3 perlop - Perl operators and precedence
7 Perl operators have the following associativity and precedence,
8 listed from highest precedence to lowest. Operators borrowed from
9 C keep the same precedence relationship with each other, even where
10 C's precedence is slightly screwy. (This makes learning Perl easier
11 for C folks.) With very few exceptions, these all operate on scalar
12 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 include 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,
68 but the commas on the left are evaluated after. In other words,
69 list operators tend to gobble up all arguments that follow, and
70 then act like a simple TERM with regard to the preceding expression.
71 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 "C<< -> >>" is an infix dereference operator, just as it is in C
99 and C++. If the right side is either a C<[...]>, C<{...}>, or a
100 C<(...)> subscript, then the left side must be either a hard or
101 symbolic reference to an array, a hash, or a subroutine respectively.
102 (Or technically speaking, a location capable of holding a hard
103 reference, if it's an array or hash reference being used for
104 assignment.) See L<perlreftut> and L<perlref>.
106 Otherwise, the right side is a method name or a simple scalar
107 variable containing either the method name or a subroutine reference,
108 and the left side must be either an object (a blessed reference)
109 or a class name (that is, a package name). See L<perlobj>.
111 =head2 Auto-increment and Auto-decrement
113 "++" and "--" work as in C. That is, if placed before a variable, they
114 increment or decrement the variable before returning the value, and if
115 placed after, increment or decrement the variable after returning the value.
117 The auto-increment operator has a little extra builtin magic to it. If
118 you increment a variable that is numeric, or that has ever been used in
119 a numeric context, you get a normal increment. If, however, the
120 variable has been used in only string contexts since it was set, and
121 has a value that is not the empty string and matches the pattern
122 C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
123 character within its range, with carry:
125 print ++($foo = '99'); # prints '100'
126 print ++($foo = 'a0'); # prints 'a1'
127 print ++($foo = 'Az'); # prints 'Ba'
128 print ++($foo = 'zz'); # prints 'aaa'
130 The auto-decrement operator is not magical.
132 =head2 Exponentiation
134 Binary "**" is the exponentiation operator. It binds even more
135 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
136 implemented using C's pow(3) function, which actually works on doubles
139 =head2 Symbolic Unary Operators
141 Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
142 precedence version of this.
144 Unary "-" performs arithmetic negation if the operand is numeric. If
145 the operand is an identifier, a string consisting of a minus sign
146 concatenated with the identifier is returned. Otherwise, if the string
147 starts with a plus or minus, a string starting with the opposite sign
148 is returned. One effect of these rules is that C<-bareword> is equivalent
151 Unary "~" performs bitwise negation, i.e., 1's complement. For
152 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
153 L<Bitwise String Operators>.) Note that the width of the result is
154 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
155 bits wide on a 64-bit platform, so if you are expecting a certain bit
156 width, remember use the & operator to mask off the excess bits.
158 Unary "+" has no effect whatsoever, even on strings. It is useful
159 syntactically for separating a function name from a parenthesized expression
160 that would otherwise be interpreted as the complete list of function
161 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
163 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
164 and L<perlref>. Do not confuse this behavior with the behavior of
165 backslash within a string, although both forms do convey the notion
166 of protecting the next thing from interpolation.
168 =head2 Binding Operators
170 Binary "=~" binds a scalar expression to a pattern match. Certain operations
171 search or modify the string $_ by default. This operator makes that kind
172 of operation work on some other string. The right argument is a search
173 pattern, substitution, or transliteration. The left argument is what is
174 supposed to be searched, substituted, or transliterated instead of the default
175 $_. When used in scalar context, the return value generally indicates the
176 success of the operation. Behavior in list context depends on the particular
177 operator. See L</"Regexp Quote-Like Operators"> for details.
179 If the right argument is an expression rather than a search pattern,
180 substitution, or transliteration, it is interpreted as a search pattern at run
181 time. This can be less efficient than an explicit search, because the
182 pattern must be compiled every time the expression is evaluated.
184 Binary "!~" is just like "=~" except the return value is negated in
187 =head2 Multiplicative Operators
189 Binary "*" multiplies two numbers.
191 Binary "/" divides two numbers.
193 Binary "%" computes the modulus of two numbers. Given integer
194 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
195 C<$a> minus the largest multiple of C<$b> that is not greater than
196 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
197 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
198 result will be less than or equal to zero).
199 Note than when C<use integer> is in scope, "%" gives you direct access
200 to the modulus operator as implemented by your C compiler. This
201 operator is not as well defined for negative operands, but it will
204 Binary "x" is the repetition operator. In scalar context or if the left
205 operand is not enclosed in parentheses, it returns a string consisting
206 of the left operand repeated the number of times specified by the right
207 operand. In list context, if the left operand is enclosed in
208 parentheses, it repeats the list.
210 print '-' x 80; # print row of dashes
212 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
214 @ones = (1) x 80; # a list of 80 1's
215 @ones = (5) x @ones; # set all elements to 5
218 =head2 Additive Operators
220 Binary "+" returns the sum of two numbers.
222 Binary "-" returns the difference of two numbers.
224 Binary "." concatenates two strings.
226 =head2 Shift Operators
228 Binary "<<" returns the value of its left argument shifted left by the
229 number of bits specified by the right argument. Arguments should be
230 integers. (See also L<Integer Arithmetic>.)
232 Binary ">>" returns the value of its left argument shifted right by
233 the number of bits specified by the right argument. Arguments should
234 be integers. (See also L<Integer Arithmetic>.)
236 Note that both "<<" and ">>" in Perl are implemented directly using
237 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
238 in force then signed C integers are used, else unsigned C integers are
239 used. Either way, the implementation isn't going to generate results
240 larger than the size of the integer type Perl was built with (32 bits
243 The result of overflowing the range of the integers is undefined
244 because it is undefined also in C. In other words, using 32-bit
245 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
246 of bits is also undefined.
248 =head2 Named Unary Operators
250 The various named unary operators are treated as functions with one
251 argument, with optional parentheses. These include the filetest
252 operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
254 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
255 is followed by a left parenthesis as the next token, the operator and
256 arguments within parentheses are taken to be of highest precedence,
257 just like a normal function call. For example,
258 because named unary operators are higher precedence than ||:
260 chdir $foo || die; # (chdir $foo) || die
261 chdir($foo) || die; # (chdir $foo) || die
262 chdir ($foo) || die; # (chdir $foo) || die
263 chdir +($foo) || die; # (chdir $foo) || die
265 but, because * is higher precedence than named operators:
267 chdir $foo * 20; # chdir ($foo * 20)
268 chdir($foo) * 20; # (chdir $foo) * 20
269 chdir ($foo) * 20; # (chdir $foo) * 20
270 chdir +($foo) * 20; # chdir ($foo * 20)
272 rand 10 * 20; # rand (10 * 20)
273 rand(10) * 20; # (rand 10) * 20
274 rand (10) * 20; # (rand 10) * 20
275 rand +(10) * 20; # rand (10 * 20)
277 See also L<"Terms and List Operators (Leftward)">.
279 =head2 Relational Operators
281 Binary "<" returns true if the left argument is numerically less than
284 Binary ">" returns true if the left argument is numerically greater
285 than the right argument.
287 Binary "<=" returns true if the left argument is numerically less than
288 or equal to the right argument.
290 Binary ">=" returns true if the left argument is numerically greater
291 than or equal to the right argument.
293 Binary "lt" returns true if the left argument is stringwise less than
296 Binary "gt" returns true if the left argument is stringwise greater
297 than the right argument.
299 Binary "le" returns true if the left argument is stringwise less than
300 or equal to the right argument.
302 Binary "ge" returns true if the left argument is stringwise greater
303 than or equal to the right argument.
305 =head2 Equality Operators
307 Binary "==" returns true if the left argument is numerically equal to
310 Binary "!=" returns true if the left argument is numerically not equal
311 to the right argument.
313 Binary "<=>" returns -1, 0, or 1 depending on whether the left
314 argument is numerically less than, equal to, or greater than the right
315 argument. If your platform supports NaNs (not-a-numbers) as numeric
316 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
317 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
318 returns true, as does NaN != anything else. If your platform doesn't
319 support NaNs then NaN is just a string with numeric value 0.
321 perl -le '$a = NaN; print "No NaN support here" if $a == $a'
322 perl -le '$a = NaN; print "NaN support here" if $a != $a'
324 Binary "eq" returns true if the left argument is stringwise equal to
327 Binary "ne" returns true if the left argument is stringwise not equal
328 to the right argument.
330 Binary "cmp" returns -1, 0, or 1 depending on whether the left
331 argument is stringwise less than, equal to, or greater than the right
334 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
335 by the current locale if C<use locale> is in effect. See L<perllocale>.
339 Binary "&" returns its operators ANDed together bit by bit.
340 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
342 =head2 Bitwise Or and Exclusive Or
344 Binary "|" returns its operators ORed together bit by bit.
345 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
347 Binary "^" returns its operators XORed together bit by bit.
348 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
350 =head2 C-style Logical And
352 Binary "&&" performs a short-circuit logical AND operation. That is,
353 if the left operand is false, the right operand is not even evaluated.
354 Scalar or list context propagates down to the right operand if it
357 =head2 C-style Logical Or
359 Binary "||" performs a short-circuit logical OR operation. That is,
360 if the left operand is true, the right operand is not even evaluated.
361 Scalar or list context propagates down to the right operand if it
364 The C<||> and C<&&> operators differ from C's in that, rather than returning
365 0 or 1, they return the last value evaluated. Thus, a reasonably portable
366 way to find out the home directory (assuming it's not "0") might be:
368 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
369 (getpwuid($<))[7] || die "You're homeless!\n";
371 In particular, this means that you shouldn't use this
372 for selecting between two aggregates for assignment:
374 @a = @b || @c; # this is wrong
375 @a = scalar(@b) || @c; # really meant this
376 @a = @b ? @b : @c; # this works fine, though
378 As more readable alternatives to C<&&> and C<||> when used for
379 control flow, Perl provides C<and> and C<or> operators (see below).
380 The short-circuit behavior is identical. The precedence of "and" and
381 "or" is much lower, however, so that you can safely use them after a
382 list operator without the need for parentheses:
384 unlink "alpha", "beta", "gamma"
385 or gripe(), next LINE;
387 With the C-style operators that would have been written like this:
389 unlink("alpha", "beta", "gamma")
390 || (gripe(), next LINE);
392 Using "or" for assignment is unlikely to do what you want; see below.
394 =head2 Range Operators
396 Binary ".." is the range operator, which is really two different
397 operators depending on the context. In list context, it returns an
398 array of values counting (up by ones) from the left value to the right
399 value. If the left value is greater than the right value then it
400 returns the empty array. The range operator is useful for writing
401 C<foreach (1..10)> loops and for doing slice operations on arrays. In
402 the current implementation, no temporary array is created when the
403 range operator is used as the expression in C<foreach> loops, but older
404 versions of Perl might burn a lot of memory when you write something
407 for (1 .. 1_000_000) {
411 In scalar context, ".." returns a boolean value. The operator is
412 bistable, like a flip-flop, and emulates the line-range (comma) operator
413 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
414 own boolean state. It is false as long as its left operand is false.
415 Once the left operand is true, the range operator stays true until the
416 right operand is true, I<AFTER> which the range operator becomes false
417 again. It doesn't become false till the next time the range operator is
418 evaluated. It can test the right operand and become false on the same
419 evaluation it became true (as in B<awk>), but it still returns true once.
420 If you don't want it to test the right operand till the next
421 evaluation, as in B<sed>, just use three dots ("...") instead of
422 two. In all other regards, "..." behaves just like ".." does.
424 The right operand is not evaluated while the operator is in the
425 "false" state, and the left operand is not evaluated while the
426 operator is in the "true" state. The precedence is a little lower
427 than || and &&. The value returned is either the empty string for
428 false, or a sequence number (beginning with 1) for true. The
429 sequence number is reset for each range encountered. The final
430 sequence number in a range has the string "E0" appended to it, which
431 doesn't affect its numeric value, but gives you something to search
432 for if you want to exclude the endpoint. You can exclude the
433 beginning point by waiting for the sequence number to be greater
434 than 1. If either operand of scalar ".." is a constant expression,
435 that operand is implicitly compared to the C<$.> variable, the
436 current line number. Examples:
438 As a scalar operator:
440 if (101 .. 200) { print; } # print 2nd hundred lines
441 next line if (1 .. /^$/); # skip header lines
442 s/^/> / if (/^$/ .. eof()); # quote body
444 # parse mail messages
446 $in_header = 1 .. /^$/;
447 $in_body = /^$/ .. eof();
448 # do something based on those
450 close ARGV if eof; # reset $. each file
455 for (101 .. 200) { print; } # print $_ 100 times
456 @foo = @foo[0 .. $#foo]; # an expensive no-op
457 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
459 The range operator (in list context) makes use of the magical
460 auto-increment algorithm if the operands are strings. You
463 @alphabet = ('A' .. 'Z');
465 to get all normal letters of the alphabet, or
467 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
469 to get a hexadecimal digit, or
471 @z2 = ('01' .. '31'); print $z2[$mday];
473 to get dates with leading zeros. If the final value specified is not
474 in the sequence that the magical increment would produce, the sequence
475 goes until the next value would be longer than the final value
478 =head2 Conditional Operator
480 Ternary "?:" is the conditional operator, just as in C. It works much
481 like an if-then-else. If the argument before the ? is true, the
482 argument before the : is returned, otherwise the argument after the :
483 is returned. For example:
485 printf "I have %d dog%s.\n", $n,
486 ($n == 1) ? '' : "s";
488 Scalar or list context propagates downward into the 2nd
489 or 3rd argument, whichever is selected.
491 $a = $ok ? $b : $c; # get a scalar
492 @a = $ok ? @b : @c; # get an array
493 $a = $ok ? @b : @c; # oops, that's just a count!
495 The operator may be assigned to if both the 2nd and 3rd arguments are
496 legal lvalues (meaning that you can assign to them):
498 ($a_or_b ? $a : $b) = $c;
500 Because this operator produces an assignable result, using assignments
501 without parentheses will get you in trouble. For example, this:
503 $a % 2 ? $a += 10 : $a += 2
507 (($a % 2) ? ($a += 10) : $a) += 2
511 ($a % 2) ? ($a += 10) : ($a += 2)
513 That should probably be written more simply as:
515 $a += ($a % 2) ? 10 : 2;
517 =head2 Assignment Operators
519 "=" is the ordinary assignment operator.
521 Assignment operators work as in C. That is,
529 although without duplicating any side effects that dereferencing the lvalue
530 might trigger, such as from tie(). Other assignment operators work similarly.
531 The following are recognized:
538 Although these are grouped by family, they all have the precedence
541 Unlike in C, the scalar assignment operator produces a valid lvalue.
542 Modifying an assignment is equivalent to doing the assignment and
543 then modifying the variable that was assigned to. This is useful
544 for modifying a copy of something, like this:
546 ($tmp = $global) =~ tr [A-Z] [a-z];
557 Similarly, a list assignment in list context produces the list of
558 lvalues assigned to, and a list assignment in scalar context returns
559 the number of elements produced by the expression on the right hand
560 side of the assignment.
562 =head2 Comma Operator
564 Binary "," is the comma operator. In scalar context it evaluates
565 its left argument, throws that value away, then evaluates its right
566 argument and returns that value. This is just like C's comma operator.
568 In list context, it's just the list argument separator, and inserts
569 both its arguments into the list.
571 The => digraph is mostly just a synonym for the comma operator. It's useful for
572 documenting arguments that come in pairs. As of release 5.001, it also forces
573 any word to the left of it to be interpreted as a string.
575 =head2 List Operators (Rightward)
577 On the right side of a list operator, it has very low precedence,
578 such that it controls all comma-separated expressions found there.
579 The only operators with lower precedence are the logical operators
580 "and", "or", and "not", which may be used to evaluate calls to list
581 operators without the need for extra parentheses:
583 open HANDLE, "filename"
584 or die "Can't open: $!\n";
586 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
590 Unary "not" returns the logical negation of the expression to its right.
591 It's the equivalent of "!" except for the very low precedence.
595 Binary "and" returns the logical conjunction of the two surrounding
596 expressions. It's equivalent to && except for the very low
597 precedence. This means that it short-circuits: i.e., the right
598 expression is evaluated only if the left expression is true.
600 =head2 Logical or and Exclusive Or
602 Binary "or" returns the logical disjunction of the two surrounding
603 expressions. It's equivalent to || except for the very low precedence.
604 This makes it useful for control flow
606 print FH $data or die "Can't write to FH: $!";
608 This means that it short-circuits: i.e., the right expression is evaluated
609 only if the left expression is false. Due to its precedence, you should
610 probably avoid using this for assignment, only for control flow.
612 $a = $b or $c; # bug: this is wrong
613 ($a = $b) or $c; # really means this
614 $a = $b || $c; # better written this way
616 However, when it's a list-context assignment and you're trying to use
617 "||" for control flow, you probably need "or" so that the assignment
618 takes higher precedence.
620 @info = stat($file) || die; # oops, scalar sense of stat!
621 @info = stat($file) or die; # better, now @info gets its due
623 Then again, you could always use parentheses.
625 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
626 It cannot short circuit, of course.
628 =head2 C Operators Missing From Perl
630 Here is what C has that Perl doesn't:
636 Address-of operator. (But see the "\" operator for taking a reference.)
640 Dereference-address operator. (Perl's prefix dereferencing
641 operators are typed: $, @, %, and &.)
645 Type-casting operator.
649 =head2 Quote and Quote-like Operators
651 While we usually think of quotes as literal values, in Perl they
652 function as operators, providing various kinds of interpolating and
653 pattern matching capabilities. Perl provides customary quote characters
654 for these behaviors, but also provides a way for you to choose your
655 quote character for any of them. In the following table, a C<{}> represents
656 any pair of delimiters you choose.
658 Customary Generic Meaning Interpolates
661 `` qx{} Command yes (unless '' is delimiter)
663 // m{} Pattern match yes (unless '' is delimiter)
664 qr{} Pattern yes (unless '' is delimiter)
665 s{}{} Substitution yes (unless '' is delimiter)
666 tr{}{} Transliteration no (but see below)
668 Non-bracketing delimiters use the same character fore and aft, but the four
669 sorts of brackets (round, angle, square, curly) will all nest, which means
678 Note, however, that this does not always work for quoting Perl code:
680 $s = q{ if($a eq "}") ... }; # WRONG
682 is a syntax error. The C<Text::Balanced> module (from CPAN, and
683 starting from Perl 5.8 part of the standard distribution) is able
686 There can be whitespace between the operator and the quoting
687 characters, except when C<#> is being used as the quoting character.
688 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
689 operator C<q> followed by a comment. Its argument will be taken
690 from the next line. This allows you to write:
692 s {foo} # Replace foo
695 For constructs that do interpolate, variables beginning with "C<$>"
696 or "C<@>" are interpolated, as are the following escape sequences. Within
697 a transliteration, the first eleven of these sequences may be used.
704 \a alarm (bell) (BEL)
706 \033 octal char (ESC)
708 \x{263a} wide hex char (SMILEY)
709 \c[ control char (ESC)
712 \l lowercase next char
713 \u uppercase next char
716 \E end case modification
717 \Q quote non-word characters till \E
719 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
720 and C<\U> is taken from the current locale. See L<perllocale>. For
721 documentation of C<\N{name}>, see L<charnames>.
723 All systems use the virtual C<"\n"> to represent a line terminator,
724 called a "newline". There is no such thing as an unvarying, physical
725 newline character. It is only an illusion that the operating system,
726 device drivers, C libraries, and Perl all conspire to preserve. Not all
727 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
728 on a Mac, these are reversed, and on systems without line terminator,
729 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
730 you mean a "newline" for your system, but use the literal ASCII when you
731 need an exact character. For example, most networking protocols expect
732 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
733 and although they often accept just C<"\012">, they seldom tolerate just
734 C<"\015">. If you get in the habit of using C<"\n"> for networking,
735 you may be burned some day.
737 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
738 An unescaped C<$> or C<@> interpolates the corresponding variable,
739 while escaping will cause the literal string C<\$> to be inserted.
740 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
742 Patterns are subject to an additional level of interpretation as a
743 regular expression. This is done as a second pass, after variables are
744 interpolated, so that regular expressions may be incorporated into the
745 pattern from the variables. If this is not what you want, use C<\Q> to
746 interpolate a variable literally.
748 Apart from the behavior described above, Perl does not expand
749 multiple levels of interpolation. In particular, contrary to the
750 expectations of shell programmers, back-quotes do I<NOT> interpolate
751 within double quotes, nor do single quotes impede evaluation of
752 variables when used within double quotes.
754 =head2 Regexp Quote-Like Operators
756 Here are the quote-like operators that apply to pattern
757 matching and related activities.
763 This is just like the C</pattern/> search, except that it matches only
764 once between calls to the reset() operator. This is a useful
765 optimization when you want to see only the first occurrence of
766 something in each file of a set of files, for instance. Only C<??>
767 patterns local to the current package are reset.
771 # blank line between header and body
774 reset if eof; # clear ?? status for next file
777 This usage is vaguely deprecated, which means it just might possibly
778 be removed in some distant future version of Perl, perhaps somewhere
779 around the year 2168.
781 =item m/PATTERN/cgimosx
783 =item /PATTERN/cgimosx
785 Searches a string for a pattern match, and in scalar context returns
786 true if it succeeds, false if it fails. If no string is specified
787 via the C<=~> or C<!~> operator, the $_ string is searched. (The
788 string specified with C<=~> need not be an lvalue--it may be the
789 result of an expression evaluation, but remember the C<=~> binds
790 rather tightly.) See also L<perlre>. See L<perllocale> for
791 discussion of additional considerations that apply when C<use locale>
796 c Do not reset search position on a failed match when /g is in effect.
797 g Match globally, i.e., find all occurrences.
798 i Do case-insensitive pattern matching.
799 m Treat string as multiple lines.
800 o Compile pattern only once.
801 s Treat string as single line.
802 x Use extended regular expressions.
804 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
805 you can use any pair of non-alphanumeric, non-whitespace characters
806 as delimiters. This is particularly useful for matching path names
807 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
808 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
809 If "'" is the delimiter, no interpolation is performed on the PATTERN.
811 PATTERN may contain variables, which will be interpolated (and the
812 pattern recompiled) every time the pattern search is evaluated, except
813 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
814 C<$|> are not interpolated because they look like end-of-string tests.)
815 If you want such a pattern to be compiled only once, add a C</o> after
816 the trailing delimiter. This avoids expensive run-time recompilations,
817 and is useful when the value you are interpolating won't change over
818 the life of the script. However, mentioning C</o> constitutes a promise
819 that you won't change the variables in the pattern. If you change them,
820 Perl won't even notice. See also L<"qr/STRING/imosx">.
822 If the PATTERN evaluates to the empty string, the last
823 I<successfully> matched regular expression is used instead.
825 If the C</g> option is not used, C<m//> in list context returns a
826 list consisting of the subexpressions matched by the parentheses in the
827 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
828 also set, and that this differs from Perl 4's behavior.) When there are
829 no parentheses in the pattern, the return value is the list C<(1)> for
830 success. With or without parentheses, an empty list is returned upon
835 open(TTY, '/dev/tty');
836 <TTY> =~ /^y/i && foo(); # do foo if desired
838 if (/Version: *([0-9.]*)/) { $version = $1; }
840 next if m#^/usr/spool/uucp#;
845 print if /$arg/o; # compile only once
848 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
850 This last example splits $foo into the first two words and the
851 remainder of the line, and assigns those three fields to $F1, $F2, and
852 $Etc. The conditional is true if any variables were assigned, i.e., if
855 The C</g> modifier specifies global pattern matching--that is,
856 matching as many times as possible within the string. How it behaves
857 depends on the context. In list context, it returns a list of the
858 substrings matched by any capturing parentheses in the regular
859 expression. If there are no parentheses, it returns a list of all
860 the matched strings, as if there were parentheses around the whole
863 In scalar context, each execution of C<m//g> finds the next match,
864 returning true if it matches, and false if there is no further match.
865 The position after the last match can be read or set using the pos()
866 function; see L<perlfunc/pos>. A failed match normally resets the
867 search position to the beginning of the string, but you can avoid that
868 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
869 string also resets the search position.
871 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
872 zero-width assertion that matches the exact position where the previous
873 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
874 still anchors at pos(), but the match is of course only attempted once.
875 Using C<\G> without C</g> on a target string that has not previously had a
876 C</g> match applied to it is the same as using the C<\A> assertion to match
877 the beginning of the string.
882 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
886 while (defined($paragraph = <>)) {
887 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
891 print "$sentences\n";
893 # using m//gc with \G
897 print $1 while /(o)/gc; print "', pos=", pos, "\n";
899 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
901 print $1 while /(p)/gc; print "', pos=", pos, "\n";
903 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
905 The last example should print:
915 Notice that the final match matched C<q> instead of C<p>, which a match
916 without the C<\G> anchor would have done. Also note that the final match
917 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
918 final match did indeed match C<p>, it's a good bet that you're running an
919 older (pre-5.6.0) Perl.
921 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
922 combine several regexps like this to process a string part-by-part,
923 doing different actions depending on which regexp matched. Each
924 regexp tries to match where the previous one leaves off.
927 $url = new URI::URL "http://www/"; die if $url eq "xXx";
931 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
932 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
933 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
934 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
935 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
936 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
937 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
938 print ". That's all!\n";
941 Here is the output (split into several lines):
943 line-noise lowercase line-noise lowercase UPPERCASE line-noise
944 UPPERCASE line-noise lowercase line-noise lowercase line-noise
945 lowercase lowercase line-noise lowercase lowercase line-noise
946 MiXeD line-noise. That's all!
952 A single-quoted, literal string. A backslash represents a backslash
953 unless followed by the delimiter or another backslash, in which case
954 the delimiter or backslash is interpolated.
956 $foo = q!I said, "You said, 'She said it.'"!;
957 $bar = q('This is it.');
958 $baz = '\n'; # a two-character string
964 A double-quoted, interpolated string.
967 (*** The previous line contains the naughty word "$1".\n)
968 if /\b(tcl|java|python)\b/i; # :-)
969 $baz = "\n"; # a one-character string
971 =item qr/STRING/imosx
973 This operator quotes (and possibly compiles) its I<STRING> as a regular
974 expression. I<STRING> is interpolated the same way as I<PATTERN>
975 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
976 is done. Returns a Perl value which may be used instead of the
977 corresponding C</STRING/imosx> expression.
981 $rex = qr/my.STRING/is;
988 The result may be used as a subpattern in a match:
991 $string =~ /foo${re}bar/; # can be interpolated in other patterns
992 $string =~ $re; # or used standalone
993 $string =~ /$re/; # or this way
995 Since Perl may compile the pattern at the moment of execution of qr()
996 operator, using qr() may have speed advantages in some situations,
997 notably if the result of qr() is used standalone:
1000 my $patterns = shift;
1001 my @compiled = map qr/$_/i, @$patterns;
1004 foreach my $pat (@compiled) {
1005 $success = 1, last if /$pat/;
1011 Precompilation of the pattern into an internal representation at
1012 the moment of qr() avoids a need to recompile the pattern every
1013 time a match C</$pat/> is attempted. (Perl has many other internal
1014 optimizations, but none would be triggered in the above example if
1015 we did not use qr() operator.)
1019 i Do case-insensitive pattern matching.
1020 m Treat string as multiple lines.
1021 o Compile pattern only once.
1022 s Treat string as single line.
1023 x Use extended regular expressions.
1025 See L<perlre> for additional information on valid syntax for STRING, and
1026 for a detailed look at the semantics of regular expressions.
1032 A string which is (possibly) interpolated and then executed as a
1033 system command with C</bin/sh> or its equivalent. Shell wildcards,
1034 pipes, and redirections will be honored. The collected standard
1035 output of the command is returned; standard error is unaffected. In
1036 scalar context, it comes back as a single (potentially multi-line)
1037 string, or undef if the command failed. In list context, returns a
1038 list of lines (however you've defined lines with $/ or
1039 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1041 Because backticks do not affect standard error, use shell file descriptor
1042 syntax (assuming the shell supports this) if you care to address this.
1043 To capture a command's STDERR and STDOUT together:
1045 $output = `cmd 2>&1`;
1047 To capture a command's STDOUT but discard its STDERR:
1049 $output = `cmd 2>/dev/null`;
1051 To capture a command's STDERR but discard its STDOUT (ordering is
1054 $output = `cmd 2>&1 1>/dev/null`;
1056 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1057 but leave its STDOUT to come out the old STDERR:
1059 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1061 To read both a command's STDOUT and its STDERR separately, it's easiest
1062 and safest to redirect them separately to files, and then read from those
1063 files when the program is done:
1065 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
1067 Using single-quote as a delimiter protects the command from Perl's
1068 double-quote interpolation, passing it on to the shell instead:
1070 $perl_info = qx(ps $$); # that's Perl's $$
1071 $shell_info = qx'ps $$'; # that's the new shell's $$
1073 How that string gets evaluated is entirely subject to the command
1074 interpreter on your system. On most platforms, you will have to protect
1075 shell metacharacters if you want them treated literally. This is in
1076 practice difficult to do, as it's unclear how to escape which characters.
1077 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1078 to emulate backticks safely.
1080 On some platforms (notably DOS-like ones), the shell may not be
1081 capable of dealing with multiline commands, so putting newlines in
1082 the string may not get you what you want. You may be able to evaluate
1083 multiple commands in a single line by separating them with the command
1084 separator character, if your shell supports that (e.g. C<;> on many Unix
1085 shells; C<&> on the Windows NT C<cmd> shell).
1087 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1088 output before starting the child process, but this may not be supported
1089 on some platforms (see L<perlport>). To be safe, you may need to set
1090 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1091 C<IO::Handle> on any open handles.
1093 Beware that some command shells may place restrictions on the length
1094 of the command line. You must ensure your strings don't exceed this
1095 limit after any necessary interpolations. See the platform-specific
1096 release notes for more details about your particular environment.
1098 Using this operator can lead to programs that are difficult to port,
1099 because the shell commands called vary between systems, and may in
1100 fact not be present at all. As one example, the C<type> command under
1101 the POSIX shell is very different from the C<type> command under DOS.
1102 That doesn't mean you should go out of your way to avoid backticks
1103 when they're the right way to get something done. Perl was made to be
1104 a glue language, and one of the things it glues together is commands.
1105 Just understand what you're getting yourself into.
1107 See L<"I/O Operators"> for more discussion.
1111 Evaluates to a list of the words extracted out of STRING, using embedded
1112 whitespace as the word delimiters. It can be understood as being roughly
1115 split(' ', q/STRING/);
1117 the difference being that it generates a real list at compile time. So
1122 is semantically equivalent to the list:
1126 Some frequently seen examples:
1128 use POSIX qw( setlocale localeconv )
1129 @EXPORT = qw( foo bar baz );
1131 A common mistake is to try to separate the words with comma or to
1132 put comments into a multi-line C<qw>-string. For this reason, the
1133 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1134 produces warnings if the STRING contains the "," or the "#" character.
1136 =item s/PATTERN/REPLACEMENT/egimosx
1138 Searches a string for a pattern, and if found, replaces that pattern
1139 with the replacement text and returns the number of substitutions
1140 made. Otherwise it returns false (specifically, the empty string).
1142 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1143 variable is searched and modified. (The string specified with C<=~> must
1144 be scalar variable, an array element, a hash element, or an assignment
1145 to one of those, i.e., an lvalue.)
1147 If the delimiter chosen is a single quote, no interpolation is
1148 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1149 PATTERN contains a $ that looks like a variable rather than an
1150 end-of-string test, the variable will be interpolated into the pattern
1151 at run-time. If you want the pattern compiled only once the first time
1152 the variable is interpolated, use the C</o> option. If the pattern
1153 evaluates to the empty string, the last successfully executed regular
1154 expression is used instead. See L<perlre> for further explanation on these.
1155 See L<perllocale> for discussion of additional considerations that apply
1156 when C<use locale> is in effect.
1160 e Evaluate the right side as an expression.
1161 g Replace globally, i.e., all occurrences.
1162 i Do case-insensitive pattern matching.
1163 m Treat string as multiple lines.
1164 o Compile pattern only once.
1165 s Treat string as single line.
1166 x Use extended regular expressions.
1168 Any non-alphanumeric, non-whitespace delimiter may replace the
1169 slashes. If single quotes are used, no interpretation is done on the
1170 replacement string (the C</e> modifier overrides this, however). Unlike
1171 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1172 text is not evaluated as a command. If the
1173 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1174 pair of quotes, which may or may not be bracketing quotes, e.g.,
1175 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1176 replacement portion to be treated as a full-fledged Perl expression
1177 and evaluated right then and there. It is, however, syntax checked at
1178 compile-time. A second C<e> modifier will cause the replacement portion
1179 to be C<eval>ed before being run as a Perl expression.
1183 s/\bgreen\b/mauve/g; # don't change wintergreen
1185 $path =~ s|/usr/bin|/usr/local/bin|;
1187 s/Login: $foo/Login: $bar/; # run-time pattern
1189 ($foo = $bar) =~ s/this/that/; # copy first, then change
1191 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1194 s/\d+/$&*2/e; # yields 'abc246xyz'
1195 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1196 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1198 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1199 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1200 s/^=(\w+)/&pod($1)/ge; # use function call
1202 # expand variables in $_, but dynamics only, using
1203 # symbolic dereferencing
1206 # Add one to the value of any numbers in the string
1209 # This will expand any embedded scalar variable
1210 # (including lexicals) in $_ : First $1 is interpolated
1211 # to the variable name, and then evaluated
1214 # Delete (most) C comments.
1216 /\* # Match the opening delimiter.
1217 .*? # Match a minimal number of characters.
1218 \*/ # Match the closing delimiter.
1221 s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
1223 for ($variable) { # trim white space in $variable, cheap
1228 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1230 Note the use of $ instead of \ in the last example. Unlike
1231 B<sed>, we use the \<I<digit>> form in only the left hand side.
1232 Anywhere else it's $<I<digit>>.
1234 Occasionally, you can't use just a C</g> to get all the changes
1235 to occur that you might want. Here are two common cases:
1237 # put commas in the right places in an integer
1238 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1240 # expand tabs to 8-column spacing
1241 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1243 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1245 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1247 Transliterates all occurrences of the characters found in the search list
1248 with the corresponding character in the replacement list. It returns
1249 the number of characters replaced or deleted. If no string is
1250 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1251 string specified with =~ must be a scalar variable, an array element, a
1252 hash element, or an assignment to one of those, i.e., an lvalue.)
1254 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1255 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1256 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1257 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1258 its own pair of quotes, which may or may not be bracketing quotes,
1259 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1261 Note that C<tr> does B<not> do regular expression character classes
1262 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1263 the tr(1) utility. If you want to map strings between lower/upper
1264 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1265 using the C<s> operator if you need regular expressions.
1267 Note also that the whole range idea is rather unportable between
1268 character sets--and even within character sets they may cause results
1269 you probably didn't expect. A sound principle is to use only ranges
1270 that begin from and end at either alphabets of equal case (a-e, A-E),
1271 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1272 character sets in full.
1276 c Complement the SEARCHLIST.
1277 d Delete found but unreplaced characters.
1278 s Squash duplicate replaced characters.
1280 If the C</c> modifier is specified, the SEARCHLIST character set
1281 is complemented. If the C</d> modifier is specified, any characters
1282 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1283 (Note that this is slightly more flexible than the behavior of some
1284 B<tr> programs, which delete anything they find in the SEARCHLIST,
1285 period.) If the C</s> modifier is specified, sequences of characters
1286 that were transliterated to the same character are squashed down
1287 to a single instance of the character.
1289 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1290 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1291 than the SEARCHLIST, the final character is replicated till it is long
1292 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1293 This latter is useful for counting characters in a class or for
1294 squashing character sequences in a class.
1298 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1300 $cnt = tr/*/*/; # count the stars in $_
1302 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1304 $cnt = tr/0-9//; # count the digits in $_
1306 tr/a-zA-Z//s; # bookkeeper -> bokeper
1308 ($HOST = $host) =~ tr/a-z/A-Z/;
1310 tr/a-zA-Z/ /cs; # change non-alphas to single space
1313 [\000-\177]; # delete 8th bit
1315 If multiple transliterations are given for a character, only the
1320 will transliterate any A to X.
1322 Because the transliteration table is built at compile time, neither
1323 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1324 interpolation. That means that if you want to use variables, you
1327 eval "tr/$oldlist/$newlist/";
1330 eval "tr/$oldlist/$newlist/, 1" or die $@;
1334 =head2 Gory details of parsing quoted constructs
1336 When presented with something that might have several different
1337 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1338 principle to pick the most probable interpretation. This strategy
1339 is so successful that Perl programmers often do not suspect the
1340 ambivalence of what they write. But from time to time, Perl's
1341 notions differ substantially from what the author honestly meant.
1343 This section hopes to clarify how Perl handles quoted constructs.
1344 Although the most common reason to learn this is to unravel labyrinthine
1345 regular expressions, because the initial steps of parsing are the
1346 same for all quoting operators, they are all discussed together.
1348 The most important Perl parsing rule is the first one discussed
1349 below: when processing a quoted construct, Perl first finds the end
1350 of that construct, then interprets its contents. If you understand
1351 this rule, you may skip the rest of this section on the first
1352 reading. The other rules are likely to contradict the user's
1353 expectations much less frequently than this first one.
1355 Some passes discussed below are performed concurrently, but because
1356 their results are the same, we consider them individually. For different
1357 quoting constructs, Perl performs different numbers of passes, from
1358 one to five, but these passes are always performed in the same order.
1362 =item Finding the end
1364 The first pass is finding the end of the quoted construct, whether
1365 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1366 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1367 terminates C<qq[]> construct, or a C<< > >> which terminates a
1368 fileglob started with C<< < >>.
1370 When searching for single-character non-pairing delimiters, such
1371 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1372 when searching for single-character pairing delimiter like C<[>,
1373 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1374 C<[>, C<]> are skipped as well. When searching for multicharacter
1375 delimiters, nothing is skipped.
1377 For constructs with three-part delimiters (C<s///>, C<y///>, and
1378 C<tr///>), the search is repeated once more.
1380 During this search no attention is paid to the semantics of the construct.
1383 "$hash{"$foo/$bar"}"
1388 bar # NOT a comment, this slash / terminated m//!
1391 do not form legal quoted expressions. The quoted part ends on the
1392 first C<"> and C</>, and the rest happens to be a syntax error.
1393 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1394 the example above is not C<m//x>, but rather C<m//> with no C</x>
1395 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1397 =item Removal of backslashes before delimiters
1399 During the second pass, text between the starting and ending
1400 delimiters is copied to a safe location, and the C<\> is removed
1401 from combinations consisting of C<\> and delimiter--or delimiters,
1402 meaning both starting and ending delimiters will should these differ.
1403 This removal does not happen for multi-character delimiters.
1404 Note that the combination C<\\> is left intact, just as it was.
1406 Starting from this step no information about the delimiters is
1411 The next step is interpolation in the text obtained, which is now
1412 delimiter-independent. There are four different cases.
1416 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1418 No interpolation is performed.
1422 The only interpolation is removal of C<\> from pairs C<\\>.
1424 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1426 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1427 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1428 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1429 The other combinations are replaced with appropriate expansions.
1431 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1432 is interpolated in the usual way. Something like C<"\Q\\E"> has
1433 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1434 result is the same as for C<"\\\\E">. As a general rule, backslashes
1435 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1436 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1437 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1442 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1444 Interpolated scalars and arrays are converted internally to the C<join> and
1445 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1447 $foo . " XXX '" . (join $", @arr) . "'";
1449 All operations above are performed simultaneously, left to right.
1451 Because the result of C<"\Q STRING \E"> has all metacharacters
1452 quoted, there is no way to insert a literal C<$> or C<@> inside a
1453 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1454 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1457 Note also that the interpolation code needs to make a decision on
1458 where the interpolated scalar ends. For instance, whether
1459 C<< "a $b -> {c}" >> really means:
1461 "a " . $b . " -> {c}";
1467 Most of the time, the longest possible text that does not include
1468 spaces between components and which contains matching braces or
1469 brackets. because the outcome may be determined by voting based
1470 on heuristic estimators, the result is not strictly predictable.
1471 Fortunately, it's usually correct for ambiguous cases.
1473 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1475 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1476 happens (almost) as with C<qq//> constructs, but the substitution
1477 of C<\> followed by RE-special chars (including C<\>) is not
1478 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1479 a C<#>-comment in a C<//x>-regular expression, no processing is
1480 performed whatsoever. This is the first step at which the presence
1481 of the C<//x> modifier is relevant.
1483 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1484 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1485 different estimators) to be either an array element or C<$var>
1486 followed by an RE alternative. This is where the notation
1487 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1488 array element C<-9>, not as a regular expression from the variable
1489 C<$arr> followed by a digit, which would be the interpretation of
1490 C</$arr[0-9]/>. Since voting among different estimators may occur,
1491 the result is not predictable.
1493 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1494 the replacement text of C<s///> to correct the incorrigible
1495 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1496 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1497 (that is, the C<$^W> variable) was set.
1499 The lack of processing of C<\\> creates specific restrictions on
1500 the post-processed text. If the delimiter is C</>, one cannot get
1501 the combination C<\/> into the result of this step. C</> will
1502 finish the regular expression, C<\/> will be stripped to C</> on
1503 the previous step, and C<\\/> will be left as is. Because C</> is
1504 equivalent to C<\/> inside a regular expression, this does not
1505 matter unless the delimiter happens to be character special to the
1506 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1507 alphanumeric char, as in:
1511 In the RE above, which is intentionally obfuscated for illustration, the
1512 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1513 RE is the same as for C<m/ ^ a s* b /mx>). There's more than one
1514 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1515 non-whitespace choices.
1519 This step is the last one for all constructs except regular expressions,
1520 which are processed further.
1522 =item Interpolation of regular expressions
1524 Previous steps were performed during the compilation of Perl code,
1525 but this one happens at run time--although it may be optimized to
1526 be calculated at compile time if appropriate. After preprocessing
1527 described above, and possibly after evaluation if catenation,
1528 joining, casing translation, or metaquoting are involved, the
1529 resulting I<string> is passed to the RE engine for compilation.
1531 Whatever happens in the RE engine might be better discussed in L<perlre>,
1532 but for the sake of continuity, we shall do so here.
1534 This is another step where the presence of the C<//x> modifier is
1535 relevant. The RE engine scans the string from left to right and
1536 converts it to a finite automaton.
1538 Backslashed characters are either replaced with corresponding
1539 literal strings (as with C<\{>), or else they generate special nodes
1540 in the finite automaton (as with C<\b>). Characters special to the
1541 RE engine (such as C<|>) generate corresponding nodes or groups of
1542 nodes. C<(?#...)> comments are ignored. All the rest is either
1543 converted to literal strings to match, or else is ignored (as is
1544 whitespace and C<#>-style comments if C<//x> is present).
1546 Parsing of the bracketed character class construct, C<[...]>, is
1547 rather different than the rule used for the rest of the pattern.
1548 The terminator of this construct is found using the same rules as
1549 for finding the terminator of a C<{}>-delimited construct, the only
1550 exception being that C<]> immediately following C<[> is treated as
1551 though preceded by a backslash. Similarly, the terminator of
1552 C<(?{...})> is found using the same rules as for finding the
1553 terminator of a C<{}>-delimited construct.
1555 It is possible to inspect both the string given to RE engine and the
1556 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1557 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1558 switch documented in L<perlrun/"Command Switches">.
1560 =item Optimization of regular expressions
1562 This step is listed for completeness only. Since it does not change
1563 semantics, details of this step are not documented and are subject
1564 to change without notice. This step is performed over the finite
1565 automaton that was generated during the previous pass.
1567 It is at this stage that C<split()> silently optimizes C</^/> to
1572 =head2 I/O Operators
1574 There are several I/O operators you should know about.
1576 A string enclosed by backticks (grave accents) first undergoes
1577 double-quote interpolation. It is then interpreted as an external
1578 command, and the output of that command is the value of the
1579 backtick string, like in a shell. In scalar context, a single string
1580 consisting of all output is returned. In list context, a list of
1581 values is returned, one per line of output. (You can set C<$/> to use
1582 a different line terminator.) The command is executed each time the
1583 pseudo-literal is evaluated. The status value of the command is
1584 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
1585 Unlike in B<csh>, no translation is done on the return data--newlines
1586 remain newlines. Unlike in any of the shells, single quotes do not
1587 hide variable names in the command from interpretation. To pass a
1588 literal dollar-sign through to the shell you need to hide it with a
1589 backslash. The generalized form of backticks is C<qx//>. (Because
1590 backticks always undergo shell expansion as well, see L<perlsec> for
1593 In scalar context, evaluating a filehandle in angle brackets yields
1594 the next line from that file (the newline, if any, included), or
1595 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1596 (sometimes known as file-slurp mode) and the file is empty, it
1597 returns C<''> the first time, followed by C<undef> subsequently.
1599 Ordinarily you must assign the returned value to a variable, but
1600 there is one situation where an automatic assignment happens. If
1601 and only if the input symbol is the only thing inside the conditional
1602 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1603 the value is automatically assigned to the global variable $_,
1604 destroying whatever was there previously. (This may seem like an
1605 odd thing to you, but you'll use the construct in almost every Perl
1606 script you write.) The $_ variable is not implicitly localized.
1607 You'll have to put a C<local $_;> before the loop if you want that
1610 The following lines are equivalent:
1612 while (defined($_ = <STDIN>)) { print; }
1613 while ($_ = <STDIN>) { print; }
1614 while (<STDIN>) { print; }
1615 for (;<STDIN>;) { print; }
1616 print while defined($_ = <STDIN>);
1617 print while ($_ = <STDIN>);
1618 print while <STDIN>;
1620 This also behaves similarly, but avoids $_ :
1622 while (my $line = <STDIN>) { print $line }
1624 In these loop constructs, the assigned value (whether assignment
1625 is automatic or explicit) is then tested to see whether it is
1626 defined. The defined test avoids problems where line has a string
1627 value that would be treated as false by Perl, for example a "" or
1628 a "0" with no trailing newline. If you really mean for such values
1629 to terminate the loop, they should be tested for explicitly:
1631 while (($_ = <STDIN>) ne '0') { ... }
1632 while (<STDIN>) { last unless $_; ... }
1634 In other boolean contexts, C<< <I<filehandle>> >> without an
1635 explicit C<defined> test or comparison elicit a warning if the
1636 C<use warnings> pragma or the B<-w>
1637 command-line switch (the C<$^W> variable) is in effect.
1639 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1640 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
1641 in packages, where they would be interpreted as local identifiers
1642 rather than global.) Additional filehandles may be created with
1643 the open() function, amongst others. See L<perlopentut> and
1644 L<perlfunc/open> for details on this.
1646 If a <FILEHANDLE> is used in a context that is looking for
1647 a list, a list comprising all input lines is returned, one line per
1648 list element. It's easy to grow to a rather large data space this
1649 way, so use with care.
1651 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
1652 See L<perlfunc/readline>.
1654 The null filehandle <> is special: it can be used to emulate the
1655 behavior of B<sed> and B<awk>. Input from <> comes either from
1656 standard input, or from each file listed on the command line. Here's
1657 how it works: the first time <> is evaluated, the @ARGV array is
1658 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
1659 gives you standard input. The @ARGV array is then processed as a list
1660 of filenames. The loop
1663 ... # code for each line
1666 is equivalent to the following Perl-like pseudo code:
1668 unshift(@ARGV, '-') unless @ARGV;
1669 while ($ARGV = shift) {
1672 ... # code for each line
1676 except that it isn't so cumbersome to say, and will actually work.
1677 It really does shift the @ARGV array and put the current filename
1678 into the $ARGV variable. It also uses filehandle I<ARGV>
1679 internally--<> is just a synonym for <ARGV>, which
1680 is magical. (The pseudo code above doesn't work because it treats
1681 <ARGV> as non-magical.)
1683 You can modify @ARGV before the first <> as long as the array ends up
1684 containing the list of filenames you really want. Line numbers (C<$.>)
1685 continue as though the input were one big happy file. See the example
1686 in L<perlfunc/eof> for how to reset line numbers on each file.
1688 If you want to set @ARGV to your own list of files, go right ahead.
1689 This sets @ARGV to all plain text files if no @ARGV was given:
1691 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
1693 You can even set them to pipe commands. For example, this automatically
1694 filters compressed arguments through B<gzip>:
1696 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1698 If you want to pass switches into your script, you can use one of the
1699 Getopts modules or put a loop on the front like this:
1701 while ($_ = $ARGV[0], /^-/) {
1704 if (/^-D(.*)/) { $debug = $1 }
1705 if (/^-v/) { $verbose++ }
1706 # ... # other switches
1710 # ... # code for each line
1713 The <> symbol will return C<undef> for end-of-file only once.
1714 If you call it again after this, it will assume you are processing another
1715 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
1717 If what the angle brackets contain is a simple scalar variable (e.g.,
1718 <$foo>), then that variable contains the name of the
1719 filehandle to input from, or its typeglob, or a reference to the
1725 If what's within the angle brackets is neither a filehandle nor a simple
1726 scalar variable containing a filehandle name, typeglob, or typeglob
1727 reference, it is interpreted as a filename pattern to be globbed, and
1728 either a list of filenames or the next filename in the list is returned,
1729 depending on context. This distinction is determined on syntactic
1730 grounds alone. That means C<< <$x> >> is always a readline() from
1731 an indirect handle, but C<< <$hash{key}> >> is always a glob().
1732 That's because $x is a simple scalar variable, but C<$hash{key}> is
1733 not--it's a hash element.
1735 One level of double-quote interpretation is done first, but you can't
1736 say C<< <$foo> >> because that's an indirect filehandle as explained
1737 in the previous paragraph. (In older versions of Perl, programmers
1738 would insert curly brackets to force interpretation as a filename glob:
1739 C<< <${foo}> >>. These days, it's considered cleaner to call the
1740 internal function directly as C<glob($foo)>, which is probably the right
1741 way to have done it in the first place.) For example:
1747 is roughly equivalent to:
1749 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1755 except that the globbing is actually done internally using the standard
1756 C<File::Glob> extension. Of course, the shortest way to do the above is:
1760 A (file)glob evaluates its (embedded) argument only when it is
1761 starting a new list. All values must be read before it will start
1762 over. In list context, this isn't important because you automatically
1763 get them all anyway. However, in scalar context the operator returns
1764 the next value each time it's called, or C<undef> when the list has
1765 run out. As with filehandle reads, an automatic C<defined> is
1766 generated when the glob occurs in the test part of a C<while>,
1767 because legal glob returns (e.g. a file called F<0>) would otherwise
1768 terminate the loop. Again, C<undef> is returned only once. So if
1769 you're expecting a single value from a glob, it is much better to
1772 ($file) = <blurch*>;
1778 because the latter will alternate between returning a filename and
1781 If you're trying to do variable interpolation, it's definitely better
1782 to use the glob() function, because the older notation can cause people
1783 to become confused with the indirect filehandle notation.
1785 @files = glob("$dir/*.[ch]");
1786 @files = glob($files[$i]);
1788 =head2 Constant Folding
1790 Like C, Perl does a certain amount of expression evaluation at
1791 compile time whenever it determines that all arguments to an
1792 operator are static and have no side effects. In particular, string
1793 concatenation happens at compile time between literals that don't do
1794 variable substitution. Backslash interpolation also happens at
1795 compile time. You can say
1797 'Now is the time for all' . "\n" .
1798 'good men to come to.'
1800 and this all reduces to one string internally. Likewise, if
1803 foreach $file (@filenames) {
1804 if (-s $file > 5 + 100 * 2**16) { }
1807 the compiler will precompute the number which that expression
1808 represents so that the interpreter won't have to.
1810 =head2 Bitwise String Operators
1812 Bitstrings of any size may be manipulated by the bitwise operators
1815 If the operands to a binary bitwise op are strings of different
1816 sizes, B<|> and B<^> ops act as though the shorter operand had
1817 additional zero bits on the right, while the B<&> op acts as though
1818 the longer operand were truncated to the length of the shorter.
1819 The granularity for such extension or truncation is one or more
1822 # ASCII-based examples
1823 print "j p \n" ^ " a h"; # prints "JAPH\n"
1824 print "JA" | " ph\n"; # prints "japh\n"
1825 print "japh\nJunk" & '_____'; # prints "JAPH\n";
1826 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
1828 If you are intending to manipulate bitstrings, be certain that
1829 you're supplying bitstrings: If an operand is a number, that will imply
1830 a B<numeric> bitwise operation. You may explicitly show which type of
1831 operation you intend by using C<""> or C<0+>, as in the examples below.
1833 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
1834 $foo = '150' | 105 ; # yields 255
1835 $foo = 150 | '105'; # yields 255
1836 $foo = '150' | '105'; # yields string '155' (under ASCII)
1838 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
1839 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
1841 See L<perlfunc/vec> for information on how to manipulate individual bits
1844 =head2 Integer Arithmetic
1846 By default, Perl assumes that it must do most of its arithmetic in
1847 floating point. But by saying
1851 you may tell the compiler that it's okay to use integer operations
1852 (if it feels like it) from here to the end of the enclosing BLOCK.
1853 An inner BLOCK may countermand this by saying
1857 which lasts until the end of that BLOCK. Note that this doesn't
1858 mean everything is only an integer, merely that Perl may use integer
1859 operations if it is so inclined. For example, even under C<use
1860 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
1863 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
1864 and ">>") always produce integral results. (But see also
1865 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
1866 them. By default, their results are interpreted as unsigned integers, but
1867 if C<use integer> is in effect, their results are interpreted
1868 as signed integers. For example, C<~0> usually evaluates to a large
1869 integral value. However, C<use integer; ~0> is C<-1> on twos-complement
1872 =head2 Floating-point Arithmetic
1874 While C<use integer> provides integer-only arithmetic, there is no
1875 analogous mechanism to provide automatic rounding or truncation to a
1876 certain number of decimal places. For rounding to a certain number
1877 of digits, sprintf() or printf() is usually the easiest route.
1880 Floating-point numbers are only approximations to what a mathematician
1881 would call real numbers. There are infinitely more reals than floats,
1882 so some corners must be cut. For example:
1884 printf "%.20g\n", 123456789123456789;
1885 # produces 123456789123456784
1887 Testing for exact equality of floating-point equality or inequality is
1888 not a good idea. Here's a (relatively expensive) work-around to compare
1889 whether two floating-point numbers are equal to a particular number of
1890 decimal places. See Knuth, volume II, for a more robust treatment of
1894 my ($X, $Y, $POINTS) = @_;
1896 $tX = sprintf("%.${POINTS}g", $X);
1897 $tY = sprintf("%.${POINTS}g", $Y);
1901 The POSIX module (part of the standard perl distribution) implements
1902 ceil(), floor(), and other mathematical and trigonometric functions.
1903 The Math::Complex module (part of the standard perl distribution)
1904 defines mathematical functions that work on both the reals and the
1905 imaginary numbers. Math::Complex not as efficient as POSIX, but
1906 POSIX can't work with complex numbers.
1908 Rounding in financial applications can have serious implications, and
1909 the rounding method used should be specified precisely. In these
1910 cases, it probably pays not to trust whichever system rounding is
1911 being used by Perl, but to instead implement the rounding function you
1914 =head2 Bigger Numbers
1916 The standard Math::BigInt and Math::BigFloat modules provide
1917 variable-precision arithmetic and overloaded operators, although
1918 they're currently pretty slow. At the cost of some space and
1919 considerable speed, they avoid the normal pitfalls associated with
1920 limited-precision representations.
1923 $x = Math::BigInt->new('123456789123456789');
1926 # prints +15241578780673678515622620750190521
1928 There are several modules that let you calculate with (bound only by
1929 memory and cpu-time) unlimited or fixed precision. There are also
1930 some non-standard modules that provide faster implementations via
1931 external C libraries.
1933 Here is a short, but incomplete summary:
1935 Math::Fraction big, unlimited fractions like 9973 / 12967
1936 Math::String treat string sequences like numbers
1937 Math::FixedPrecision calculate with a fixed precision
1938 Math::Currency for currency calculations
1939 Bit::Vector manipulate bit vectors fast (uses C)
1940 Math::BigIntFast Bit::Vector wrapper for big numbers
1941 Math::Pari provides access to the Pari C library
1942 Math::BigInteger uses an external C library
1943 Math::Cephes uses external Cephes C library (no big numbers)
1944 Math::Cephes::Fraction fractions via the Cephes library
1945 Math::GMP another one using an external C library