3 perlop - Perl operators and precedence
7 Perl operators have the following associativity and precedence,
8 listed from highest precedence to lowest. Note that all operators
9 borrowed from C keep the same precedence relationship with each other,
10 even where C's precedence is slightly screwy. (This makes learning
11 Perl easier for C folks.) With very few exceptions, these all
12 operate on scalar values only, not array values.
14 left terms and list operators (leftward)
18 right ! ~ \ and unary + and -
23 nonassoc named unary operators
24 nonassoc < > <= >= lt gt le ge
25 nonassoc == != <=> eq ne cmp
34 nonassoc list operators (rightward)
39 In the following sections, these operators are covered in precedence order.
41 Many operators can be overloaded for objects. See L<overload>.
45 =head2 Terms and List Operators (Leftward)
47 A TERM has the highest precedence in Perl. They includes variables,
48 quote and quote-like operators, any expression in parentheses,
49 and any function whose arguments are parenthesized. Actually, there
50 aren't really functions in this sense, just list operators and unary
51 operators behaving as functions because you put parentheses around
52 the arguments. These are all documented in L<perlfunc>.
54 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
55 is followed by a left parenthesis as the next token, the operator and
56 arguments within parentheses are taken to be of highest precedence,
57 just like a normal function call.
59 In the absence of parentheses, the precedence of list operators such as
60 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
61 whether you are looking at the left side or the right side of the operator.
64 @ary = (1, 3, sort 4, 2);
65 print @ary; # prints 1324
67 the commas on the right of the sort are evaluated before the sort, but
68 the commas on the left are evaluated after. In other words, list
69 operators tend to gobble up all the arguments that follow them, and
70 then act like a simple TERM with regard to the preceding expression.
71 Note that you have to be careful with parentheses:
73 # These evaluate exit before doing the print:
74 print($foo, exit); # Obviously not what you want.
75 print $foo, exit; # Nor is this.
77 # These do the print before evaluating exit:
78 (print $foo), exit; # This is what you want.
79 print($foo), exit; # Or this.
80 print ($foo), exit; # Or even this.
84 print ($foo & 255) + 1, "\n";
86 probably doesn't do what you expect at first glance. See
87 L<Named Unary Operators> for more discussion of this.
89 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
90 well as subroutine and method calls, and the anonymous
91 constructors C<[]> and C<{}>.
93 See also L<Quote and Quote-like Operators> toward the end of this section,
94 as well as L<"I/O Operators">.
96 =head2 The Arrow Operator
98 Just as in C and C++, "C<-E<gt>>" is an infix dereference operator. If the
99 right side is either a C<[...]> or C<{...}> subscript, then the left side
100 must be either a hard or symbolic reference to an array or hash (or
101 a location capable of holding a hard reference, if it's an lvalue (assignable)).
104 Otherwise, the right side is a method name or a simple scalar variable
105 containing the method name, and the left side must either be an object
106 (a blessed reference) or a class name (that is, a package name).
109 =head2 Auto-increment and Auto-decrement
111 "++" and "--" work as in C. That is, if placed before a variable, they
112 increment or decrement the variable before returning the value, and if
113 placed after, increment or decrement the variable after returning the value.
115 The auto-increment operator has a little extra builtin magic to it. If
116 you increment a variable that is numeric, or that has ever been used in
117 a numeric context, you get a normal increment. If, however, the
118 variable has been used in only string contexts since it was set, and
119 has a value that is not the empty string and matches the pattern
120 C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each
121 character within its range, with carry:
123 print ++($foo = '99'); # prints '100'
124 print ++($foo = 'a0'); # prints 'a1'
125 print ++($foo = 'Az'); # prints 'Ba'
126 print ++($foo = 'zz'); # prints 'aaa'
128 The auto-decrement operator is not magical.
130 =head2 Exponentiation
132 Binary "**" is the exponentiation operator. Note that it binds even more
133 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
134 implemented using C's pow(3) function, which actually works on doubles
137 =head2 Symbolic Unary Operators
139 Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
140 precedence version of this.
142 Unary "-" performs arithmetic negation if the operand is numeric. If
143 the operand is an identifier, a string consisting of a minus sign
144 concatenated with the identifier is returned. Otherwise, if the string
145 starts with a plus or minus, a string starting with the opposite sign
146 is returned. One effect of these rules is that C<-bareword> is equivalent
149 Unary "~" performs bitwise negation, i.e., 1's complement. For example,
150 C<0666 &~ 027> is 0640. (See also L<Integer Arithmetic> and L<Bitwise
153 Unary "+" has no effect whatsoever, even on strings. It is useful
154 syntactically for separating a function name from a parenthesized expression
155 that would otherwise be interpreted as the complete list of function
156 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
158 Unary "\" creates a reference to whatever follows it. See L<perlref>.
159 Do not confuse this behavior with the behavior of backslash within a
160 string, although both forms do convey the notion of protecting the next
161 thing from interpretation.
163 =head2 Binding Operators
165 Binary "=~" binds a scalar expression to a pattern match. Certain operations
166 search or modify the string $_ by default. This operator makes that kind
167 of operation work on some other string. The right argument is a search
168 pattern, substitution, or transliteration. The left argument is what is
169 supposed to be searched, substituted, or transliterated instead of the default
170 $_. The return value indicates the success of the operation. (If the
171 right argument is an expression rather than a search pattern,
172 substitution, or transliteration, it is interpreted as a search pattern at run
173 time. This can be is less efficient than an explicit search, because the
174 pattern must be compiled every time the expression is evaluated.
176 Binary "!~" is just like "=~" except the return value is negated in
179 =head2 Multiplicative Operators
181 Binary "*" multiplies two numbers.
183 Binary "/" divides two numbers.
185 Binary "%" computes the modulus of two numbers. Given integer
186 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
187 C<$a> minus the largest multiple of C<$b> that is not greater than
188 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
189 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
190 result will be less than or equal to zero).
191 Note than when C<use integer> is in scope, "%" give you direct access
192 to the modulus operator as implemented by your C compiler. This
193 operator is not as well defined for negative operands, but it will
196 Binary "x" is the repetition operator. In scalar context, it
197 returns a string consisting of the left operand repeated the number of
198 times specified by the right operand. In list context, if the left
199 operand is a list in parentheses, it repeats the list.
201 print '-' x 80; # print row of dashes
203 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
205 @ones = (1) x 80; # a list of 80 1's
206 @ones = (5) x @ones; # set all elements to 5
209 =head2 Additive Operators
211 Binary "+" returns the sum of two numbers.
213 Binary "-" returns the difference of two numbers.
215 Binary "." concatenates two strings.
217 =head2 Shift Operators
219 Binary "<<" returns the value of its left argument shifted left by the
220 number of bits specified by the right argument. Arguments should be
221 integers. (See also L<Integer Arithmetic>.)
223 Binary ">>" returns the value of its left argument shifted right by
224 the number of bits specified by the right argument. Arguments should
225 be integers. (See also L<Integer Arithmetic>.)
227 =head2 Named Unary Operators
229 The various named unary operators are treated as functions with one
230 argument, with optional parentheses. These include the filetest
231 operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
233 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
234 is followed by a left parenthesis as the next token, the operator and
235 arguments within parentheses are taken to be of highest precedence,
236 just like a normal function call. Examples:
238 chdir $foo || die; # (chdir $foo) || die
239 chdir($foo) || die; # (chdir $foo) || die
240 chdir ($foo) || die; # (chdir $foo) || die
241 chdir +($foo) || die; # (chdir $foo) || die
243 but, because * is higher precedence than ||:
245 chdir $foo * 20; # chdir ($foo * 20)
246 chdir($foo) * 20; # (chdir $foo) * 20
247 chdir ($foo) * 20; # (chdir $foo) * 20
248 chdir +($foo) * 20; # chdir ($foo * 20)
250 rand 10 * 20; # rand (10 * 20)
251 rand(10) * 20; # (rand 10) * 20
252 rand (10) * 20; # (rand 10) * 20
253 rand +(10) * 20; # rand (10 * 20)
255 See also L<"Terms and List Operators (Leftward)">.
257 =head2 Relational Operators
259 Binary "E<lt>" returns true if the left argument is numerically less than
262 Binary "E<gt>" returns true if the left argument is numerically greater
263 than the right argument.
265 Binary "E<lt>=" returns true if the left argument is numerically less than
266 or equal to the right argument.
268 Binary "E<gt>=" returns true if the left argument is numerically greater
269 than or equal to the right argument.
271 Binary "lt" returns true if the left argument is stringwise less than
274 Binary "gt" returns true if the left argument is stringwise greater
275 than the right argument.
277 Binary "le" returns true if the left argument is stringwise less than
278 or equal to the right argument.
280 Binary "ge" returns true if the left argument is stringwise greater
281 than or equal to the right argument.
283 =head2 Equality Operators
285 Binary "==" returns true if the left argument is numerically equal to
288 Binary "!=" returns true if the left argument is numerically not equal
289 to the right argument.
291 Binary "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left
292 argument is numerically less than, equal to, or greater than the right
295 Binary "eq" returns true if the left argument is stringwise equal to
298 Binary "ne" returns true if the left argument is stringwise not equal
299 to the right argument.
301 Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise
302 less than, equal to, or greater than the right argument.
304 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
305 by the current locale if C<use locale> is in effect. See L<perllocale>.
309 Binary "&" returns its operators ANDed together bit by bit.
310 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
312 =head2 Bitwise Or and Exclusive Or
314 Binary "|" returns its operators ORed together bit by bit.
315 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
317 Binary "^" returns its operators XORed together bit by bit.
318 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
320 =head2 C-style Logical And
322 Binary "&&" performs a short-circuit logical AND operation. That is,
323 if the left operand is false, the right operand is not even evaluated.
324 Scalar or list context propagates down to the right operand if it
327 =head2 C-style Logical Or
329 Binary "||" performs a short-circuit logical OR operation. That is,
330 if the left operand is true, the right operand is not even evaluated.
331 Scalar or list context propagates down to the right operand if it
334 The C<||> and C<&&> operators differ from C's in that, rather than returning
335 0 or 1, they return the last value evaluated. Thus, a reasonably portable
336 way to find out the home directory (assuming it's not "0") might be:
338 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
339 (getpwuid($<))[7] || die "You're homeless!\n";
341 In particular, this means that you shouldn't use this
342 for selecting between two aggregates for assignment:
344 @a = @b || @c; # this is wrong
345 @a = scalar(@b) || @c; # really meant this
346 @a = @b ? @b : @c; # this works fine, though
348 As more readable alternatives to C<&&> and C<||> when used for
349 control flow, Perl provides C<and> and C<or> operators (see below).
350 The short-circuit behavior is identical. The precedence of "and" and
351 "or" is much lower, however, so that you can safely use them after a
352 list operator without the need for parentheses:
354 unlink "alpha", "beta", "gamma"
355 or gripe(), next LINE;
357 With the C-style operators that would have been written like this:
359 unlink("alpha", "beta", "gamma")
360 || (gripe(), next LINE);
362 Use "or" for assignment is unlikely to do what you want; see below.
364 =head2 Range Operators
366 Binary ".." is the range operator, which is really two different
367 operators depending on the context. In list context, it returns an
368 array of values counting (by ones) from the left value to the right
369 value. This is useful for writing C<foreach (1..10)> loops and for
370 doing slice operations on arrays. In the current implementation, no
371 temporary array is created when the range operator is used as the
372 expression in C<foreach> loops, but older versions of Perl might burn
373 a lot of memory when you write something like this:
375 for (1 .. 1_000_000) {
379 In scalar context, ".." returns a boolean value. The operator is
380 bistable, like a flip-flop, and emulates the line-range (comma) operator
381 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
382 own boolean state. It is false as long as its left operand is false.
383 Once the left operand is true, the range operator stays true until the
384 right operand is true, I<AFTER> which the range operator becomes false
385 again. (It doesn't become false till the next time the range operator is
386 evaluated. It can test the right operand and become false on the same
387 evaluation it became true (as in B<awk>), but it still returns true once.
388 If you don't want it to test the right operand till the next evaluation
389 (as in B<sed>), use three dots ("...") instead of two.) The right
390 operand is not evaluated while the operator is in the "false" state, and
391 the left operand is not evaluated while the operator is in the "true"
392 state. The precedence is a little lower than || and &&. The value
393 returned is either the empty string for false, or a sequence number
394 (beginning with 1) for true. The sequence number is reset for each range
395 encountered. The final sequence number in a range has the string "E0"
396 appended to it, which doesn't affect its numeric value, but gives you
397 something to search for if you want to exclude the endpoint. You can
398 exclude the beginning point by waiting for the sequence number to be
399 greater than 1. If either operand of scalar ".." is a constant expression,
400 that operand is implicitly compared to the C<$.> variable, the current
401 line number. Examples:
403 As a scalar operator:
405 if (101 .. 200) { print; } # print 2nd hundred lines
406 next line if (1 .. /^$/); # skip header lines
407 s/^/> / if (/^$/ .. eof()); # quote body
409 # parse mail messages
411 $in_header = 1 .. /^$/;
412 $in_body = /^$/ .. eof();
413 # do something based on those
415 close ARGV if eof; # reset $. each file
420 for (101 .. 200) { print; } # print $_ 100 times
421 @foo = @foo[0 .. $#foo]; # an expensive no-op
422 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
424 The range operator (in list context) makes use of the magical
425 auto-increment algorithm if the operands are strings. You
428 @alphabet = ('A' .. 'Z');
430 to get all the letters of the alphabet, or
432 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
434 to get a hexadecimal digit, or
436 @z2 = ('01' .. '31'); print $z2[$mday];
438 to get dates with leading zeros. If the final value specified is not
439 in the sequence that the magical increment would produce, the sequence
440 goes until the next value would be longer than the final value
443 =head2 Conditional Operator
445 Ternary "?:" is the conditional operator, just as in C. It works much
446 like an if-then-else. If the argument before the ? is true, the
447 argument before the : is returned, otherwise the argument after the :
448 is returned. For example:
450 printf "I have %d dog%s.\n", $n,
451 ($n == 1) ? '' : "s";
453 Scalar or list context propagates downward into the 2nd
454 or 3rd argument, whichever is selected.
456 $a = $ok ? $b : $c; # get a scalar
457 @a = $ok ? @b : @c; # get an array
458 $a = $ok ? @b : @c; # oops, that's just a count!
460 The operator may be assigned to if both the 2nd and 3rd arguments are
461 legal lvalues (meaning that you can assign to them):
463 ($a_or_b ? $a : $b) = $c;
465 This is not necessarily guaranteed to contribute to the readability of your program.
467 Because this operator produces an assignable result, using assignments
468 without parentheses will get you in trouble. For example, this:
470 $a % 2 ? $a += 10 : $a += 2
474 (($a % 2) ? ($a += 10) : $a) += 2
478 ($a % 2) ? ($a += 10) : ($a += 2)
480 =head2 Assignment Operators
482 "=" is the ordinary assignment operator.
484 Assignment operators work as in C. That is,
492 although without duplicating any side effects that dereferencing the lvalue
493 might trigger, such as from tie(). Other assignment operators work similarly.
494 The following are recognized:
501 Note that while these are grouped by family, they all have the precedence
504 Unlike in C, the assignment operator produces a valid lvalue. Modifying
505 an assignment is equivalent to doing the assignment and then modifying
506 the variable that was assigned to. This is useful for modifying
507 a copy of something, like this:
509 ($tmp = $global) =~ tr [A-Z] [a-z];
520 =head2 Comma Operator
522 Binary "," is the comma operator. In scalar context it evaluates
523 its left argument, throws that value away, then evaluates its right
524 argument and returns that value. This is just like C's comma operator.
526 In list context, it's just the list argument separator, and inserts
527 both its arguments into the list.
529 The =E<gt> digraph is mostly just a synonym for the comma operator. It's useful for
530 documenting arguments that come in pairs. As of release 5.001, it also forces
531 any word to the left of it to be interpreted as a string.
533 =head2 List Operators (Rightward)
535 On the right side of a list operator, it has very low precedence,
536 such that it controls all comma-separated expressions found there.
537 The only operators with lower precedence are the logical operators
538 "and", "or", and "not", which may be used to evaluate calls to list
539 operators without the need for extra parentheses:
541 open HANDLE, "filename"
542 or die "Can't open: $!\n";
544 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
548 Unary "not" returns the logical negation of the expression to its right.
549 It's the equivalent of "!" except for the very low precedence.
553 Binary "and" returns the logical conjunction of the two surrounding
554 expressions. It's equivalent to && except for the very low
555 precedence. This means that it short-circuits: i.e., the right
556 expression is evaluated only if the left expression is true.
558 =head2 Logical or and Exclusive Or
560 Binary "or" returns the logical disjunction of the two surrounding
561 expressions. It's equivalent to || except for the very low precedence.
562 This makes it useful for control flow
564 print FH $data or die "Can't write to FH: $!";
566 This means that it short-circuits: i.e., the right expression is evaluated
567 only if the left expression is false. Due to its precedence, you should
568 probably avoid using this for assignment, only for control flow.
570 $a = $b or $c; # bug: this is wrong
571 ($a = $b) or $c; # really means this
572 $a = $b || $c; # better written this way
574 However, when it's a list context assignment and you're trying to use
575 "||" for control flow, you probably need "or" so that the assignment
576 takes higher precedence.
578 @info = stat($file) || die; # oops, scalar sense of stat!
579 @info = stat($file) or die; # better, now @info gets its due
581 Then again, you could always use parentheses.
583 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
584 It cannot short circuit, of course.
586 =head2 C Operators Missing From Perl
588 Here is what C has that Perl doesn't:
594 Address-of operator. (But see the "\" operator for taking a reference.)
598 Dereference-address operator. (Perl's prefix dereferencing
599 operators are typed: $, @, %, and &.)
603 Type casting operator.
607 =head2 Quote and Quote-like Operators
609 While we usually think of quotes as literal values, in Perl they
610 function as operators, providing various kinds of interpolating and
611 pattern matching capabilities. Perl provides customary quote characters
612 for these behaviors, but also provides a way for you to choose your
613 quote character for any of them. In the following table, a C<{}> represents
614 any pair of delimiters you choose. Non-bracketing delimiters use
615 the same character fore and aft, but the 4 sorts of brackets
616 (round, angle, square, curly) will all nest.
618 Customary Generic Meaning Interpolates
621 `` qx{} Command yes (unless '' is delimiter)
623 // m{} Pattern match yes
625 s{}{} Substitution yes
626 tr{}{} Transliteration no (but see below)
628 Note that there can be whitespace between the operator and the quoting
629 characters, except when C<#> is being used as the quoting character.
630 C<q#foo#> is parsed as being the string C<foo>, while C<q #foo#> is the
631 operator C<q> followed by a comment. Its argument will be taken from the
632 next line. This allows you to write:
634 s {foo} # Replace foo
637 For constructs that do interpolation, variables beginning with "C<$>"
638 or "C<@>" are interpolated, as are the following sequences. Within
639 a transliteration, the first eleven of these sequences may be used.
646 \a alarm (bell) (BEL)
648 \033 octal char (ESC)
650 \x{263a} wide hex char (SMILEY)
653 \l lowercase next char
654 \u uppercase next char
657 \E end case modification
658 \Q quote non-word characters till \E
660 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
661 and C<\U> is taken from the current locale. See L<perllocale>.
663 All systems use the virtual C<"\n"> to represent a line terminator,
664 called a "newline". There is no such thing as an unvarying, physical
665 newline character. It is an illusion that the operating system,
666 device drivers, C libraries, and Perl all conspire to preserve. Not all
667 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
668 on a Mac, these are reversed, and on systems without line terminator,
669 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
670 you mean a "newline" for your system, but use the literal ASCII when you
671 need an exact character. For example, most networking protocols expect
672 and prefer a CR+LF (C<"\012\015"> or C<"\cJ\cM">) for line terminators,
673 and although they often accept just C<"\012">, they seldom tolerate just
674 C<"\015">. If you get in the habit of using C<"\n"> for networking,
675 you may be burned some day.
677 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
678 An unescaped C<$> or C<@> interpolates the corresponding variable,
679 while escaping will cause the literal string C<\$> to be inserted.
680 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
682 Patterns are subject to an additional level of interpretation as a
683 regular expression. This is done as a second pass, after variables are
684 interpolated, so that regular expressions may be incorporated into the
685 pattern from the variables. If this is not what you want, use C<\Q> to
686 interpolate a variable literally.
688 Apart from the above, there are no multiple levels of interpolation. In
689 particular, contrary to the expectations of shell programmers, back-quotes
690 do I<NOT> interpolate within double quotes, nor do single quotes impede
691 evaluation of variables when used within double quotes.
693 =head2 Regexp Quote-Like Operators
695 Here are the quote-like operators that apply to pattern
696 matching and related activities.
698 Most of this section is related to use of regular expressions from Perl.
699 Such a use may be considered from two points of view: Perl handles a
700 a string and a "pattern" to RE (regular expression) engine to match,
701 RE engine finds (or does not find) the match, and Perl uses the findings
702 of RE engine for its operation, possibly asking the engine for other matches.
704 RE engine has no idea what Perl is going to do with what it finds,
705 similarly, the rest of Perl has no idea what a particular regular expression
706 means to RE engine. This creates a clean separation, and in this section
707 we discuss matching from Perl point of view only. The other point of
708 view may be found in L<perlre>.
714 This is just like the C</pattern/> search, except that it matches only
715 once between calls to the reset() operator. This is a useful
716 optimization when you want to see only the first occurrence of
717 something in each file of a set of files, for instance. Only C<??>
718 patterns local to the current package are reset.
722 # blank line between header and body
725 reset if eof; # clear ?? status for next file
728 This usage is vaguely deprecated, and may be removed in some future
731 =item m/PATTERN/cgimosx
733 =item /PATTERN/cgimosx
735 Searches a string for a pattern match, and in scalar context returns
736 true (1) or false (''). If no string is specified via the C<=~> or
737 C<!~> operator, the $_ string is searched. (The string specified with
738 C<=~> need not be an lvalue--it may be the result of an expression
739 evaluation, but remember the C<=~> binds rather tightly.) See also
741 See L<perllocale> for discussion of additional considerations that apply
742 when C<use locale> is in effect.
746 c Do not reset search position on a failed match when /g is in effect.
747 g Match globally, i.e., find all occurrences.
748 i Do case-insensitive pattern matching.
749 m Treat string as multiple lines.
750 o Compile pattern only once.
751 s Treat string as single line.
752 x Use extended regular expressions.
754 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
755 you can use any pair of non-alphanumeric, non-whitespace characters
756 as delimiters (if single quotes are used, no interpretation is done
757 on the replacement string. Unlike Perl 4, Perl 5 treats backticks as normal
758 delimiters; the replacement text is not evaluated as a command).
759 This is particularly useful for matching Unix path names
760 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
761 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
763 PATTERN may contain variables, which will be interpolated (and the
764 pattern recompiled) every time the pattern search is evaluated. (Note
765 that C<$)> and C<$|> might not be interpolated because they look like
766 end-of-string tests.) If you want such a pattern to be compiled only
767 once, add a C</o> after the trailing delimiter. This avoids expensive
768 run-time recompilations, and is useful when the value you are
769 interpolating won't change over the life of the script. However, mentioning
770 C</o> constitutes a promise that you won't change the variables in the pattern.
771 If you change them, Perl won't even notice.
773 If the PATTERN evaluates to the empty string, the last
774 I<successfully> matched regular expression is used instead.
776 If the C</g> option is not used, C<m//> in a list context returns a
777 list consisting of the subexpressions matched by the parentheses in the
778 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
779 also set, and that this differs from Perl 4's behavior.) When there are
780 no parentheses in the pattern, the return value is the list C<(1)> for
781 success. With or without parentheses, an empty list is returned upon
786 open(TTY, '/dev/tty');
787 <TTY> =~ /^y/i && foo(); # do foo if desired
789 if (/Version: *([0-9.]*)/) { $version = $1; }
791 next if m#^/usr/spool/uucp#;
796 print if /$arg/o; # compile only once
799 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
801 This last example splits $foo into the first two words and the
802 remainder of the line, and assigns those three fields to $F1, $F2, and
803 $Etc. The conditional is true if any variables were assigned, i.e., if
806 The C</g> modifier specifies global pattern matching--that is, matching
807 as many times as possible within the string. How it behaves depends on
808 the context. In list context, it returns a list of all the
809 substrings matched by all the parentheses in the regular expression.
810 If there are no parentheses, it returns a list of all the matched
811 strings, as if there were parentheses around the whole pattern.
813 In scalar context, each execution of C<m//g> finds the next match,
814 returning TRUE if it matches, and FALSE if there is no further match.
815 The position after the last match can be read or set using the pos()
816 function; see L<perlfunc/pos>. A failed match normally resets the
817 search position to the beginning of the string, but you can avoid that
818 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
819 string also resets the search position.
821 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
822 zero-width assertion that matches the exact position where the previous
823 C<m//g>, if any, left off. The C<\G> assertion is not supported without
824 the C</g> modifier; currently, without C</g>, C<\G> behaves just like
825 C<\A>, but that's accidental and may change in the future.
830 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
833 $/ = ""; $* = 1; # $* deprecated in modern perls
834 while (defined($paragraph = <>)) {
835 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
839 print "$sentences\n";
841 # using m//gc with \G
845 print $1 while /(o)/gc; print "', pos=", pos, "\n";
847 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
849 print $1 while /(p)/gc; print "', pos=", pos, "\n";
852 The last example should print:
861 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
862 combine several regexps like this to process a string part-by-part,
863 doing different actions depending on which regexp matched. Each
864 regexp tries to match where the previous one leaves off.
867 $url = new URI::URL "http://www/"; die if $url eq "xXx";
871 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
872 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
873 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
874 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
875 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
876 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
877 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
878 print ". That's all!\n";
881 Here is the output (split into several lines):
883 line-noise lowercase line-noise lowercase UPPERCASE line-noise
884 UPPERCASE line-noise lowercase line-noise lowercase line-noise
885 lowercase lowercase line-noise lowercase lowercase line-noise
886 MiXeD line-noise. That's all!
892 A single-quoted, literal string. A backslash represents a backslash
893 unless followed by the delimiter or another backslash, in which case
894 the delimiter or backslash is interpolated.
896 $foo = q!I said, "You said, 'She said it.'"!;
897 $bar = q('This is it.');
898 $baz = '\n'; # a two-character string
904 A double-quoted, interpolated string.
907 (*** The previous line contains the naughty word "$1".\n)
908 if /(tcl|rexx|python)/; # :-)
909 $baz = "\n"; # a one-character string
911 =item qr/STRING/imosx
913 Quote-as-a-regular-expression operator. I<STRING> is interpolated the
914 same way as I<PATTERN> in C<m/PATTERN/>. Returns a Perl value which
915 may be used instead of the corresponding C</STRING/imosx> expression.
919 $rex = qr/my.STRING/is;
926 The result may be used as a subpattern in a match:
929 $string =~ /foo${re}bar/; # can be interpolated in other patterns
930 $string =~ $re; # or used standalone
931 $string =~ /$re/; # or this way
933 Since Perl may compile the pattern at the moment of execution of qr()
934 operator, using qr() may have speed advantages in I<some> situations,
935 notably if the result of qr() is used standalone:
938 my $patterns = shift;
939 my @compiled = map qr/$_/i, @$patterns;
942 foreach my $pat @compiled {
943 $success = 1, last if /$pat/;
949 Precompilation of the pattern into an internal representation at the
950 moment of qr() avoids a need to recompile the pattern every time a
951 match C</$pat/> is attempted. (Note that Perl has many other
952 internal optimizations, but none would be triggered in the above
953 example if we did not use qr() operator.)
957 i Do case-insensitive pattern matching.
958 m Treat string as multiple lines.
959 o Compile pattern only once.
960 s Treat string as single line.
961 x Use extended regular expressions.
963 See L<perlre> for additional information on valid syntax for STRING, and
964 for a detailed look at the semantics of regular expressions.
970 A string which is (possibly) interpolated and then executed as a system
971 command with C</bin/sh> or its equivalent. Shell wildcards, pipes,
972 and redirections will be honored. The collected standard output of the
973 command is returned; standard error is unaffected. In scalar context,
974 it comes back as a single (potentially multi-line) string. In list
975 context, returns a list of lines (however you've defined lines with $/
976 or $INPUT_RECORD_SEPARATOR).
978 Because backticks do not affect standard error, use shell file descriptor
979 syntax (assuming the shell supports this) if you care to address this.
980 To capture a command's STDERR and STDOUT together:
982 $output = `cmd 2>&1`;
984 To capture a command's STDOUT but discard its STDERR:
986 $output = `cmd 2>/dev/null`;
988 To capture a command's STDERR but discard its STDOUT (ordering is
991 $output = `cmd 2>&1 1>/dev/null`;
993 To exchange a command's STDOUT and STDERR in order to capture the STDERR
994 but leave its STDOUT to come out the old STDERR:
996 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
998 To read both a command's STDOUT and its STDERR separately, it's easiest
999 and safest to redirect them separately to files, and then read from those
1000 files when the program is done:
1002 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
1004 Using single-quote as a delimiter protects the command from Perl's
1005 double-quote interpolation, passing it on to the shell instead:
1007 $perl_info = qx(ps $$); # that's Perl's $$
1008 $shell_info = qx'ps $$'; # that's the new shell's $$
1010 Note that how the string gets evaluated is entirely subject to the command
1011 interpreter on your system. On most platforms, you will have to protect
1012 shell metacharacters if you want them treated literally. This is in
1013 practice difficult to do, as it's unclear how to escape which characters.
1014 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1015 to emulate backticks safely.
1017 On some platforms (notably DOS-like ones), the shell may not be
1018 capable of dealing with multiline commands, so putting newlines in
1019 the string may not get you what you want. You may be able to evaluate
1020 multiple commands in a single line by separating them with the command
1021 separator character, if your shell supports that (e.g. C<;> on many Unix
1022 shells; C<&> on the Windows NT C<cmd> shell).
1024 Beware that some command shells may place restrictions on the length
1025 of the command line. You must ensure your strings don't exceed this
1026 limit after any necessary interpolations. See the platform-specific
1027 release notes for more details about your particular environment.
1029 Using this operator can lead to programs that are difficult to port,
1030 because the shell commands called vary between systems, and may in
1031 fact not be present at all. As one example, the C<type> command under
1032 the POSIX shell is very different from the C<type> command under DOS.
1033 That doesn't mean you should go out of your way to avoid backticks
1034 when they're the right way to get something done. Perl was made to be
1035 a glue language, and one of the things it glues together is commands.
1036 Just understand what you're getting yourself into.
1038 See L<"I/O Operators"> for more discussion.
1042 Returns a list of the words extracted out of STRING, using embedded
1043 whitespace as the word delimiters. It is exactly equivalent to
1045 split(' ', q/STRING/);
1047 This equivalency means that if used in scalar context, you'll get split's
1048 (unfortunate) scalar context behavior, complete with mysterious warnings.
1050 Some frequently seen examples:
1052 use POSIX qw( setlocale localeconv )
1053 @EXPORT = qw( foo bar baz );
1055 A common mistake is to try to separate the words with comma or to put
1056 comments into a multi-line C<qw>-string. For this reason the C<-w>
1057 switch produce warnings if the STRING contains the "," or the "#"
1060 =item s/PATTERN/REPLACEMENT/egimosx
1062 Searches a string for a pattern, and if found, replaces that pattern
1063 with the replacement text and returns the number of substitutions
1064 made. Otherwise it returns false (specifically, the empty string).
1066 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1067 variable is searched and modified. (The string specified with C<=~> must
1068 be scalar variable, an array element, a hash element, or an assignment
1069 to one of those, i.e., an lvalue.)
1071 If the delimiter chosen is single quote, no variable interpolation is
1072 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1073 PATTERN contains a $ that looks like a variable rather than an
1074 end-of-string test, the variable will be interpolated into the pattern
1075 at run-time. If you want the pattern compiled only once the first time
1076 the variable is interpolated, use the C</o> option. If the pattern
1077 evaluates to the empty string, the last successfully executed regular
1078 expression is used instead. See L<perlre> for further explanation on these.
1079 See L<perllocale> for discussion of additional considerations that apply
1080 when C<use locale> is in effect.
1084 e Evaluate the right side as an expression.
1085 g Replace globally, i.e., all occurrences.
1086 i Do case-insensitive pattern matching.
1087 m Treat string as multiple lines.
1088 o Compile pattern only once.
1089 s Treat string as single line.
1090 x Use extended regular expressions.
1092 Any non-alphanumeric, non-whitespace delimiter may replace the
1093 slashes. If single quotes are used, no interpretation is done on the
1094 replacement string (the C</e> modifier overrides this, however). Unlike
1095 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1096 text is not evaluated as a command. If the
1097 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1098 pair of quotes, which may or may not be bracketing quotes, e.g.,
1099 C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the
1100 replacement portion to be interpreted as a full-fledged Perl expression
1101 and eval()ed right then and there. It is, however, syntax checked at
1106 s/\bgreen\b/mauve/g; # don't change wintergreen
1108 $path =~ s|/usr/bin|/usr/local/bin|;
1110 s/Login: $foo/Login: $bar/; # run-time pattern
1112 ($foo = $bar) =~ s/this/that/; # copy first, then change
1114 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1117 s/\d+/$&*2/e; # yields 'abc246xyz'
1118 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1119 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1121 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1122 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1123 s/^=(\w+)/&pod($1)/ge; # use function call
1125 # expand variables in $_, but dynamics only, using
1126 # symbolic dereferencing
1129 # /e's can even nest; this will expand
1130 # any embedded scalar variable (including lexicals) in $_
1133 # Delete (most) C comments.
1135 /\* # Match the opening delimiter.
1136 .*? # Match a minimal number of characters.
1137 \*/ # Match the closing delimiter.
1140 s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
1142 for ($variable) { # trim white space in $variable, cheap
1147 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1149 Note the use of $ instead of \ in the last example. Unlike
1150 B<sed>, we use the \E<lt>I<digit>E<gt> form in only the left hand side.
1151 Anywhere else it's $E<lt>I<digit>E<gt>.
1153 Occasionally, you can't use just a C</g> to get all the changes
1154 to occur. Here are two common cases:
1156 # put commas in the right places in an integer
1157 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4
1158 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5
1160 # expand tabs to 8-column spacing
1161 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1164 =item tr/SEARCHLIST/REPLACEMENTLIST/cdsUC
1166 =item y/SEARCHLIST/REPLACEMENTLIST/cdsUC
1168 Transliterates all occurrences of the characters found in the search list
1169 with the corresponding character in the replacement list. It returns
1170 the number of characters replaced or deleted. If no string is
1171 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1172 string specified with =~ must be a scalar variable, an array element, a
1173 hash element, or an assignment to one of those, i.e., an lvalue.)
1175 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1176 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1177 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1178 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1179 its own pair of quotes, which may or may not be bracketing quotes,
1180 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1182 Note also that the whole range idea is rather unportable between
1183 character sets--and even within character sets they may cause results
1184 you probably didn't expect. A sound principle is to use only ranges
1185 that begin from and end at either alphabets of equal case (a-e, A-E),
1186 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1187 character sets in full.
1191 c Complement the SEARCHLIST.
1192 d Delete found but unreplaced characters.
1193 s Squash duplicate replaced characters.
1194 U Translate to/from UTF-8.
1195 C Translate to/from 8-bit char (octet).
1197 If the C</c> modifier is specified, the SEARCHLIST character set is
1198 complemented. If the C</d> modifier is specified, any characters specified
1199 by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note
1200 that this is slightly more flexible than the behavior of some B<tr>
1201 programs, which delete anything they find in the SEARCHLIST, period.)
1202 If the C</s> modifier is specified, sequences of characters that were
1203 transliterated to the same character are squashed down to a single instance of the
1206 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1207 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1208 than the SEARCHLIST, the final character is replicated till it is long
1209 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1210 This latter is useful for counting characters in a class or for
1211 squashing character sequences in a class.
1213 The first C</U> or C</C> modifier applies to the left side of the translation.
1214 The second one applies to the right side. If present, these modifiers override
1215 the current utf8 state.
1219 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1221 $cnt = tr/*/*/; # count the stars in $_
1223 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1225 $cnt = tr/0-9//; # count the digits in $_
1227 tr/a-zA-Z//s; # bookkeeper -> bokeper
1229 ($HOST = $host) =~ tr/a-z/A-Z/;
1231 tr/a-zA-Z/ /cs; # change non-alphas to single space
1234 [\000-\177]; # delete 8th bit
1236 tr/\0-\xFF//CU; # translate Latin-1 to Unicode
1237 tr/\0-\x{FF}//UC; # translate Unicode to Latin-1
1239 If multiple transliterations are given for a character, only the first one is used:
1243 will transliterate any A to X.
1245 Note that because the transliteration table is built at compile time, neither
1246 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1247 interpolation. That means that if you want to use variables, you must use
1250 eval "tr/$oldlist/$newlist/";
1253 eval "tr/$oldlist/$newlist/, 1" or die $@;
1257 =head2 Gory details of parsing quoted constructs
1259 When presented with something which may have several different
1260 interpretations, Perl uses the principle B<DWIM> (expanded to Do What I Mean
1261 - not what I wrote) to pick up the most probable interpretation of the
1262 source. This strategy is so successful that Perl users usually do not
1263 suspect ambivalence of what they write. However, time to time Perl's ideas
1264 differ from what the author meant.
1266 The target of this section is to clarify the Perl's way of interpreting
1267 quoted constructs. The most frequent reason one may have to want to know the
1268 details discussed in this section is hairy regular expressions. However, the
1269 first steps of parsing are the same for all Perl quoting operators, so here
1270 they are discussed together.
1272 Some of the passes discussed below are performed concurrently, but as
1273 far as results are the same, we consider them one-by-one. For different
1274 quoting constructs Perl performs different number of passes, from
1275 one to five, but they are always performed in the same order.
1279 =item Finding the end
1281 First pass is finding the end of the quoted construct, be it multichar ender
1282 C<"\nEOF\n"> of C<<<EOF> construct, C</> which terminates C<qq/> construct,
1283 C<]> which terminates C<qq[> construct, or C<E<gt>> which terminates a
1284 fileglob started with C<<>.
1286 When searching for multichar construct no skipping is performed. When
1287 searching for one-char non-matching delimiter, such as C</>, combinations
1288 C<\\> and C<\/> are skipped. When searching for one-char matching delimiter,
1289 such as C<]>, combinations C<\\>, C<\]> and C<\[> are skipped, and
1290 nested C<[>, C<]> are skipped as well.
1292 For 3-parts constructs, C<s///> etc. the search is repeated once more.
1294 During this search no attention is paid to the semantic of the construct, thus
1296 "$hash{"$foo/$bar"}"
1301 bar # This is not a comment, this slash / terminated m//!
1304 do not form legal quoted expressions. Note that since the slash which
1305 terminated C<m//> was followed by a C<SPACE>, this is not C<m//x>,
1306 thus C<#> was interpreted as a literal C<#>.
1308 =item Removal of backslashes before delimiters
1310 During the second pass the text between the starting delimiter and
1311 the ending delimiter is copied to a safe location, and the C<\> is
1312 removed from combinations consisting of C<\> and delimiter(s) (both starting
1313 and ending delimiter if they differ).
1315 The removal does not happen for multi-char delimiters.
1317 Note that the combination C<\\> is left as it was!
1319 Starting from this step no information about the delimiter(s) is used in the
1324 Next step is interpolation in the obtained delimiter-independent text.
1325 There are four different cases.
1329 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1331 No interpolation is performed.
1335 The only interpolation is removal of C<\> from pairs C<\\>.
1337 =item C<"">, C<``>, C<qq//>, C<qx//>, C<<file*globE<gt>>
1339 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are converted
1340 to corresponding Perl constructs, thus C<"$foo\Qbaz$bar"> is converted to
1342 $foo . (quotemeta("baz" . $bar));
1344 Other combinations of C<\> with following chars are substituted with
1345 appropriate expansions.
1347 Interpolated scalars and arrays are converted to C<join> and C<.> Perl
1348 constructs, thus C<"'@arr'"> becomes
1350 "'" . (join $", @arr) . "'";
1352 Since all three above steps are performed simultaneously left-to-right,
1353 the is no way to insert a literal C<$> or C<@> inside C<\Q\E> pair: it
1354 cannot be protected by C<\>, since any C<\> (except in C<\E>) is
1355 interpreted as a literal inside C<\Q\E>, and any C<$> is
1356 interpreted as starting an interpolated scalar.
1358 Note also that the interpolating code needs to make decision where the
1359 interpolated scalar ends, say, whether C<"a $b -E<gt> {c}"> means
1361 "a " . $b . " -> {c}";
1367 Most the time the decision is to take the longest possible text which does
1368 not include spaces between components and contains matching braces/brackets.
1370 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1372 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> and interpolation happens
1373 (almost) as with C<qq//> constructs, but I<the substitution of C<\> followed by
1374 other chars is not performed>! Moreover, inside C<(?{BLOCK})> no processing
1375 is performed at all.
1377 Interpolation has several quirks: C<$|>, C<$(> and C<$)> are not interpolated, and
1378 constructs C<$var[SOMETHING]> are I<voted> (by several different estimators)
1379 to be an array element or C<$var> followed by a RE alternative. This is
1380 the place where the notation C<${arr[$bar]}> comes handy: C</${arr[0-9]}/>
1381 is interpreted as an array element C<-9>, not as a regular expression from
1382 variable C<$arr> followed by a digit, which is the interpretation of
1385 Note that absence of processing of C<\\> creates specific restrictions on the
1386 post-processed text: if the delimiter is C</>, one cannot get the combination
1387 C<\/> into the result of this step: C</> will finish the regular expression,
1388 C<\/> will be stripped to C</> on the previous step, and C<\\/> will be left
1389 as is. Since C</> is equivalent to C<\/> inside a regular expression, this
1390 does not matter unless the delimiter is special character for the RE engine, as
1391 in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>.
1395 This step is the last one for all the constructs except regular expressions,
1396 which are processed further.
1398 =item Interpolation of regular expressions
1400 All the previous steps were performed during the compilation of Perl code,
1401 this one happens in run time (though it may be optimized to be calculated
1402 at compile time if appropriate). After all the preprocessing performed
1403 above (and possibly after evaluation if catenation, joining, up/down-casing
1404 and C<quotemeta()>ing are involved) the resulting I<string> is passed to RE
1405 engine for compilation.
1407 Whatever happens in the RE engine is better be discussed in L<perlre>,
1408 but for the sake of continuity let us do it here.
1410 This is the first step where presence of the C<//x> switch is relevant.
1411 The RE engine scans the string left-to-right, and converts it to a finite
1414 Backslashed chars are either substituted by corresponding literal
1415 strings, or generate special nodes of the finite automaton. Characters
1416 which are special to the RE engine generate corresponding nodes. C<(?#...)>
1417 comments are ignored. All the rest is either converted to literal strings
1418 to match, or is ignored (as is whitespace and C<#>-style comments if
1421 Note that the parsing of the construct C<[...]> is performed using
1422 absolutely different rules than the rest of the regular expression.
1423 Similarly, the C<(?{...})> is only checked for matching braces.
1425 =item Optimization of regular expressions
1427 This step is listed for completeness only. Since it does not change
1428 semantics, details of this step are not documented and are subject
1433 =head2 I/O Operators
1435 There are several I/O operators you should know about.
1436 A string enclosed by backticks (grave accents) first undergoes
1437 variable substitution just like a double quoted string. It is then
1438 interpreted as a command, and the output of that command is the value
1439 of the pseudo-literal, like in a shell. In scalar context, a single
1440 string consisting of all the output is returned. In list context,
1441 a list of values is returned, one for each line of output. (You can
1442 set C<$/> to use a different line terminator.) The command is executed
1443 each time the pseudo-literal is evaluated. The status value of the
1444 command is returned in C<$?> (see L<perlvar> for the interpretation
1445 of C<$?>). Unlike in B<csh>, no translation is done on the return
1446 data--newlines remain newlines. Unlike in any of the shells, single
1447 quotes do not hide variable names in the command from interpretation.
1448 To pass a $ through to the shell you need to hide it with a backslash.
1449 The generalized form of backticks is C<qx//>. (Because backticks
1450 always undergo shell expansion as well, see L<perlsec> for
1453 Evaluating a filehandle in angle brackets yields the next line from
1454 that file (newline, if any, included), or C<undef> at end of file.
1455 Ordinarily you must assign that value to a variable, but there is one
1456 situation where an automatic assignment happens. I<If and ONLY if> the
1457 input symbol is the only thing inside the conditional of a C<while> or
1458 C<for(;;)> loop, the value is automatically assigned to the variable
1459 C<$_>. In these loop constructs, the assigned value (whether assignment
1460 is automatic or explicit) is then tested to see if it is defined.
1461 The defined test avoids problems where line has a string value
1462 that would be treated as false by perl e.g. "" or "0" with no trailing
1463 newline. (This may seem like an odd thing to you, but you'll use the
1464 construct in almost every Perl script you write.) Anyway, the following
1465 lines are equivalent to each other:
1467 while (defined($_ = <STDIN>)) { print; }
1468 while ($_ = <STDIN>) { print; }
1469 while (<STDIN>) { print; }
1470 for (;<STDIN>;) { print; }
1471 print while defined($_ = <STDIN>);
1472 print while ($_ = <STDIN>);
1473 print while <STDIN>;
1475 and this also behaves similarly, but avoids the use of $_ :
1477 while (my $line = <STDIN>) { print $line }
1479 If you really mean such values to terminate the loop they should be
1480 tested for explicitly:
1482 while (($_ = <STDIN>) ne '0') { ... }
1483 while (<STDIN>) { last unless $_; ... }
1485 In other boolean contexts, C<E<lt>I<filehandle>E<gt>> without explicit C<defined>
1486 test or comparison will solicit a warning if C<-w> is in effect.
1488 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1489 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in
1490 packages, where they would be interpreted as local identifiers rather
1491 than global.) Additional filehandles may be created with the open()
1492 function. See L<perlfunc/open()> for details on this.
1494 If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a
1495 list consisting of all the input lines is returned, one line per list
1496 element. It's easy to make a I<LARGE> data space this way, so use with
1499 The null filehandle E<lt>E<gt> is special and can be used to emulate the
1500 behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from
1501 standard input, or from each file listed on the command line. Here's
1502 how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is
1503 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
1504 gives you standard input. The @ARGV array is then processed as a list
1505 of filenames. The loop
1508 ... # code for each line
1511 is equivalent to the following Perl-like pseudo code:
1513 unshift(@ARGV, '-') unless @ARGV;
1514 while ($ARGV = shift) {
1517 ... # code for each line
1521 except that it isn't so cumbersome to say, and will actually work. It
1522 really does shift array @ARGV and put the current filename into variable
1523 $ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a
1524 synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above
1525 doesn't work because it treats E<lt>ARGVE<gt> as non-magical.)
1527 You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up
1528 containing the list of filenames you really want. Line numbers (C<$.>)
1529 continue as if the input were one big happy file. (But see example
1530 under C<eof> for how to reset line numbers on each file.)
1532 If you want to set @ARGV to your own list of files, go right ahead.
1533 This sets @ARGV to all plain text files if no @ARGV was given:
1535 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
1537 You can even set them to pipe commands. For example, this automatically
1538 filters compressed arguments through B<gzip>:
1540 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1542 If you want to pass switches into your script, you can use one of the
1543 Getopts modules or put a loop on the front like this:
1545 while ($_ = $ARGV[0], /^-/) {
1548 if (/^-D(.*)/) { $debug = $1 }
1549 if (/^-v/) { $verbose++ }
1550 # ... # other switches
1554 # ... # code for each line
1557 The E<lt>E<gt> symbol will return C<undef> for end-of-file only once.
1558 If you call it again after this it will assume you are processing another
1559 @ARGV list, and if you haven't set @ARGV, will input from STDIN.
1561 If the string inside the angle brackets is a reference to a scalar
1562 variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the
1563 filehandle to input from, or its typeglob, or a reference to the same. For example:
1568 If what's within the angle brackets is neither a filehandle nor a simple
1569 scalar variable containing a filehandle name, typeglob, or typeglob
1570 reference, it is interpreted as a filename pattern to be globbed, and
1571 either a list of filenames or the next filename in the list is returned,
1572 depending on context. This distinction is determined on syntactic
1573 grounds alone. That means C<E<lt>$xE<gt>> is always a readline from
1574 an indirect handle, but C<E<lt>$hash{key}E<gt>> is always a glob.
1575 That's because $x is a simple scalar variable, but C<$hash{key}> is
1576 not--it's a hash element.
1578 One level of double-quote interpretation is done first, but you can't
1579 say C<E<lt>$fooE<gt>> because that's an indirect filehandle as explained
1580 in the previous paragraph. (In older versions of Perl, programmers
1581 would insert curly brackets to force interpretation as a filename glob:
1582 C<E<lt>${foo}E<gt>>. These days, it's considered cleaner to call the
1583 internal function directly as C<glob($foo)>, which is probably the right
1584 way to have done it in the first place.) Example:
1592 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1598 In fact, it's currently implemented that way. (Which means it will not
1599 work on filenames with spaces in them unless you have csh(1) on your
1600 machine.) Of course, the shortest way to do the above is:
1604 Because globbing invokes a shell, it's often faster to call readdir() yourself
1605 and do your own grep() on the filenames. Furthermore, due to its current
1606 implementation of using a shell, the glob() routine may get "Arg list too
1607 long" errors (unless you've installed tcsh(1L) as F</bin/csh>).
1609 A glob evaluates its (embedded) argument only when it is starting a new
1610 list. All values must be read before it will start over. In a list
1611 context this isn't important, because you automatically get them all
1612 anyway. In scalar context, however, the operator returns the next value
1613 each time it is called, or a C<undef> value if you've just run out. As
1614 for filehandles an automatic C<defined> is generated when the glob
1615 occurs in the test part of a C<while> or C<for> - because legal glob returns
1616 (e.g. a file called F<0>) would otherwise terminate the loop.
1617 Again, C<undef> is returned only once. So if you're expecting a single value
1618 from a glob, it is much better to say
1620 ($file) = <blurch*>;
1626 because the latter will alternate between returning a filename and
1629 It you're trying to do variable interpolation, it's definitely better
1630 to use the glob() function, because the older notation can cause people
1631 to become confused with the indirect filehandle notation.
1633 @files = glob("$dir/*.[ch]");
1634 @files = glob($files[$i]);
1636 =head2 Constant Folding
1638 Like C, Perl does a certain amount of expression evaluation at
1639 compile time, whenever it determines that all arguments to an
1640 operator are static and have no side effects. In particular, string
1641 concatenation happens at compile time between literals that don't do
1642 variable substitution. Backslash interpretation also happens at
1643 compile time. You can say
1645 'Now is the time for all' . "\n" .
1646 'good men to come to.'
1648 and this all reduces to one string internally. Likewise, if
1651 foreach $file (@filenames) {
1652 if (-s $file > 5 + 100 * 2**16) { }
1655 the compiler will precompute the number that
1656 expression represents so that the interpreter
1659 =head2 Bitwise String Operators
1661 Bitstrings of any size may be manipulated by the bitwise operators
1664 If the operands to a binary bitwise op are strings of different sizes,
1665 B<or> and B<xor> ops will act as if the shorter operand had additional
1666 zero bits on the right, while the B<and> op will act as if the longer
1667 operand were truncated to the length of the shorter.
1669 # ASCII-based examples
1670 print "j p \n" ^ " a h"; # prints "JAPH\n"
1671 print "JA" | " ph\n"; # prints "japh\n"
1672 print "japh\nJunk" & '_____'; # prints "JAPH\n";
1673 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
1675 If you are intending to manipulate bitstrings, you should be certain that
1676 you're supplying bitstrings: If an operand is a number, that will imply
1677 a B<numeric> bitwise operation. You may explicitly show which type of
1678 operation you intend by using C<""> or C<0+>, as in the examples below.
1680 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
1681 $foo = '150' | 105 ; # yields 255
1682 $foo = 150 | '105'; # yields 255
1683 $foo = '150' | '105'; # yields string '155' (under ASCII)
1685 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
1686 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
1688 =head2 Integer Arithmetic
1690 By default Perl assumes that it must do most of its arithmetic in
1691 floating point. But by saying
1695 you may tell the compiler that it's okay to use integer operations
1696 from here to the end of the enclosing BLOCK. An inner BLOCK may
1697 countermand this by saying
1701 which lasts until the end of that BLOCK.
1703 The bitwise operators ("&", "|", "^", "~", "<<", and ">>") always
1704 produce integral results. (But see also L<Bitwise String Operators>.)
1705 However, C<use integer> still has meaning
1706 for them. By default, their results are interpreted as unsigned
1707 integers. However, if C<use integer> is in effect, their results are
1708 interpreted as signed integers. For example, C<~0> usually evaluates
1709 to a large integral value. However, C<use integer; ~0> is -1 on twos-complement machines.
1711 =head2 Floating-point Arithmetic
1713 While C<use integer> provides integer-only arithmetic, there is no
1714 similar ways to provide rounding or truncation at a certain number of
1715 decimal places. For rounding to a certain number of digits, sprintf()
1716 or printf() is usually the easiest route.
1718 Floating-point numbers are only approximations to what a mathematician
1719 would call real numbers. There are infinitely more reals than floats,
1720 so some corners must be cut. For example:
1722 printf "%.20g\n", 123456789123456789;
1723 # produces 123456789123456784
1725 Testing for exact equality of floating-point equality or inequality is
1726 not a good idea. Here's a (relatively expensive) work-around to compare
1727 whether two floating-point numbers are equal to a particular number of
1728 decimal places. See Knuth, volume II, for a more robust treatment of
1732 my ($X, $Y, $POINTS) = @_;
1734 $tX = sprintf("%.${POINTS}g", $X);
1735 $tY = sprintf("%.${POINTS}g", $Y);
1739 The POSIX module (part of the standard perl distribution) implements
1740 ceil(), floor(), and a number of other mathematical and trigonometric
1741 functions. The Math::Complex module (part of the standard perl
1742 distribution) defines a number of mathematical functions that can also
1743 work on real numbers. Math::Complex not as efficient as POSIX, but
1744 POSIX can't work with complex numbers.
1746 Rounding in financial applications can have serious implications, and
1747 the rounding method used should be specified precisely. In these
1748 cases, it probably pays not to trust whichever system rounding is
1749 being used by Perl, but to instead implement the rounding function you
1752 =head2 Bigger Numbers
1754 The standard Math::BigInt and Math::BigFloat modules provide
1755 variable precision arithmetic and overloaded operators.
1756 At the cost of some space and considerable speed, they
1757 avoid the normal pitfalls associated with limited-precision
1761 $x = Math::BigInt->new('123456789123456789');
1764 # prints +15241578780673678515622620750190521