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 $_. When used in scalar context, the return value generally indicates the
238 success of the operation. Behavior in list context depends on the particular
239 operator. See L</"Regexp Quote-Like Operators"> for details and
240 L<perlretut> for examples using these operators.
242 If the right argument is an expression rather than a search pattern,
243 substitution, or transliteration, it is interpreted as a search pattern at run
246 Binary "!~" is just like "=~" except the return value is negated in
249 =head2 Multiplicative Operators
250 X<operator, multiplicative>
252 Binary "*" multiplies two numbers.
255 Binary "/" divides two numbers.
258 Binary "%" computes the modulus of two numbers. Given integer
259 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
260 C<$a> minus the largest multiple of C<$b> that is not greater than
261 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
262 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
263 result will be less than or equal to zero).
264 Note that when C<use integer> is in scope, "%" gives you direct access
265 to the modulus operator as implemented by your C compiler. This
266 operator is not as well defined for negative operands, but it will
268 X<%> X<remainder> X<modulus> X<mod>
270 Binary "x" is the repetition operator. In scalar context or if the left
271 operand is not enclosed in parentheses, it returns a string consisting
272 of the left operand repeated the number of times specified by the right
273 operand. In list context, if the left operand is enclosed in
274 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
275 If the right operand is zero or negative, it returns an empty string
276 or an empty list, depending on the context.
279 print '-' x 80; # print row of dashes
281 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
283 @ones = (1) x 80; # a list of 80 1's
284 @ones = (5) x @ones; # set all elements to 5
287 =head2 Additive Operators
288 X<operator, additive>
290 Binary "+" returns the sum of two numbers.
293 Binary "-" returns the difference of two numbers.
296 Binary "." concatenates two strings.
297 X<string, concatenation> X<concatenation>
298 X<cat> X<concat> X<concatenate> X<.>
300 =head2 Shift Operators
301 X<shift operator> X<operator, shift> X<<< << >>>
302 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
303 X<shl> X<shr> X<shift, right> X<shift, left>
305 Binary "<<" returns the value of its left argument shifted left by the
306 number of bits specified by the right argument. Arguments should be
307 integers. (See also L<Integer Arithmetic>.)
309 Binary ">>" returns the value of its left argument shifted right by
310 the number of bits specified by the right argument. Arguments should
311 be integers. (See also L<Integer Arithmetic>.)
313 Note that both "<<" and ">>" in Perl are implemented directly using
314 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
315 in force then signed C integers are used, else unsigned C integers are
316 used. Either way, the implementation isn't going to generate results
317 larger than the size of the integer type Perl was built with (32 bits
320 The result of overflowing the range of the integers is undefined
321 because it is undefined also in C. In other words, using 32-bit
322 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
323 of bits is also undefined.
325 =head2 Named Unary Operators
326 X<operator, named unary>
328 The various named unary operators are treated as functions with one
329 argument, with optional parentheses.
331 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
332 is followed by a left parenthesis as the next token, the operator and
333 arguments within parentheses are taken to be of highest precedence,
334 just like a normal function call. For example,
335 because named unary operators are higher precedence than ||:
337 chdir $foo || die; # (chdir $foo) || die
338 chdir($foo) || die; # (chdir $foo) || die
339 chdir ($foo) || die; # (chdir $foo) || die
340 chdir +($foo) || die; # (chdir $foo) || die
342 but, because * is higher precedence than named operators:
344 chdir $foo * 20; # chdir ($foo * 20)
345 chdir($foo) * 20; # (chdir $foo) * 20
346 chdir ($foo) * 20; # (chdir $foo) * 20
347 chdir +($foo) * 20; # chdir ($foo * 20)
349 rand 10 * 20; # rand (10 * 20)
350 rand(10) * 20; # (rand 10) * 20
351 rand (10) * 20; # (rand 10) * 20
352 rand +(10) * 20; # rand (10 * 20)
354 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
355 treated like named unary operators, but they don't follow this functional
356 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
357 equivalent to C<-f "$file.bak">.
358 X<-X> X<filetest> X<operator, filetest>
360 See also L<"Terms and List Operators (Leftward)">.
362 =head2 Relational Operators
363 X<relational operator> X<operator, relational>
365 Binary "<" returns true if the left argument is numerically less than
369 Binary ">" returns true if the left argument is numerically greater
370 than the right argument.
373 Binary "<=" returns true if the left argument is numerically less than
374 or equal to the right argument.
377 Binary ">=" returns true if the left argument is numerically greater
378 than or equal to the right argument.
381 Binary "lt" returns true if the left argument is stringwise less than
385 Binary "gt" returns true if the left argument is stringwise greater
386 than the right argument.
389 Binary "le" returns true if the left argument is stringwise less than
390 or equal to the right argument.
393 Binary "ge" returns true if the left argument is stringwise greater
394 than or equal to the right argument.
397 =head2 Equality Operators
398 X<equality> X<equal> X<equals> X<operator, equality>
400 Binary "==" returns true if the left argument is numerically equal to
404 Binary "!=" returns true if the left argument is numerically not equal
405 to the right argument.
408 Binary "<=>" returns -1, 0, or 1 depending on whether the left
409 argument is numerically less than, equal to, or greater than the right
410 argument. If your platform supports NaNs (not-a-numbers) as numeric
411 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
412 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
413 returns true, as does NaN != anything else. If your platform doesn't
414 support NaNs then NaN is just a string with numeric value 0.
415 X<< <=> >> X<spaceship>
417 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
418 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
420 Binary "eq" returns true if the left argument is stringwise equal to
424 Binary "ne" returns true if the left argument is stringwise not equal
425 to the right argument.
428 Binary "cmp" returns -1, 0, or 1 depending on whether the left
429 argument is stringwise less than, equal to, or greater than the right
433 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
434 by the current locale if C<use locale> is in effect. See L<perllocale>.
437 X<operator, bitwise, and> X<bitwise and> X<&>
439 Binary "&" returns its operands ANDed together bit by bit.
440 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
442 Note that "&" has lower priority than relational operators, so for example
443 the brackets are essential in a test like
445 print "Even\n" if ($x & 1) == 0;
447 =head2 Bitwise Or and Exclusive Or
448 X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
451 Binary "|" returns its operands ORed together bit by bit.
452 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
454 Binary "^" returns its operands XORed together bit by bit.
455 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
457 Note that "|" and "^" have lower priority than relational operators, so
458 for example the brackets are essential in a test like
460 print "false\n" if (8 | 2) != 10;
462 =head2 C-style Logical And
463 X<&&> X<logical and> X<operator, logical, and>
465 Binary "&&" performs a short-circuit logical AND operation. That is,
466 if the left operand is false, the right operand is not even evaluated.
467 Scalar or list context propagates down to the right operand if it
470 =head2 C-style Logical Or
471 X<||> X<operator, logical, or>
473 Binary "||" performs a short-circuit logical OR operation. That is,
474 if the left operand is true, the right operand is not even evaluated.
475 Scalar or list context propagates down to the right operand if it
478 =head2 C-style Logical Defined-Or
479 X<//> X<operator, logical, defined-or>
481 Although it has no direct equivalent in C, Perl's C<//> operator is related
482 to its C-style or. In fact, it's exactly the same as C<||>, except that it
483 tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
484 is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
485 rather than the value of C<defined($a)>) and is exactly equivalent to
486 C<defined($a) ? $a : $b>. This is very useful for providing default values
487 for variables. If you actually want to test if at least one of C<$a> and
488 C<$b> is defined, use C<defined($a // $b)>.
490 The C<||>, C<//> and C<&&> operators return the last value evaluated
491 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
492 portable way to find out the home directory might be:
494 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
495 (getpwuid($<))[7] // die "You're homeless!\n";
497 In particular, this means that you shouldn't use this
498 for selecting between two aggregates for assignment:
500 @a = @b || @c; # this is wrong
501 @a = scalar(@b) || @c; # really meant this
502 @a = @b ? @b : @c; # this works fine, though
504 As more readable alternatives to C<&&>, C<//> and C<||> when used for
505 control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
506 The short-circuit behavior is identical. The precedence of "and", "err"
507 and "or" is much lower, however, so that you can safely use them after a
508 list operator without the need for parentheses:
510 unlink "alpha", "beta", "gamma"
511 or gripe(), next LINE;
513 With the C-style operators that would have been written like this:
515 unlink("alpha", "beta", "gamma")
516 || (gripe(), next LINE);
518 Using "or" for assignment is unlikely to do what you want; see below.
520 =head2 Range Operators
521 X<operator, range> X<range> X<..> X<...>
523 Binary ".." is the range operator, which is really two different
524 operators depending on the context. In list context, it returns a
525 list of values counting (up by ones) from the left value to the right
526 value. If the left value is greater than the right value then it
527 returns the empty list. The range operator is useful for writing
528 C<foreach (1..10)> loops and for doing slice operations on arrays. In
529 the current implementation, no temporary array is created when the
530 range operator is used as the expression in C<foreach> loops, but older
531 versions of Perl might burn a lot of memory when you write something
534 for (1 .. 1_000_000) {
538 The range operator also works on strings, using the magical auto-increment,
541 In scalar context, ".." returns a boolean value. The operator is
542 bistable, like a flip-flop, and emulates the line-range (comma) operator
543 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
544 own boolean state. It is false as long as its left operand is false.
545 Once the left operand is true, the range operator stays true until the
546 right operand is true, I<AFTER> which the range operator becomes false
547 again. It doesn't become false till the next time the range operator is
548 evaluated. It can test the right operand and become false on the same
549 evaluation it became true (as in B<awk>), but it still returns true once.
550 If you don't want it to test the right operand till the next
551 evaluation, as in B<sed>, just use three dots ("...") instead of
552 two. In all other regards, "..." behaves just like ".." does.
554 The right operand is not evaluated while the operator is in the
555 "false" state, and the left operand is not evaluated while the
556 operator is in the "true" state. The precedence is a little lower
557 than || and &&. The value returned is either the empty string for
558 false, or a sequence number (beginning with 1) for true. The
559 sequence number is reset for each range encountered. The final
560 sequence number in a range has the string "E0" appended to it, which
561 doesn't affect its numeric value, but gives you something to search
562 for if you want to exclude the endpoint. You can exclude the
563 beginning point by waiting for the sequence number to be greater
566 If either operand of scalar ".." is a constant expression,
567 that operand is considered true if it is equal (C<==>) to the current
568 input line number (the C<$.> variable).
570 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
571 but that is only an issue if you use a floating point expression; when
572 implicitly using C<$.> as described in the previous paragraph, the
573 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
574 is set to a floating point value and you are not reading from a file.
575 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
576 you want in scalar context because each of the operands are evaluated
577 using their integer representation.
581 As a scalar operator:
583 if (101 .. 200) { print; } # print 2nd hundred lines, short for
584 # if ($. == 101 .. $. == 200) ...
585 next line if (1 .. /^$/); # skip header lines, short for
586 # ... if ($. == 1 .. /^$/);
587 s/^/> / if (/^$/ .. eof()); # quote body
589 # parse mail messages
591 $in_header = 1 .. /^$/;
592 $in_body = /^$/ .. eof;
599 close ARGV if eof; # reset $. each file
602 Here's a simple example to illustrate the difference between
603 the two range operators:
618 This program will print only the line containing "Bar". If
619 the range operator is changed to C<...>, it will also print the
622 And now some examples as a list operator:
624 for (101 .. 200) { print; } # print $_ 100 times
625 @foo = @foo[0 .. $#foo]; # an expensive no-op
626 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
628 The range operator (in list context) makes use of the magical
629 auto-increment algorithm if the operands are strings. You
632 @alphabet = ('A' .. 'Z');
634 to get all normal letters of the English alphabet, or
636 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
638 to get a hexadecimal digit, or
640 @z2 = ('01' .. '31'); print $z2[$mday];
642 to get dates with leading zeros. If the final value specified is not
643 in the sequence that the magical increment would produce, the sequence
644 goes until the next value would be longer than the final value
647 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
648 return two elements in list context.
650 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
652 =head2 Conditional Operator
653 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
655 Ternary "?:" is the conditional operator, just as in C. It works much
656 like an if-then-else. If the argument before the ? is true, the
657 argument before the : is returned, otherwise the argument after the :
658 is returned. For example:
660 printf "I have %d dog%s.\n", $n,
661 ($n == 1) ? '' : "s";
663 Scalar or list context propagates downward into the 2nd
664 or 3rd argument, whichever is selected.
666 $a = $ok ? $b : $c; # get a scalar
667 @a = $ok ? @b : @c; # get an array
668 $a = $ok ? @b : @c; # oops, that's just a count!
670 The operator may be assigned to if both the 2nd and 3rd arguments are
671 legal lvalues (meaning that you can assign to them):
673 ($a_or_b ? $a : $b) = $c;
675 Because this operator produces an assignable result, using assignments
676 without parentheses will get you in trouble. For example, this:
678 $a % 2 ? $a += 10 : $a += 2
682 (($a % 2) ? ($a += 10) : $a) += 2
686 ($a % 2) ? ($a += 10) : ($a += 2)
688 That should probably be written more simply as:
690 $a += ($a % 2) ? 10 : 2;
692 =head2 Assignment Operators
693 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
694 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
697 "=" is the ordinary assignment operator.
699 Assignment operators work as in C. That is,
707 although without duplicating any side effects that dereferencing the lvalue
708 might trigger, such as from tie(). Other assignment operators work similarly.
709 The following are recognized:
716 Although these are grouped by family, they all have the precedence
719 Unlike in C, the scalar assignment operator produces a valid lvalue.
720 Modifying an assignment is equivalent to doing the assignment and
721 then modifying the variable that was assigned to. This is useful
722 for modifying a copy of something, like this:
724 ($tmp = $global) =~ tr [A-Z] [a-z];
735 Similarly, a list assignment in list context produces the list of
736 lvalues assigned to, and a list assignment in scalar context returns
737 the number of elements produced by the expression on the right hand
738 side of the assignment.
740 =head2 Comma Operator
741 X<comma> X<operator, comma> X<,>
743 Binary "," is the comma operator. In scalar context it evaluates
744 its left argument, throws that value away, then evaluates its right
745 argument and returns that value. This is just like C's comma operator.
747 In list context, it's just the list argument separator, and inserts
748 both its arguments into the list.
750 The C<< => >> operator is a synonym for the comma, but forces any word
751 (consisting entirely of word characters) to its left to be interpreted
752 as a string (as of 5.001). This includes words that might otherwise be
753 considered a constant or function call.
755 use constant FOO => "something";
757 my %h = ( FOO => 23 );
765 my %h = ("something", 23);
767 If the argument on the left is not a word, it is first interpreted as
768 an expression, and then the string value of that is used.
770 The C<< => >> operator is helpful in documenting the correspondence
771 between keys and values in hashes, and other paired elements in lists.
773 %hash = ( $key => $value );
774 login( $username => $password );
776 =head2 List Operators (Rightward)
777 X<operator, list, rightward> X<list operator>
779 On the right side of a list operator, it has very low precedence,
780 such that it controls all comma-separated expressions found there.
781 The only operators with lower precedence are the logical operators
782 "and", "or", and "not", which may be used to evaluate calls to list
783 operators without the need for extra parentheses:
785 open HANDLE, "filename"
786 or die "Can't open: $!\n";
788 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
791 X<operator, logical, not> X<not>
793 Unary "not" returns the logical negation of the expression to its right.
794 It's the equivalent of "!" except for the very low precedence.
797 X<operator, logical, and> X<and>
799 Binary "and" returns the logical conjunction of the two surrounding
800 expressions. It's equivalent to && except for the very low
801 precedence. This means that it short-circuits: i.e., the right
802 expression is evaluated only if the left expression is true.
804 =head2 Logical or, Defined or, and Exclusive Or
805 X<operator, logical, or> X<operator, logical, xor> X<operator, logical, err>
806 X<operator, logical, defined or> X<operator, logical, exclusive or>
809 Binary "or" returns the logical disjunction of the two surrounding
810 expressions. It's equivalent to || except for the very low precedence.
811 This makes it useful for control flow
813 print FH $data or die "Can't write to FH: $!";
815 This means that it short-circuits: i.e., the right expression is evaluated
816 only if the left expression is false. Due to its precedence, you should
817 probably avoid using this for assignment, only for control flow.
819 $a = $b or $c; # bug: this is wrong
820 ($a = $b) or $c; # really means this
821 $a = $b || $c; # better written this way
823 However, when it's a list-context assignment and you're trying to use
824 "||" for control flow, you probably need "or" so that the assignment
825 takes higher precedence.
827 @info = stat($file) || die; # oops, scalar sense of stat!
828 @info = stat($file) or die; # better, now @info gets its due
830 Then again, you could always use parentheses.
832 Binary "err" is equivalent to C<//>--it's just like binary "or", except it tests
833 its left argument's definedness instead of its truth. There are two ways to
834 remember "err": either because many functions return C<undef> on an B<err>or,
835 or as a sort of correction: C<$a=($b err 'default')>
837 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
838 It cannot short circuit, of course.
840 =head2 C Operators Missing From Perl
841 X<operator, missing from perl> X<&> X<*>
842 X<typecasting> X<(TYPE)>
844 Here is what C has that Perl doesn't:
850 Address-of operator. (But see the "\" operator for taking a reference.)
854 Dereference-address operator. (Perl's prefix dereferencing
855 operators are typed: $, @, %, and &.)
859 Type-casting operator.
863 =head2 Quote and Quote-like Operators
864 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
865 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
866 X<escape sequence> X<escape>
869 While we usually think of quotes as literal values, in Perl they
870 function as operators, providing various kinds of interpolating and
871 pattern matching capabilities. Perl provides customary quote characters
872 for these behaviors, but also provides a way for you to choose your
873 quote character for any of them. In the following table, a C<{}> represents
874 any pair of delimiters you choose.
876 Customary Generic Meaning Interpolates
881 // m{} Pattern match yes*
883 s{}{} Substitution yes*
884 tr{}{} Transliteration no (but see below)
887 * unless the delimiter is ''.
889 Non-bracketing delimiters use the same character fore and aft, but the four
890 sorts of brackets (round, angle, square, curly) will all nest, which means
899 Note, however, that this does not always work for quoting Perl code:
901 $s = q{ if($a eq "}") ... }; # WRONG
903 is a syntax error. The C<Text::Balanced> module (from CPAN, and
904 starting from Perl 5.8 part of the standard distribution) is able
907 There can be whitespace between the operator and the quoting
908 characters, except when C<#> is being used as the quoting character.
909 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
910 operator C<q> followed by a comment. Its argument will be taken
911 from the next line. This allows you to write:
913 s {foo} # Replace foo
916 The following escape sequences are available in constructs that interpolate
917 and in transliterations.
918 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N>
925 \a alarm (bell) (BEL)
927 \033 octal char (ESC)
929 \x{263a} wide hex char (SMILEY)
930 \c[ control char (ESC)
931 \N{name} named Unicode character
933 B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
934 the vertical tab (VT - ASCII 11).
936 The following escape sequences are available in constructs that interpolate
937 but not in transliterations.
938 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
940 \l lowercase next char
941 \u uppercase next char
944 \E end case modification
945 \Q quote non-word characters till \E
947 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
948 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
949 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
950 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
951 C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
954 All systems use the virtual C<"\n"> to represent a line terminator,
955 called a "newline". There is no such thing as an unvarying, physical
956 newline character. It is only an illusion that the operating system,
957 device drivers, C libraries, and Perl all conspire to preserve. Not all
958 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
959 on a Mac, these are reversed, and on systems without line terminator,
960 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
961 you mean a "newline" for your system, but use the literal ASCII when you
962 need an exact character. For example, most networking protocols expect
963 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
964 and although they often accept just C<"\012">, they seldom tolerate just
965 C<"\015">. If you get in the habit of using C<"\n"> for networking,
966 you may be burned some day.
967 X<newline> X<line terminator> X<eol> X<end of line>
970 For constructs that do interpolate, variables beginning with "C<$>"
971 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
972 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
973 But method calls such as C<< $obj->meth >> are not.
975 Interpolating an array or slice interpolates the elements in order,
976 separated by the value of C<$">, so is equivalent to interpolating
977 C<join $", @array>. "Punctuation" arrays such as C<@+> are only
978 interpolated if the name is enclosed in braces C<@{+}>.
980 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
981 An unescaped C<$> or C<@> interpolates the corresponding variable,
982 while escaping will cause the literal string C<\$> to be inserted.
983 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
985 Patterns are subject to an additional level of interpretation as a
986 regular expression. This is done as a second pass, after variables are
987 interpolated, so that regular expressions may be incorporated into the
988 pattern from the variables. If this is not what you want, use C<\Q> to
989 interpolate a variable literally.
991 Apart from the behavior described above, Perl does not expand
992 multiple levels of interpolation. In particular, contrary to the
993 expectations of shell programmers, back-quotes do I<NOT> interpolate
994 within double quotes, nor do single quotes impede evaluation of
995 variables when used within double quotes.
997 =head2 Regexp Quote-Like Operators
1000 Here are the quote-like operators that apply to pattern
1001 matching and related activities.
1008 This is just like the C</pattern/> search, except that it matches only
1009 once between calls to the reset() operator. This is a useful
1010 optimization when you want to see only the first occurrence of
1011 something in each file of a set of files, for instance. Only C<??>
1012 patterns local to the current package are reset.
1016 # blank line between header and body
1019 reset if eof; # clear ?? status for next file
1022 This usage is vaguely deprecated, which means it just might possibly
1023 be removed in some distant future version of Perl, perhaps somewhere
1024 around the year 2168.
1026 =item m/PATTERN/cgimosx
1027 X<m> X<operator, match>
1028 X<regexp, options> X<regexp> X<regex, options> X<regex>
1029 X</c> X</i> X</m> X</o> X</s> X</x>
1031 =item /PATTERN/cgimosx
1033 Searches a string for a pattern match, and in scalar context returns
1034 true if it succeeds, false if it fails. If no string is specified
1035 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1036 string specified with C<=~> need not be an lvalue--it may be the
1037 result of an expression evaluation, but remember the C<=~> binds
1038 rather tightly.) See also L<perlre>. See L<perllocale> for
1039 discussion of additional considerations that apply when C<use locale>
1044 c Do not reset search position on a failed match when /g is in effect.
1045 g Match globally, i.e., find all occurrences.
1046 i Do case-insensitive pattern matching.
1047 m Treat string as multiple lines.
1048 o Compile pattern only once.
1049 s Treat string as single line.
1050 x Use extended regular expressions.
1052 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1053 you can use any pair of non-alphanumeric, non-whitespace characters
1054 as delimiters. This is particularly useful for matching path names
1055 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1056 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
1057 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1059 PATTERN may contain variables, which will be interpolated (and the
1060 pattern recompiled) every time the pattern search is evaluated, except
1061 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1062 C<$|> are not interpolated because they look like end-of-string tests.)
1063 If you want such a pattern to be compiled only once, add a C</o> after
1064 the trailing delimiter. This avoids expensive run-time recompilations,
1065 and is useful when the value you are interpolating won't change over
1066 the life of the script. However, mentioning C</o> constitutes a promise
1067 that you won't change the variables in the pattern. If you change them,
1068 Perl won't even notice. See also L<"qr/STRING/imosx">.
1070 If the PATTERN evaluates to the empty string, the last
1071 I<successfully> matched regular expression is used instead. In this
1072 case, only the C<g> and C<c> flags on the empty pattern is honoured -
1073 the other flags are taken from the original pattern. If no match has
1074 previously succeeded, this will (silently) act instead as a genuine
1075 empty pattern (which will always match).
1077 Note that it's possible to confuse Perl into thinking C<//> (the empty
1078 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1079 good about this, but some pathological cases might trigger this, such as
1080 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1081 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1082 will assume you meant defined-or. If you meant the empty regex, just
1083 use parentheses or spaces to disambiguate, or even prefix the empty
1084 regex with an C<m> (so C<//> becomes C<m//>).
1086 If the C</g> option is not used, C<m//> in list context returns a
1087 list consisting of the subexpressions matched by the parentheses in the
1088 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1089 also set, and that this differs from Perl 4's behavior.) When there are
1090 no parentheses in the pattern, the return value is the list C<(1)> for
1091 success. With or without parentheses, an empty list is returned upon
1096 open(TTY, '/dev/tty');
1097 <TTY> =~ /^y/i && foo(); # do foo if desired
1099 if (/Version: *([0-9.]*)/) { $version = $1; }
1101 next if m#^/usr/spool/uucp#;
1106 print if /$arg/o; # compile only once
1109 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1111 This last example splits $foo into the first two words and the
1112 remainder of the line, and assigns those three fields to $F1, $F2, and
1113 $Etc. The conditional is true if any variables were assigned, i.e., if
1114 the pattern matched.
1116 The C</g> modifier specifies global pattern matching--that is,
1117 matching as many times as possible within the string. How it behaves
1118 depends on the context. In list context, it returns a list of the
1119 substrings matched by any capturing parentheses in the regular
1120 expression. If there are no parentheses, it returns a list of all
1121 the matched strings, as if there were parentheses around the whole
1124 In scalar context, each execution of C<m//g> finds the next match,
1125 returning true if it matches, and false if there is no further match.
1126 The position after the last match can be read or set using the pos()
1127 function; see L<perlfunc/pos>. A failed match normally resets the
1128 search position to the beginning of the string, but you can avoid that
1129 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1130 string also resets the search position.
1132 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1133 zero-width assertion that matches the exact position where the previous
1134 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1135 still anchors at pos(), but the match is of course only attempted once.
1136 Using C<\G> without C</g> on a target string that has not previously had a
1137 C</g> match applied to it is the same as using the C<\A> assertion to match
1138 the beginning of the string. Note also that, currently, C<\G> is only
1139 properly supported when anchored at the very beginning of the pattern.
1144 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1148 while (defined($paragraph = <>)) {
1149 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1153 print "$sentences\n";
1155 # using m//gc with \G
1159 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1161 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1163 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1165 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1167 The last example should print:
1177 Notice that the final match matched C<q> instead of C<p>, which a match
1178 without the C<\G> anchor would have done. Also note that the final match
1179 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1180 final match did indeed match C<p>, it's a good bet that you're running an
1181 older (pre-5.6.0) Perl.
1183 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1184 combine several regexps like this to process a string part-by-part,
1185 doing different actions depending on which regexp matched. Each
1186 regexp tries to match where the previous one leaves off.
1189 $url = new URI::URL "http://www/"; die if $url eq "xXx";
1193 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1194 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1195 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1196 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1197 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1198 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1199 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1200 print ". That's all!\n";
1203 Here is the output (split into several lines):
1205 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1206 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1207 lowercase lowercase line-noise lowercase lowercase line-noise
1208 MiXeD line-noise. That's all!
1211 X<q> X<quote, double> X<'> X<''>
1215 A single-quoted, literal string. A backslash represents a backslash
1216 unless followed by the delimiter or another backslash, in which case
1217 the delimiter or backslash is interpolated.
1219 $foo = q!I said, "You said, 'She said it.'"!;
1220 $bar = q('This is it.');
1221 $baz = '\n'; # a two-character string
1224 X<qq> X<quote, double> X<"> X<"">
1228 A double-quoted, interpolated string.
1231 (*** The previous line contains the naughty word "$1".\n)
1232 if /\b(tcl|java|python)\b/i; # :-)
1233 $baz = "\n"; # a one-character string
1235 =item qr/STRING/imosx
1236 X<qr> X</i> X</m> X</o> X</s> X</x>
1238 This operator quotes (and possibly compiles) its I<STRING> as a regular
1239 expression. I<STRING> is interpolated the same way as I<PATTERN>
1240 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1241 is done. Returns a Perl value which may be used instead of the
1242 corresponding C</STRING/imosx> expression.
1246 $rex = qr/my.STRING/is;
1253 The result may be used as a subpattern in a match:
1256 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1257 $string =~ $re; # or used standalone
1258 $string =~ /$re/; # or this way
1260 Since Perl may compile the pattern at the moment of execution of qr()
1261 operator, using qr() may have speed advantages in some situations,
1262 notably if the result of qr() is used standalone:
1265 my $patterns = shift;
1266 my @compiled = map qr/$_/i, @$patterns;
1269 foreach my $pat (@compiled) {
1270 $success = 1, last if /$pat/;
1276 Precompilation of the pattern into an internal representation at
1277 the moment of qr() avoids a need to recompile the pattern every
1278 time a match C</$pat/> is attempted. (Perl has many other internal
1279 optimizations, but none would be triggered in the above example if
1280 we did not use qr() operator.)
1284 i Do case-insensitive pattern matching.
1285 m Treat string as multiple lines.
1286 o Compile pattern only once.
1287 s Treat string as single line.
1288 x Use extended regular expressions.
1290 See L<perlre> for additional information on valid syntax for STRING, and
1291 for a detailed look at the semantics of regular expressions.
1294 X<qx> X<`> X<``> X<backtick>
1298 A string which is (possibly) interpolated and then executed as a
1299 system command with C</bin/sh> or its equivalent. Shell wildcards,
1300 pipes, and redirections will be honored. The collected standard
1301 output of the command is returned; standard error is unaffected. In
1302 scalar context, it comes back as a single (potentially multi-line)
1303 string, or undef if the command failed. In list context, returns a
1304 list of lines (however you've defined lines with $/ or
1305 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1307 Because backticks do not affect standard error, use shell file descriptor
1308 syntax (assuming the shell supports this) if you care to address this.
1309 To capture a command's STDERR and STDOUT together:
1311 $output = `cmd 2>&1`;
1313 To capture a command's STDOUT but discard its STDERR:
1315 $output = `cmd 2>/dev/null`;
1317 To capture a command's STDERR but discard its STDOUT (ordering is
1320 $output = `cmd 2>&1 1>/dev/null`;
1322 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1323 but leave its STDOUT to come out the old STDERR:
1325 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1327 To read both a command's STDOUT and its STDERR separately, it's easiest
1328 to redirect them separately to files, and then read from those files
1329 when the program is done:
1331 system("program args 1>program.stdout 2>program.stderr");
1333 Using single-quote as a delimiter protects the command from Perl's
1334 double-quote interpolation, passing it on to the shell instead:
1336 $perl_info = qx(ps $$); # that's Perl's $$
1337 $shell_info = qx'ps $$'; # that's the new shell's $$
1339 How that string gets evaluated is entirely subject to the command
1340 interpreter on your system. On most platforms, you will have to protect
1341 shell metacharacters if you want them treated literally. This is in
1342 practice difficult to do, as it's unclear how to escape which characters.
1343 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1344 to emulate backticks safely.
1346 On some platforms (notably DOS-like ones), the shell may not be
1347 capable of dealing with multiline commands, so putting newlines in
1348 the string may not get you what you want. You may be able to evaluate
1349 multiple commands in a single line by separating them with the command
1350 separator character, if your shell supports that (e.g. C<;> on many Unix
1351 shells; C<&> on the Windows NT C<cmd> shell).
1353 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1354 output before starting the child process, but this may not be supported
1355 on some platforms (see L<perlport>). To be safe, you may need to set
1356 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1357 C<IO::Handle> on any open handles.
1359 Beware that some command shells may place restrictions on the length
1360 of the command line. You must ensure your strings don't exceed this
1361 limit after any necessary interpolations. See the platform-specific
1362 release notes for more details about your particular environment.
1364 Using this operator can lead to programs that are difficult to port,
1365 because the shell commands called vary between systems, and may in
1366 fact not be present at all. As one example, the C<type> command under
1367 the POSIX shell is very different from the C<type> command under DOS.
1368 That doesn't mean you should go out of your way to avoid backticks
1369 when they're the right way to get something done. Perl was made to be
1370 a glue language, and one of the things it glues together is commands.
1371 Just understand what you're getting yourself into.
1373 See L<"I/O Operators"> for more discussion.
1376 X<qw> X<quote, list> X<quote, words>
1378 Evaluates to a list of the words extracted out of STRING, using embedded
1379 whitespace as the word delimiters. It can be understood as being roughly
1382 split(' ', q/STRING/);
1384 the differences being that it generates a real list at compile time, and
1385 in scalar context it returns the last element in the list. So
1390 is semantically equivalent to the list:
1394 Some frequently seen examples:
1396 use POSIX qw( setlocale localeconv )
1397 @EXPORT = qw( foo bar baz );
1399 A common mistake is to try to separate the words with comma or to
1400 put comments into a multi-line C<qw>-string. For this reason, the
1401 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1402 produces warnings if the STRING contains the "," or the "#" character.
1404 =item s/PATTERN/REPLACEMENT/egimosx
1405 X<substitute> X<substitution> X<replace> X<regexp, replace>
1406 X<regexp, substitute> X</e> X</g> X</i> X</m> X</o> X</s> X</x>
1408 Searches a string for a pattern, and if found, replaces that pattern
1409 with the replacement text and returns the number of substitutions
1410 made. Otherwise it returns false (specifically, the empty string).
1412 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1413 variable is searched and modified. (The string specified with C<=~> must
1414 be scalar variable, an array element, a hash element, or an assignment
1415 to one of those, i.e., an lvalue.)
1417 If the delimiter chosen is a single quote, no interpolation is
1418 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1419 PATTERN contains a $ that looks like a variable rather than an
1420 end-of-string test, the variable will be interpolated into the pattern
1421 at run-time. If you want the pattern compiled only once the first time
1422 the variable is interpolated, use the C</o> option. If the pattern
1423 evaluates to the empty string, the last successfully executed regular
1424 expression is used instead. See L<perlre> for further explanation on these.
1425 See L<perllocale> for discussion of additional considerations that apply
1426 when C<use locale> is in effect.
1430 e Evaluate the right side as an expression.
1431 g Replace globally, i.e., all occurrences.
1432 i Do case-insensitive pattern matching.
1433 m Treat string as multiple lines.
1434 o Compile pattern only once.
1435 s Treat string as single line.
1436 x Use extended regular expressions.
1438 Any non-alphanumeric, non-whitespace delimiter may replace the
1439 slashes. If single quotes are used, no interpretation is done on the
1440 replacement string (the C</e> modifier overrides this, however). Unlike
1441 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1442 text is not evaluated as a command. If the
1443 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1444 pair of quotes, which may or may not be bracketing quotes, e.g.,
1445 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1446 replacement portion to be treated as a full-fledged Perl expression
1447 and evaluated right then and there. It is, however, syntax checked at
1448 compile-time. A second C<e> modifier will cause the replacement portion
1449 to be C<eval>ed before being run as a Perl expression.
1453 s/\bgreen\b/mauve/g; # don't change wintergreen
1455 $path =~ s|/usr/bin|/usr/local/bin|;
1457 s/Login: $foo/Login: $bar/; # run-time pattern
1459 ($foo = $bar) =~ s/this/that/; # copy first, then change
1461 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1464 s/\d+/$&*2/e; # yields 'abc246xyz'
1465 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1466 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1468 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1469 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1470 s/^=(\w+)/&pod($1)/ge; # use function call
1472 # expand variables in $_, but dynamics only, using
1473 # symbolic dereferencing
1476 # Add one to the value of any numbers in the string
1479 # This will expand any embedded scalar variable
1480 # (including lexicals) in $_ : First $1 is interpolated
1481 # to the variable name, and then evaluated
1484 # Delete (most) C comments.
1486 /\* # Match the opening delimiter.
1487 .*? # Match a minimal number of characters.
1488 \*/ # Match the closing delimiter.
1491 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1493 for ($variable) { # trim whitespace in $variable, cheap
1498 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1500 Note the use of $ instead of \ in the last example. Unlike
1501 B<sed>, we use the \<I<digit>> form in only the left hand side.
1502 Anywhere else it's $<I<digit>>.
1504 Occasionally, you can't use just a C</g> to get all the changes
1505 to occur that you might want. Here are two common cases:
1507 # put commas in the right places in an integer
1508 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1510 # expand tabs to 8-column spacing
1511 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1513 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1514 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
1516 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1518 Transliterates all occurrences of the characters found in the search list
1519 with the corresponding character in the replacement list. It returns
1520 the number of characters replaced or deleted. If no string is
1521 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1522 string specified with =~ must be a scalar variable, an array element, a
1523 hash element, or an assignment to one of those, i.e., an lvalue.)
1525 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1526 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1527 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1528 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1529 its own pair of quotes, which may or may not be bracketing quotes,
1530 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1532 Note that C<tr> does B<not> do regular expression character classes
1533 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1534 the tr(1) utility. If you want to map strings between lower/upper
1535 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1536 using the C<s> operator if you need regular expressions.
1538 Note also that the whole range idea is rather unportable between
1539 character sets--and even within character sets they may cause results
1540 you probably didn't expect. A sound principle is to use only ranges
1541 that begin from and end at either alphabets of equal case (a-e, A-E),
1542 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1543 character sets in full.
1547 c Complement the SEARCHLIST.
1548 d Delete found but unreplaced characters.
1549 s Squash duplicate replaced characters.
1551 If the C</c> modifier is specified, the SEARCHLIST character set
1552 is complemented. If the C</d> modifier is specified, any characters
1553 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1554 (Note that this is slightly more flexible than the behavior of some
1555 B<tr> programs, which delete anything they find in the SEARCHLIST,
1556 period.) If the C</s> modifier is specified, sequences of characters
1557 that were transliterated to the same character are squashed down
1558 to a single instance of the character.
1560 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1561 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1562 than the SEARCHLIST, the final character is replicated till it is long
1563 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1564 This latter is useful for counting characters in a class or for
1565 squashing character sequences in a class.
1569 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1571 $cnt = tr/*/*/; # count the stars in $_
1573 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1575 $cnt = tr/0-9//; # count the digits in $_
1577 tr/a-zA-Z//s; # bookkeeper -> bokeper
1579 ($HOST = $host) =~ tr/a-z/A-Z/;
1581 tr/a-zA-Z/ /cs; # change non-alphas to single space
1584 [\000-\177]; # delete 8th bit
1586 If multiple transliterations are given for a character, only the
1591 will transliterate any A to X.
1593 Because the transliteration table is built at compile time, neither
1594 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1595 interpolation. That means that if you want to use variables, you
1598 eval "tr/$oldlist/$newlist/";
1601 eval "tr/$oldlist/$newlist/, 1" or die $@;
1604 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
1606 A line-oriented form of quoting is based on the shell "here-document"
1607 syntax. Following a C<< << >> you specify a string to terminate
1608 the quoted material, and all lines following the current line down to
1609 the terminating string are the value of the item. The terminating
1610 string may be either an identifier (a word), or some quoted text. If
1611 quoted, the type of quotes you use determines the treatment of the
1612 text, just as in regular quoting. An unquoted identifier works like
1613 double quotes. There must be no space between the C<< << >> and
1614 the identifier, unless the identifier is quoted. (If you put a space it
1615 will be treated as a null identifier, which is valid, and matches the first
1616 empty line.) The terminating string must appear by itself (unquoted and
1617 with no surrounding whitespace) on the terminating line.
1620 The price is $Price.
1623 print << "EOF"; # same as above
1624 The price is $Price.
1627 print << `EOC`; # execute commands
1632 print <<"foo", <<"bar"; # you can stack them
1638 myfunc(<< "THIS", 23, <<'THAT');
1645 Just don't forget that you have to put a semicolon on the end
1646 to finish the statement, as Perl doesn't know you're not going to
1654 If you want your here-docs to be indented with the
1655 rest of the code, you'll need to remove leading whitespace
1656 from each line manually:
1658 ($quote = <<'FINIS') =~ s/^\s+//gm;
1659 The Road goes ever on and on,
1660 down from the door where it began.
1663 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1664 the quoted material must come on the lines following the final delimiter.
1679 If the terminating identifier is on the last line of the program, you
1680 must be sure there is a newline after it; otherwise, Perl will give the
1681 warning B<Can't find string terminator "END" anywhere before EOF...>.
1683 Additionally, the quoting rules for the identifier are not related to
1684 Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
1685 in place of C<''> and C<"">, and the only interpolation is for backslashing
1686 the quoting character:
1688 print << "abc\"def";
1692 Finally, quoted strings cannot span multiple lines. The general rule is
1693 that the identifier must be a string literal. Stick with that, and you
1698 =head2 Gory details of parsing quoted constructs
1699 X<quote, gory details>
1701 When presented with something that might have several different
1702 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1703 principle to pick the most probable interpretation. This strategy
1704 is so successful that Perl programmers often do not suspect the
1705 ambivalence of what they write. But from time to time, Perl's
1706 notions differ substantially from what the author honestly meant.
1708 This section hopes to clarify how Perl handles quoted constructs.
1709 Although the most common reason to learn this is to unravel labyrinthine
1710 regular expressions, because the initial steps of parsing are the
1711 same for all quoting operators, they are all discussed together.
1713 The most important Perl parsing rule is the first one discussed
1714 below: when processing a quoted construct, Perl first finds the end
1715 of that construct, then interprets its contents. If you understand
1716 this rule, you may skip the rest of this section on the first
1717 reading. The other rules are likely to contradict the user's
1718 expectations much less frequently than this first one.
1720 Some passes discussed below are performed concurrently, but because
1721 their results are the same, we consider them individually. For different
1722 quoting constructs, Perl performs different numbers of passes, from
1723 one to five, but these passes are always performed in the same order.
1727 =item Finding the end
1729 The first pass is finding the end of the quoted construct, whether
1730 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1731 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1732 terminates C<qq[]> construct, or a C<< > >> which terminates a
1733 fileglob started with C<< < >>.
1735 When searching for single-character non-pairing delimiters, such
1736 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1737 when searching for single-character pairing delimiter like C<[>,
1738 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1739 C<[>, C<]> are skipped as well. When searching for multicharacter
1740 delimiters, nothing is skipped.
1742 For constructs with three-part delimiters (C<s///>, C<y///>, and
1743 C<tr///>), the search is repeated once more.
1745 During this search no attention is paid to the semantics of the construct.
1748 "$hash{"$foo/$bar"}"
1753 bar # NOT a comment, this slash / terminated m//!
1756 do not form legal quoted expressions. The quoted part ends on the
1757 first C<"> and C</>, and the rest happens to be a syntax error.
1758 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1759 the example above is not C<m//x>, but rather C<m//> with no C</x>
1760 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1762 Also no attention is paid to C<\c\> during this search.
1763 Thus the second C<\> in C<qq/\c\/> is interpreted as a part of C<\/>,
1764 and the following C</> is not recognized as a delimiter.
1765 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1767 =item Removal of backslashes before delimiters
1769 During the second pass, text between the starting and ending
1770 delimiters is copied to a safe location, and the C<\> is removed
1771 from combinations consisting of C<\> and delimiter--or delimiters,
1772 meaning both starting and ending delimiters will should these differ.
1773 This removal does not happen for multi-character delimiters.
1774 Note that the combination C<\\> is left intact, just as it was.
1776 Starting from this step no information about the delimiters is
1782 The next step is interpolation in the text obtained, which is now
1783 delimiter-independent. There are four different cases.
1787 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1789 No interpolation is performed.
1793 The only interpolation is removal of C<\> from pairs C<\\>.
1795 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1797 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1798 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1799 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1800 The other combinations are replaced with appropriate expansions.
1802 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1803 is interpolated in the usual way. Something like C<"\Q\\E"> has
1804 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1805 result is the same as for C<"\\\\E">. As a general rule, backslashes
1806 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1807 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1808 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1813 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1815 Interpolated scalars and arrays are converted internally to the C<join> and
1816 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1818 $foo . " XXX '" . (join $", @arr) . "'";
1820 All operations above are performed simultaneously, left to right.
1822 Because the result of C<"\Q STRING \E"> has all metacharacters
1823 quoted, there is no way to insert a literal C<$> or C<@> inside a
1824 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1825 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1828 Note also that the interpolation code needs to make a decision on
1829 where the interpolated scalar ends. For instance, whether
1830 C<< "a $b -> {c}" >> really means:
1832 "a " . $b . " -> {c}";
1838 Most of the time, the longest possible text that does not include
1839 spaces between components and which contains matching braces or
1840 brackets. because the outcome may be determined by voting based
1841 on heuristic estimators, the result is not strictly predictable.
1842 Fortunately, it's usually correct for ambiguous cases.
1844 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1846 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1847 happens (almost) as with C<qq//> constructs, but the substitution
1848 of C<\> followed by RE-special chars (including C<\>) is not
1849 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1850 a C<#>-comment in a C<//x>-regular expression, no processing is
1851 performed whatsoever. This is the first step at which the presence
1852 of the C<//x> modifier is relevant.
1854 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1855 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1856 different estimators) to be either an array element or C<$var>
1857 followed by an RE alternative. This is where the notation
1858 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1859 array element C<-9>, not as a regular expression from the variable
1860 C<$arr> followed by a digit, which would be the interpretation of
1861 C</$arr[0-9]/>. Since voting among different estimators may occur,
1862 the result is not predictable.
1864 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1865 the replacement text of C<s///> to correct the incorrigible
1866 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1867 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1868 (that is, the C<$^W> variable) was set.
1870 The lack of processing of C<\\> creates specific restrictions on
1871 the post-processed text. If the delimiter is C</>, one cannot get
1872 the combination C<\/> into the result of this step. C</> will
1873 finish the regular expression, C<\/> will be stripped to C</> on
1874 the previous step, and C<\\/> will be left as is. Because C</> is
1875 equivalent to C<\/> inside a regular expression, this does not
1876 matter unless the delimiter happens to be character special to the
1877 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1878 alphanumeric char, as in:
1882 In the RE above, which is intentionally obfuscated for illustration, the
1883 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1884 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
1885 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1886 non-whitespace choices.
1890 This step is the last one for all constructs except regular expressions,
1891 which are processed further.
1893 =item Interpolation of regular expressions
1894 X<regexp, interpolation>
1896 Previous steps were performed during the compilation of Perl code,
1897 but this one happens at run time--although it may be optimized to
1898 be calculated at compile time if appropriate. After preprocessing
1899 described above, and possibly after evaluation if catenation,
1900 joining, casing translation, or metaquoting are involved, the
1901 resulting I<string> is passed to the RE engine for compilation.
1903 Whatever happens in the RE engine might be better discussed in L<perlre>,
1904 but for the sake of continuity, we shall do so here.
1906 This is another step where the presence of the C<//x> modifier is
1907 relevant. The RE engine scans the string from left to right and
1908 converts it to a finite automaton.
1910 Backslashed characters are either replaced with corresponding
1911 literal strings (as with C<\{>), or else they generate special nodes
1912 in the finite automaton (as with C<\b>). Characters special to the
1913 RE engine (such as C<|>) generate corresponding nodes or groups of
1914 nodes. C<(?#...)> comments are ignored. All the rest is either
1915 converted to literal strings to match, or else is ignored (as is
1916 whitespace and C<#>-style comments if C<//x> is present).
1918 Parsing of the bracketed character class construct, C<[...]>, is
1919 rather different than the rule used for the rest of the pattern.
1920 The terminator of this construct is found using the same rules as
1921 for finding the terminator of a C<{}>-delimited construct, the only
1922 exception being that C<]> immediately following C<[> is treated as
1923 though preceded by a backslash. Similarly, the terminator of
1924 C<(?{...})> is found using the same rules as for finding the
1925 terminator of a C<{}>-delimited construct.
1927 It is possible to inspect both the string given to RE engine and the
1928 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1929 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1930 switch documented in L<perlrun/"Command Switches">.
1932 =item Optimization of regular expressions
1933 X<regexp, optimization>
1935 This step is listed for completeness only. Since it does not change
1936 semantics, details of this step are not documented and are subject
1937 to change without notice. This step is performed over the finite
1938 automaton that was generated during the previous pass.
1940 It is at this stage that C<split()> silently optimizes C</^/> to
1945 =head2 I/O Operators
1946 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
1949 There are several I/O operators you should know about.
1951 A string enclosed by backticks (grave accents) first undergoes
1952 double-quote interpolation. It is then interpreted as an external
1953 command, and the output of that command is the value of the
1954 backtick string, like in a shell. In scalar context, a single string
1955 consisting of all output is returned. In list context, a list of
1956 values is returned, one per line of output. (You can set C<$/> to use
1957 a different line terminator.) The command is executed each time the
1958 pseudo-literal is evaluated. The status value of the command is
1959 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
1960 Unlike in B<csh>, no translation is done on the return data--newlines
1961 remain newlines. Unlike in any of the shells, single quotes do not
1962 hide variable names in the command from interpretation. To pass a
1963 literal dollar-sign through to the shell you need to hide it with a
1964 backslash. The generalized form of backticks is C<qx//>. (Because
1965 backticks always undergo shell expansion as well, see L<perlsec> for
1967 X<qx> X<`> X<``> X<backtick> X<glob>
1969 In scalar context, evaluating a filehandle in angle brackets yields
1970 the next line from that file (the newline, if any, included), or
1971 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1972 (sometimes known as file-slurp mode) and the file is empty, it
1973 returns C<''> the first time, followed by C<undef> subsequently.
1975 Ordinarily you must assign the returned value to a variable, but
1976 there is one situation where an automatic assignment happens. If
1977 and only if the input symbol is the only thing inside the conditional
1978 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1979 the value is automatically assigned to the global variable $_,
1980 destroying whatever was there previously. (This may seem like an
1981 odd thing to you, but you'll use the construct in almost every Perl
1982 script you write.) The $_ variable is not implicitly localized.
1983 You'll have to put a C<local $_;> before the loop if you want that
1986 The following lines are equivalent:
1988 while (defined($_ = <STDIN>)) { print; }
1989 while ($_ = <STDIN>) { print; }
1990 while (<STDIN>) { print; }
1991 for (;<STDIN>;) { print; }
1992 print while defined($_ = <STDIN>);
1993 print while ($_ = <STDIN>);
1994 print while <STDIN>;
1996 This also behaves similarly, but avoids $_ :
1998 while (my $line = <STDIN>) { print $line }
2000 In these loop constructs, the assigned value (whether assignment
2001 is automatic or explicit) is then tested to see whether it is
2002 defined. The defined test avoids problems where line has a string
2003 value that would be treated as false by Perl, for example a "" or
2004 a "0" with no trailing newline. If you really mean for such values
2005 to terminate the loop, they should be tested for explicitly:
2007 while (($_ = <STDIN>) ne '0') { ... }
2008 while (<STDIN>) { last unless $_; ... }
2010 In other boolean contexts, C<< <I<filehandle>> >> without an
2011 explicit C<defined> test or comparison elicit a warning if the
2012 C<use warnings> pragma or the B<-w>
2013 command-line switch (the C<$^W> variable) is in effect.
2015 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2016 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2017 in packages, where they would be interpreted as local identifiers
2018 rather than global.) Additional filehandles may be created with
2019 the open() function, amongst others. See L<perlopentut> and
2020 L<perlfunc/open> for details on this.
2021 X<stdin> X<stdout> X<sterr>
2023 If a <FILEHANDLE> is used in a context that is looking for
2024 a list, a list comprising all input lines is returned, one line per
2025 list element. It's easy to grow to a rather large data space this
2026 way, so use with care.
2028 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2029 See L<perlfunc/readline>.
2031 The null filehandle <> is special: it can be used to emulate the
2032 behavior of B<sed> and B<awk>. Input from <> comes either from
2033 standard input, or from each file listed on the command line. Here's
2034 how it works: the first time <> is evaluated, the @ARGV array is
2035 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2036 gives you standard input. The @ARGV array is then processed as a list
2037 of filenames. The loop
2040 ... # code for each line
2043 is equivalent to the following Perl-like pseudo code:
2045 unshift(@ARGV, '-') unless @ARGV;
2046 while ($ARGV = shift) {
2049 ... # code for each line
2053 except that it isn't so cumbersome to say, and will actually work.
2054 It really does shift the @ARGV array and put the current filename
2055 into the $ARGV variable. It also uses filehandle I<ARGV>
2056 internally--<> is just a synonym for <ARGV>, which
2057 is magical. (The pseudo code above doesn't work because it treats
2058 <ARGV> as non-magical.)
2060 You can modify @ARGV before the first <> as long as the array ends up
2061 containing the list of filenames you really want. Line numbers (C<$.>)
2062 continue as though the input were one big happy file. See the example
2063 in L<perlfunc/eof> for how to reset line numbers on each file.
2065 If you want to set @ARGV to your own list of files, go right ahead.
2066 This sets @ARGV to all plain text files if no @ARGV was given:
2068 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2070 You can even set them to pipe commands. For example, this automatically
2071 filters compressed arguments through B<gzip>:
2073 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2075 If you want to pass switches into your script, you can use one of the
2076 Getopts modules or put a loop on the front like this:
2078 while ($_ = $ARGV[0], /^-/) {
2081 if (/^-D(.*)/) { $debug = $1 }
2082 if (/^-v/) { $verbose++ }
2083 # ... # other switches
2087 # ... # code for each line
2090 The <> symbol will return C<undef> for end-of-file only once.
2091 If you call it again after this, it will assume you are processing another
2092 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2094 If what the angle brackets contain is a simple scalar variable (e.g.,
2095 <$foo>), then that variable contains the name of the
2096 filehandle to input from, or its typeglob, or a reference to the
2102 If what's within the angle brackets is neither a filehandle nor a simple
2103 scalar variable containing a filehandle name, typeglob, or typeglob
2104 reference, it is interpreted as a filename pattern to be globbed, and
2105 either a list of filenames or the next filename in the list is returned,
2106 depending on context. This distinction is determined on syntactic
2107 grounds alone. That means C<< <$x> >> is always a readline() from
2108 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2109 That's because $x is a simple scalar variable, but C<$hash{key}> is
2110 not--it's a hash element. Even C<< <$x > >> (note the extra space)
2111 is treated as C<glob("$x ")>, not C<readline($x)>.
2113 One level of double-quote interpretation is done first, but you can't
2114 say C<< <$foo> >> because that's an indirect filehandle as explained
2115 in the previous paragraph. (In older versions of Perl, programmers
2116 would insert curly brackets to force interpretation as a filename glob:
2117 C<< <${foo}> >>. These days, it's considered cleaner to call the
2118 internal function directly as C<glob($foo)>, which is probably the right
2119 way to have done it in the first place.) For example:
2125 is roughly equivalent to:
2127 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2133 except that the globbing is actually done internally using the standard
2134 C<File::Glob> extension. Of course, the shortest way to do the above is:
2138 A (file)glob evaluates its (embedded) argument only when it is
2139 starting a new list. All values must be read before it will start
2140 over. In list context, this isn't important because you automatically
2141 get them all anyway. However, in scalar context the operator returns
2142 the next value each time it's called, or C<undef> when the list has
2143 run out. As with filehandle reads, an automatic C<defined> is
2144 generated when the glob occurs in the test part of a C<while>,
2145 because legal glob returns (e.g. a file called F<0>) would otherwise
2146 terminate the loop. Again, C<undef> is returned only once. So if
2147 you're expecting a single value from a glob, it is much better to
2150 ($file) = <blurch*>;
2156 because the latter will alternate between returning a filename and
2159 If you're trying to do variable interpolation, it's definitely better
2160 to use the glob() function, because the older notation can cause people
2161 to become confused with the indirect filehandle notation.
2163 @files = glob("$dir/*.[ch]");
2164 @files = glob($files[$i]);
2166 =head2 Constant Folding
2167 X<constant folding> X<folding>
2169 Like C, Perl does a certain amount of expression evaluation at
2170 compile time whenever it determines that all arguments to an
2171 operator are static and have no side effects. In particular, string
2172 concatenation happens at compile time between literals that don't do
2173 variable substitution. Backslash interpolation also happens at
2174 compile time. You can say
2176 'Now is the time for all' . "\n" .
2177 'good men to come to.'
2179 and this all reduces to one string internally. Likewise, if
2182 foreach $file (@filenames) {
2183 if (-s $file > 5 + 100 * 2**16) { }
2186 the compiler will precompute the number which that expression
2187 represents so that the interpreter won't have to.
2192 Perl doesn't officially have a no-op operator, but the bare constants
2193 C<0> and C<1> are special-cased to not produce a warning in a void
2194 context, so you can for example safely do
2198 =head2 Bitwise String Operators
2199 X<operator, bitwise, string>
2201 Bitstrings of any size may be manipulated by the bitwise operators
2204 If the operands to a binary bitwise op are strings of different
2205 sizes, B<|> and B<^> ops act as though the shorter operand had
2206 additional zero bits on the right, while the B<&> op acts as though
2207 the longer operand were truncated to the length of the shorter.
2208 The granularity for such extension or truncation is one or more
2211 # ASCII-based examples
2212 print "j p \n" ^ " a h"; # prints "JAPH\n"
2213 print "JA" | " ph\n"; # prints "japh\n"
2214 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2215 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2217 If you are intending to manipulate bitstrings, be certain that
2218 you're supplying bitstrings: If an operand is a number, that will imply
2219 a B<numeric> bitwise operation. You may explicitly show which type of
2220 operation you intend by using C<""> or C<0+>, as in the examples below.
2222 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2223 $foo = '150' | 105; # yields 255
2224 $foo = 150 | '105'; # yields 255
2225 $foo = '150' | '105'; # yields string '155' (under ASCII)
2227 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2228 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2230 See L<perlfunc/vec> for information on how to manipulate individual bits
2233 =head2 Integer Arithmetic
2236 By default, Perl assumes that it must do most of its arithmetic in
2237 floating point. But by saying
2241 you may tell the compiler that it's okay to use integer operations
2242 (if it feels like it) from here to the end of the enclosing BLOCK.
2243 An inner BLOCK may countermand this by saying
2247 which lasts until the end of that BLOCK. Note that this doesn't
2248 mean everything is only an integer, merely that Perl may use integer
2249 operations if it is so inclined. For example, even under C<use
2250 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2253 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2254 and ">>") always produce integral results. (But see also
2255 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2256 them. By default, their results are interpreted as unsigned integers, but
2257 if C<use integer> is in effect, their results are interpreted
2258 as signed integers. For example, C<~0> usually evaluates to a large
2259 integral value. However, C<use integer; ~0> is C<-1> on twos-complement
2262 =head2 Floating-point Arithmetic
2263 X<floating-point> X<floating point> X<float> X<real>
2265 While C<use integer> provides integer-only arithmetic, there is no
2266 analogous mechanism to provide automatic rounding or truncation to a
2267 certain number of decimal places. For rounding to a certain number
2268 of digits, sprintf() or printf() is usually the easiest route.
2271 Floating-point numbers are only approximations to what a mathematician
2272 would call real numbers. There are infinitely more reals than floats,
2273 so some corners must be cut. For example:
2275 printf "%.20g\n", 123456789123456789;
2276 # produces 123456789123456784
2278 Testing for exact equality of floating-point equality or inequality is
2279 not a good idea. Here's a (relatively expensive) work-around to compare
2280 whether two floating-point numbers are equal to a particular number of
2281 decimal places. See Knuth, volume II, for a more robust treatment of
2285 my ($X, $Y, $POINTS) = @_;
2287 $tX = sprintf("%.${POINTS}g", $X);
2288 $tY = sprintf("%.${POINTS}g", $Y);
2292 The POSIX module (part of the standard perl distribution) implements
2293 ceil(), floor(), and other mathematical and trigonometric functions.
2294 The Math::Complex module (part of the standard perl distribution)
2295 defines mathematical functions that work on both the reals and the
2296 imaginary numbers. Math::Complex not as efficient as POSIX, but
2297 POSIX can't work with complex numbers.
2299 Rounding in financial applications can have serious implications, and
2300 the rounding method used should be specified precisely. In these
2301 cases, it probably pays not to trust whichever system rounding is
2302 being used by Perl, but to instead implement the rounding function you
2305 =head2 Bigger Numbers
2306 X<number, arbitrary precision>
2308 The standard Math::BigInt and Math::BigFloat modules provide
2309 variable-precision arithmetic and overloaded operators, although
2310 they're currently pretty slow. At the cost of some space and
2311 considerable speed, they avoid the normal pitfalls associated with
2312 limited-precision representations.
2315 $x = Math::BigInt->new('123456789123456789');
2318 # prints +15241578780673678515622620750190521
2320 There are several modules that let you calculate with (bound only by
2321 memory and cpu-time) unlimited or fixed precision. There are also
2322 some non-standard modules that provide faster implementations via
2323 external C libraries.
2325 Here is a short, but incomplete summary:
2327 Math::Fraction big, unlimited fractions like 9973 / 12967
2328 Math::String treat string sequences like numbers
2329 Math::FixedPrecision calculate with a fixed precision
2330 Math::Currency for currency calculations
2331 Bit::Vector manipulate bit vectors fast (uses C)
2332 Math::BigIntFast Bit::Vector wrapper for big numbers
2333 Math::Pari provides access to the Pari C library
2334 Math::BigInteger uses an external C library
2335 Math::Cephes uses external Cephes C library (no big numbers)
2336 Math::Cephes::Fraction fractions via the Cephes library
2337 Math::GMP another one using an external C library
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