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 floting point values, only the integer portion
270 of C<$a> and C<$b> will be used in the operation.
271 Note that when C<use integer> is in scope, "%" gives you direct access
272 to the modulus operator as implemented by your C compiler. This
273 operator is not as well defined for negative operands, but it will
275 X<%> X<remainder> X<modulus> X<mod>
277 Binary "x" is the repetition operator. In scalar context or if the left
278 operand is not enclosed in parentheses, it returns a string consisting
279 of the left operand repeated the number of times specified by the right
280 operand. In list context, if the left operand is enclosed in
281 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
282 If the right operand is zero or negative, it returns an empty string
283 or an empty list, depending on the context.
286 print '-' x 80; # print row of dashes
288 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
290 @ones = (1) x 80; # a list of 80 1's
291 @ones = (5) x @ones; # set all elements to 5
294 =head2 Additive Operators
295 X<operator, additive>
297 Binary "+" returns the sum of two numbers.
300 Binary "-" returns the difference of two numbers.
303 Binary "." concatenates two strings.
304 X<string, concatenation> X<concatenation>
305 X<cat> X<concat> X<concatenate> X<.>
307 =head2 Shift Operators
308 X<shift operator> X<operator, shift> X<<< << >>>
309 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
310 X<shl> X<shr> X<shift, right> X<shift, left>
312 Binary "<<" returns the value of its left argument shifted left by the
313 number of bits specified by the right argument. Arguments should be
314 integers. (See also L<Integer Arithmetic>.)
316 Binary ">>" returns the value of its left argument shifted right by
317 the number of bits specified by the right argument. Arguments should
318 be integers. (See also L<Integer Arithmetic>.)
320 Note that both "<<" and ">>" in Perl are implemented directly using
321 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
322 in force then signed C integers are used, else unsigned C integers are
323 used. Either way, the implementation isn't going to generate results
324 larger than the size of the integer type Perl was built with (32 bits
327 The result of overflowing the range of the integers is undefined
328 because it is undefined also in C. In other words, using 32-bit
329 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
330 of bits is also undefined.
332 =head2 Named Unary Operators
333 X<operator, named unary>
335 The various named unary operators are treated as functions with one
336 argument, with optional parentheses.
338 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
339 is followed by a left parenthesis as the next token, the operator and
340 arguments within parentheses are taken to be of highest precedence,
341 just like a normal function call. For example,
342 because named unary operators are higher precedence than ||:
344 chdir $foo || die; # (chdir $foo) || die
345 chdir($foo) || die; # (chdir $foo) || die
346 chdir ($foo) || die; # (chdir $foo) || die
347 chdir +($foo) || die; # (chdir $foo) || die
349 but, because * is higher precedence than named operators:
351 chdir $foo * 20; # chdir ($foo * 20)
352 chdir($foo) * 20; # (chdir $foo) * 20
353 chdir ($foo) * 20; # (chdir $foo) * 20
354 chdir +($foo) * 20; # chdir ($foo * 20)
356 rand 10 * 20; # rand (10 * 20)
357 rand(10) * 20; # (rand 10) * 20
358 rand (10) * 20; # (rand 10) * 20
359 rand +(10) * 20; # rand (10 * 20)
361 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
362 treated like named unary operators, but they don't follow this functional
363 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
364 equivalent to C<-f "$file.bak">.
365 X<-X> X<filetest> X<operator, filetest>
367 See also L<"Terms and List Operators (Leftward)">.
369 =head2 Relational Operators
370 X<relational operator> X<operator, relational>
372 Binary "<" returns true if the left argument is numerically less than
376 Binary ">" returns true if the left argument is numerically greater
377 than the right argument.
380 Binary "<=" returns true if the left argument is numerically less than
381 or equal to the right argument.
384 Binary ">=" returns true if the left argument is numerically greater
385 than or equal to the right argument.
388 Binary "lt" returns true if the left argument is stringwise less than
392 Binary "gt" returns true if the left argument is stringwise greater
393 than the right argument.
396 Binary "le" returns true if the left argument is stringwise less than
397 or equal to the right argument.
400 Binary "ge" returns true if the left argument is stringwise greater
401 than or equal to the right argument.
404 =head2 Equality Operators
405 X<equality> X<equal> X<equals> X<operator, equality>
407 Binary "==" returns true if the left argument is numerically equal to
411 Binary "!=" returns true if the left argument is numerically not equal
412 to the right argument.
415 Binary "<=>" returns -1, 0, or 1 depending on whether the left
416 argument is numerically less than, equal to, or greater than the right
417 argument. If your platform supports NaNs (not-a-numbers) as numeric
418 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
419 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
420 returns true, as does NaN != anything else. If your platform doesn't
421 support NaNs then NaN is just a string with numeric value 0.
422 X<< <=> >> X<spaceship>
424 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
425 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
427 Binary "eq" returns true if the left argument is stringwise equal to
431 Binary "ne" returns true if the left argument is stringwise not equal
432 to the right argument.
435 Binary "cmp" returns -1, 0, or 1 depending on whether the left
436 argument is stringwise less than, equal to, or greater than the right
440 Binary "~~" does a smart match between its arguments. Smart matching
441 is described in L<perlsyn/"Smart Matching in Detail">.
442 This operator is only available if you enable the "~~" feature:
443 see L<feature> for more information.
446 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
447 by the current locale if C<use locale> is in effect. See L<perllocale>.
450 X<operator, bitwise, and> X<bitwise and> X<&>
452 Binary "&" returns its operands ANDed together bit by bit.
453 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
455 Note that "&" has lower priority than relational operators, so for example
456 the brackets are essential in a test like
458 print "Even\n" if ($x & 1) == 0;
460 =head2 Bitwise Or and Exclusive Or
461 X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
464 Binary "|" returns its operands ORed together bit by bit.
465 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
467 Binary "^" returns its operands XORed together bit by bit.
468 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
470 Note that "|" and "^" have lower priority than relational operators, so
471 for example the brackets are essential in a test like
473 print "false\n" if (8 | 2) != 10;
475 =head2 C-style Logical And
476 X<&&> X<logical and> X<operator, logical, and>
478 Binary "&&" performs a short-circuit logical AND operation. That is,
479 if the left operand is false, the right operand is not even evaluated.
480 Scalar or list context propagates down to the right operand if it
483 =head2 C-style Logical Or
484 X<||> X<operator, logical, or>
486 Binary "||" performs a short-circuit logical OR operation. That is,
487 if the left operand is true, 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 Defined-Or
492 X<//> X<operator, logical, defined-or>
494 Although it has no direct equivalent in C, Perl's C<//> operator is related
495 to its C-style or. In fact, it's exactly the same as C<||>, except that it
496 tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
497 is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
498 rather than the value of C<defined($a)>) and is exactly equivalent to
499 C<defined($a) ? $a : $b>. This is very useful for providing default values
500 for variables. If you actually want to test if at least one of C<$a> and
501 C<$b> is defined, use C<defined($a // $b)>.
503 The C<||>, C<//> and C<&&> operators return the last value evaluated
504 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
505 portable way to find out the home directory might be:
507 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
508 (getpwuid($<))[7] // die "You're homeless!\n";
510 In particular, this means that you shouldn't use this
511 for selecting between two aggregates for assignment:
513 @a = @b || @c; # this is wrong
514 @a = scalar(@b) || @c; # really meant this
515 @a = @b ? @b : @c; # this works fine, though
517 As more readable alternatives to C<&&>, C<//> and C<||> when used for
518 control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
519 The short-circuit behavior is identical. The precedence of "and", "err"
520 and "or" is much lower, however, so that you can safely use them after a
521 list operator without the need for parentheses:
523 unlink "alpha", "beta", "gamma"
524 or gripe(), next LINE;
526 With the C-style operators that would have been written like this:
528 unlink("alpha", "beta", "gamma")
529 || (gripe(), next LINE);
531 Using "or" for assignment is unlikely to do what you want; see below.
533 =head2 Range Operators
534 X<operator, range> X<range> X<..> X<...>
536 Binary ".." is the range operator, which is really two different
537 operators depending on the context. In list context, it returns a
538 list of values counting (up by ones) from the left value to the right
539 value. If the left value is greater than the right value then it
540 returns the empty list. The range operator is useful for writing
541 C<foreach (1..10)> loops and for doing slice operations on arrays. In
542 the current implementation, no temporary array is created when the
543 range operator is used as the expression in C<foreach> loops, but older
544 versions of Perl might burn a lot of memory when you write something
547 for (1 .. 1_000_000) {
551 The range operator also works on strings, using the magical auto-increment,
554 In scalar context, ".." returns a boolean value. The operator is
555 bistable, like a flip-flop, and emulates the line-range (comma) operator
556 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
557 own boolean state. It is false as long as its left operand is false.
558 Once the left operand is true, the range operator stays true until the
559 right operand is true, I<AFTER> which the range operator becomes false
560 again. It doesn't become false till the next time the range operator is
561 evaluated. It can test the right operand and become false on the same
562 evaluation it became true (as in B<awk>), but it still returns true once.
563 If you don't want it to test the right operand till the next
564 evaluation, as in B<sed>, just use three dots ("...") instead of
565 two. In all other regards, "..." behaves just like ".." does.
567 The right operand is not evaluated while the operator is in the
568 "false" state, and the left operand is not evaluated while the
569 operator is in the "true" state. The precedence is a little lower
570 than || and &&. The value returned is either the empty string for
571 false, or a sequence number (beginning with 1) for true. The
572 sequence number is reset for each range encountered. The final
573 sequence number in a range has the string "E0" appended to it, which
574 doesn't affect its numeric value, but gives you something to search
575 for if you want to exclude the endpoint. You can exclude the
576 beginning point by waiting for the sequence number to be greater
579 If either operand of scalar ".." is a constant expression,
580 that operand is considered true if it is equal (C<==>) to the current
581 input line number (the C<$.> variable).
583 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
584 but that is only an issue if you use a floating point expression; when
585 implicitly using C<$.> as described in the previous paragraph, the
586 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
587 is set to a floating point value and you are not reading from a file.
588 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
589 you want in scalar context because each of the operands are evaluated
590 using their integer representation.
594 As a scalar operator:
596 if (101 .. 200) { print; } # print 2nd hundred lines, short for
597 # if ($. == 101 .. $. == 200) ...
599 next LINE if (1 .. /^$/); # skip header lines, short for
600 # ... if ($. == 1 .. /^$/);
601 # (typically in a loop labeled LINE)
603 s/^/> / if (/^$/ .. eof()); # quote body
605 # parse mail messages
607 $in_header = 1 .. /^$/;
608 $in_body = /^$/ .. eof;
615 close ARGV if eof; # reset $. each file
618 Here's a simple example to illustrate the difference between
619 the two range operators:
632 This program will print only the line containing "Bar". If
633 the range operator is changed to C<...>, it will also print the
636 And now some examples as a list operator:
638 for (101 .. 200) { print; } # print $_ 100 times
639 @foo = @foo[0 .. $#foo]; # an expensive no-op
640 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
642 The range operator (in list context) makes use of the magical
643 auto-increment algorithm if the operands are strings. You
646 @alphabet = ('A' .. 'Z');
648 to get all normal letters of the English alphabet, or
650 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
652 to get a hexadecimal digit, or
654 @z2 = ('01' .. '31'); print $z2[$mday];
656 to get dates with leading zeros.
658 If the final value specified is not in the sequence that the magical
659 increment would produce, the sequence goes until the next value would
660 be longer than the final value specified.
662 If the initial value specified isn't part of a magical increment
663 sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
664 only the initial value will be returned. So the following will only
667 use charnames 'greek';
668 my @greek_small = ("\N{alpha}" .. "\N{omega}");
670 To get lower-case greek letters, use this instead:
672 my @greek_small = map { chr } ( ord("\N{alpha}") .. ord("\N{omega}") );
674 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
675 return two elements in list context.
677 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
679 =head2 Conditional Operator
680 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
682 Ternary "?:" is the conditional operator, just as in C. It works much
683 like an if-then-else. If the argument before the ? is true, the
684 argument before the : is returned, otherwise the argument after the :
685 is returned. For example:
687 printf "I have %d dog%s.\n", $n,
688 ($n == 1) ? '' : "s";
690 Scalar or list context propagates downward into the 2nd
691 or 3rd argument, whichever is selected.
693 $a = $ok ? $b : $c; # get a scalar
694 @a = $ok ? @b : @c; # get an array
695 $a = $ok ? @b : @c; # oops, that's just a count!
697 The operator may be assigned to if both the 2nd and 3rd arguments are
698 legal lvalues (meaning that you can assign to them):
700 ($a_or_b ? $a : $b) = $c;
702 Because this operator produces an assignable result, using assignments
703 without parentheses will get you in trouble. For example, this:
705 $a % 2 ? $a += 10 : $a += 2
709 (($a % 2) ? ($a += 10) : $a) += 2
713 ($a % 2) ? ($a += 10) : ($a += 2)
715 That should probably be written more simply as:
717 $a += ($a % 2) ? 10 : 2;
719 =head2 Assignment Operators
720 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
721 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
724 "=" is the ordinary assignment operator.
726 Assignment operators work as in C. That is,
734 although without duplicating any side effects that dereferencing the lvalue
735 might trigger, such as from tie(). Other assignment operators work similarly.
736 The following are recognized:
743 Although these are grouped by family, they all have the precedence
746 Unlike in C, the scalar assignment operator produces a valid lvalue.
747 Modifying an assignment is equivalent to doing the assignment and
748 then modifying the variable that was assigned to. This is useful
749 for modifying a copy of something, like this:
751 ($tmp = $global) =~ tr [A-Z] [a-z];
762 Similarly, a list assignment in list context produces the list of
763 lvalues assigned to, and a list assignment in scalar context returns
764 the number of elements produced by the expression on the right hand
765 side of the assignment.
767 =head2 Comma Operator
768 X<comma> X<operator, comma> X<,>
770 Binary "," is the comma operator. In scalar context it evaluates
771 its left argument, throws that value away, then evaluates its right
772 argument and returns that value. This is just like C's comma operator.
774 In list context, it's just the list argument separator, and inserts
775 both its arguments into the list. These arguments are also evaluated
778 The C<< => >> operator is a synonym for the comma, but forces any word
779 (consisting entirely of word characters) to its left to be interpreted
780 as a string (as of 5.001). This includes words that might otherwise be
781 considered a constant or function call.
783 use constant FOO => "something";
785 my %h = ( FOO => 23 );
793 my %h = ("something", 23);
795 If the argument on the left is not a word, it is first interpreted as
796 an expression, and then the string value of that is used.
798 The C<< => >> operator is helpful in documenting the correspondence
799 between keys and values in hashes, and other paired elements in lists.
801 %hash = ( $key => $value );
802 login( $username => $password );
804 =head2 List Operators (Rightward)
805 X<operator, list, rightward> X<list operator>
807 On the right side of a list operator, it has very low precedence,
808 such that it controls all comma-separated expressions found there.
809 The only operators with lower precedence are the logical operators
810 "and", "or", and "not", which may be used to evaluate calls to list
811 operators without the need for extra parentheses:
813 open HANDLE, "filename"
814 or die "Can't open: $!\n";
816 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
819 X<operator, logical, not> X<not>
821 Unary "not" returns the logical negation of the expression to its right.
822 It's the equivalent of "!" except for the very low precedence.
825 X<operator, logical, and> X<and>
827 Binary "and" returns the logical conjunction of the two surrounding
828 expressions. It's equivalent to && except for the very low
829 precedence. This means that it short-circuits: i.e., the right
830 expression is evaluated only if the left expression is true.
832 =head2 Logical or, Defined or, and Exclusive Or
833 X<operator, logical, or> X<operator, logical, xor> X<operator, logical, err>
834 X<operator, logical, defined or> X<operator, logical, exclusive or>
837 Binary "or" returns the logical disjunction of the two surrounding
838 expressions. It's equivalent to || except for the very low precedence.
839 This makes it useful for control flow
841 print FH $data or die "Can't write to FH: $!";
843 This means that it short-circuits: i.e., the right expression is evaluated
844 only if the left expression is false. Due to its precedence, you should
845 probably avoid using this for assignment, only for control flow.
847 $a = $b or $c; # bug: this is wrong
848 ($a = $b) or $c; # really means this
849 $a = $b || $c; # better written this way
851 However, when it's a list-context assignment and you're trying to use
852 "||" for control flow, you probably need "or" so that the assignment
853 takes higher precedence.
855 @info = stat($file) || die; # oops, scalar sense of stat!
856 @info = stat($file) or die; # better, now @info gets its due
858 Then again, you could always use parentheses.
860 Binary "err" is equivalent to C<//>--it's just like binary "or", except it
861 tests its left argument's definedness instead of its truth. There are two
862 ways to remember "err": either because many functions return C<undef> on
863 an B<err>or, or as a sort of correction: C<$a = ($b err 'default')>. This
864 keyword is only available when the 'err' feature is enabled: see
865 L<feature> for more information.
867 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
868 It cannot short circuit, of course.
870 =head2 C Operators Missing From Perl
871 X<operator, missing from perl> X<&> X<*>
872 X<typecasting> X<(TYPE)>
874 Here is what C has that Perl doesn't:
880 Address-of operator. (But see the "\" operator for taking a reference.)
884 Dereference-address operator. (Perl's prefix dereferencing
885 operators are typed: $, @, %, and &.)
889 Type-casting operator.
893 =head2 Quote and Quote-like Operators
894 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
895 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
896 X<escape sequence> X<escape>
899 While we usually think of quotes as literal values, in Perl they
900 function as operators, providing various kinds of interpolating and
901 pattern matching capabilities. Perl provides customary quote characters
902 for these behaviors, but also provides a way for you to choose your
903 quote character for any of them. In the following table, a C<{}> represents
904 any pair of delimiters you choose.
906 Customary Generic Meaning Interpolates
911 // m{} Pattern match yes*
913 s{}{} Substitution yes*
914 tr{}{} Transliteration no (but see below)
917 * unless the delimiter is ''.
919 Non-bracketing delimiters use the same character fore and aft, but the four
920 sorts of brackets (round, angle, square, curly) will all nest, which means
929 Note, however, that this does not always work for quoting Perl code:
931 $s = q{ if($a eq "}") ... }; # WRONG
933 is a syntax error. The C<Text::Balanced> module (from CPAN, and
934 starting from Perl 5.8 part of the standard distribution) is able
937 There can be whitespace between the operator and the quoting
938 characters, except when C<#> is being used as the quoting character.
939 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
940 operator C<q> followed by a comment. Its argument will be taken
941 from the next line. This allows you to write:
943 s {foo} # Replace foo
946 The following escape sequences are available in constructs that interpolate
947 and in transliterations.
948 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N>
955 \a alarm (bell) (BEL)
957 \033 octal char (ESC)
959 \x{263a} wide hex char (SMILEY)
960 \c[ control char (ESC)
961 \N{name} named Unicode character
963 B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
964 the vertical tab (VT - ASCII 11).
966 The following escape sequences are available in constructs that interpolate
967 but not in transliterations.
968 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
970 \l lowercase next char
971 \u uppercase next char
974 \E end case modification
975 \Q quote non-word characters till \E
977 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
978 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
979 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
980 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
981 C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
984 All systems use the virtual C<"\n"> to represent a line terminator,
985 called a "newline". There is no such thing as an unvarying, physical
986 newline character. It is only an illusion that the operating system,
987 device drivers, C libraries, and Perl all conspire to preserve. Not all
988 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
989 on a Mac, these are reversed, and on systems without line terminator,
990 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
991 you mean a "newline" for your system, but use the literal ASCII when you
992 need an exact character. For example, most networking protocols expect
993 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
994 and although they often accept just C<"\012">, they seldom tolerate just
995 C<"\015">. If you get in the habit of using C<"\n"> for networking,
996 you may be burned some day.
997 X<newline> X<line terminator> X<eol> X<end of line>
1000 For constructs that do interpolate, variables beginning with "C<$>"
1001 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
1002 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1003 But method calls such as C<< $obj->meth >> are not.
1005 Interpolating an array or slice interpolates the elements in order,
1006 separated by the value of C<$">, so is equivalent to interpolating
1007 C<join $", @array>. "Punctuation" arrays such as C<@+> are only
1008 interpolated if the name is enclosed in braces C<@{+}>.
1010 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
1011 An unescaped C<$> or C<@> interpolates the corresponding variable,
1012 while escaping will cause the literal string C<\$> to be inserted.
1013 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
1015 Patterns are subject to an additional level of interpretation as a
1016 regular expression. This is done as a second pass, after variables are
1017 interpolated, so that regular expressions may be incorporated into the
1018 pattern from the variables. If this is not what you want, use C<\Q> to
1019 interpolate a variable literally.
1021 Apart from the behavior described above, Perl does not expand
1022 multiple levels of interpolation. In particular, contrary to the
1023 expectations of shell programmers, back-quotes do I<NOT> interpolate
1024 within double quotes, nor do single quotes impede evaluation of
1025 variables when used within double quotes.
1027 =head2 Regexp Quote-Like Operators
1030 Here are the quote-like operators that apply to pattern
1031 matching and related activities.
1038 This is just like the C</pattern/> search, except that it matches only
1039 once between calls to the reset() operator. This is a useful
1040 optimization when you want to see only the first occurrence of
1041 something in each file of a set of files, for instance. Only C<??>
1042 patterns local to the current package are reset.
1046 # blank line between header and body
1049 reset if eof; # clear ?? status for next file
1052 This usage is vaguely deprecated, which means it just might possibly
1053 be removed in some distant future version of Perl, perhaps somewhere
1054 around the year 2168.
1056 =item m/PATTERN/cgimosx
1057 X<m> X<operator, match>
1058 X<regexp, options> X<regexp> X<regex, options> X<regex>
1059 X</c> X</i> X</m> X</o> X</s> X</x>
1061 =item /PATTERN/cgimosx
1063 Searches a string for a pattern match, and in scalar context returns
1064 true if it succeeds, false if it fails. If no string is specified
1065 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1066 string specified with C<=~> need not be an lvalue--it may be the
1067 result of an expression evaluation, but remember the C<=~> binds
1068 rather tightly.) See also L<perlre>. See L<perllocale> for
1069 discussion of additional considerations that apply when C<use locale>
1074 c Do not reset search position on a failed match when /g is in effect.
1075 g Match globally, i.e., find all occurrences.
1076 i Do case-insensitive pattern matching.
1077 m Treat string as multiple lines.
1078 o Compile pattern only once.
1079 s Treat string as single line.
1080 x Use extended regular expressions.
1082 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1083 you can use any pair of non-alphanumeric, non-whitespace characters
1084 as delimiters. This is particularly useful for matching path names
1085 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1086 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
1087 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1089 PATTERN may contain variables, which will be interpolated (and the
1090 pattern recompiled) every time the pattern search is evaluated, except
1091 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1092 C<$|> are not interpolated because they look like end-of-string tests.)
1093 If you want such a pattern to be compiled only once, add a C</o> after
1094 the trailing delimiter. This avoids expensive run-time recompilations,
1095 and is useful when the value you are interpolating won't change over
1096 the life of the script. However, mentioning C</o> constitutes a promise
1097 that you won't change the variables in the pattern. If you change them,
1098 Perl won't even notice. See also L<"qr/STRING/imosx">.
1100 If the PATTERN evaluates to the empty string, the last
1101 I<successfully> matched regular expression is used instead. In this
1102 case, only the C<g> and C<c> flags on the empty pattern is honoured -
1103 the other flags are taken from the original pattern. If no match has
1104 previously succeeded, this will (silently) act instead as a genuine
1105 empty pattern (which will always match).
1107 Note that it's possible to confuse Perl into thinking C<//> (the empty
1108 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1109 good about this, but some pathological cases might trigger this, such as
1110 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1111 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1112 will assume you meant defined-or. If you meant the empty regex, just
1113 use parentheses or spaces to disambiguate, or even prefix the empty
1114 regex with an C<m> (so C<//> becomes C<m//>).
1116 If the C</g> option is not used, C<m//> in list context returns a
1117 list consisting of the subexpressions matched by the parentheses in the
1118 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1119 also set, and that this differs from Perl 4's behavior.) When there are
1120 no parentheses in the pattern, the return value is the list C<(1)> for
1121 success. With or without parentheses, an empty list is returned upon
1126 open(TTY, '/dev/tty');
1127 <TTY> =~ /^y/i && foo(); # do foo if desired
1129 if (/Version: *([0-9.]*)/) { $version = $1; }
1131 next if m#^/usr/spool/uucp#;
1136 print if /$arg/o; # compile only once
1139 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1141 This last example splits $foo into the first two words and the
1142 remainder of the line, and assigns those three fields to $F1, $F2, and
1143 $Etc. The conditional is true if any variables were assigned, i.e., if
1144 the pattern matched.
1146 The C</g> modifier specifies global pattern matching--that is,
1147 matching as many times as possible within the string. How it behaves
1148 depends on the context. In list context, it returns a list of the
1149 substrings matched by any capturing parentheses in the regular
1150 expression. If there are no parentheses, it returns a list of all
1151 the matched strings, as if there were parentheses around the whole
1154 In scalar context, each execution of C<m//g> finds the next match,
1155 returning true if it matches, and false if there is no further match.
1156 The position after the last match can be read or set using the pos()
1157 function; see L<perlfunc/pos>. A failed match normally resets the
1158 search position to the beginning of the string, but you can avoid that
1159 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1160 string also resets the search position.
1162 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1163 zero-width assertion that matches the exact position where the previous
1164 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1165 still anchors at pos(), but the match is of course only attempted once.
1166 Using C<\G> without C</g> on a target string that has not previously had a
1167 C</g> match applied to it is the same as using the C<\A> assertion to match
1168 the beginning of the string. Note also that, currently, C<\G> is only
1169 properly supported when anchored at the very beginning of the pattern.
1174 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1178 while (defined($paragraph = <>)) {
1179 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1183 print "$sentences\n";
1185 # using m//gc with \G
1189 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1191 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1193 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1195 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1197 The last example should print:
1207 Notice that the final match matched C<q> instead of C<p>, which a match
1208 without the C<\G> anchor would have done. Also note that the final match
1209 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1210 final match did indeed match C<p>, it's a good bet that you're running an
1211 older (pre-5.6.0) Perl.
1213 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1214 combine several regexps like this to process a string part-by-part,
1215 doing different actions depending on which regexp matched. Each
1216 regexp tries to match where the previous one leaves off.
1219 $url = new URI::URL "http://www/"; die if $url eq "xXx";
1223 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1224 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1225 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1226 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1227 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1228 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1229 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1230 print ". That's all!\n";
1233 Here is the output (split into several lines):
1235 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1236 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1237 lowercase lowercase line-noise lowercase lowercase line-noise
1238 MiXeD line-noise. That's all!
1241 X<q> X<quote, single> X<'> X<''>
1245 A single-quoted, literal string. A backslash represents a backslash
1246 unless followed by the delimiter or another backslash, in which case
1247 the delimiter or backslash is interpolated.
1249 $foo = q!I said, "You said, 'She said it.'"!;
1250 $bar = q('This is it.');
1251 $baz = '\n'; # a two-character string
1254 X<qq> X<quote, double> X<"> X<"">
1258 A double-quoted, interpolated string.
1261 (*** The previous line contains the naughty word "$1".\n)
1262 if /\b(tcl|java|python)\b/i; # :-)
1263 $baz = "\n"; # a one-character string
1265 =item qr/STRING/imosx
1266 X<qr> X</i> X</m> X</o> X</s> X</x>
1268 This operator quotes (and possibly compiles) its I<STRING> as a regular
1269 expression. I<STRING> is interpolated the same way as I<PATTERN>
1270 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1271 is done. Returns a Perl value which may be used instead of the
1272 corresponding C</STRING/imosx> expression.
1276 $rex = qr/my.STRING/is;
1283 The result may be used as a subpattern in a match:
1286 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1287 $string =~ $re; # or used standalone
1288 $string =~ /$re/; # or this way
1290 Since Perl may compile the pattern at the moment of execution of qr()
1291 operator, using qr() may have speed advantages in some situations,
1292 notably if the result of qr() is used standalone:
1295 my $patterns = shift;
1296 my @compiled = map qr/$_/i, @$patterns;
1299 foreach my $pat (@compiled) {
1300 $success = 1, last if /$pat/;
1306 Precompilation of the pattern into an internal representation at
1307 the moment of qr() avoids a need to recompile the pattern every
1308 time a match C</$pat/> is attempted. (Perl has many other internal
1309 optimizations, but none would be triggered in the above example if
1310 we did not use qr() operator.)
1314 i Do case-insensitive pattern matching.
1315 m Treat string as multiple lines.
1316 o Compile pattern only once.
1317 s Treat string as single line.
1318 x Use extended regular expressions.
1320 See L<perlre> for additional information on valid syntax for STRING, and
1321 for a detailed look at the semantics of regular expressions.
1324 X<qx> X<`> X<``> X<backtick>
1328 A string which is (possibly) interpolated and then executed as a
1329 system command with C</bin/sh> or its equivalent. Shell wildcards,
1330 pipes, and redirections will be honored. The collected standard
1331 output of the command is returned; standard error is unaffected. In
1332 scalar context, it comes back as a single (potentially multi-line)
1333 string, or undef if the command failed. In list context, returns a
1334 list of lines (however you've defined lines with $/ or
1335 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1337 Because backticks do not affect standard error, use shell file descriptor
1338 syntax (assuming the shell supports this) if you care to address this.
1339 To capture a command's STDERR and STDOUT together:
1341 $output = `cmd 2>&1`;
1343 To capture a command's STDOUT but discard its STDERR:
1345 $output = `cmd 2>/dev/null`;
1347 To capture a command's STDERR but discard its STDOUT (ordering is
1350 $output = `cmd 2>&1 1>/dev/null`;
1352 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1353 but leave its STDOUT to come out the old STDERR:
1355 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1357 To read both a command's STDOUT and its STDERR separately, it's easiest
1358 to redirect them separately to files, and then read from those files
1359 when the program is done:
1361 system("program args 1>program.stdout 2>program.stderr");
1363 The STDIN filehandle used by the command is inherited from Perl's STDIN.
1366 open BLAM, "blam" || die "Can't open: $!";
1367 open STDIN, "<&BLAM";
1370 will print the sorted contents of the file "blam".
1372 Using single-quote as a delimiter protects the command from Perl's
1373 double-quote interpolation, passing it on to the shell instead:
1375 $perl_info = qx(ps $$); # that's Perl's $$
1376 $shell_info = qx'ps $$'; # that's the new shell's $$
1378 How that string gets evaluated is entirely subject to the command
1379 interpreter on your system. On most platforms, you will have to protect
1380 shell metacharacters if you want them treated literally. This is in
1381 practice difficult to do, as it's unclear how to escape which characters.
1382 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1383 to emulate backticks safely.
1385 On some platforms (notably DOS-like ones), the shell may not be
1386 capable of dealing with multiline commands, so putting newlines in
1387 the string may not get you what you want. You may be able to evaluate
1388 multiple commands in a single line by separating them with the command
1389 separator character, if your shell supports that (e.g. C<;> on many Unix
1390 shells; C<&> on the Windows NT C<cmd> shell).
1392 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1393 output before starting the child process, but this may not be supported
1394 on some platforms (see L<perlport>). To be safe, you may need to set
1395 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1396 C<IO::Handle> on any open handles.
1398 Beware that some command shells may place restrictions on the length
1399 of the command line. You must ensure your strings don't exceed this
1400 limit after any necessary interpolations. See the platform-specific
1401 release notes for more details about your particular environment.
1403 Using this operator can lead to programs that are difficult to port,
1404 because the shell commands called vary between systems, and may in
1405 fact not be present at all. As one example, the C<type> command under
1406 the POSIX shell is very different from the C<type> command under DOS.
1407 That doesn't mean you should go out of your way to avoid backticks
1408 when they're the right way to get something done. Perl was made to be
1409 a glue language, and one of the things it glues together is commands.
1410 Just understand what you're getting yourself into.
1412 See L</"I/O Operators"> for more discussion.
1415 X<qw> X<quote, list> X<quote, words>
1417 Evaluates to a list of the words extracted out of STRING, using embedded
1418 whitespace as the word delimiters. It can be understood as being roughly
1421 split(' ', q/STRING/);
1423 the differences being that it generates a real list at compile time, and
1424 in scalar context it returns the last element in the list. So
1429 is semantically equivalent to the list:
1433 Some frequently seen examples:
1435 use POSIX qw( setlocale localeconv )
1436 @EXPORT = qw( foo bar baz );
1438 A common mistake is to try to separate the words with comma or to
1439 put comments into a multi-line C<qw>-string. For this reason, the
1440 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1441 produces warnings if the STRING contains the "," or the "#" character.
1443 =item s/PATTERN/REPLACEMENT/egimosx
1444 X<substitute> X<substitution> X<replace> X<regexp, replace>
1445 X<regexp, substitute> X</e> X</g> X</i> X</m> X</o> X</s> X</x>
1447 Searches a string for a pattern, and if found, replaces that pattern
1448 with the replacement text and returns the number of substitutions
1449 made. Otherwise it returns false (specifically, the empty string).
1451 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1452 variable is searched and modified. (The string specified with C<=~> must
1453 be scalar variable, an array element, a hash element, or an assignment
1454 to one of those, i.e., an lvalue.)
1456 If the delimiter chosen is a single quote, no interpolation is
1457 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1458 PATTERN contains a $ that looks like a variable rather than an
1459 end-of-string test, the variable will be interpolated into the pattern
1460 at run-time. If you want the pattern compiled only once the first time
1461 the variable is interpolated, use the C</o> option. If the pattern
1462 evaluates to the empty string, the last successfully executed regular
1463 expression is used instead. See L<perlre> for further explanation on these.
1464 See L<perllocale> for discussion of additional considerations that apply
1465 when C<use locale> is in effect.
1469 e Evaluate the right side as an expression.
1470 g Replace globally, i.e., all occurrences.
1471 i Do case-insensitive pattern matching.
1472 m Treat string as multiple lines.
1473 o Compile pattern only once.
1474 s Treat string as single line.
1475 x Use extended regular expressions.
1477 Any non-alphanumeric, non-whitespace delimiter may replace the
1478 slashes. If single quotes are used, no interpretation is done on the
1479 replacement string (the C</e> modifier overrides this, however). Unlike
1480 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1481 text is not evaluated as a command. If the
1482 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1483 pair of quotes, which may or may not be bracketing quotes, e.g.,
1484 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1485 replacement portion to be treated as a full-fledged Perl expression
1486 and evaluated right then and there. It is, however, syntax checked at
1487 compile-time. A second C<e> modifier will cause the replacement portion
1488 to be C<eval>ed before being run as a Perl expression.
1492 s/\bgreen\b/mauve/g; # don't change wintergreen
1494 $path =~ s|/usr/bin|/usr/local/bin|;
1496 s/Login: $foo/Login: $bar/; # run-time pattern
1498 ($foo = $bar) =~ s/this/that/; # copy first, then change
1500 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1503 s/\d+/$&*2/e; # yields 'abc246xyz'
1504 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1505 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1507 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1508 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1509 s/^=(\w+)/&pod($1)/ge; # use function call
1511 # expand variables in $_, but dynamics only, using
1512 # symbolic dereferencing
1515 # Add one to the value of any numbers in the string
1518 # This will expand any embedded scalar variable
1519 # (including lexicals) in $_ : First $1 is interpolated
1520 # to the variable name, and then evaluated
1523 # Delete (most) C comments.
1525 /\* # Match the opening delimiter.
1526 .*? # Match a minimal number of characters.
1527 \*/ # Match the closing delimiter.
1530 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1532 for ($variable) { # trim whitespace in $variable, cheap
1537 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1539 Note the use of $ instead of \ in the last example. Unlike
1540 B<sed>, we use the \<I<digit>> form in only the left hand side.
1541 Anywhere else it's $<I<digit>>.
1543 Occasionally, you can't use just a C</g> to get all the changes
1544 to occur that you might want. Here are two common cases:
1546 # put commas in the right places in an integer
1547 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1549 # expand tabs to 8-column spacing
1550 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1552 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1553 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
1555 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1557 Transliterates all occurrences of the characters found in the search list
1558 with the corresponding character in the replacement list. It returns
1559 the number of characters replaced or deleted. If no string is
1560 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1561 string specified with =~ must be a scalar variable, an array element, a
1562 hash element, or an assignment to one of those, i.e., an lvalue.)
1564 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1565 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1566 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1567 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1568 its own pair of quotes, which may or may not be bracketing quotes,
1569 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1571 Note that C<tr> does B<not> do regular expression character classes
1572 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1573 the tr(1) utility. If you want to map strings between lower/upper
1574 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1575 using the C<s> operator if you need regular expressions.
1577 Note also that the whole range idea is rather unportable between
1578 character sets--and even within character sets they may cause results
1579 you probably didn't expect. A sound principle is to use only ranges
1580 that begin from and end at either alphabets of equal case (a-e, A-E),
1581 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1582 character sets in full.
1586 c Complement the SEARCHLIST.
1587 d Delete found but unreplaced characters.
1588 s Squash duplicate replaced characters.
1590 If the C</c> modifier is specified, the SEARCHLIST character set
1591 is complemented. If the C</d> modifier is specified, any characters
1592 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1593 (Note that this is slightly more flexible than the behavior of some
1594 B<tr> programs, which delete anything they find in the SEARCHLIST,
1595 period.) If the C</s> modifier is specified, sequences of characters
1596 that were transliterated to the same character are squashed down
1597 to a single instance of the character.
1599 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1600 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1601 than the SEARCHLIST, the final character is replicated till it is long
1602 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1603 This latter is useful for counting characters in a class or for
1604 squashing character sequences in a class.
1608 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1610 $cnt = tr/*/*/; # count the stars in $_
1612 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1614 $cnt = tr/0-9//; # count the digits in $_
1616 tr/a-zA-Z//s; # bookkeeper -> bokeper
1618 ($HOST = $host) =~ tr/a-z/A-Z/;
1620 tr/a-zA-Z/ /cs; # change non-alphas to single space
1623 [\000-\177]; # delete 8th bit
1625 If multiple transliterations are given for a character, only the
1630 will transliterate any A to X.
1632 Because the transliteration table is built at compile time, neither
1633 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1634 interpolation. That means that if you want to use variables, you
1637 eval "tr/$oldlist/$newlist/";
1640 eval "tr/$oldlist/$newlist/, 1" or die $@;
1643 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
1645 A line-oriented form of quoting is based on the shell "here-document"
1646 syntax. Following a C<< << >> you specify a string to terminate
1647 the quoted material, and all lines following the current line down to
1648 the terminating string are the value of the item.
1650 The terminating string may be either an identifier (a word), or some
1651 quoted text. An unquoted identifier works like double quotes.
1652 There may not be a space between the C<< << >> and the identifier,
1653 unless the identifier is explicitly quoted. (If you put a space it
1654 will be treated as a null identifier, which is valid, and matches the
1655 first empty line.) The terminating string must appear by itself
1656 (unquoted and with no surrounding whitespace) on the terminating line.
1658 If the terminating string is quoted, the type of quotes used determine
1659 the treatment of the text.
1665 Double quotes indicate that the text will be interpolated using exactly
1666 the same rules as normal double quoted strings.
1669 The price is $Price.
1672 print << "EOF"; # same as above
1673 The price is $Price.
1679 Single quotes indicate the text is to be treated literally with no
1680 interpolation of its content. This is similar to single quoted
1681 strings except that backslashes have no special meaning, with C<\\>
1682 being treated as two backslashes and not one as they would in every
1683 other quoting construct.
1685 This is the only form of quoting in perl where there is no need
1686 to worry about escaping content, something that code generators
1687 can and do make good use of.
1691 The content of the here doc is treated just as it would be if the
1692 string were embedded in backticks. Thus the content is interpolated
1693 as though it were double quoted and then executed via the shell, with
1694 the results of the execution returned.
1696 print << `EOC`; # execute command and get results
1702 It is possible to stack multiple here-docs in a row:
1704 print <<"foo", <<"bar"; # you can stack them
1710 myfunc(<< "THIS", 23, <<'THAT');
1717 Just don't forget that you have to put a semicolon on the end
1718 to finish the statement, as Perl doesn't know you're not going to
1726 If you want to remove the line terminator from your here-docs,
1729 chomp($string = <<'END');
1733 If you want your here-docs to be indented with the rest of the code,
1734 you'll need to remove leading whitespace from each line manually:
1736 ($quote = <<'FINIS') =~ s/^\s+//gm;
1737 The Road goes ever on and on,
1738 down from the door where it began.
1741 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1742 the quoted material must come on the lines following the final delimiter.
1757 If the terminating identifier is on the last line of the program, you
1758 must be sure there is a newline after it; otherwise, Perl will give the
1759 warning B<Can't find string terminator "END" anywhere before EOF...>.
1761 Additionally, the quoting rules for the end of string identifier are not
1762 related to Perl's quoting rules -- C<q()>, C<qq()>, and the like are not
1763 supported in place of C<''> and C<"">, and the only interpolation is for
1764 backslashing the quoting character:
1766 print << "abc\"def";
1770 Finally, quoted strings cannot span multiple lines. The general rule is
1771 that the identifier must be a string literal. Stick with that, and you
1776 =head2 Gory details of parsing quoted constructs
1777 X<quote, gory details>
1779 When presented with something that might have several different
1780 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1781 principle to pick the most probable interpretation. This strategy
1782 is so successful that Perl programmers often do not suspect the
1783 ambivalence of what they write. But from time to time, Perl's
1784 notions differ substantially from what the author honestly meant.
1786 This section hopes to clarify how Perl handles quoted constructs.
1787 Although the most common reason to learn this is to unravel labyrinthine
1788 regular expressions, because the initial steps of parsing are the
1789 same for all quoting operators, they are all discussed together.
1791 The most important Perl parsing rule is the first one discussed
1792 below: when processing a quoted construct, Perl first finds the end
1793 of that construct, then interprets its contents. If you understand
1794 this rule, you may skip the rest of this section on the first
1795 reading. The other rules are likely to contradict the user's
1796 expectations much less frequently than this first one.
1798 Some passes discussed below are performed concurrently, but because
1799 their results are the same, we consider them individually. For different
1800 quoting constructs, Perl performs different numbers of passes, from
1801 one to five, but these passes are always performed in the same order.
1805 =item Finding the end
1807 The first pass is finding the end of the quoted construct, whether
1808 it be a multicharacter delimiter C<"EOF\n"> in the C<<<EOF>
1809 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1810 terminates C<qq[]> construct, or a C<< > >> which terminates a
1811 fileglob started with C<< < >>.
1813 When searching for single-character non-pairing delimiters, such
1814 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1815 when searching for single-character pairing delimiter like C<[>,
1816 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1817 C<[>, C<]> are skipped as well. When searching for multicharacter
1818 delimiters like C<"EOF\n">, nothing is skipped, though the delimiter
1819 must start from the first column of the terminating line.
1821 For constructs with three-part delimiters (C<s///>, C<y///>, and
1822 C<tr///>), the search is repeated once more.
1824 During this search no attention is paid to the semantics of the construct.
1827 "$hash{"$foo/$bar"}"
1832 bar # NOT a comment, this slash / terminated m//!
1835 do not form legal quoted expressions. The quoted part ends on the
1836 first C<"> and C</>, and the rest happens to be a syntax error.
1837 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1838 the example above is not C<m//x>, but rather C<m//> with no C</x>
1839 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1841 Also no attention is paid to C<\c\> (multichar control char syntax) during
1842 this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
1843 of C<\/>, and the following C</> is not recognized as a delimiter.
1844 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1846 =item Removal of backslashes before delimiters
1848 During the second pass, text between the starting and ending
1849 delimiters is copied to a safe location, and the C<\> is removed
1850 from combinations consisting of C<\> and delimiter--or delimiters,
1851 meaning both starting and ending delimiters will be handled,
1852 should these differ. This removal does not happen for multi-character
1853 delimiters. Note that the combination C<\\> is left intact.
1855 Starting from this step no information about the delimiters is
1861 The next step is interpolation in the text obtained, which is now
1862 delimiter-independent. There are multiple cases.
1868 No interpolation is performed.
1870 =item C<m''>, C<s'''>
1872 No interpolation is performed at this stage, see
1873 L</"Interpolation of regular expressions"> for comments on later
1874 processing of their contents.
1878 The only interpolation is removal of C<\> from pairs of C<\\>.
1880 =item C<tr///>, C<y///>
1882 No variable interpolation occurs. Escape sequences such as \200
1883 and the common escapes such as \t for tab are converted to literals.
1884 The character C<-> is treated specially and therefore C<\-> is treated
1887 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
1889 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1890 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1891 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1892 The other combinations are replaced with appropriate expansions.
1894 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1895 is interpolated in the usual way. Something like C<"\Q\\E"> has
1896 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1897 result is the same as for C<"\\\\E">. As a general rule, backslashes
1898 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1899 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1900 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1905 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1907 Interpolated scalars and arrays are converted internally to the C<join> and
1908 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1910 $foo . " XXX '" . (join $", @arr) . "'";
1912 All operations above are performed simultaneously, left to right.
1914 Because the result of C<"\Q STRING \E"> has all metacharacters
1915 quoted, there is no way to insert a literal C<$> or C<@> inside a
1916 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1917 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1920 Note also that the interpolation code needs to make a decision on
1921 where the interpolated scalar ends. For instance, whether
1922 C<< "a $b -> {c}" >> really means:
1924 "a " . $b . " -> {c}";
1930 Most of the time, the longest possible text that does not include
1931 spaces between components and which contains matching braces or
1932 brackets. because the outcome may be determined by voting based
1933 on heuristic estimators, the result is not strictly predictable.
1934 Fortunately, it's usually correct for ambiguous cases.
1936 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1938 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1939 happens (almost) as with C<qq//> constructs, but the substitution
1940 of C<\> followed by RE-special chars (including C<\>) is not
1941 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1942 a C<#>-comment in a C<//x>-regular expression, no processing is
1943 performed whatsoever. This is the first step at which the presence
1944 of the C<//x> modifier is relevant.
1946 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1947 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1948 different estimators) to be either an array element or C<$var>
1949 followed by an RE alternative. This is where the notation
1950 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1951 array element C<-9>, not as a regular expression from the variable
1952 C<$arr> followed by a digit, which would be the interpretation of
1953 C</$arr[0-9]/>. Since voting among different estimators may occur,
1954 the result is not predictable.
1956 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1957 the replacement text of C<s///> to correct the incorrigible
1958 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1959 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1960 (that is, the C<$^W> variable) was set.
1962 The lack of processing of C<\\> creates specific restrictions on
1963 the post-processed text. If the delimiter is C</>, one cannot get
1964 the combination C<\/> into the result of this step. C</> will
1965 finish the regular expression, C<\/> will be stripped to C</> on
1966 the previous step, and C<\\/> will be left as is. Because C</> is
1967 equivalent to C<\/> inside a regular expression, this does not
1968 matter unless the delimiter happens to be character special to the
1969 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1970 alphanumeric char, as in:
1974 In the RE above, which is intentionally obfuscated for illustration, the
1975 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1976 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
1977 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1978 non-whitespace choices.
1982 This step is the last one for all constructs except regular expressions,
1983 which are processed further.
1985 =item Interpolation of regular expressions
1986 X<regexp, interpolation>
1988 Previous steps were performed during the compilation of Perl code,
1989 but this one happens at run time--although it may be optimized to
1990 be calculated at compile time if appropriate. After preprocessing
1991 described above, and possibly after evaluation if catenation,
1992 joining, casing translation, or metaquoting are involved, the
1993 resulting I<string> is passed to the RE engine for compilation.
1995 Whatever happens in the RE engine might be better discussed in L<perlre>,
1996 but for the sake of continuity, we shall do so here.
1998 This is another step where the presence of the C<//x> modifier is
1999 relevant. The RE engine scans the string from left to right and
2000 converts it to a finite automaton.
2002 Backslashed characters are either replaced with corresponding
2003 literal strings (as with C<\{>), or else they generate special nodes
2004 in the finite automaton (as with C<\b>). Characters special to the
2005 RE engine (such as C<|>) generate corresponding nodes or groups of
2006 nodes. C<(?#...)> comments are ignored. All the rest is either
2007 converted to literal strings to match, or else is ignored (as is
2008 whitespace and C<#>-style comments if C<//x> is present).
2010 Parsing of the bracketed character class construct, C<[...]>, is
2011 rather different than the rule used for the rest of the pattern.
2012 The terminator of this construct is found using the same rules as
2013 for finding the terminator of a C<{}>-delimited construct, the only
2014 exception being that C<]> immediately following C<[> is treated as
2015 though preceded by a backslash. Similarly, the terminator of
2016 C<(?{...})> is found using the same rules as for finding the
2017 terminator of a C<{}>-delimited construct.
2019 It is possible to inspect both the string given to RE engine and the
2020 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2021 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
2022 switch documented in L<perlrun/"Command Switches">.
2024 =item Optimization of regular expressions
2025 X<regexp, optimization>
2027 This step is listed for completeness only. Since it does not change
2028 semantics, details of this step are not documented and are subject
2029 to change without notice. This step is performed over the finite
2030 automaton that was generated during the previous pass.
2032 It is at this stage that C<split()> silently optimizes C</^/> to
2037 =head2 I/O Operators
2038 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2041 There are several I/O operators you should know about.
2043 A string enclosed by backticks (grave accents) first undergoes
2044 double-quote interpolation. It is then interpreted as an external
2045 command, and the output of that command is the value of the
2046 backtick string, like in a shell. In scalar context, a single string
2047 consisting of all output is returned. In list context, a list of
2048 values is returned, one per line of output. (You can set C<$/> to use
2049 a different line terminator.) The command is executed each time the
2050 pseudo-literal is evaluated. The status value of the command is
2051 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2052 Unlike in B<csh>, no translation is done on the return data--newlines
2053 remain newlines. Unlike in any of the shells, single quotes do not
2054 hide variable names in the command from interpretation. To pass a
2055 literal dollar-sign through to the shell you need to hide it with a
2056 backslash. The generalized form of backticks is C<qx//>. (Because
2057 backticks always undergo shell expansion as well, see L<perlsec> for
2059 X<qx> X<`> X<``> X<backtick> X<glob>
2061 In scalar context, evaluating a filehandle in angle brackets yields
2062 the next line from that file (the newline, if any, included), or
2063 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2064 (sometimes known as file-slurp mode) and the file is empty, it
2065 returns C<''> the first time, followed by C<undef> subsequently.
2067 Ordinarily you must assign the returned value to a variable, but
2068 there is one situation where an automatic assignment happens. If
2069 and only if the input symbol is the only thing inside the conditional
2070 of a C<while> statement (even if disguised as a C<for(;;)> loop),
2071 the value is automatically assigned to the global variable $_,
2072 destroying whatever was there previously. (This may seem like an
2073 odd thing to you, but you'll use the construct in almost every Perl
2074 script you write.) The $_ variable is not implicitly localized.
2075 You'll have to put a C<local $_;> before the loop if you want that
2078 The following lines are equivalent:
2080 while (defined($_ = <STDIN>)) { print; }
2081 while ($_ = <STDIN>) { print; }
2082 while (<STDIN>) { print; }
2083 for (;<STDIN>;) { print; }
2084 print while defined($_ = <STDIN>);
2085 print while ($_ = <STDIN>);
2086 print while <STDIN>;
2088 This also behaves similarly, but avoids $_ :
2090 while (my $line = <STDIN>) { print $line }
2092 In these loop constructs, the assigned value (whether assignment
2093 is automatic or explicit) is then tested to see whether it is
2094 defined. The defined test avoids problems where line has a string
2095 value that would be treated as false by Perl, for example a "" or
2096 a "0" with no trailing newline. If you really mean for such values
2097 to terminate the loop, they should be tested for explicitly:
2099 while (($_ = <STDIN>) ne '0') { ... }
2100 while (<STDIN>) { last unless $_; ... }
2102 In other boolean contexts, C<< <I<filehandle>> >> without an
2103 explicit C<defined> test or comparison elicit a warning if the
2104 C<use warnings> pragma or the B<-w>
2105 command-line switch (the C<$^W> variable) is in effect.
2107 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2108 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2109 in packages, where they would be interpreted as local identifiers
2110 rather than global.) Additional filehandles may be created with
2111 the open() function, amongst others. See L<perlopentut> and
2112 L<perlfunc/open> for details on this.
2113 X<stdin> X<stdout> X<sterr>
2115 If a <FILEHANDLE> is used in a context that is looking for
2116 a list, a list comprising all input lines is returned, one line per
2117 list element. It's easy to grow to a rather large data space this
2118 way, so use with care.
2120 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2121 See L<perlfunc/readline>.
2123 The null filehandle <> is special: it can be used to emulate the
2124 behavior of B<sed> and B<awk>. Input from <> comes either from
2125 standard input, or from each file listed on the command line. Here's
2126 how it works: the first time <> is evaluated, the @ARGV array is
2127 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2128 gives you standard input. The @ARGV array is then processed as a list
2129 of filenames. The loop
2132 ... # code for each line
2135 is equivalent to the following Perl-like pseudo code:
2137 unshift(@ARGV, '-') unless @ARGV;
2138 while ($ARGV = shift) {
2141 ... # code for each line
2145 except that it isn't so cumbersome to say, and will actually work.
2146 It really does shift the @ARGV array and put the current filename
2147 into the $ARGV variable. It also uses filehandle I<ARGV>
2148 internally--<> is just a synonym for <ARGV>, which
2149 is magical. (The pseudo code above doesn't work because it treats
2150 <ARGV> as non-magical.)
2152 You can modify @ARGV before the first <> as long as the array ends up
2153 containing the list of filenames you really want. Line numbers (C<$.>)
2154 continue as though the input were one big happy file. See the example
2155 in L<perlfunc/eof> for how to reset line numbers on each file.
2157 If you want to set @ARGV to your own list of files, go right ahead.
2158 This sets @ARGV to all plain text files if no @ARGV was given:
2160 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2162 You can even set them to pipe commands. For example, this automatically
2163 filters compressed arguments through B<gzip>:
2165 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2167 If you want to pass switches into your script, you can use one of the
2168 Getopts modules or put a loop on the front like this:
2170 while ($_ = $ARGV[0], /^-/) {
2173 if (/^-D(.*)/) { $debug = $1 }
2174 if (/^-v/) { $verbose++ }
2175 # ... # other switches
2179 # ... # code for each line
2182 The <> symbol will return C<undef> for end-of-file only once.
2183 If you call it again after this, it will assume you are processing another
2184 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2186 If what the angle brackets contain is a simple scalar variable (e.g.,
2187 <$foo>), then that variable contains the name of the
2188 filehandle to input from, or its typeglob, or a reference to the
2194 If what's within the angle brackets is neither a filehandle nor a simple
2195 scalar variable containing a filehandle name, typeglob, or typeglob
2196 reference, it is interpreted as a filename pattern to be globbed, and
2197 either a list of filenames or the next filename in the list is returned,
2198 depending on context. This distinction is determined on syntactic
2199 grounds alone. That means C<< <$x> >> is always a readline() from
2200 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2201 That's because $x is a simple scalar variable, but C<$hash{key}> is
2202 not--it's a hash element. Even C<< <$x > >> (note the extra space)
2203 is treated as C<glob("$x ")>, not C<readline($x)>.
2205 One level of double-quote interpretation is done first, but you can't
2206 say C<< <$foo> >> because that's an indirect filehandle as explained
2207 in the previous paragraph. (In older versions of Perl, programmers
2208 would insert curly brackets to force interpretation as a filename glob:
2209 C<< <${foo}> >>. These days, it's considered cleaner to call the
2210 internal function directly as C<glob($foo)>, which is probably the right
2211 way to have done it in the first place.) For example:
2217 is roughly equivalent to:
2219 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2225 except that the globbing is actually done internally using the standard
2226 C<File::Glob> extension. Of course, the shortest way to do the above is:
2230 A (file)glob evaluates its (embedded) argument only when it is
2231 starting a new list. All values must be read before it will start
2232 over. In list context, this isn't important because you automatically
2233 get them all anyway. However, in scalar context the operator returns
2234 the next value each time it's called, or C<undef> when the list has
2235 run out. As with filehandle reads, an automatic C<defined> is
2236 generated when the glob occurs in the test part of a C<while>,
2237 because legal glob returns (e.g. a file called F<0>) would otherwise
2238 terminate the loop. Again, C<undef> is returned only once. So if
2239 you're expecting a single value from a glob, it is much better to
2242 ($file) = <blurch*>;
2248 because the latter will alternate between returning a filename and
2251 If you're trying to do variable interpolation, it's definitely better
2252 to use the glob() function, because the older notation can cause people
2253 to become confused with the indirect filehandle notation.
2255 @files = glob("$dir/*.[ch]");
2256 @files = glob($files[$i]);
2258 =head2 Constant Folding
2259 X<constant folding> X<folding>
2261 Like C, Perl does a certain amount of expression evaluation at
2262 compile time whenever it determines that all arguments to an
2263 operator are static and have no side effects. In particular, string
2264 concatenation happens at compile time between literals that don't do
2265 variable substitution. Backslash interpolation also happens at
2266 compile time. You can say
2268 'Now is the time for all' . "\n" .
2269 'good men to come to.'
2271 and this all reduces to one string internally. Likewise, if
2274 foreach $file (@filenames) {
2275 if (-s $file > 5 + 100 * 2**16) { }
2278 the compiler will precompute the number which that expression
2279 represents so that the interpreter won't have to.
2284 Perl doesn't officially have a no-op operator, but the bare constants
2285 C<0> and C<1> are special-cased to not produce a warning in a void
2286 context, so you can for example safely do
2290 =head2 Bitwise String Operators
2291 X<operator, bitwise, string>
2293 Bitstrings of any size may be manipulated by the bitwise operators
2296 If the operands to a binary bitwise op are strings of different
2297 sizes, B<|> and B<^> ops act as though the shorter operand had
2298 additional zero bits on the right, while the B<&> op acts as though
2299 the longer operand were truncated to the length of the shorter.
2300 The granularity for such extension or truncation is one or more
2303 # ASCII-based examples
2304 print "j p \n" ^ " a h"; # prints "JAPH\n"
2305 print "JA" | " ph\n"; # prints "japh\n"
2306 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2307 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2309 If you are intending to manipulate bitstrings, be certain that
2310 you're supplying bitstrings: If an operand is a number, that will imply
2311 a B<numeric> bitwise operation. You may explicitly show which type of
2312 operation you intend by using C<""> or C<0+>, as in the examples below.
2314 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2315 $foo = '150' | 105; # yields 255
2316 $foo = 150 | '105'; # yields 255
2317 $foo = '150' | '105'; # yields string '155' (under ASCII)
2319 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2320 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2322 See L<perlfunc/vec> for information on how to manipulate individual bits
2325 =head2 Integer Arithmetic
2328 By default, Perl assumes that it must do most of its arithmetic in
2329 floating point. But by saying
2333 you may tell the compiler that it's okay to use integer operations
2334 (if it feels like it) from here to the end of the enclosing BLOCK.
2335 An inner BLOCK may countermand this by saying
2339 which lasts until the end of that BLOCK. Note that this doesn't
2340 mean everything is only an integer, merely that Perl may use integer
2341 operations if it is so inclined. For example, even under C<use
2342 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2345 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2346 and ">>") always produce integral results. (But see also
2347 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2348 them. By default, their results are interpreted as unsigned integers, but
2349 if C<use integer> is in effect, their results are interpreted
2350 as signed integers. For example, C<~0> usually evaluates to a large
2351 integral value. However, C<use integer; ~0> is C<-1> on two's-complement
2354 =head2 Floating-point Arithmetic
2355 X<floating-point> X<floating point> X<float> X<real>
2357 While C<use integer> provides integer-only arithmetic, there is no
2358 analogous mechanism to provide automatic rounding or truncation to a
2359 certain number of decimal places. For rounding to a certain number
2360 of digits, sprintf() or printf() is usually the easiest route.
2363 Floating-point numbers are only approximations to what a mathematician
2364 would call real numbers. There are infinitely more reals than floats,
2365 so some corners must be cut. For example:
2367 printf "%.20g\n", 123456789123456789;
2368 # produces 123456789123456784
2370 Testing for exact equality of floating-point equality or inequality is
2371 not a good idea. Here's a (relatively expensive) work-around to compare
2372 whether two floating-point numbers are equal to a particular number of
2373 decimal places. See Knuth, volume II, for a more robust treatment of
2377 my ($X, $Y, $POINTS) = @_;
2379 $tX = sprintf("%.${POINTS}g", $X);
2380 $tY = sprintf("%.${POINTS}g", $Y);
2384 The POSIX module (part of the standard perl distribution) implements
2385 ceil(), floor(), and other mathematical and trigonometric functions.
2386 The Math::Complex module (part of the standard perl distribution)
2387 defines mathematical functions that work on both the reals and the
2388 imaginary numbers. Math::Complex not as efficient as POSIX, but
2389 POSIX can't work with complex numbers.
2391 Rounding in financial applications can have serious implications, and
2392 the rounding method used should be specified precisely. In these
2393 cases, it probably pays not to trust whichever system rounding is
2394 being used by Perl, but to instead implement the rounding function you
2397 =head2 Bigger Numbers
2398 X<number, arbitrary precision>
2400 The standard Math::BigInt and Math::BigFloat modules provide
2401 variable-precision arithmetic and overloaded operators, although
2402 they're currently pretty slow. At the cost of some space and
2403 considerable speed, they avoid the normal pitfalls associated with
2404 limited-precision representations.
2407 $x = Math::BigInt->new('123456789123456789');
2410 # prints +15241578780673678515622620750190521
2412 There are several modules that let you calculate with (bound only by
2413 memory and cpu-time) unlimited or fixed precision. There are also
2414 some non-standard modules that provide faster implementations via
2415 external C libraries.
2417 Here is a short, but incomplete summary:
2419 Math::Fraction big, unlimited fractions like 9973 / 12967
2420 Math::String treat string sequences like numbers
2421 Math::FixedPrecision calculate with a fixed precision
2422 Math::Currency for currency calculations
2423 Bit::Vector manipulate bit vectors fast (uses C)
2424 Math::BigIntFast Bit::Vector wrapper for big numbers
2425 Math::Pari provides access to the Pari C library
2426 Math::BigInteger uses an external C library
2427 Math::Cephes uses external Cephes C library (no big numbers)
2428 Math::Cephes::Fraction fractions via the Cephes library
2429 Math::GMP another one using an external C library
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