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.
43 =head2 Terms and List Operators (Leftward)
45 A TERM has the highest precedence in Perl. They includes variables,
46 quote and quote-like operators, any expression in parentheses,
47 and any function whose arguments are parenthesized. Actually, there
48 aren't really functions in this sense, just list operators and unary
49 operators behaving as functions because you put parentheses around
50 the arguments. These are all documented in L<perlfunc>.
52 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
53 is followed by a left parenthesis as the next token, the operator and
54 arguments within parentheses are taken to be of highest precedence,
55 just like a normal function call.
57 In the absence of parentheses, the precedence of list operators such as
58 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
59 whether you are looking at the left side or the right side of the operator.
62 @ary = (1, 3, sort 4, 2);
63 print @ary; # prints 1324
65 the commas on the right of the sort are evaluated before the sort, but
66 the commas on the left are evaluated after. In other words, list
67 operators tend to gobble up all the arguments that follow them, and
68 then act like a simple TERM with regard to the preceding expression.
69 Note that you have to be careful with parentheses:
71 # These evaluate exit before doing the print:
72 print($foo, exit); # Obviously not what you want.
73 print $foo, exit; # Nor is this.
75 # These do the print before evaluating exit:
76 (print $foo), exit; # This is what you want.
77 print($foo), exit; # Or this.
78 print ($foo), exit; # Or even this.
82 print ($foo & 255) + 1, "\n";
84 probably doesn't do what you expect at first glance. See
85 L<Named Unary Operators> for more discussion of this.
87 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
88 well as subroutine and method calls, and the anonymous
89 constructors C<[]> and C<{}>.
91 See also L<Quote and Quote-like Operators> toward the end of this section,
92 as well as L<"I/O Operators">.
94 =head2 The Arrow Operator
96 Just as in C and C++, "C<-E<gt>>" is an infix dereference operator. If the
97 right side is either a C<[...]> or C<{...}> subscript, then the left side
98 must be either a hard or symbolic reference to an array or hash (or
99 a location capable of holding a hard reference, if it's an lvalue (assignable)).
102 Otherwise, the right side is a method name or a simple scalar variable
103 containing the method name, and the left side must either be an object
104 (a blessed reference) or a class name (that is, a package name).
107 =head2 Auto-increment and Auto-decrement
109 "++" and "--" work as in C. That is, if placed before a variable, they
110 increment or decrement the variable before returning the value, and if
111 placed after, increment or decrement the variable after returning the value.
113 The auto-increment operator has a little extra builtin magic to it. If
114 you increment a variable that is numeric, or that has ever been used in
115 a numeric context, you get a normal increment. If, however, the
116 variable has been used in only string contexts since it was set, and
117 has a value that is not null and matches the pattern
118 C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each
119 character within its range, with carry:
121 print ++($foo = '99'); # prints '100'
122 print ++($foo = 'a0'); # prints 'a1'
123 print ++($foo = 'Az'); # prints 'Ba'
124 print ++($foo = 'zz'); # prints 'aaa'
126 The auto-decrement operator is not magical.
128 =head2 Exponentiation
130 Binary "**" is the exponentiation operator. Note that it binds even more
131 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
132 implemented using C's pow(3) function, which actually works on doubles
135 =head2 Symbolic Unary Operators
137 Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
138 precedence version of this.
140 Unary "-" performs arithmetic negation if the operand is numeric. If
141 the operand is an identifier, a string consisting of a minus sign
142 concatenated with the identifier is returned. Otherwise, if the string
143 starts with a plus or minus, a string starting with the opposite sign
144 is returned. One effect of these rules is that C<-bareword> is equivalent
147 Unary "~" performs bitwise negation, i.e., 1's complement.
148 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
150 Unary "+" has no effect whatsoever, even on strings. It is useful
151 syntactically for separating a function name from a parenthesized expression
152 that would otherwise be interpreted as the complete list of function
153 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
155 Unary "\" creates a reference to whatever follows it. See L<perlref>.
156 Do not confuse this behavior with the behavior of backslash within a
157 string, although both forms do convey the notion of protecting the next
158 thing from interpretation.
160 =head2 Binding Operators
162 Binary "=~" binds a scalar expression to a pattern match. Certain operations
163 search or modify the string $_ by default. This operator makes that kind
164 of operation work on some other string. The right argument is a search
165 pattern, substitution, or transliteration. The left argument is what is
166 supposed to be searched, substituted, or transliterated instead of the default
167 $_. The return value indicates the success of the operation. (If the
168 right argument is an expression rather than a search pattern,
169 substitution, or transliteration, it is interpreted as a search pattern at run
170 time. This can be is less efficient than an explicit search, because the
171 pattern must be compiled every time the expression is evaluated.
173 Binary "!~" is just like "=~" except the return value is negated in
176 =head2 Multiplicative Operators
178 Binary "*" multiplies two numbers.
180 Binary "/" divides two numbers.
182 Binary "%" computes the modulus of two numbers. Given integer
183 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
184 C<$a> minus the largest multiple of C<$b> that is not greater than
185 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
186 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
187 result will be less than or equal to zero).
189 Note than when C<use integer> is in scope "%" give you direct access
190 to the modulus operator as implemented by your C compiler. This
191 operator is not as well defined for negative operands, but it will
194 Binary "x" is the repetition operator. In a scalar context, it
195 returns a string consisting of the left operand repeated the number of
196 times specified by the right operand. In a list context, if the left
197 operand is a list in parentheses, it repeats the list.
199 print '-' x 80; # print row of dashes
201 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
203 @ones = (1) x 80; # a list of 80 1's
204 @ones = (5) x @ones; # set all elements to 5
207 =head2 Additive Operators
209 Binary "+" returns the sum of two numbers.
211 Binary "-" returns the difference of two numbers.
213 Binary "." concatenates two strings.
215 =head2 Shift Operators
217 Binary "<<" returns the value of its left argument shifted left by the
218 number of bits specified by the right argument. Arguments should be
219 integers. (See also L<Integer Arithmetic>.)
221 Binary ">>" returns the value of its left argument shifted right by
222 the number of bits specified by the right argument. Arguments should
223 be integers. (See also L<Integer Arithmetic>.)
225 =head2 Named Unary Operators
227 The various named unary operators are treated as functions with one
228 argument, with optional parentheses. These include the filetest
229 operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
231 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
232 is followed by a left parenthesis as the next token, the operator and
233 arguments within parentheses are taken to be of highest precedence,
234 just like a normal function call. Examples:
236 chdir $foo || die; # (chdir $foo) || die
237 chdir($foo) || die; # (chdir $foo) || die
238 chdir ($foo) || die; # (chdir $foo) || die
239 chdir +($foo) || die; # (chdir $foo) || die
241 but, because * is higher precedence than ||:
243 chdir $foo * 20; # chdir ($foo * 20)
244 chdir($foo) * 20; # (chdir $foo) * 20
245 chdir ($foo) * 20; # (chdir $foo) * 20
246 chdir +($foo) * 20; # chdir ($foo * 20)
248 rand 10 * 20; # rand (10 * 20)
249 rand(10) * 20; # (rand 10) * 20
250 rand (10) * 20; # (rand 10) * 20
251 rand +(10) * 20; # rand (10 * 20)
253 See also L<"Terms and List Operators (Leftward)">.
255 =head2 Relational Operators
257 Binary "E<lt>" returns true if the left argument is numerically less than
260 Binary "E<gt>" returns true if the left argument is numerically greater
261 than the right argument.
263 Binary "E<lt>=" returns true if the left argument is numerically less than
264 or equal to the right argument.
266 Binary "E<gt>=" returns true if the left argument is numerically greater
267 than or equal to the right argument.
269 Binary "lt" returns true if the left argument is stringwise less than
272 Binary "gt" returns true if the left argument is stringwise greater
273 than the right argument.
275 Binary "le" returns true if the left argument is stringwise less than
276 or equal to the right argument.
278 Binary "ge" returns true if the left argument is stringwise greater
279 than or equal to the right argument.
281 =head2 Equality Operators
283 Binary "==" returns true if the left argument is numerically equal to
286 Binary "!=" returns true if the left argument is numerically not equal
287 to the right argument.
289 Binary "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left
290 argument is numerically less than, equal to, or greater than the right
293 Binary "eq" returns true if the left argument is stringwise equal to
296 Binary "ne" returns true if the left argument is stringwise not equal
297 to the right argument.
299 Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise
300 less than, equal to, or greater than the right argument.
302 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
303 by the current locale if C<use locale> is in effect. See L<perllocale>.
307 Binary "&" returns its operators ANDed together bit by bit.
308 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
310 =head2 Bitwise Or and Exclusive Or
312 Binary "|" returns its operators ORed together bit by bit.
313 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
315 Binary "^" returns its operators XORed together bit by bit.
316 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
318 =head2 C-style Logical And
320 Binary "&&" performs a short-circuit logical AND operation. That is,
321 if the left operand is false, the right operand is not even evaluated.
322 Scalar or list context propagates down to the right operand if it
325 =head2 C-style Logical Or
327 Binary "||" performs a short-circuit logical OR operation. That is,
328 if the left operand is true, the right operand is not even evaluated.
329 Scalar or list context propagates down to the right operand if it
332 The C<||> and C<&&> operators differ from C's in that, rather than returning
333 0 or 1, they return the last value evaluated. Thus, a reasonably portable
334 way to find out the home directory (assuming it's not "0") might be:
336 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
337 (getpwuid($<))[7] || die "You're homeless!\n";
339 As more readable alternatives to C<&&> and C<||>, Perl provides "and" and
340 "or" operators (see below). The short-circuit behavior is identical. The
341 precedence of "and" and "or" is much lower, however, so that you can
342 safely use them after a list operator without the need for
345 unlink "alpha", "beta", "gamma"
346 or gripe(), next LINE;
348 With the C-style operators that would have been written like this:
350 unlink("alpha", "beta", "gamma")
351 || (gripe(), next LINE);
353 =head2 Range Operator
355 Binary ".." is the range operator, which is really two different
356 operators depending on the context. In a list context, it returns an
357 array of values counting (by ones) from the left value to the right
358 value. This is useful for writing C<for (1..10)> loops and for doing
359 slice operations on arrays. Be aware that under the current implementation,
360 a temporary array is created, so you'll burn a lot of memory if you
361 write something like this:
363 for (1 .. 1_000_000) {
367 In a scalar context, ".." returns a boolean value. The operator is
368 bistable, like a flip-flop, and emulates the line-range (comma) operator
369 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
370 own boolean state. It is false as long as its left operand is false.
371 Once the left operand is true, the range operator stays true until the
372 right operand is true, I<AFTER> which the range operator becomes false
373 again. (It doesn't become false till the next time the range operator is
374 evaluated. It can test the right operand and become false on the same
375 evaluation it became true (as in B<awk>), but it still returns true once.
376 If you don't want it to test the right operand till the next evaluation
377 (as in B<sed>), use three dots ("...") instead of two.) The right
378 operand is not evaluated while the operator is in the "false" state, and
379 the left operand is not evaluated while the operator is in the "true"
380 state. The precedence is a little lower than || and &&. The value
381 returned is either the null string for false, or a sequence number
382 (beginning with 1) for true. The sequence number is reset for each range
383 encountered. The final sequence number in a range has the string "E0"
384 appended to it, which doesn't affect its numeric value, but gives you
385 something to search for if you want to exclude the endpoint. You can
386 exclude the beginning point by waiting for the sequence number to be
387 greater than 1. If either operand of scalar ".." is a numeric literal,
388 that operand is implicitly compared to the C<$.> variable, the current
389 line number. Examples:
391 As a scalar operator:
393 if (101 .. 200) { print; } # print 2nd hundred lines
394 next line if (1 .. /^$/); # skip header lines
395 s/^/> / if (/^$/ .. eof()); # quote body
399 for (101 .. 200) { print; } # print $_ 100 times
400 @foo = @foo[0 .. $#foo]; # an expensive no-op
401 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
403 The range operator (in a list context) makes use of the magical
404 auto-increment algorithm if the operands are strings. You
407 @alphabet = ('A' .. 'Z');
409 to get all the letters of the alphabet, or
411 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
413 to get a hexadecimal digit, or
415 @z2 = ('01' .. '31'); print $z2[$mday];
417 to get dates with leading zeros. If the final value specified is not
418 in the sequence that the magical increment would produce, the sequence
419 goes until the next value would be longer than the final value
422 =head2 Conditional Operator
424 Ternary "?:" is the conditional operator, just as in C. It works much
425 like an if-then-else. If the argument before the ? is true, the
426 argument before the : is returned, otherwise the argument after the :
427 is returned. For example:
429 printf "I have %d dog%s.\n", $n,
430 ($n == 1) ? '' : "s";
432 Scalar or list context propagates downward into the 2nd
433 or 3rd argument, whichever is selected.
435 $a = $ok ? $b : $c; # get a scalar
436 @a = $ok ? @b : @c; # get an array
437 $a = $ok ? @b : @c; # oops, that's just a count!
439 The operator may be assigned to if both the 2nd and 3rd arguments are
440 legal lvalues (meaning that you can assign to them):
442 ($a_or_b ? $a : $b) = $c;
444 This is not necessarily guaranteed to contribute to the readability of your program.
446 =head2 Assignment Operators
448 "=" is the ordinary assignment operator.
450 Assignment operators work as in C. That is,
458 although without duplicating any side effects that dereferencing the lvalue
459 might trigger, such as from tie(). Other assignment operators work similarly.
460 The following are recognized:
467 Note that while these are grouped by family, they all have the precedence
470 Unlike in C, the assignment operator produces a valid lvalue. Modifying
471 an assignment is equivalent to doing the assignment and then modifying
472 the variable that was assigned to. This is useful for modifying
473 a copy of something, like this:
475 ($tmp = $global) =~ tr [A-Z] [a-z];
486 =head2 Comma Operator
488 Binary "," is the comma operator. In a scalar context it evaluates
489 its left argument, throws that value away, then evaluates its right
490 argument and returns that value. This is just like C's comma operator.
492 In a list context, it's just the list argument separator, and inserts
493 both its arguments into the list.
495 The =E<gt> digraph is mostly just a synonym for the comma operator. It's useful for
496 documenting arguments that come in pairs. As of release 5.001, it also forces
497 any word to the left of it to be interpreted as a string.
499 =head2 List Operators (Rightward)
501 On the right side of a list operator, it has very low precedence,
502 such that it controls all comma-separated expressions found there.
503 The only operators with lower precedence are the logical operators
504 "and", "or", and "not", which may be used to evaluate calls to list
505 operators without the need for extra parentheses:
507 open HANDLE, "filename"
508 or die "Can't open: $!\n";
510 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
514 Unary "not" returns the logical negation of the expression to its right.
515 It's the equivalent of "!" except for the very low precedence.
519 Binary "and" returns the logical conjunction of the two surrounding
520 expressions. It's equivalent to && except for the very low
521 precedence. This means that it short-circuits: i.e., the right
522 expression is evaluated only if the left expression is true.
524 =head2 Logical or and Exclusive Or
526 Binary "or" returns the logical disjunction of the two surrounding
527 expressions. It's equivalent to || except for the very low
528 precedence. This means that it short-circuits: i.e., the right
529 expression is evaluated only if the left expression is false.
531 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
532 It cannot short circuit, of course.
534 =head2 C Operators Missing From Perl
536 Here is what C has that Perl doesn't:
542 Address-of operator. (But see the "\" operator for taking a reference.)
546 Dereference-address operator. (Perl's prefix dereferencing
547 operators are typed: $, @, %, and &.)
551 Type casting operator.
555 =head2 Quote and Quote-like Operators
557 While we usually think of quotes as literal values, in Perl they
558 function as operators, providing various kinds of interpolating and
559 pattern matching capabilities. Perl provides customary quote characters
560 for these behaviors, but also provides a way for you to choose your
561 quote character for any of them. In the following table, a C<{}> represents
562 any pair of delimiters you choose. Non-bracketing delimiters use
563 the same character fore and aft, but the 4 sorts of brackets
564 (round, angle, square, curly) will all nest.
566 Customary Generic Meaning Interpolates
571 // m{} Pattern match yes
572 s{}{} Substitution yes
573 tr{}{} Transliteration no (but see below)
575 Note that there can be whitespace between the operator and the quoting
576 characters, except when C<#> is being used as the quoting character.
577 C<q#foo#> is parsed as being the string C<foo>, while C<q #foo#> is the
578 operator C<q> followed by a comment. Its argument will be taken from the
579 next line. This allows you to write:
581 s {foo} # Replace foo
584 For constructs that do interpolation, variables beginning with "C<$>"
585 or "C<@>" are interpolated, as are the following sequences. Within
586 a transliteration, the first ten of these sequences may be used.
593 \a alarm (bell) (BEL)
599 \l lowercase next char
600 \u uppercase next char
603 \E end case modification
604 \Q quote non-word characters till \E
606 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
607 and C<\U> is taken from the current locale. See L<perllocale>.
609 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
610 An unescaped C<$> or C<@> interpolates the corresponding variable,
611 while escaping will cause the literal string C<\$> to be inserted.
612 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
614 Patterns are subject to an additional level of interpretation as a
615 regular expression. This is done as a second pass, after variables are
616 interpolated, so that regular expressions may be incorporated into the
617 pattern from the variables. If this is not what you want, use C<\Q> to
618 interpolate a variable literally.
620 Apart from the above, there are no multiple levels of interpolation. In
621 particular, contrary to the expectations of shell programmers, back-quotes
622 do I<NOT> interpolate within double quotes, nor do single quotes impede
623 evaluation of variables when used within double quotes.
625 =head2 Regexp Quote-Like Operators
627 Here are the quote-like operators that apply to pattern
628 matching and related activities.
634 This is just like the C</pattern/> search, except that it matches only
635 once between calls to the reset() operator. This is a useful
636 optimization when you want to see only the first occurrence of
637 something in each file of a set of files, for instance. Only C<??>
638 patterns local to the current package are reset.
640 This usage is vaguely deprecated, and may be removed in some future
643 =item m/PATTERN/cgimosx
645 =item /PATTERN/cgimosx
647 Searches a string for a pattern match, and in a scalar context returns
648 true (1) or false (''). If no string is specified via the C<=~> or
649 C<!~> operator, the $_ string is searched. (The string specified with
650 C<=~> need not be an lvalue--it may be the result of an expression
651 evaluation, but remember the C<=~> binds rather tightly.) See also
653 See L<perllocale> for discussion of additional considerations which apply
654 when C<use locale> is in effect.
658 c Do not reset search position on a failed match when /g is in effect.
659 g Match globally, i.e., find all occurrences.
660 i Do case-insensitive pattern matching.
661 m Treat string as multiple lines.
662 o Compile pattern only once.
663 s Treat string as single line.
664 t Taint $1 etc. if target string is tainted.
665 x Use extended regular expressions.
667 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
668 you can use any pair of non-alphanumeric, non-whitespace characters as
669 delimiters. This is particularly useful for matching Unix path names
670 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
671 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
673 PATTERN may contain variables, which will be interpolated (and the
674 pattern recompiled) every time the pattern search is evaluated. (Note
675 that C<$)> and C<$|> might not be interpolated because they look like
676 end-of-string tests.) If you want such a pattern to be compiled only
677 once, add a C</o> after the trailing delimiter. This avoids expensive
678 run-time recompilations, and is useful when the value you are
679 interpolating won't change over the life of the script. However, mentioning
680 C</o> constitutes a promise that you won't change the variables in the pattern.
681 If you change them, Perl won't even notice.
683 If the PATTERN evaluates to a null string, the last
684 successfully matched regular expression is used instead.
686 If used in a context that requires a list value, a pattern match returns a
687 list consisting of the subexpressions matched by the parentheses in the
688 pattern, i.e., (C<$1>, $2, $3...). (Note that here $1 etc. are also set, and
689 that this differs from Perl 4's behavior.) If there are no parentheses,
690 the return value is the list C<(1)> for success or C<('')> upon failure.
691 With parentheses, C<()> is returned upon failure.
695 open(TTY, '/dev/tty');
696 <TTY> =~ /^y/i && foo(); # do foo if desired
698 if (/Version: *([0-9.]*)/) { $version = $1; }
700 next if m#^/usr/spool/uucp#;
705 print if /$arg/o; # compile only once
708 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
710 This last example splits $foo into the first two words and the
711 remainder of the line, and assigns those three fields to $F1, $F2, and
712 $Etc. The conditional is true if any variables were assigned, i.e., if
715 The C</g> modifier specifies global pattern matching--that is, matching
716 as many times as possible within the string. How it behaves depends on
717 the context. In a list context, it returns a list of all the
718 substrings matched by all the parentheses in the regular expression.
719 If there are no parentheses, it returns a list of all the matched
720 strings, as if there were parentheses around the whole pattern.
722 In a scalar context, C<m//g> iterates through the string, returning TRUE
723 each time it matches, and FALSE when it eventually runs out of matches.
724 (In other words, it remembers where it left off last time and restarts
725 the search at that point. You can actually find the current match
726 position of a string or set it using the pos() function; see
727 L<perlfunc/pos>.) A failed match normally resets the search position to
728 the beginning of the string, but you can avoid that by adding the C</c>
729 modifier (e.g. C<m//gc>). Modifying the target string also resets the
732 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
733 zero-width assertion that matches the exact position where the previous
734 C<m//g>, if any, left off. The C<\G> assertion is not supported without
735 the C</g> modifier; currently, without C</g>, C<\G> behaves just like
736 C<\A>, but that's accidental and may change in the future.
741 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
744 $/ = ""; $* = 1; # $* deprecated in modern perls
745 while (defined($paragraph = <>)) {
746 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
750 print "$sentences\n";
752 # using m//gc with \G
756 print $1 while /(o)/gc; print "', pos=", pos, "\n";
758 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
760 print $1 while /(p)/gc; print "', pos=", pos, "\n";
763 The last example should print:
772 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
773 combine several regexps like this to process a string part-by-part,
774 doing different actions depending on which regexp matched. Each
775 regexp tries to match where the previous one leaves off.
778 $url = new URI::URL "http://www/"; die if $url eq "xXx";
782 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
783 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
784 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
785 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
786 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
787 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
788 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
789 print ". That's all!\n";
792 Here is the output (split into several lines):
794 line-noise lowercase line-noise lowercase UPPERCASE line-noise
795 UPPERCASE line-noise lowercase line-noise lowercase line-noise
796 lowercase lowercase line-noise lowercase lowercase line-noise
797 MiXeD line-noise. That's all!
803 A single-quoted, literal string. A backslash represents a backslash
804 unless followed by the delimiter or another backslash, in which case
805 the delimiter or backslash is interpolated.
807 $foo = q!I said, "You said, 'She said it.'"!;
808 $bar = q('This is it.');
809 $baz = '\n'; # a two-character string
815 A double-quoted, interpolated string.
818 (*** The previous line contains the naughty word "$1".\n)
819 if /(tcl|rexx|python)/; # :-)
820 $baz = "\n"; # a one-character string
826 A string which is interpolated and then executed as a system command.
827 The collected standard output of the command is returned. In scalar
828 context, it comes back as a single (potentially multi-line) string.
829 In list context, returns a list of lines (however you've defined lines
830 with $/ or $INPUT_RECORD_SEPARATOR).
834 Note that how the string gets evaluated is entirely subject to the
835 command interpreter on your system. On most platforms, you will have
836 to protect shell metacharacters if you want them treated literally.
837 On some platforms (notably DOS-like ones), the shell may not be
838 capable of dealing with multiline commands, so putting newlines in
839 the string may not get you what you want. You may be able to evaluate
840 multiple commands in a single line by separating them with the command
841 separator character, if your shell supports that (e.g. C<;> on many Unix
842 shells; C<&> on the Windows NT C<cmd> shell).
844 Beware that some command shells may place restrictions on the length
845 of the command line. You must ensure your strings don't exceed this
846 limit after any necessary interpolations. See the platform-specific
847 release notes for more details about your particular environment.
849 Also realize that using this operator frequently leads to unportable
852 See L<"I/O Operators"> for more discussion.
856 Returns a list of the words extracted out of STRING, using embedded
857 whitespace as the word delimiters. It is exactly equivalent to
859 split(' ', q/STRING/);
861 Some frequently seen examples:
863 use POSIX qw( setlocale localeconv )
864 @EXPORT = qw( foo bar baz );
866 A common mistake is to try to separate the words with comma or to put
867 comments into a multi-line qw-string. For this reason the C<-w>
868 switch produce warnings if the STRING contains the "," or the "#"
871 =item s/PATTERN/REPLACEMENT/egimosx
873 Searches a string for a pattern, and if found, replaces that pattern
874 with the replacement text and returns the number of substitutions
875 made. Otherwise it returns false (specifically, the empty string).
877 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
878 variable is searched and modified. (The string specified with C<=~> must
879 be a scalar variable, an array element, a hash element, or an assignment
880 to one of those, i.e., an lvalue.)
882 If the delimiter chosen is single quote, no variable interpolation is
883 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
884 PATTERN contains a $ that looks like a variable rather than an
885 end-of-string test, the variable will be interpolated into the pattern
886 at run-time. If you want the pattern compiled only once the first time
887 the variable is interpolated, use the C</o> option. If the pattern
888 evaluates to a null string, the last successfully executed regular
889 expression is used instead. See L<perlre> for further explanation on these.
890 See L<perllocale> for discussion of additional considerations which apply
891 when C<use locale> is in effect.
895 e Evaluate the right side as an expression.
896 g Replace globally, i.e., all occurrences.
897 i Do case-insensitive pattern matching.
898 m Treat string as multiple lines.
899 o Compile pattern only once.
900 s Treat string as single line.
901 x Use extended regular expressions.
903 Any non-alphanumeric, non-whitespace delimiter may replace the
904 slashes. If single quotes are used, no interpretation is done on the
905 replacement string (the C</e> modifier overrides this, however). Unlike
906 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
907 text is not evaluated as a command. If the
908 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
909 pair of quotes, which may or may not be bracketing quotes, e.g.,
910 C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the
911 replacement portion to be interpreted as a full-fledged Perl expression
912 and eval()ed right then and there. It is, however, syntax checked at
917 s/\bgreen\b/mauve/g; # don't change wintergreen
919 $path =~ s|/usr/bin|/usr/local/bin|;
921 s/Login: $foo/Login: $bar/; # run-time pattern
923 ($foo = $bar) =~ s/this/that/;
925 $count = ($paragraph =~ s/Mister\b/Mr./g);
928 s/\d+/$&*2/e; # yields 'abc246xyz'
929 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
930 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
932 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
933 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
934 s/^=(\w+)/&pod($1)/ge; # use function call
936 # /e's can even nest; this will expand
937 # simple embedded variables in $_
942 /\* # Match the opening delimiter.
943 .*? # Match a minimal number of characters.
944 \*/ # Match the closing delimiter.
947 s/^\s*(.*?)\s*$/$1/; # trim white space
949 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
951 Note the use of $ instead of \ in the last example. Unlike
952 B<sed>, we use the \E<lt>I<digit>E<gt> form in only the left hand side.
953 Anywhere else it's $E<lt>I<digit>E<gt>.
955 Occasionally, you can't use just a C</g> to get all the changes
956 to occur. Here are two common cases:
958 # put commas in the right places in an integer
959 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4
960 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5
962 # expand tabs to 8-column spacing
963 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
966 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
968 =item y/SEARCHLIST/REPLACEMENTLIST/cds
970 Transliterates all occurrences of the characters found in the search list
971 with the corresponding character in the replacement list. It returns
972 the number of characters replaced or deleted. If no string is
973 specified via the =~ or !~ operator, the $_ string is transliterated. (The
974 string specified with =~ must be a scalar variable, an array element, a
975 hash element, or an assignment to one of those, i.e., an lvalue.)
976 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
977 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
978 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
979 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
980 its own pair of quotes, which may or may not be bracketing quotes,
981 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
985 c Complement the SEARCHLIST.
986 d Delete found but unreplaced characters.
987 s Squash duplicate replaced characters.
989 If the C</c> modifier is specified, the SEARCHLIST character set is
990 complemented. If the C</d> modifier is specified, any characters specified
991 by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note
992 that this is slightly more flexible than the behavior of some B<tr>
993 programs, which delete anything they find in the SEARCHLIST, period.)
994 If the C</s> modifier is specified, sequences of characters that were
995 transliterated to the same character are squashed down to a single instance of the
998 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
999 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1000 than the SEARCHLIST, the final character is replicated till it is long
1001 enough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated.
1002 This latter is useful for counting characters in a class or for
1003 squashing character sequences in a class.
1007 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1009 $cnt = tr/*/*/; # count the stars in $_
1011 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1013 $cnt = tr/0-9//; # count the digits in $_
1015 tr/a-zA-Z//s; # bookkeeper -> bokeper
1017 ($HOST = $host) =~ tr/a-z/A-Z/;
1019 tr/a-zA-Z/ /cs; # change non-alphas to single space
1022 [\000-\177]; # delete 8th bit
1024 If multiple transliterations are given for a character, only the first one is used:
1028 will transliterate any A to X.
1030 Note that because the transliteration table is built at compile time, neither
1031 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1032 interpolation. That means that if you want to use variables, you must use
1035 eval "tr/$oldlist/$newlist/";
1038 eval "tr/$oldlist/$newlist/, 1" or die $@;
1042 =head2 I/O Operators
1044 There are several I/O operators you should know about.
1045 A string enclosed by backticks (grave accents) first undergoes
1046 variable substitution just like a double quoted string. It is then
1047 interpreted as a command, and the output of that command is the value
1048 of the pseudo-literal, like in a shell. In a scalar context, a single
1049 string consisting of all the output is returned. In a list context,
1050 a list of values is returned, one for each line of output. (You can
1051 set C<$/> to use a different line terminator.) The command is executed
1052 each time the pseudo-literal is evaluated. The status value of the
1053 command is returned in C<$?> (see L<perlvar> for the interpretation
1054 of C<$?>). Unlike in B<csh>, no translation is done on the return
1055 data--newlines remain newlines. Unlike in any of the shells, single
1056 quotes do not hide variable names in the command from interpretation.
1057 To pass a $ through to the shell you need to hide it with a backslash.
1058 The generalized form of backticks is C<qx//>. (Because backticks
1059 always undergo shell expansion as well, see L<perlsec> for
1062 Evaluating a filehandle in angle brackets yields the next line from
1063 that file (newline, if any, included), or C<undef> at end of file.
1064 Ordinarily you must assign that value to a variable, but there is one
1065 situation where an automatic assignment happens. I<If and ONLY if> the
1066 input symbol is the only thing inside the conditional of a C<while> or
1067 C<for(;;)> loop, the value is automatically assigned to the variable
1068 C<$_>. In these loop constructs, the assigned value (whether assignment
1069 is automatic or explcit) is then tested to see if it is defined.
1070 The defined test avoids problems where line has a string value
1071 that would be treated as false by perl e.g. "" or "0" with no trailing
1072 newline. (This may seem like an odd thing to you, but you'll use the
1073 construct in almost every Perl script you write.) Anyway, the following
1074 lines are equivalent to each other:
1076 while (defined($_ = <STDIN>)) { print; }
1077 while ($_ = <STDIN>) { print; }
1078 while (<STDIN>) { print; }
1079 for (;<STDIN>;) { print; }
1080 print while defined($_ = <STDIN>);
1081 print while ($_ = <STDIN>);
1082 print while <STDIN>;
1084 and this also behaves similarly, but avoids the use of $_ :
1086 while (my $line = <STDIN>) { print $line }
1088 If you really mean such values to terminate the loop they should be
1089 tested for explcitly:
1091 while (($_ = <STDIN>) ne '0') { ... }
1092 while (<STDIN>) { last unless $_; ... }
1094 In other boolean contexts C<E<lt>I<filehandle>E<gt>> without explcit C<defined>
1095 test or comparison will solicit a warning if C<-w> is in effect.
1097 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1098 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in
1099 packages, where they would be interpreted as local identifiers rather
1100 than global.) Additional filehandles may be created with the open()
1101 function. See L<perlfunc/open()> for details on this.
1103 If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a
1104 list consisting of all the input lines is returned, one line per list
1105 element. It's easy to make a I<LARGE> data space this way, so use with
1108 The null filehandle E<lt>E<gt> is special and can be used to emulate the
1109 behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from
1110 standard input, or from each file listed on the command line. Here's
1111 how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is
1112 checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened
1113 gives you standard input. The @ARGV array is then processed as a list
1114 of filenames. The loop
1117 ... # code for each line
1120 is equivalent to the following Perl-like pseudo code:
1122 unshift(@ARGV, '-') unless @ARGV;
1123 while ($ARGV = shift) {
1126 ... # code for each line
1130 except that it isn't so cumbersome to say, and will actually work. It
1131 really does shift array @ARGV and put the current filename into variable
1132 $ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a
1133 synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above
1134 doesn't work because it treats E<lt>ARGVE<gt> as non-magical.)
1136 You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up
1137 containing the list of filenames you really want. Line numbers (C<$.>)
1138 continue as if the input were one big happy file. (But see example
1139 under eof() for how to reset line numbers on each file.)
1141 If you want to set @ARGV to your own list of files, go right ahead. If
1142 you want to pass switches into your script, you can use one of the
1143 Getopts modules or put a loop on the front like this:
1145 while ($_ = $ARGV[0], /^-/) {
1148 if (/^-D(.*)/) { $debug = $1 }
1149 if (/^-v/) { $verbose++ }
1150 ... # other switches
1153 ... # code for each line
1156 The E<lt>E<gt> symbol will return C<undef> for end-of-file only once.
1157 If you call it again after this it will assume you are processing another
1158 @ARGV list, and if you haven't set @ARGV, will input from STDIN.
1160 If the string inside the angle brackets is a reference to a scalar
1161 variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the
1162 filehandle to input from, or a reference to the same. For example:
1167 If the string inside angle brackets is not a filehandle or a scalar
1168 variable containing a filehandle name or reference, then it is interpreted
1169 as a filename pattern to be globbed, and either a list of filenames or the
1170 next filename in the list is returned, depending on context. One level of
1171 $ interpretation is done first, but you can't say C<E<lt>$fooE<gt>>
1172 because that's an indirect filehandle as explained in the previous
1173 paragraph. (In older versions of Perl, programmers would insert curly
1174 brackets to force interpretation as a filename glob: C<E<lt>${foo}E<gt>>.
1175 These days, it's considered cleaner to call the internal function directly
1176 as C<glob($foo)>, which is probably the right way to have done it in the
1177 first place.) Example:
1185 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1191 In fact, it's currently implemented that way. (Which means it will not
1192 work on filenames with spaces in them unless you have csh(1) on your
1193 machine.) Of course, the shortest way to do the above is:
1197 Because globbing invokes a shell, it's often faster to call readdir() yourself
1198 and do your own grep() on the filenames. Furthermore, due to its current
1199 implementation of using a shell, the glob() routine may get "Arg list too
1200 long" errors (unless you've installed tcsh(1L) as F</bin/csh>).
1202 A glob evaluates its (embedded) argument only when it is starting a new
1203 list. All values must be read before it will start over. In a list
1204 context this isn't important, because you automatically get them all
1205 anyway. In a scalar context, however, the operator returns the next value
1206 each time it is called, or a C<undef> value if you've just run out. As
1207 for filehandles an automatic C<defined> is generated when the glob
1208 occurs in the test part of a C<while> or C<for> - because legal glob returns
1209 (e.g. a file called F<0>) would otherwise terminate the loop.
1210 Again, C<undef> is returned only once. So if you're expecting a single value
1211 from a glob, it is much better to say
1213 ($file) = <blurch*>;
1219 because the latter will alternate between returning a filename and
1222 It you're trying to do variable interpolation, it's definitely better
1223 to use the glob() function, because the older notation can cause people
1224 to become confused with the indirect filehandle notation.
1226 @files = glob("$dir/*.[ch]");
1227 @files = glob($files[$i]);
1229 =head2 Constant Folding
1231 Like C, Perl does a certain amount of expression evaluation at
1232 compile time, whenever it determines that all of the arguments to an
1233 operator are static and have no side effects. In particular, string
1234 concatenation happens at compile time between literals that don't do
1235 variable substitution. Backslash interpretation also happens at
1236 compile time. You can say
1238 'Now is the time for all' . "\n" .
1239 'good men to come to.'
1241 and this all reduces to one string internally. Likewise, if
1244 foreach $file (@filenames) {
1245 if (-s $file > 5 + 100 * 2**16) { ... }
1248 the compiler will precompute the number that
1249 expression represents so that the interpreter
1252 =head2 Bitwise String Operators
1254 Bitstrings of any size may be manipulated by the bitwise operators
1257 If the operands to a binary bitwise op are strings of different sizes,
1258 B<or> and B<xor> ops will act as if the shorter operand had additional
1259 zero bits on the right, while the B<and> op will act as if the longer
1260 operand were truncated to the length of the shorter.
1262 # ASCII-based examples
1263 print "j p \n" ^ " a h"; # prints "JAPH\n"
1264 print "JA" | " ph\n"; # prints "japh\n"
1265 print "japh\nJunk" & '_____'; # prints "JAPH\n";
1266 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
1268 If you are intending to manipulate bitstrings, you should be certain that
1269 you're supplying bitstrings: If an operand is a number, that will imply
1270 a B<numeric> bitwise operation. You may explicitly show which type of
1271 operation you intend by using C<""> or C<0+>, as in the examples below.
1273 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
1274 $foo = '150' | 105 ; # yields 255
1275 $foo = 150 | '105'; # yields 255
1276 $foo = '150' | '105'; # yields string '155' (under ASCII)
1278 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
1279 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
1281 =head2 Integer Arithmetic
1283 By default Perl assumes that it must do most of its arithmetic in
1284 floating point. But by saying
1288 you may tell the compiler that it's okay to use integer operations
1289 from here to the end of the enclosing BLOCK. An inner BLOCK may
1290 countermand this by saying
1294 which lasts until the end of that BLOCK.
1296 The bitwise operators ("&", "|", "^", "~", "<<", and ">>") always
1297 produce integral results. (But see also L<Bitwise String Operators>.)
1298 However, C<use integer> still has meaning
1299 for them. By default, their results are interpreted as unsigned
1300 integers. However, if C<use integer> is in effect, their results are
1301 interpreted as signed integers. For example, C<~0> usually evaluates
1302 to a large integral value. However, C<use integer; ~0> is -1.
1304 =head2 Floating-point Arithmetic
1306 While C<use integer> provides integer-only arithmetic, there is no
1307 similar ways to provide rounding or truncation at a certain number of
1308 decimal places. For rounding to a certain number of digits, sprintf()
1309 or printf() is usually the easiest route.
1311 The POSIX module (part of the standard perl distribution) implements
1312 ceil(), floor(), and a number of other mathematical and trigonometric
1313 functions. The Math::Complex module (part of the standard perl
1314 distribution) defines a number of mathematical functions that can also
1315 work on real numbers. Math::Complex not as efficient as POSIX, but
1316 POSIX can't work with complex numbers.
1318 Rounding in financial applications can have serious implications, and
1319 the rounding method used should be specified precisely. In these
1320 cases, it probably pays not to trust whichever system rounding is
1321 being used by Perl, but to instead implement the rounding function you