4 perlop - Perl operators and precedence
8 =head2 Operator Precedence and Associativity
9 X<operator, precedence> X<precedence> X<associativity>
11 Operator precedence and associativity work in Perl more or less like
12 they do in mathematics.
14 I<Operator precedence> means some operators are evaluated before
15 others. For example, in C<2 + 4 * 5>, the multiplication has higher
16 precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
17 22> and not C<6 * 5 == 30>.
19 I<Operator associativity> defines what happens if a sequence of the
20 same operators is used one after another: whether the evaluator will
21 evaluate the left operations first or the right. For example, in C<8
22 - 4 - 2>, subtraction is left associative so Perl evaluates the
23 expression left to right. C<8 - 4> is evaluated first making the
24 expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
26 Perl operators have the following associativity and precedence,
27 listed from highest precedence to lowest. Operators borrowed from
28 C keep the same precedence relationship with each other, even where
29 C's precedence is slightly screwy. (This makes learning Perl easier
30 for C folks.) With very few exceptions, these all operate on scalar
31 values only, not array values.
33 left terms and list operators (leftward)
37 right ! ~ \ and unary + and -
42 nonassoc named unary operators
43 nonassoc < > <= >= lt gt le ge
44 nonassoc == != <=> eq ne cmp ~~
53 nonassoc list operators (rightward)
58 In the following sections, these operators are covered in precedence order.
60 Many operators can be overloaded for objects. See L<overload>.
62 =head2 Terms and List Operators (Leftward)
63 X<list operator> X<operator, list> X<term>
65 A TERM has the highest precedence in Perl. They include variables,
66 quote and quote-like operators, any expression in parentheses,
67 and any function whose arguments are parenthesized. Actually, there
68 aren't really functions in this sense, just list operators and unary
69 operators behaving as functions because you put parentheses around
70 the arguments. These are all documented in L<perlfunc>.
72 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
73 is followed by a left parenthesis as the next token, the operator and
74 arguments within parentheses are taken to be of highest precedence,
75 just like a normal function call.
77 In the absence of parentheses, the precedence of list operators such as
78 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
79 whether you are looking at the left side or the right side of the operator.
82 @ary = (1, 3, sort 4, 2);
83 print @ary; # prints 1324
85 the commas on the right of the sort are evaluated before the sort,
86 but the commas on the left are evaluated after. In other words,
87 list operators tend to gobble up all arguments that follow, and
88 then act like a simple TERM with regard to the preceding expression.
89 Be careful with parentheses:
91 # These evaluate exit before doing the print:
92 print($foo, exit); # Obviously not what you want.
93 print $foo, exit; # Nor is this.
95 # These do the print before evaluating exit:
96 (print $foo), exit; # This is what you want.
97 print($foo), exit; # Or this.
98 print ($foo), exit; # Or even this.
102 print ($foo & 255) + 1, "\n";
104 probably doesn't do what you expect at first glance. The parentheses
105 enclose the argument list for C<print> which is evaluated (printing
106 the result of C<$foo & 255>). Then one is added to the return value
107 of C<print> (usually 1). The result is something like this:
109 1 + 1, "\n"; # Obviously not what you meant.
111 To do what you meant properly, you must write:
113 print(($foo & 255) + 1, "\n");
115 See L<Named Unary Operators> for more discussion of this.
117 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
118 well as subroutine and method calls, and the anonymous
119 constructors C<[]> and C<{}>.
121 See also L<Quote and Quote-like Operators> toward the end of this section,
122 as well as L</"I/O Operators">.
124 =head2 The Arrow Operator
125 X<arrow> X<dereference> X<< -> >>
127 "C<< -> >>" is an infix dereference operator, just as it is in C
128 and C++. If the right side is either a C<[...]>, C<{...}>, or a
129 C<(...)> subscript, then the left side must be either a hard or
130 symbolic reference to an array, a hash, or a subroutine respectively.
131 (Or technically speaking, a location capable of holding a hard
132 reference, if it's an array or hash reference being used for
133 assignment.) See L<perlreftut> and L<perlref>.
135 Otherwise, the right side is a method name or a simple scalar
136 variable containing either the method name or a subroutine reference,
137 and the left side must be either an object (a blessed reference)
138 or a class name (that is, a package name). See L<perlobj>.
140 =head2 Auto-increment and Auto-decrement
141 X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
143 "++" and "--" work as in C. That is, if placed before a variable,
144 they increment or decrement the variable by one before returning the
145 value, and if placed after, increment or decrement after returning the
149 print $i++; # prints 0
150 print ++$j; # prints 1
152 Note that just as in C, Perl doesn't define B<when> the variable is
153 incremented or decremented. You just know it will be done sometime
154 before or after the value is returned. This also means that modifying
155 a variable twice in the same statement will lead to undefined behaviour.
156 Avoid statements like:
161 Perl will not guarantee what the result of the above statements is.
163 The auto-increment operator has a little extra builtin magic to it. If
164 you increment a variable that is numeric, or that has ever been used in
165 a numeric context, you get a normal increment. If, however, the
166 variable has been used in only string contexts since it was set, and
167 has a value that is not the empty string and matches the pattern
168 C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
169 character within its range, with carry:
171 print ++($foo = '99'); # prints '100'
172 print ++($foo = 'a0'); # prints 'a1'
173 print ++($foo = 'Az'); # prints 'Ba'
174 print ++($foo = 'zz'); # prints 'aaa'
176 C<undef> is always treated as numeric, and in particular is changed
177 to C<0> before incrementing (so that a post-increment of an undef value
178 will return C<0> rather than C<undef>).
180 The auto-decrement operator is not magical.
182 =head2 Exponentiation
183 X<**> X<exponentiation> X<power>
185 Binary "**" is the exponentiation operator. It binds even more
186 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
187 implemented using C's pow(3) function, which actually works on doubles
190 =head2 Symbolic Unary Operators
191 X<unary operator> X<operator, unary>
193 Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
194 precedence version of this.
197 Unary "-" performs arithmetic negation if the operand is numeric. If
198 the operand is an identifier, a string consisting of a minus sign
199 concatenated with the identifier is returned. Otherwise, if the string
200 starts with a plus or minus, a string starting with the opposite sign
201 is returned. One effect of these rules is that -bareword is equivalent
202 to the string "-bareword". If, however, the string begins with a
203 non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
204 the string to a numeric and the arithmetic negation is performed. If the
205 string cannot be cleanly converted to a numeric, Perl will give the warning
206 B<Argument "the string" isn't numeric in negation (-) at ...>.
207 X<-> X<negation, arithmetic>
209 Unary "~" performs bitwise negation, i.e., 1's complement. For
210 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
211 L<Bitwise String Operators>.) Note that the width of the result is
212 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
213 bits wide on a 64-bit platform, so if you are expecting a certain bit
214 width, remember to use the & operator to mask off the excess bits.
215 X<~> X<negation, binary>
217 Unary "+" has no effect whatsoever, even on strings. It is useful
218 syntactically for separating a function name from a parenthesized expression
219 that would otherwise be interpreted as the complete list of function
220 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
223 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
224 and L<perlref>. Do not confuse this behavior with the behavior of
225 backslash within a string, although both forms do convey the notion
226 of protecting the next thing from interpolation.
227 X<\> X<reference> X<backslash>
229 =head2 Binding Operators
230 X<binding> X<operator, binding> X<=~> X<!~>
232 Binary "=~" binds a scalar expression to a pattern match. Certain operations
233 search or modify the string $_ by default. This operator makes that kind
234 of operation work on some other string. The right argument is a search
235 pattern, substitution, or transliteration. The left argument is what is
236 supposed to be searched, substituted, or transliterated instead of the default
237 $_. When used in scalar context, the return value generally indicates the
238 success of the operation. Behavior in list context depends on the particular
239 operator. See L</"Regexp Quote-Like Operators"> for details and
240 L<perlretut> for examples using these operators.
242 If the right argument is an expression rather than a search pattern,
243 substitution, or transliteration, it is interpreted as a search pattern at run
244 time. Note that this means that its contents will be interpolated twice, so
248 is not ok, as the regex engine will end up trying to compile the
249 pattern C<\>, which it will consider a syntax error.
251 Binary "!~" is just like "=~" except the return value is negated in
254 =head2 Multiplicative Operators
255 X<operator, multiplicative>
257 Binary "*" multiplies two numbers.
260 Binary "/" divides two numbers.
263 Binary "%" is the modulo operator, which computes the division
264 remainder of its first argument with respect to its second argument.
266 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
267 C<$a> minus the largest multiple of C<$b> less than or equal to
268 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
269 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
270 result will be less than or equal to zero). If the operands
271 C<$a> and C<$b> are floating point values and the absolute value of
272 C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
273 the integer portion of C<$a> and C<$b> will be used in the operation
274 (Note: here C<UV_MAX> means the maximum of the unsigned integer type).
275 If the absolute value of the right operand (C<abs($b)>) is greater than
276 or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
277 C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
278 integer that makes C<$r> have the same sign as the right operand
279 C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
280 and the absolute value less than that of C<$b>.
281 Note that when C<use integer> is in scope, "%" gives you direct access
282 to the modulo operator as implemented by your C compiler. This
283 operator is not as well defined for negative operands, but it will
285 X<%> X<remainder> X<modulo> X<mod>
287 Binary "x" is the repetition operator. In scalar context or if the left
288 operand is not enclosed in parentheses, it returns a string consisting
289 of the left operand repeated the number of times specified by the right
290 operand. In list context, if the left operand is enclosed in
291 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
292 If the right operand is zero or negative, it returns an empty string
293 or an empty list, depending on the context.
296 print '-' x 80; # print row of dashes
298 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
300 @ones = (1) x 80; # a list of 80 1's
301 @ones = (5) x @ones; # set all elements to 5
304 =head2 Additive Operators
305 X<operator, additive>
307 Binary "+" returns the sum of two numbers.
310 Binary "-" returns the difference of two numbers.
313 Binary "." concatenates two strings.
314 X<string, concatenation> X<concatenation>
315 X<cat> X<concat> X<concatenate> X<.>
317 =head2 Shift Operators
318 X<shift operator> X<operator, shift> X<<< << >>>
319 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
320 X<shl> X<shr> X<shift, right> X<shift, left>
322 Binary "<<" returns the value of its left argument shifted left by the
323 number of bits specified by the right argument. Arguments should be
324 integers. (See also L<Integer Arithmetic>.)
326 Binary ">>" returns the value of its left argument shifted right by
327 the number of bits specified by the right argument. Arguments should
328 be integers. (See also L<Integer Arithmetic>.)
330 Note that both "<<" and ">>" in Perl are implemented directly using
331 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
332 in force then signed C integers are used, else unsigned C integers are
333 used. Either way, the implementation isn't going to generate results
334 larger than the size of the integer type Perl was built with (32 bits
337 The result of overflowing the range of the integers is undefined
338 because it is undefined also in C. In other words, using 32-bit
339 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
340 of bits is also undefined.
342 =head2 Named Unary Operators
343 X<operator, named unary>
345 The various named unary operators are treated as functions with one
346 argument, with optional parentheses.
348 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
349 is followed by a left parenthesis as the next token, the operator and
350 arguments within parentheses are taken to be of highest precedence,
351 just like a normal function call. For example,
352 because named unary operators are higher precedence than ||:
354 chdir $foo || die; # (chdir $foo) || die
355 chdir($foo) || die; # (chdir $foo) || die
356 chdir ($foo) || die; # (chdir $foo) || die
357 chdir +($foo) || die; # (chdir $foo) || die
359 but, because * is higher precedence than named operators:
361 chdir $foo * 20; # chdir ($foo * 20)
362 chdir($foo) * 20; # (chdir $foo) * 20
363 chdir ($foo) * 20; # (chdir $foo) * 20
364 chdir +($foo) * 20; # chdir ($foo * 20)
366 rand 10 * 20; # rand (10 * 20)
367 rand(10) * 20; # (rand 10) * 20
368 rand (10) * 20; # (rand 10) * 20
369 rand +(10) * 20; # rand (10 * 20)
371 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
372 treated like named unary operators, but they don't follow this functional
373 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
374 equivalent to C<-f "$file.bak">.
375 X<-X> X<filetest> X<operator, filetest>
377 See also L<"Terms and List Operators (Leftward)">.
379 =head2 Relational Operators
380 X<relational operator> X<operator, relational>
382 Binary "<" returns true if the left argument is numerically less than
386 Binary ">" returns true if the left argument is numerically greater
387 than the right argument.
390 Binary "<=" returns true if the left argument is numerically less than
391 or equal to the right argument.
394 Binary ">=" returns true if the left argument is numerically greater
395 than or equal to the right argument.
398 Binary "lt" returns true if the left argument is stringwise less than
402 Binary "gt" returns true if the left argument is stringwise greater
403 than the right argument.
406 Binary "le" returns true if the left argument is stringwise less than
407 or equal to the right argument.
410 Binary "ge" returns true if the left argument is stringwise greater
411 than or equal to the right argument.
414 =head2 Equality Operators
415 X<equality> X<equal> X<equals> X<operator, equality>
417 Binary "==" returns true if the left argument is numerically equal to
421 Binary "!=" returns true if the left argument is numerically not equal
422 to the right argument.
425 Binary "<=>" returns -1, 0, or 1 depending on whether the left
426 argument is numerically less than, equal to, or greater than the right
427 argument. If your platform supports NaNs (not-a-numbers) as numeric
428 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
429 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
430 returns true, as does NaN != anything else. If your platform doesn't
431 support NaNs then NaN is just a string with numeric value 0.
432 X<< <=> >> X<spaceship>
434 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
435 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
437 Binary "eq" returns true if the left argument is stringwise equal to
441 Binary "ne" returns true if the left argument is stringwise not equal
442 to the right argument.
445 Binary "cmp" returns -1, 0, or 1 depending on whether the left
446 argument is stringwise less than, equal to, or greater than the right
450 Binary "~~" does a smart match between its arguments. Smart matching
451 is described in L<perlsyn/"Smart matching in detail">.
454 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
455 by the current locale if C<use locale> is in effect. See L<perllocale>.
458 X<operator, bitwise, and> X<bitwise and> X<&>
460 Binary "&" returns its operands ANDed together bit by bit.
461 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
463 Note that "&" has lower priority than relational operators, so for example
464 the brackets are essential in a test like
466 print "Even\n" if ($x & 1) == 0;
468 =head2 Bitwise Or and Exclusive Or
469 X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
472 Binary "|" returns its operands ORed together bit by bit.
473 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
475 Binary "^" returns its operands XORed together bit by bit.
476 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
478 Note that "|" and "^" have lower priority than relational operators, so
479 for example the brackets are essential in a test like
481 print "false\n" if (8 | 2) != 10;
483 =head2 C-style Logical And
484 X<&&> X<logical and> X<operator, logical, and>
486 Binary "&&" performs a short-circuit logical AND operation. That is,
487 if the left operand is false, the right operand is not even evaluated.
488 Scalar or list context propagates down to the right operand if it
491 =head2 C-style Logical Or
492 X<||> X<operator, logical, or>
494 Binary "||" performs a short-circuit logical OR operation. That is,
495 if the left operand is true, the right operand is not even evaluated.
496 Scalar or list context propagates down to the right operand if it
499 =head2 C-style Logical Defined-Or
500 X<//> X<operator, logical, defined-or>
502 Although it has no direct equivalent in C, Perl's C<//> operator is related
503 to its C-style or. In fact, it's exactly the same as C<||>, except that it
504 tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
505 is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
506 rather than the value of C<defined($a)>) and is exactly equivalent to
507 C<defined($a) ? $a : $b>. This is very useful for providing default values
508 for variables. If you actually want to test if at least one of C<$a> and
509 C<$b> is defined, use C<defined($a // $b)>.
511 The C<||>, C<//> and C<&&> operators return the last value evaluated
512 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
513 portable way to find out the home directory might be:
515 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
516 (getpwuid($<))[7] // die "You're homeless!\n";
518 In particular, this means that you shouldn't use this
519 for selecting between two aggregates for assignment:
521 @a = @b || @c; # this is wrong
522 @a = scalar(@b) || @c; # really meant this
523 @a = @b ? @b : @c; # this works fine, though
525 As more readable alternatives to C<&&> and C<||> when used for
526 control flow, Perl provides the C<and> and C<or> operators (see below).
527 The short-circuit behavior is identical. The precedence of "and"
528 and "or" is much lower, however, so that you can safely use them after a
529 list operator without the need for parentheses:
531 unlink "alpha", "beta", "gamma"
532 or gripe(), next LINE;
534 With the C-style operators that would have been written like this:
536 unlink("alpha", "beta", "gamma")
537 || (gripe(), next LINE);
539 Using "or" for assignment is unlikely to do what you want; see below.
541 =head2 Range Operators
542 X<operator, range> X<range> X<..> X<...>
544 Binary ".." is the range operator, which is really two different
545 operators depending on the context. In list context, it returns a
546 list of values counting (up by ones) from the left value to the right
547 value. If the left value is greater than the right value then it
548 returns the empty list. The range operator is useful for writing
549 C<foreach (1..10)> loops and for doing slice operations on arrays. In
550 the current implementation, no temporary array is created when the
551 range operator is used as the expression in C<foreach> loops, but older
552 versions of Perl might burn a lot of memory when you write something
555 for (1 .. 1_000_000) {
559 The range operator also works on strings, using the magical
560 auto-increment, see below.
562 In scalar context, ".." returns a boolean value. The operator is
563 bistable, like a flip-flop, and emulates the line-range (comma)
564 operator of B<sed>, B<awk>, and various editors. Each ".." operator
565 maintains its own boolean state, even across calls to a subroutine
566 that contains it. It is false as long as its left operand is false.
567 Once the left operand is true, the range operator stays true until the
568 right operand is true, I<AFTER> which the range operator becomes false
569 again. It doesn't become false till the next time the range operator
570 is evaluated. It can test the right operand and become false on the
571 same evaluation it became true (as in B<awk>), but it still returns
572 true once. If you don't want it to test the right operand until the
573 next evaluation, as in B<sed>, just use three dots ("...") instead of
574 two. In all other regards, "..." behaves just like ".." does.
576 The right operand is not evaluated while the operator is in the
577 "false" state, and the left operand is not evaluated while the
578 operator is in the "true" state. The precedence is a little lower
579 than || and &&. The value returned is either the empty string for
580 false, or a sequence number (beginning with 1) for true. The sequence
581 number is reset for each range encountered. The final sequence number
582 in a range has the string "E0" appended to it, which doesn't affect
583 its numeric value, but gives you something to search for if you want
584 to exclude the endpoint. You can exclude the beginning point by
585 waiting for the sequence number to be greater than 1.
587 If either operand of scalar ".." is a constant expression,
588 that operand is considered true if it is equal (C<==>) to the current
589 input line number (the C<$.> variable).
591 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
592 but that is only an issue if you use a floating point expression; when
593 implicitly using C<$.> as described in the previous paragraph, the
594 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
595 is set to a floating point value and you are not reading from a file.
596 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
597 you want in scalar context because each of the operands are evaluated
598 using their integer representation.
602 As a scalar operator:
604 if (101 .. 200) { print; } # print 2nd hundred lines, short for
605 # if ($. == 101 .. $. == 200) { print; }
607 next LINE if (1 .. /^$/); # skip header lines, short for
608 # next LINE if ($. == 1 .. /^$/);
609 # (typically in a loop labeled LINE)
611 s/^/> / if (/^$/ .. eof()); # quote body
613 # parse mail messages
615 $in_header = 1 .. /^$/;
616 $in_body = /^$/ .. eof;
623 close ARGV if eof; # reset $. each file
626 Here's a simple example to illustrate the difference between
627 the two range operators:
640 This program will print only the line containing "Bar". If
641 the range operator is changed to C<...>, it will also print the
644 And now some examples as a list operator:
646 for (101 .. 200) { print; } # print $_ 100 times
647 @foo = @foo[0 .. $#foo]; # an expensive no-op
648 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
650 The range operator (in list context) makes use of the magical
651 auto-increment algorithm if the operands are strings. You
654 @alphabet = ('A' .. 'Z');
656 to get all normal letters of the English alphabet, or
658 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
660 to get a hexadecimal digit, or
662 @z2 = ('01' .. '31'); print $z2[$mday];
664 to get dates with leading zeros.
666 If the final value specified is not in the sequence that the magical
667 increment would produce, the sequence goes until the next value would
668 be longer than the final value specified.
670 If the initial value specified isn't part of a magical increment
671 sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
672 only the initial value will be returned. So the following will only
675 use charnames 'greek';
676 my @greek_small = ("\N{alpha}" .. "\N{omega}");
678 To get lower-case greek letters, use this instead:
680 my @greek_small = map { chr } ( ord("\N{alpha}") .. ord("\N{omega}") );
682 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
683 return two elements in list context.
685 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
687 =head2 Conditional Operator
688 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
690 Ternary "?:" is the conditional operator, just as in C. It works much
691 like an if-then-else. If the argument before the ? is true, the
692 argument before the : is returned, otherwise the argument after the :
693 is returned. For example:
695 printf "I have %d dog%s.\n", $n,
696 ($n == 1) ? '' : "s";
698 Scalar or list context propagates downward into the 2nd
699 or 3rd argument, whichever is selected.
701 $a = $ok ? $b : $c; # get a scalar
702 @a = $ok ? @b : @c; # get an array
703 $a = $ok ? @b : @c; # oops, that's just a count!
705 The operator may be assigned to if both the 2nd and 3rd arguments are
706 legal lvalues (meaning that you can assign to them):
708 ($a_or_b ? $a : $b) = $c;
710 Because this operator produces an assignable result, using assignments
711 without parentheses will get you in trouble. For example, this:
713 $a % 2 ? $a += 10 : $a += 2
717 (($a % 2) ? ($a += 10) : $a) += 2
721 ($a % 2) ? ($a += 10) : ($a += 2)
723 That should probably be written more simply as:
725 $a += ($a % 2) ? 10 : 2;
727 =head2 Assignment Operators
728 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
729 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
732 "=" is the ordinary assignment operator.
734 Assignment operators work as in C. That is,
742 although without duplicating any side effects that dereferencing the lvalue
743 might trigger, such as from tie(). Other assignment operators work similarly.
744 The following are recognized:
751 Although these are grouped by family, they all have the precedence
754 Unlike in C, the scalar assignment operator produces a valid lvalue.
755 Modifying an assignment is equivalent to doing the assignment and
756 then modifying the variable that was assigned to. This is useful
757 for modifying a copy of something, like this:
759 ($tmp = $global) =~ tr [A-Z] [a-z];
770 Similarly, a list assignment in list context produces the list of
771 lvalues assigned to, and a list assignment in scalar context returns
772 the number of elements produced by the expression on the right hand
773 side of the assignment.
775 =head2 Comma Operator
776 X<comma> X<operator, comma> X<,>
778 Binary "," is the comma operator. In scalar context it evaluates
779 its left argument, throws that value away, then evaluates its right
780 argument and returns that value. This is just like C's comma operator.
782 In list context, it's just the list argument separator, and inserts
783 both its arguments into the list. These arguments are also evaluated
786 The C<< => >> operator is a synonym for the comma except that it causes
787 its left operand to be interpreted as a string if it begins with a letter
788 or underscore and is composed only of letters, digits and underscores.
789 This includes operands that might otherwise be interpreted as operators,
790 constants, single number v-strings or function calls. If in doubt about
791 this behaviour, the left operand can be quoted explicitly.
793 Otherwise, the C<< => >> operator behaves exactly as the comma operator
794 or list argument separator, according to context.
798 use constant FOO => "something";
800 my %h = ( FOO => 23 );
808 my %h = ("something", 23);
810 The C<< => >> operator is helpful in documenting the correspondence
811 between keys and values in hashes, and other paired elements in lists.
813 %hash = ( $key => $value );
814 login( $username => $password );
816 =head2 Yada Yada Operator
817 X<...> X<... operator> X<yada yada operator>
819 The yada yada operator (noted C<...>) is a placeholder for code. Perl
820 parses it without error, but when you try to execute a yada yada, it
821 throws an exception with the text C<Unimplemented>:
823 sub unimplemented { ... }
825 eval { unimplemented() };
826 if( $@ eq 'Unimplemented' ) {
827 print "I found the yada yada!\n";
830 You can only use the yada yada to stand in for a complete statement.
831 These examples of the yada yada work:
847 do { my $n; ...; print 'Hurrah!' };
849 The yada yada cannot stand in for an expression that is part of a
850 larger statement since the C<...> is also the three-dot version of the
851 range operator (see L<Range Operators>). These examples of the yada
852 yada are still syntax errors:
856 open my($fh), '>', '/dev/passwd' or ...;
858 if( $condition && ... ) { print "Hello\n" };
860 There are some cases where Perl can't immediately tell the difference
861 between an expression and a statement. For instance, the syntax for a
862 block and an anonymous hash reference constructor look the same unless
863 there's something in the braces that give Perl a hint. The yada yada
864 is a syntax error if Perl doesn't guess that the C<{ ... }> is a
865 block. In that case, it doesn't think the C<...> is the yada yada
866 because it's expecting an expression instead of a statement:
868 my @transformed = map { ... } @input; # syntax error
870 You can use a C<;> inside your block to denote that the C<{ ... }> is
871 a block and not a hash reference constructor. Now the yada yada works:
873 my @transformed = map {; ... } @input; # ; disambiguates
875 my @transformed = map { ...; } @input; # ; disambiguates
877 =head2 List Operators (Rightward)
878 X<operator, list, rightward> X<list operator>
880 On the right side of a list operator, it has very low precedence,
881 such that it controls all comma-separated expressions found there.
882 The only operators with lower precedence are the logical operators
883 "and", "or", and "not", which may be used to evaluate calls to list
884 operators without the need for extra parentheses:
886 open HANDLE, "filename"
887 or die "Can't open: $!\n";
889 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
892 X<operator, logical, not> X<not>
894 Unary "not" returns the logical negation of the expression to its right.
895 It's the equivalent of "!" except for the very low precedence.
898 X<operator, logical, and> X<and>
900 Binary "and" returns the logical conjunction of the two surrounding
901 expressions. It's equivalent to && except for the very low
902 precedence. This means that it short-circuits: i.e., the right
903 expression is evaluated only if the left expression is true.
905 =head2 Logical or, Defined or, and Exclusive Or
906 X<operator, logical, or> X<operator, logical, xor>
907 X<operator, logical, defined or> X<operator, logical, exclusive or>
910 Binary "or" returns the logical disjunction of the two surrounding
911 expressions. It's equivalent to || except for the very low precedence.
912 This makes it useful for control flow
914 print FH $data or die "Can't write to FH: $!";
916 This means that it short-circuits: i.e., the right expression is evaluated
917 only if the left expression is false. Due to its precedence, you should
918 probably avoid using this for assignment, only for control flow.
920 $a = $b or $c; # bug: this is wrong
921 ($a = $b) or $c; # really means this
922 $a = $b || $c; # better written this way
924 However, when it's a list-context assignment and you're trying to use
925 "||" for control flow, you probably need "or" so that the assignment
926 takes higher precedence.
928 @info = stat($file) || die; # oops, scalar sense of stat!
929 @info = stat($file) or die; # better, now @info gets its due
931 Then again, you could always use parentheses.
933 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
934 It cannot short circuit, of course.
936 =head2 C Operators Missing From Perl
937 X<operator, missing from perl> X<&> X<*>
938 X<typecasting> X<(TYPE)>
940 Here is what C has that Perl doesn't:
946 Address-of operator. (But see the "\" operator for taking a reference.)
950 Dereference-address operator. (Perl's prefix dereferencing
951 operators are typed: $, @, %, and &.)
955 Type-casting operator.
959 =head2 Quote and Quote-like Operators
960 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
961 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
962 X<escape sequence> X<escape>
965 While we usually think of quotes as literal values, in Perl they
966 function as operators, providing various kinds of interpolating and
967 pattern matching capabilities. Perl provides customary quote characters
968 for these behaviors, but also provides a way for you to choose your
969 quote character for any of them. In the following table, a C<{}> represents
970 any pair of delimiters you choose.
972 Customary Generic Meaning Interpolates
977 // m{} Pattern match yes*
979 s{}{} Substitution yes*
980 tr{}{} Transliteration no (but see below)
983 * unless the delimiter is ''.
985 Non-bracketing delimiters use the same character fore and aft, but the four
986 sorts of brackets (round, angle, square, curly) will all nest, which means
995 Note, however, that this does not always work for quoting Perl code:
997 $s = q{ if($a eq "}") ... }; # WRONG
999 is a syntax error. The C<Text::Balanced> module (from CPAN, and
1000 starting from Perl 5.8 part of the standard distribution) is able
1001 to do this properly.
1003 There can be whitespace between the operator and the quoting
1004 characters, except when C<#> is being used as the quoting character.
1005 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
1006 operator C<q> followed by a comment. Its argument will be taken
1007 from the next line. This allows you to write:
1009 s {foo} # Replace foo
1012 The following escape sequences are available in constructs that interpolate
1013 and in transliterations.
1014 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N>
1021 \a alarm (bell) (BEL)
1023 \033 octal char (example: ESC)
1024 \x1b hex char (example: ESC)
1025 \x{263a} wide hex char (example: SMILEY)
1026 \c[ control char (example: ESC)
1027 \N{name} named Unicode character
1029 The character following C<\c> is mapped to some other character by
1030 converting letters to upper case and then (on ASCII systems) by inverting
1031 the 7th bit (0x40). The most interesting range is from '@' to '_'
1032 (0x40 through 0x5F), resulting in a control character from 0x00
1033 through 0x1F. A '?' maps to the DEL character. On EBCDIC systems only
1034 '@', the letters, '[', '\', ']', '^', '_' and '?' will work, resulting
1035 in 0x00 through 0x1F and 0x7F.
1037 B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
1038 the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>.
1040 The following escape sequences are available in constructs that interpolate
1041 but not in transliterations.
1042 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
1044 \l lowercase next char
1045 \u uppercase next char
1046 \L lowercase till \E
1047 \U uppercase till \E
1048 \E end case modification
1049 \Q quote non-word characters till \E
1051 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
1052 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
1053 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
1054 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
1055 C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
1058 All systems use the virtual C<"\n"> to represent a line terminator,
1059 called a "newline". There is no such thing as an unvarying, physical
1060 newline character. It is only an illusion that the operating system,
1061 device drivers, C libraries, and Perl all conspire to preserve. Not all
1062 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1063 on a Mac, these are reversed, and on systems without line terminator,
1064 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
1065 you mean a "newline" for your system, but use the literal ASCII when you
1066 need an exact character. For example, most networking protocols expect
1067 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
1068 and although they often accept just C<"\012">, they seldom tolerate just
1069 C<"\015">. If you get in the habit of using C<"\n"> for networking,
1070 you may be burned some day.
1071 X<newline> X<line terminator> X<eol> X<end of line>
1074 For constructs that do interpolate, variables beginning with "C<$>"
1075 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
1076 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1077 But method calls such as C<< $obj->meth >> are not.
1079 Interpolating an array or slice interpolates the elements in order,
1080 separated by the value of C<$">, so is equivalent to interpolating
1081 C<join $", @array>. "Punctuation" arrays such as C<@*> are only
1082 interpolated if the name is enclosed in braces C<@{*}>, but special
1083 arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
1085 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
1086 An unescaped C<$> or C<@> interpolates the corresponding variable,
1087 while escaping will cause the literal string C<\$> to be inserted.
1088 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
1090 Patterns are subject to an additional level of interpretation as a
1091 regular expression. This is done as a second pass, after variables are
1092 interpolated, so that regular expressions may be incorporated into the
1093 pattern from the variables. If this is not what you want, use C<\Q> to
1094 interpolate a variable literally.
1096 Apart from the behavior described above, Perl does not expand
1097 multiple levels of interpolation. In particular, contrary to the
1098 expectations of shell programmers, back-quotes do I<NOT> interpolate
1099 within double quotes, nor do single quotes impede evaluation of
1100 variables when used within double quotes.
1102 =head2 Regexp Quote-Like Operators
1105 Here are the quote-like operators that apply to pattern
1106 matching and related activities.
1110 =item qr/STRING/msixpo
1111 X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
1113 This operator quotes (and possibly compiles) its I<STRING> as a regular
1114 expression. I<STRING> is interpolated the same way as I<PATTERN>
1115 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1116 is done. Returns a Perl value which may be used instead of the
1117 corresponding C</STRING/msixpo> expression. The returned value is a
1118 normalized version of the original pattern. It magically differs from
1119 a string containing the same characters: C<ref(qr/x/)> returns "Regexp",
1120 even though dereferencing the result returns undef.
1124 $rex = qr/my.STRING/is;
1125 print $rex; # prints (?si-xm:my.STRING)
1132 The result may be used as a subpattern in a match:
1135 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1136 $string =~ $re; # or used standalone
1137 $string =~ /$re/; # or this way
1139 Since Perl may compile the pattern at the moment of execution of qr()
1140 operator, using qr() may have speed advantages in some situations,
1141 notably if the result of qr() is used standalone:
1144 my $patterns = shift;
1145 my @compiled = map qr/$_/i, @$patterns;
1148 foreach my $pat (@compiled) {
1149 $success = 1, last if /$pat/;
1155 Precompilation of the pattern into an internal representation at
1156 the moment of qr() avoids a need to recompile the pattern every
1157 time a match C</$pat/> is attempted. (Perl has many other internal
1158 optimizations, but none would be triggered in the above example if
1159 we did not use qr() operator.)
1163 m Treat string as multiple lines.
1164 s Treat string as single line. (Make . match a newline)
1165 i Do case-insensitive pattern matching.
1166 x Use extended regular expressions.
1167 p When matching preserve a copy of the matched string so
1168 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
1169 o Compile pattern only once.
1171 If a precompiled pattern is embedded in a larger pattern then the effect
1172 of 'msixp' will be propagated appropriately. The effect of the 'o'
1173 modifier has is not propagated, being restricted to those patterns
1174 explicitly using it.
1176 See L<perlre> for additional information on valid syntax for STRING, and
1177 for a detailed look at the semantics of regular expressions.
1179 =item m/PATTERN/msixpogc
1180 X<m> X<operator, match>
1181 X<regexp, options> X<regexp> X<regex, options> X<regex>
1182 X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
1184 =item /PATTERN/msixpogc
1186 Searches a string for a pattern match, and in scalar context returns
1187 true if it succeeds, false if it fails. If no string is specified
1188 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1189 string specified with C<=~> need not be an lvalue--it may be the
1190 result of an expression evaluation, but remember the C<=~> binds
1191 rather tightly.) See also L<perlre>. See L<perllocale> for
1192 discussion of additional considerations that apply when C<use locale>
1195 Options are as described in C<qr//>; in addition, the following match
1196 process modifiers are available:
1198 g Match globally, i.e., find all occurrences.
1199 c Do not reset search position on a failed match when /g is in effect.
1201 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1202 you can use any pair of non-whitespace characters
1203 as delimiters. This is particularly useful for matching path names
1204 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1205 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
1206 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1207 When using a character valid in an identifier, whitespace is required
1210 PATTERN may contain variables, which will be interpolated (and the
1211 pattern recompiled) every time the pattern search is evaluated, except
1212 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1213 C<$|> are not interpolated because they look like end-of-string tests.)
1214 If you want such a pattern to be compiled only once, add a C</o> after
1215 the trailing delimiter. This avoids expensive run-time recompilations,
1216 and is useful when the value you are interpolating won't change over
1217 the life of the script. However, mentioning C</o> constitutes a promise
1218 that you won't change the variables in the pattern. If you change them,
1219 Perl won't even notice. See also L<"qr/STRING/msixpo">.
1221 =item The empty pattern //
1223 If the PATTERN evaluates to the empty string, the last
1224 I<successfully> matched regular expression is used instead. In this
1225 case, only the C<g> and C<c> flags on the empty pattern is honoured -
1226 the other flags are taken from the original pattern. If no match has
1227 previously succeeded, this will (silently) act instead as a genuine
1228 empty pattern (which will always match).
1230 Note that it's possible to confuse Perl into thinking C<//> (the empty
1231 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1232 good about this, but some pathological cases might trigger this, such as
1233 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1234 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1235 will assume you meant defined-or. If you meant the empty regex, just
1236 use parentheses or spaces to disambiguate, or even prefix the empty
1237 regex with an C<m> (so C<//> becomes C<m//>).
1239 =item Matching in list context
1241 If the C</g> option is not used, C<m//> in list context returns a
1242 list consisting of the subexpressions matched by the parentheses in the
1243 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1244 also set, and that this differs from Perl 4's behavior.) When there are
1245 no parentheses in the pattern, the return value is the list C<(1)> for
1246 success. With or without parentheses, an empty list is returned upon
1251 open(TTY, '/dev/tty');
1252 <TTY> =~ /^y/i && foo(); # do foo if desired
1254 if (/Version: *([0-9.]*)/) { $version = $1; }
1256 next if m#^/usr/spool/uucp#;
1261 print if /$arg/o; # compile only once
1264 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1266 This last example splits $foo into the first two words and the
1267 remainder of the line, and assigns those three fields to $F1, $F2, and
1268 $Etc. The conditional is true if any variables were assigned, i.e., if
1269 the pattern matched.
1271 The C</g> modifier specifies global pattern matching--that is,
1272 matching as many times as possible within the string. How it behaves
1273 depends on the context. In list context, it returns a list of the
1274 substrings matched by any capturing parentheses in the regular
1275 expression. If there are no parentheses, it returns a list of all
1276 the matched strings, as if there were parentheses around the whole
1279 In scalar context, each execution of C<m//g> finds the next match,
1280 returning true if it matches, and false if there is no further match.
1281 The position after the last match can be read or set using the pos()
1282 function; see L<perlfunc/pos>. A failed match normally resets the
1283 search position to the beginning of the string, but you can avoid that
1284 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1285 string also resets the search position.
1289 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1290 zero-width assertion that matches the exact position where the previous
1291 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1292 still anchors at pos(), but the match is of course only attempted once.
1293 Using C<\G> without C</g> on a target string that has not previously had a
1294 C</g> match applied to it is the same as using the C<\A> assertion to match
1295 the beginning of the string. Note also that, currently, C<\G> is only
1296 properly supported when anchored at the very beginning of the pattern.
1301 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1305 while (defined($paragraph = <>)) {
1306 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1310 print "$sentences\n";
1312 # using m//gc with \G
1316 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1318 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1320 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1322 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1324 The last example should print:
1334 Notice that the final match matched C<q> instead of C<p>, which a match
1335 without the C<\G> anchor would have done. Also note that the final match
1336 did not update C<pos>. C<pos> is only updated on a C</g> match. If the
1337 final match did indeed match C<p>, it's a good bet that you're running an
1338 older (pre-5.6.0) Perl.
1340 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1341 combine several regexps like this to process a string part-by-part,
1342 doing different actions depending on which regexp matched. Each
1343 regexp tries to match where the previous one leaves off.
1346 $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx";
1350 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1351 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1352 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1353 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1354 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1355 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1356 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1357 print ". That's all!\n";
1360 Here is the output (split into several lines):
1362 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1363 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1364 lowercase lowercase line-noise lowercase lowercase line-noise
1365 MiXeD line-noise. That's all!
1370 This is just like the C</pattern/> search, except that it matches only
1371 once between calls to the reset() operator. This is a useful
1372 optimization when you want to see only the first occurrence of
1373 something in each file of a set of files, for instance. Only C<??>
1374 patterns local to the current package are reset.
1378 # blank line between header and body
1381 reset if eof; # clear ?? status for next file
1384 This usage is vaguely deprecated, which means it just might possibly
1385 be removed in some distant future version of Perl, perhaps somewhere
1386 around the year 2168.
1388 =item s/PATTERN/REPLACEMENT/msixpogce
1389 X<substitute> X<substitution> X<replace> X<regexp, replace>
1390 X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e>
1392 Searches a string for a pattern, and if found, replaces that pattern
1393 with the replacement text and returns the number of substitutions
1394 made. Otherwise it returns false (specifically, the empty string).
1396 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1397 variable is searched and modified. (The string specified with C<=~> must
1398 be scalar variable, an array element, a hash element, or an assignment
1399 to one of those, i.e., an lvalue.)
1401 If the delimiter chosen is a single quote, no interpolation is
1402 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1403 PATTERN contains a $ that looks like a variable rather than an
1404 end-of-string test, the variable will be interpolated into the pattern
1405 at run-time. If you want the pattern compiled only once the first time
1406 the variable is interpolated, use the C</o> option. If the pattern
1407 evaluates to the empty string, the last successfully executed regular
1408 expression is used instead. See L<perlre> for further explanation on these.
1409 See L<perllocale> for discussion of additional considerations that apply
1410 when C<use locale> is in effect.
1412 Options are as with m// with the addition of the following replacement
1415 e Evaluate the right side as an expression.
1416 ee Evaluate the right side as a string then eval the result
1418 Any non-whitespace delimiter may replace the slashes. Add space after
1419 the C<s> when using a character allowed in identifiers. If single quotes
1420 are used, no interpretation is done on the replacement string (the C</e>
1421 modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks
1422 as normal delimiters; the replacement text is not evaluated as a command.
1423 If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
1424 its own pair of quotes, which may or may not be bracketing quotes, e.g.,
1425 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1426 replacement portion to be treated as a full-fledged Perl expression
1427 and evaluated right then and there. It is, however, syntax checked at
1428 compile-time. A second C<e> modifier will cause the replacement portion
1429 to be C<eval>ed before being run as a Perl expression.
1433 s/\bgreen\b/mauve/g; # don't change wintergreen
1435 $path =~ s|/usr/bin|/usr/local/bin|;
1437 s/Login: $foo/Login: $bar/; # run-time pattern
1439 ($foo = $bar) =~ s/this/that/; # copy first, then change
1441 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1444 s/\d+/$&*2/e; # yields 'abc246xyz'
1445 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1446 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1448 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1449 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1450 s/^=(\w+)/pod($1)/ge; # use function call
1452 # expand variables in $_, but dynamics only, using
1453 # symbolic dereferencing
1456 # Add one to the value of any numbers in the string
1459 # This will expand any embedded scalar variable
1460 # (including lexicals) in $_ : First $1 is interpolated
1461 # to the variable name, and then evaluated
1464 # Delete (most) C comments.
1466 /\* # Match the opening delimiter.
1467 .*? # Match a minimal number of characters.
1468 \*/ # Match the closing delimiter.
1471 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1473 for ($variable) { # trim whitespace in $variable, cheap
1478 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1480 Note the use of $ instead of \ in the last example. Unlike
1481 B<sed>, we use the \<I<digit>> form in only the left hand side.
1482 Anywhere else it's $<I<digit>>.
1484 Occasionally, you can't use just a C</g> to get all the changes
1485 to occur that you might want. Here are two common cases:
1487 # put commas in the right places in an integer
1488 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1490 # expand tabs to 8-column spacing
1491 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1495 =head2 Quote-Like Operators
1496 X<operator, quote-like>
1501 X<q> X<quote, single> X<'> X<''>
1505 A single-quoted, literal string. A backslash represents a backslash
1506 unless followed by the delimiter or another backslash, in which case
1507 the delimiter or backslash is interpolated.
1509 $foo = q!I said, "You said, 'She said it.'"!;
1510 $bar = q('This is it.');
1511 $baz = '\n'; # a two-character string
1514 X<qq> X<quote, double> X<"> X<"">
1518 A double-quoted, interpolated string.
1521 (*** The previous line contains the naughty word "$1".\n)
1522 if /\b(tcl|java|python)\b/i; # :-)
1523 $baz = "\n"; # a one-character string
1526 X<qx> X<`> X<``> X<backtick>
1530 A string which is (possibly) interpolated and then executed as a
1531 system command with C</bin/sh> or its equivalent. Shell wildcards,
1532 pipes, and redirections will be honored. The collected standard
1533 output of the command is returned; standard error is unaffected. In
1534 scalar context, it comes back as a single (potentially multi-line)
1535 string, or undef if the command failed. In list context, returns a
1536 list of lines (however you've defined lines with $/ or
1537 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1539 Because backticks do not affect standard error, use shell file descriptor
1540 syntax (assuming the shell supports this) if you care to address this.
1541 To capture a command's STDERR and STDOUT together:
1543 $output = `cmd 2>&1`;
1545 To capture a command's STDOUT but discard its STDERR:
1547 $output = `cmd 2>/dev/null`;
1549 To capture a command's STDERR but discard its STDOUT (ordering is
1552 $output = `cmd 2>&1 1>/dev/null`;
1554 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1555 but leave its STDOUT to come out the old STDERR:
1557 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1559 To read both a command's STDOUT and its STDERR separately, it's easiest
1560 to redirect them separately to files, and then read from those files
1561 when the program is done:
1563 system("program args 1>program.stdout 2>program.stderr");
1565 The STDIN filehandle used by the command is inherited from Perl's STDIN.
1568 open BLAM, "blam" || die "Can't open: $!";
1569 open STDIN, "<&BLAM";
1572 will print the sorted contents of the file "blam".
1574 Using single-quote as a delimiter protects the command from Perl's
1575 double-quote interpolation, passing it on to the shell instead:
1577 $perl_info = qx(ps $$); # that's Perl's $$
1578 $shell_info = qx'ps $$'; # that's the new shell's $$
1580 How that string gets evaluated is entirely subject to the command
1581 interpreter on your system. On most platforms, you will have to protect
1582 shell metacharacters if you want them treated literally. This is in
1583 practice difficult to do, as it's unclear how to escape which characters.
1584 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1585 to emulate backticks safely.
1587 On some platforms (notably DOS-like ones), the shell may not be
1588 capable of dealing with multiline commands, so putting newlines in
1589 the string may not get you what you want. You may be able to evaluate
1590 multiple commands in a single line by separating them with the command
1591 separator character, if your shell supports that (e.g. C<;> on many Unix
1592 shells; C<&> on the Windows NT C<cmd> shell).
1594 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1595 output before starting the child process, but this may not be supported
1596 on some platforms (see L<perlport>). To be safe, you may need to set
1597 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1598 C<IO::Handle> on any open handles.
1600 Beware that some command shells may place restrictions on the length
1601 of the command line. You must ensure your strings don't exceed this
1602 limit after any necessary interpolations. See the platform-specific
1603 release notes for more details about your particular environment.
1605 Using this operator can lead to programs that are difficult to port,
1606 because the shell commands called vary between systems, and may in
1607 fact not be present at all. As one example, the C<type> command under
1608 the POSIX shell is very different from the C<type> command under DOS.
1609 That doesn't mean you should go out of your way to avoid backticks
1610 when they're the right way to get something done. Perl was made to be
1611 a glue language, and one of the things it glues together is commands.
1612 Just understand what you're getting yourself into.
1614 See L</"I/O Operators"> for more discussion.
1617 X<qw> X<quote, list> X<quote, words>
1619 Evaluates to a list of the words extracted out of STRING, using embedded
1620 whitespace as the word delimiters. It can be understood as being roughly
1623 split(' ', q/STRING/);
1625 the differences being that it generates a real list at compile time, and
1626 in scalar context it returns the last element in the list. So
1631 is semantically equivalent to the list:
1635 Some frequently seen examples:
1637 use POSIX qw( setlocale localeconv )
1638 @EXPORT = qw( foo bar baz );
1640 A common mistake is to try to separate the words with comma or to
1641 put comments into a multi-line C<qw>-string. For this reason, the
1642 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1643 produces warnings if the STRING contains the "," or the "#" character.
1646 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1647 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
1649 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1651 Transliterates all occurrences of the characters found in the search list
1652 with the corresponding character in the replacement list. It returns
1653 the number of characters replaced or deleted. If no string is
1654 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1655 string specified with =~ must be a scalar variable, an array element, a
1656 hash element, or an assignment to one of those, i.e., an lvalue.)
1658 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1659 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1660 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1661 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1662 its own pair of quotes, which may or may not be bracketing quotes,
1663 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1665 Note that C<tr> does B<not> do regular expression character classes
1666 such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to
1667 the tr(1) utility. If you want to map strings between lower/upper
1668 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1669 using the C<s> operator if you need regular expressions.
1671 Note also that the whole range idea is rather unportable between
1672 character sets--and even within character sets they may cause results
1673 you probably didn't expect. A sound principle is to use only ranges
1674 that begin from and end at either alphabets of equal case (a-e, A-E),
1675 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1676 character sets in full.
1680 c Complement the SEARCHLIST.
1681 d Delete found but unreplaced characters.
1682 s Squash duplicate replaced characters.
1684 If the C</c> modifier is specified, the SEARCHLIST character set
1685 is complemented. If the C</d> modifier is specified, any characters
1686 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1687 (Note that this is slightly more flexible than the behavior of some
1688 B<tr> programs, which delete anything they find in the SEARCHLIST,
1689 period.) If the C</s> modifier is specified, sequences of characters
1690 that were transliterated to the same character are squashed down
1691 to a single instance of the character.
1693 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1694 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1695 than the SEARCHLIST, the final character is replicated till it is long
1696 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1697 This latter is useful for counting characters in a class or for
1698 squashing character sequences in a class.
1702 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1704 $cnt = tr/*/*/; # count the stars in $_
1706 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1708 $cnt = tr/0-9//; # count the digits in $_
1710 tr/a-zA-Z//s; # bookkeeper -> bokeper
1712 ($HOST = $host) =~ tr/a-z/A-Z/;
1714 tr/a-zA-Z/ /cs; # change non-alphas to single space
1717 [\000-\177]; # delete 8th bit
1719 If multiple transliterations are given for a character, only the
1724 will transliterate any A to X.
1726 Because the transliteration table is built at compile time, neither
1727 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1728 interpolation. That means that if you want to use variables, you
1731 eval "tr/$oldlist/$newlist/";
1734 eval "tr/$oldlist/$newlist/, 1" or die $@;
1737 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
1739 A line-oriented form of quoting is based on the shell "here-document"
1740 syntax. Following a C<< << >> you specify a string to terminate
1741 the quoted material, and all lines following the current line down to
1742 the terminating string are the value of the item.
1744 The terminating string may be either an identifier (a word), or some
1745 quoted text. An unquoted identifier works like double quotes.
1746 There may not be a space between the C<< << >> and the identifier,
1747 unless the identifier is explicitly quoted. (If you put a space it
1748 will be treated as a null identifier, which is valid, and matches the
1749 first empty line.) The terminating string must appear by itself
1750 (unquoted and with no surrounding whitespace) on the terminating line.
1752 If the terminating string is quoted, the type of quotes used determine
1753 the treatment of the text.
1759 Double quotes indicate that the text will be interpolated using exactly
1760 the same rules as normal double quoted strings.
1763 The price is $Price.
1766 print << "EOF"; # same as above
1767 The price is $Price.
1773 Single quotes indicate the text is to be treated literally with no
1774 interpolation of its content. This is similar to single quoted
1775 strings except that backslashes have no special meaning, with C<\\>
1776 being treated as two backslashes and not one as they would in every
1777 other quoting construct.
1779 This is the only form of quoting in perl where there is no need
1780 to worry about escaping content, something that code generators
1781 can and do make good use of.
1785 The content of the here doc is treated just as it would be if the
1786 string were embedded in backticks. Thus the content is interpolated
1787 as though it were double quoted and then executed via the shell, with
1788 the results of the execution returned.
1790 print << `EOC`; # execute command and get results
1796 It is possible to stack multiple here-docs in a row:
1798 print <<"foo", <<"bar"; # you can stack them
1804 myfunc(<< "THIS", 23, <<'THAT');
1811 Just don't forget that you have to put a semicolon on the end
1812 to finish the statement, as Perl doesn't know you're not going to
1820 If you want to remove the line terminator from your here-docs,
1823 chomp($string = <<'END');
1827 If you want your here-docs to be indented with the rest of the code,
1828 you'll need to remove leading whitespace from each line manually:
1830 ($quote = <<'FINIS') =~ s/^\s+//gm;
1831 The Road goes ever on and on,
1832 down from the door where it began.
1835 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1836 the quoted material must come on the lines following the final delimiter.
1851 If the terminating identifier is on the last line of the program, you
1852 must be sure there is a newline after it; otherwise, Perl will give the
1853 warning B<Can't find string terminator "END" anywhere before EOF...>.
1855 Additionally, the quoting rules for the end of string identifier are not
1856 related to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
1857 supported in place of C<''> and C<"">, and the only interpolation is for
1858 backslashing the quoting character:
1860 print << "abc\"def";
1864 Finally, quoted strings cannot span multiple lines. The general rule is
1865 that the identifier must be a string literal. Stick with that, and you
1870 =head2 Gory details of parsing quoted constructs
1871 X<quote, gory details>
1873 When presented with something that might have several different
1874 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1875 principle to pick the most probable interpretation. This strategy
1876 is so successful that Perl programmers often do not suspect the
1877 ambivalence of what they write. But from time to time, Perl's
1878 notions differ substantially from what the author honestly meant.
1880 This section hopes to clarify how Perl handles quoted constructs.
1881 Although the most common reason to learn this is to unravel labyrinthine
1882 regular expressions, because the initial steps of parsing are the
1883 same for all quoting operators, they are all discussed together.
1885 The most important Perl parsing rule is the first one discussed
1886 below: when processing a quoted construct, Perl first finds the end
1887 of that construct, then interprets its contents. If you understand
1888 this rule, you may skip the rest of this section on the first
1889 reading. The other rules are likely to contradict the user's
1890 expectations much less frequently than this first one.
1892 Some passes discussed below are performed concurrently, but because
1893 their results are the same, we consider them individually. For different
1894 quoting constructs, Perl performs different numbers of passes, from
1895 one to four, but these passes are always performed in the same order.
1899 =item Finding the end
1901 The first pass is finding the end of the quoted construct, where
1902 the information about the delimiters is used in parsing.
1903 During this search, text between the starting and ending delimiters
1904 is copied to a safe location. The text copied gets delimiter-independent.
1906 If the construct is a here-doc, the ending delimiter is a line
1907 that has a terminating string as the content. Therefore C<<<EOF> is
1908 terminated by C<EOF> immediately followed by C<"\n"> and starting
1909 from the first column of the terminating line.
1910 When searching for the terminating line of a here-doc, nothing
1911 is skipped. In other words, lines after the here-doc syntax
1912 are compared with the terminating string line by line.
1914 For the constructs except here-docs, single characters are used as starting
1915 and ending delimiters. If the starting delimiter is an opening punctuation
1916 (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
1917 corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
1918 If the starting delimiter is an unpaired character like C</> or a closing
1919 punctuation, the ending delimiter is same as the starting delimiter.
1920 Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
1921 C<qq[]> and C<qq]]> constructs.
1923 When searching for single-character delimiters, escaped delimiters
1924 and C<\\> are skipped. For example, while searching for terminating C</>,
1925 combinations of C<\\> and C<\/> are skipped. If the delimiters are
1926 bracketing, nested pairs are also skipped. For example, while searching
1927 for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
1928 and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
1929 However, when backslashes are used as the delimiters (like C<qq\\> and
1930 C<tr\\\>), nothing is skipped.
1931 During the search for the end, backslashes that escape delimiters
1932 are removed (exactly speaking, they are not copied to the safe location).
1934 For constructs with three-part delimiters (C<s///>, C<y///>, and
1935 C<tr///>), the search is repeated once more.
1936 If the first delimiter is not an opening punctuation, three delimiters must
1937 be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
1938 terminates the left part and starts the right part at once.
1939 If the left part is delimited by bracketing punctuations (that is C<()>,
1940 C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
1941 delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces
1942 and comments are allowed between both parts, though the comment must follow
1943 at least one whitespace; otherwise a character expected as the start of
1944 the comment may be regarded as the starting delimiter of the right part.
1946 During this search no attention is paid to the semantics of the construct.
1949 "$hash{"$foo/$bar"}"
1954 bar # NOT a comment, this slash / terminated m//!
1957 do not form legal quoted expressions. The quoted part ends on the
1958 first C<"> and C</>, and the rest happens to be a syntax error.
1959 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1960 the example above is not C<m//x>, but rather C<m//> with no C</x>
1961 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1963 Also no attention is paid to C<\c\> (multichar control char syntax) during
1964 this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
1965 of C<\/>, and the following C</> is not recognized as a delimiter.
1966 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1971 The next step is interpolation in the text obtained, which is now
1972 delimiter-independent. There are multiple cases.
1978 No interpolation is performed.
1979 Note that the combination C<\\> is left intact, since escaped delimiters
1980 are not available for here-docs.
1982 =item C<m''>, the pattern of C<s'''>
1984 No interpolation is performed at this stage.
1985 Any backslashed sequences including C<\\> are treated at the stage
1986 to L</"parsing regular expressions">.
1988 =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
1990 The only interpolation is removal of C<\> from pairs of C<\\>.
1991 Therefore C<-> in C<tr'''> and C<y'''> is treated literally
1992 as a hyphen and no character range is available.
1993 C<\1> in the replacement of C<s'''> does not work as C<$1>.
1995 =item C<tr///>, C<y///>
1997 No variable interpolation occurs. String modifying combinations for
1998 case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
1999 The other escape sequences such as C<\200> and C<\t> and backslashed
2000 characters such as C<\\> and C<\-> are converted to appropriate literals.
2001 The character C<-> is treated specially and therefore C<\-> is treated
2004 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
2006 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
2007 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
2008 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
2009 The other escape sequences such as C<\200> and C<\t> and backslashed
2010 characters such as C<\\> and C<\-> are replaced with appropriate
2013 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2014 is interpolated in the usual way. Something like C<"\Q\\E"> has
2015 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
2016 result is the same as for C<"\\\\E">. As a general rule, backslashes
2017 between C<\Q> and C<\E> may lead to counterintuitive results. So,
2018 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2019 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2024 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2026 Interpolated scalars and arrays are converted internally to the C<join> and
2027 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
2029 $foo . " XXX '" . (join $", @arr) . "'";
2031 All operations above are performed simultaneously, left to right.
2033 Because the result of C<"\Q STRING \E"> has all metacharacters
2034 quoted, there is no way to insert a literal C<$> or C<@> inside a
2035 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
2036 C<"\\\$">; if not, it is interpreted as the start of an interpolated
2039 Note also that the interpolation code needs to make a decision on
2040 where the interpolated scalar ends. For instance, whether
2041 C<< "a $b -> {c}" >> really means:
2043 "a " . $b . " -> {c}";
2049 Most of the time, the longest possible text that does not include
2050 spaces between components and which contains matching braces or
2051 brackets. because the outcome may be determined by voting based
2052 on heuristic estimators, the result is not strictly predictable.
2053 Fortunately, it's usually correct for ambiguous cases.
2055 =item the replacement of C<s///>
2057 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
2058 happens as with C<qq//> constructs.
2060 It is at this step that C<\1> is begrudgingly converted to C<$1> in
2061 the replacement text of C<s///>, in order to correct the incorrigible
2062 I<sed> hackers who haven't picked up the saner idiom yet. A warning
2063 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2064 (that is, the C<$^W> variable) was set.
2066 =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2068 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
2069 and interpolation happens (almost) as with C<qq//> constructs.
2071 However any other combinations of C<\> followed by a character
2072 are not substituted but only skipped, in order to parse them
2073 as regular expressions at the following step.
2074 As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
2075 treated as an array symbol (for example C<@foo>),
2076 even though the same text in C<qq//> gives interpolation of C<\c@>.
2078 Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
2079 a C<#>-comment in a C<//x>-regular expression, no processing is
2080 performed whatsoever. This is the first step at which the presence
2081 of the C<//x> modifier is relevant.
2083 Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2084 and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2085 voted (by several different estimators) to be either an array element
2086 or C<$var> followed by an RE alternative. This is where the notation
2087 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2088 array element C<-9>, not as a regular expression from the variable
2089 C<$arr> followed by a digit, which would be the interpretation of
2090 C</$arr[0-9]/>. Since voting among different estimators may occur,
2091 the result is not predictable.
2093 The lack of processing of C<\\> creates specific restrictions on
2094 the post-processed text. If the delimiter is C</>, one cannot get
2095 the combination C<\/> into the result of this step. C</> will
2096 finish the regular expression, C<\/> will be stripped to C</> on
2097 the previous step, and C<\\/> will be left as is. Because C</> is
2098 equivalent to C<\/> inside a regular expression, this does not
2099 matter unless the delimiter happens to be character special to the
2100 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2101 alphanumeric char, as in:
2105 In the RE above, which is intentionally obfuscated for illustration, the
2106 delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
2107 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
2108 reason you're encouraged to restrict your delimiters to non-alphanumeric,
2109 non-whitespace choices.
2113 This step is the last one for all constructs except regular expressions,
2114 which are processed further.
2116 =item parsing regular expressions
2119 Previous steps were performed during the compilation of Perl code,
2120 but this one happens at run time, although it may be optimized to
2121 be calculated at compile time if appropriate. After preprocessing
2122 described above, and possibly after evaluation if concatenation,
2123 joining, casing translation, or metaquoting are involved, the
2124 resulting I<string> is passed to the RE engine for compilation.
2126 Whatever happens in the RE engine might be better discussed in L<perlre>,
2127 but for the sake of continuity, we shall do so here.
2129 This is another step where the presence of the C<//x> modifier is
2130 relevant. The RE engine scans the string from left to right and
2131 converts it to a finite automaton.
2133 Backslashed characters are either replaced with corresponding
2134 literal strings (as with C<\{>), or else they generate special nodes
2135 in the finite automaton (as with C<\b>). Characters special to the
2136 RE engine (such as C<|>) generate corresponding nodes or groups of
2137 nodes. C<(?#...)> comments are ignored. All the rest is either
2138 converted to literal strings to match, or else is ignored (as is
2139 whitespace and C<#>-style comments if C<//x> is present).
2141 Parsing of the bracketed character class construct, C<[...]>, is
2142 rather different than the rule used for the rest of the pattern.
2143 The terminator of this construct is found using the same rules as
2144 for finding the terminator of a C<{}>-delimited construct, the only
2145 exception being that C<]> immediately following C<[> is treated as
2146 though preceded by a backslash. Similarly, the terminator of
2147 C<(?{...})> is found using the same rules as for finding the
2148 terminator of a C<{}>-delimited construct.
2150 It is possible to inspect both the string given to RE engine and the
2151 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2152 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
2153 switch documented in L<perlrun/"Command Switches">.
2155 =item Optimization of regular expressions
2156 X<regexp, optimization>
2158 This step is listed for completeness only. Since it does not change
2159 semantics, details of this step are not documented and are subject
2160 to change without notice. This step is performed over the finite
2161 automaton that was generated during the previous pass.
2163 It is at this stage that C<split()> silently optimizes C</^/> to
2168 =head2 I/O Operators
2169 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2172 There are several I/O operators you should know about.
2174 A string enclosed by backticks (grave accents) first undergoes
2175 double-quote interpolation. It is then interpreted as an external
2176 command, and the output of that command is the value of the
2177 backtick string, like in a shell. In scalar context, a single string
2178 consisting of all output is returned. In list context, a list of
2179 values is returned, one per line of output. (You can set C<$/> to use
2180 a different line terminator.) The command is executed each time the
2181 pseudo-literal is evaluated. The status value of the command is
2182 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2183 Unlike in B<csh>, no translation is done on the return data--newlines
2184 remain newlines. Unlike in any of the shells, single quotes do not
2185 hide variable names in the command from interpretation. To pass a
2186 literal dollar-sign through to the shell you need to hide it with a
2187 backslash. The generalized form of backticks is C<qx//>. (Because
2188 backticks always undergo shell expansion as well, see L<perlsec> for
2190 X<qx> X<`> X<``> X<backtick> X<glob>
2192 In scalar context, evaluating a filehandle in angle brackets yields
2193 the next line from that file (the newline, if any, included), or
2194 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2195 (sometimes known as file-slurp mode) and the file is empty, it
2196 returns C<''> the first time, followed by C<undef> subsequently.
2198 Ordinarily you must assign the returned value to a variable, but
2199 there is one situation where an automatic assignment happens. If
2200 and only if the input symbol is the only thing inside the conditional
2201 of a C<while> statement (even if disguised as a C<for(;;)> loop),
2202 the value is automatically assigned to the global variable $_,
2203 destroying whatever was there previously. (This may seem like an
2204 odd thing to you, but you'll use the construct in almost every Perl
2205 script you write.) The $_ variable is not implicitly localized.
2206 You'll have to put a C<local $_;> before the loop if you want that
2209 The following lines are equivalent:
2211 while (defined($_ = <STDIN>)) { print; }
2212 while ($_ = <STDIN>) { print; }
2213 while (<STDIN>) { print; }
2214 for (;<STDIN>;) { print; }
2215 print while defined($_ = <STDIN>);
2216 print while ($_ = <STDIN>);
2217 print while <STDIN>;
2219 This also behaves similarly, but avoids $_ :
2221 while (my $line = <STDIN>) { print $line }
2223 In these loop constructs, the assigned value (whether assignment
2224 is automatic or explicit) is then tested to see whether it is
2225 defined. The defined test avoids problems where line has a string
2226 value that would be treated as false by Perl, for example a "" or
2227 a "0" with no trailing newline. If you really mean for such values
2228 to terminate the loop, they should be tested for explicitly:
2230 while (($_ = <STDIN>) ne '0') { ... }
2231 while (<STDIN>) { last unless $_; ... }
2233 In other boolean contexts, C<< <filehandle> >> without an
2234 explicit C<defined> test or comparison elicits a warning if the
2235 C<use warnings> pragma or the B<-w>
2236 command-line switch (the C<$^W> variable) is in effect.
2238 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2239 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2240 in packages, where they would be interpreted as local identifiers
2241 rather than global.) Additional filehandles may be created with
2242 the open() function, amongst others. See L<perlopentut> and
2243 L<perlfunc/open> for details on this.
2244 X<stdin> X<stdout> X<sterr>
2246 If a <FILEHANDLE> is used in a context that is looking for
2247 a list, a list comprising all input lines is returned, one line per
2248 list element. It's easy to grow to a rather large data space this
2249 way, so use with care.
2251 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2252 See L<perlfunc/readline>.
2254 The null filehandle <> is special: it can be used to emulate the
2255 behavior of B<sed> and B<awk>. Input from <> comes either from
2256 standard input, or from each file listed on the command line. Here's
2257 how it works: the first time <> is evaluated, the @ARGV array is
2258 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2259 gives you standard input. The @ARGV array is then processed as a list
2260 of filenames. The loop
2263 ... # code for each line
2266 is equivalent to the following Perl-like pseudo code:
2268 unshift(@ARGV, '-') unless @ARGV;
2269 while ($ARGV = shift) {
2272 ... # code for each line
2276 except that it isn't so cumbersome to say, and will actually work.
2277 It really does shift the @ARGV array and put the current filename
2278 into the $ARGV variable. It also uses filehandle I<ARGV>
2279 internally. <> is just a synonym for <ARGV>, which
2280 is magical. (The pseudo code above doesn't work because it treats
2281 <ARGV> as non-magical.)
2283 Since the null filehandle uses the two argument form of L<perlfunc/open>
2284 it interprets special characters, so if you have a script like this:
2290 and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2291 pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2292 If you want all items in C<@ARGV> to be interpreted as file names, you
2293 can use the module C<ARGV::readonly> from CPAN.
2295 You can modify @ARGV before the first <> as long as the array ends up
2296 containing the list of filenames you really want. Line numbers (C<$.>)
2297 continue as though the input were one big happy file. See the example
2298 in L<perlfunc/eof> for how to reset line numbers on each file.
2300 If you want to set @ARGV to your own list of files, go right ahead.
2301 This sets @ARGV to all plain text files if no @ARGV was given:
2303 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2305 You can even set them to pipe commands. For example, this automatically
2306 filters compressed arguments through B<gzip>:
2308 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2310 If you want to pass switches into your script, you can use one of the
2311 Getopts modules or put a loop on the front like this:
2313 while ($_ = $ARGV[0], /^-/) {
2316 if (/^-D(.*)/) { $debug = $1 }
2317 if (/^-v/) { $verbose++ }
2318 # ... # other switches
2322 # ... # code for each line
2325 The <> symbol will return C<undef> for end-of-file only once.
2326 If you call it again after this, it will assume you are processing another
2327 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2329 If what the angle brackets contain is a simple scalar variable (e.g.,
2330 <$foo>), then that variable contains the name of the
2331 filehandle to input from, or its typeglob, or a reference to the
2337 If what's within the angle brackets is neither a filehandle nor a simple
2338 scalar variable containing a filehandle name, typeglob, or typeglob
2339 reference, it is interpreted as a filename pattern to be globbed, and
2340 either a list of filenames or the next filename in the list is returned,
2341 depending on context. This distinction is determined on syntactic
2342 grounds alone. That means C<< <$x> >> is always a readline() from
2343 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2344 That's because $x is a simple scalar variable, but C<$hash{key}> is
2345 not--it's a hash element. Even C<< <$x > >> (note the extra space)
2346 is treated as C<glob("$x ")>, not C<readline($x)>.
2348 One level of double-quote interpretation is done first, but you can't
2349 say C<< <$foo> >> because that's an indirect filehandle as explained
2350 in the previous paragraph. (In older versions of Perl, programmers
2351 would insert curly brackets to force interpretation as a filename glob:
2352 C<< <${foo}> >>. These days, it's considered cleaner to call the
2353 internal function directly as C<glob($foo)>, which is probably the right
2354 way to have done it in the first place.) For example:
2360 is roughly equivalent to:
2362 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2368 except that the globbing is actually done internally using the standard
2369 C<File::Glob> extension. Of course, the shortest way to do the above is:
2373 A (file)glob evaluates its (embedded) argument only when it is
2374 starting a new list. All values must be read before it will start
2375 over. In list context, this isn't important because you automatically
2376 get them all anyway. However, in scalar context the operator returns
2377 the next value each time it's called, or C<undef> when the list has
2378 run out. As with filehandle reads, an automatic C<defined> is
2379 generated when the glob occurs in the test part of a C<while>,
2380 because legal glob returns (e.g. a file called F<0>) would otherwise
2381 terminate the loop. Again, C<undef> is returned only once. So if
2382 you're expecting a single value from a glob, it is much better to
2385 ($file) = <blurch*>;
2391 because the latter will alternate between returning a filename and
2394 If you're trying to do variable interpolation, it's definitely better
2395 to use the glob() function, because the older notation can cause people
2396 to become confused with the indirect filehandle notation.
2398 @files = glob("$dir/*.[ch]");
2399 @files = glob($files[$i]);
2401 =head2 Constant Folding
2402 X<constant folding> X<folding>
2404 Like C, Perl does a certain amount of expression evaluation at
2405 compile time whenever it determines that all arguments to an
2406 operator are static and have no side effects. In particular, string
2407 concatenation happens at compile time between literals that don't do
2408 variable substitution. Backslash interpolation also happens at
2409 compile time. You can say
2411 'Now is the time for all' . "\n" .
2412 'good men to come to.'
2414 and this all reduces to one string internally. Likewise, if
2417 foreach $file (@filenames) {
2418 if (-s $file > 5 + 100 * 2**16) { }
2421 the compiler will precompute the number which that expression
2422 represents so that the interpreter won't have to.
2427 Perl doesn't officially have a no-op operator, but the bare constants
2428 C<0> and C<1> are special-cased to not produce a warning in a void
2429 context, so you can for example safely do
2433 =head2 Bitwise String Operators
2434 X<operator, bitwise, string>
2436 Bitstrings of any size may be manipulated by the bitwise operators
2439 If the operands to a binary bitwise op are strings of different
2440 sizes, B<|> and B<^> ops act as though the shorter operand had
2441 additional zero bits on the right, while the B<&> op acts as though
2442 the longer operand were truncated to the length of the shorter.
2443 The granularity for such extension or truncation is one or more
2446 # ASCII-based examples
2447 print "j p \n" ^ " a h"; # prints "JAPH\n"
2448 print "JA" | " ph\n"; # prints "japh\n"
2449 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2450 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2452 If you are intending to manipulate bitstrings, be certain that
2453 you're supplying bitstrings: If an operand is a number, that will imply
2454 a B<numeric> bitwise operation. You may explicitly show which type of
2455 operation you intend by using C<""> or C<0+>, as in the examples below.
2457 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2458 $foo = '150' | 105; # yields 255
2459 $foo = 150 | '105'; # yields 255
2460 $foo = '150' | '105'; # yields string '155' (under ASCII)
2462 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2463 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2465 See L<perlfunc/vec> for information on how to manipulate individual bits
2468 =head2 Integer Arithmetic
2471 By default, Perl assumes that it must do most of its arithmetic in
2472 floating point. But by saying
2476 you may tell the compiler that it's okay to use integer operations
2477 (if it feels like it) from here to the end of the enclosing BLOCK.
2478 An inner BLOCK may countermand this by saying
2482 which lasts until the end of that BLOCK. Note that this doesn't
2483 mean everything is only an integer, merely that Perl may use integer
2484 operations if it is so inclined. For example, even under C<use
2485 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2488 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2489 and ">>") always produce integral results. (But see also
2490 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2491 them. By default, their results are interpreted as unsigned integers, but
2492 if C<use integer> is in effect, their results are interpreted
2493 as signed integers. For example, C<~0> usually evaluates to a large
2494 integral value. However, C<use integer; ~0> is C<-1> on two's-complement
2497 =head2 Floating-point Arithmetic
2498 X<floating-point> X<floating point> X<float> X<real>
2500 While C<use integer> provides integer-only arithmetic, there is no
2501 analogous mechanism to provide automatic rounding or truncation to a
2502 certain number of decimal places. For rounding to a certain number
2503 of digits, sprintf() or printf() is usually the easiest route.
2506 Floating-point numbers are only approximations to what a mathematician
2507 would call real numbers. There are infinitely more reals than floats,
2508 so some corners must be cut. For example:
2510 printf "%.20g\n", 123456789123456789;
2511 # produces 123456789123456784
2513 Testing for exact equality of floating-point equality or inequality is
2514 not a good idea. Here's a (relatively expensive) work-around to compare
2515 whether two floating-point numbers are equal to a particular number of
2516 decimal places. See Knuth, volume II, for a more robust treatment of
2520 my ($X, $Y, $POINTS) = @_;
2522 $tX = sprintf("%.${POINTS}g", $X);
2523 $tY = sprintf("%.${POINTS}g", $Y);
2527 The POSIX module (part of the standard perl distribution) implements
2528 ceil(), floor(), and other mathematical and trigonometric functions.
2529 The Math::Complex module (part of the standard perl distribution)
2530 defines mathematical functions that work on both the reals and the
2531 imaginary numbers. Math::Complex not as efficient as POSIX, but
2532 POSIX can't work with complex numbers.
2534 Rounding in financial applications can have serious implications, and
2535 the rounding method used should be specified precisely. In these
2536 cases, it probably pays not to trust whichever system rounding is
2537 being used by Perl, but to instead implement the rounding function you
2540 =head2 Bigger Numbers
2541 X<number, arbitrary precision>
2543 The standard Math::BigInt and Math::BigFloat modules provide
2544 variable-precision arithmetic and overloaded operators, although
2545 they're currently pretty slow. At the cost of some space and
2546 considerable speed, they avoid the normal pitfalls associated with
2547 limited-precision representations.
2550 $x = Math::BigInt->new('123456789123456789');
2553 # prints +15241578780673678515622620750190521
2555 There are several modules that let you calculate with (bound only by
2556 memory and cpu-time) unlimited or fixed precision. There are also
2557 some non-standard modules that provide faster implementations via
2558 external C libraries.
2560 Here is a short, but incomplete summary:
2562 Math::Fraction big, unlimited fractions like 9973 / 12967
2563 Math::String treat string sequences like numbers
2564 Math::FixedPrecision calculate with a fixed precision
2565 Math::Currency for currency calculations
2566 Bit::Vector manipulate bit vectors fast (uses C)
2567 Math::BigIntFast Bit::Vector wrapper for big numbers
2568 Math::Pari provides access to the Pari C library
2569 Math::BigInteger uses an external C library
2570 Math::Cephes uses external Cephes C library (no big numbers)
2571 Math::Cephes::Fraction fractions via the Cephes library
2572 Math::GMP another one using an external C library