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.
41 Many operators can be overloaded for objects. See L<overload>.
45 =head2 Terms and List Operators (Leftward)
47 A TERM has the highest precedence in Perl. They includes variables,
48 quote and quote-like operators, any expression in parentheses,
49 and any function whose arguments are parenthesized. Actually, there
50 aren't really functions in this sense, just list operators and unary
51 operators behaving as functions because you put parentheses around
52 the arguments. These are all documented in L<perlfunc>.
54 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
55 is followed by a left parenthesis as the next token, the operator and
56 arguments within parentheses are taken to be of highest precedence,
57 just like a normal function call.
59 In the absence of parentheses, the precedence of list operators such as
60 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
61 whether you are looking at the left side or the right side of the operator.
64 @ary = (1, 3, sort 4, 2);
65 print @ary; # prints 1324
67 the commas on the right of the sort are evaluated before the sort, but
68 the commas on the left are evaluated after. In other words, list
69 operators tend to gobble up all the arguments that follow them, and
70 then act like a simple TERM with regard to the preceding expression.
71 Note that you have to be careful with parentheses:
73 # These evaluate exit before doing the print:
74 print($foo, exit); # Obviously not what you want.
75 print $foo, exit; # Nor is this.
77 # These do the print before evaluating exit:
78 (print $foo), exit; # This is what you want.
79 print($foo), exit; # Or this.
80 print ($foo), exit; # Or even this.
84 print ($foo & 255) + 1, "\n";
86 probably doesn't do what you expect at first glance. See
87 L<Named Unary Operators> for more discussion of this.
89 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
90 well as subroutine and method calls, and the anonymous
91 constructors C<[]> and C<{}>.
93 See also L<Quote and Quote-like Operators> toward the end of this section,
94 as well as L<"I/O Operators">.
96 =head2 The Arrow Operator
98 Just as in C and C++, "C<-E<gt>>" is an infix dereference operator. If the
99 right side is either a C<[...]> or C<{...}> subscript, then the left side
100 must be either a hard or symbolic reference to an array or hash (or
101 a location capable of holding a hard reference, if it's an lvalue (assignable)).
104 Otherwise, the right side is a method name or a simple scalar variable
105 containing the method name, and the left side must either be an object
106 (a blessed reference) or a class name (that is, a package name).
109 =head2 Auto-increment and Auto-decrement
111 "++" and "--" work as in C. That is, if placed before a variable, they
112 increment or decrement the variable before returning the value, and if
113 placed after, increment or decrement the variable after returning the value.
115 The auto-increment operator has a little extra builtin magic to it. If
116 you increment a variable that is numeric, or that has ever been used in
117 a numeric context, you get a normal increment. If, however, the
118 variable has been used in only string contexts since it was set, and
119 has a value that is not the empty string and matches the pattern
120 C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each
121 character within its range, with carry:
123 print ++($foo = '99'); # prints '100'
124 print ++($foo = 'a0'); # prints 'a1'
125 print ++($foo = 'Az'); # prints 'Ba'
126 print ++($foo = 'zz'); # prints 'aaa'
128 The auto-decrement operator is not magical.
130 =head2 Exponentiation
132 Binary "**" is the exponentiation operator. Note that it binds even more
133 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
134 implemented using C's pow(3) function, which actually works on doubles
137 =head2 Symbolic Unary Operators
139 Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
140 precedence version of this.
142 Unary "-" performs arithmetic negation if the operand is numeric. If
143 the operand is an identifier, a string consisting of a minus sign
144 concatenated with the identifier is returned. Otherwise, if the string
145 starts with a plus or minus, a string starting with the opposite sign
146 is returned. One effect of these rules is that C<-bareword> is equivalent
149 Unary "~" performs bitwise negation, i.e., 1's complement. For example,
150 C<0666 &~ 027> is 0640. (See also L<Integer Arithmetic> and L<Bitwise
153 Unary "+" has no effect whatsoever, even on strings. It is useful
154 syntactically for separating a function name from a parenthesized expression
155 that would otherwise be interpreted as the complete list of function
156 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
158 Unary "\" creates a reference to whatever follows it. See L<perlref>.
159 Do not confuse this behavior with the behavior of backslash within a
160 string, although both forms do convey the notion of protecting the next
161 thing from interpretation.
163 =head2 Binding Operators
165 Binary "=~" binds a scalar expression to a pattern match. Certain operations
166 search or modify the string $_ by default. This operator makes that kind
167 of operation work on some other string. The right argument is a search
168 pattern, substitution, or transliteration. The left argument is what is
169 supposed to be searched, substituted, or transliterated instead of the default
170 $_. The return value indicates the success of the operation. (If the
171 right argument is an expression rather than a search pattern,
172 substitution, or transliteration, it is interpreted as a search pattern at run
173 time. This can be is less efficient than an explicit search, because the
174 pattern must be compiled every time the expression is evaluated.
176 Binary "!~" is just like "=~" except the return value is negated in
179 =head2 Multiplicative Operators
181 Binary "*" multiplies two numbers.
183 Binary "/" divides two numbers.
185 Binary "%" computes the modulus of two numbers. Given integer
186 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
187 C<$a> minus the largest multiple of C<$b> that is not greater than
188 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
189 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
190 result will be less than or equal to zero). If C<use integer> is
191 in effect, the native hardware will be used instead of this rule,
192 which may be construed a bug that will be fixed at some point.
194 Note than when C<use integer> is in scope, "%" give you direct access
195 to the modulus operator as implemented by your C compiler. This
196 operator is not as well defined for negative operands, but it will
199 Binary "x" is the repetition operator. In scalar context, it
200 returns a string consisting of the left operand repeated the number of
201 times specified by the right operand. In list context, if the left
202 operand is a list in parentheses, it repeats the list.
204 print '-' x 80; # print row of dashes
206 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
208 @ones = (1) x 80; # a list of 80 1's
209 @ones = (5) x @ones; # set all elements to 5
212 =head2 Additive Operators
214 Binary "+" returns the sum of two numbers.
216 Binary "-" returns the difference of two numbers.
218 Binary "." concatenates two strings.
220 =head2 Shift Operators
222 Binary "<<" returns the value of its left argument shifted left by the
223 number of bits specified by the right argument. Arguments should be
224 integers. (See also L<Integer Arithmetic>.)
226 Binary ">>" returns the value of its left argument shifted right by
227 the number of bits specified by the right argument. Arguments should
228 be integers. (See also L<Integer Arithmetic>.)
230 =head2 Named Unary Operators
232 The various named unary operators are treated as functions with one
233 argument, with optional parentheses. These include the filetest
234 operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
236 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
237 is followed by a left parenthesis as the next token, the operator and
238 arguments within parentheses are taken to be of highest precedence,
239 just like a normal function call. Examples:
241 chdir $foo || die; # (chdir $foo) || die
242 chdir($foo) || die; # (chdir $foo) || die
243 chdir ($foo) || die; # (chdir $foo) || die
244 chdir +($foo) || die; # (chdir $foo) || die
246 but, because * is higher precedence than ||:
248 chdir $foo * 20; # chdir ($foo * 20)
249 chdir($foo) * 20; # (chdir $foo) * 20
250 chdir ($foo) * 20; # (chdir $foo) * 20
251 chdir +($foo) * 20; # chdir ($foo * 20)
253 rand 10 * 20; # rand (10 * 20)
254 rand(10) * 20; # (rand 10) * 20
255 rand (10) * 20; # (rand 10) * 20
256 rand +(10) * 20; # rand (10 * 20)
258 See also L<"Terms and List Operators (Leftward)">.
260 =head2 Relational Operators
262 Binary "E<lt>" returns true if the left argument is numerically less than
265 Binary "E<gt>" returns true if the left argument is numerically greater
266 than the right argument.
268 Binary "E<lt>=" returns true if the left argument is numerically less than
269 or equal to the right argument.
271 Binary "E<gt>=" returns true if the left argument is numerically greater
272 than or equal to the right argument.
274 Binary "lt" returns true if the left argument is stringwise less than
277 Binary "gt" returns true if the left argument is stringwise greater
278 than the right argument.
280 Binary "le" returns true if the left argument is stringwise less than
281 or equal to the right argument.
283 Binary "ge" returns true if the left argument is stringwise greater
284 than or equal to the right argument.
286 =head2 Equality Operators
288 Binary "==" returns true if the left argument is numerically equal to
291 Binary "!=" returns true if the left argument is numerically not equal
292 to the right argument.
294 Binary "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left
295 argument is numerically less than, equal to, or greater than the right
298 Binary "eq" returns true if the left argument is stringwise equal to
301 Binary "ne" returns true if the left argument is stringwise not equal
302 to the right argument.
304 Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise
305 less than, equal to, or greater than the right argument.
307 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
308 by the current locale if C<use locale> is in effect. See L<perllocale>.
312 Binary "&" returns its operators ANDed together bit by bit.
313 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
315 =head2 Bitwise Or and Exclusive Or
317 Binary "|" returns its operators ORed together bit by bit.
318 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
320 Binary "^" returns its operators XORed together bit by bit.
321 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
323 =head2 C-style Logical And
325 Binary "&&" performs a short-circuit logical AND operation. That is,
326 if the left operand is false, the right operand is not even evaluated.
327 Scalar or list context propagates down to the right operand if it
330 =head2 C-style Logical Or
332 Binary "||" performs a short-circuit logical OR operation. That is,
333 if the left operand is true, the right operand is not even evaluated.
334 Scalar or list context propagates down to the right operand if it
337 The C<||> and C<&&> operators differ from C's in that, rather than returning
338 0 or 1, they return the last value evaluated. Thus, a reasonably portable
339 way to find out the home directory (assuming it's not "0") might be:
341 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
342 (getpwuid($<))[7] || die "You're homeless!\n";
344 In particular, this means that you shouldn't use this
345 for selecting between two aggregates for assignment:
347 @a = @b || @c; # this is wrong
348 @a = scalar(@b) || @c; # really meant this
349 @a = @b ? @b : @c; # this works fine, though
351 As more readable alternatives to C<&&> and C<||> when used for
352 control flow, Perl provides C<and> and C<or> operators (see below).
353 The short-circuit behavior is identical. The precedence of "and" and
354 "or" is much lower, however, so that you can safely use them after a
355 list operator without the need for parentheses:
357 unlink "alpha", "beta", "gamma"
358 or gripe(), next LINE;
360 With the C-style operators that would have been written like this:
362 unlink("alpha", "beta", "gamma")
363 || (gripe(), next LINE);
365 Use "or" for assignment is unlikely to do what you want; see below.
367 =head2 Range Operators
369 Binary ".." is the range operator, which is really two different
370 operators depending on the context. In list context, it returns an
371 array of values counting (by ones) from the left value to the right
372 value. This is useful for writing C<foreach (1..10)> loops and for
373 doing slice operations on arrays. In the current implementation, no
374 temporary array is created when the range operator is used as the
375 expression in C<foreach> loops, but older versions of Perl might burn
376 a lot of memory when you write something like this:
378 for (1 .. 1_000_000) {
382 In scalar context, ".." returns a boolean value. The operator is
383 bistable, like a flip-flop, and emulates the line-range (comma) operator
384 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
385 own boolean state. It is false as long as its left operand is false.
386 Once the left operand is true, the range operator stays true until the
387 right operand is true, I<AFTER> which the range operator becomes false
388 again. (It doesn't become false till the next time the range operator is
389 evaluated. It can test the right operand and become false on the same
390 evaluation it became true (as in B<awk>), but it still returns true once.
391 If you don't want it to test the right operand till the next evaluation
392 (as in B<sed>), use three dots ("...") instead of two.) The right
393 operand is not evaluated while the operator is in the "false" state, and
394 the left operand is not evaluated while the operator is in the "true"
395 state. The precedence is a little lower than || and &&. The value
396 returned is either the empty string for false, or a sequence number
397 (beginning with 1) for true. The sequence number is reset for each range
398 encountered. The final sequence number in a range has the string "E0"
399 appended to it, which doesn't affect its numeric value, but gives you
400 something to search for if you want to exclude the endpoint. You can
401 exclude the beginning point by waiting for the sequence number to be
402 greater than 1. If either operand of scalar ".." is a constant expression,
403 that operand is implicitly compared to the C<$.> variable, the current
404 line number. Examples:
406 As a scalar operator:
408 if (101 .. 200) { print; } # print 2nd hundred lines
409 next line if (1 .. /^$/); # skip header lines
410 s/^/> / if (/^$/ .. eof()); # quote body
412 # parse mail messages
414 $in_header = 1 .. /^$/;
415 $in_body = /^$/ .. eof();
416 # do something based on those
418 close ARGV if eof; # reset $. each file
423 for (101 .. 200) { print; } # print $_ 100 times
424 @foo = @foo[0 .. $#foo]; # an expensive no-op
425 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
427 The range operator (in list context) makes use of the magical
428 auto-increment algorithm if the operands are strings. You
431 @alphabet = ('A' .. 'Z');
433 to get all the letters of the alphabet, or
435 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
437 to get a hexadecimal digit, or
439 @z2 = ('01' .. '31'); print $z2[$mday];
441 to get dates with leading zeros. If the final value specified is not
442 in the sequence that the magical increment would produce, the sequence
443 goes until the next value would be longer than the final value
446 =head2 Conditional Operator
448 Ternary "?:" is the conditional operator, just as in C. It works much
449 like an if-then-else. If the argument before the ? is true, the
450 argument before the : is returned, otherwise the argument after the :
451 is returned. For example:
453 printf "I have %d dog%s.\n", $n,
454 ($n == 1) ? '' : "s";
456 Scalar or list context propagates downward into the 2nd
457 or 3rd argument, whichever is selected.
459 $a = $ok ? $b : $c; # get a scalar
460 @a = $ok ? @b : @c; # get an array
461 $a = $ok ? @b : @c; # oops, that's just a count!
463 The operator may be assigned to if both the 2nd and 3rd arguments are
464 legal lvalues (meaning that you can assign to them):
466 ($a_or_b ? $a : $b) = $c;
468 This is not necessarily guaranteed to contribute to the readability of your program.
470 Because this operator produces an assignable result, using assignments
471 without parentheses will get you in trouble. For example, this:
473 $a % 2 ? $a += 10 : $a += 2
477 (($a % 2) ? ($a += 10) : $a) += 2
481 ($a % 2) ? ($a += 10) : ($a += 2)
483 =head2 Assignment Operators
485 "=" is the ordinary assignment operator.
487 Assignment operators work as in C. That is,
495 although without duplicating any side effects that dereferencing the lvalue
496 might trigger, such as from tie(). Other assignment operators work similarly.
497 The following are recognized:
504 Note that while these are grouped by family, they all have the precedence
507 Unlike in C, the assignment operator produces a valid lvalue. Modifying
508 an assignment is equivalent to doing the assignment and then modifying
509 the variable that was assigned to. This is useful for modifying
510 a copy of something, like this:
512 ($tmp = $global) =~ tr [A-Z] [a-z];
523 =head2 Comma Operator
525 Binary "," is the comma operator. In scalar context it evaluates
526 its left argument, throws that value away, then evaluates its right
527 argument and returns that value. This is just like C's comma operator.
529 In list context, it's just the list argument separator, and inserts
530 both its arguments into the list.
532 The =E<gt> digraph is mostly just a synonym for the comma operator. It's useful for
533 documenting arguments that come in pairs. As of release 5.001, it also forces
534 any word to the left of it to be interpreted as a string.
536 =head2 List Operators (Rightward)
538 On the right side of a list operator, it has very low precedence,
539 such that it controls all comma-separated expressions found there.
540 The only operators with lower precedence are the logical operators
541 "and", "or", and "not", which may be used to evaluate calls to list
542 operators without the need for extra parentheses:
544 open HANDLE, "filename"
545 or die "Can't open: $!\n";
547 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
551 Unary "not" returns the logical negation of the expression to its right.
552 It's the equivalent of "!" except for the very low precedence.
556 Binary "and" returns the logical conjunction of the two surrounding
557 expressions. It's equivalent to && except for the very low
558 precedence. This means that it short-circuits: i.e., the right
559 expression is evaluated only if the left expression is true.
561 =head2 Logical or and Exclusive Or
563 Binary "or" returns the logical disjunction of the two surrounding
564 expressions. It's equivalent to || except for the very low precedence.
565 This makes it useful for control flow
567 print FH $data or die "Can't write to FH: $!";
569 This means that it short-circuits: i.e., the right expression is evaluated
570 only if the left expression is false. Due to its precedence, you should
571 probably avoid using this for assignment, only for control flow.
573 $a = $b or $c; # bug: this is wrong
574 ($a = $b) or $c; # really means this
575 $a = $b || $c; # better written this way
577 However, when it's a list context assignment and you're trying to use
578 "||" for control flow, you probably need "or" so that the assignment
579 takes higher precedence.
581 @info = stat($file) || die; # oops, scalar sense of stat!
582 @info = stat($file) or die; # better, now @info gets its due
584 Then again, you could always use parentheses.
586 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
587 It cannot short circuit, of course.
589 =head2 C Operators Missing From Perl
591 Here is what C has that Perl doesn't:
597 Address-of operator. (But see the "\" operator for taking a reference.)
601 Dereference-address operator. (Perl's prefix dereferencing
602 operators are typed: $, @, %, and &.)
606 Type casting operator.
610 =head2 Quote and Quote-like Operators
612 While we usually think of quotes as literal values, in Perl they
613 function as operators, providing various kinds of interpolating and
614 pattern matching capabilities. Perl provides customary quote characters
615 for these behaviors, but also provides a way for you to choose your
616 quote character for any of them. In the following table, a C<{}> represents
617 any pair of delimiters you choose. Non-bracketing delimiters use
618 the same character fore and aft, but the 4 sorts of brackets
619 (round, angle, square, curly) will all nest.
621 Customary Generic Meaning Interpolates
624 `` qx{} Command yes (unless '' is delimiter)
626 // m{} Pattern match yes
628 s{}{} Substitution yes
629 tr{}{} Transliteration no (but see below)
631 Note that there can be whitespace between the operator and the quoting
632 characters, except when C<#> is being used as the quoting character.
633 C<q#foo#> is parsed as being the string C<foo>, while C<q #foo#> is the
634 operator C<q> followed by a comment. Its argument will be taken from the
635 next line. This allows you to write:
637 s {foo} # Replace foo
640 For constructs that do interpolation, variables beginning with "C<$>"
641 or "C<@>" are interpolated, as are the following sequences. Within
642 a transliteration, the first ten of these sequences may be used.
649 \a alarm (bell) (BEL)
655 \l lowercase next char
656 \u uppercase next char
659 \E end case modification
660 \Q quote non-word characters till \E
662 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
663 and C<\U> is taken from the current locale. See L<perllocale>.
665 All systems use the virtual C<"\n"> to represent a line terminator,
666 called a "newline". There is no such thing as an unvarying, physical
667 newline character. It is an illusion that the operating system,
668 device drivers, C libraries, and Perl all conspire to preserve. Not all
669 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
670 on a Mac, these are reversed, and on systems without line terminator,
671 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
672 you mean a "newline" for your system, but use the literal ASCII when you
673 need an exact character. For example, most networking protocols expect
674 and prefer a CR+LF (C<"\012\015"> or C<"\cJ\cM">) for line terminators,
675 and although they often accept just C<"\012">, they seldom tolerate just
676 C<"\015">. If you get in the habit of using C<"\n"> for networking,
677 you may be burned some day.
679 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
680 An unescaped C<$> or C<@> interpolates the corresponding variable,
681 while escaping will cause the literal string C<\$> to be inserted.
682 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
684 Patterns are subject to an additional level of interpretation as a
685 regular expression. This is done as a second pass, after variables are
686 interpolated, so that regular expressions may be incorporated into the
687 pattern from the variables. If this is not what you want, use C<\Q> to
688 interpolate a variable literally.
690 Apart from the above, there are no multiple levels of interpolation. In
691 particular, contrary to the expectations of shell programmers, back-quotes
692 do I<NOT> interpolate within double quotes, nor do single quotes impede
693 evaluation of variables when used within double quotes.
695 =head2 Regexp Quote-Like Operators
697 Here are the quote-like operators that apply to pattern
698 matching and related activities.
700 Most of this section is related to use of regular expressions from Perl.
701 Such a use may be considered from two points of view: Perl handles a
702 a string and a "pattern" to RE (regular expression) engine to match,
703 RE engine finds (or does not find) the match, and Perl uses the findings
704 of RE engine for its operation, possibly asking the engine for other matches.
706 RE engine has no idea what Perl is going to do with what it finds,
707 similarly, the rest of Perl has no idea what a particular regular expression
708 means to RE engine. This creates a clean separation, and in this section
709 we discuss matching from Perl point of view only. The other point of
710 view may be found in L<perlre>.
716 This is just like the C</pattern/> search, except that it matches only
717 once between calls to the reset() operator. This is a useful
718 optimization when you want to see only the first occurrence of
719 something in each file of a set of files, for instance. Only C<??>
720 patterns local to the current package are reset.
724 # blank line between header and body
727 reset if eof; # clear ?? status for next file
730 This usage is vaguely deprecated, and may be removed in some future
733 =item m/PATTERN/cgimosx
735 =item /PATTERN/cgimosx
737 Searches a string for a pattern match, and in scalar context returns
738 true (1) or false (''). If no string is specified via the C<=~> or
739 C<!~> operator, the $_ string is searched. (The string specified with
740 C<=~> need not be an lvalue--it may be the result of an expression
741 evaluation, but remember the C<=~> binds rather tightly.) See also
743 See L<perllocale> for discussion of additional considerations that apply
744 when C<use locale> is in effect.
748 c Do not reset search position on a failed match when /g is in effect.
749 g Match globally, i.e., find all occurrences.
750 i Do case-insensitive pattern matching.
751 m Treat string as multiple lines.
752 o Compile pattern only once.
753 s Treat string as single line.
754 x Use extended regular expressions.
756 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
757 you can use any pair of non-alphanumeric, non-whitespace characters
758 as delimiters (if single quotes are used, no interpretation is done
759 on the replacement string. Unlike Perl 4, Perl 5 treats backticks as normal
760 delimiters; the replacement text is not evaluated as a command).
761 This is particularly useful for matching Unix path names
762 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
763 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
765 PATTERN may contain variables, which will be interpolated (and the
766 pattern recompiled) every time the pattern search is evaluated. (Note
767 that C<$)> and C<$|> might not be interpolated because they look like
768 end-of-string tests.) If you want such a pattern to be compiled only
769 once, add a C</o> after the trailing delimiter. This avoids expensive
770 run-time recompilations, and is useful when the value you are
771 interpolating won't change over the life of the script. However, mentioning
772 C</o> constitutes a promise that you won't change the variables in the pattern.
773 If you change them, Perl won't even notice.
775 If the PATTERN evaluates to the empty string, the last
776 I<successfully> matched regular expression is used instead.
778 If the C</g> option is not used, C<m//> in a list context returns a
779 list consisting of the subexpressions matched by the parentheses in the
780 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here
781 C<$1> etc. are also set, and
782 that this differs from Perl 4's behavior.) If there are no parentheses,
783 the return value is the list C<(1)> for success or C<('')> upon failure.
784 With parentheses, C<()> is returned upon failure.
788 open(TTY, '/dev/tty');
789 <TTY> =~ /^y/i && foo(); # do foo if desired
791 if (/Version: *([0-9.]*)/) { $version = $1; }
793 next if m#^/usr/spool/uucp#;
798 print if /$arg/o; # compile only once
801 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
803 This last example splits $foo into the first two words and the
804 remainder of the line, and assigns those three fields to $F1, $F2, and
805 $Etc. The conditional is true if any variables were assigned, i.e., if
808 The C</g> modifier specifies global pattern matching--that is, matching
809 as many times as possible within the string. How it behaves depends on
810 the context. In list context, it returns a list of all the
811 substrings matched by all the parentheses in the regular expression.
812 If there are no parentheses, it returns a list of all the matched
813 strings, as if there were parentheses around the whole pattern.
815 In scalar context, each execution of C<m//g> finds the next match,
816 returning TRUE if it matches, and FALSE if there is no further match.
817 The position after the last match can be read or set using the pos()
818 function; see L<perlfunc/pos>. A failed match normally resets the
819 search position to the beginning of the string, but you can avoid that
820 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
821 string also resets the search position.
823 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
824 zero-width assertion that matches the exact position where the previous
825 C<m//g>, if any, left off. The C<\G> assertion is not supported without
826 the C</g> modifier; currently, without C</g>, C<\G> behaves just like
827 C<\A>, but that's accidental and may change in the future.
832 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
835 $/ = ""; $* = 1; # $* deprecated in modern perls
836 while (defined($paragraph = <>)) {
837 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
841 print "$sentences\n";
843 # using m//gc with \G
847 print $1 while /(o)/gc; print "', pos=", pos, "\n";
849 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
851 print $1 while /(p)/gc; print "', pos=", pos, "\n";
854 The last example should print:
863 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
864 combine several regexps like this to process a string part-by-part,
865 doing different actions depending on which regexp matched. Each
866 regexp tries to match where the previous one leaves off.
869 $url = new URI::URL "http://www/"; die if $url eq "xXx";
873 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
874 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
875 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
876 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
877 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
878 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
879 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
880 print ". That's all!\n";
883 Here is the output (split into several lines):
885 line-noise lowercase line-noise lowercase UPPERCASE line-noise
886 UPPERCASE line-noise lowercase line-noise lowercase line-noise
887 lowercase lowercase line-noise lowercase lowercase line-noise
888 MiXeD line-noise. That's all!
894 A single-quoted, literal string. A backslash represents a backslash
895 unless followed by the delimiter or another backslash, in which case
896 the delimiter or backslash is interpolated.
898 $foo = q!I said, "You said, 'She said it.'"!;
899 $bar = q('This is it.');
900 $baz = '\n'; # a two-character string
906 A double-quoted, interpolated string.
909 (*** The previous line contains the naughty word "$1".\n)
910 if /(tcl|rexx|python)/; # :-)
911 $baz = "\n"; # a one-character string
913 =item qr/STRING/imosx
915 A string which is (possibly) interpolated and then compiled as a
916 regular expression. The result may be used as a pattern in a match
923 i Do case-insensitive pattern matching.
924 m Treat string as multiple lines.
925 o Compile pattern only once.
926 s Treat string as single line.
927 x Use extended regular expressions.
929 The benefit from this is that the pattern is compiled into an internal
930 representation by the C<qr//> operator and not by the match operator.
932 foreach $pattern (@pattern_list) {
933 my $re = qr/$pattern/;
934 foreach $line (@lines) {
945 A string which is (possibly) interpolated and then executed as a system
946 command with C</bin/sh> or its equivalent. Shell wildcards, pipes,
947 and redirections will be honored. The collected standard output of the
948 command is returned; standard error is unaffected. In scalar context,
949 it comes back as a single (potentially multi-line) string. In list
950 context, returns a list of lines (however you've defined lines with $/
951 or $INPUT_RECORD_SEPARATOR).
953 Because backticks do not affect standard error, use shell file descriptor
954 syntax (assuming the shell supports this) if you care to address this.
955 To capture a command's STDERR and STDOUT together:
957 $output = `cmd 2>&1`;
959 To capture a command's STDOUT but discard its STDERR:
961 $output = `cmd 2>/dev/null`;
963 To capture a command's STDERR but discard its STDOUT (ordering is
966 $output = `cmd 2>&1 1>/dev/null`;
968 To exchange a command's STDOUT and STDERR in order to capture the STDERR
969 but leave its STDOUT to come out the old STDERR:
971 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
973 To read both a command's STDOUT and its STDERR separately, it's easiest
974 and safest to redirect them separately to files, and then read from those
975 files when the program is done:
977 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
979 Using single-quote as a delimiter protects the command from Perl's
980 double-quote interpolation, passing it on to the shell instead:
982 $perl_info = qx(ps $$); # that's Perl's $$
983 $shell_info = qx'ps $$'; # that's the new shell's $$
985 Note that how the string gets evaluated is entirely subject to the command
986 interpreter on your system. On most platforms, you will have to protect
987 shell metacharacters if you want them treated literally. This is in
988 practice difficult to do, as it's unclear how to escape which characters.
989 See L<perlsec> for a clean and safe example of a manual fork() and exec()
990 to emulate backticks safely.
992 On some platforms (notably DOS-like ones), the shell may not be
993 capable of dealing with multiline commands, so putting newlines in
994 the string may not get you what you want. You may be able to evaluate
995 multiple commands in a single line by separating them with the command
996 separator character, if your shell supports that (e.g. C<;> on many Unix
997 shells; C<&> on the Windows NT C<cmd> shell).
999 Beware that some command shells may place restrictions on the length
1000 of the command line. You must ensure your strings don't exceed this
1001 limit after any necessary interpolations. See the platform-specific
1002 release notes for more details about your particular environment.
1004 Using this operator can lead to programs that are difficult to port,
1005 because the shell commands called vary between systems, and may in
1006 fact not be present at all. As one example, the C<type> command under
1007 the POSIX shell is very different from the C<type> command under DOS.
1008 That doesn't mean you should go out of your way to avoid backticks
1009 when they're the right way to get something done. Perl was made to be
1010 a glue language, and one of the things it glues together is commands.
1011 Just understand what you're getting yourself into.
1013 See L<"I/O Operators"> for more discussion.
1017 Returns a list of the words extracted out of STRING, using embedded
1018 whitespace as the word delimiters. It is exactly equivalent to
1020 split(' ', q/STRING/);
1022 This equivalency means that if used in scalar context, you'll get split's
1023 (unfortunate) scalar context behavior, complete with mysterious warnings.
1025 Some frequently seen examples:
1027 use POSIX qw( setlocale localeconv )
1028 @EXPORT = qw( foo bar baz );
1030 A common mistake is to try to separate the words with comma or to put
1031 comments into a multi-line C<qw>-string. For this reason the C<-w>
1032 switch produce warnings if the STRING contains the "," or the "#"
1035 =item s/PATTERN/REPLACEMENT/egimosx
1037 Searches a string for a pattern, and if found, replaces that pattern
1038 with the replacement text and returns the number of substitutions
1039 made. Otherwise it returns false (specifically, the empty string).
1041 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1042 variable is searched and modified. (The string specified with C<=~> must
1043 be scalar variable, an array element, a hash element, or an assignment
1044 to one of those, i.e., an lvalue.)
1046 If the delimiter chosen is single quote, no variable interpolation is
1047 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1048 PATTERN contains a $ that looks like a variable rather than an
1049 end-of-string test, the variable will be interpolated into the pattern
1050 at run-time. If you want the pattern compiled only once the first time
1051 the variable is interpolated, use the C</o> option. If the pattern
1052 evaluates to the empty string, the last successfully executed regular
1053 expression is used instead. See L<perlre> for further explanation on these.
1054 See L<perllocale> for discussion of additional considerations that apply
1055 when C<use locale> is in effect.
1059 e Evaluate the right side as an expression.
1060 g Replace globally, i.e., all occurrences.
1061 i Do case-insensitive pattern matching.
1062 m Treat string as multiple lines.
1063 o Compile pattern only once.
1064 s Treat string as single line.
1065 x Use extended regular expressions.
1067 Any non-alphanumeric, non-whitespace delimiter may replace the
1068 slashes. If single quotes are used, no interpretation is done on the
1069 replacement string (the C</e> modifier overrides this, however). Unlike
1070 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1071 text is not evaluated as a command. If the
1072 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1073 pair of quotes, which may or may not be bracketing quotes, e.g.,
1074 C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the
1075 replacement portion to be interpreted as a full-fledged Perl expression
1076 and eval()ed right then and there. It is, however, syntax checked at
1081 s/\bgreen\b/mauve/g; # don't change wintergreen
1083 $path =~ s|/usr/bin|/usr/local/bin|;
1085 s/Login: $foo/Login: $bar/; # run-time pattern
1087 ($foo = $bar) =~ s/this/that/; # copy first, then change
1089 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1092 s/\d+/$&*2/e; # yields 'abc246xyz'
1093 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1094 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1096 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1097 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1098 s/^=(\w+)/&pod($1)/ge; # use function call
1100 # expand variables in $_, but dynamics only, using
1101 # symbolic dereferencing
1104 # /e's can even nest; this will expand
1105 # any embedded scalar variable (including lexicals) in $_
1108 # Delete (most) C comments.
1110 /\* # Match the opening delimiter.
1111 .*? # Match a minimal number of characters.
1112 \*/ # Match the closing delimiter.
1115 s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
1117 for ($variable) { # trim white space in $variable, cheap
1122 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1124 Note the use of $ instead of \ in the last example. Unlike
1125 B<sed>, we use the \E<lt>I<digit>E<gt> form in only the left hand side.
1126 Anywhere else it's $E<lt>I<digit>E<gt>.
1128 Occasionally, you can't use just a C</g> to get all the changes
1129 to occur. Here are two common cases:
1131 # put commas in the right places in an integer
1132 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4
1133 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5
1135 # expand tabs to 8-column spacing
1136 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1139 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1141 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1143 Transliterates all occurrences of the characters found in the search list
1144 with the corresponding character in the replacement list. It returns
1145 the number of characters replaced or deleted. If no string is
1146 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1147 string specified with =~ must be a scalar variable, an array element, a
1148 hash element, or an assignment to one of those, i.e., an lvalue.)
1149 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1150 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1151 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1152 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1153 its own pair of quotes, which may or may not be bracketing quotes,
1154 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1158 c Complement the SEARCHLIST.
1159 d Delete found but unreplaced characters.
1160 s Squash duplicate replaced characters.
1162 If the C</c> modifier is specified, the SEARCHLIST character set is
1163 complemented. If the C</d> modifier is specified, any characters specified
1164 by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note
1165 that this is slightly more flexible than the behavior of some B<tr>
1166 programs, which delete anything they find in the SEARCHLIST, period.)
1167 If the C</s> modifier is specified, sequences of characters that were
1168 transliterated to the same character are squashed down to a single instance of the
1171 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1172 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1173 than the SEARCHLIST, the final character is replicated till it is long
1174 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1175 This latter is useful for counting characters in a class or for
1176 squashing character sequences in a class.
1180 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1182 $cnt = tr/*/*/; # count the stars in $_
1184 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1186 $cnt = tr/0-9//; # count the digits in $_
1188 tr/a-zA-Z//s; # bookkeeper -> bokeper
1190 ($HOST = $host) =~ tr/a-z/A-Z/;
1192 tr/a-zA-Z/ /cs; # change non-alphas to single space
1195 [\000-\177]; # delete 8th bit
1197 If multiple transliterations are given for a character, only the first one is used:
1201 will transliterate any A to X.
1203 Note that because the transliteration table is built at compile time, neither
1204 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1205 interpolation. That means that if you want to use variables, you must use
1208 eval "tr/$oldlist/$newlist/";
1211 eval "tr/$oldlist/$newlist/, 1" or die $@;
1215 =head2 Gory details of parsing quoted constructs
1217 When presented with something which may have several different
1218 interpretations, Perl uses the principle B<DWIM> (expanded to Do What I Mean
1219 - not what I wrote) to pick up the most probable interpretation of the
1220 source. This strategy is so successful that Perl users usually do not
1221 suspect ambivalence of what they write. However, time to time Perl's ideas
1222 differ from what the author meant.
1224 The target of this section is to clarify the Perl's way of interpreting
1225 quoted constructs. The most frequent reason one may have to want to know the
1226 details discussed in this section is hairy regular expressions. However, the
1227 first steps of parsing are the same for all Perl quoting operators, so here
1228 they are discussed together.
1230 Some of the passes discussed below are performed concurrently, but as
1231 far as results are the same, we consider them one-by-one. For different
1232 quoting constructs Perl performs different number of passes, from
1233 one to five, but they are always performed in the same order.
1237 =item Finding the end
1239 First pass is finding the end of the quoted construct, be it multichar ender
1240 C<"\nEOF\n"> of C<<<EOF> construct, C</> which terminates C<qq/> construct,
1241 C<E<]>> which terminates C<qq[> construct, or C<E<gt>> which terminates a
1242 fileglob started with C<<>.
1244 When searching for multichar construct no skipping is performed. When
1245 searching for one-char non-matching delimiter, such as C</>, combinations
1246 C<\\> and C<\/> are skipped. When searching for one-char matching delimiter,
1247 such as C<]>, combinations C<\\>, C<\]> and C<\[> are skipped, and
1248 nested C<[>, C<]> are skipped as well.
1250 For 3-parts constructs C<s///> etc. the search is repeated once more.
1252 During this search no attension is paid to the semantic of the construct, thus
1254 "$hash{"$foo/$bar"}"
1259 bar # This is not a comment, this slash / terminated m//!
1262 do not form legal quoted expressions. Note that since the slash which
1263 terminated C<m//> was followed by a C<SPACE>, this is not C<m//x>,
1264 thus C<#> was interpreted as a literal C<#>.
1266 =item Removal of backslashes before delimiters
1268 During the second pass the text between the starting delimiter and
1269 the ending delimiter is copied to a safe location, and the C<\> is
1270 removed from combinations consisting of C<\> and delimiter(s) (both starting
1271 and ending delimiter if they differ).
1273 The removal does not happen for multi-char delimiters.
1275 Note that the combination C<\\> is left as it was!
1277 Starting from this step no information about the delimiter(s) is used in the
1282 Next step is interpolation in the obtained delimiter-independent text.
1283 There are many different cases.
1287 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1289 No interpolation is performed.
1293 The only interpolation is removal of C<\> from pairs C<\\>.
1295 =item C<"">, C<``>, C<qq//>, C<qx//>, C<<file*globE<gt>>
1297 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are converted
1298 to corresponding Perl constructs, thus C<"$foo\Qbaz$bar"> is converted to
1300 $foo . (quotemeta("baz" . $bar));
1302 Other combinations of C<\> with following chars are substituted with
1303 appropriate expansions.
1305 Interpolated scalars and arrays are converted to C<join> and C<.> Perl
1306 constructs, thus C<"'@arr'"> becomes
1308 "'" . (join $", @arr) . "'";
1310 Since all three above steps are performed simultaneously left-to-right,
1311 the is no way to insert a literal C<$> or C<@> inside C<\Q\E> pair: it
1312 cannot be protected by C<\>, since any C<\> (except in C<\E>) is
1313 interpreted as a literal inside C<\Q\E>, and any $ is
1314 interpreted as starting an interpolated scalar.
1316 Note also that the interpolating code needs to make decision where the
1317 interpolated scalar ends, say, whether C<"a $b -> {c}"> means
1319 "a " . $b . " -> {c}";
1325 Most the time the decision is to take the longest possible text which does
1326 not include spaces between components and contains matching braces/brackets.
1328 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1330 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> and interpolation happens
1331 (almost) as with qq// constructs, but I<the substitution of C<\> followed by
1332 other chars is not performed>! Moreover, inside C<(?{BLOCK})> no processing
1333 is performed at all.
1335 Interpolation has several quirks: $|, $( and $) are not interpolated, and
1336 constructs C<$var[SOMETHING]> are I<voted> (by several different estimators)
1337 to be an array element or $var followed by a RE alternative. This is
1338 the place where the notation C<${arr[$bar]}> comes handy: C</${arr[0-9]}/>
1339 is interpreted as an array element -9, not as a regular expression from
1340 variable $arr followed by a digit, which is the interpretation of
1343 Note that absense of processing of C<\\> creates specific restrictions on the
1344 post-processed text: if the delimeter is C</>, one cannot get the combination
1345 C<\/> into the result of this step: C</> will finish the regular expression,
1346 C<\/> will be stripped to C</> on the previous step, and C<\\/> will be left
1347 as is. Since C</> is equivalent to C<\/> inside a regular expression, this
1348 does not matter unless the delimiter is special character for RE engine, as
1349 in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>.
1353 This step is the last one for all the constructs except regular expressions,
1354 which are processed further.
1356 =item Interpolation of regular expressions
1358 All the previous steps were performed during the compilation of Perl code,
1359 this one happens in run time (though it may be optimized to be calculated
1360 at compile time if appropriate). After all the preprocessing performed
1361 above (and possibly after evaluation if catenation, joining, up/down-casing
1362 and quotemeta()ing are involved) the resulting I<string> is passed to RE
1363 engine for compilation.
1365 Whatever happens in the RE engine is better be discussed in L<perlre>,
1366 but for the sake of continuity let us do it here.
1368 This is the first step where presense of the C<//x> switch is relevant.
1369 RE engine scans the string left-to-right, and converts it to a finite
1372 Backslashed chars are either substituted by corresponding literal
1373 strings, or generate special nodes of the finite automaton. Characters
1374 which are special to RE engine generate corresponding nodes. C<(?#...)>
1375 comments are ignored. All the rest is either converted to literal strings
1376 to match, or is ignored (as is whitespace and C<#>-style comments if
1379 Note that the parsing of the construct C<[...]> is performed using
1380 absolutely different rules than the rest of the regular expression.
1381 Similarly, the C<(?{...})> is only checked for matching braces.
1383 =item Optimization of regular expressions
1385 This step is listed for compeleteness only. Since it does not change
1386 semantics, details of this step are not documented and are subject
1391 =head2 I/O Operators
1393 There are several I/O operators you should know about.
1394 A string enclosed by backticks (grave accents) first undergoes
1395 variable substitution just like a double quoted string. It is then
1396 interpreted as a command, and the output of that command is the value
1397 of the pseudo-literal, like in a shell. In scalar context, a single
1398 string consisting of all the output is returned. In list context,
1399 a list of values is returned, one for each line of output. (You can
1400 set C<$/> to use a different line terminator.) The command is executed
1401 each time the pseudo-literal is evaluated. The status value of the
1402 command is returned in C<$?> (see L<perlvar> for the interpretation
1403 of C<$?>). Unlike in B<csh>, no translation is done on the return
1404 data--newlines remain newlines. Unlike in any of the shells, single
1405 quotes do not hide variable names in the command from interpretation.
1406 To pass a $ through to the shell you need to hide it with a backslash.
1407 The generalized form of backticks is C<qx//>. (Because backticks
1408 always undergo shell expansion as well, see L<perlsec> for
1411 Evaluating a filehandle in angle brackets yields the next line from
1412 that file (newline, if any, included), or C<undef> at end of file.
1413 Ordinarily you must assign that value to a variable, but there is one
1414 situation where an automatic assignment happens. I<If and ONLY if> the
1415 input symbol is the only thing inside the conditional of a C<while> or
1416 C<for(;;)> loop, the value is automatically assigned to the variable
1417 C<$_>. In these loop constructs, the assigned value (whether assignment
1418 is automatic or explicit) is then tested to see if it is defined.
1419 The defined test avoids problems where line has a string value
1420 that would be treated as false by perl e.g. "" or "0" with no trailing
1421 newline. (This may seem like an odd thing to you, but you'll use the
1422 construct in almost every Perl script you write.) Anyway, the following
1423 lines are equivalent to each other:
1425 while (defined($_ = <STDIN>)) { print; }
1426 while ($_ = <STDIN>) { print; }
1427 while (<STDIN>) { print; }
1428 for (;<STDIN>;) { print; }
1429 print while defined($_ = <STDIN>);
1430 print while ($_ = <STDIN>);
1431 print while <STDIN>;
1433 and this also behaves similarly, but avoids the use of $_ :
1435 while (my $line = <STDIN>) { print $line }
1437 If you really mean such values to terminate the loop they should be
1438 tested for explicitly:
1440 while (($_ = <STDIN>) ne '0') { ... }
1441 while (<STDIN>) { last unless $_; ... }
1443 In other boolean contexts, C<E<lt>I<filehandle>E<gt>> without explicit C<defined>
1444 test or comparison will solicit a warning if C<-w> is in effect.
1446 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1447 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in
1448 packages, where they would be interpreted as local identifiers rather
1449 than global.) Additional filehandles may be created with the open()
1450 function. See L<perlfunc/open()> for details on this.
1452 If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a
1453 list consisting of all the input lines is returned, one line per list
1454 element. It's easy to make a I<LARGE> data space this way, so use with
1457 The null filehandle E<lt>E<gt> is special and can be used to emulate the
1458 behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from
1459 standard input, or from each file listed on the command line. Here's
1460 how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is
1461 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
1462 gives you standard input. The @ARGV array is then processed as a list
1463 of filenames. The loop
1466 ... # code for each line
1469 is equivalent to the following Perl-like pseudo code:
1471 unshift(@ARGV, '-') unless @ARGV;
1472 while ($ARGV = shift) {
1475 ... # code for each line
1479 except that it isn't so cumbersome to say, and will actually work. It
1480 really does shift array @ARGV and put the current filename into variable
1481 $ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a
1482 synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above
1483 doesn't work because it treats E<lt>ARGVE<gt> as non-magical.)
1485 You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up
1486 containing the list of filenames you really want. Line numbers (C<$.>)
1487 continue as if the input were one big happy file. (But see example
1488 under C<eof> for how to reset line numbers on each file.)
1490 If you want to set @ARGV to your own list of files, go right ahead.
1491 This sets @ARGV to all plain text files if no @ARGV was given:
1493 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
1495 You can even set them to pipe commands. For example, this automatically
1496 filters compressed arguments through B<gzip>:
1498 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1500 If you want to pass switches into your script, you can use one of the
1501 Getopts modules or put a loop on the front like this:
1503 while ($_ = $ARGV[0], /^-/) {
1506 if (/^-D(.*)/) { $debug = $1 }
1507 if (/^-v/) { $verbose++ }
1508 # ... # other switches
1512 # ... # code for each line
1515 The E<lt>E<gt> symbol will return C<undef> for end-of-file only once.
1516 If you call it again after this it will assume you are processing another
1517 @ARGV list, and if you haven't set @ARGV, will input from STDIN.
1519 If the string inside the angle brackets is a reference to a scalar
1520 variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the
1521 filehandle to input from, or its typeglob, or a reference to the same. For example:
1526 If what's within the angle brackets is neither a filehandle nor a simple
1527 scalar variable containing a filehandle name, typeglob, or typeglob
1528 reference, it is interpreted as a filename pattern to be globbed, and
1529 either a list of filenames or the next filename in the list is returned,
1530 depending on context. This distinction is determined on syntactic
1531 grounds alone. That means C<E<lt>$xE<gt>> is always a readline from
1532 an indirect handle, but C<E<lt>$hash{key}E<gt>> is always a glob.
1533 That's because $x is a simple scalar variable, but C<$hash{key}> is
1534 not--it's a hash element.
1536 One level of double-quote interpretation is done first, but you can't
1537 say C<E<lt>$fooE<gt>> because that's an indirect filehandle as explained
1538 in the previous paragraph. (In older versions of Perl, programmers
1539 would insert curly brackets to force interpretation as a filename glob:
1540 C<E<lt>${foo}E<gt>>. These days, it's considered cleaner to call the
1541 internal function directly as C<glob($foo)>, which is probably the right
1542 way to have done it in the first place.) Example:
1550 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1556 In fact, it's currently implemented that way. (Which means it will not
1557 work on filenames with spaces in them unless you have csh(1) on your
1558 machine.) Of course, the shortest way to do the above is:
1562 Because globbing invokes a shell, it's often faster to call readdir() yourself
1563 and do your own grep() on the filenames. Furthermore, due to its current
1564 implementation of using a shell, the glob() routine may get "Arg list too
1565 long" errors (unless you've installed tcsh(1L) as F</bin/csh>).
1567 A glob evaluates its (embedded) argument only when it is starting a new
1568 list. All values must be read before it will start over. In a list
1569 context this isn't important, because you automatically get them all
1570 anyway. In scalar context, however, the operator returns the next value
1571 each time it is called, or a C<undef> value if you've just run out. As
1572 for filehandles an automatic C<defined> is generated when the glob
1573 occurs in the test part of a C<while> or C<for> - because legal glob returns
1574 (e.g. a file called F<0>) would otherwise terminate the loop.
1575 Again, C<undef> is returned only once. So if you're expecting a single value
1576 from a glob, it is much better to say
1578 ($file) = <blurch*>;
1584 because the latter will alternate between returning a filename and
1587 It you're trying to do variable interpolation, it's definitely better
1588 to use the glob() function, because the older notation can cause people
1589 to become confused with the indirect filehandle notation.
1591 @files = glob("$dir/*.[ch]");
1592 @files = glob($files[$i]);
1594 =head2 Constant Folding
1596 Like C, Perl does a certain amount of expression evaluation at
1597 compile time, whenever it determines that all arguments to an
1598 operator are static and have no side effects. In particular, string
1599 concatenation happens at compile time between literals that don't do
1600 variable substitution. Backslash interpretation also happens at
1601 compile time. You can say
1603 'Now is the time for all' . "\n" .
1604 'good men to come to.'
1606 and this all reduces to one string internally. Likewise, if
1609 foreach $file (@filenames) {
1610 if (-s $file > 5 + 100 * 2**16) { }
1613 the compiler will precompute the number that
1614 expression represents so that the interpreter
1617 =head2 Bitwise String Operators
1619 Bitstrings of any size may be manipulated by the bitwise operators
1622 If the operands to a binary bitwise op are strings of different sizes,
1623 B<or> and B<xor> ops will act as if the shorter operand had additional
1624 zero bits on the right, while the B<and> op will act as if the longer
1625 operand were truncated to the length of the shorter.
1627 # ASCII-based examples
1628 print "j p \n" ^ " a h"; # prints "JAPH\n"
1629 print "JA" | " ph\n"; # prints "japh\n"
1630 print "japh\nJunk" & '_____'; # prints "JAPH\n";
1631 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
1633 If you are intending to manipulate bitstrings, you should be certain that
1634 you're supplying bitstrings: If an operand is a number, that will imply
1635 a B<numeric> bitwise operation. You may explicitly show which type of
1636 operation you intend by using C<""> or C<0+>, as in the examples below.
1638 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
1639 $foo = '150' | 105 ; # yields 255
1640 $foo = 150 | '105'; # yields 255
1641 $foo = '150' | '105'; # yields string '155' (under ASCII)
1643 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
1644 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
1646 =head2 Integer Arithmetic
1648 By default Perl assumes that it must do most of its arithmetic in
1649 floating point. But by saying
1653 you may tell the compiler that it's okay to use integer operations
1654 from here to the end of the enclosing BLOCK. An inner BLOCK may
1655 countermand this by saying
1659 which lasts until the end of that BLOCK.
1661 The bitwise operators ("&", "|", "^", "~", "<<", and ">>") always
1662 produce integral results. (But see also L<Bitwise String Operators>.)
1663 However, C<use integer> still has meaning
1664 for them. By default, their results are interpreted as unsigned
1665 integers. However, if C<use integer> is in effect, their results are
1666 interpreted as signed integers. For example, C<~0> usually evaluates
1667 to a large integral value. However, C<use integer; ~0> is -1 on twos-complement machines.
1669 =head2 Floating-point Arithmetic
1671 While C<use integer> provides integer-only arithmetic, there is no
1672 similar ways to provide rounding or truncation at a certain number of
1673 decimal places. For rounding to a certain number of digits, sprintf()
1674 or printf() is usually the easiest route.
1676 Floating-point numbers are only approximations to what a mathematician
1677 would call real numbers. There are infinitely more reals than floats,
1678 so some corners must be cut. For example:
1680 printf "%.20g\n", 123456789123456789;
1681 # produces 123456789123456784
1683 Testing for exact equality of floating-point equality or inequality is
1684 not a good idea. Here's a (relatively expensive) work-around to compare
1685 whether two floating-point numbers are equal to a particular number of
1686 decimal places. See Knuth, volume II, for a more robust treatment of
1690 my ($X, $Y, $POINTS) = @_;
1692 $tX = sprintf("%.${POINTS}g", $X);
1693 $tY = sprintf("%.${POINTS}g", $Y);
1697 The POSIX module (part of the standard perl distribution) implements
1698 ceil(), floor(), and a number of other mathematical and trigonometric
1699 functions. The Math::Complex module (part of the standard perl
1700 distribution) defines a number of mathematical functions that can also
1701 work on real numbers. Math::Complex not as efficient as POSIX, but
1702 POSIX can't work with complex numbers.
1704 Rounding in financial applications can have serious implications, and
1705 the rounding method used should be specified precisely. In these
1706 cases, it probably pays not to trust whichever system rounding is
1707 being used by Perl, but to instead implement the rounding function you
1710 =head2 Bigger Numbers
1712 The standard Math::BigInt and Math::BigFloat modules provide
1713 variable precision arithmetic and overloaded operators.
1714 At the cost of some space and considerable speed, they
1715 avoid the normal pitfalls associated with limited-precision
1719 $x = Math::BigInt->new('123456789123456789');
1722 # prints +15241578780673678515622620750190521