of exceptions to the above rule. S<C<\N{U+I<hex number>}>> is
1483 always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even
1484 on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the
1485 number is considered to be in that encoding, and is translated from that into
1486 the platform's native encoding if there is a corresponding native character;
1487 otherwise to Unicode.
1491 B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
1492 the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may
1495 does have meaning in regular expression patterns in Perl, see L<perlre>.)
1497 The following escape sequences are available in constructs that interpolate,
1498 but not in transliterations.
1499 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> X<\F>
1501 \l lowercase next character only
1502 \u titlecase (not uppercase!) next character only
1503 \L lowercase all characters till \E or end of string
1504 \U uppercase all characters till \E or end of string
1505 \F foldcase all characters till \E or end of string
1506 \Q quote (disable) pattern metacharacters till \E or
1508 \E end either case modification or quoted section
1509 (whichever was last seen)
1511 See L<perlfunc/quotemeta> for the exact definition of characters that
1512 are quoted by C<\Q>.
1514 C<\L>, C<\U>, C<\F>, and C<\Q> can stack, in which case you need one
1515 C<\E> for each. For example:
1517 say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
1518 This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?
1520 If C<use locale> is in effect (but not C<use locale ':not_characters'>),
1521 the case map used by C<\l>, C<\L>,
1522 C<\u>, and C<\U> is taken from the current locale. See L<perllocale>.
1523 If Unicode (for example, C<\N{}> or code points of 0x100 or
1524 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u>, and
1525 C<\U> is as defined by Unicode. That means that case-mapping
1526 a single character can sometimes produce several characters.
1527 Under C<use locale>, C<\F> produces the same results as C<\L>.
1529 All systems use the virtual C<"\n"> to represent a line terminator,
1530 called a "newline". There is no such thing as an unvarying, physical
1531 newline character. It is only an illusion that the operating system,
1532 device drivers, C libraries, and Perl all conspire to preserve. Not all
1533 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1534 on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,
1535 and on systems without line terminator,
1536 printing C<"\n"> might emit no actual data. In general, use C<"\n"> when
1537 you mean a "newline" for your system, but use the literal ASCII when you
1538 need an exact character. For example, most networking protocols expect
1539 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
1540 and although they often accept just C<"\012">, they seldom tolerate just
1541 C<"\015">. If you get in the habit of using C<"\n"> for networking,
1542 you may be burned some day.
1543 X<newline> X<line terminator> X<eol> X<end of line>
1546 For constructs that do interpolate, variables beginning with "C<$>"
1547 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
1548 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1549 But method calls such as C<< $obj->meth >> are not.
1551 Interpolating an array or slice interpolates the elements in order,
1552 separated by the value of C<$">, so is equivalent to interpolating
1553 C<join $", @array>. "Punctuation" arrays such as C<@*> are usually
1554 interpolated only if the name is enclosed in braces C<@{*}>, but the
1555 arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces.
1557 For double-quoted strings, the quoting from C<\Q> is applied after
1558 interpolation and escapes are processed.
1560 "abc\Qfoo\tbar$s\Exyz"
1564 "abc" . quotemeta("foo\tbar$s") . "xyz"
1566 For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
1567 the quoting from C<\Q> is applied after interpolation is processed,
1568 but before escapes are processed. This allows the pattern to match
1569 literally (except for C<$> and C<@>). For example, the following matches:
1573 Because C<$> or C<@> trigger interpolation, you'll need to use something
1574 like C</\Quser\E\@\Qhost/> to match them literally.
1576 Patterns are subject to an additional level of interpretation as a
1577 regular expression. This is done as a second pass, after variables are
1578 interpolated, so that regular expressions may be incorporated into the
1579 pattern from the variables. If this is not what you want, use C<\Q> to
1580 interpolate a variable literally.
1582 Apart from the behavior described above, Perl does not expand
1583 multiple levels of interpolation. In particular, contrary to the
1584 expectations of shell programmers, back-quotes do I<NOT> interpolate
1585 within double quotes, nor do single quotes impede evaluation of
1586 variables when used within double quotes.
1588 =head2 Regexp Quote-Like Operators
1591 Here are the quote-like operators that apply to pattern
1592 matching and related activities.
1596 =item qr/STRING/msixpodual
1597 X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
1599 This operator quotes (and possibly compiles) its I<STRING> as a regular
1600 expression. I<STRING> is interpolated the same way as I<PATTERN>
1601 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1602 is done. Returns a Perl value which may be used instead of the
1603 corresponding C</STRING/msixpodual> expression. The returned value is a
1604 normalized version of the original pattern. It magically differs from
1605 a string containing the same characters: C<ref(qr/x/)> returns "Regexp";
1606 however, dereferencing it is not well defined (you currently get the
1607 normalized version of the original pattern, but this may change).
1612 $rex = qr/my.STRING/is;
1613 print $rex; # prints (?si-xm:my.STRING)
1620 The result may be used as a subpattern in a match:
1623 $string =~ /foo${re}bar/; # can be interpolated in other
1625 $string =~ $re; # or used standalone
1626 $string =~ /$re/; # or this way
1628 Since Perl may compile the pattern at the moment of execution of the qr()
1629 operator, using qr() may have speed advantages in some situations,
1630 notably if the result of qr() is used standalone:
1633 my $patterns = shift;
1634 my @compiled = map qr/$_/i, @$patterns;
1637 foreach my $pat (@compiled) {
1638 $success = 1, last if /$pat/;
1644 Precompilation of the pattern into an internal representation at
1645 the moment of qr() avoids a need to recompile the pattern every
1646 time a match C</$pat/> is attempted. (Perl has many other internal
1647 optimizations, but none would be triggered in the above example if
1648 we did not use qr() operator.)
1650 Options (specified by the following modifiers) are:
1652 m Treat string as multiple lines.
1653 s Treat string as single line. (Make . match a newline)
1654 i Do case-insensitive pattern matching.
1655 x Use extended regular expressions.
1656 p When matching preserve a copy of the matched string so
1657 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be
1659 o Compile pattern only once.
1660 a ASCII-restrict: Use ASCII for \d, \s, \w; specifying two
1661 a's further restricts /i matching so that no ASCII
1662 character will match a non-ASCII one.
1664 u Use Unicode rules.
1665 d Use Unicode or native charset, as in 5.12 and earlier.
1667 If a precompiled pattern is embedded in a larger pattern then the effect
1668 of "msixpluad" will be propagated appropriately. The effect the "o"
1669 modifier has is not propagated, being restricted to those patterns
1670 explicitly using it.
1672 The last four modifiers listed above, added in Perl 5.14,
1673 control the character set semantics, but C</a> is the only one you are likely
1674 to want to specify explicitly; the other three are selected
1675 automatically by various pragmas.
1677 See L<perlre> for additional information on valid syntax for STRING, and
1678 for a detailed look at the semantics of regular expressions. In
1679 particular, all modifiers except the largely obsolete C</o> are further
1680 explained in L<perlre/Modifiers>. C</o> is described in the next section.
1682 =item m/PATTERN/msixpodualgc
1683 X<m> X<operator, match>
1684 X<regexp, options> X<regexp> X<regex, options> X<regex>
1685 X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
1687 =item /PATTERN/msixpodualgc
1689 Searches a string for a pattern match, and in scalar context returns
1690 true if it succeeds, false if it fails. If no string is specified
1691 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1692 string specified with C<=~> need not be an lvalue--it may be the
1693 result of an expression evaluation, but remember the C<=~> binds
1694 rather tightly.) See also L<perlre>.
1696 Options are as described in C<qr//> above; in addition, the following match
1697 process modifiers are available:
1699 g Match globally, i.e., find all occurrences.
1700 c Do not reset search position on a failed match when /g is
1703 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1704 you can use any pair of non-whitespace (ASCII) characters
1705 as delimiters. This is particularly useful for matching path names
1706 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1707 the delimiter, then a match-only-once rule applies,
1708 described in C<m?PATTERN?> below.
1709 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1710 When using a character valid in an identifier, whitespace is required
1713 PATTERN may contain variables, which will be interpolated
1714 every time the pattern search is evaluated, except
1715 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1716 C<$|> are not interpolated because they look like end-of-string tests.)
1717 Perl will not recompile the pattern unless an interpolated
1718 variable that it contains changes. You can force Perl to skip the
1719 test and never recompile by adding a C</o> (which stands for "once")
1720 after the trailing delimiter.
1721 Once upon a time, Perl would recompile regular expressions
1722 unnecessarily, and this modifier was useful to tell it not to do so, in the
1723 interests of speed. But now, the only reasons to use C</o> are either:
1729 The variables are thousands of characters long and you know that they
1730 don't change, and you need to wring out the last little bit of speed by
1731 having Perl skip testing for that. (There is a maintenance penalty for
1732 doing this, as mentioning C</o> constitutes a promise that you won't
1733 change the variables in the pattern. If you do change them, Perl won't
1738 you want the pattern to use the initial values of the variables
1739 regardless of whether they change or not. (But there are saner ways
1740 of accomplishing this than using C</o>.)
1744 If the pattern contains embedded code, such as
1747 $code = 'foo(?{ $x })';
1750 then perl will recompile each time, even though the pattern string hasn't
1751 changed, to ensure that the current value of C<$x> is seen each time.
1752 Use C</o> if you want to avoid this.
1756 The bottom line is that using C</o> is almost never a good idea.
1758 =item The empty pattern //
1760 If the PATTERN evaluates to the empty string, the last
1761 I<successfully> matched regular expression is used instead. In this
1762 case, only the C<g> and C<c> flags on the empty pattern are honored;
1763 the other flags are taken from the original pattern. If no match has
1764 previously succeeded, this will (silently) act instead as a genuine
1765 empty pattern (which will always match).
1767 Note that it's possible to confuse Perl into thinking C<//> (the empty
1768 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1769 good about this, but some pathological cases might trigger this, such as
1770 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1771 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1772 will assume you meant defined-or. If you meant the empty regex, just
1773 use parentheses or spaces to disambiguate, or even prefix the empty
1774 regex with an C<m> (so C<//> becomes C<m//>).
1776 =item Matching in list context
1778 If the C</g> option is not used, C<m//> in list context returns a
1779 list consisting of the subexpressions matched by the parentheses in the
1780 pattern, that is, (C<$1>, C<$2>, C<$3>...) (Note that here C<$1> etc. are
1781 also set). When there are no parentheses in the pattern, the return
1782 value is the list C<(1)> for success.
1783 With or without parentheses, an empty list is returned upon failure.
1787 open(TTY, "+</dev/tty")
1788 || die "can't access /dev/tty: $!";
1790 <TTY> =~ /^y/i && foo(); # do foo if desired
1792 if (/Version: *([0-9.]*)/) { $version = $1; }
1794 next if m#^/usr/spool/uucp#;
1799 print if /$arg/o; # compile only once (no longer needed!)
1802 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1804 This last example splits $foo into the first two words and the
1805 remainder of the line, and assigns those three fields to $F1, $F2, and
1806 $Etc. The conditional is true if any variables were assigned; that is,
1807 if the pattern matched.
1809 The C</g> modifier specifies global pattern matching--that is,
1810 matching as many times as possible within the string. How it behaves
1811 depends on the context. In list context, it returns a list of the
1812 substrings matched by any capturing parentheses in the regular
1813 expression. If there are no parentheses, it returns a list of all
1814 the matched strings, as if there were parentheses around the whole
1817 In scalar context, each execution of C<m//g> finds the next match,
1818 returning true if it matches, and false if there is no further match.
1819 The position after the last match can be read or set using the C<pos()>
1820 function; see L<perlfunc/pos>. A failed match normally resets the
1821 search position to the beginning of the string, but you can avoid that
1822 by adding the C</c> modifier (for example, C<m//gc>). Modifying the target
1823 string also resets the search position.
1827 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1828 zero-width assertion that matches the exact position where the
1829 previous C<m//g>, if any, left off. Without the C</g> modifier, the
1830 C<\G> assertion still anchors at C<pos()> as it was at the start of
1831 the operation (see L<perlfunc/pos>), but the match is of course only
1832 attempted once. Using C<\G> without C</g> on a target string that has
1833 not previously had a C</g> match applied to it is the same as using
1834 the C<\A> assertion to match the beginning of the string. Note also
1835 that, currently, C<\G> is only properly supported when anchored at the
1836 very beginning of the pattern.
1841 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1845 while ($paragraph = <>) {
1846 while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
1852 Here's another way to check for sentences in a paragraph:
1854 my $sentence_rx = qr{
1855 (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or
1857 \p{Lu} # capital letter
1858 .*? # a bunch of anything
1859 (?<= \S ) # that ends in non-
1861 (?<! \b [DMS]r ) # but isn't a common abbr.
1865 [.?!] # followed by a sentence
1867 (?= $ | \s ) # in front of end-of-string
1871 while (my $paragraph = <>) {
1872 say "NEW PARAGRAPH";
1874 while ($paragraph =~ /($sentence_rx)/g) {
1875 printf "\tgot sentence %d: <%s>\n", ++$count, $1;
1879 Here's how to use C<m//gc> with C<\G>:
1884 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1886 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1888 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1890 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1892 The last example should print:
1902 Notice that the final match matched C<q> instead of C<p>, which a match
1903 without the C<\G> anchor would have done. Also note that the final match
1904 did not update C<pos>. C<pos> is only updated on a C</g> match. If the
1905 final match did indeed match C<p>, it's a good bet that you're running a
1906 very old (pre-5.6.0) version of Perl.
1908 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1909 combine several regexps like this to process a string part-by-part,
1910 doing different actions depending on which regexp matched. Each
1911 regexp tries to match where the previous one leaves off.
1914 $url = URI::URL->new( "http://example.com/" );
1915 die if $url eq "xXx";
1919 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1920 print(" lowercase"), redo LOOP
1921 if /\G\p{Ll}+\b[,.;]?\s*/gc;
1922 print(" UPPERCASE"), redo LOOP
1923 if /\G\p{Lu}+\b[,.;]?\s*/gc;
1924 print(" Capitalized"), redo LOOP
1925 if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
1926 print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
1927 print(" alphanumeric"), redo LOOP
1928 if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
1929 print(" line-noise"), redo LOOP if /\G\W+/gc;
1930 print ". That's all!\n";
1933 Here is the output (split into several lines):
1935 line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
1936 line-noise lowercase line-noise lowercase line-noise lowercase
1937 lowercase line-noise lowercase lowercase line-noise lowercase
1938 lowercase line-noise MiXeD line-noise. That's all!
1940 =item m?PATTERN?msixpodualgc
1941 X<?> X<operator, match-once>
1943 =item ?PATTERN?msixpodualgc
1945 This is just like the C<m/PATTERN/> search, except that it matches
1946 only once between calls to the reset() operator. This is a useful
1947 optimization when you want to see only the first occurrence of
1948 something in each file of a set of files, for instance. Only C<m??>
1949 patterns local to the current package are reset.
1953 # blank line between header and body
1956 reset if eof; # clear m?? status for next file
1959 Another example switched the first "latin1" encoding it finds
1960 to "utf8" in a pod file:
1962 s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
1964 The match-once behavior is controlled by the match delimiter being
1965 C<?>; with any other delimiter this is the normal C<m//> operator.
1967 For historical reasons, the leading C<m> in C<m?PATTERN?> is optional,
1968 but the resulting C<?PATTERN?> syntax is deprecated, will warn on
1969 usage and might be removed from a future stable release of Perl (without
1972 =item s/PATTERN/REPLACEMENT/msixpodualgcer
1973 X<substitute> X<substitution> X<replace> X<regexp, replace>
1974 X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
1976 Searches a string for a pattern, and if found, replaces that pattern
1977 with the replacement text and returns the number of substitutions
1978 made. Otherwise it returns false (specifically, the empty string).
1980 If the C</r> (non-destructive) option is used then it runs the
1981 substitution on a copy of the string and instead of returning the
1982 number of substitutions, it returns the copy whether or not a
1983 substitution occurred. The original string is never changed when
1984 C</r> is used. The copy will always be a plain string, even if the
1985 input is an object or a tied variable.
1987 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1988 variable is searched and modified. Unless the C</r> option is used,
1989 the string specified must be a scalar variable, an array element, a
1990 hash element, or an assignment to one of those; that is, some sort of
1993 If the delimiter chosen is a single quote, no interpolation is
1994 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1995 PATTERN contains a $ that looks like a variable rather than an
1996 end-of-string test, the variable will be interpolated into the pattern
1997 at run-time. If you want the pattern compiled only once the first time
1998 the variable is interpolated, use the C</o> option. If the pattern
1999 evaluates to the empty string, the last successfully executed regular
2000 expression is used instead. See L<perlre> for further explanation on these.
2002 Options are as with m// with the addition of the following replacement
2005 e Evaluate the right side as an expression.
2006 ee Evaluate the right side as a string then eval the
2008 r Return substitution and leave the original string
2011 Any non-whitespace delimiter may replace the slashes. Add space after
2012 the C<s> when using a character allowed in identifiers. If single quotes
2013 are used, no interpretation is done on the replacement string (the C</e>
2014 modifier overrides this, however). Note that Perl treats backticks
2015 as normal delimiters; the replacement text is not evaluated as a command.
2016 If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
2017 its own pair of quotes, which may or may not be bracketing quotes, for example,
2018 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
2019 replacement portion to be treated as a full-fledged Perl expression
2020 and evaluated right then and there. It is, however, syntax checked at
2021 compile-time. A second C<e> modifier will cause the replacement portion
2022 to be C<eval>ed before being run as a Perl expression.
2026 s/\bgreen\b/mauve/g; # don't change wintergreen
2028 $path =~ s|/usr/bin|/usr/local/bin|;
2030 s/Login: $foo/Login: $bar/; # run-time pattern
2032 ($foo = $bar) =~ s/this/that/; # copy first, then
2034 ($foo = "$bar") =~ s/this/that/; # convert to string,
2036 $foo = $bar =~ s/this/that/r; # Same as above using /r
2037 $foo = $bar =~ s/this/that/r
2038 =~ s/that/the other/r; # Chained substitutes
2040 @foo = map { s/this/that/r } @bar # /r is very useful in
2043 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-cnt
2046 s/\d+/$&*2/e; # yields 'abc246xyz'
2047 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
2048 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
2050 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
2051 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
2052 s/^=(\w+)/pod($1)/ge; # use function call
2055 $a = s/abc/def/r; # $a is 'def123xyz' and
2056 # $_ remains 'abc123xyz'.
2058 # expand variables in $_, but dynamics only, using
2059 # symbolic dereferencing
2062 # Add one to the value of any numbers in the string
2065 # Titlecase words in the last 30 characters only
2066 substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;
2068 # This will expand any embedded scalar variable
2069 # (including lexicals) in $_ : First $1 is interpolated
2070 # to the variable name, and then evaluated
2073 # Delete (most) C comments.
2075 /\* # Match the opening delimiter.
2076 .*? # Match a minimal number of characters.
2077 \*/ # Match the closing delimiter.
2080 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_,
2083 for ($variable) { # trim whitespace in $variable,
2089 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
2091 Note the use of $ instead of \ in the last example. Unlike
2092 B<sed>, we use the \<I<digit>> form in only the left hand side.
2093 Anywhere else it's $<I<digit>>.
2095 Occasionally, you can't use just a C</g> to get all the changes
2096 to occur that you might want. Here are two common cases:
2098 # put commas in the right places in an integer
2099 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
2101 # expand tabs to 8-column spacing
2102 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
2106 =head2 Quote-Like Operators
2107 X<operator, quote-like>
2112 X<q> X<quote, single> X<'> X<''>
2116 A single-quoted, literal string. A backslash represents a backslash
2117 unless followed by the delimiter or another backslash, in which case
2118 the delimiter or backslash is interpolated.
2120 $foo = q!I said, "You said, 'She said it.'"!;
2121 $bar = q('This is it.');
2122 $baz = '\n'; # a two-character string
2125 X<qq> X<quote, double> X<"> X<"">
2129 A double-quoted, interpolated string.
2132 (*** The previous line contains the naughty word "$1".\n)
2133 if /\b(tcl|java|python)\b/i; # :-)
2134 $baz = "\n"; # a one-character string
2137 X<qx> X<`> X<``> X<backtick>
2141 A string which is (possibly) interpolated and then executed as a
2142 system command with F</bin/sh> or its equivalent. Shell wildcards,
2143 pipes, and redirections will be honored. The collected standard
2144 output of the command is returned; standard error is unaffected. In
2145 scalar context, it comes back as a single (potentially multi-line)
2146 string, or undef if the command failed. In list context, returns a
2147 list of lines (however you've defined lines with $/ or
2148 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
2150 Because backticks do not affect standard error, use shell file descriptor
2151 syntax (assuming the shell supports this) if you care to address this.
2152 To capture a command's STDERR and STDOUT together:
2154 $output = `cmd 2>&1`;
2156 To capture a command's STDOUT but discard its STDERR:
2158 $output = `cmd 2>/dev/null`;
2160 To capture a command's STDERR but discard its STDOUT (ordering is
2163 $output = `cmd 2>&1 1>/dev/null`;
2165 To exchange a command's STDOUT and STDERR in order to capture the STDERR
2166 but leave its STDOUT to come out the old STDERR:
2168 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
2170 To read both a command's STDOUT and its STDERR separately, it's easiest
2171 to redirect them separately to files, and then read from those files
2172 when the program is done:
2174 system("program args 1>program.stdout 2>program.stderr");
2176 The STDIN filehandle used by the command is inherited from Perl's STDIN.
2179 open(SPLAT, "stuff") || die "can't open stuff: $!";
2180 open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
2181 print STDOUT `sort`;
2183 will print the sorted contents of the file named F<"stuff">.
2185 Using single-quote as a delimiter protects the command from Perl's
2186 double-quote interpolation, passing it on to the shell instead:
2188 $perl_info = qx(ps $$); # that's Perl's $$
2189 $shell_info = qx'ps $$'; # that's the new shell's $$
2191 How that string gets evaluated is entirely subject to the command
2192 interpreter on your system. On most platforms, you will have to protect
2193 shell metacharacters if you want them treated literally. This is in
2194 practice difficult to do, as it's unclear how to escape which characters.
2195 See L<perlsec> for a clean and safe example of a manual fork() and exec()
2196 to emulate backticks safely.
2198 On some platforms (notably DOS-like ones), the shell may not be
2199 capable of dealing with multiline commands, so putting newlines in
2200 the string may not get you what you want. You may be able to evaluate
2201 multiple commands in a single line by separating them with the command
2202 separator character, if your shell supports that (for example, C<;> on
2203 many Unix shells and C<&> on the Windows NT C<cmd> shell).
2205 Perl will attempt to flush all files opened for
2206 output before starting the child process, but this may not be supported
2207 on some platforms (see L<perlport>). To be safe, you may need to set
2208 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2209 C<IO::Handle> on any open handles.
2211 Beware that some command shells may place restrictions on the length
2212 of the command line. You must ensure your strings don't exceed this
2213 limit after any necessary interpolations. See the platform-specific
2214 release notes for more details about your particular environment.
2216 Using this operator can lead to programs that are difficult to port,
2217 because the shell commands called vary between systems, and may in
2218 fact not be present at all. As one example, the C<type> command under
2219 the POSIX shell is very different from the C<type> command under DOS.
2220 That doesn't mean you should go out of your way to avoid backticks
2221 when they're the right way to get something done. Perl was made to be
2222 a glue language, and one of the things it glues together is commands.
2223 Just understand what you're getting yourself into.
2225 See L</"I/O Operators"> for more discussion.
2228 X<qw> X<quote, list> X<quote, words>
2230 Evaluates to a list of the words extracted out of STRING, using embedded
2231 whitespace as the word delimiters. It can be understood as being roughly
2234 split(" ", q/STRING/);
2236 the differences being that it generates a real list at compile time, and
2237 in scalar context it returns the last element in the list. So
2242 is semantically equivalent to the list:
2246 Some frequently seen examples:
2248 use POSIX qw( setlocale localeconv )
2249 @EXPORT = qw( foo bar baz );
2251 A common mistake is to try to separate the words with comma or to
2252 put comments into a multi-line C<qw>-string. For this reason, the
2253 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
2254 produces warnings if the STRING contains the "," or the "#" character.
2256 =item tr/SEARCHLIST/REPLACEMENTLIST/cdsr
2257 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
2259 =item y/SEARCHLIST/REPLACEMENTLIST/cdsr
2261 Transliterates all occurrences of the characters found in the search list
2262 with the corresponding character in the replacement list. It returns
2263 the number of characters replaced or deleted. If no string is
2264 specified via the C<=~> or C<!~> operator, the $_ string is transliterated.
2266 If the C</r> (non-destructive) option is present, a new copy of the string
2267 is made and its characters transliterated, and this copy is returned no
2268 matter whether it was modified or not: the original string is always
2269 left unchanged. The new copy is always a plain string, even if the input
2270 string is an object or a tied variable.
2272 Unless the C</r> option is used, the string specified with C<=~> must be a
2273 scalar variable, an array element, a hash element, or an assignment to one
2274 of those; in other words, an lvalue.
2276 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2277 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
2278 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
2279 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
2280 its own pair of quotes, which may or may not be bracketing quotes;
2281 for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>.
2283 Note that C<tr> does B<not> do regular expression character classes such as
2284 C<\d> or C<\pL>. The C<tr> operator is not equivalent to the tr(1)
2285 utility. If you want to map strings between lower/upper cases, see
2286 L<perlfunc/lc> and L<perlfunc/uc>, and in general consider using the C<s>
2287 operator if you need regular expressions. The C<\U>, C<\u>, C<\L>, and
2288 C<\l> string-interpolation escapes on the right side of a substitution
2289 operator will perform correct case-mappings, but C<tr[a-z][A-Z]> will not
2290 (except sometimes on legacy 7-bit data).
2292 Note also that the whole range idea is rather unportable between
2293 character sets--and even within character sets they may cause results
2294 you probably didn't expect. A sound principle is to use only ranges
2295 that begin from and end at either alphabets of equal case (a-e, A-E),
2296 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
2297 character sets in full.
2301 c Complement the SEARCHLIST.
2302 d Delete found but unreplaced characters.
2303 s Squash duplicate replaced characters.
2304 r Return the modified string and leave the original string
2307 If the C</c> modifier is specified, the SEARCHLIST character set
2308 is complemented. If the C</d> modifier is specified, any characters
2309 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
2310 (Note that this is slightly more flexible than the behavior of some
2311 B<tr> programs, which delete anything they find in the SEARCHLIST,
2312 period.) If the C</s> modifier is specified, sequences of characters
2313 that were transliterated to the same character are squashed down
2314 to a single instance of the character.
2316 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
2317 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
2318 than the SEARCHLIST, the final character is replicated till it is long
2319 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
2320 This latter is useful for counting characters in a class or for
2321 squashing character sequences in a class.
2325 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
2327 $cnt = tr/*/*/; # count the stars in $_
2329 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
2331 $cnt = tr/0-9//; # count the digits in $_
2333 tr/a-zA-Z//s; # bookkeeper -> bokeper
2335 ($HOST = $host) =~ tr/a-z/A-Z/;
2336 $HOST = $host =~ tr/a-z/A-Z/r; # same thing
2338 $HOST = $host =~ tr/a-z/A-Z/r # chained with s///r
2341 tr/a-zA-Z/ /cs; # change non-alphas to single space
2343 @stripped = map tr/a-zA-Z/ /csr, @original;
2347 [\000-\177]; # wickedly delete 8th bit
2349 If multiple transliterations are given for a character, only the
2354 will transliterate any A to X.
2356 Because the transliteration table is built at compile time, neither
2357 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
2358 interpolation. That means that if you want to use variables, you
2361 eval "tr/$oldlist/$newlist/";
2364 eval "tr/$oldlist/$newlist/, 1" or die $@;
2367 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
2369 A line-oriented form of quoting is based on the shell "here-document"
2370 syntax. Following a C<< << >> you specify a string to terminate
2371 the quoted material, and all lines following the current line down to
2372 the terminating string are the value of the item.
2374 The terminating string may be either an identifier (a word), or some
2375 quoted text. An unquoted identifier works like double quotes.
2376 There may not be a space between the C<< << >> and the identifier,
2377 unless the identifier is explicitly quoted. (If you put a space it
2378 will be treated as a null identifier, which is valid, and matches the
2379 first empty line.) The terminating string must appear by itself
2380 (unquoted and with no surrounding whitespace) on the terminating line.
2382 If the terminating string is quoted, the type of quotes used determine
2383 the treatment of the text.
2389 Double quotes indicate that the text will be interpolated using exactly
2390 the same rules as normal double quoted strings.
2393 The price is $Price.
2396 print << "EOF"; # same as above
2397 The price is $Price.
2403 Single quotes indicate the text is to be treated literally with no
2404 interpolation of its content. This is similar to single quoted
2405 strings except that backslashes have no special meaning, with C<\\>
2406 being treated as two backslashes and not one as they would in every
2407 other quoting construct.
2409 Just as in the shell, a backslashed bareword following the C<<< << >>>
2410 means the same thing as a single-quoted string does:
2412 $cost = <<'VISTA'; # hasta la ...
2413 That'll be $10 please, ma'am.
2416 $cost = <<\VISTA; # Same thing!
2417 That'll be $10 please, ma'am.
2420 This is the only form of quoting in perl where there is no need
2421 to worry about escaping content, something that code generators
2422 can and do make good use of.
2426 The content of the here doc is treated just as it would be if the
2427 string were embedded in backticks. Thus the content is interpolated
2428 as though it were double quoted and then executed via the shell, with
2429 the results of the execution returned.
2431 print << `EOC`; # execute command and get results
2437 It is possible to stack multiple here-docs in a row:
2439 print <<"foo", <<"bar"; # you can stack them
2445 myfunc(<< "THIS", 23, <<'THAT');
2452 Just don't forget that you have to put a semicolon on the end
2453 to finish the statement, as Perl doesn't know you're not going to
2461 If you want to remove the line terminator from your here-docs,
2464 chomp($string = <<'END');
2468 If you want your here-docs to be indented with the rest of the code,
2469 you'll need to remove leading whitespace from each line manually:
2471 ($quote = <<'FINIS') =~ s/^\s+//gm;
2472 The Road goes ever on and on,
2473 down from the door where it began.
2476 If you use a here-doc within a delimited construct, such as in C<s///eg>,
2477 the quoted material must still come on the line following the
2478 C<<< <<FOO >>> marker, which means it may be inside the delimited
2486 It works this way as of Perl 5.18. Historically, it was inconsistent, and
2487 you would have to write
2494 outside of string evals.
2496 Additionally, quoting rules for the end-of-string identifier are
2497 unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
2498 supported in place of C<''> and C<"">, and the only interpolation is for
2499 backslashing the quoting character:
2501 print << "abc\"def";
2505 Finally, quoted strings cannot span multiple lines. The general rule is
2506 that the identifier must be a string literal. Stick with that, and you
2511 =head2 Gory details of parsing quoted constructs
2512 X<quote, gory details>
2514 When presented with something that might have several different
2515 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2516 principle to pick the most probable interpretation. This strategy
2517 is so successful that Perl programmers often do not suspect the
2518 ambivalence of what they write. But from time to time, Perl's
2519 notions differ substantially from what the author honestly meant.
2521 This section hopes to clarify how Perl handles quoted constructs.
2522 Although the most common reason to learn this is to unravel labyrinthine
2523 regular expressions, because the initial steps of parsing are the
2524 same for all quoting operators, they are all discussed together.
2526 The most important Perl parsing rule is the first one discussed
2527 below: when processing a quoted construct, Perl first finds the end
2528 of that construct, then interprets its contents. If you understand
2529 this rule, you may skip the rest of this section on the first
2530 reading. The other rules are likely to contradict the user's
2531 expectations much less frequently than this first one.
2533 Some passes discussed below are performed concurrently, but because
2534 their results are the same, we consider them individually. For different
2535 quoting constructs, Perl performs different numbers of passes, from
2536 one to four, but these passes are always performed in the same order.
2540 =item Finding the end
2542 The first pass is finding the end of the quoted construct, where
2543 the information about the delimiters is used in parsing.
2544 During this search, text between the starting and ending delimiters
2545 is copied to a safe location. The text copied gets delimiter-independent.
2547 If the construct is a here-doc, the ending delimiter is a line
2548 that has a terminating string as the content. Therefore C<<<EOF> is
2549 terminated by C<EOF> immediately followed by C<"\n"> and starting
2550 from the first column of the terminating line.
2551 When searching for the terminating line of a here-doc, nothing
2552 is skipped. In other words, lines after the here-doc syntax
2553 are compared with the terminating string line by line.
2555 For the constructs except here-docs, single characters are used as starting
2556 and ending delimiters. If the starting delimiter is an opening punctuation
2557 (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2558 corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2559 If the starting delimiter is an unpaired character like C</> or a closing
2560 punctuation, the ending delimiter is same as the starting delimiter.
2561 Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
2562 C<qq[]> and C<qq]]> constructs.
2564 When searching for single-character delimiters, escaped delimiters
2565 and C<\\> are skipped. For example, while searching for terminating C</>,
2566 combinations of C<\\> and C<\/> are skipped. If the delimiters are
2567 bracketing, nested pairs are also skipped. For example, while searching
2568 for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
2569 and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2570 However, when backslashes are used as the delimiters (like C<qq\\> and
2571 C<tr\\\>), nothing is skipped.
2572 During the search for the end, backslashes that escape delimiters or
2573 other backslashes are removed (exactly speaking, they are not copied to the
2576 For constructs with three-part delimiters (C<s///>, C<y///>, and
2577 C<tr///>), the search is repeated once more.
2578 If the first delimiter is not an opening punctuation, three delimiters must
2579 be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
2580 terminates the left part and starts the right part at once.
2581 If the left part is delimited by bracketing punctuation (that is C<()>,
2582 C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
2583 delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace
2584 and comments are allowed between both parts, though the comment must follow
2585 at least one whitespace character; otherwise a character expected as the
2586 start of the comment may be regarded as the starting delimiter of the right part.
2588 During this search no attention is paid to the semantics of the construct.
2591 "$hash{"$foo/$bar"}"
2596 bar # NOT a comment, this slash / terminated m//!
2599 do not form legal quoted expressions. The quoted part ends on the
2600 first C<"> and C</>, and the rest happens to be a syntax error.
2601 Because the slash that terminated C<m//> was followed by a C<SPACE>,
2602 the example above is not C<m//x>, but rather C<m//> with no C</x>
2603 modifier. So the embedded C<#> is interpreted as a literal C<#>.
2605 Also no attention is paid to C<\c\> (multichar control char syntax) during
2606 this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
2607 of C<\/>, and the following C</> is not recognized as a delimiter.
2608 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2613 The next step is interpolation in the text obtained, which is now
2614 delimiter-independent. There are multiple cases.
2620 No interpolation is performed.
2621 Note that the combination C<\\> is left intact, since escaped delimiters
2622 are not available for here-docs.
2624 =item C<m''>, the pattern of C<s'''>
2626 No interpolation is performed at this stage.
2627 Any backslashed sequences including C<\\> are treated at the stage
2628 to L</"parsing regular expressions">.
2630 =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
2632 The only interpolation is removal of C<\> from pairs of C<\\>.
2633 Therefore C<-> in C<tr'''> and C<y'''> is treated literally
2634 as a hyphen and no character range is available.
2635 C<\1> in the replacement of C<s'''> does not work as C<$1>.
2637 =item C<tr///>, C<y///>
2639 No variable interpolation occurs. String modifying combinations for
2640 case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2641 The other escape sequences such as C<\200> and C<\t> and backslashed
2642 characters such as C<\\> and C<\-> are converted to appropriate literals.
2643 The character C<-> is treated specially and therefore C<\-> is treated
2646 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
2648 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> (possibly paired with C<\E>) are
2649 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
2650 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
2651 The other escape sequences such as C<\200> and C<\t> and backslashed
2652 characters such as C<\\> and C<\-> are replaced with appropriate
2655 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2656 is interpolated in the usual way. Something like C<"\Q\\E"> has
2657 no C<\E> inside. Instead, it has C<\Q>, C<\\>, and C<E>, so the
2658 result is the same as for C<"\\\\E">. As a general rule, backslashes
2659 between C<\Q> and C<\E> may lead to counterintuitive results. So,
2660 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2661 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2666 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2668 Interpolated scalars and arrays are converted internally to the C<join> and
2669 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
2671 $foo . " XXX '" . (join $", @arr) . "'";
2673 All operations above are performed simultaneously, left to right.
2675 Because the result of C<"\Q STRING \E"> has all metacharacters
2676 quoted, there is no way to insert a literal C<$> or C<@> inside a
2677 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
2678 C<"\\\$">; if not, it is interpreted as the start of an interpolated
2681 Note also that the interpolation code needs to make a decision on
2682 where the interpolated scalar ends. For instance, whether
2683 C<< "a $b -> {c}" >> really means:
2685 "a " . $b . " -> {c}";
2691 Most of the time, the longest possible text that does not include
2692 spaces between components and which contains matching braces or
2693 brackets. because the outcome may be determined by voting based
2694 on heuristic estimators, the result is not strictly predictable.
2695 Fortunately, it's usually correct for ambiguous cases.
2697 =item the replacement of C<s///>
2699 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> and interpolation
2700 happens as with C<qq//> constructs.
2702 It is at this step that C<\1> is begrudgingly converted to C<$1> in
2703 the replacement text of C<s///>, in order to correct the incorrigible
2704 I<sed> hackers who haven't picked up the saner idiom yet. A warning
2705 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2706 (that is, the C<$^W> variable) was set.
2708 =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2710 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F>, C<\E>,
2711 and interpolation happens (almost) as with C<qq//> constructs.
2713 Processing of C<\N{...}> is also done here, and compiled into an intermediate
2714 form for the regex compiler. (This is because, as mentioned below, the regex
2715 compilation may be done at execution time, and C<\N{...}> is a compile-time
2718 However any other combinations of C<\> followed by a character
2719 are not substituted but only skipped, in order to parse them
2720 as regular expressions at the following step.
2721 As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
2722 treated as an array symbol (for example C<@foo>),
2723 even though the same text in C<qq//> gives interpolation of C<\c@>.
2725 Code blocks such as C<(?{BLOCK})> are handled by temporarily passing control
2726 back to the perl parser, in a similar way that an interpolated array
2727 subscript expression such as C<"foo$array[1+f("[xyz")]bar"> would be.
2729 Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
2730 a C<#>-comment in a C<//x>-regular expression, no processing is
2731 performed whatsoever. This is the first step at which the presence
2732 of the C<//x> modifier is relevant.
2734 Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2735 and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2736 voted (by several different estimators) to be either an array element
2737 or C<$var> followed by an RE alternative. This is where the notation
2738 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2739 array element C<-9>, not as a regular expression from the variable
2740 C<$arr> followed by a digit, which would be the interpretation of
2741 C</$arr[0-9]/>. Since voting among different estimators may occur,
2742 the result is not predictable.
2744 The lack of processing of C<\\> creates specific restrictions on
2745 the post-processed text. If the delimiter is C</>, one cannot get
2746 the combination C<\/> into the result of this step. C</> will
2747 finish the regular expression, C<\/> will be stripped to C</> on
2748 the previous step, and C<\\/> will be left as is. Because C</> is
2749 equivalent to C<\/> inside a regular expression, this does not
2750 matter unless the delimiter happens to be character special to the
2751 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2752 alphanumeric char, as in:
2756 In the RE above, which is intentionally obfuscated for illustration, the
2757 delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
2758 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
2759 reason you're encouraged to restrict your delimiters to non-alphanumeric,
2760 non-whitespace choices.
2764 This step is the last one for all constructs except regular expressions,
2765 which are processed further.
2767 =item parsing regular expressions
2770 Previous steps were performed during the compilation of Perl code,
2771 but this one happens at run time, although it may be optimized to
2772 be calculated at compile time if appropriate. After preprocessing
2773 described above, and possibly after evaluation if concatenation,
2774 joining, casing translation, or metaquoting are involved, the
2775 resulting I<string> is passed to the RE engine for compilation.
2777 Whatever happens in the RE engine might be better discussed in L<perlre>,
2778 but for the sake of continuity, we shall do so here.
2780 This is another step where the presence of the C<//x> modifier is
2781 relevant. The RE engine scans the string from left to right and
2782 converts it to a finite automaton.
2784 Backslashed characters are either replaced with corresponding
2785 literal strings (as with C<\{>), or else they generate special nodes
2786 in the finite automaton (as with C<\b>). Characters special to the
2787 RE engine (such as C<|>) generate corresponding nodes or groups of
2788 nodes. C<(?#...)> comments are ignored. All the rest is either
2789 converted to literal strings to match, or else is ignored (as is
2790 whitespace and C<#>-style comments if C<//x> is present).
2792 Parsing of the bracketed character class construct, C<[...]>, is
2793 rather different than the rule used for the rest of the pattern.
2794 The terminator of this construct is found using the same rules as
2795 for finding the terminator of a C<{}>-delimited construct, the only
2796 exception being that C<]> immediately following C<[> is treated as
2797 though preceded by a backslash.
2799 The terminator of runtime C<(?{...})> is found by temporarily switching
2800 control to the perl parser, which should stop at the point where the
2801 logically balancing terminating C<}> is found.
2803 It is possible to inspect both the string given to RE engine and the
2804 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2805 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
2806 switch documented in L<perlrun/"Command Switches">.
2808 =item Optimization of regular expressions
2809 X<regexp, optimization>
2811 This step is listed for completeness only. Since it does not change
2812 semantics, details of this step are not documented and are subject
2813 to change without notice. This step is performed over the finite
2814 automaton that was generated during the previous pass.
2816 It is at this stage that C<split()> silently optimizes C</^/> to
2821 =head2 I/O Operators
2822 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2825 There are several I/O operators you should know about.
2827 A string enclosed by backticks (grave accents) first undergoes
2828 double-quote interpolation. It is then interpreted as an external
2829 command, and the output of that command is the value of the
2830 backtick string, like in a shell. In scalar context, a single string
2831 consisting of all output is returned. In list context, a list of
2832 values is returned, one per line of output. (You can set C<$/> to use
2833 a different line terminator.) The command is executed each time the
2834 pseudo-literal is evaluated. The status value of the command is
2835 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2836 Unlike in B<csh>, no translation is done on the return data--newlines
2837 remain newlines. Unlike in any of the shells, single quotes do not
2838 hide variable names in the command from interpretation. To pass a
2839 literal dollar-sign through to the shell you need to hide it with a
2840 backslash. The generalized form of backticks is C<qx//>. (Because
2841 backticks always undergo shell expansion as well, see L<perlsec> for
2843 X<qx> X<`> X<``> X<backtick> X<glob>
2845 In scalar context, evaluating a filehandle in angle brackets yields
2846 the next line from that file (the newline, if any, included), or
2847 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2848 (sometimes known as file-slurp mode) and the file is empty, it
2849 returns C<''> the first time, followed by C<undef> subsequently.
2851 Ordinarily you must assign the returned value to a variable, but
2852 there is one situation where an automatic assignment happens. If
2853 and only if the input symbol is the only thing inside the conditional
2854 of a C<while> statement (even if disguised as a C<for(;;)> loop),
2855 the value is automatically assigned to the global variable $_,
2856 destroying whatever was there previously. (This may seem like an
2857 odd thing to you, but you'll use the construct in almost every Perl
2858 script you write.) The $_ variable is not implicitly localized.
2859 You'll have to put a C<local $_;> before the loop if you want that
2862 The following lines are equivalent:
2864 while (defined($_ = <STDIN>)) { print; }
2865 while ($_ = <STDIN>) { print; }
2866 while (<STDIN>) { print; }
2867 for (;<STDIN>;) { print; }
2868 print while defined($_ = <STDIN>);
2869 print while ($_ = <STDIN>);
2870 print while <STDIN>;
2872 This also behaves similarly, but assigns to a lexical variable
2873 instead of to C<$_>:
2875 while (my $line = <STDIN>) { print $line }
2877 In these loop constructs, the assigned value (whether assignment
2878 is automatic or explicit) is then tested to see whether it is
2879 defined. The defined test avoids problems where the line has a string
2880 value that would be treated as false by Perl; for example a "" or
2881 a "0" with no trailing newline. If you really mean for such values
2882 to terminate the loop, they should be tested for explicitly:
2884 while (($_ = <STDIN>) ne '0') { ... }
2885 while (<STDIN>) { last unless $_; ... }
2887 In other boolean contexts, C<< <FILEHANDLE> >> without an
2888 explicit C<defined> test or comparison elicits a warning if the
2889 C<use warnings> pragma or the B<-w>
2890 command-line switch (the C<$^W> variable) is in effect.
2892 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2893 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2894 in packages, where they would be interpreted as local identifiers
2895 rather than global.) Additional filehandles may be created with
2896 the open() function, amongst others. See L<perlopentut> and
2897 L<perlfunc/open> for details on this.
2898 X<stdin> X<stdout> X<sterr>
2900 If a <FILEHANDLE> is used in a context that is looking for
2901 a list, a list comprising all input lines is returned, one line per
2902 list element. It's easy to grow to a rather large data space this
2903 way, so use with care.
2905 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2906 See L<perlfunc/readline>.
2908 The null filehandle <> is special: it can be used to emulate the
2909 behavior of B<sed> and B<awk>, and any other Unix filter program
2910 that takes a list of filenames, doing the same to each line
2911 of input from all of them. Input from <> comes either from
2912 standard input, or from each file listed on the command line. Here's
2913 how it works: the first time <> is evaluated, the @ARGV array is
2914 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2915 gives you standard input. The @ARGV array is then processed as a list
2916 of filenames. The loop
2919 ... # code for each line
2922 is equivalent to the following Perl-like pseudo code:
2924 unshift(@ARGV, '-') unless @ARGV;
2925 while ($ARGV = shift) {
2928 ... # code for each line
2932 except that it isn't so cumbersome to say, and will actually work.
2933 It really does shift the @ARGV array and put the current filename
2934 into the $ARGV variable. It also uses filehandle I<ARGV>
2935 internally. <> is just a synonym for <ARGV>, which
2936 is magical. (The pseudo code above doesn't work because it treats
2937 <ARGV> as non-magical.)
2939 Since the null filehandle uses the two argument form of L<perlfunc/open>
2940 it interprets special characters, so if you have a script like this:
2946 and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2947 pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2948 If you want all items in C<@ARGV> to be interpreted as file names, you
2949 can use the module C<ARGV::readonly> from CPAN.
2951 You can modify @ARGV before the first <> as long as the array ends up
2952 containing the list of filenames you really want. Line numbers (C<$.>)
2953 continue as though the input were one big happy file. See the example
2954 in L<perlfunc/eof> for how to reset line numbers on each file.
2956 If you want to set @ARGV to your own list of files, go right ahead.
2957 This sets @ARGV to all plain text files if no @ARGV was given:
2959 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2961 You can even set them to pipe commands. For example, this automatically
2962 filters compressed arguments through B<gzip>:
2964 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2966 If you want to pass switches into your script, you can use one of the
2967 Getopts modules or put a loop on the front like this:
2969 while ($_ = $ARGV[0], /^-/) {
2972 if (/^-D(.*)/) { $debug = $1 }
2973 if (/^-v/) { $verbose++ }
2974 # ... # other switches
2978 # ... # code for each line
2981 The <> symbol will return C<undef> for end-of-file only once.
2982 If you call it again after this, it will assume you are processing another
2983 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2985 If what the angle brackets contain is a simple scalar variable (for example,
2986 <$foo>), then that variable contains the name of the
2987 filehandle to input from, or its typeglob, or a reference to the
2993 If what's within the angle brackets is neither a filehandle nor a simple
2994 scalar variable containing a filehandle name, typeglob, or typeglob
2995 reference, it is interpreted as a filename pattern to be globbed, and
2996 either a list of filenames or the next filename in the list is returned,
2997 depending on context. This distinction is determined on syntactic
2998 grounds alone. That means C<< <$x> >> is always a readline() from
2999 an indirect handle, but C<< <$hash{key}> >> is always a glob().
3000 That's because $x is a simple scalar variable, but C<$hash{key}> is
3001 not--it's a hash element. Even C<< <$x > >> (note the extra space)
3002 is treated as C<glob("$x ")>, not C<readline($x)>.
3004 One level of double-quote interpretation is done first, but you can't
3005 say C<< <$foo> >> because that's an indirect filehandle as explained
3006 in the previous paragraph. (In older versions of Perl, programmers
3007 would insert curly brackets to force interpretation as a filename glob:
3008 C<< <${foo}> >>. These days, it's considered cleaner to call the
3009 internal function directly as C<glob($foo)>, which is probably the right
3010 way to have done it in the first place.) For example:
3016 is roughly equivalent to:
3018 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
3024 except that the globbing is actually done internally using the standard
3025 C<File::Glob> extension. Of course, the shortest way to do the above is:
3029 A (file)glob evaluates its (embedded) argument only when it is
3030 starting a new list. All values must be read before it will start
3031 over. In list context, this isn't important because you automatically
3032 get them all anyway. However, in scalar context the operator returns
3033 the next value each time it's called, or C<undef> when the list has
3034 run out. As with filehandle reads, an automatic C<defined> is
3035 generated when the glob occurs in the test part of a C<while>,
3036 because legal glob returns (for example,
3037 a file called F<0>) would otherwise
3038 terminate the loop. Again, C<undef> is returned only once. So if
3039 you're expecting a single value from a glob, it is much better to
3042 ($file) = <blurch*>;
3048 because the latter will alternate between returning a filename and
3051 If you're trying to do variable interpolation, it's definitely better
3052 to use the glob() function, because the older notation can cause people
3053 to become confused with the indirect filehandle notation.
3055 @files = glob("$dir/*.[ch]");
3056 @files = glob($files[$i]);
3058 =head2 Constant Folding
3059 X<constant folding> X<folding>
3061 Like C, Perl does a certain amount of expression evaluation at
3062 compile time whenever it determines that all arguments to an
3063 operator are static and have no side effects. In particular, string
3064 concatenation happens at compile time between literals that don't do
3065 variable substitution. Backslash interpolation also happens at
3066 compile time. You can say
3068 'Now is the time for all'
3070 . 'good men to come to.'
3072 and this all reduces to one string internally. Likewise, if
3075 foreach $file (@filenames) {
3076 if (-s $file > 5 + 100 * 2**16) { }
3079 the compiler precomputes the number which that expression
3080 represents so that the interpreter won't have to.
3085 Perl doesn't officially have a no-op operator, but the bare constants
3086 C<0> and C<1> are special-cased not to produce a warning in void
3087 context, so you can for example safely do
3091 =head2 Bitwise String Operators
3092 X<operator, bitwise, string>
3094 Bitstrings of any size may be manipulated by the bitwise operators
3097 If the operands to a binary bitwise op are strings of different
3098 sizes, B<|> and B<^> ops act as though the shorter operand had
3099 additional zero bits on the right, while the B<&> op acts as though
3100 the longer operand were truncated to the length of the shorter.
3101 The granularity for such extension or truncation is one or more
3104 # ASCII-based examples
3105 print "j p \n" ^ " a h"; # prints "JAPH\n"
3106 print "JA" | " ph\n"; # prints "japh\n"
3107 print "japh\nJunk" & '_____'; # prints "JAPH\n";
3108 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
3110 If you are intending to manipulate bitstrings, be certain that
3111 you're supplying bitstrings: If an operand is a number, that will imply
3112 a B<numeric> bitwise operation. You may explicitly show which type of
3113 operation you intend by using C<""> or C<0+>, as in the examples below.
3115 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
3116 $foo = '150' | 105; # yields 255
3117 $foo = 150 | '105'; # yields 255
3118 $foo = '150' | '105'; # yields string '155' (under ASCII)
3120 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
3121 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
3123 See L<perlfunc/vec> for information on how to manipulate individual bits
3126 =head2 Integer Arithmetic
3129 By default, Perl assumes that it must do most of its arithmetic in
3130 floating point. But by saying
3134 you may tell the compiler to use integer operations
3135 (see L<integer> for a detailed explanation) from here to the end of
3136 the enclosing BLOCK. An inner BLOCK may countermand this by saying
3140 which lasts until the end of that BLOCK. Note that this doesn't
3141 mean everything is an integer, merely that Perl will use integer
3142 operations for arithmetic, comparison, and bitwise operators. For
3143 example, even under C<use integer>, if you take the C<sqrt(2)>, you'll
3144 still get C<1.4142135623731> or so.
3146 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
3147 and ">>") always produce integral results. (But see also
3148 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
3149 them. By default, their results are interpreted as unsigned integers, but
3150 if C<use integer> is in effect, their results are interpreted
3151 as signed integers. For example, C<~0> usually evaluates to a large
3152 integral value. However, C<use integer; ~0> is C<-1> on two's-complement
3155 =head2 Floating-point Arithmetic
3157 X<floating-point> X<floating point> X<float> X<real>
3159 While C<use integer> provides integer-only arithmetic, there is no
3160 analogous mechanism to provide automatic rounding or truncation to a
3161 certain number of decimal places. For rounding to a certain number
3162 of digits, sprintf() or printf() is usually the easiest route.
3165 Floating-point numbers are only approximations to what a mathematician
3166 would call real numbers. There are infinitely more reals than floats,
3167 so some corners must be cut. For example:
3169 printf "%.20g\n", 123456789123456789;
3170 # produces 123456789123456784
3172 Testing for exact floating-point equality or inequality is not a
3173 good idea. Here's a (relatively expensive) work-around to compare
3174 whether two floating-point numbers are equal to a particular number of
3175 decimal places. See Knuth, volume II, for a more robust treatment of
3179 my ($X, $Y, $POINTS) = @_;
3181 $tX = sprintf("%.${POINTS}g", $X);
3182 $tY = sprintf("%.${POINTS}g", $Y);
3186 The POSIX module (part of the standard perl distribution) implements
3187 ceil(), floor(), and other mathematical and trigonometric functions.
3188 The Math::Complex module (part of the standard perl distribution)
3189 defines mathematical functions that work on both the reals and the
3190 imaginary numbers. Math::Complex not as efficient as POSIX, but
3191 POSIX can't work with complex numbers.
3193 Rounding in financial applications can have serious implications, and
3194 the rounding method used should be specified precisely. In these
3195 cases, it probably pays not to trust whichever system rounding is
3196 being used by Perl, but to instead implement the rounding function you
3199 =head2 Bigger Numbers
3200 X<number, arbitrary precision>
3202 The standard C<Math::BigInt>, C<Math::BigRat>, and C<Math::BigFloat> modules,
3203 along with the C<bignum>, C<bigint>, and C<bigrat> pragmas, provide
3204 variable-precision arithmetic and overloaded operators, although
3205 they're currently pretty slow. At the cost of some space and
3206 considerable speed, they avoid the normal pitfalls associated with
3207 limited-precision representations.
3210 use bigint; # easy interface to Math::BigInt
3211 $x = 123456789123456789;
3213 +15241578780673678515622620750190521
3221 say "a/b is ", $a/$b;
3222 say "a*b is ", $a*$b;
3226 Several modules let you calculate with (bound only by memory and CPU time)
3227 unlimited or fixed precision. There are also some non-standard modules that
3228 provide faster implementations via external C libraries.
3230 Here is a short, but incomplete summary:
3232 Math::String treat string sequences like numbers
3233 Math::FixedPrecision calculate with a fixed precision
3234 Math::Currency for currency calculations
3235 Bit::Vector manipulate bit vectors fast (uses C)
3236 Math::BigIntFast Bit::Vector wrapper for big numbers
3237 Math::Pari provides access to the Pari C library
3238 Math::Cephes uses the external Cephes C library (no
3240 Math::Cephes::Fraction fractions via the Cephes library
3241 Math::GMP another one using an external C library
3242 Math::GMPz an alternative interface to libgmp's big ints
3243 Math::GMPq an interface to libgmp's fraction numbers
3244 Math::GMPf an interface to libgmp's floating point numbers