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 (exluding "+" 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 $_. When used in scalar context, the return value generally indicates the
238 success of the operation. Behavior in list context depends on the particular
239 operator. See L</"Regexp Quote-Like Operators"> for details and
240 L<perlretut> for examples using these operators.
242 If the right argument is an expression rather than a search pattern,
243 substitution, or transliteration, it is interpreted as a search pattern at run
244 time. Note that this means that its contents will be interpolated twice, so
248 is not ok, as the regex engine will end up trying to compile the
249 pattern C<\>, which it will consider a syntax error.
251 Binary "!~" is just like "=~" except the return value is negated in
254 =head2 Multiplicative Operators
255 X<operator, multiplicative>
257 Binary "*" multiplies two numbers.
260 Binary "/" divides two numbers.
263 Binary "%" computes the modulus of two numbers. Given integer
264 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
265 C<$a> minus the largest multiple of C<$b> that is not greater than
266 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
267 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
268 result will be less than or equal to zero). If the operands
269 C<$a> and C<$b> are floating point values and the absolute value of
270 C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
271 the integer portion of C<$a> and C<$b> will be used in the operation
272 (Note: here C<UV_MAX> means the maximum of the unsigned integer type).
273 If the absolute value of the right operand (C<abs($b)>) is greater than
274 or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
275 C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
276 integer that makes C<$r> should have the same sign as the right operand
277 C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
278 and the absolute value less than that of C<$b>.
279 Note that when C<use integer> is in scope, "%" gives you direct access
280 to the modulus operator as implemented by your C compiler. This
281 operator is not as well defined for negative operands, but it will
283 X<%> X<remainder> X<modulus> X<mod>
285 Binary "x" is the repetition operator. In scalar context or if the left
286 operand is not enclosed in parentheses, it returns a string consisting
287 of the left operand repeated the number of times specified by the right
288 operand. In list context, if the left operand is enclosed in
289 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
290 If the right operand is zero or negative, it returns an empty string
291 or an empty list, depending on the context.
294 print '-' x 80; # print row of dashes
296 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
298 @ones = (1) x 80; # a list of 80 1's
299 @ones = (5) x @ones; # set all elements to 5
302 =head2 Additive Operators
303 X<operator, additive>
305 Binary "+" returns the sum of two numbers.
308 Binary "-" returns the difference of two numbers.
311 Binary "." concatenates two strings.
312 X<string, concatenation> X<concatenation>
313 X<cat> X<concat> X<concatenate> X<.>
315 =head2 Shift Operators
316 X<shift operator> X<operator, shift> X<<< << >>>
317 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
318 X<shl> X<shr> X<shift, right> X<shift, left>
320 Binary "<<" returns the value of its left argument shifted left by the
321 number of bits specified by the right argument. Arguments should be
322 integers. (See also L<Integer Arithmetic>.)
324 Binary ">>" returns the value of its left argument shifted right by
325 the number of bits specified by the right argument. Arguments should
326 be integers. (See also L<Integer Arithmetic>.)
328 Note that both "<<" and ">>" in Perl are implemented directly using
329 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
330 in force then signed C integers are used, else unsigned C integers are
331 used. Either way, the implementation isn't going to generate results
332 larger than the size of the integer type Perl was built with (32 bits
335 The result of overflowing the range of the integers is undefined
336 because it is undefined also in C. In other words, using 32-bit
337 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
338 of bits is also undefined.
340 =head2 Named Unary Operators
341 X<operator, named unary>
343 The various named unary operators are treated as functions with one
344 argument, with optional parentheses.
346 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
347 is followed by a left parenthesis as the next token, the operator and
348 arguments within parentheses are taken to be of highest precedence,
349 just like a normal function call. For example,
350 because named unary operators are higher precedence than ||:
352 chdir $foo || die; # (chdir $foo) || die
353 chdir($foo) || die; # (chdir $foo) || die
354 chdir ($foo) || die; # (chdir $foo) || die
355 chdir +($foo) || die; # (chdir $foo) || die
357 but, because * is higher precedence than named operators:
359 chdir $foo * 20; # chdir ($foo * 20)
360 chdir($foo) * 20; # (chdir $foo) * 20
361 chdir ($foo) * 20; # (chdir $foo) * 20
362 chdir +($foo) * 20; # chdir ($foo * 20)
364 rand 10 * 20; # rand (10 * 20)
365 rand(10) * 20; # (rand 10) * 20
366 rand (10) * 20; # (rand 10) * 20
367 rand +(10) * 20; # rand (10 * 20)
369 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
370 treated like named unary operators, but they don't follow this functional
371 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
372 equivalent to C<-f "$file.bak">.
373 X<-X> X<filetest> X<operator, filetest>
375 See also L<"Terms and List Operators (Leftward)">.
377 =head2 Relational Operators
378 X<relational operator> X<operator, relational>
380 Binary "<" returns true if the left argument is numerically less than
384 Binary ">" returns true if the left argument is numerically greater
385 than the right argument.
388 Binary "<=" returns true if the left argument is numerically less than
389 or equal to the right argument.
392 Binary ">=" returns true if the left argument is numerically greater
393 than or equal to the right argument.
396 Binary "lt" returns true if the left argument is stringwise less than
400 Binary "gt" returns true if the left argument is stringwise greater
401 than the right argument.
404 Binary "le" returns true if the left argument is stringwise less than
405 or equal to the right argument.
408 Binary "ge" returns true if the left argument is stringwise greater
409 than or equal to the right argument.
412 =head2 Equality Operators
413 X<equality> X<equal> X<equals> X<operator, equality>
415 Binary "==" returns true if the left argument is numerically equal to
419 Binary "!=" returns true if the left argument is numerically not equal
420 to the right argument.
423 Binary "<=>" returns -1, 0, or 1 depending on whether the left
424 argument is numerically less than, equal to, or greater than the right
425 argument. If your platform supports NaNs (not-a-numbers) as numeric
426 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
427 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
428 returns true, as does NaN != anything else. If your platform doesn't
429 support NaNs then NaN is just a string with numeric value 0.
430 X<< <=> >> X<spaceship>
432 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
433 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
435 Binary "eq" returns true if the left argument is stringwise equal to
439 Binary "ne" returns true if the left argument is stringwise not equal
440 to the right argument.
443 Binary "cmp" returns -1, 0, or 1 depending on whether the left
444 argument is stringwise less than, equal to, or greater than the right
448 Binary "~~" does a smart match between its arguments. Smart matching
449 is described in L<perlsyn/"Smart matching in detail">.
450 This operator is only available if you enable the "~~" feature:
451 see L<feature> for more information.
454 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
455 by the current locale if C<use locale> is in effect. See L<perllocale>.
458 X<operator, bitwise, and> X<bitwise and> X<&>
460 Binary "&" returns its operands ANDed together bit by bit.
461 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
463 Note that "&" has lower priority than relational operators, so for example
464 the brackets are essential in a test like
466 print "Even\n" if ($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<&&>, C<//> and C<||> when used for
526 control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
527 The short-circuit behavior is identical. The precedence of "and", "err"
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 auto-increment,
562 In scalar context, ".." returns a boolean value. The operator is
563 bistable, like a flip-flop, and emulates the line-range (comma) operator
564 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
565 own boolean state. It is false as long as its left operand is false.
566 Once the left operand is true, the range operator stays true until the
567 right operand is true, I<AFTER> which the range operator becomes false
568 again. It doesn't become false till the next time the range operator is
569 evaluated. It can test the right operand and become false on the same
570 evaluation it became true (as in B<awk>), but it still returns true once.
571 If you don't want it to test the right operand till the next
572 evaluation, as in B<sed>, just use three dots ("...") instead of
573 two. In all other regards, "..." behaves just like ".." does.
575 The right operand is not evaluated while the operator is in the
576 "false" state, and the left operand is not evaluated while the
577 operator is in the "true" state. The precedence is a little lower
578 than || and &&. The value returned is either the empty string for
579 false, or a sequence number (beginning with 1) for true. The
580 sequence number is reset for each range encountered. The final
581 sequence number in a range has the string "E0" appended to it, which
582 doesn't affect its numeric value, but gives you something to search
583 for if you want to exclude the endpoint. You can exclude the
584 beginning point by waiting for the sequence number to be greater
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 .. $. == 200) ...
607 next LINE if (1 .. /^$/); # skip header lines, short for
608 # ... 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, but forces any word
787 (consisting entirely of word characters) to its left to be interpreted
788 as a string (as of 5.001). This includes words that might otherwise be
789 considered a constant or function call.
791 use constant FOO => "something";
793 my %h = ( FOO => 23 );
801 my %h = ("something", 23);
803 If the argument on the left is not a word, it is first interpreted as
804 an expression, and then the string value of that is used.
806 The C<< => >> operator is helpful in documenting the correspondence
807 between keys and values in hashes, and other paired elements in lists.
809 %hash = ( $key => $value );
810 login( $username => $password );
812 =head2 List Operators (Rightward)
813 X<operator, list, rightward> X<list operator>
815 On the right side of a list operator, it has very low precedence,
816 such that it controls all comma-separated expressions found there.
817 The only operators with lower precedence are the logical operators
818 "and", "or", and "not", which may be used to evaluate calls to list
819 operators without the need for extra parentheses:
821 open HANDLE, "filename"
822 or die "Can't open: $!\n";
824 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
827 X<operator, logical, not> X<not>
829 Unary "not" returns the logical negation of the expression to its right.
830 It's the equivalent of "!" except for the very low precedence.
833 X<operator, logical, and> X<and>
835 Binary "and" returns the logical conjunction of the two surrounding
836 expressions. It's equivalent to && except for the very low
837 precedence. This means that it short-circuits: i.e., the right
838 expression is evaluated only if the left expression is true.
840 =head2 Logical or, Defined or, and Exclusive Or
841 X<operator, logical, or> X<operator, logical, xor> X<operator, logical, err>
842 X<operator, logical, defined or> X<operator, logical, exclusive or>
845 Binary "or" returns the logical disjunction of the two surrounding
846 expressions. It's equivalent to || except for the very low precedence.
847 This makes it useful for control flow
849 print FH $data or die "Can't write to FH: $!";
851 This means that it short-circuits: i.e., the right expression is evaluated
852 only if the left expression is false. Due to its precedence, you should
853 probably avoid using this for assignment, only for control flow.
855 $a = $b or $c; # bug: this is wrong
856 ($a = $b) or $c; # really means this
857 $a = $b || $c; # better written this way
859 However, when it's a list-context assignment and you're trying to use
860 "||" for control flow, you probably need "or" so that the assignment
861 takes higher precedence.
863 @info = stat($file) || die; # oops, scalar sense of stat!
864 @info = stat($file) or die; # better, now @info gets its due
866 Then again, you could always use parentheses.
868 Binary "err" is equivalent to C<//>--it's just like binary "or", except it
869 tests its left argument's definedness instead of its truth. There are two
870 ways to remember "err": either because many functions return C<undef> on
871 an B<err>or, or as a sort of correction: C<$a = ($b err 'default')>. This
872 keyword is only available when the 'err' feature is enabled: see
873 L<feature> for more information.
875 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
876 It cannot short circuit, of course.
878 =head2 C Operators Missing From Perl
879 X<operator, missing from perl> X<&> X<*>
880 X<typecasting> X<(TYPE)>
882 Here is what C has that Perl doesn't:
888 Address-of operator. (But see the "\" operator for taking a reference.)
892 Dereference-address operator. (Perl's prefix dereferencing
893 operators are typed: $, @, %, and &.)
897 Type-casting operator.
901 =head2 Quote and Quote-like Operators
902 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
903 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
904 X<escape sequence> X<escape>
907 While we usually think of quotes as literal values, in Perl they
908 function as operators, providing various kinds of interpolating and
909 pattern matching capabilities. Perl provides customary quote characters
910 for these behaviors, but also provides a way for you to choose your
911 quote character for any of them. In the following table, a C<{}> represents
912 any pair of delimiters you choose.
914 Customary Generic Meaning Interpolates
919 // m{} Pattern match yes*
921 s{}{} Substitution yes*
922 tr{}{} Transliteration no (but see below)
925 * unless the delimiter is ''.
927 Non-bracketing delimiters use the same character fore and aft, but the four
928 sorts of brackets (round, angle, square, curly) will all nest, which means
937 Note, however, that this does not always work for quoting Perl code:
939 $s = q{ if($a eq "}") ... }; # WRONG
941 is a syntax error. The C<Text::Balanced> module (from CPAN, and
942 starting from Perl 5.8 part of the standard distribution) is able
945 There can be whitespace between the operator and the quoting
946 characters, except when C<#> is being used as the quoting character.
947 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
948 operator C<q> followed by a comment. Its argument will be taken
949 from the next line. This allows you to write:
951 s {foo} # Replace foo
954 The following escape sequences are available in constructs that interpolate
955 and in transliterations.
956 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N>
963 \a alarm (bell) (BEL)
965 \033 octal char (ESC)
967 \x{263a} wide hex char (SMILEY)
968 \c[ control char (ESC)
969 \N{name} named Unicode character
971 B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
972 the vertical tab (VT - ASCII 11).
974 The following escape sequences are available in constructs that interpolate
975 but not in transliterations.
976 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
978 \l lowercase next char
979 \u uppercase next char
982 \E end case modification
983 \Q quote non-word characters till \E
985 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
986 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
987 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
988 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
989 C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
992 All systems use the virtual C<"\n"> to represent a line terminator,
993 called a "newline". There is no such thing as an unvarying, physical
994 newline character. It is only an illusion that the operating system,
995 device drivers, C libraries, and Perl all conspire to preserve. Not all
996 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
997 on a Mac, these are reversed, and on systems without line terminator,
998 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
999 you mean a "newline" for your system, but use the literal ASCII when you
1000 need an exact character. For example, most networking protocols expect
1001 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
1002 and although they often accept just C<"\012">, they seldom tolerate just
1003 C<"\015">. If you get in the habit of using C<"\n"> for networking,
1004 you may be burned some day.
1005 X<newline> X<line terminator> X<eol> X<end of line>
1008 For constructs that do interpolate, variables beginning with "C<$>"
1009 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
1010 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1011 But method calls such as C<< $obj->meth >> are not.
1013 Interpolating an array or slice interpolates the elements in order,
1014 separated by the value of C<$">, so is equivalent to interpolating
1015 C<join $", @array>. "Punctuation" arrays such as C<@*> are only
1016 interpolated if the name is enclosed in braces C<@{*}>, but special
1017 arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
1019 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
1020 An unescaped C<$> or C<@> interpolates the corresponding variable,
1021 while escaping will cause the literal string C<\$> to be inserted.
1022 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
1024 Patterns are subject to an additional level of interpretation as a
1025 regular expression. This is done as a second pass, after variables are
1026 interpolated, so that regular expressions may be incorporated into the
1027 pattern from the variables. If this is not what you want, use C<\Q> to
1028 interpolate a variable literally.
1030 Apart from the behavior described above, Perl does not expand
1031 multiple levels of interpolation. In particular, contrary to the
1032 expectations of shell programmers, back-quotes do I<NOT> interpolate
1033 within double quotes, nor do single quotes impede evaluation of
1034 variables when used within double quotes.
1036 =head2 Regexp Quote-Like Operators
1039 Here are the quote-like operators that apply to pattern
1040 matching and related activities.
1047 This is just like the C</pattern/> search, except that it matches only
1048 once between calls to the reset() operator. This is a useful
1049 optimization when you want to see only the first occurrence of
1050 something in each file of a set of files, for instance. Only C<??>
1051 patterns local to the current package are reset.
1055 # blank line between header and body
1058 reset if eof; # clear ?? status for next file
1061 This usage is vaguely deprecated, which means it just might possibly
1062 be removed in some distant future version of Perl, perhaps somewhere
1063 around the year 2168.
1065 =item m/PATTERN/cgimosx
1066 X<m> X<operator, match>
1067 X<regexp, options> X<regexp> X<regex, options> X<regex>
1068 X</c> X</i> X</m> X</o> X</s> X</x>
1070 =item /PATTERN/cgimosxk
1072 Searches a string for a pattern match, and in scalar context returns
1073 true if it succeeds, false if it fails. If no string is specified
1074 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1075 string specified with C<=~> need not be an lvalue--it may be the
1076 result of an expression evaluation, but remember the C<=~> binds
1077 rather tightly.) See also L<perlre>. See L<perllocale> for
1078 discussion of additional considerations that apply when C<use locale>
1083 i Do case-insensitive pattern matching.
1084 m Treat string as multiple lines.
1085 s Treat string as single line.
1086 x Use extended regular expressions.
1087 g Match globally, i.e., find all occurrences.
1088 c Do not reset search position on a failed match when /g is in effect.
1089 o Compile pattern only once.
1090 k Keep a copy of the matched string so that ${^MATCH} and friends
1093 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1094 you can use any pair of non-alphanumeric, non-whitespace characters
1095 as delimiters. This is particularly useful for matching path names
1096 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1097 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
1098 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1100 PATTERN may contain variables, which will be interpolated (and the
1101 pattern recompiled) every time the pattern search is evaluated, except
1102 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1103 C<$|> are not interpolated because they look like end-of-string tests.)
1104 If you want such a pattern to be compiled only once, add a C</o> after
1105 the trailing delimiter. This avoids expensive run-time recompilations,
1106 and is useful when the value you are interpolating won't change over
1107 the life of the script. However, mentioning C</o> constitutes a promise
1108 that you won't change the variables in the pattern. If you change them,
1109 Perl won't even notice. See also L<"qr/STRING/imosx">.
1111 If the PATTERN evaluates to the empty string, the last
1112 I<successfully> matched regular expression is used instead. In this
1113 case, only the C<g> and C<c> flags on the empty pattern is honoured -
1114 the other flags are taken from the original pattern. If no match has
1115 previously succeeded, this will (silently) act instead as a genuine
1116 empty pattern (which will always match).
1118 Note that it's possible to confuse Perl into thinking C<//> (the empty
1119 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1120 good about this, but some pathological cases might trigger this, such as
1121 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1122 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1123 will assume you meant defined-or. If you meant the empty regex, just
1124 use parentheses or spaces to disambiguate, or even prefix the empty
1125 regex with an C<m> (so C<//> becomes C<m//>).
1127 If the C</g> option is not used, C<m//> in list context returns a
1128 list consisting of the subexpressions matched by the parentheses in the
1129 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1130 also set, and that this differs from Perl 4's behavior.) When there are
1131 no parentheses in the pattern, the return value is the list C<(1)> for
1132 success. With or without parentheses, an empty list is returned upon
1137 open(TTY, '/dev/tty');
1138 <TTY> =~ /^y/i && foo(); # do foo if desired
1140 if (/Version: *([0-9.]*)/) { $version = $1; }
1142 next if m#^/usr/spool/uucp#;
1147 print if /$arg/o; # compile only once
1150 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1152 This last example splits $foo into the first two words and the
1153 remainder of the line, and assigns those three fields to $F1, $F2, and
1154 $Etc. The conditional is true if any variables were assigned, i.e., if
1155 the pattern matched.
1157 The C</g> modifier specifies global pattern matching--that is,
1158 matching as many times as possible within the string. How it behaves
1159 depends on the context. In list context, it returns a list of the
1160 substrings matched by any capturing parentheses in the regular
1161 expression. If there are no parentheses, it returns a list of all
1162 the matched strings, as if there were parentheses around the whole
1165 In scalar context, each execution of C<m//g> finds the next match,
1166 returning true if it matches, and false if there is no further match.
1167 The position after the last match can be read or set using the pos()
1168 function; see L<perlfunc/pos>. A failed match normally resets the
1169 search position to the beginning of the string, but you can avoid that
1170 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1171 string also resets the search position.
1173 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1174 zero-width assertion that matches the exact position where the previous
1175 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1176 still anchors at pos(), but the match is of course only attempted once.
1177 Using C<\G> without C</g> on a target string that has not previously had a
1178 C</g> match applied to it is the same as using the C<\A> assertion to match
1179 the beginning of the string. Note also that, currently, C<\G> is only
1180 properly supported when anchored at the very beginning of the pattern.
1185 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1189 while (defined($paragraph = <>)) {
1190 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1194 print "$sentences\n";
1196 # using m//gc with \G
1200 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1202 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1204 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1206 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1208 The last example should print:
1218 Notice that the final match matched C<q> instead of C<p>, which a match
1219 without the C<\G> anchor would have done. Also note that the final match
1220 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1221 final match did indeed match C<p>, it's a good bet that you're running an
1222 older (pre-5.6.0) Perl.
1224 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1225 combine several regexps like this to process a string part-by-part,
1226 doing different actions depending on which regexp matched. Each
1227 regexp tries to match where the previous one leaves off.
1230 $url = URI::URL->new( "http://www/" ); die if $url eq "xXx";
1234 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1235 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1236 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1237 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1238 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1239 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1240 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1241 print ". That's all!\n";
1244 Here is the output (split into several lines):
1246 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1247 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1248 lowercase lowercase line-noise lowercase lowercase line-noise
1249 MiXeD line-noise. That's all!
1252 X<q> X<quote, single> X<'> X<''>
1256 A single-quoted, literal string. A backslash represents a backslash
1257 unless followed by the delimiter or another backslash, in which case
1258 the delimiter or backslash is interpolated.
1260 $foo = q!I said, "You said, 'She said it.'"!;
1261 $bar = q('This is it.');
1262 $baz = '\n'; # a two-character string
1265 X<qq> X<quote, double> X<"> X<"">
1269 A double-quoted, interpolated string.
1272 (*** The previous line contains the naughty word "$1".\n)
1273 if /\b(tcl|java|python)\b/i; # :-)
1274 $baz = "\n"; # a one-character string
1276 =item qr/STRING/imosx
1277 X<qr> X</i> X</m> X</o> X</s> X</x>
1279 This operator quotes (and possibly compiles) its I<STRING> as a regular
1280 expression. I<STRING> is interpolated the same way as I<PATTERN>
1281 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1282 is done. Returns a Perl value which may be used instead of the
1283 corresponding C</STRING/imosx> expression.
1287 $rex = qr/my.STRING/is;
1294 The result may be used as a subpattern in a match:
1297 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1298 $string =~ $re; # or used standalone
1299 $string =~ /$re/; # or this way
1301 Since Perl may compile the pattern at the moment of execution of qr()
1302 operator, using qr() may have speed advantages in some situations,
1303 notably if the result of qr() is used standalone:
1306 my $patterns = shift;
1307 my @compiled = map qr/$_/i, @$patterns;
1310 foreach my $pat (@compiled) {
1311 $success = 1, last if /$pat/;
1317 Precompilation of the pattern into an internal representation at
1318 the moment of qr() avoids a need to recompile the pattern every
1319 time a match C</$pat/> is attempted. (Perl has many other internal
1320 optimizations, but none would be triggered in the above example if
1321 we did not use qr() operator.)
1325 i Do case-insensitive pattern matching.
1326 m Treat string as multiple lines.
1327 o Compile pattern only once.
1328 s Treat string as single line.
1329 x Use extended regular expressions.
1331 See L<perlre> for additional information on valid syntax for STRING, and
1332 for a detailed look at the semantics of regular expressions.
1335 X<qx> X<`> X<``> X<backtick>
1339 A string which is (possibly) interpolated and then executed as a
1340 system command with C</bin/sh> or its equivalent. Shell wildcards,
1341 pipes, and redirections will be honored. The collected standard
1342 output of the command is returned; standard error is unaffected. In
1343 scalar context, it comes back as a single (potentially multi-line)
1344 string, or undef if the command failed. In list context, returns a
1345 list of lines (however you've defined lines with $/ or
1346 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1348 Because backticks do not affect standard error, use shell file descriptor
1349 syntax (assuming the shell supports this) if you care to address this.
1350 To capture a command's STDERR and STDOUT together:
1352 $output = `cmd 2>&1`;
1354 To capture a command's STDOUT but discard its STDERR:
1356 $output = `cmd 2>/dev/null`;
1358 To capture a command's STDERR but discard its STDOUT (ordering is
1361 $output = `cmd 2>&1 1>/dev/null`;
1363 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1364 but leave its STDOUT to come out the old STDERR:
1366 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1368 To read both a command's STDOUT and its STDERR separately, it's easiest
1369 to redirect them separately to files, and then read from those files
1370 when the program is done:
1372 system("program args 1>program.stdout 2>program.stderr");
1374 The STDIN filehandle used by the command is inherited from Perl's STDIN.
1377 open BLAM, "blam" || die "Can't open: $!";
1378 open STDIN, "<&BLAM";
1381 will print the sorted contents of the file "blam".
1383 Using single-quote as a delimiter protects the command from Perl's
1384 double-quote interpolation, passing it on to the shell instead:
1386 $perl_info = qx(ps $$); # that's Perl's $$
1387 $shell_info = qx'ps $$'; # that's the new shell's $$
1389 How that string gets evaluated is entirely subject to the command
1390 interpreter on your system. On most platforms, you will have to protect
1391 shell metacharacters if you want them treated literally. This is in
1392 practice difficult to do, as it's unclear how to escape which characters.
1393 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1394 to emulate backticks safely.
1396 On some platforms (notably DOS-like ones), the shell may not be
1397 capable of dealing with multiline commands, so putting newlines in
1398 the string may not get you what you want. You may be able to evaluate
1399 multiple commands in a single line by separating them with the command
1400 separator character, if your shell supports that (e.g. C<;> on many Unix
1401 shells; C<&> on the Windows NT C<cmd> shell).
1403 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1404 output before starting the child process, but this may not be supported
1405 on some platforms (see L<perlport>). To be safe, you may need to set
1406 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1407 C<IO::Handle> on any open handles.
1409 Beware that some command shells may place restrictions on the length
1410 of the command line. You must ensure your strings don't exceed this
1411 limit after any necessary interpolations. See the platform-specific
1412 release notes for more details about your particular environment.
1414 Using this operator can lead to programs that are difficult to port,
1415 because the shell commands called vary between systems, and may in
1416 fact not be present at all. As one example, the C<type> command under
1417 the POSIX shell is very different from the C<type> command under DOS.
1418 That doesn't mean you should go out of your way to avoid backticks
1419 when they're the right way to get something done. Perl was made to be
1420 a glue language, and one of the things it glues together is commands.
1421 Just understand what you're getting yourself into.
1423 See L</"I/O Operators"> for more discussion.
1426 X<qw> X<quote, list> X<quote, words>
1428 Evaluates to a list of the words extracted out of STRING, using embedded
1429 whitespace as the word delimiters. It can be understood as being roughly
1432 split(' ', q/STRING/);
1434 the differences being that it generates a real list at compile time, and
1435 in scalar context it returns the last element in the list. So
1440 is semantically equivalent to the list:
1444 Some frequently seen examples:
1446 use POSIX qw( setlocale localeconv )
1447 @EXPORT = qw( foo bar baz );
1449 A common mistake is to try to separate the words with comma or to
1450 put comments into a multi-line C<qw>-string. For this reason, the
1451 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1452 produces warnings if the STRING contains the "," or the "#" character.
1454 =item s/PATTERN/REPLACEMENT/egimosxk
1455 X<substitute> X<substitution> X<replace> X<regexp, replace>
1456 X<regexp, substitute> X</e> X</g> X</i> X</m> X</o> X</s> X</x>
1458 Searches a string for a pattern, and if found, replaces that pattern
1459 with the replacement text and returns the number of substitutions
1460 made. Otherwise it returns false (specifically, the empty string).
1462 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1463 variable is searched and modified. (The string specified with C<=~> must
1464 be scalar variable, an array element, a hash element, or an assignment
1465 to one of those, i.e., an lvalue.)
1467 If the delimiter chosen is a single quote, no interpolation is
1468 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1469 PATTERN contains a $ that looks like a variable rather than an
1470 end-of-string test, the variable will be interpolated into the pattern
1471 at run-time. If you want the pattern compiled only once the first time
1472 the variable is interpolated, use the C</o> option. If the pattern
1473 evaluates to the empty string, the last successfully executed regular
1474 expression is used instead. See L<perlre> for further explanation on these.
1475 See L<perllocale> for discussion of additional considerations that apply
1476 when C<use locale> is in effect.
1480 i Do case-insensitive pattern matching.
1481 m Treat string as multiple lines.
1482 s Treat string as single line.
1483 x Use extended regular expressions.
1484 g Replace globally, i.e., all occurrences.
1485 o Compile pattern only once.
1486 k Keep a copy of the original string so ${^MATCH} and friends
1488 e Evaluate the right side as an expression.
1491 Any non-alphanumeric, non-whitespace delimiter may replace the
1492 slashes. If single quotes are used, no interpretation is done on the
1493 replacement string (the C</e> modifier overrides this, however). Unlike
1494 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1495 text is not evaluated as a command. If the
1496 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1497 pair of quotes, which may or may not be bracketing quotes, e.g.,
1498 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1499 replacement portion to be treated as a full-fledged Perl expression
1500 and evaluated right then and there. It is, however, syntax checked at
1501 compile-time. A second C<e> modifier will cause the replacement portion
1502 to be C<eval>ed before being run as a Perl expression.
1506 s/\bgreen\b/mauve/g; # don't change wintergreen
1508 $path =~ s|/usr/bin|/usr/local/bin|;
1510 s/Login: $foo/Login: $bar/; # run-time pattern
1512 ($foo = $bar) =~ s/this/that/; # copy first, then change
1514 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1517 s/\d+/$&*2/e; # yields 'abc246xyz'
1518 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1519 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1521 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1522 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1523 s/^=(\w+)/pod($1)/ge; # use function call
1525 # expand variables in $_, but dynamics only, using
1526 # symbolic dereferencing
1529 # Add one to the value of any numbers in the string
1532 # This will expand any embedded scalar variable
1533 # (including lexicals) in $_ : First $1 is interpolated
1534 # to the variable name, and then evaluated
1537 # Delete (most) C comments.
1539 /\* # Match the opening delimiter.
1540 .*? # Match a minimal number of characters.
1541 \*/ # Match the closing delimiter.
1544 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1546 for ($variable) { # trim whitespace in $variable, cheap
1551 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1553 Note the use of $ instead of \ in the last example. Unlike
1554 B<sed>, we use the \<I<digit>> form in only the left hand side.
1555 Anywhere else it's $<I<digit>>.
1557 Occasionally, you can't use just a C</g> to get all the changes
1558 to occur that you might want. Here are two common cases:
1560 # put commas in the right places in an integer
1561 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1563 # expand tabs to 8-column spacing
1564 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1566 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1567 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
1569 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1571 Transliterates all occurrences of the characters found in the search list
1572 with the corresponding character in the replacement list. It returns
1573 the number of characters replaced or deleted. If no string is
1574 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1575 string specified with =~ must be a scalar variable, an array element, a
1576 hash element, or an assignment to one of those, i.e., an lvalue.)
1578 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1579 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1580 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1581 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1582 its own pair of quotes, which may or may not be bracketing quotes,
1583 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1585 Note that C<tr> does B<not> do regular expression character classes
1586 such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to
1587 the tr(1) utility. If you want to map strings between lower/upper
1588 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1589 using the C<s> operator if you need regular expressions.
1591 Note also that the whole range idea is rather unportable between
1592 character sets--and even within character sets they may cause results
1593 you probably didn't expect. A sound principle is to use only ranges
1594 that begin from and end at either alphabets of equal case (a-e, A-E),
1595 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1596 character sets in full.
1600 c Complement the SEARCHLIST.
1601 d Delete found but unreplaced characters.
1602 s Squash duplicate replaced characters.
1604 If the C</c> modifier is specified, the SEARCHLIST character set
1605 is complemented. If the C</d> modifier is specified, any characters
1606 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1607 (Note that this is slightly more flexible than the behavior of some
1608 B<tr> programs, which delete anything they find in the SEARCHLIST,
1609 period.) If the C</s> modifier is specified, sequences of characters
1610 that were transliterated to the same character are squashed down
1611 to a single instance of the character.
1613 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1614 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1615 than the SEARCHLIST, the final character is replicated till it is long
1616 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1617 This latter is useful for counting characters in a class or for
1618 squashing character sequences in a class.
1622 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1624 $cnt = tr/*/*/; # count the stars in $_
1626 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1628 $cnt = tr/0-9//; # count the digits in $_
1630 tr/a-zA-Z//s; # bookkeeper -> bokeper
1632 ($HOST = $host) =~ tr/a-z/A-Z/;
1634 tr/a-zA-Z/ /cs; # change non-alphas to single space
1637 [\000-\177]; # delete 8th bit
1639 If multiple transliterations are given for a character, only the
1644 will transliterate any A to X.
1646 Because the transliteration table is built at compile time, neither
1647 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1648 interpolation. That means that if you want to use variables, you
1651 eval "tr/$oldlist/$newlist/";
1654 eval "tr/$oldlist/$newlist/, 1" or die $@;
1657 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
1659 A line-oriented form of quoting is based on the shell "here-document"
1660 syntax. Following a C<< << >> you specify a string to terminate
1661 the quoted material, and all lines following the current line down to
1662 the terminating string are the value of the item.
1664 The terminating string may be either an identifier (a word), or some
1665 quoted text. An unquoted identifier works like double quotes.
1666 There may not be a space between the C<< << >> and the identifier,
1667 unless the identifier is explicitly quoted. (If you put a space it
1668 will be treated as a null identifier, which is valid, and matches the
1669 first empty line.) The terminating string must appear by itself
1670 (unquoted and with no surrounding whitespace) on the terminating line.
1672 If the terminating string is quoted, the type of quotes used determine
1673 the treatment of the text.
1679 Double quotes indicate that the text will be interpolated using exactly
1680 the same rules as normal double quoted strings.
1683 The price is $Price.
1686 print << "EOF"; # same as above
1687 The price is $Price.
1693 Single quotes indicate the text is to be treated literally with no
1694 interpolation of its content. This is similar to single quoted
1695 strings except that backslashes have no special meaning, with C<\\>
1696 being treated as two backslashes and not one as they would in every
1697 other quoting construct.
1699 This is the only form of quoting in perl where there is no need
1700 to worry about escaping content, something that code generators
1701 can and do make good use of.
1705 The content of the here doc is treated just as it would be if the
1706 string were embedded in backticks. Thus the content is interpolated
1707 as though it were double quoted and then executed via the shell, with
1708 the results of the execution returned.
1710 print << `EOC`; # execute command and get results
1716 It is possible to stack multiple here-docs in a row:
1718 print <<"foo", <<"bar"; # you can stack them
1724 myfunc(<< "THIS", 23, <<'THAT');
1731 Just don't forget that you have to put a semicolon on the end
1732 to finish the statement, as Perl doesn't know you're not going to
1740 If you want to remove the line terminator from your here-docs,
1743 chomp($string = <<'END');
1747 If you want your here-docs to be indented with the rest of the code,
1748 you'll need to remove leading whitespace from each line manually:
1750 ($quote = <<'FINIS') =~ s/^\s+//gm;
1751 The Road goes ever on and on,
1752 down from the door where it began.
1755 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1756 the quoted material must come on the lines following the final delimiter.
1771 If the terminating identifier is on the last line of the program, you
1772 must be sure there is a newline after it; otherwise, Perl will give the
1773 warning B<Can't find string terminator "END" anywhere before EOF...>.
1775 Additionally, the quoting rules for the end of string identifier are not
1776 related to Perl's quoting rules -- C<q()>, C<qq()>, and the like are not
1777 supported in place of C<''> and C<"">, and the only interpolation is for
1778 backslashing the quoting character:
1780 print << "abc\"def";
1784 Finally, quoted strings cannot span multiple lines. The general rule is
1785 that the identifier must be a string literal. Stick with that, and you
1790 =head2 Gory details of parsing quoted constructs
1791 X<quote, gory details>
1793 When presented with something that might have several different
1794 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1795 principle to pick the most probable interpretation. This strategy
1796 is so successful that Perl programmers often do not suspect the
1797 ambivalence of what they write. But from time to time, Perl's
1798 notions differ substantially from what the author honestly meant.
1800 This section hopes to clarify how Perl handles quoted constructs.
1801 Although the most common reason to learn this is to unravel labyrinthine
1802 regular expressions, because the initial steps of parsing are the
1803 same for all quoting operators, they are all discussed together.
1805 The most important Perl parsing rule is the first one discussed
1806 below: when processing a quoted construct, Perl first finds the end
1807 of that construct, then interprets its contents. If you understand
1808 this rule, you may skip the rest of this section on the first
1809 reading. The other rules are likely to contradict the user's
1810 expectations much less frequently than this first one.
1812 Some passes discussed below are performed concurrently, but because
1813 their results are the same, we consider them individually. For different
1814 quoting constructs, Perl performs different numbers of passes, from
1815 one to four, but these passes are always performed in the same order.
1819 =item Finding the end
1821 The first pass is finding the end of the quoted construct, where
1822 the information about the delimiters is used in parsing.
1823 During this search, text between the starting and ending delimiters
1824 is copied to a safe location. The text copied gets delimiter-independent.
1826 If the construct is a here-doc, the ending delimiter is a line
1827 that has a terminating string as the content. Therefore C<<<EOF> is
1828 terminated by C<EOF> immediately followed by C<"\n"> and starting
1829 from the first column of the terminating line.
1830 When searching for the terminating line of a here-doc, nothing
1831 is skipped. In other words, lines after the here-doc syntax
1832 are compared with the terminating string line by line.
1834 For the constructs except here-docs, single characters are used as starting
1835 and ending delimiters. If the starting delimiter is an opening punctuation
1836 (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
1837 corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
1838 If the starting delimiter is an unpaired character like C</> or a closing
1839 punctuation, the ending delimiter is same as the starting delimiter.
1840 Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
1841 C<qq[]> and C<qq]]> constructs.
1843 When searching for single-character delimiters, escaped delimiters
1844 and C<\\> are skipped. For example, while searching for terminating C</>,
1845 combinations of C<\\> and C<\/> are skipped. If the delimiters are
1846 bracketing, nested pairs are also skipped. For example, while searching
1847 for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
1848 and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
1849 However, when backslashes are used as the delimiters (like C<qq\\> and
1850 C<tr\\\>), nothing is skipped.
1851 During the search for the end, backslashes that escape delimiters
1852 are removed (exactly speaking, they are not copied to the safe location).
1854 For constructs with three-part delimiters (C<s///>, C<y///>, and
1855 C<tr///>), the search is repeated once more.
1856 If the first delimiter is not an opening punctuation, three delimiters must
1857 be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
1858 terminates the left part and starts the right part at once.
1859 If the left part is delimited by bracketing punctuations (that is C<()>,
1860 C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
1861 delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces
1862 and comments are allowed between both parts, though the comment must follow
1863 at least one whitespace; otherwise a character expected as the start of
1864 the comment may be regarded as the starting delimiter of the right part.
1866 During this search no attention is paid to the semantics of the construct.
1869 "$hash{"$foo/$bar"}"
1874 bar # NOT a comment, this slash / terminated m//!
1877 do not form legal quoted expressions. The quoted part ends on the
1878 first C<"> and C</>, and the rest happens to be a syntax error.
1879 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1880 the example above is not C<m//x>, but rather C<m//> with no C</x>
1881 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1883 Also no attention is paid to C<\c\> (multichar control char syntax) during
1884 this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
1885 of C<\/>, and the following C</> is not recognized as a delimiter.
1886 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1891 The next step is interpolation in the text obtained, which is now
1892 delimiter-independent. There are multiple cases.
1898 No interpolation is performed.
1899 Note that the combination C<\\> is left intact, since escaped delimiters
1900 are not available for here-docs.
1902 =item C<m''>, the pattern of C<s'''>
1904 No interpolation is performed at this stage.
1905 Any backslashed sequences including C<\\> are treated at the stage
1906 to L</"parsing regular expressions">.
1908 =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
1910 The only interpolation is removal of C<\> from pairs of C<\\>.
1911 Therefore C<-> in C<tr'''> and C<y'''> is treated literally
1912 as a hyphen and no character range is available.
1913 C<\1> in the replacement of C<s'''> does not work as C<$1>.
1915 =item C<tr///>, C<y///>
1917 No variable interpolation occurs. String modifying combinations for
1918 case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
1919 The other escape sequences such as C<\200> and C<\t> and backslashed
1920 characters such as C<\\> and C<\-> are converted to appropriate literals.
1921 The character C<-> is treated specially and therefore C<\-> is treated
1924 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
1926 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1927 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1928 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1929 The other escape sequences such as C<\200> and C<\t> and backslashed
1930 characters such as C<\\> and C<\-> are replaced with appropriate
1933 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1934 is interpolated in the usual way. Something like C<"\Q\\E"> has
1935 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1936 result is the same as for C<"\\\\E">. As a general rule, backslashes
1937 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1938 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1939 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1944 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1946 Interpolated scalars and arrays are converted internally to the C<join> and
1947 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1949 $foo . " XXX '" . (join $", @arr) . "'";
1951 All operations above are performed simultaneously, left to right.
1953 Because the result of C<"\Q STRING \E"> has all metacharacters
1954 quoted, there is no way to insert a literal C<$> or C<@> inside a
1955 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1956 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1959 Note also that the interpolation code needs to make a decision on
1960 where the interpolated scalar ends. For instance, whether
1961 C<< "a $b -> {c}" >> really means:
1963 "a " . $b . " -> {c}";
1969 Most of the time, the longest possible text that does not include
1970 spaces between components and which contains matching braces or
1971 brackets. because the outcome may be determined by voting based
1972 on heuristic estimators, the result is not strictly predictable.
1973 Fortunately, it's usually correct for ambiguous cases.
1975 =item the replacement of C<s///>
1977 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1978 happens as with C<qq//> constructs.
1980 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1981 the replacement text of C<s///>, in order to correct the incorrigible
1982 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1983 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1984 (that is, the C<$^W> variable) was set.
1986 =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1988 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
1989 and interpolation happens (almost) as with C<qq//> constructs.
1991 However any other combinations of C<\> followed by a character
1992 are not substituted but only skipped, in order to parse them
1993 as regular expressions at the following step.
1994 As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
1995 treated as an array symbol (for example C<@foo>),
1996 even though the same text in C<qq//> gives interpolation of C<\c@>.
1998 Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1999 a C<#>-comment in a C<//x>-regular expression, no processing is
2000 performed whatsoever. This is the first step at which the presence
2001 of the C<//x> modifier is relevant.
2003 Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2004 and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2005 voted (by several different estimators) to be either an array element
2006 or C<$var> followed by an RE alternative. This is where the notation
2007 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2008 array element C<-9>, not as a regular expression from the variable
2009 C<$arr> followed by a digit, which would be the interpretation of
2010 C</$arr[0-9]/>. Since voting among different estimators may occur,
2011 the result is not predictable.
2013 The lack of processing of C<\\> creates specific restrictions on
2014 the post-processed text. If the delimiter is C</>, one cannot get
2015 the combination C<\/> into the result of this step. C</> will
2016 finish the regular expression, C<\/> will be stripped to C</> on
2017 the previous step, and C<\\/> will be left as is. Because C</> is
2018 equivalent to C<\/> inside a regular expression, this does not
2019 matter unless the delimiter happens to be character special to the
2020 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2021 alphanumeric char, as in:
2025 In the RE above, which is intentionally obfuscated for illustration, the
2026 delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
2027 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
2028 reason you're encouraged to restrict your delimiters to non-alphanumeric,
2029 non-whitespace choices.
2033 This step is the last one for all constructs except regular expressions,
2034 which are processed further.
2036 =item parsing regular expressions
2039 Previous steps were performed during the compilation of Perl code,
2040 but this one happens at run time--although it may be optimized to
2041 be calculated at compile time if appropriate. After preprocessing
2042 described above, and possibly after evaluation if concatenation,
2043 joining, casing translation, or metaquoting are involved, the
2044 resulting I<string> is passed to the RE engine for compilation.
2046 Whatever happens in the RE engine might be better discussed in L<perlre>,
2047 but for the sake of continuity, we shall do so here.
2049 This is another step where the presence of the C<//x> modifier is
2050 relevant. The RE engine scans the string from left to right and
2051 converts it to a finite automaton.
2053 Backslashed characters are either replaced with corresponding
2054 literal strings (as with C<\{>), or else they generate special nodes
2055 in the finite automaton (as with C<\b>). Characters special to the
2056 RE engine (such as C<|>) generate corresponding nodes or groups of
2057 nodes. C<(?#...)> comments are ignored. All the rest is either
2058 converted to literal strings to match, or else is ignored (as is
2059 whitespace and C<#>-style comments if C<//x> is present).
2061 Parsing of the bracketed character class construct, C<[...]>, is
2062 rather different than the rule used for the rest of the pattern.
2063 The terminator of this construct is found using the same rules as
2064 for finding the terminator of a C<{}>-delimited construct, the only
2065 exception being that C<]> immediately following C<[> is treated as
2066 though preceded by a backslash. Similarly, the terminator of
2067 C<(?{...})> is found using the same rules as for finding the
2068 terminator of a C<{}>-delimited construct.
2070 It is possible to inspect both the string given to RE engine and the
2071 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2072 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
2073 switch documented in L<perlrun/"Command Switches">.
2075 =item Optimization of regular expressions
2076 X<regexp, optimization>
2078 This step is listed for completeness only. Since it does not change
2079 semantics, details of this step are not documented and are subject
2080 to change without notice. This step is performed over the finite
2081 automaton that was generated during the previous pass.
2083 It is at this stage that C<split()> silently optimizes C</^/> to
2088 =head2 I/O Operators
2089 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2092 There are several I/O operators you should know about.
2094 A string enclosed by backticks (grave accents) first undergoes
2095 double-quote interpolation. It is then interpreted as an external
2096 command, and the output of that command is the value of the
2097 backtick string, like in a shell. In scalar context, a single string
2098 consisting of all output is returned. In list context, a list of
2099 values is returned, one per line of output. (You can set C<$/> to use
2100 a different line terminator.) The command is executed each time the
2101 pseudo-literal is evaluated. The status value of the command is
2102 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2103 Unlike in B<csh>, no translation is done on the return data--newlines
2104 remain newlines. Unlike in any of the shells, single quotes do not
2105 hide variable names in the command from interpretation. To pass a
2106 literal dollar-sign through to the shell you need to hide it with a
2107 backslash. The generalized form of backticks is C<qx//>. (Because
2108 backticks always undergo shell expansion as well, see L<perlsec> for
2110 X<qx> X<`> X<``> X<backtick> X<glob>
2112 In scalar context, evaluating a filehandle in angle brackets yields
2113 the next line from that file (the newline, if any, included), or
2114 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2115 (sometimes known as file-slurp mode) and the file is empty, it
2116 returns C<''> the first time, followed by C<undef> subsequently.
2118 Ordinarily you must assign the returned value to a variable, but
2119 there is one situation where an automatic assignment happens. If
2120 and only if the input symbol is the only thing inside the conditional
2121 of a C<while> statement (even if disguised as a C<for(;;)> loop),
2122 the value is automatically assigned to the global variable $_,
2123 destroying whatever was there previously. (This may seem like an
2124 odd thing to you, but you'll use the construct in almost every Perl
2125 script you write.) The $_ variable is not implicitly localized.
2126 You'll have to put a C<local $_;> before the loop if you want that
2129 The following lines are equivalent:
2131 while (defined($_ = <STDIN>)) { print; }
2132 while ($_ = <STDIN>) { print; }
2133 while (<STDIN>) { print; }
2134 for (;<STDIN>;) { print; }
2135 print while defined($_ = <STDIN>);
2136 print while ($_ = <STDIN>);
2137 print while <STDIN>;
2139 This also behaves similarly, but avoids $_ :
2141 while (my $line = <STDIN>) { print $line }
2143 In these loop constructs, the assigned value (whether assignment
2144 is automatic or explicit) is then tested to see whether it is
2145 defined. The defined test avoids problems where line has a string
2146 value that would be treated as false by Perl, for example a "" or
2147 a "0" with no trailing newline. If you really mean for such values
2148 to terminate the loop, they should be tested for explicitly:
2150 while (($_ = <STDIN>) ne '0') { ... }
2151 while (<STDIN>) { last unless $_; ... }
2153 In other boolean contexts, C<< <I<filehandle>> >> without an
2154 explicit C<defined> test or comparison elicit a warning if the
2155 C<use warnings> pragma or the B<-w>
2156 command-line switch (the C<$^W> variable) is in effect.
2158 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2159 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2160 in packages, where they would be interpreted as local identifiers
2161 rather than global.) Additional filehandles may be created with
2162 the open() function, amongst others. See L<perlopentut> and
2163 L<perlfunc/open> for details on this.
2164 X<stdin> X<stdout> X<sterr>
2166 If a <FILEHANDLE> is used in a context that is looking for
2167 a list, a list comprising all input lines is returned, one line per
2168 list element. It's easy to grow to a rather large data space this
2169 way, so use with care.
2171 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2172 See L<perlfunc/readline>.
2174 The null filehandle <> is special: it can be used to emulate the
2175 behavior of B<sed> and B<awk>. Input from <> comes either from
2176 standard input, or from each file listed on the command line. Here's
2177 how it works: the first time <> is evaluated, the @ARGV array is
2178 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2179 gives you standard input. The @ARGV array is then processed as a list
2180 of filenames. The loop
2183 ... # code for each line
2186 is equivalent to the following Perl-like pseudo code:
2188 unshift(@ARGV, '-') unless @ARGV;
2189 while ($ARGV = shift) {
2192 ... # code for each line
2196 except that it isn't so cumbersome to say, and will actually work.
2197 It really does shift the @ARGV array and put the current filename
2198 into the $ARGV variable. It also uses filehandle I<ARGV>
2199 internally--<> is just a synonym for <ARGV>, which
2200 is magical. (The pseudo code above doesn't work because it treats
2201 <ARGV> as non-magical.)
2203 You can modify @ARGV before the first <> as long as the array ends up
2204 containing the list of filenames you really want. Line numbers (C<$.>)
2205 continue as though the input were one big happy file. See the example
2206 in L<perlfunc/eof> for how to reset line numbers on each file.
2208 If you want to set @ARGV to your own list of files, go right ahead.
2209 This sets @ARGV to all plain text files if no @ARGV was given:
2211 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2213 You can even set them to pipe commands. For example, this automatically
2214 filters compressed arguments through B<gzip>:
2216 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2218 If you want to pass switches into your script, you can use one of the
2219 Getopts modules or put a loop on the front like this:
2221 while ($_ = $ARGV[0], /^-/) {
2224 if (/^-D(.*)/) { $debug = $1 }
2225 if (/^-v/) { $verbose++ }
2226 # ... # other switches
2230 # ... # code for each line
2233 The <> symbol will return C<undef> for end-of-file only once.
2234 If you call it again after this, it will assume you are processing another
2235 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2237 If what the angle brackets contain is a simple scalar variable (e.g.,
2238 <$foo>), then that variable contains the name of the
2239 filehandle to input from, or its typeglob, or a reference to the
2245 If what's within the angle brackets is neither a filehandle nor a simple
2246 scalar variable containing a filehandle name, typeglob, or typeglob
2247 reference, it is interpreted as a filename pattern to be globbed, and
2248 either a list of filenames or the next filename in the list is returned,
2249 depending on context. This distinction is determined on syntactic
2250 grounds alone. That means C<< <$x> >> is always a readline() from
2251 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2252 That's because $x is a simple scalar variable, but C<$hash{key}> is
2253 not--it's a hash element. Even C<< <$x > >> (note the extra space)
2254 is treated as C<glob("$x ")>, not C<readline($x)>.
2256 One level of double-quote interpretation is done first, but you can't
2257 say C<< <$foo> >> because that's an indirect filehandle as explained
2258 in the previous paragraph. (In older versions of Perl, programmers
2259 would insert curly brackets to force interpretation as a filename glob:
2260 C<< <${foo}> >>. These days, it's considered cleaner to call the
2261 internal function directly as C<glob($foo)>, which is probably the right
2262 way to have done it in the first place.) For example:
2268 is roughly equivalent to:
2270 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2276 except that the globbing is actually done internally using the standard
2277 C<File::Glob> extension. Of course, the shortest way to do the above is:
2281 A (file)glob evaluates its (embedded) argument only when it is
2282 starting a new list. All values must be read before it will start
2283 over. In list context, this isn't important because you automatically
2284 get them all anyway. However, in scalar context the operator returns
2285 the next value each time it's called, or C<undef> when the list has
2286 run out. As with filehandle reads, an automatic C<defined> is
2287 generated when the glob occurs in the test part of a C<while>,
2288 because legal glob returns (e.g. a file called F<0>) would otherwise
2289 terminate the loop. Again, C<undef> is returned only once. So if
2290 you're expecting a single value from a glob, it is much better to
2293 ($file) = <blurch*>;
2299 because the latter will alternate between returning a filename and
2302 If you're trying to do variable interpolation, it's definitely better
2303 to use the glob() function, because the older notation can cause people
2304 to become confused with the indirect filehandle notation.
2306 @files = glob("$dir/*.[ch]");
2307 @files = glob($files[$i]);
2309 =head2 Constant Folding
2310 X<constant folding> X<folding>
2312 Like C, Perl does a certain amount of expression evaluation at
2313 compile time whenever it determines that all arguments to an
2314 operator are static and have no side effects. In particular, string
2315 concatenation happens at compile time between literals that don't do
2316 variable substitution. Backslash interpolation also happens at
2317 compile time. You can say
2319 'Now is the time for all' . "\n" .
2320 'good men to come to.'
2322 and this all reduces to one string internally. Likewise, if
2325 foreach $file (@filenames) {
2326 if (-s $file > 5 + 100 * 2**16) { }
2329 the compiler will precompute the number which that expression
2330 represents so that the interpreter won't have to.
2335 Perl doesn't officially have a no-op operator, but the bare constants
2336 C<0> and C<1> are special-cased to not produce a warning in a void
2337 context, so you can for example safely do
2341 =head2 Bitwise String Operators
2342 X<operator, bitwise, string>
2344 Bitstrings of any size may be manipulated by the bitwise operators
2347 If the operands to a binary bitwise op are strings of different
2348 sizes, B<|> and B<^> ops act as though the shorter operand had
2349 additional zero bits on the right, while the B<&> op acts as though
2350 the longer operand were truncated to the length of the shorter.
2351 The granularity for such extension or truncation is one or more
2354 # ASCII-based examples
2355 print "j p \n" ^ " a h"; # prints "JAPH\n"
2356 print "JA" | " ph\n"; # prints "japh\n"
2357 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2358 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2360 If you are intending to manipulate bitstrings, be certain that
2361 you're supplying bitstrings: If an operand is a number, that will imply
2362 a B<numeric> bitwise operation. You may explicitly show which type of
2363 operation you intend by using C<""> or C<0+>, as in the examples below.
2365 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2366 $foo = '150' | 105; # yields 255
2367 $foo = 150 | '105'; # yields 255
2368 $foo = '150' | '105'; # yields string '155' (under ASCII)
2370 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2371 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2373 See L<perlfunc/vec> for information on how to manipulate individual bits
2376 =head2 Integer Arithmetic
2379 By default, Perl assumes that it must do most of its arithmetic in
2380 floating point. But by saying
2384 you may tell the compiler that it's okay to use integer operations
2385 (if it feels like it) from here to the end of the enclosing BLOCK.
2386 An inner BLOCK may countermand this by saying
2390 which lasts until the end of that BLOCK. Note that this doesn't
2391 mean everything is only an integer, merely that Perl may use integer
2392 operations if it is so inclined. For example, even under C<use
2393 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2396 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2397 and ">>") always produce integral results. (But see also
2398 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2399 them. By default, their results are interpreted as unsigned integers, but
2400 if C<use integer> is in effect, their results are interpreted
2401 as signed integers. For example, C<~0> usually evaluates to a large
2402 integral value. However, C<use integer; ~0> is C<-1> on two's-complement
2405 =head2 Floating-point Arithmetic
2406 X<floating-point> X<floating point> X<float> X<real>
2408 While C<use integer> provides integer-only arithmetic, there is no
2409 analogous mechanism to provide automatic rounding or truncation to a
2410 certain number of decimal places. For rounding to a certain number
2411 of digits, sprintf() or printf() is usually the easiest route.
2414 Floating-point numbers are only approximations to what a mathematician
2415 would call real numbers. There are infinitely more reals than floats,
2416 so some corners must be cut. For example:
2418 printf "%.20g\n", 123456789123456789;
2419 # produces 123456789123456784
2421 Testing for exact equality of floating-point equality or inequality is
2422 not a good idea. Here's a (relatively expensive) work-around to compare
2423 whether two floating-point numbers are equal to a particular number of
2424 decimal places. See Knuth, volume II, for a more robust treatment of
2428 my ($X, $Y, $POINTS) = @_;
2430 $tX = sprintf("%.${POINTS}g", $X);
2431 $tY = sprintf("%.${POINTS}g", $Y);
2435 The POSIX module (part of the standard perl distribution) implements
2436 ceil(), floor(), and other mathematical and trigonometric functions.
2437 The Math::Complex module (part of the standard perl distribution)
2438 defines mathematical functions that work on both the reals and the
2439 imaginary numbers. Math::Complex not as efficient as POSIX, but
2440 POSIX can't work with complex numbers.
2442 Rounding in financial applications can have serious implications, and
2443 the rounding method used should be specified precisely. In these
2444 cases, it probably pays not to trust whichever system rounding is
2445 being used by Perl, but to instead implement the rounding function you
2448 =head2 Bigger Numbers
2449 X<number, arbitrary precision>
2451 The standard Math::BigInt and Math::BigFloat modules provide
2452 variable-precision arithmetic and overloaded operators, although
2453 they're currently pretty slow. At the cost of some space and
2454 considerable speed, they avoid the normal pitfalls associated with
2455 limited-precision representations.
2458 $x = Math::BigInt->new('123456789123456789');
2461 # prints +15241578780673678515622620750190521
2463 There are several modules that let you calculate with (bound only by
2464 memory and cpu-time) unlimited or fixed precision. There are also
2465 some non-standard modules that provide faster implementations via
2466 external C libraries.
2468 Here is a short, but incomplete summary:
2470 Math::Fraction big, unlimited fractions like 9973 / 12967
2471 Math::String treat string sequences like numbers
2472 Math::FixedPrecision calculate with a fixed precision
2473 Math::Currency for currency calculations
2474 Bit::Vector manipulate bit vectors fast (uses C)
2475 Math::BigIntFast Bit::Vector wrapper for big numbers
2476 Math::Pari provides access to the Pari C library
2477 Math::BigInteger uses an external C library
2478 Math::Cephes uses external Cephes C library (no big numbers)
2479 Math::Cephes::Fraction fractions via the Cephes library
2480 Math::GMP another one using an external C library