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 "E<lt>E<lt>" 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 "E<gt>E<gt>" 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 "E<lt>" returns true if the left argument is numerically less than
250 Binary "E<gt>" returns true if the left argument is numerically greater
251 than the right argument.
253 Binary "E<lt>=" returns true if the left argument is numerically less than
254 or equal to the right argument.
256 Binary "E<gt>=" 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 "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left
280 argument is numerically less than, equal to, or greater than the right
283 Binary "eq" returns true if the left argument is stringwise equal to
286 Binary "ne" returns true if the left argument is stringwise not equal
287 to the right argument.
289 Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise
290 less than, equal to, or greater than the right argument.
294 Binary "&" returns its operators ANDed together bit by bit.
296 =head2 Bitwise Or and Exclusive Or
298 Binary "|" returns its operators ORed together bit by bit.
300 Binary "^" returns its operators XORed together bit by bit.
302 =head2 C-style Logical And
304 Binary "&&" performs a short-circuit logical AND operation. That is,
305 if the left operand is false, the right operand is not even evaluated.
306 Scalar or list context propagates down to the right operand if it
309 =head2 C-style Logical Or
311 Binary "||" performs a short-circuit logical OR operation. That is,
312 if the left operand is true, the right operand is not even evaluated.
313 Scalar or list context propagates down to the right operand if it
316 The C<||> and C<&&> operators differ from C's in that, rather than returning
317 0 or 1, they return the last value evaluated. Thus, a reasonably portable
318 way to find out the home directory (assuming it's not "0") might be:
320 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
321 (getpwuid($<))[7] || die "You're homeless!\n";
323 As more readable alternatives to C<&&> and C<||>, Perl provides "and" and
324 "or" operators (see below). The short-circuit behavior is identical. The
325 precedence of "and" and "or" is much lower, however, so that you can
326 safely use them after a list operator without the need for
329 unlink "alpha", "beta", "gamma"
330 or gripe(), next LINE;
332 With the C-style operators that would have been written like this:
334 unlink("alpha", "beta", "gamma")
335 || (gripe(), next LINE);
337 =head2 Range Operator
339 Binary ".." is the range operator, which is really two different
340 operators depending on the context. In a list context, it returns an
341 array of values counting (by ones) from the left value to the right
342 value. This is useful for writing C<for (1..10)> loops and for doing
343 slice operations on arrays. Be aware that under the current implementation,
344 a temporary array is created, so you'll burn a lot of memory if you
345 write something like this:
347 for (1 .. 1_000_000) {
351 In a scalar context, ".." returns a boolean value. The operator is
352 bistable, like a flip-flop, and emulates the line-range (comma) operator
353 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
354 own boolean state. It is false as long as its left operand is false.
355 Once the left operand is true, the range operator stays true until the
356 right operand is true, I<AFTER> which the range operator becomes false
357 again. (It doesn't become false till the next time the range operator is
358 evaluated. It can test the right operand and become false on the same
359 evaluation it became true (as in B<awk>), but it still returns true once.
360 If you don't want it to test the right operand till the next evaluation
361 (as in B<sed>), use three dots ("...") instead of two.) The right
362 operand is not evaluated while the operator is in the "false" state, and
363 the left operand is not evaluated while the operator is in the "true"
364 state. The precedence is a little lower than || and &&. The value
365 returned is either the null string for false, or a sequence number
366 (beginning with 1) for true. The sequence number is reset for each range
367 encountered. The final sequence number in a range has the string "E0"
368 appended to it, which doesn't affect its numeric value, but gives you
369 something to search for if you want to exclude the endpoint. You can
370 exclude the beginning point by waiting for the sequence number to be
371 greater than 1. If either operand of scalar ".." is a numeric literal,
372 that operand is implicitly compared to the C<$.> variable, the current
373 line number. Examples:
375 As a scalar operator:
377 if (101 .. 200) { print; } # print 2nd hundred lines
378 next line if (1 .. /^$/); # skip header lines
379 s/^/> / if (/^$/ .. eof()); # quote body
383 for (101 .. 200) { print; } # print $_ 100 times
384 @foo = @foo[$[ .. $#foo]; # an expensive no-op
385 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
387 The range operator (in a list context) makes use of the magical
388 autoincrement algorithm if the operands are strings. You
391 @alphabet = ('A' .. 'Z');
393 to get all the letters of the alphabet, or
395 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
397 to get a hexadecimal digit, or
399 @z2 = ('01' .. '31'); print $z2[$mday];
401 to get dates with leading zeros. If the final value specified is not
402 in the sequence that the magical increment would produce, the sequence
403 goes until the next value would be longer than the final value
406 =head2 Conditional Operator
408 Ternary "?:" is the conditional operator, just as in C. It works much
409 like an if-then-else. If the argument before the ? is true, the
410 argument before the : is returned, otherwise the argument after the :
411 is returned. For example:
413 printf "I have %d dog%s.\n", $n,
414 ($n == 1) ? '' : "s";
416 Scalar or list context propagates downward into the 2nd
417 or 3rd argument, whichever is selected.
419 $a = $ok ? $b : $c; # get a scalar
420 @a = $ok ? @b : @c; # get an array
421 $a = $ok ? @b : @c; # oops, that's just a count!
423 The operator may be assigned to if both the 2nd and 3rd arguments are
424 legal lvalues (meaning that you can assign to them):
426 ($a_or_b ? $a : $b) = $c;
428 This is not necessarily guaranteed to contribute to the readability of your program.
430 =head2 Assignment Operators
432 "=" is the ordinary assignment operator.
434 Assignment operators work as in C. That is,
442 although without duplicating any side effects that dereferencing the lvalue
443 might trigger, such as from tie(). Other assignment operators work similarly.
444 The following are recognized:
451 Note that while these are grouped by family, they all have the precedence
454 Unlike in C, the assignment operator produces a valid lvalue. Modifying
455 an assignment is equivalent to doing the assignment and then modifying
456 the variable that was assigned to. This is useful for modifying
457 a copy of something, like this:
459 ($tmp = $global) =~ tr [A-Z] [a-z];
470 =head2 Comma Operator
472 Binary "," is the comma operator. In a scalar context it evaluates
473 its left argument, throws that value away, then evaluates its right
474 argument and returns that value. This is just like C's comma operator.
476 In a list context, it's just the list argument separator, and inserts
477 both its arguments into the list.
479 The =E<gt> digraph is mostly just a synonym for the comma operator. It's useful for
480 documenting arguments that come in pairs. As of release 5.001, it also forces
481 any word to the left of it to be interpreted as a string.
483 =head2 List Operators (Rightward)
485 On the right side of a list operator, it has very low precedence,
486 such that it controls all comma-separated expressions found there.
487 The only operators with lower precedence are the logical operators
488 "and", "or", and "not", which may be used to evaluate calls to list
489 operators without the need for extra parentheses:
491 open HANDLE, "filename"
492 or die "Can't open: $!\n";
494 See also discussion of list operators in L<List Operators (Leftward)>.
498 Unary "not" returns the logical negation of the expression to its right.
499 It's the equivalent of "!" except for the very low precedence.
503 Binary "and" returns the logical conjunction of the two surrounding
504 expressions. It's equivalent to && except for the very low
505 precedence. This means that it short-circuits: i.e. the right
506 expression is evaluated only if the left expression is true.
508 =head2 Logical or and Exclusive Or
510 Binary "or" returns the logical disjunction of the two surrounding
511 expressions. It's equivalent to || except for the very low
512 precedence. This means that it short-circuits: i.e. the right
513 expression is evaluated only if the left expression is false.
515 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
516 It cannot short circuit, of course.
518 =head2 C Operators Missing From Perl
520 Here is what C has that Perl doesn't:
526 Address-of operator. (But see the "\" operator for taking a reference.)
530 Dereference-address operator. (Perl's prefix dereferencing
531 operators are typed: $, @, %, and &.)
535 Type casting operator.
539 =head2 Quote and Quotelike Operators
541 While we usually think of quotes as literal values, in Perl they
542 function as operators, providing various kinds of interpolating and
543 pattern matching capabilities. Perl provides customary quote characters
544 for these behaviors, but also provides a way for you to choose your
545 quote character for any of them. In the following table, a C<{}> represents
546 any pair of delimiters you choose. Non-bracketing delimiters use
547 the same character fore and aft, but the 4 sorts of brackets
548 (round, angle, square, curly) will all nest.
550 Customary Generic Meaning Interpolates
555 // m{} Pattern match yes
556 s{}{} Substitution yes
557 tr{}{} Translation no
559 For constructs that do interpolation, variables beginning with "C<$>" or "C<@>"
560 are interpolated, as are the following sequences:
567 \a alarm (bell) (BEL)
572 \l lowercase next char
573 \u uppercase next char
576 \E end case modification
577 \Q quote regexp metacharacters till \E
579 Patterns are subject to an additional level of interpretation as a
580 regular expression. This is done as a second pass, after variables are
581 interpolated, so that regular expressions may be incorporated into the
582 pattern from the variables. If this is not what you want, use C<\Q> to
583 interpolate a variable literally.
585 Apart from the above, there are no multiple levels of interpolation. In
586 particular, contrary to the expectations of shell programmers, backquotes
587 do I<NOT> interpolate within double quotes, nor do single quotes impede
588 evaluation of variables when used within double quotes.
590 =head2 Regexp Quotelike Operators
592 Here are the quotelike operators that apply to pattern
593 matching and related activities.
599 This is just like the C</pattern/> search, except that it matches only
600 once between calls to the reset() operator. This is a useful
601 optimization when you only want to see the first occurrence of
602 something in each file of a set of files, for instance. Only C<??>
603 patterns local to the current package are reset.
605 This usage is vaguely deprecated, and may be removed in some future
608 =item m/PATTERN/gimosx
610 =item /PATTERN/gimosx
612 Searches a string for a pattern match, and in a scalar context returns
613 true (1) or false (''). If no string is specified via the C<=~> or
614 C<!~> operator, the $_ string is searched. (The string specified with
615 C<=~> need not be an lvalue--it may be the result of an expression
616 evaluation, but remember the C<=~> binds rather tightly.) See also
621 g Match globally, i.e. find all occurrences.
622 i Do case-insensitive pattern matching.
623 m Treat string as multiple lines.
624 o Only compile pattern once.
625 s Treat string as single line.
626 x Use extended regular expressions.
628 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
629 you can use any pair of non-alphanumeric, non-whitespace characters as
630 delimiters. This is particularly useful for matching Unix path names
631 that contain "/", to avoid LTS (leaning toothpick syndrome).
633 PATTERN may contain variables, which will be interpolated (and the
634 pattern recompiled) every time the pattern search is evaluated. (Note
635 that C<$)> and C<$|> might not be interpolated because they look like
636 end-of-string tests.) If you want such a pattern to be compiled only
637 once, add a C</o> after the trailing delimiter. This avoids expensive
638 run-time recompilations, and is useful when the value you are
639 interpolating won't change over the life of the script. However, mentioning
640 C</o> constitutes a promise that you won't change the variables in the pattern.
641 If you change them, Perl won't even notice.
643 If the PATTERN evaluates to a null string, the last
644 successfully executed regular expression is used instead.
646 If used in a context that requires a list value, a pattern match returns a
647 list consisting of the subexpressions matched by the parentheses in the
648 pattern, i.e. (C<$1>, $2, $3...). (Note that here $1 etc. are also set, and
649 that this differs from Perl 4's behavior.) If the match fails, a null
650 array is returned. If the match succeeds, but there were no parentheses,
651 a list value of (1) is returned.
655 open(TTY, '/dev/tty');
656 <TTY> =~ /^y/i && foo(); # do foo if desired
658 if (/Version: *([0-9.]*)/) { $version = $1; }
660 next if m#^/usr/spool/uucp#;
665 print if /$arg/o; # compile only once
668 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
670 This last example splits $foo into the first two words and the
671 remainder of the line, and assigns those three fields to $F1, $F2 and
672 $Etc. The conditional is true if any variables were assigned, i.e. if
675 The C</g> modifier specifies global pattern matching--that is, matching
676 as many times as possible within the string. How it behaves depends on
677 the context. In a list context, it returns a list of all the
678 substrings matched by all the parentheses in the regular expression.
679 If there are no parentheses, it returns a list of all the matched
680 strings, as if there were parentheses around the whole pattern.
682 In a scalar context, C<m//g> iterates through the string, returning TRUE
683 each time it matches, and FALSE when it eventually runs out of
684 matches. (In other words, it remembers where it left off last time and
685 restarts the search at that point. You can actually find the current
686 match position of a string using the pos() function--see L<perlfunc>.)
687 If you modify the string in any way, the match position is reset to the
691 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
694 $/ = ""; $* = 1; # $* deprecated in Perl 5
695 while ($paragraph = <>) {
696 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
700 print "$sentences\n";
706 A single-quoted, literal string. Backslashes are ignored, unless
707 followed by the delimiter or another backslash, in which case the
708 delimiter or backslash is interpolated.
710 $foo = q!I said, "You said, 'She said it.'"!;
711 $bar = q('This is it.');
717 A double-quoted, interpolated string.
720 (*** The previous line contains the naughty word "$1".\n)
721 if /(tcl|rexx|python)/; # :-)
727 A string which is interpolated and then executed as a system command.
728 The collected standard output of the command is returned. In scalar
729 context, it comes back as a single (potentially multi-line) string.
730 In list context, returns a list of lines (however you've defined lines
731 with $/ or $INPUT_RECORD_SEPARATOR).
735 See L<I/O Operators> for more discussion.
739 Returns a list of the words extracted out of STRING, using embedded
740 whitespace as the word delimiters. It is exactly equivalent to
742 split(' ', q/STRING/);
744 Some frequently seen examples:
746 use POSIX qw( setlocale localeconv )
747 @EXPORT = qw( foo bar baz );
749 =item s/PATTERN/REPLACEMENT/egimosx
751 Searches a string for a pattern, and if found, replaces that pattern
752 with the replacement text and returns the number of substitutions
753 made. Otherwise it returns false (specifically, the empty string).
755 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
756 variable is searched and modified. (The string specified with C<=~> must
757 be a scalar variable, an array element, a hash element, or an assignment
758 to one of those, i.e. an lvalue.)
760 If the delimiter chosen is single quote, no variable interpolation is
761 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
762 PATTERN contains a $ that looks like a variable rather than an
763 end-of-string test, the variable will be interpolated into the pattern
764 at run-time. If you only want the pattern compiled once the first time
765 the variable is interpolated, use the C</o> option. If the pattern
766 evaluates to a null string, the last successfully executed regular
767 expression is used instead. See L<perlre> for further explanation on these.
771 e Evaluate the right side as an expression.
772 g Replace globally, i.e. all occurrences.
773 i Do case-insensitive pattern matching.
774 m Treat string as multiple lines.
775 o Only compile pattern once.
776 s Treat string as single line.
777 x Use extended regular expressions.
779 Any non-alphanumeric, non-whitespace delimiter may replace the
780 slashes. If single quotes are used, no interpretation is done on the
781 replacement string (the C</e> modifier overrides this, however). Unlike
782 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
783 text is not evaluated as a command. If the
784 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
785 pair of quotes, which may or may not be bracketing quotes, e.g.
786 C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the
787 replacement portion to be interpreter as a full-fledged Perl expression
788 and eval()ed right then and there. It is, however, syntax checked at
793 s/\bgreen\b/mauve/g; # don't change wintergreen
795 $path =~ s|/usr/bin|/usr/local/bin|;
797 s/Login: $foo/Login: $bar/; # run-time pattern
799 ($foo = $bar) =~ s/this/that/;
801 $count = ($paragraph =~ s/Mister\b/Mr./g);
804 s/\d+/$&*2/e; # yields 'abc246xyz'
805 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
806 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
808 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
809 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
810 s/^=(\w+)/&pod($1)/ge; # use function call
812 # /e's can even nest; this will expand
813 # simple embedded variables in $_
818 /\* # Match the opening delimiter.
819 .*? # Match a minimal number of characters.
820 \*/ # Match the closing delimiter.
823 s/^\s*(.*?)\s*$/$1/; # trim white space
825 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
827 Note the use of $ instead of \ in the last example. Unlike
828 B<sed>, we only use the \E<lt>I<digit>E<gt> form in the left hand side.
829 Anywhere else it's $E<lt>I<digit>E<gt>.
831 Occasionally, you can't just use a C</g> to get all the changes
832 to occur. Here are two common cases:
834 # put commas in the right places in an integer
835 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4
836 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5
838 # expand tabs to 8-column spacing
839 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
842 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
844 =item y/SEARCHLIST/REPLACEMENTLIST/cds
846 Translates all occurrences of the characters found in the search list
847 with the corresponding character in the replacement list. It returns
848 the number of characters replaced or deleted. If no string is
849 specified via the =~ or !~ operator, the $_ string is translated. (The
850 string specified with =~ must be a scalar variable, an array element,
851 or an assignment to one of those, i.e. an lvalue.) For B<sed> devotees,
852 C<y> is provided as a synonym for C<tr>. If the SEARCHLIST is
853 delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of
854 quotes, which may or may not be bracketing quotes, e.g. C<tr[A-Z][a-z]>
855 or C<tr(+-*/)/ABCD/>.
859 c Complement the SEARCHLIST.
860 d Delete found but unreplaced characters.
861 s Squash duplicate replaced characters.
863 If the C</c> modifier is specified, the SEARCHLIST character set is
864 complemented. If the C</d> modifier is specified, any characters specified
865 by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note
866 that this is slightly more flexible than the behavior of some B<tr>
867 programs, which delete anything they find in the SEARCHLIST, period.)
868 If the C</s> modifier is specified, sequences of characters that were
869 translated to the same character are squashed down to a single instance of the
872 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
873 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
874 than the SEARCHLIST, the final character is replicated till it is long
875 enough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated.
876 This latter is useful for counting characters in a class or for
877 squashing character sequences in a class.
881 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
883 $cnt = tr/*/*/; # count the stars in $_
885 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
887 $cnt = tr/0-9//; # count the digits in $_
889 tr/a-zA-Z//s; # bookkeeper -> bokeper
891 ($HOST = $host) =~ tr/a-z/A-Z/;
893 tr/a-zA-Z/ /cs; # change non-alphas to single space
896 [\000-\177]; # delete 8th bit
898 If multiple translations are given for a character, only the first one is used:
902 will translate any A to X.
904 Note that because the translation table is built at compile time, neither
905 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
906 interpolation. That means that if you want to use variables, you must use
909 eval "tr/$oldlist/$newlist/";
912 eval "tr/$oldlist/$newlist/, 1" or die $@;
918 There are several I/O operators you should know about.
919 A string is enclosed by backticks (grave accents) first undergoes
920 variable substitution just like a double quoted string. It is then
921 interpreted as a command, and the output of that command is the value
922 of the pseudo-literal, like in a shell. In a scalar context, a single
923 string consisting of all the output is returned. In a list context,
924 a list of values is returned, one for each line of output. (You can
925 set C<$/> to use a different line terminator.) The command is executed
926 each time the pseudo-literal is evaluated. The status value of the
927 command is returned in C<$?> (see L<perlvar> for the interpretation
928 of C<$?>). Unlike in B<csh>, no translation is done on the return
929 data--newlines remain newlines. Unlike in any of the shells, single
930 quotes do not hide variable names in the command from interpretation.
931 To pass a $ through to the shell you need to hide it with a backslash.
932 The generalized form of backticks is C<qx//>. (Because backticks
933 always undergo shell expansion as well, see L<perlsec> for
936 Evaluating a filehandle in angle brackets yields the next line from
937 that file (newline included, so it's never false until end of file, at
938 which time an undefined value is returned). Ordinarily you must assign
939 that value to a variable, but there is one situation where an automatic
940 assignment happens. I<If and ONLY if> the input symbol is the only
941 thing inside the conditional of a C<while> loop, the value is
942 automatically assigned to the variable C<$_>. The assigned value is
943 then tested to see if it is defined. (This may seem like an odd thing
944 to you, but you'll use the construct in almost every Perl script you
945 write.) Anyway, the following lines are equivalent to each other:
947 while (defined($_ = <STDIN>)) { print; }
948 while (<STDIN>) { print; }
949 for (;<STDIN>;) { print; }
950 print while defined($_ = <STDIN>);
953 The filehandles STDIN, STDOUT and STDERR are predefined. (The
954 filehandles C<stdin>, C<stdout> and C<stderr> will also work except in
955 packages, where they would be interpreted as local identifiers rather
956 than global.) Additional filehandles may be created with the open()
957 function. See L<perlfunc/open()> for details on this.
959 If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a
960 list consisting of all the input lines is returned, one line per list
961 element. It's easy to make a I<LARGE> data space this way, so use with
964 The null filehandle E<lt>E<gt> is special and can be used to emulate the
965 behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from
966 standard input, or from each file listed on the command line. Here's
967 how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is
968 checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened
969 gives you standard input. The @ARGV array is then processed as a list
970 of filenames. The loop
973 ... # code for each line
976 is equivalent to the following Perl-like pseudo code:
978 unshift(@ARGV, '-') if $#ARGV < $[;
979 while ($ARGV = shift) {
982 ... # code for each line
986 except that it isn't so cumbersome to say, and will actually work. It
987 really does shift array @ARGV and put the current filename into variable
988 $ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a synonym
989 for E<lt>ARGVE<gt>, which is magical. (The pseudo code above doesn't work
990 because it treats E<lt>ARGVE<gt> as non-magical.)
992 You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up
993 containing the list of filenames you really want. Line numbers (C<$.>)
994 continue as if the input were one big happy file. (But see example
995 under eof() for how to reset line numbers on each file.)
997 If you want to set @ARGV to your own list of files, go right ahead. If
998 you want to pass switches into your script, you can use one of the
999 Getopts modules or put a loop on the front like this:
1001 while ($_ = $ARGV[0], /^-/) {
1004 if (/^-D(.*)/) { $debug = $1 }
1005 if (/^-v/) { $verbose++ }
1006 ... # other switches
1009 ... # code for each line
1012 The E<lt>E<gt> symbol will return FALSE only once. If you call it again after
1013 this it will assume you are processing another @ARGV list, and if you
1014 haven't set @ARGV, will input from STDIN.
1016 If the string inside the angle brackets is a reference to a scalar
1017 variable (e.g. E<lt>$fooE<gt>), then that variable contains the name of the
1018 filehandle to input from, or a reference to the same. For example:
1023 If the string inside angle brackets is not a filehandle or a scalar
1024 variable containing a filehandle name or reference, then it is interpreted
1025 as a filename pattern to be globbed, and either a list of filenames or the
1026 next filename in the list is returned, depending on context. One level of
1027 $ interpretation is done first, but you can't say C<E<lt>$fooE<gt>>
1028 because that's an indirect filehandle as explained in the previous
1029 paragraph. (In older versions of Perl, programmers would insert curly
1030 brackets to force interpretation as a filename glob: C<E<lt>${foo}E<gt>>.
1031 These days, it's considered cleaner to call the internal function directly
1032 as C<glob($foo)>, which is probably the right way to have done it in the
1033 first place.) Example:
1041 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1047 In fact, it's currently implemented that way. (Which means it will not
1048 work on filenames with spaces in them unless you have csh(1) on your
1049 machine.) Of course, the shortest way to do the above is:
1053 Because globbing invokes a shell, it's often faster to call readdir() yourself
1054 and just do your own grep() on the filenames. Furthermore, due to its current
1055 implementation of using a shell, the glob() routine may get "Arg list too
1056 long" errors (unless you've installed tcsh(1L) as F</bin/csh>).
1058 A glob only evaluates its (embedded) argument when it is starting a new
1059 list. All values must be read before it will start over. In a list
1060 context this isn't important, because you automatically get them all
1061 anyway. In a scalar context, however, the operator returns the next value
1062 each time it is called, or a FALSE value if you've just run out. Again,
1063 FALSE is returned only once. So if you're expecting a single value from
1064 a glob, it is much better to say
1066 ($file) = <blurch*>;
1072 because the latter will alternate between returning a filename and
1075 It you're trying to do variable interpolation, it's definitely better
1076 to use the glob() function, because the older notation can cause people
1077 to become confused with the indirect filehandle notation.
1079 @files = glob("$dir/*.[ch]");
1080 @files = glob($files[$i]);
1082 =head2 Constant Folding
1084 Like C, Perl does a certain amount of expression evaluation at
1085 compile time, whenever it determines that all of the arguments to an
1086 operator are static and have no side effects. In particular, string
1087 concatenation happens at compile time between literals that don't do
1088 variable substitution. Backslash interpretation also happens at
1089 compile time. You can say
1091 'Now is the time for all' . "\n" .
1092 'good men to come to.'
1094 and this all reduces to one string internally. Likewise, if
1097 foreach $file (@filenames) {
1098 if (-s $file > 5 + 100 * 2**16) { ... }
1101 the compiler will pre-compute the number that
1102 expression represents so that the interpreter
1106 =head2 Integer arithmetic
1108 By default Perl assumes that it must do most of its arithmetic in
1109 floating point. But by saying
1113 you may tell the compiler that it's okay to use integer operations
1114 from here to the end of the enclosing BLOCK. An inner BLOCK may
1115 countermand this by saying
1119 which lasts until the end of that BLOCK.