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.)
13 left terms and list operators (leftward)
17 right ! ~ \ and unary + and -
22 nonassoc named unary operators
23 nonassoc < > <= >= lt gt le ge
24 nonassoc == != <=> eq ne cmp
33 nonassoc list operators (rightward)
38 In the following sections, these operators are covered in precedence order.
42 =head2 Terms and List Operators (Leftward)
44 Any TERM is of highest precedence of Perl. These includes variables,
45 quote and quotelike operators, any expression in parentheses,
46 and any function whose arguments are parenthesized. Actually, there
47 aren't really functions in this sense, just list operators and unary
48 operators behaving as functions because you put parentheses around
49 the arguments. These are all documented in L<perlfunc>.
51 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
52 is followed by a left parenthesis as the next token, the operator and
53 arguments within parentheses are taken to be of highest precedence,
54 just like a normal function call.
56 In the absence of parentheses, the precedence of list operators such as
57 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
58 whether you look at the left side of operator or the right side of it.
61 @ary = (1, 3, sort 4, 2);
62 print @ary; # prints 1324
64 the commas on the right of the sort are evaluated before the sort, but
65 the commas on the left are evaluated after. In other words, list
66 operators tend to gobble up all the arguments that follow them, and
67 then act like a simple TERM with regard to the preceding expression.
68 Note that you have to be careful with parens:
70 # These evaluate exit before doing the print:
71 print($foo, exit); # Obviously not what you want.
72 print $foo, exit; # Nor is this.
74 # These do the print before evaluating exit:
75 (print $foo), exit; # This is what you want.
76 print($foo), exit; # Or this.
77 print ($foo), exit; # Or even this.
81 print ($foo & 255) + 1, "\n";
83 probably doesn't do what you expect at first glance. See
84 L<Named Unary Operators> for more discussion of this.
86 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
87 well as subroutine and method calls, and the anonymous
88 constructors C<[]> and C<{}>.
90 See also L<Quote and Quotelike Operators> toward the end of this section,
91 as well as L<I/O Operators>.
93 =head2 The Arrow Operator
95 Just as in C and C++, "C<-E<gt>>" is an infix dereference operator. If the
96 right side is either a C<[...]> or C<{...}> subscript, then the left side
97 must be either a hard or symbolic reference to an array or hash (or
98 a location capable of holding a hard reference, if it's an lvalue (assignable)).
101 Otherwise, the right side is a method name or a simple scalar variable
102 containing the method name, and the left side must either be an object
103 (a blessed reference) or a class name (that is, a package name).
106 =head2 Autoincrement and Autodecrement
108 "++" and "--" work as in C. That is, if placed before a variable, they
109 increment or decrement the variable before returning the value, and if
110 placed after, increment or decrement the variable after returning the value.
112 The autoincrement operator has a little extra built-in magic to it. If
113 you increment a variable that is numeric, or that has ever been used in
114 a numeric context, you get a normal increment. If, however, the
115 variable has only been used in string contexts since it was set, and
116 has a value that is not null and matches the pattern
117 C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each
118 character within its range, with carry:
120 print ++($foo = '99'); # prints '100'
121 print ++($foo = 'a0'); # prints 'a1'
122 print ++($foo = 'Az'); # prints 'Ba'
123 print ++($foo = 'zz'); # prints 'aaa'
125 The autodecrement operator is not magical.
127 =head2 Exponentiation
129 Binary "**" is the exponentiation operator. Note that it binds even more
130 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4.
132 =head2 Symbolic Unary Operators
134 Unary "!" performs logical negation, i.e. "not". See also C<not> for a lower
135 precedence version of this.
137 Unary "-" performs arithmetic negation if the operand is numeric. If
138 the operand is an identifier, a string consisting of a minus sign
139 concatenated with the identifier is returned. Otherwise, if the string
140 starts with a plus or minus, a string starting with the opposite sign
141 is returned. One effect of these rules is that C<-bareword> is equivalent
144 Unary "~" performs bitwise negation, i.e. 1's complement.
146 Unary "+" has no effect whatsoever, even on strings. It is useful
147 syntactically for separating a function name from a parenthesized expression
148 that would otherwise be interpreted as the complete list of function
149 arguments. (See examples above under L<List Operators>.)
151 Unary "\" creates a reference to whatever follows it. See L<perlref>.
152 Do not confuse this behavior with the behavior of backslash within a
153 string, although both forms do convey the notion of protecting the next
154 thing from interpretation.
156 =head2 Binding Operators
158 Binary "=~" binds an expression to a pattern match.
159 Certain operations search or modify the string $_ by default. This
160 operator makes that kind of operation work on some other string. The
161 right argument is a search pattern, substitution, or translation. The
162 left argument is what is supposed to be searched, substituted, or
163 translated instead of the default $_. The return value indicates the
164 success of the operation. (If the right argument is an expression
165 rather than a search pattern, substitution, or translation, it is
166 interpreted as a search pattern at run time. This is less efficient
167 than an explicit search, since the pattern must be compiled every time
168 the expression is evaluated--unless you've used C</o>.)
170 Binary "!~" is just like "=~" except the return value is negated in
173 =head2 Multiplicative Operators
175 Binary "*" multiplies two numbers.
177 Binary "/" divides two numbers.
179 Binary "%" computes the modulus of the two numbers.
181 Binary "x" is the repetition operator. In a scalar context, it
182 returns a string consisting of the left operand repeated the number of
183 times specified by the right operand. In a list context, if the left
184 operand is a list in parens, it repeats the list.
186 print '-' x 80; # print row of dashes
188 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
190 @ones = (1) x 80; # a list of 80 1's
191 @ones = (5) x @ones; # set all elements to 5
194 =head2 Additive Operators
196 Binary "+" returns the sum of two numbers.
198 Binary "-" returns the difference of two numbers.
200 Binary "." concatenates two strings.
202 =head2 Shift Operators
204 Binary "<<" returns the value of its left argument shifted left by the
205 number of bits specified by the right argument. Arguments should be
208 Binary ">>" returns the value of its left argument shifted right by the
209 number of bits specified by the right argument. Arguments should be
212 =head2 Named Unary Operators
214 The various named unary operators are treated as functions with one
215 argument, with optional parentheses. These include the filetest
216 operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
218 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
219 is followed by a left parenthesis as the next token, the operator and
220 arguments within parentheses are taken to be of highest precedence,
221 just like a normal function call. Examples:
223 chdir $foo || die; # (chdir $foo) || die
224 chdir($foo) || die; # (chdir $foo) || die
225 chdir ($foo) || die; # (chdir $foo) || die
226 chdir +($foo) || die; # (chdir $foo) || die
228 but, because * is higher precedence than ||:
230 chdir $foo * 20; # chdir ($foo * 20)
231 chdir($foo) * 20; # (chdir $foo) * 20
232 chdir ($foo) * 20; # (chdir $foo) * 20
233 chdir +($foo) * 20; # chdir ($foo * 20)
235 rand 10 * 20; # rand (10 * 20)
236 rand(10) * 20; # (rand 10) * 20
237 rand (10) * 20; # (rand 10) * 20
238 rand +(10) * 20; # rand (10 * 20)
240 See also L<"List Operators">.
242 =head2 Relational Operators
244 Binary "<" returns true if the left argument is numerically less than
247 Binary ">" returns true if the left argument is numerically greater
248 than the right argument.
250 Binary "<=" returns true if the left argument is numerically less than
251 or equal to the right argument.
253 Binary ">=" returns true if the left argument is numerically greater
254 than or equal to the right argument.
256 Binary "lt" returns true if the left argument is stringwise less than
259 Binary "gt" returns true if the left argument is stringwise greater
260 than the right argument.
262 Binary "le" returns true if the left argument is stringwise less than
263 or equal to the right argument.
265 Binary "ge" returns true if the left argument is stringwise greater
266 than or equal to the right argument.
268 =head2 Equality Operators
270 Binary "==" returns true if the left argument is numerically equal to
273 Binary "!=" returns true if the left argument is numerically not equal
274 to the right argument.
276 Binary "<=>" returns -1, 0, or 1 depending on whether the left argument is numerically
277 less than, equal to, or greater than the right argument.
279 Binary "eq" returns true if the left argument is stringwise equal to
282 Binary "ne" returns true if the left argument is stringwise not equal
283 to the right argument.
285 Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise
286 less than, equal to, or greater than the right argument.
290 Binary "&" returns its operators ANDed together bit by bit.
292 =head2 Bitwise Or and Exclusive Or
294 Binary "|" returns its operators ORed together bit by bit.
296 Binary "^" returns its operators XORed together bit by bit.
298 =head2 C-style Logical And
300 Binary "&&" performs a short-circuit logical AND operation. That is,
301 if the left operand is false, the right operand is not even evaluated.
302 Scalar or list context propagates down to the right operand if it
305 =head2 C-style Logical Or
307 Binary "||" performs a short-circuit logical OR operation. That is,
308 if the left operand is true, the right operand is not even evaluated.
309 Scalar or list context propagates down to the right operand if it
312 The C<||> and C<&&> operators differ from C's in that, rather than returning
313 0 or 1, they return the last value evaluated. Thus, a reasonably portable
314 way to find out the home directory (assuming it's not "0") might be:
316 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
317 (getpwuid($<))[7] || die "You're homeless!\n";
319 As more readable alternatives to C<&&> and C<||>, Perl provides "and" and
320 "or" operators (see below). The short-circuit behavior is identical. The
321 precedence of "and" and "or" is much lower, however, so that you can
322 safely use them after a list operator without the need for
325 unlink "alpha", "beta", "gamma"
326 or gripe(), next LINE;
328 With the C-style operators that would have been written like this:
330 unlink("alpha", "beta", "gamma")
331 || (gripe(), next LINE);
333 =head2 Range Operator
335 Binary ".." is the range operator, which is really two different
336 operators depending on the context. In a list context, it returns an
337 array of values counting (by ones) from the left value to the right
338 value. This is useful for writing C<for (1..10)> loops and for doing
339 slice operations on arrays. Be aware that under the current implementation,
340 a temporary array is created, so you'll burn a lot of memory if you
341 write something like this:
343 for (1 .. 1_000_000) {
347 In a scalar context, ".." returns a boolean value. The operator is
348 bistable, like a flip-flop, and emulates the line-range (comma) operator
349 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
350 own boolean state. It is false as long as its left operand is false.
351 Once the left operand is true, the range operator stays true until the
352 right operand is true, I<AFTER> which the range operator becomes false
353 again. (It doesn't become false till the next time the range operator is
354 evaluated. It can test the right operand and become false on the same
355 evaluation it became true (as in B<awk>), but it still returns true once.
356 If you don't want it to test the right operand till the next evaluation
357 (as in B<sed>), use three dots ("...") instead of two.) The right
358 operand is not evaluated while the operator is in the "false" state, and
359 the left operand is not evaluated while the operator is in the "true"
360 state. The precedence is a little lower than || and &&. The value
361 returned is either the null string for false, or a sequence number
362 (beginning with 1) for true. The sequence number is reset for each range
363 encountered. The final sequence number in a range has the string "E0"
364 appended to it, which doesn't affect its numeric value, but gives you
365 something to search for if you want to exclude the endpoint. You can
366 exclude the beginning point by waiting for the sequence number to be
367 greater than 1. If either operand of scalar ".." is a numeric literal,
368 that operand is implicitly compared to the C<$.> variable, the current
369 line number. Examples:
371 As a scalar operator:
373 if (101 .. 200) { print; } # print 2nd hundred lines
374 next line if (1 .. /^$/); # skip header lines
375 s/^/> / if (/^$/ .. eof()); # quote body
379 for (101 .. 200) { print; } # print $_ 100 times
380 @foo = @foo[$[ .. $#foo]; # an expensive no-op
381 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
383 The range operator (in a list context) makes use of the magical
384 autoincrement algorithm if the operaands are strings. You
387 @alphabet = ('A' .. 'Z');
389 to get all the letters of the alphabet, or
391 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
393 to get a hexadecimal digit, or
395 @z2 = ('01' .. '31'); print $z2[$mday];
397 to get dates with leading zeros. If the final value specified is not
398 in the sequence that the magical increment would produce, the sequence
399 goes until the next value would be longer than the final value
402 =head2 Conditional Operator
404 Ternary "?:" is the conditional operator, just as in C. It works much
405 like an if-then-else. If the argument before the ? is true, the
406 argument before the : is returned, otherwise the argument after the :
407 is returned. Scalar or list context propagates downward into the 2nd
408 or 3rd argument, whichever is selected. The operator may be assigned
409 to if both the 2nd and 3rd arguments are legal lvalues (meaning that you
412 ($a_or_b ? $a : $b) = $c;
414 Note that this is not guaranteed to contribute to the readability of
417 =head2 Assigment Operators
419 "=" is the ordinary assignment operator.
421 Assignment operators work as in C. That is,
429 although without duplicating any side effects that dereferencing the lvalue
430 might trigger, such as from tie(). Other assignment operators work similarly.
431 The following are recognized:
438 Note that while these are grouped by family, they all have the precedence
441 Unlike in C, the assignment operator produces a valid lvalue. Modifying
442 an assignment is equivalent to doing the assignment and then modifying
443 the variable that was assigned to. This is useful for modifying
444 a copy of something, like this:
446 ($tmp = $global) =~ tr [A-Z] [a-z];
457 =head2 Comma Operator
459 Binary "," is the comma operator. In a scalar context it evaluates
460 its left argument, throws that value away, then evaluates its right
461 argument and returns that value. This is just like C's comma operator.
463 In a list context, it's just the list argument separator, and inserts
464 both its arguments into the list.
466 The => digraph is simply a synonym for the comma operator. It's useful
467 for documenting arguments that come in pairs.
469 =head2 List Operators (Rightward)
471 On the right side of a list operator, it has very low precedence,
472 such that it controls all comma-separated expressions found there.
473 The only operators with lower precedence are the logical operators
474 "and", "or", and "not", which may be used to evaluate calls to list
475 operators without the need for extra parentheses:
477 open HANDLE, "filename"
478 or die "Can't open: $!\n";
480 See also discussion of list operators in L<List Operators (Leftward)>.
484 Unary "not" returns the logical negation of the expression to its right.
485 It's the equivalent of "!" except for the very low precedence.
489 Binary "and" returns the logical conjunction of the two surrounding
490 expressions. It's equivalent to && except for the very low
491 precedence. This means that it short-circuits: i.e. the right
492 expression is evaluated only if the left expression is true.
494 =head2 Logical or and Exclusive Or
496 Binary "or" returns the logical disjunction of the two surrounding
497 expressions. It's equivalent to || except for the very low
498 precedence. This means that it short-circuits: i.e. the right
499 expression is evaluated only if the left expression is false.
501 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
502 It cannot short circuit, of course.
504 =head2 C Operators Missing From Perl
506 Here is what C has that Perl doesn't:
512 Address-of operator. (But see the "\" operator for taking a reference.)
516 Dereference-address operator. (Perl's prefix dereferencing
517 operators are typed: $, @, %, and &.)
521 Type casting operator.
525 =head2 Quote and Quotelike Operators
527 While we usually think of quotes as literal values, in Perl they
528 function as operators, providing various kinds of interpolating and
529 pattern matching capabilities. Perl provides customary quote characters
530 for these behaviors, but also provides a way for you to choose your
531 quote character for any of them. In the following table, a C<{}> represents
532 any pair of delimiters you choose. Non-bracketing delimiters use
533 the same character fore and aft, but the 4 sorts of brackets
534 (round, angle, square, curly) will all nest.
536 Customary Generic Meaning Interpolates
541 // m{} Pattern match yes
542 s{}{} Substitution yes
543 tr{}{} Translation no
545 For constructs that do interpolation, variables beginning with "C<$> or "C<@>"
546 are interpolated, as are the following sequences:
552 \v vertical tab, whatever that is
559 \l lowercase next char
560 \u uppercase next char
563 \E end case modification
564 \Q quote regexp metacharacters till \E
566 Patterns are subject to an additional level of interpretation as a
567 regular expression. This is done as a second pass, after variables are
568 interpolated, so that regular expressions may be incorporated into the
569 pattern from the variables. If this is not what you want, use C<\Q> to
570 interpolate a variable literally.
572 Apart from the above, there are no multiple levels of interpolation. In
573 particular, contrary to the expectations of shell programmers, backquotes
574 do I<NOT> interpolate within double quotes, nor do single quotes impede
575 evaluation of variables when used within double quotes.
581 This is just like the C</pattern/> search, except that it matches only
582 once between calls to the reset() operator. This is a useful
583 optimization when you only want to see the first occurrence of
584 something in each file of a set of files, for instance. Only C<??>
585 patterns local to the current package are reset.
587 This usage is vaguely deprecated, and may be removed in some future
590 =item m/PATTERN/gimosx
592 =item /PATTERN/gimosx
594 Searches a string for a pattern match, and in a scalar context returns
595 true (1) or false (''). If no string is specified via the C<=~> or
596 C<!~> operator, the $_ string is searched. (The string specified with
597 C<=~> need not be an lvalue--it may be the result of an expression
598 evaluation, but remember the C<=~> binds rather tightly.) See also
603 g Match globally, i.e. find all occurrences.
604 i Do case-insensitive pattern matching.
605 m Treat string as multiple lines.
606 o Only compile pattern once.
607 s Treat string as single line.
608 x Use extended regular expressions.
610 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
611 you can use any pair of non-alphanumeric, non-whitespace characters as
612 delimiters. This is particularly useful for matching Unix path names
613 that contain "/", to avoid LTS (leaning toothpick syndrome).
615 PATTERN may contain variables, which will be interpolated (and the
616 pattern recompiled) every time the pattern search is evaluated. (Note
617 that C<$)> and C<$|> might not be interpolated because they look like
618 end-of-string tests.) If you want such a pattern to be compiled only
619 once, add a C</o> after the trailing delimiter. This avoids expensive
620 run-time recompilations, and is useful when the value you are
621 interpolating won't change over the life of the script. However, mentioning
622 C</o> constitutes a promise that you won't change the variables in the pattern.
623 If you change them, Perl won't even notice.
625 If the PATTERN evaluates to a null string, the most recently executed
626 (and successfully compiled) regular expression is used instead.
628 If used in a context that requires a list value, a pattern match returns a
629 list consisting of the subexpressions matched by the parentheses in the
630 pattern, i.e. ($1, $2, $3...). (Note that here $1 etc. are also set, and
631 that this differs from Perl 4's behavior.) If the match fails, a null
632 array is returned. If the match succeeds, but there were no parentheses,
633 a list value of (1) is returned.
637 open(TTY, '/dev/tty');
638 <TTY> =~ /^y/i && foo(); # do foo if desired
640 if (/Version: *([0-9.]*)/) { $version = $1; }
642 next if m#^/usr/spool/uucp#;
647 print if /$arg/o; # compile only once
650 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
652 This last example splits $foo into the first two words and the
653 remainder of the line, and assigns those three fields to $F1, $F2 and
654 $Etc. The conditional is true if any variables were assigned, i.e. if
657 The C</g> modifier specifies global pattern matching--that is, matching
658 as many times as possible within the string. How it behaves depends on
659 the context. In a list context, it returns a list of all the
660 substrings matched by all the parentheses in the regular expression.
661 If there are no parentheses, it returns a list of all the matched
662 strings, as if there were parentheses around the whole pattern.
664 In a scalar context, C<m//g> iterates through the string, returning TRUE
665 each time it matches, and FALSE when it eventually runs out of
666 matches. (In other words, it remembers where it left off last time and
667 restarts the search at that point. You can actually find the current
668 match position of a string using the pos() function--see L<perlfunc>.)
669 If you modify the string in any way, the match position is reset to the
673 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
676 $/ = ""; $* = 1; # $* deprecated in Perl 5
677 while ($paragraph = <>) {
678 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
682 print "$sentences\n";
688 A single-quoted, literal string. Backslashes are ignored, unless
689 followed by the delimiter or another backslash, in which case the
690 delimiter or backslash is interpolated.
692 $foo = q!I said, "You said, 'She said it.'"!;
693 $bar = q('This is it.');
699 A double-quoted, interpolated string.
702 (*** The previous line contains the naughty word "$1".\n)
703 if /(tcl|rexx|python)/; # :-)
709 A string which is interpolated and then executed as a system command.
710 The collected standard output of the command is returned. In scalar
711 context, it comes back as a single (potentially multi-line) string.
712 In list context, returns a list of lines (however you've defined lines
713 with $/ or $INPUT_RECORD_SEPARATOR).
717 See L<I/O Operators> for more discussion.
721 Returns a list of the words extracted out of STRING, using embedded
722 whitespace as the word delimiters. It is exactly equivalent to
724 split(' ', q/STRING/);
726 Some frequently seen examples:
728 use POSIX qw( setlocale localeconv )
729 @EXPORT = qw( foo bar baz );
731 =item s/PATTERN/REPLACEMENT/egimosx
733 Searches a string for a pattern, and if found, replaces that pattern
734 with the replacement text and returns the number of substitutions
735 made. Otherwise it returns false (0).
737 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
738 variable is searched and modified. (The string specified with C<=~> must
739 be a scalar variable, an array element, a hash element, or an assignment
740 to one of those, i.e. an lvalue.)
742 If the delimiter chosen is single quote, no variable interpolation is
743 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
744 PATTERN contains a $ that looks like a variable rather than an
745 end-of-string test, the variable will be interpolated into the pattern
746 at run-time. If you only want the pattern compiled once the first time
747 the variable is interpolated, use the C</o> option. If the pattern
748 evaluates to a null string, the most recently executed (and successfully compiled) regular
749 expression is used instead. See L<perlre> for further explanation on these.
753 e Evaluate the right side as an expression.
754 g Replace globally, i.e. all occurrences.
755 i Do case-insensitive pattern matching.
756 m Treat string as multiple lines.
757 o Only compile pattern once.
758 s Treat string as single line.
759 x Use extended regular expressions.
761 Any non-alphanumeric, non-whitespace delimiter may replace the
762 slashes. If single quotes are used, no interpretation is done on the
763 replacement string (the C</e> modifier overrides this, however). If
764 backquotes are used, the replacement string is a command to execute
765 whose output will be used as the actual replacement text. If the
766 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
767 pair of quotes, which may or may not be bracketing quotes, e.g.
768 C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the
769 replacement portion to be interpreter as a full-fledged Perl expression
770 and eval()ed right then and there. It is, however, syntax checked at
775 s/\bgreen\b/mauve/g; # don't change wintergreen
777 $path =~ s|/usr/bin|/usr/local/bin|;
779 s/Login: $foo/Login: $bar/; # run-time pattern
781 ($foo = $bar) =~ s/this/that/;
783 $count = ($paragraph =~ s/Mister\b/Mr./g);
786 s/\d+/$&*2/e; # yields 'abc246xyz'
787 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
788 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
790 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
791 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
792 s/^=(\w+)/&pod($1)/ge; # use function call
794 # /e's can even nest; this will expand
795 # simple embedded variables in $_
800 /\* (?# Match the opening delimiter.)
801 .*? (?# Match a minimal number of characters.)
802 \*/ (?# Match the closing delimiter.)
805 s/^\s*(.*?)\s*$/$1/; # trim white space
807 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
809 Note the use of $ instead of \ in the last example. Unlike
810 B<sed>, we only use the \<I<digit>> form in the left hand side.
811 Anywhere else it's $<I<digit>>.
813 Occasionally, you can't just use a C</g> to get all the changes
814 to occur. Here are two common cases:
816 # put commas in the right places in an integer
817 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4
818 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5
820 # expand tabs to 8-column spacing
821 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
824 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
826 =item y/SEARCHLIST/REPLACEMENTLIST/cds
828 Translates all occurrences of the characters found in the search list
829 with the corresponding character in the replacement list. It returns
830 the number of characters replaced or deleted. If no string is
831 specified via the =~ or !~ operator, the $_ string is translated. (The
832 string specified with =~ must be a scalar variable, an array element,
833 or an assignment to one of those, i.e. an lvalue.) For B<sed> devotees,
834 C<y> is provided as a synonym for C<tr>. If the SEARCHLIST is
835 delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of
836 quotes, which may or may not be bracketing quotes, e.g. C<tr[A-Z][a-z]>
837 or C<tr(+-*/)/ABCD/>.
841 c Complement the SEARCHLIST.
842 d Delete found but unreplaced characters.
843 s Squash duplicate replaced characters.
845 If the C</c> modifier is specified, the SEARCHLIST character set is
846 complemented. If the C</d> modifier is specified, any characters specified
847 by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note
848 that this is slightly more flexible than the behavior of some B<tr>
849 programs, which delete anything they find in the SEARCHLIST, period.)
850 If the C</s> modifier is specified, sequences of characters that were
851 translated to the same character are squashed down to a single instance of the
854 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
855 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
856 than the SEARCHLIST, the final character is replicated till it is long
857 enough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated.
858 This latter is useful for counting characters in a class or for
859 squashing character sequences in a class.
863 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
865 $cnt = tr/*/*/; # count the stars in $_
867 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
869 $cnt = tr/0-9//; # count the digits in $_
871 tr/a-zA-Z//s; # bookkeeper -> bokeper
873 ($HOST = $host) =~ tr/a-z/A-Z/;
875 tr/a-zA-Z/ /cs; # change non-alphas to single space
878 [\000-\177]; # delete 8th bit
880 If multiple translations are given for a character, only the first one is used:
884 will translate any A to X.
886 Note that because the translation table is built at compile time, neither
887 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
888 interpolation. That means that if you want to use variables, you must use
891 eval "tr/$oldlist/$newlist/";
894 eval "tr/$oldlist/$newlist/, 1" or die $@;
900 There are several I/O operators you should know about.
901 A string is enclosed by backticks (grave accents) first undergoes
902 variable substitution just like a double quoted string. It is then
903 interpreted as a command, and the output of that command is the value
904 of the pseudo-literal, like in a shell. In a scalar context, a single
905 string consisting of all the output is returned. In a list context,
906 a list of values is returned, one for each line of output. (You can
907 set C<$/> to use a different line terminator.) The command is executed
908 each time the pseudo-literal is evaluated. The status value of the
909 command is returned in C<$?> (see L<perlvar> for the interpretation
910 of C<$?>). Unlike in B<csh>, no translation is done on the return
911 data--newlines remain newlines. Unlike in any of the shells, single
912 quotes do not hide variable names in the command from interpretation.
913 To pass a $ through to the shell you need to hide it with a backslash.
914 The generalized form of backticks is C<qx//>.
916 Evaluating a filehandle in angle brackets yields the next line from
917 that file (newline included, so it's never false until end of file, at
918 which time an undefined value is returned). Ordinarily you must assign
919 that value to a variable, but there is one situation where an automatic
920 assignment happens. I<If and ONLY if> the input symbol is the only
921 thing inside the conditional of a C<while> loop, the value is
922 automatically assigned to the variable C<$_>. The assigned value is
923 then tested to see if it is defined. (This may seem like an odd thing
924 to you, but you'll use the construct in almost every Perl script you
925 write.) Anyway, the following lines are equivalent to each other:
927 while (defined($_ = <STDIN>)) { print; }
928 while (<STDIN>) { print; }
929 for (;<STDIN>;) { print; }
930 print while defined($_ = <STDIN>);
933 The filehandles STDIN, STDOUT and STDERR are predefined. (The
934 filehandles C<stdin>, C<stdout> and C<stderr> will also work except in
935 packages, where they would be interpreted as local identifiers rather
936 than global.) Additional filehandles may be created with the open()
939 If a <FILEHANDLE> is used in a context that is looking for a list, a
940 list consisting of all the input lines is returned, one line per list
941 element. It's easy to make a I<LARGE> data space this way, so use with
944 The null filehandle <> is special and can be used to emulate the
945 behavior of B<sed> and B<awk>. Input from <> comes either from
946 standard input, or from each file listed on the command line. Here's
947 how it works: the first time <> is evaluated, the @ARGV array is
948 checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened
949 gives you standard input. The @ARGV array is then processed as a list
950 of filenames. The loop
953 ... # code for each line
956 is equivalent to the following Perl-like pseudo code:
958 unshift(@ARGV, '-') if $#ARGV < $[;
959 while ($ARGV = shift) {
962 ... # code for each line
966 except that it isn't so cumbersome to say, and will actually work. It
967 really does shift array @ARGV and put the current filename into variable
968 $ARGV. It also uses filehandle I<ARGV> internally--<> is just a synonym
969 for <ARGV>, which is magical. (The pseudo code above doesn't work
970 because it treats <ARGV> as non-magical.)
972 You can modify @ARGV before the first <> as long as the array ends up
973 containing the list of filenames you really want. Line numbers (C<$.>)
974 continue as if the input were one big happy file. (But see example
975 under eof() for how to reset line numbers on each file.)
977 If you want to set @ARGV to your own list of files, go right ahead. If
978 you want to pass switches into your script, you can use one of the
979 Getopts modules or put a loop on the front like this:
981 while ($_ = $ARGV[0], /^-/) {
984 if (/^-D(.*)/) { $debug = $1 }
985 if (/^-v/) { $verbose++ }
989 ... # code for each line
992 The <> symbol will return FALSE only once. If you call it again after
993 this it will assume you are processing another @ARGV list, and if you
994 haven't set @ARGV, will input from STDIN.
996 If the string inside the angle brackets is a reference to a scalar
997 variable (e.g. <$foo>), then that variable contains the name of the
998 filehandle to input from.
1000 If the string inside angle brackets is not a filehandle, it is
1001 interpreted as a filename pattern to be globbed, and either a list of
1002 filenames or the next filename in the list is returned, depending on
1003 context. One level of $ interpretation is done first, but you can't
1004 say C<E<lt>$fooE<gt>> because that's an indirect filehandle as explained in the
1005 previous paragraph. You could insert curly brackets to force
1006 interpretation as a filename glob: C<E<lt>${foo}E<gt>>. (Alternately, you can
1007 call the internal function directly as C<glob($foo)>, which is probably
1008 the right way to have done it in the first place.) Example:
1016 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1022 In fact, it's currently implemented that way. (Which means it will not
1023 work on filenames with spaces in them unless you have csh(1) on your
1024 machine.) Of course, the shortest way to do the above is:
1028 Because globbing invokes a shell, it's often faster to call readdir() yourself
1029 and just do your own grep() on the filenames. Furthermore, due to its current
1030 implementation of using a shell, the glob() routine may get "Arg list too
1031 long" errors (unless you've installed tcsh(1L) as F</bin/csh>).
1033 =head2 Constant Folding
1035 Like C, Perl does a certain amount of expression evaluation at
1036 compile time, whenever it determines that all of the arguments to an
1037 operator are static and have no side effects. In particular, string
1038 concatenation happens at compile time between literals that don't do
1039 variable substitution. Backslash interpretation also happens at
1040 compile time. You can say
1042 'Now is the time for all' . "\n" .
1043 'good men to come to.'
1045 and this all reduces to one string internally. Likewise, if
1048 foreach $file (@filenames) {
1049 if (-s $file > 5 + 100 * 2**16) { ... }
1052 the compiler will pre-compute the number that
1053 expression represents so that the interpreter
1057 =head2 Integer arithmetic
1059 By default Perl assumes that it must do most of its arithmetic in
1060 floating point. But by saying
1064 you may tell the compiler that it's okay to use integer operations
1065 from here to the end of the enclosing BLOCK. An inner BLOCK may
1066 countermand this by saying
1070 which lasts until the end of that BLOCK.