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 Any TERM is of highest precedence of Perl. These includes variables,
46 quote and quotelike 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 look at the left side of operator or the right side of it.
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 parens:
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 Quotelike 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 Autoincrement and Autodecrement
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 autoincrement operator has a little extra built-in 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 only been used in 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 autodecrement 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.
149 Unary "+" has no effect whatsoever, even on strings. It is useful
150 syntactically for separating a function name from a parenthesized expression
151 that would otherwise be interpreted as the complete list of function
152 arguments. (See examples above under L<List Operators>.)
154 Unary "\" creates a reference to whatever follows it. See L<perlref>.
155 Do not confuse this behavior with the behavior of backslash within a
156 string, although both forms do convey the notion of protecting the next
157 thing from interpretation.
159 =head2 Binding Operators
161 Binary "=~" binds a scalar expression to a pattern match. Certain operations
162 search or modify the string $_ by default. This operator makes that kind
163 of operation work on some other string. The right argument is a search
164 pattern, substitution, or translation. The left argument is what is
165 supposed to be searched, substituted, or translated instead of the default
166 $_. The return value indicates the success of the operation. (If the
167 right argument is an expression rather than a search pattern,
168 substitution, or translation, it is interpreted as a search pattern at run
169 time. This is less efficient than an explicit search, since the pattern
170 must be compiled every time the expression is evaluated--unless you've
173 Binary "!~" is just like "=~" except the return value is negated in
176 =head2 Multiplicative Operators
178 Binary "*" multiplies two numbers.
180 Binary "/" divides two numbers.
182 Binary "%" computes the modulus of the two numbers.
184 Binary "x" is the repetition operator. In a scalar context, it
185 returns a string consisting of the left operand repeated the number of
186 times specified by the right operand. In a list context, if the left
187 operand is a list in parens, it repeats the list.
189 print '-' x 80; # print row of dashes
191 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
193 @ones = (1) x 80; # a list of 80 1's
194 @ones = (5) x @ones; # set all elements to 5
197 =head2 Additive Operators
199 Binary "+" returns the sum of two numbers.
201 Binary "-" returns the difference of two numbers.
203 Binary "." concatenates two strings.
205 =head2 Shift Operators
207 Binary "<<" returns the value of its left argument shifted left by the
208 number of bits specified by the right argument. Arguments should be
211 Binary ">>" returns the value of its left argument shifted right by the
212 number of bits specified by the right argument. Arguments should be
215 =head2 Named Unary Operators
217 The various named unary operators are treated as functions with one
218 argument, with optional parentheses. These include the filetest
219 operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
221 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
222 is followed by a left parenthesis as the next token, the operator and
223 arguments within parentheses are taken to be of highest precedence,
224 just like a normal function call. Examples:
226 chdir $foo || die; # (chdir $foo) || die
227 chdir($foo) || die; # (chdir $foo) || die
228 chdir ($foo) || die; # (chdir $foo) || die
229 chdir +($foo) || die; # (chdir $foo) || die
231 but, because * is higher precedence than ||:
233 chdir $foo * 20; # chdir ($foo * 20)
234 chdir($foo) * 20; # (chdir $foo) * 20
235 chdir ($foo) * 20; # (chdir $foo) * 20
236 chdir +($foo) * 20; # chdir ($foo * 20)
238 rand 10 * 20; # rand (10 * 20)
239 rand(10) * 20; # (rand 10) * 20
240 rand (10) * 20; # (rand 10) * 20
241 rand +(10) * 20; # rand (10 * 20)
243 See also L<"List Operators">.
245 =head2 Relational Operators
247 Binary "<" returns true if the left argument is numerically less than
250 Binary ">" returns true if the left argument is numerically greater
251 than the right argument.
253 Binary "<=" returns true if the left argument is numerically less than
254 or equal to the right argument.
256 Binary ">=" returns true if the left argument is numerically greater
257 than or equal to the right argument.
259 Binary "lt" returns true if the left argument is stringwise less than
262 Binary "gt" returns true if the left argument is stringwise greater
263 than the right argument.
265 Binary "le" returns true if the left argument is stringwise less than
266 or equal to the right argument.
268 Binary "ge" returns true if the left argument is stringwise greater
269 than or equal to the right argument.
271 =head2 Equality Operators
273 Binary "==" returns true if the left argument is numerically equal to
276 Binary "!=" returns true if the left argument is numerically not equal
277 to the right argument.
279 Binary "<=>" returns -1, 0, or 1 depending on whether the left argument is numerically
280 less than, equal to, or greater than the right argument.
282 Binary "eq" returns true if the left argument is stringwise equal to
285 Binary "ne" returns true if the left argument is stringwise not equal
286 to the right argument.
288 Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise
289 less than, equal to, or greater than the right argument.
293 Binary "&" returns its operators ANDed together bit by bit.
295 =head2 Bitwise Or and Exclusive Or
297 Binary "|" returns its operators ORed together bit by bit.
299 Binary "^" returns its operators XORed together bit by bit.
301 =head2 C-style Logical And
303 Binary "&&" performs a short-circuit logical AND operation. That is,
304 if the left operand is false, the right operand is not even evaluated.
305 Scalar or list context propagates down to the right operand if it
308 =head2 C-style Logical Or
310 Binary "||" performs a short-circuit logical OR operation. That is,
311 if the left operand is true, the right operand is not even evaluated.
312 Scalar or list context propagates down to the right operand if it
315 The C<||> and C<&&> operators differ from C's in that, rather than returning
316 0 or 1, they return the last value evaluated. Thus, a reasonably portable
317 way to find out the home directory (assuming it's not "0") might be:
319 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
320 (getpwuid($<))[7] || die "You're homeless!\n";
322 As more readable alternatives to C<&&> and C<||>, Perl provides "and" and
323 "or" operators (see below). The short-circuit behavior is identical. The
324 precedence of "and" and "or" is much lower, however, so that you can
325 safely use them after a list operator without the need for
328 unlink "alpha", "beta", "gamma"
329 or gripe(), next LINE;
331 With the C-style operators that would have been written like this:
333 unlink("alpha", "beta", "gamma")
334 || (gripe(), next LINE);
336 =head2 Range Operator
338 Binary ".." is the range operator, which is really two different
339 operators depending on the context. In a list context, it returns an
340 array of values counting (by ones) from the left value to the right
341 value. This is useful for writing C<for (1..10)> loops and for doing
342 slice operations on arrays. Be aware that under the current implementation,
343 a temporary array is created, so you'll burn a lot of memory if you
344 write something like this:
346 for (1 .. 1_000_000) {
350 In a scalar context, ".." returns a boolean value. The operator is
351 bistable, like a flip-flop, and emulates the line-range (comma) operator
352 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
353 own boolean state. It is false as long as its left operand is false.
354 Once the left operand is true, the range operator stays true until the
355 right operand is true, I<AFTER> which the range operator becomes false
356 again. (It doesn't become false till the next time the range operator is
357 evaluated. It can test the right operand and become false on the same
358 evaluation it became true (as in B<awk>), but it still returns true once.
359 If you don't want it to test the right operand till the next evaluation
360 (as in B<sed>), use three dots ("...") instead of two.) The right
361 operand is not evaluated while the operator is in the "false" state, and
362 the left operand is not evaluated while the operator is in the "true"
363 state. The precedence is a little lower than || and &&. The value
364 returned is either the null string for false, or a sequence number
365 (beginning with 1) for true. The sequence number is reset for each range
366 encountered. The final sequence number in a range has the string "E0"
367 appended to it, which doesn't affect its numeric value, but gives you
368 something to search for if you want to exclude the endpoint. You can
369 exclude the beginning point by waiting for the sequence number to be
370 greater than 1. If either operand of scalar ".." is a numeric literal,
371 that operand is implicitly compared to the C<$.> variable, the current
372 line number. Examples:
374 As a scalar operator:
376 if (101 .. 200) { print; } # print 2nd hundred lines
377 next line if (1 .. /^$/); # skip header lines
378 s/^/> / if (/^$/ .. eof()); # quote body
382 for (101 .. 200) { print; } # print $_ 100 times
383 @foo = @foo[$[ .. $#foo]; # an expensive no-op
384 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
386 The range operator (in a list context) makes use of the magical
387 autoincrement algorithm if the operands are strings. You
390 @alphabet = ('A' .. 'Z');
392 to get all the letters of the alphabet, or
394 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
396 to get a hexadecimal digit, or
398 @z2 = ('01' .. '31'); print $z2[$mday];
400 to get dates with leading zeros. If the final value specified is not
401 in the sequence that the magical increment would produce, the sequence
402 goes until the next value would be longer than the final value
405 =head2 Conditional Operator
407 Ternary "?:" is the conditional operator, just as in C. It works much
408 like an if-then-else. If the argument before the ? is true, the
409 argument before the : is returned, otherwise the argument after the :
410 is returned. For example:
412 printf "I have %d dog%s.\n", $n,
413 ($n == 1) ? '' : "s";
415 Scalar or list context propagates downward into the 2nd
416 or 3rd argument, whichever is selected.
418 $a = $ok ? $b : $c; # get a scalar
419 @a = $ok ? @b : @c; # get an array
420 $a = $ok ? @b : @c; # oops, that's just a count!
422 The operator may be assigned to if both the 2nd and 3rd arguments are
423 legal lvalues (meaning that you can assign to them):
425 ($a_or_b ? $a : $b) = $c;
427 This is not necessarily guaranteed to contribute to the readability of your program.
429 =head2 Assignment Operators
431 "=" is the ordinary assignment operator.
433 Assignment operators work as in C. That is,
441 although without duplicating any side effects that dereferencing the lvalue
442 might trigger, such as from tie(). Other assignment operators work similarly.
443 The following are recognized:
450 Note that while these are grouped by family, they all have the precedence
453 Unlike in C, the assignment operator produces a valid lvalue. Modifying
454 an assignment is equivalent to doing the assignment and then modifying
455 the variable that was assigned to. This is useful for modifying
456 a copy of something, like this:
458 ($tmp = $global) =~ tr [A-Z] [a-z];
469 =head2 Comma Operator
471 Binary "," is the comma operator. In a scalar context it evaluates
472 its left argument, throws that value away, then evaluates its right
473 argument and returns that value. This is just like C's comma operator.
475 In a list context, it's just the list argument separator, and inserts
476 both its arguments into the list.
478 The => digraph is mostly just a synonym for the comma operator. It's useful for
479 documenting arguments that come in pairs. As of release 5.001, it also forces
480 any word to the left of it to be interpreted as a string.
482 =head2 List Operators (Rightward)
484 On the right side of a list operator, it has very low precedence,
485 such that it controls all comma-separated expressions found there.
486 The only operators with lower precedence are the logical operators
487 "and", "or", and "not", which may be used to evaluate calls to list
488 operators without the need for extra parentheses:
490 open HANDLE, "filename"
491 or die "Can't open: $!\n";
493 See also discussion of list operators in L<List Operators (Leftward)>.
497 Unary "not" returns the logical negation of the expression to its right.
498 It's the equivalent of "!" except for the very low precedence.
502 Binary "and" returns the logical conjunction of the two surrounding
503 expressions. It's equivalent to && except for the very low
504 precedence. This means that it short-circuits: i.e. the right
505 expression is evaluated only if the left expression is true.
507 =head2 Logical or and Exclusive Or
509 Binary "or" returns the logical disjunction of the two surrounding
510 expressions. It's equivalent to || except for the very low
511 precedence. This means that it short-circuits: i.e. the right
512 expression is evaluated only if the left expression is false.
514 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
515 It cannot short circuit, of course.
517 =head2 C Operators Missing From Perl
519 Here is what C has that Perl doesn't:
525 Address-of operator. (But see the "\" operator for taking a reference.)
529 Dereference-address operator. (Perl's prefix dereferencing
530 operators are typed: $, @, %, and &.)
534 Type casting operator.
538 =head2 Quote and Quotelike Operators
540 While we usually think of quotes as literal values, in Perl they
541 function as operators, providing various kinds of interpolating and
542 pattern matching capabilities. Perl provides customary quote characters
543 for these behaviors, but also provides a way for you to choose your
544 quote character for any of them. In the following table, a C<{}> represents
545 any pair of delimiters you choose. Non-bracketing delimiters use
546 the same character fore and aft, but the 4 sorts of brackets
547 (round, angle, square, curly) will all nest.
549 Customary Generic Meaning Interpolates
554 // m{} Pattern match yes
555 s{}{} Substitution yes
556 tr{}{} Translation no
558 For constructs that do interpolation, variables beginning with "C<$>" or "C<@>"
559 are interpolated, as are the following sequences:
571 \l lowercase next char
572 \u uppercase next char
575 \E end case modification
576 \Q quote regexp metacharacters till \E
578 Patterns are subject to an additional level of interpretation as a
579 regular expression. This is done as a second pass, after variables are
580 interpolated, so that regular expressions may be incorporated into the
581 pattern from the variables. If this is not what you want, use C<\Q> to
582 interpolate a variable literally.
584 Apart from the above, there are no multiple levels of interpolation. In
585 particular, contrary to the expectations of shell programmers, backquotes
586 do I<NOT> interpolate within double quotes, nor do single quotes impede
587 evaluation of variables when used within double quotes.
589 =head2 Regexp Quotelike Operators
591 Here are the quotelike operators that apply to pattern
592 matching and related activities.
598 This is just like the C</pattern/> search, except that it matches only
599 once between calls to the reset() operator. This is a useful
600 optimization when you only want to see the first occurrence of
601 something in each file of a set of files, for instance. Only C<??>
602 patterns local to the current package are reset.
604 This usage is vaguely deprecated, and may be removed in some future
607 =item m/PATTERN/gimosx
609 =item /PATTERN/gimosx
611 Searches a string for a pattern match, and in a scalar context returns
612 true (1) or false (''). If no string is specified via the C<=~> or
613 C<!~> operator, the $_ string is searched. (The string specified with
614 C<=~> need not be an lvalue--it may be the result of an expression
615 evaluation, but remember the C<=~> binds rather tightly.) See also
620 g Match globally, i.e. find all occurrences.
621 i Do case-insensitive pattern matching.
622 m Treat string as multiple lines.
623 o Only compile pattern once.
624 s Treat string as single line.
625 x Use extended regular expressions.
627 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
628 you can use any pair of non-alphanumeric, non-whitespace characters as
629 delimiters. This is particularly useful for matching Unix path names
630 that contain "/", to avoid LTS (leaning toothpick syndrome).
632 PATTERN may contain variables, which will be interpolated (and the
633 pattern recompiled) every time the pattern search is evaluated. (Note
634 that C<$)> and C<$|> might not be interpolated because they look like
635 end-of-string tests.) If you want such a pattern to be compiled only
636 once, add a C</o> after the trailing delimiter. This avoids expensive
637 run-time recompilations, and is useful when the value you are
638 interpolating won't change over the life of the script. However, mentioning
639 C</o> constitutes a promise that you won't change the variables in the pattern.
640 If you change them, Perl won't even notice.
642 If the PATTERN evaluates to a null string, the last
643 successfully executed regular expression is used instead.
645 If used in a context that requires a list value, a pattern match returns a
646 list consisting of the subexpressions matched by the parentheses in the
647 pattern, i.e. ($1, $2, $3...). (Note that here $1 etc. are also set, and
648 that this differs from Perl 4's behavior.) If the match fails, a null
649 array is returned. If the match succeeds, but there were no parentheses,
650 a list value of (1) is returned.
654 open(TTY, '/dev/tty');
655 <TTY> =~ /^y/i && foo(); # do foo if desired
657 if (/Version: *([0-9.]*)/) { $version = $1; }
659 next if m#^/usr/spool/uucp#;
664 print if /$arg/o; # compile only once
667 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
669 This last example splits $foo into the first two words and the
670 remainder of the line, and assigns those three fields to $F1, $F2 and
671 $Etc. The conditional is true if any variables were assigned, i.e. if
674 The C</g> modifier specifies global pattern matching--that is, matching
675 as many times as possible within the string. How it behaves depends on
676 the context. In a list context, it returns a list of all the
677 substrings matched by all the parentheses in the regular expression.
678 If there are no parentheses, it returns a list of all the matched
679 strings, as if there were parentheses around the whole pattern.
681 In a scalar context, C<m//g> iterates through the string, returning TRUE
682 each time it matches, and FALSE when it eventually runs out of
683 matches. (In other words, it remembers where it left off last time and
684 restarts the search at that point. You can actually find the current
685 match position of a string using the pos() function--see L<perlfunc>.)
686 If you modify the string in any way, the match position is reset to the
690 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
693 $/ = ""; $* = 1; # $* deprecated in Perl 5
694 while ($paragraph = <>) {
695 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
699 print "$sentences\n";
705 A single-quoted, literal string. Backslashes are ignored, unless
706 followed by the delimiter or another backslash, in which case the
707 delimiter or backslash is interpolated.
709 $foo = q!I said, "You said, 'She said it.'"!;
710 $bar = q('This is it.');
716 A double-quoted, interpolated string.
719 (*** The previous line contains the naughty word "$1".\n)
720 if /(tcl|rexx|python)/; # :-)
726 A string which is interpolated and then executed as a system command.
727 The collected standard output of the command is returned. In scalar
728 context, it comes back as a single (potentially multi-line) string.
729 In list context, returns a list of lines (however you've defined lines
730 with $/ or $INPUT_RECORD_SEPARATOR).
734 See L<I/O Operators> for more discussion.
738 Returns a list of the words extracted out of STRING, using embedded
739 whitespace as the word delimiters. It is exactly equivalent to
741 split(' ', q/STRING/);
743 Some frequently seen examples:
745 use POSIX qw( setlocale localeconv )
746 @EXPORT = qw( foo bar baz );
748 =item s/PATTERN/REPLACEMENT/egimosx
750 Searches a string for a pattern, and if found, replaces that pattern
751 with the replacement text and returns the number of substitutions
752 made. Otherwise it returns false (0).
754 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
755 variable is searched and modified. (The string specified with C<=~> must
756 be a scalar variable, an array element, a hash element, or an assignment
757 to one of those, i.e. an lvalue.)
759 If the delimiter chosen is single quote, no variable interpolation is
760 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
761 PATTERN contains a $ that looks like a variable rather than an
762 end-of-string test, the variable will be interpolated into the pattern
763 at run-time. If you only want the pattern compiled once the first time
764 the variable is interpolated, use the C</o> option. If the pattern
765 evaluates to a null string, the last successfully executed regular
766 expression is used instead. See L<perlre> for further explanation on these.
770 e Evaluate the right side as an expression.
771 g Replace globally, i.e. all occurrences.
772 i Do case-insensitive pattern matching.
773 m Treat string as multiple lines.
774 o Only compile pattern once.
775 s Treat string as single line.
776 x Use extended regular expressions.
778 Any non-alphanumeric, non-whitespace delimiter may replace the
779 slashes. If single quotes are used, no interpretation is done on the
780 replacement string (the C</e> modifier overrides this, however). If
781 backquotes are used, the replacement string is a command to execute
782 whose output will be used as the actual replacement text. If the
783 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
784 pair of quotes, which may or may not be bracketing quotes, e.g.
785 C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the
786 replacement portion to be interpreter as a full-fledged Perl expression
787 and eval()ed right then and there. It is, however, syntax checked at
792 s/\bgreen\b/mauve/g; # don't change wintergreen
794 $path =~ s|/usr/bin|/usr/local/bin|;
796 s/Login: $foo/Login: $bar/; # run-time pattern
798 ($foo = $bar) =~ s/this/that/;
800 $count = ($paragraph =~ s/Mister\b/Mr./g);
803 s/\d+/$&*2/e; # yields 'abc246xyz'
804 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
805 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
807 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
808 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
809 s/^=(\w+)/&pod($1)/ge; # use function call
811 # /e's can even nest; this will expand
812 # simple embedded variables in $_
817 /\* # Match the opening delimiter.
818 .*? # Match a minimal number of characters.
819 \*/ # Match the closing delimiter.
822 s/^\s*(.*?)\s*$/$1/; # trim white space
824 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
826 Note the use of $ instead of \ in the last example. Unlike
827 B<sed>, we only use the \<I<digit>> form in the left hand side.
828 Anywhere else it's $<I<digit>>.
830 Occasionally, you can't just use a C</g> to get all the changes
831 to occur. Here are two common cases:
833 # put commas in the right places in an integer
834 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4
835 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5
837 # expand tabs to 8-column spacing
838 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
841 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
843 =item y/SEARCHLIST/REPLACEMENTLIST/cds
845 Translates all occurrences of the characters found in the search list
846 with the corresponding character in the replacement list. It returns
847 the number of characters replaced or deleted. If no string is
848 specified via the =~ or !~ operator, the $_ string is translated. (The
849 string specified with =~ must be a scalar variable, an array element,
850 or an assignment to one of those, i.e. an lvalue.) For B<sed> devotees,
851 C<y> is provided as a synonym for C<tr>. If the SEARCHLIST is
852 delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of
853 quotes, which may or may not be bracketing quotes, e.g. C<tr[A-Z][a-z]>
854 or C<tr(+-*/)/ABCD/>.
858 c Complement the SEARCHLIST.
859 d Delete found but unreplaced characters.
860 s Squash duplicate replaced characters.
862 If the C</c> modifier is specified, the SEARCHLIST character set is
863 complemented. If the C</d> modifier is specified, any characters specified
864 by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note
865 that this is slightly more flexible than the behavior of some B<tr>
866 programs, which delete anything they find in the SEARCHLIST, period.)
867 If the C</s> modifier is specified, sequences of characters that were
868 translated to the same character are squashed down to a single instance of the
871 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
872 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
873 than the SEARCHLIST, the final character is replicated till it is long
874 enough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated.
875 This latter is useful for counting characters in a class or for
876 squashing character sequences in a class.
880 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
882 $cnt = tr/*/*/; # count the stars in $_
884 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
886 $cnt = tr/0-9//; # count the digits in $_
888 tr/a-zA-Z//s; # bookkeeper -> bokeper
890 ($HOST = $host) =~ tr/a-z/A-Z/;
892 tr/a-zA-Z/ /cs; # change non-alphas to single space
895 [\000-\177]; # delete 8th bit
897 If multiple translations are given for a character, only the first one is used:
901 will translate any A to X.
903 Note that because the translation table is built at compile time, neither
904 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
905 interpolation. That means that if you want to use variables, you must use
908 eval "tr/$oldlist/$newlist/";
911 eval "tr/$oldlist/$newlist/, 1" or die $@;
917 There are several I/O operators you should know about.
918 A string is enclosed by backticks (grave accents) first undergoes
919 variable substitution just like a double quoted string. It is then
920 interpreted as a command, and the output of that command is the value
921 of the pseudo-literal, like in a shell. In a scalar context, a single
922 string consisting of all the output is returned. In a list context,
923 a list of values is returned, one for each line of output. (You can
924 set C<$/> to use a different line terminator.) The command is executed
925 each time the pseudo-literal is evaluated. The status value of the
926 command is returned in C<$?> (see L<perlvar> for the interpretation
927 of C<$?>). Unlike in B<csh>, no translation is done on the return
928 data--newlines remain newlines. Unlike in any of the shells, single
929 quotes do not hide variable names in the command from interpretation.
930 To pass a $ through to the shell you need to hide it with a backslash.
931 The generalized form of backticks is C<qx//>. (Because backticks
932 always undergo shell expansion as well, see L<perlsec> for
935 Evaluating a filehandle in angle brackets yields the next line from
936 that file (newline included, so it's never false until end of file, at
937 which time an undefined value is returned). Ordinarily you must assign
938 that value to a variable, but there is one situation where an automatic
939 assignment happens. I<If and ONLY if> the input symbol is the only
940 thing inside the conditional of a C<while> loop, the value is
941 automatically assigned to the variable C<$_>. The assigned value is
942 then tested to see if it is defined. (This may seem like an odd thing
943 to you, but you'll use the construct in almost every Perl script you
944 write.) Anyway, the following lines are equivalent to each other:
946 while (defined($_ = <STDIN>)) { print; }
947 while (<STDIN>) { print; }
948 for (;<STDIN>;) { print; }
949 print while defined($_ = <STDIN>);
952 The filehandles STDIN, STDOUT and STDERR are predefined. (The
953 filehandles C<stdin>, C<stdout> and C<stderr> will also work except in
954 packages, where they would be interpreted as local identifiers rather
955 than global.) Additional filehandles may be created with the open()
956 function. See L<perlfunc/open()> for details on this.
958 If a <FILEHANDLE> is used in a context that is looking for a list, a
959 list consisting of all the input lines is returned, one line per list
960 element. It's easy to make a I<LARGE> data space this way, so use with
963 The null filehandle E<lt>E<gt> is special and can be used to emulate the
964 behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from
965 standard input, or from each file listed on the command line. Here's
966 how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is
967 checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened
968 gives you standard input. The @ARGV array is then processed as a list
969 of filenames. The loop
972 ... # code for each line
975 is equivalent to the following Perl-like pseudo code:
977 unshift(@ARGV, '-') if $#ARGV < $[;
978 while ($ARGV = shift) {
981 ... # code for each line
985 except that it isn't so cumbersome to say, and will actually work. It
986 really does shift array @ARGV and put the current filename into variable
987 $ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a synonym
988 for <ARGV>, which is magical. (The pseudo code above doesn't work
989 because it treats <ARGV> as non-magical.)
991 You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up
992 containing the list of filenames you really want. Line numbers (C<$.>)
993 continue as if the input were one big happy file. (But see example
994 under eof() for how to reset line numbers on each file.)
996 If you want to set @ARGV to your own list of files, go right ahead. If
997 you want to pass switches into your script, you can use one of the
998 Getopts modules or put a loop on the front like this:
1000 while ($_ = $ARGV[0], /^-/) {
1003 if (/^-D(.*)/) { $debug = $1 }
1004 if (/^-v/) { $verbose++ }
1005 ... # other switches
1008 ... # code for each line
1011 The E<lt>E<gt> symbol will return FALSE only once. If you call it again after
1012 this it will assume you are processing another @ARGV list, and if you
1013 haven't set @ARGV, will input from STDIN.
1015 If the string inside the angle brackets is a reference to a scalar
1016 variable (e.g. <$foo>), then that variable contains the name of the
1017 filehandle to input from, or a reference to the same. For example:
1022 If the string inside angle brackets is not a filehandle or a scalar
1023 variable containing a filehandle name or reference, then it is interpreted
1024 as a filename pattern to be globbed, and either a list of filenames or the
1025 next filename in the list is returned, depending on context. One level of
1026 $ interpretation is done first, but you can't say C<E<lt>$fooE<gt>>
1027 because that's an indirect filehandle as explained in the previous
1028 paragraph. In older version of Perl, programmers would insert curly
1029 brackets to force interpretation as a filename glob: C<E<lt>${foo}E<gt>>.
1030 These days, it's considered cleaner to call the internal function directly
1031 as C<glob($foo)>, which is probably the right way to have done it in the
1032 first place.) Example:
1040 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1046 In fact, it's currently implemented that way. (Which means it will not
1047 work on filenames with spaces in them unless you have csh(1) on your
1048 machine.) Of course, the shortest way to do the above is:
1052 Because globbing invokes a shell, it's often faster to call readdir() yourself
1053 and just do your own grep() on the filenames. Furthermore, due to its current
1054 implementation of using a shell, the glob() routine may get "Arg list too
1055 long" errors (unless you've installed tcsh(1L) as F</bin/csh>).
1057 A glob only evaluates its (embedded) argument when it is starting a new
1058 list. All values must be read before it will start over. In a list
1059 context this isn't important, because you automatically get them all
1060 anyway. In a scalar context, however, the operator returns the next value
1061 each time it is called, or a FALSE value if you've just run out. Again,
1062 FALSE is returned only once. So if you're expecting a single value from
1063 a glob, it is much better to say
1065 ($file) = <blurch*>;
1071 because the latter will alternate between returning a filename and
1074 It you're trying to do variable interpolation, it's definitely better
1075 to use the glob() function, because the older notation can cause people
1076 to become confused with the indirect filehandle notatin.
1078 @files = glob("$dir/*.[ch]");
1079 @files = glob($files[$i]);
1081 =head2 Constant Folding
1083 Like C, Perl does a certain amount of expression evaluation at
1084 compile time, whenever it determines that all of the arguments to an
1085 operator are static and have no side effects. In particular, string
1086 concatenation happens at compile time between literals that don't do
1087 variable substitution. Backslash interpretation also happens at
1088 compile time. You can say
1090 'Now is the time for all' . "\n" .
1091 'good men to come to.'
1093 and this all reduces to one string internally. Likewise, if
1096 foreach $file (@filenames) {
1097 if (-s $file > 5 + 100 * 2**16) { ... }
1100 the compiler will pre-compute the number that
1101 expression represents so that the interpreter
1105 =head2 Integer arithmetic
1107 By default Perl assumes that it must do most of its arithmetic in
1108 floating point. But by saying
1112 you may tell the compiler that it's okay to use integer operations
1113 from here to the end of the enclosing BLOCK. An inner BLOCK may
1114 countermand this by saying
1118 which lasts until the end of that BLOCK.