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 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 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 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 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 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>.)
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 translation. The left argument is what is
166 supposed to be searched, substituted, or translated instead of the default
167 $_. The return value indicates the success of the operation. (If the
168 right argument is an expression rather than a search pattern,
169 substitution, or translation, it is interpreted as a search pattern at run
170 time. This is less efficient than an explicit search, because the pattern
171 must be compiled every time the expression is evaluated--unless you've
174 Binary "!~" is just like "=~" except the return value is negated in
177 =head2 Multiplicative Operators
179 Binary "*" multiplies two numbers.
181 Binary "/" divides two numbers.
183 Binary "%" computes the modulus of the two numbers.
185 Binary "x" is the repetition operator. In a scalar context, it
186 returns a string consisting of the left operand repeated the number of
187 times specified by the right operand. In a list context, if the left
188 operand is a list in parentheses, it repeats the list.
190 print '-' x 80; # print row of dashes
192 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
194 @ones = (1) x 80; # a list of 80 1's
195 @ones = (5) x @ones; # set all elements to 5
198 =head2 Additive Operators
200 Binary "+" returns the sum of two numbers.
202 Binary "-" returns the difference of two numbers.
204 Binary "." concatenates two strings.
206 =head2 Shift Operators
208 Binary "<<" returns the value of its left argument shifted left by the
209 number of bits specified by the right argument. Arguments should be
210 integers. (See also L<Integer Arithmetic>.)
212 Binary ">>" returns the value of its left argument shifted right by
213 the number of bits specified by the right argument. Arguments should
214 be integers. (See also L<Integer Arithmetic>.)
216 =head2 Named Unary Operators
218 The various named unary operators are treated as functions with one
219 argument, with optional parentheses. These include the filetest
220 operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
222 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
223 is followed by a left parenthesis as the next token, the operator and
224 arguments within parentheses are taken to be of highest precedence,
225 just like a normal function call. Examples:
227 chdir $foo || die; # (chdir $foo) || die
228 chdir($foo) || die; # (chdir $foo) || die
229 chdir ($foo) || die; # (chdir $foo) || die
230 chdir +($foo) || die; # (chdir $foo) || die
232 but, because * is higher precedence than ||:
234 chdir $foo * 20; # chdir ($foo * 20)
235 chdir($foo) * 20; # (chdir $foo) * 20
236 chdir ($foo) * 20; # (chdir $foo) * 20
237 chdir +($foo) * 20; # chdir ($foo * 20)
239 rand 10 * 20; # rand (10 * 20)
240 rand(10) * 20; # (rand 10) * 20
241 rand (10) * 20; # (rand 10) * 20
242 rand +(10) * 20; # rand (10 * 20)
244 See also L<"Terms and List Operators (Leftward)">.
246 =head2 Relational Operators
248 Binary "E<lt>" returns true if the left argument is numerically less than
251 Binary "E<gt>" returns true if the left argument is numerically greater
252 than the right argument.
254 Binary "E<lt>=" returns true if the left argument is numerically less than
255 or equal to the right argument.
257 Binary "E<gt>=" returns true if the left argument is numerically greater
258 than or equal to the right argument.
260 Binary "lt" returns true if the left argument is stringwise less than
263 Binary "gt" returns true if the left argument is stringwise greater
264 than the right argument.
266 Binary "le" returns true if the left argument is stringwise less than
267 or equal to the right argument.
269 Binary "ge" returns true if the left argument is stringwise greater
270 than or equal to the right argument.
272 =head2 Equality Operators
274 Binary "==" returns true if the left argument is numerically equal to
277 Binary "!=" returns true if the left argument is numerically not equal
278 to the right argument.
280 Binary "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left
281 argument is numerically less than, equal to, or greater than the right
284 Binary "eq" returns true if the left argument is stringwise equal to
287 Binary "ne" returns true if the left argument is stringwise not equal
288 to the right argument.
290 Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise
291 less than, equal to, or greater than the right argument.
295 Binary "&" returns its operators ANDed together bit by bit.
296 (See also L<Integer Arithmetic>.)
298 =head2 Bitwise Or and Exclusive Or
300 Binary "|" returns its operators ORed together bit by bit.
301 (See also L<Integer Arithmetic>.)
303 Binary "^" returns its operators XORed together bit by bit.
304 (See also L<Integer Arithmetic>.)
306 =head2 C-style Logical And
308 Binary "&&" performs a short-circuit logical AND operation. That is,
309 if the left operand is false, the right operand is not even evaluated.
310 Scalar or list context propagates down to the right operand if it
313 =head2 C-style Logical Or
315 Binary "||" performs a short-circuit logical OR operation. That is,
316 if the left operand is true, the right operand is not even evaluated.
317 Scalar or list context propagates down to the right operand if it
320 The C<||> and C<&&> operators differ from C's in that, rather than returning
321 0 or 1, they return the last value evaluated. Thus, a reasonably portable
322 way to find out the home directory (assuming it's not "0") might be:
324 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
325 (getpwuid($<))[7] || die "You're homeless!\n";
327 As more readable alternatives to C<&&> and C<||>, Perl provides "and" and
328 "or" operators (see below). The short-circuit behavior is identical. The
329 precedence of "and" and "or" is much lower, however, so that you can
330 safely use them after a list operator without the need for
333 unlink "alpha", "beta", "gamma"
334 or gripe(), next LINE;
336 With the C-style operators that would have been written like this:
338 unlink("alpha", "beta", "gamma")
339 || (gripe(), next LINE);
341 =head2 Range Operator
343 Binary ".." is the range operator, which is really two different
344 operators depending on the context. In a list context, it returns an
345 array of values counting (by ones) from the left value to the right
346 value. This is useful for writing C<for (1..10)> loops and for doing
347 slice operations on arrays. Be aware that under the current implementation,
348 a temporary array is created, so you'll burn a lot of memory if you
349 write something like this:
351 for (1 .. 1_000_000) {
355 In a scalar context, ".." returns a boolean value. The operator is
356 bistable, like a flip-flop, and emulates the line-range (comma) operator
357 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
358 own boolean state. It is false as long as its left operand is false.
359 Once the left operand is true, the range operator stays true until the
360 right operand is true, I<AFTER> which the range operator becomes false
361 again. (It doesn't become false till the next time the range operator is
362 evaluated. It can test the right operand and become false on the same
363 evaluation it became true (as in B<awk>), but it still returns true once.
364 If you don't want it to test the right operand till the next evaluation
365 (as in B<sed>), use three dots ("...") instead of two.) The right
366 operand is not evaluated while the operator is in the "false" state, and
367 the left operand is not evaluated while the operator is in the "true"
368 state. The precedence is a little lower than || and &&. The value
369 returned is either the null string for false, or a sequence number
370 (beginning with 1) for true. The sequence number is reset for each range
371 encountered. The final sequence number in a range has the string "E0"
372 appended to it, which doesn't affect its numeric value, but gives you
373 something to search for if you want to exclude the endpoint. You can
374 exclude the beginning point by waiting for the sequence number to be
375 greater than 1. If either operand of scalar ".." is a numeric literal,
376 that operand is implicitly compared to the C<$.> variable, the current
377 line number. Examples:
379 As a scalar operator:
381 if (101 .. 200) { print; } # print 2nd hundred lines
382 next line if (1 .. /^$/); # skip header lines
383 s/^/> / if (/^$/ .. eof()); # quote body
387 for (101 .. 200) { print; } # print $_ 100 times
388 @foo = @foo[$[ .. $#foo]; # an expensive no-op
389 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
391 The range operator (in a list context) makes use of the magical
392 auto-increment algorithm if the operands are strings. You
395 @alphabet = ('A' .. 'Z');
397 to get all the letters of the alphabet, or
399 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
401 to get a hexadecimal digit, or
403 @z2 = ('01' .. '31'); print $z2[$mday];
405 to get dates with leading zeros. If the final value specified is not
406 in the sequence that the magical increment would produce, the sequence
407 goes until the next value would be longer than the final value
410 =head2 Conditional Operator
412 Ternary "?:" is the conditional operator, just as in C. It works much
413 like an if-then-else. If the argument before the ? is true, the
414 argument before the : is returned, otherwise the argument after the :
415 is returned. For example:
417 printf "I have %d dog%s.\n", $n,
418 ($n == 1) ? '' : "s";
420 Scalar or list context propagates downward into the 2nd
421 or 3rd argument, whichever is selected.
423 $a = $ok ? $b : $c; # get a scalar
424 @a = $ok ? @b : @c; # get an array
425 $a = $ok ? @b : @c; # oops, that's just a count!
427 The operator may be assigned to if both the 2nd and 3rd arguments are
428 legal lvalues (meaning that you can assign to them):
430 ($a_or_b ? $a : $b) = $c;
432 This is not necessarily guaranteed to contribute to the readability of your program.
434 =head2 Assignment Operators
436 "=" is the ordinary assignment operator.
438 Assignment operators work as in C. That is,
446 although without duplicating any side effects that dereferencing the lvalue
447 might trigger, such as from tie(). Other assignment operators work similarly.
448 The following are recognized:
455 Note that while these are grouped by family, they all have the precedence
458 Unlike in C, the assignment operator produces a valid lvalue. Modifying
459 an assignment is equivalent to doing the assignment and then modifying
460 the variable that was assigned to. This is useful for modifying
461 a copy of something, like this:
463 ($tmp = $global) =~ tr [A-Z] [a-z];
474 =head2 Comma Operator
476 Binary "," is the comma operator. In a scalar context it evaluates
477 its left argument, throws that value away, then evaluates its right
478 argument and returns that value. This is just like C's comma operator.
480 In a list context, it's just the list argument separator, and inserts
481 both its arguments into the list.
483 The =E<gt> digraph is mostly just a synonym for the comma operator. It's useful for
484 documenting arguments that come in pairs. As of release 5.001, it also forces
485 any word to the left of it to be interpreted as a string.
487 =head2 List Operators (Rightward)
489 On the right side of a list operator, it has very low precedence,
490 such that it controls all comma-separated expressions found there.
491 The only operators with lower precedence are the logical operators
492 "and", "or", and "not", which may be used to evaluate calls to list
493 operators without the need for extra parentheses:
495 open HANDLE, "filename"
496 or die "Can't open: $!\n";
498 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
502 Unary "not" returns the logical negation of the expression to its right.
503 It's the equivalent of "!" except for the very low precedence.
507 Binary "and" returns the logical conjunction of the two surrounding
508 expressions. It's equivalent to && except for the very low
509 precedence. This means that it short-circuits: i.e., the right
510 expression is evaluated only if the left expression is true.
512 =head2 Logical or and Exclusive Or
514 Binary "or" returns the logical disjunction of the two surrounding
515 expressions. It's equivalent to || except for the very low
516 precedence. This means that it short-circuits: i.e., the right
517 expression is evaluated only if the left expression is false.
519 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
520 It cannot short circuit, of course.
522 =head2 C Operators Missing From Perl
524 Here is what C has that Perl doesn't:
530 Address-of operator. (But see the "\" operator for taking a reference.)
534 Dereference-address operator. (Perl's prefix dereferencing
535 operators are typed: $, @, %, and &.)
539 Type casting operator.
543 =head2 Quote and Quote-like Operators
545 While we usually think of quotes as literal values, in Perl they
546 function as operators, providing various kinds of interpolating and
547 pattern matching capabilities. Perl provides customary quote characters
548 for these behaviors, but also provides a way for you to choose your
549 quote character for any of them. In the following table, a C<{}> represents
550 any pair of delimiters you choose. Non-bracketing delimiters use
551 the same character fore and aft, but the 4 sorts of brackets
552 (round, angle, square, curly) will all nest.
554 Customary Generic Meaning Interpolates
559 // m{} Pattern match yes
560 s{}{} Substitution yes
561 tr{}{} Translation no
563 For constructs that do interpolation, variables beginning with "C<$>" or "C<@>"
564 are interpolated, as are the following sequences:
571 \a alarm (bell) (BEL)
576 \l lowercase next char
577 \u uppercase next char
580 \E end case modification
581 \Q quote regexp metacharacters till \E
583 Patterns are subject to an additional level of interpretation as a
584 regular expression. This is done as a second pass, after variables are
585 interpolated, so that regular expressions may be incorporated into the
586 pattern from the variables. If this is not what you want, use C<\Q> to
587 interpolate a variable literally.
589 Apart from the above, there are no multiple levels of interpolation. In
590 particular, contrary to the expectations of shell programmers, back-quotes
591 do I<NOT> interpolate within double quotes, nor do single quotes impede
592 evaluation of variables when used within double quotes.
594 =head2 Regexp Quote-Like Operators
596 Here are the quote-like operators that apply to pattern
597 matching and related activities.
603 This is just like the C</pattern/> search, except that it matches only
604 once between calls to the reset() operator. This is a useful
605 optimization when you want to see only the first occurrence of
606 something in each file of a set of files, for instance. Only C<??>
607 patterns local to the current package are reset.
609 This usage is vaguely deprecated, and may be removed in some future
612 =item m/PATTERN/gimosx
614 =item /PATTERN/gimosx
616 Searches a string for a pattern match, and in a scalar context returns
617 true (1) or false (''). If no string is specified via the C<=~> or
618 C<!~> operator, the $_ string is searched. (The string specified with
619 C<=~> need not be an lvalue--it may be the result of an expression
620 evaluation, but remember the C<=~> binds rather tightly.) See also
625 g Match globally, i.e., find all occurrences.
626 i Do case-insensitive pattern matching.
627 m Treat string as multiple lines.
628 o Compile pattern only once.
629 s Treat string as single line.
630 x Use extended regular expressions.
632 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
633 you can use any pair of non-alphanumeric, non-whitespace characters as
634 delimiters. This is particularly useful for matching Unix path names
635 that contain "/", to avoid LTS (leaning toothpick syndrome).
637 PATTERN may contain variables, which will be interpolated (and the
638 pattern recompiled) every time the pattern search is evaluated. (Note
639 that C<$)> and C<$|> might not be interpolated because they look like
640 end-of-string tests.) If you want such a pattern to be compiled only
641 once, add a C</o> after the trailing delimiter. This avoids expensive
642 run-time recompilations, and is useful when the value you are
643 interpolating won't change over the life of the script. However, mentioning
644 C</o> constitutes a promise that you won't change the variables in the pattern.
645 If you change them, Perl won't even notice.
647 If the PATTERN evaluates to a null string, the last
648 successfully executed regular expression is used instead.
650 If used in a context that requires a list value, a pattern match returns a
651 list consisting of the subexpressions matched by the parentheses in the
652 pattern, i.e., (C<$1>, $2, $3...). (Note that here $1 etc. are also set, and
653 that this differs from Perl 4's behavior.) If the match fails, a null
654 array is returned. If the match succeeds, but there were no parentheses,
655 a list value of (1) is returned.
659 open(TTY, '/dev/tty');
660 <TTY> =~ /^y/i && foo(); # do foo if desired
662 if (/Version: *([0-9.]*)/) { $version = $1; }
664 next if m#^/usr/spool/uucp#;
669 print if /$arg/o; # compile only once
672 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
674 This last example splits $foo into the first two words and the
675 remainder of the line, and assigns those three fields to $F1, $F2 and
676 $Etc. The conditional is true if any variables were assigned, i.e., if
679 The C</g> modifier specifies global pattern matching--that is, matching
680 as many times as possible within the string. How it behaves depends on
681 the context. In a list context, it returns a list of all the
682 substrings matched by all the parentheses in the regular expression.
683 If there are no parentheses, it returns a list of all the matched
684 strings, as if there were parentheses around the whole pattern.
686 In a scalar context, C<m//g> iterates through the string, returning TRUE
687 each time it matches, and FALSE when it eventually runs out of
688 matches. (In other words, it remembers where it left off last time and
689 restarts the search at that point. You can actually find the current
690 match position of a string using the pos() function--see L<perlfunc>.)
691 If you modify the string in any way, the match position is reset to the
695 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
698 $/ = ""; $* = 1; # $* deprecated in Perl 5
699 while ($paragraph = <>) {
700 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
704 print "$sentences\n";
710 A single-quoted, literal string. Backslashes are ignored, unless
711 followed by the delimiter or another backslash, in which case the
712 delimiter or backslash is interpolated.
714 $foo = q!I said, "You said, 'She said it.'"!;
715 $bar = q('This is it.');
721 A double-quoted, interpolated string.
724 (*** The previous line contains the naughty word "$1".\n)
725 if /(tcl|rexx|python)/; # :-)
731 A string which is interpolated and then executed as a system command.
732 The collected standard output of the command is returned. In scalar
733 context, it comes back as a single (potentially multi-line) string.
734 In list context, returns a list of lines (however you've defined lines
735 with $/ or $INPUT_RECORD_SEPARATOR).
739 See L<I/O Operators> for more discussion.
743 Returns a list of the words extracted out of STRING, using embedded
744 whitespace as the word delimiters. It is exactly equivalent to
746 split(' ', q/STRING/);
748 Some frequently seen examples:
750 use POSIX qw( setlocale localeconv )
751 @EXPORT = qw( foo bar baz );
753 =item s/PATTERN/REPLACEMENT/egimosx
755 Searches a string for a pattern, and if found, replaces that pattern
756 with the replacement text and returns the number of substitutions
757 made. Otherwise it returns false (specifically, the empty string).
759 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
760 variable is searched and modified. (The string specified with C<=~> must
761 be a scalar variable, an array element, a hash element, or an assignment
762 to one of those, i.e., an lvalue.)
764 If the delimiter chosen is single quote, no variable interpolation is
765 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
766 PATTERN contains a $ that looks like a variable rather than an
767 end-of-string test, the variable will be interpolated into the pattern
768 at run-time. If you want the pattern compiled only once the first time
769 the variable is interpolated, use the C</o> option. If the pattern
770 evaluates to a null string, the last successfully executed regular
771 expression is used instead. See L<perlre> for further explanation on these.
775 e Evaluate the right side as an expression.
776 g Replace globally, i.e., all occurrences.
777 i Do case-insensitive pattern matching.
778 m Treat string as multiple lines.
779 o Compile pattern only once.
780 s Treat string as single line.
781 x Use extended regular expressions.
783 Any non-alphanumeric, non-whitespace delimiter may replace the
784 slashes. If single quotes are used, no interpretation is done on the
785 replacement string (the C</e> modifier overrides this, however). Unlike
786 Perl 4, Perl 5 treats back-ticks as normal delimiters; the replacement
787 text is not evaluated as a command. If the
788 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
789 pair of quotes, which may or may not be bracketing quotes, e.g.,
790 C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the
791 replacement portion to be interpreter as a full-fledged Perl expression
792 and eval()ed right then and there. It is, however, syntax checked at
797 s/\bgreen\b/mauve/g; # don't change wintergreen
799 $path =~ s|/usr/bin|/usr/local/bin|;
801 s/Login: $foo/Login: $bar/; # run-time pattern
803 ($foo = $bar) =~ s/this/that/;
805 $count = ($paragraph =~ s/Mister\b/Mr./g);
808 s/\d+/$&*2/e; # yields 'abc246xyz'
809 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
810 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
812 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
813 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
814 s/^=(\w+)/&pod($1)/ge; # use function call
816 # /e's can even nest; this will expand
817 # simple embedded variables in $_
822 /\* # Match the opening delimiter.
823 .*? # Match a minimal number of characters.
824 \*/ # Match the closing delimiter.
827 s/^\s*(.*?)\s*$/$1/; # trim white space
829 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
831 Note the use of $ instead of \ in the last example. Unlike
832 B<sed>, we use the \E<lt>I<digit>E<gt> form in only the left hand side.
833 Anywhere else it's $E<lt>I<digit>E<gt>.
835 Occasionally, you can't use just a C</g> to get all the changes
836 to occur. Here are two common cases:
838 # put commas in the right places in an integer
839 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4
840 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5
842 # expand tabs to 8-column spacing
843 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
846 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
848 =item y/SEARCHLIST/REPLACEMENTLIST/cds
850 Translates all occurrences of the characters found in the search list
851 with the corresponding character in the replacement list. It returns
852 the number of characters replaced or deleted. If no string is
853 specified via the =~ or !~ operator, the $_ string is translated. (The
854 string specified with =~ must be a scalar variable, an array element,
855 or an assignment to one of those, i.e., an lvalue.) For B<sed> devotees,
856 C<y> is provided as a synonym for C<tr>. If the SEARCHLIST is
857 delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of
858 quotes, which may or may not be bracketing quotes, e.g., C<tr[A-Z][a-z]>
859 or C<tr(+-*/)/ABCD/>.
863 c Complement the SEARCHLIST.
864 d Delete found but unreplaced characters.
865 s Squash duplicate replaced characters.
867 If the C</c> modifier is specified, the SEARCHLIST character set is
868 complemented. If the C</d> modifier is specified, any characters specified
869 by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note
870 that this is slightly more flexible than the behavior of some B<tr>
871 programs, which delete anything they find in the SEARCHLIST, period.)
872 If the C</s> modifier is specified, sequences of characters that were
873 translated to the same character are squashed down to a single instance of the
876 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
877 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
878 than the SEARCHLIST, the final character is replicated till it is long
879 enough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated.
880 This latter is useful for counting characters in a class or for
881 squashing character sequences in a class.
885 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
887 $cnt = tr/*/*/; # count the stars in $_
889 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
891 $cnt = tr/0-9//; # count the digits in $_
893 tr/a-zA-Z//s; # bookkeeper -> bokeper
895 ($HOST = $host) =~ tr/a-z/A-Z/;
897 tr/a-zA-Z/ /cs; # change non-alphas to single space
900 [\000-\177]; # delete 8th bit
902 If multiple translations are given for a character, only the first one is used:
906 will translate any A to X.
908 Note that because the translation table is built at compile time, neither
909 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
910 interpolation. That means that if you want to use variables, you must use
913 eval "tr/$oldlist/$newlist/";
916 eval "tr/$oldlist/$newlist/, 1" or die $@;
922 There are several I/O operators you should know about.
923 A string is enclosed by back-ticks (grave accents) first undergoes
924 variable substitution just like a double quoted string. It is then
925 interpreted as a command, and the output of that command is the value
926 of the pseudo-literal, like in a shell. In a scalar context, a single
927 string consisting of all the output is returned. In a list context,
928 a list of values is returned, one for each line of output. (You can
929 set C<$/> to use a different line terminator.) The command is executed
930 each time the pseudo-literal is evaluated. The status value of the
931 command is returned in C<$?> (see L<perlvar> for the interpretation
932 of C<$?>). Unlike in B<csh>, no translation is done on the return
933 data--newlines remain newlines. Unlike in any of the shells, single
934 quotes do not hide variable names in the command from interpretation.
935 To pass a $ through to the shell you need to hide it with a backslash.
936 The generalized form of back-ticks is C<qx//>. (Because back-ticks
937 always undergo shell expansion as well, see L<perlsec> for
940 Evaluating a filehandle in angle brackets yields the next line from
941 that file (newline included, so it's never false until end of file, at
942 which time an undefined value is returned). Ordinarily you must assign
943 that value to a variable, but there is one situation where an automatic
944 assignment happens. I<If and ONLY if> the input symbol is the only
945 thing inside the conditional of a C<while> loop, the value is
946 automatically assigned to the variable C<$_>. The assigned value is
947 then tested to see if it is defined. (This may seem like an odd thing
948 to you, but you'll use the construct in almost every Perl script you
949 write.) Anyway, the following lines are equivalent to each other:
951 while (defined($_ = <STDIN>)) { print; }
952 while (<STDIN>) { print; }
953 for (;<STDIN>;) { print; }
954 print while defined($_ = <STDIN>);
957 The filehandles STDIN, STDOUT and STDERR are predefined. (The
958 filehandles C<stdin>, C<stdout> and C<stderr> will also work except in
959 packages, where they would be interpreted as local identifiers rather
960 than global.) Additional filehandles may be created with the open()
961 function. See L<perlfunc/open()> for details on this.
963 If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a
964 list consisting of all the input lines is returned, one line per list
965 element. It's easy to make a I<LARGE> data space this way, so use with
968 The null filehandle E<lt>E<gt> is special and can be used to emulate the
969 behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from
970 standard input, or from each file listed on the command line. Here's
971 how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is
972 checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened
973 gives you standard input. The @ARGV array is then processed as a list
974 of filenames. The loop
977 ... # code for each line
980 is equivalent to the following Perl-like pseudo code:
982 unshift(@ARGV, '-') if $#ARGV < $[;
983 while ($ARGV = shift) {
986 ... # code for each line
990 except that it isn't so cumbersome to say, and will actually work. It
991 really does shift array @ARGV and put the current filename into variable
992 $ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a
993 synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above
994 doesn't work because it treats E<lt>ARGVE<gt> as non-magical.)
996 You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up
997 containing the list of filenames you really want. Line numbers (C<$.>)
998 continue as if the input were one big happy file. (But see example
999 under eof() for how to reset line numbers on each file.)
1001 If you want to set @ARGV to your own list of files, go right ahead. If
1002 you want to pass switches into your script, you can use one of the
1003 Getopts modules or put a loop on the front like this:
1005 while ($_ = $ARGV[0], /^-/) {
1008 if (/^-D(.*)/) { $debug = $1 }
1009 if (/^-v/) { $verbose++ }
1010 ... # other switches
1013 ... # code for each line
1016 The E<lt>E<gt> symbol will return FALSE only once. If you call it again after
1017 this it will assume you are processing another @ARGV list, and if you
1018 haven't set @ARGV, will input from STDIN.
1020 If the string inside the angle brackets is a reference to a scalar
1021 variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the
1022 filehandle to input from, or a reference to the same. For example:
1027 If the string inside angle brackets is not a filehandle or a scalar
1028 variable containing a filehandle name or reference, then it is interpreted
1029 as a filename pattern to be globbed, and either a list of filenames or the
1030 next filename in the list is returned, depending on context. One level of
1031 $ interpretation is done first, but you can't say C<E<lt>$fooE<gt>>
1032 because that's an indirect filehandle as explained in the previous
1033 paragraph. (In older versions of Perl, programmers would insert curly
1034 brackets to force interpretation as a filename glob: C<E<lt>${foo}E<gt>>.
1035 These days, it's considered cleaner to call the internal function directly
1036 as C<glob($foo)>, which is probably the right way to have done it in the
1037 first place.) Example:
1045 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1051 In fact, it's currently implemented that way. (Which means it will not
1052 work on filenames with spaces in them unless you have csh(1) on your
1053 machine.) Of course, the shortest way to do the above is:
1057 Because globbing invokes a shell, it's often faster to call readdir() yourself
1058 and do your own grep() on the filenames. Furthermore, due to its current
1059 implementation of using a shell, the glob() routine may get "Arg list too
1060 long" errors (unless you've installed tcsh(1L) as F</bin/csh>).
1062 A glob evaluates its (embedded) argument only when it is starting a new
1063 list. All values must be read before it will start over. In a list
1064 context this isn't important, because you automatically get them all
1065 anyway. In a scalar context, however, the operator returns the next value
1066 each time it is called, or a FALSE value if you've just run out. Again,
1067 FALSE is returned only once. So if you're expecting a single value from
1068 a glob, it is much better to say
1070 ($file) = <blurch*>;
1076 because the latter will alternate between returning a filename and
1079 It you're trying to do variable interpolation, it's definitely better
1080 to use the glob() function, because the older notation can cause people
1081 to become confused with the indirect filehandle notation.
1083 @files = glob("$dir/*.[ch]");
1084 @files = glob($files[$i]);
1086 =head2 Constant Folding
1088 Like C, Perl does a certain amount of expression evaluation at
1089 compile time, whenever it determines that all of the arguments to an
1090 operator are static and have no side effects. In particular, string
1091 concatenation happens at compile time between literals that don't do
1092 variable substitution. Backslash interpretation also happens at
1093 compile time. You can say
1095 'Now is the time for all' . "\n" .
1096 'good men to come to.'
1098 and this all reduces to one string internally. Likewise, if
1101 foreach $file (@filenames) {
1102 if (-s $file > 5 + 100 * 2**16) { ... }
1105 the compiler will pre-compute the number that
1106 expression represents so that the interpreter
1110 =head2 Integer Arithmetic
1112 By default Perl assumes that it must do most of its arithmetic in
1113 floating point. But by saying
1117 you may tell the compiler that it's okay to use integer operations
1118 from here to the end of the enclosing BLOCK. An inner BLOCK may
1119 countermand this by saying
1123 which lasts until the end of that BLOCK.
1125 The bitwise operators ("&", "|", "^", "~", "<<", and ">>") always
1126 produce integral results. However, C<use integer> still has meaning
1127 for them. By default, their results are interpreted as unsigned
1128 integers. However, if C<use integer> is in effect, their results are
1129 interpreted as signed integers. For example, C<~0> usually evaluates
1130 to a large integral value. However, C<use integer; ~0> is -1.