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
663 // m{} Pattern match yes*
665 s{}{} Substitution yes*
666 tr{}{} Transliteration no (but see below)
669 * unless the delimiter is ''.
671 Non-bracketing delimiters use the same character fore and aft, but the four
672 sorts of brackets (round, angle, square, curly) will all nest, which means
681 Note, however, that this does not always work for quoting Perl code:
683 $s = q{ if($a eq "}") ... }; # WRONG
685 is a syntax error. The C<Text::Balanced> module (from CPAN, and
686 starting from Perl 5.8 part of the standard distribution) is able
689 There can be whitespace between the operator and the quoting
690 characters, except when C<#> is being used as the quoting character.
691 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
692 operator C<q> followed by a comment. Its argument will be taken
693 from the next line. This allows you to write:
695 s {foo} # Replace foo
698 The following escape sequences are available in constructs that interpolate
699 and in transliterations.
706 \a alarm (bell) (BEL)
708 \033 octal char (ESC)
710 \x{263a} wide hex char (SMILEY)
711 \c[ control char (ESC)
714 The following escape sequences are available in constructs that interpolate
715 but not in transliterations.
717 \l lowercase next char
718 \u uppercase next char
721 \E end case modification
722 \Q quote non-word characters till \E
724 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
725 and C<\U> is taken from the current locale. See L<perllocale>. For
726 documentation of C<\N{name}>, see L<charnames>.
728 All systems use the virtual C<"\n"> to represent a line terminator,
729 called a "newline". There is no such thing as an unvarying, physical
730 newline character. It is only an illusion that the operating system,
731 device drivers, C libraries, and Perl all conspire to preserve. Not all
732 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
733 on a Mac, these are reversed, and on systems without line terminator,
734 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
735 you mean a "newline" for your system, but use the literal ASCII when you
736 need an exact character. For example, most networking protocols expect
737 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
738 and although they often accept just C<"\012">, they seldom tolerate just
739 C<"\015">. If you get in the habit of using C<"\n"> for networking,
740 you may be burned some day.
742 For constructs that do interpolate, variables beginning with "C<$>"
743 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
744 C<$href->{key}[0]> are also interpolated, as are array and hash slices.
745 But method calls such as C<$obj->meth> are not.
747 Interpolating an array or slice interpolates the elements in order,
748 separated by the value of C<$">, so is equivalent to interpolating
749 C<join $", @array>. "Punctuation" arrays such as C<@+> are only
750 interpolated if the name is enclosed in braces C<@{+}>.
752 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
753 An unescaped C<$> or C<@> interpolates the corresponding variable,
754 while escaping will cause the literal string C<\$> to be inserted.
755 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
757 Patterns are subject to an additional level of interpretation as a
758 regular expression. This is done as a second pass, after variables are
759 interpolated, so that regular expressions may be incorporated into the
760 pattern from the variables. If this is not what you want, use C<\Q> to
761 interpolate a variable literally.
763 Apart from the behavior described above, Perl does not expand
764 multiple levels of interpolation. In particular, contrary to the
765 expectations of shell programmers, back-quotes do I<NOT> interpolate
766 within double quotes, nor do single quotes impede evaluation of
767 variables when used within double quotes.
769 =head2 Regexp Quote-Like Operators
771 Here are the quote-like operators that apply to pattern
772 matching and related activities.
778 This is just like the C</pattern/> search, except that it matches only
779 once between calls to the reset() operator. This is a useful
780 optimization when you want to see only the first occurrence of
781 something in each file of a set of files, for instance. Only C<??>
782 patterns local to the current package are reset.
786 # blank line between header and body
789 reset if eof; # clear ?? status for next file
792 This usage is vaguely deprecated, which means it just might possibly
793 be removed in some distant future version of Perl, perhaps somewhere
794 around the year 2168.
796 =item m/PATTERN/cgimosx
798 =item /PATTERN/cgimosx
800 Searches a string for a pattern match, and in scalar context returns
801 true if it succeeds, false if it fails. If no string is specified
802 via the C<=~> or C<!~> operator, the $_ string is searched. (The
803 string specified with C<=~> need not be an lvalue--it may be the
804 result of an expression evaluation, but remember the C<=~> binds
805 rather tightly.) See also L<perlre>. See L<perllocale> for
806 discussion of additional considerations that apply when C<use locale>
811 c Do not reset search position on a failed match when /g is in effect.
812 g Match globally, i.e., find all occurrences.
813 i Do case-insensitive pattern matching.
814 m Treat string as multiple lines.
815 o Compile pattern only once.
816 s Treat string as single line.
817 x Use extended regular expressions.
819 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
820 you can use any pair of non-alphanumeric, non-whitespace characters
821 as delimiters. This is particularly useful for matching path names
822 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
823 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
824 If "'" is the delimiter, no interpolation is performed on the PATTERN.
826 PATTERN may contain variables, which will be interpolated (and the
827 pattern recompiled) every time the pattern search is evaluated, except
828 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
829 C<$|> are not interpolated because they look like end-of-string tests.)
830 If you want such a pattern to be compiled only once, add a C</o> after
831 the trailing delimiter. This avoids expensive run-time recompilations,
832 and is useful when the value you are interpolating won't change over
833 the life of the script. However, mentioning C</o> constitutes a promise
834 that you won't change the variables in the pattern. If you change them,
835 Perl won't even notice. See also L<"qr/STRING/imosx">.
837 If the PATTERN evaluates to the empty string, the last
838 I<successfully> matched regular expression is used instead.
840 If the C</g> option is not used, C<m//> in list context returns a
841 list consisting of the subexpressions matched by the parentheses in the
842 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
843 also set, and that this differs from Perl 4's behavior.) When there are
844 no parentheses in the pattern, the return value is the list C<(1)> for
845 success. With or without parentheses, an empty list is returned upon
850 open(TTY, '/dev/tty');
851 <TTY> =~ /^y/i && foo(); # do foo if desired
853 if (/Version: *([0-9.]*)/) { $version = $1; }
855 next if m#^/usr/spool/uucp#;
860 print if /$arg/o; # compile only once
863 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
865 This last example splits $foo into the first two words and the
866 remainder of the line, and assigns those three fields to $F1, $F2, and
867 $Etc. The conditional is true if any variables were assigned, i.e., if
870 The C</g> modifier specifies global pattern matching--that is,
871 matching as many times as possible within the string. How it behaves
872 depends on the context. In list context, it returns a list of the
873 substrings matched by any capturing parentheses in the regular
874 expression. If there are no parentheses, it returns a list of all
875 the matched strings, as if there were parentheses around the whole
878 In scalar context, each execution of C<m//g> finds the next match,
879 returning true if it matches, and false if there is no further match.
880 The position after the last match can be read or set using the pos()
881 function; see L<perlfunc/pos>. A failed match normally resets the
882 search position to the beginning of the string, but you can avoid that
883 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
884 string also resets the search position.
886 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
887 zero-width assertion that matches the exact position where the previous
888 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
889 still anchors at pos(), but the match is of course only attempted once.
890 Using C<\G> without C</g> on a target string that has not previously had a
891 C</g> match applied to it is the same as using the C<\A> assertion to match
892 the beginning of the string.
897 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
901 while (defined($paragraph = <>)) {
902 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
906 print "$sentences\n";
908 # using m//gc with \G
912 print $1 while /(o)/gc; print "', pos=", pos, "\n";
914 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
916 print $1 while /(p)/gc; print "', pos=", pos, "\n";
918 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
920 The last example should print:
930 Notice that the final match matched C<q> instead of C<p>, which a match
931 without the C<\G> anchor would have done. Also note that the final match
932 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
933 final match did indeed match C<p>, it's a good bet that you're running an
934 older (pre-5.6.0) Perl.
936 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
937 combine several regexps like this to process a string part-by-part,
938 doing different actions depending on which regexp matched. Each
939 regexp tries to match where the previous one leaves off.
942 $url = new URI::URL "http://www/"; die if $url eq "xXx";
946 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
947 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
948 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
949 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
950 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
951 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
952 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
953 print ". That's all!\n";
956 Here is the output (split into several lines):
958 line-noise lowercase line-noise lowercase UPPERCASE line-noise
959 UPPERCASE line-noise lowercase line-noise lowercase line-noise
960 lowercase lowercase line-noise lowercase lowercase line-noise
961 MiXeD line-noise. That's all!
967 A single-quoted, literal string. A backslash represents a backslash
968 unless followed by the delimiter or another backslash, in which case
969 the delimiter or backslash is interpolated.
971 $foo = q!I said, "You said, 'She said it.'"!;
972 $bar = q('This is it.');
973 $baz = '\n'; # a two-character string
979 A double-quoted, interpolated string.
982 (*** The previous line contains the naughty word "$1".\n)
983 if /\b(tcl|java|python)\b/i; # :-)
984 $baz = "\n"; # a one-character string
986 =item qr/STRING/imosx
988 This operator quotes (and possibly compiles) its I<STRING> as a regular
989 expression. I<STRING> is interpolated the same way as I<PATTERN>
990 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
991 is done. Returns a Perl value which may be used instead of the
992 corresponding C</STRING/imosx> expression.
996 $rex = qr/my.STRING/is;
1003 The result may be used as a subpattern in a match:
1006 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1007 $string =~ $re; # or used standalone
1008 $string =~ /$re/; # or this way
1010 Since Perl may compile the pattern at the moment of execution of qr()
1011 operator, using qr() may have speed advantages in some situations,
1012 notably if the result of qr() is used standalone:
1015 my $patterns = shift;
1016 my @compiled = map qr/$_/i, @$patterns;
1019 foreach my $pat (@compiled) {
1020 $success = 1, last if /$pat/;
1026 Precompilation of the pattern into an internal representation at
1027 the moment of qr() avoids a need to recompile the pattern every
1028 time a match C</$pat/> is attempted. (Perl has many other internal
1029 optimizations, but none would be triggered in the above example if
1030 we did not use qr() operator.)
1034 i Do case-insensitive pattern matching.
1035 m Treat string as multiple lines.
1036 o Compile pattern only once.
1037 s Treat string as single line.
1038 x Use extended regular expressions.
1040 See L<perlre> for additional information on valid syntax for STRING, and
1041 for a detailed look at the semantics of regular expressions.
1047 A string which is (possibly) interpolated and then executed as a
1048 system command with C</bin/sh> or its equivalent. Shell wildcards,
1049 pipes, and redirections will be honored. The collected standard
1050 output of the command is returned; standard error is unaffected. In
1051 scalar context, it comes back as a single (potentially multi-line)
1052 string, or undef if the command failed. In list context, returns a
1053 list of lines (however you've defined lines with $/ or
1054 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1056 Because backticks do not affect standard error, use shell file descriptor
1057 syntax (assuming the shell supports this) if you care to address this.
1058 To capture a command's STDERR and STDOUT together:
1060 $output = `cmd 2>&1`;
1062 To capture a command's STDOUT but discard its STDERR:
1064 $output = `cmd 2>/dev/null`;
1066 To capture a command's STDERR but discard its STDOUT (ordering is
1069 $output = `cmd 2>&1 1>/dev/null`;
1071 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1072 but leave its STDOUT to come out the old STDERR:
1074 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1076 To read both a command's STDOUT and its STDERR separately, it's easiest
1077 and safest to redirect them separately to files, and then read from those
1078 files when the program is done:
1080 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
1082 Using single-quote as a delimiter protects the command from Perl's
1083 double-quote interpolation, passing it on to the shell instead:
1085 $perl_info = qx(ps $$); # that's Perl's $$
1086 $shell_info = qx'ps $$'; # that's the new shell's $$
1088 How that string gets evaluated is entirely subject to the command
1089 interpreter on your system. On most platforms, you will have to protect
1090 shell metacharacters if you want them treated literally. This is in
1091 practice difficult to do, as it's unclear how to escape which characters.
1092 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1093 to emulate backticks safely.
1095 On some platforms (notably DOS-like ones), the shell may not be
1096 capable of dealing with multiline commands, so putting newlines in
1097 the string may not get you what you want. You may be able to evaluate
1098 multiple commands in a single line by separating them with the command
1099 separator character, if your shell supports that (e.g. C<;> on many Unix
1100 shells; C<&> on the Windows NT C<cmd> shell).
1102 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1103 output before starting the child process, but this may not be supported
1104 on some platforms (see L<perlport>). To be safe, you may need to set
1105 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1106 C<IO::Handle> on any open handles.
1108 Beware that some command shells may place restrictions on the length
1109 of the command line. You must ensure your strings don't exceed this
1110 limit after any necessary interpolations. See the platform-specific
1111 release notes for more details about your particular environment.
1113 Using this operator can lead to programs that are difficult to port,
1114 because the shell commands called vary between systems, and may in
1115 fact not be present at all. As one example, the C<type> command under
1116 the POSIX shell is very different from the C<type> command under DOS.
1117 That doesn't mean you should go out of your way to avoid backticks
1118 when they're the right way to get something done. Perl was made to be
1119 a glue language, and one of the things it glues together is commands.
1120 Just understand what you're getting yourself into.
1122 See L<"I/O Operators"> for more discussion.
1126 Evaluates to a list of the words extracted out of STRING, using embedded
1127 whitespace as the word delimiters. It can be understood as being roughly
1130 split(' ', q/STRING/);
1132 the difference being that it generates a real list at compile time. So
1137 is semantically equivalent to the list:
1141 Some frequently seen examples:
1143 use POSIX qw( setlocale localeconv )
1144 @EXPORT = qw( foo bar baz );
1146 A common mistake is to try to separate the words with comma or to
1147 put comments into a multi-line C<qw>-string. For this reason, the
1148 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1149 produces warnings if the STRING contains the "," or the "#" character.
1151 =item s/PATTERN/REPLACEMENT/egimosx
1153 Searches a string for a pattern, and if found, replaces that pattern
1154 with the replacement text and returns the number of substitutions
1155 made. Otherwise it returns false (specifically, the empty string).
1157 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1158 variable is searched and modified. (The string specified with C<=~> must
1159 be scalar variable, an array element, a hash element, or an assignment
1160 to one of those, i.e., an lvalue.)
1162 If the delimiter chosen is a single quote, no interpolation is
1163 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1164 PATTERN contains a $ that looks like a variable rather than an
1165 end-of-string test, the variable will be interpolated into the pattern
1166 at run-time. If you want the pattern compiled only once the first time
1167 the variable is interpolated, use the C</o> option. If the pattern
1168 evaluates to the empty string, the last successfully executed regular
1169 expression is used instead. See L<perlre> for further explanation on these.
1170 See L<perllocale> for discussion of additional considerations that apply
1171 when C<use locale> is in effect.
1175 e Evaluate the right side as an expression.
1176 g Replace globally, i.e., all occurrences.
1177 i Do case-insensitive pattern matching.
1178 m Treat string as multiple lines.
1179 o Compile pattern only once.
1180 s Treat string as single line.
1181 x Use extended regular expressions.
1183 Any non-alphanumeric, non-whitespace delimiter may replace the
1184 slashes. If single quotes are used, no interpretation is done on the
1185 replacement string (the C</e> modifier overrides this, however). Unlike
1186 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1187 text is not evaluated as a command. If the
1188 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1189 pair of quotes, which may or may not be bracketing quotes, e.g.,
1190 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1191 replacement portion to be treated as a full-fledged Perl expression
1192 and evaluated right then and there. It is, however, syntax checked at
1193 compile-time. A second C<e> modifier will cause the replacement portion
1194 to be C<eval>ed before being run as a Perl expression.
1198 s/\bgreen\b/mauve/g; # don't change wintergreen
1200 $path =~ s|/usr/bin|/usr/local/bin|;
1202 s/Login: $foo/Login: $bar/; # run-time pattern
1204 ($foo = $bar) =~ s/this/that/; # copy first, then change
1206 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1209 s/\d+/$&*2/e; # yields 'abc246xyz'
1210 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1211 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1213 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1214 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1215 s/^=(\w+)/&pod($1)/ge; # use function call
1217 # expand variables in $_, but dynamics only, using
1218 # symbolic dereferencing
1221 # Add one to the value of any numbers in the string
1224 # This will expand any embedded scalar variable
1225 # (including lexicals) in $_ : First $1 is interpolated
1226 # to the variable name, and then evaluated
1229 # Delete (most) C comments.
1231 /\* # Match the opening delimiter.
1232 .*? # Match a minimal number of characters.
1233 \*/ # Match the closing delimiter.
1236 s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
1238 for ($variable) { # trim white space in $variable, cheap
1243 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1245 Note the use of $ instead of \ in the last example. Unlike
1246 B<sed>, we use the \<I<digit>> form in only the left hand side.
1247 Anywhere else it's $<I<digit>>.
1249 Occasionally, you can't use just a C</g> to get all the changes
1250 to occur that you might want. Here are two common cases:
1252 # put commas in the right places in an integer
1253 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1255 # expand tabs to 8-column spacing
1256 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1258 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1260 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1262 Transliterates all occurrences of the characters found in the search list
1263 with the corresponding character in the replacement list. It returns
1264 the number of characters replaced or deleted. If no string is
1265 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1266 string specified with =~ must be a scalar variable, an array element, a
1267 hash element, or an assignment to one of those, i.e., an lvalue.)
1269 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1270 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1271 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1272 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1273 its own pair of quotes, which may or may not be bracketing quotes,
1274 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1276 Note that C<tr> does B<not> do regular expression character classes
1277 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1278 the tr(1) utility. If you want to map strings between lower/upper
1279 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1280 using the C<s> operator if you need regular expressions.
1282 Note also that the whole range idea is rather unportable between
1283 character sets--and even within character sets they may cause results
1284 you probably didn't expect. A sound principle is to use only ranges
1285 that begin from and end at either alphabets of equal case (a-e, A-E),
1286 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1287 character sets in full.
1291 c Complement the SEARCHLIST.
1292 d Delete found but unreplaced characters.
1293 s Squash duplicate replaced characters.
1295 If the C</c> modifier is specified, the SEARCHLIST character set
1296 is complemented. If the C</d> modifier is specified, any characters
1297 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1298 (Note that this is slightly more flexible than the behavior of some
1299 B<tr> programs, which delete anything they find in the SEARCHLIST,
1300 period.) If the C</s> modifier is specified, sequences of characters
1301 that were transliterated to the same character are squashed down
1302 to a single instance of the character.
1304 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1305 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1306 than the SEARCHLIST, the final character is replicated till it is long
1307 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1308 This latter is useful for counting characters in a class or for
1309 squashing character sequences in a class.
1313 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1315 $cnt = tr/*/*/; # count the stars in $_
1317 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1319 $cnt = tr/0-9//; # count the digits in $_
1321 tr/a-zA-Z//s; # bookkeeper -> bokeper
1323 ($HOST = $host) =~ tr/a-z/A-Z/;
1325 tr/a-zA-Z/ /cs; # change non-alphas to single space
1328 [\000-\177]; # delete 8th bit
1330 If multiple transliterations are given for a character, only the
1335 will transliterate any A to X.
1337 Because the transliteration table is built at compile time, neither
1338 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1339 interpolation. That means that if you want to use variables, you
1342 eval "tr/$oldlist/$newlist/";
1345 eval "tr/$oldlist/$newlist/, 1" or die $@;
1349 A line-oriented form of quoting is based on the shell "here-document"
1350 syntax. Following a C<< << >> you specify a string to terminate
1351 the quoted material, and all lines following the current line down to
1352 the terminating string are the value of the item. The terminating
1353 string may be either an identifier (a word), or some quoted text. If
1354 quoted, the type of quotes you use determines the treatment of the
1355 text, just as in regular quoting. An unquoted identifier works like
1356 double quotes. There must be no space between the C<< << >> and
1357 the identifier, unless the identifier is quoted. (If you put a space it
1358 will be treated as a null identifier, which is valid, and matches the first
1359 empty line.) The terminating string must appear by itself (unquoted and
1360 with no surrounding whitespace) on the terminating line.
1363 The price is $Price.
1366 print << "EOF"; # same as above
1367 The price is $Price.
1370 print << `EOC`; # execute commands
1375 print <<"foo", <<"bar"; # you can stack them
1381 myfunc(<< "THIS", 23, <<'THAT');
1388 Just don't forget that you have to put a semicolon on the end
1389 to finish the statement, as Perl doesn't know you're not going to
1397 If you want your here-docs to be indented with the
1398 rest of the code, you'll need to remove leading whitespace
1399 from each line manually:
1401 ($quote = <<'FINIS') =~ s/^\s+//gm;
1402 The Road goes ever on and on,
1403 down from the door where it began.
1406 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1407 the quoted material must come on the lines following the final delimiter.
1422 If the terminating identifier is on the last line of the program, you
1423 must be sure there is a newline after it; otherwise, Perl will give the
1424 warning B<Can't find string terminator "END" anywhere before EOF...>.
1426 Additionally, the quoting rules for the identifier are not related to
1427 Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
1428 in place of C<''> and C<"">, and the only interpolation is for backslashing
1429 the quoting character:
1431 print << "abc\"def";
1435 Finally, quoted strings cannot span multiple lines. The general rule is
1436 that the identifier must be a string literal. Stick with that, and you
1441 =head2 Gory details of parsing quoted constructs
1443 When presented with something that might have several different
1444 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1445 principle to pick the most probable interpretation. This strategy
1446 is so successful that Perl programmers often do not suspect the
1447 ambivalence of what they write. But from time to time, Perl's
1448 notions differ substantially from what the author honestly meant.
1450 This section hopes to clarify how Perl handles quoted constructs.
1451 Although the most common reason to learn this is to unravel labyrinthine
1452 regular expressions, because the initial steps of parsing are the
1453 same for all quoting operators, they are all discussed together.
1455 The most important Perl parsing rule is the first one discussed
1456 below: when processing a quoted construct, Perl first finds the end
1457 of that construct, then interprets its contents. If you understand
1458 this rule, you may skip the rest of this section on the first
1459 reading. The other rules are likely to contradict the user's
1460 expectations much less frequently than this first one.
1462 Some passes discussed below are performed concurrently, but because
1463 their results are the same, we consider them individually. For different
1464 quoting constructs, Perl performs different numbers of passes, from
1465 one to five, but these passes are always performed in the same order.
1469 =item Finding the end
1471 The first pass is finding the end of the quoted construct, whether
1472 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1473 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1474 terminates C<qq[]> construct, or a C<< > >> which terminates a
1475 fileglob started with C<< < >>.
1477 When searching for single-character non-pairing delimiters, such
1478 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1479 when searching for single-character pairing delimiter like C<[>,
1480 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1481 C<[>, C<]> are skipped as well. When searching for multicharacter
1482 delimiters, nothing is skipped.
1484 For constructs with three-part delimiters (C<s///>, C<y///>, and
1485 C<tr///>), the search is repeated once more.
1487 During this search no attention is paid to the semantics of the construct.
1490 "$hash{"$foo/$bar"}"
1495 bar # NOT a comment, this slash / terminated m//!
1498 do not form legal quoted expressions. The quoted part ends on the
1499 first C<"> and C</>, and the rest happens to be a syntax error.
1500 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1501 the example above is not C<m//x>, but rather C<m//> with no C</x>
1502 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1504 =item Removal of backslashes before delimiters
1506 During the second pass, text between the starting and ending
1507 delimiters is copied to a safe location, and the C<\> is removed
1508 from combinations consisting of C<\> and delimiter--or delimiters,
1509 meaning both starting and ending delimiters will should these differ.
1510 This removal does not happen for multi-character delimiters.
1511 Note that the combination C<\\> is left intact, just as it was.
1513 Starting from this step no information about the delimiters is
1518 The next step is interpolation in the text obtained, which is now
1519 delimiter-independent. There are four different cases.
1523 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1525 No interpolation is performed.
1529 The only interpolation is removal of C<\> from pairs C<\\>.
1531 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1533 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1534 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1535 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1536 The other combinations are replaced with appropriate expansions.
1538 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1539 is interpolated in the usual way. Something like C<"\Q\\E"> has
1540 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1541 result is the same as for C<"\\\\E">. As a general rule, backslashes
1542 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1543 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1544 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1549 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1551 Interpolated scalars and arrays are converted internally to the C<join> and
1552 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1554 $foo . " XXX '" . (join $", @arr) . "'";
1556 All operations above are performed simultaneously, left to right.
1558 Because the result of C<"\Q STRING \E"> has all metacharacters
1559 quoted, there is no way to insert a literal C<$> or C<@> inside a
1560 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1561 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1564 Note also that the interpolation code needs to make a decision on
1565 where the interpolated scalar ends. For instance, whether
1566 C<< "a $b -> {c}" >> really means:
1568 "a " . $b . " -> {c}";
1574 Most of the time, the longest possible text that does not include
1575 spaces between components and which contains matching braces or
1576 brackets. because the outcome may be determined by voting based
1577 on heuristic estimators, the result is not strictly predictable.
1578 Fortunately, it's usually correct for ambiguous cases.
1580 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1582 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1583 happens (almost) as with C<qq//> constructs, but the substitution
1584 of C<\> followed by RE-special chars (including C<\>) is not
1585 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1586 a C<#>-comment in a C<//x>-regular expression, no processing is
1587 performed whatsoever. This is the first step at which the presence
1588 of the C<//x> modifier is relevant.
1590 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1591 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1592 different estimators) to be either an array element or C<$var>
1593 followed by an RE alternative. This is where the notation
1594 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1595 array element C<-9>, not as a regular expression from the variable
1596 C<$arr> followed by a digit, which would be the interpretation of
1597 C</$arr[0-9]/>. Since voting among different estimators may occur,
1598 the result is not predictable.
1600 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1601 the replacement text of C<s///> to correct the incorrigible
1602 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1603 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1604 (that is, the C<$^W> variable) was set.
1606 The lack of processing of C<\\> creates specific restrictions on
1607 the post-processed text. If the delimiter is C</>, one cannot get
1608 the combination C<\/> into the result of this step. C</> will
1609 finish the regular expression, C<\/> will be stripped to C</> on
1610 the previous step, and C<\\/> will be left as is. Because C</> is
1611 equivalent to C<\/> inside a regular expression, this does not
1612 matter unless the delimiter happens to be character special to the
1613 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1614 alphanumeric char, as in:
1618 In the RE above, which is intentionally obfuscated for illustration, the
1619 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1620 RE is the same as for C<m/ ^ a s* b /mx>). There's more than one
1621 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1622 non-whitespace choices.
1626 This step is the last one for all constructs except regular expressions,
1627 which are processed further.
1629 =item Interpolation of regular expressions
1631 Previous steps were performed during the compilation of Perl code,
1632 but this one happens at run time--although it may be optimized to
1633 be calculated at compile time if appropriate. After preprocessing
1634 described above, and possibly after evaluation if catenation,
1635 joining, casing translation, or metaquoting are involved, the
1636 resulting I<string> is passed to the RE engine for compilation.
1638 Whatever happens in the RE engine might be better discussed in L<perlre>,
1639 but for the sake of continuity, we shall do so here.
1641 This is another step where the presence of the C<//x> modifier is
1642 relevant. The RE engine scans the string from left to right and
1643 converts it to a finite automaton.
1645 Backslashed characters are either replaced with corresponding
1646 literal strings (as with C<\{>), or else they generate special nodes
1647 in the finite automaton (as with C<\b>). Characters special to the
1648 RE engine (such as C<|>) generate corresponding nodes or groups of
1649 nodes. C<(?#...)> comments are ignored. All the rest is either
1650 converted to literal strings to match, or else is ignored (as is
1651 whitespace and C<#>-style comments if C<//x> is present).
1653 Parsing of the bracketed character class construct, C<[...]>, is
1654 rather different than the rule used for the rest of the pattern.
1655 The terminator of this construct is found using the same rules as
1656 for finding the terminator of a C<{}>-delimited construct, the only
1657 exception being that C<]> immediately following C<[> is treated as
1658 though preceded by a backslash. Similarly, the terminator of
1659 C<(?{...})> is found using the same rules as for finding the
1660 terminator of a C<{}>-delimited construct.
1662 It is possible to inspect both the string given to RE engine and the
1663 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1664 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1665 switch documented in L<perlrun/"Command Switches">.
1667 =item Optimization of regular expressions
1669 This step is listed for completeness only. Since it does not change
1670 semantics, details of this step are not documented and are subject
1671 to change without notice. This step is performed over the finite
1672 automaton that was generated during the previous pass.
1674 It is at this stage that C<split()> silently optimizes C</^/> to
1679 =head2 I/O Operators
1681 There are several I/O operators you should know about.
1683 A string enclosed by backticks (grave accents) first undergoes
1684 double-quote interpolation. It is then interpreted as an external
1685 command, and the output of that command is the value of the
1686 backtick string, like in a shell. In scalar context, a single string
1687 consisting of all output is returned. In list context, a list of
1688 values is returned, one per line of output. (You can set C<$/> to use
1689 a different line terminator.) The command is executed each time the
1690 pseudo-literal is evaluated. The status value of the command is
1691 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
1692 Unlike in B<csh>, no translation is done on the return data--newlines
1693 remain newlines. Unlike in any of the shells, single quotes do not
1694 hide variable names in the command from interpretation. To pass a
1695 literal dollar-sign through to the shell you need to hide it with a
1696 backslash. The generalized form of backticks is C<qx//>. (Because
1697 backticks always undergo shell expansion as well, see L<perlsec> for
1700 In scalar context, evaluating a filehandle in angle brackets yields
1701 the next line from that file (the newline, if any, included), or
1702 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1703 (sometimes known as file-slurp mode) and the file is empty, it
1704 returns C<''> the first time, followed by C<undef> subsequently.
1706 Ordinarily you must assign the returned value to a variable, but
1707 there is one situation where an automatic assignment happens. If
1708 and only if the input symbol is the only thing inside the conditional
1709 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1710 the value is automatically assigned to the global variable $_,
1711 destroying whatever was there previously. (This may seem like an
1712 odd thing to you, but you'll use the construct in almost every Perl
1713 script you write.) The $_ variable is not implicitly localized.
1714 You'll have to put a C<local $_;> before the loop if you want that
1717 The following lines are equivalent:
1719 while (defined($_ = <STDIN>)) { print; }
1720 while ($_ = <STDIN>) { print; }
1721 while (<STDIN>) { print; }
1722 for (;<STDIN>;) { print; }
1723 print while defined($_ = <STDIN>);
1724 print while ($_ = <STDIN>);
1725 print while <STDIN>;
1727 This also behaves similarly, but avoids $_ :
1729 while (my $line = <STDIN>) { print $line }
1731 In these loop constructs, the assigned value (whether assignment
1732 is automatic or explicit) is then tested to see whether it is
1733 defined. The defined test avoids problems where line has a string
1734 value that would be treated as false by Perl, for example a "" or
1735 a "0" with no trailing newline. If you really mean for such values
1736 to terminate the loop, they should be tested for explicitly:
1738 while (($_ = <STDIN>) ne '0') { ... }
1739 while (<STDIN>) { last unless $_; ... }
1741 In other boolean contexts, C<< <I<filehandle>> >> without an
1742 explicit C<defined> test or comparison elicit a warning if the
1743 C<use warnings> pragma or the B<-w>
1744 command-line switch (the C<$^W> variable) is in effect.
1746 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1747 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
1748 in packages, where they would be interpreted as local identifiers
1749 rather than global.) Additional filehandles may be created with
1750 the open() function, amongst others. See L<perlopentut> and
1751 L<perlfunc/open> for details on this.
1753 If a <FILEHANDLE> is used in a context that is looking for
1754 a list, a list comprising all input lines is returned, one line per
1755 list element. It's easy to grow to a rather large data space this
1756 way, so use with care.
1758 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
1759 See L<perlfunc/readline>.
1761 The null filehandle <> is special: it can be used to emulate the
1762 behavior of B<sed> and B<awk>. Input from <> comes either from
1763 standard input, or from each file listed on the command line. Here's
1764 how it works: the first time <> is evaluated, the @ARGV array is
1765 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
1766 gives you standard input. The @ARGV array is then processed as a list
1767 of filenames. The loop
1770 ... # code for each line
1773 is equivalent to the following Perl-like pseudo code:
1775 unshift(@ARGV, '-') unless @ARGV;
1776 while ($ARGV = shift) {
1779 ... # code for each line
1783 except that it isn't so cumbersome to say, and will actually work.
1784 It really does shift the @ARGV array and put the current filename
1785 into the $ARGV variable. It also uses filehandle I<ARGV>
1786 internally--<> is just a synonym for <ARGV>, which
1787 is magical. (The pseudo code above doesn't work because it treats
1788 <ARGV> as non-magical.)
1790 You can modify @ARGV before the first <> as long as the array ends up
1791 containing the list of filenames you really want. Line numbers (C<$.>)
1792 continue as though the input were one big happy file. See the example
1793 in L<perlfunc/eof> for how to reset line numbers on each file.
1795 If you want to set @ARGV to your own list of files, go right ahead.
1796 This sets @ARGV to all plain text files if no @ARGV was given:
1798 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
1800 You can even set them to pipe commands. For example, this automatically
1801 filters compressed arguments through B<gzip>:
1803 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1805 If you want to pass switches into your script, you can use one of the
1806 Getopts modules or put a loop on the front like this:
1808 while ($_ = $ARGV[0], /^-/) {
1811 if (/^-D(.*)/) { $debug = $1 }
1812 if (/^-v/) { $verbose++ }
1813 # ... # other switches
1817 # ... # code for each line
1820 The <> symbol will return C<undef> for end-of-file only once.
1821 If you call it again after this, it will assume you are processing another
1822 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
1824 If what the angle brackets contain is a simple scalar variable (e.g.,
1825 <$foo>), then that variable contains the name of the
1826 filehandle to input from, or its typeglob, or a reference to the
1832 If what's within the angle brackets is neither a filehandle nor a simple
1833 scalar variable containing a filehandle name, typeglob, or typeglob
1834 reference, it is interpreted as a filename pattern to be globbed, and
1835 either a list of filenames or the next filename in the list is returned,
1836 depending on context. This distinction is determined on syntactic
1837 grounds alone. That means C<< <$x> >> is always a readline() from
1838 an indirect handle, but C<< <$hash{key}> >> is always a glob().
1839 That's because $x is a simple scalar variable, but C<$hash{key}> is
1840 not--it's a hash element.
1842 One level of double-quote interpretation is done first, but you can't
1843 say C<< <$foo> >> because that's an indirect filehandle as explained
1844 in the previous paragraph. (In older versions of Perl, programmers
1845 would insert curly brackets to force interpretation as a filename glob:
1846 C<< <${foo}> >>. These days, it's considered cleaner to call the
1847 internal function directly as C<glob($foo)>, which is probably the right
1848 way to have done it in the first place.) For example:
1854 is roughly equivalent to:
1856 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1862 except that the globbing is actually done internally using the standard
1863 C<File::Glob> extension. Of course, the shortest way to do the above is:
1867 A (file)glob evaluates its (embedded) argument only when it is
1868 starting a new list. All values must be read before it will start
1869 over. In list context, this isn't important because you automatically
1870 get them all anyway. However, in scalar context the operator returns
1871 the next value each time it's called, or C<undef> when the list has
1872 run out. As with filehandle reads, an automatic C<defined> is
1873 generated when the glob occurs in the test part of a C<while>,
1874 because legal glob returns (e.g. a file called F<0>) would otherwise
1875 terminate the loop. Again, C<undef> is returned only once. So if
1876 you're expecting a single value from a glob, it is much better to
1879 ($file) = <blurch*>;
1885 because the latter will alternate between returning a filename and
1888 If you're trying to do variable interpolation, it's definitely better
1889 to use the glob() function, because the older notation can cause people
1890 to become confused with the indirect filehandle notation.
1892 @files = glob("$dir/*.[ch]");
1893 @files = glob($files[$i]);
1895 =head2 Constant Folding
1897 Like C, Perl does a certain amount of expression evaluation at
1898 compile time whenever it determines that all arguments to an
1899 operator are static and have no side effects. In particular, string
1900 concatenation happens at compile time between literals that don't do
1901 variable substitution. Backslash interpolation also happens at
1902 compile time. You can say
1904 'Now is the time for all' . "\n" .
1905 'good men to come to.'
1907 and this all reduces to one string internally. Likewise, if
1910 foreach $file (@filenames) {
1911 if (-s $file > 5 + 100 * 2**16) { }
1914 the compiler will precompute the number which that expression
1915 represents so that the interpreter won't have to.
1917 =head2 Bitwise String Operators
1919 Bitstrings of any size may be manipulated by the bitwise operators
1922 If the operands to a binary bitwise op are strings of different
1923 sizes, B<|> and B<^> ops act as though the shorter operand had
1924 additional zero bits on the right, while the B<&> op acts as though
1925 the longer operand were truncated to the length of the shorter.
1926 The granularity for such extension or truncation is one or more
1929 # ASCII-based examples
1930 print "j p \n" ^ " a h"; # prints "JAPH\n"
1931 print "JA" | " ph\n"; # prints "japh\n"
1932 print "japh\nJunk" & '_____'; # prints "JAPH\n";
1933 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
1935 If you are intending to manipulate bitstrings, be certain that
1936 you're supplying bitstrings: If an operand is a number, that will imply
1937 a B<numeric> bitwise operation. You may explicitly show which type of
1938 operation you intend by using C<""> or C<0+>, as in the examples below.
1940 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
1941 $foo = '150' | 105 ; # yields 255
1942 $foo = 150 | '105'; # yields 255
1943 $foo = '150' | '105'; # yields string '155' (under ASCII)
1945 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
1946 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
1948 See L<perlfunc/vec> for information on how to manipulate individual bits
1951 =head2 Integer Arithmetic
1953 By default, Perl assumes that it must do most of its arithmetic in
1954 floating point. But by saying
1958 you may tell the compiler that it's okay to use integer operations
1959 (if it feels like it) from here to the end of the enclosing BLOCK.
1960 An inner BLOCK may countermand this by saying
1964 which lasts until the end of that BLOCK. Note that this doesn't
1965 mean everything is only an integer, merely that Perl may use integer
1966 operations if it is so inclined. For example, even under C<use
1967 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
1970 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
1971 and ">>") always produce integral results. (But see also
1972 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
1973 them. By default, their results are interpreted as unsigned integers, but
1974 if C<use integer> is in effect, their results are interpreted
1975 as signed integers. For example, C<~0> usually evaluates to a large
1976 integral value. However, C<use integer; ~0> is C<-1> on twos-complement
1979 =head2 Floating-point Arithmetic
1981 While C<use integer> provides integer-only arithmetic, there is no
1982 analogous mechanism to provide automatic rounding or truncation to a
1983 certain number of decimal places. For rounding to a certain number
1984 of digits, sprintf() or printf() is usually the easiest route.
1987 Floating-point numbers are only approximations to what a mathematician
1988 would call real numbers. There are infinitely more reals than floats,
1989 so some corners must be cut. For example:
1991 printf "%.20g\n", 123456789123456789;
1992 # produces 123456789123456784
1994 Testing for exact equality of floating-point equality or inequality is
1995 not a good idea. Here's a (relatively expensive) work-around to compare
1996 whether two floating-point numbers are equal to a particular number of
1997 decimal places. See Knuth, volume II, for a more robust treatment of
2001 my ($X, $Y, $POINTS) = @_;
2003 $tX = sprintf("%.${POINTS}g", $X);
2004 $tY = sprintf("%.${POINTS}g", $Y);
2008 The POSIX module (part of the standard perl distribution) implements
2009 ceil(), floor(), and other mathematical and trigonometric functions.
2010 The Math::Complex module (part of the standard perl distribution)
2011 defines mathematical functions that work on both the reals and the
2012 imaginary numbers. Math::Complex not as efficient as POSIX, but
2013 POSIX can't work with complex numbers.
2015 Rounding in financial applications can have serious implications, and
2016 the rounding method used should be specified precisely. In these
2017 cases, it probably pays not to trust whichever system rounding is
2018 being used by Perl, but to instead implement the rounding function you
2021 =head2 Bigger Numbers
2023 The standard Math::BigInt and Math::BigFloat modules provide
2024 variable-precision arithmetic and overloaded operators, although
2025 they're currently pretty slow. At the cost of some space and
2026 considerable speed, they avoid the normal pitfalls associated with
2027 limited-precision representations.
2030 $x = Math::BigInt->new('123456789123456789');
2033 # prints +15241578780673678515622620750190521
2035 There are several modules that let you calculate with (bound only by
2036 memory and cpu-time) unlimited or fixed precision. There are also
2037 some non-standard modules that provide faster implementations via
2038 external C libraries.
2040 Here is a short, but incomplete summary:
2042 Math::Fraction big, unlimited fractions like 9973 / 12967
2043 Math::String treat string sequences like numbers
2044 Math::FixedPrecision calculate with a fixed precision
2045 Math::Currency for currency calculations
2046 Bit::Vector manipulate bit vectors fast (uses C)
2047 Math::BigIntFast Bit::Vector wrapper for big numbers
2048 Math::Pari provides access to the Pari C library
2049 Math::BigInteger uses an external C library
2050 Math::Cephes uses external Cephes C library (no big numbers)
2051 Math::Cephes::Fraction fractions via the Cephes library
2052 Math::GMP another one using an external C library