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".
203 X<-> X<negation, arithmetic>
205 Unary "~" performs bitwise negation, i.e., 1's complement. For
206 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
207 L<Bitwise String Operators>.) Note that the width of the result is
208 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
209 bits wide on a 64-bit platform, so if you are expecting a certain bit
210 width, remember to use the & operator to mask off the excess bits.
211 X<~> X<negation, binary>
213 Unary "+" has no effect whatsoever, even on strings. It is useful
214 syntactically for separating a function name from a parenthesized expression
215 that would otherwise be interpreted as the complete list of function
216 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
219 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
220 and L<perlref>. Do not confuse this behavior with the behavior of
221 backslash within a string, although both forms do convey the notion
222 of protecting the next thing from interpolation.
223 X<\> X<reference> X<backslash>
225 =head2 Binding Operators
226 X<binding> X<operator, binding> X<=~> X<!~>
228 Binary "=~" binds a scalar expression to a pattern match. Certain operations
229 search or modify the string $_ by default. This operator makes that kind
230 of operation work on some other string. The right argument is a search
231 pattern, substitution, or transliteration. The left argument is what is
232 supposed to be searched, substituted, or transliterated instead of the default
233 $_. When used in scalar context, the return value generally indicates the
234 success of the operation. Behavior in list context depends on the particular
235 operator. See L</"Regexp Quote-Like Operators"> for details and
236 L<perlretut> for examples using these operators.
238 If the right argument is an expression rather than a search pattern,
239 substitution, or transliteration, it is interpreted as a search pattern at run
242 Binary "!~" is just like "=~" except the return value is negated in
245 =head2 Multiplicative Operators
246 X<operator, multiplicative>
248 Binary "*" multiplies two numbers.
251 Binary "/" divides two numbers.
254 Binary "%" computes the modulus of two numbers. Given integer
255 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
256 C<$a> minus the largest multiple of C<$b> that is not greater than
257 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
258 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
259 result will be less than or equal to zero).
260 Note that when C<use integer> is in scope, "%" gives you direct access
261 to the modulus operator as implemented by your C compiler. This
262 operator is not as well defined for negative operands, but it will
264 X<%> X<remainder> X<modulus> X<mod>
266 Binary "x" is the repetition operator. In scalar context or if the left
267 operand is not enclosed in parentheses, it returns a string consisting
268 of the left operand repeated the number of times specified by the right
269 operand. In list context, if the left operand is enclosed in
270 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
271 If the right operand is zero or negative, it returns an empty string
272 or an empty list, depending on the context.
275 print '-' x 80; # print row of dashes
277 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
279 @ones = (1) x 80; # a list of 80 1's
280 @ones = (5) x @ones; # set all elements to 5
283 =head2 Additive Operators
284 X<operator, additive>
286 Binary "+" returns the sum of two numbers.
289 Binary "-" returns the difference of two numbers.
292 Binary "." concatenates two strings.
293 X<string, concatenation> X<concatenation>
294 X<cat> X<concat> X<concatenate> X<.>
296 =head2 Shift Operators
297 X<shift operator> X<operator, shift> X<<< << >>>
298 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
299 X<shl> X<shr> X<shift, right> X<shift, left>
301 Binary "<<" returns the value of its left argument shifted left by the
302 number of bits specified by the right argument. Arguments should be
303 integers. (See also L<Integer Arithmetic>.)
305 Binary ">>" returns the value of its left argument shifted right by
306 the number of bits specified by the right argument. Arguments should
307 be integers. (See also L<Integer Arithmetic>.)
309 Note that both "<<" and ">>" in Perl are implemented directly using
310 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
311 in force then signed C integers are used, else unsigned C integers are
312 used. Either way, the implementation isn't going to generate results
313 larger than the size of the integer type Perl was built with (32 bits
316 The result of overflowing the range of the integers is undefined
317 because it is undefined also in C. In other words, using 32-bit
318 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
319 of bits is also undefined.
321 =head2 Named Unary Operators
322 X<operator, named unary>
324 The various named unary operators are treated as functions with one
325 argument, with optional parentheses.
327 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
328 is followed by a left parenthesis as the next token, the operator and
329 arguments within parentheses are taken to be of highest precedence,
330 just like a normal function call. For example,
331 because named unary operators are higher precedence than ||:
333 chdir $foo || die; # (chdir $foo) || die
334 chdir($foo) || die; # (chdir $foo) || die
335 chdir ($foo) || die; # (chdir $foo) || die
336 chdir +($foo) || die; # (chdir $foo) || die
338 but, because * is higher precedence than named operators:
340 chdir $foo * 20; # chdir ($foo * 20)
341 chdir($foo) * 20; # (chdir $foo) * 20
342 chdir ($foo) * 20; # (chdir $foo) * 20
343 chdir +($foo) * 20; # chdir ($foo * 20)
345 rand 10 * 20; # rand (10 * 20)
346 rand(10) * 20; # (rand 10) * 20
347 rand (10) * 20; # (rand 10) * 20
348 rand +(10) * 20; # rand (10 * 20)
350 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
351 treated like named unary operators, but they don't follow this functional
352 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
353 equivalent to C<-f "$file.bak">.
354 X<-X> X<filetest> X<operator, filetest>
356 See also L<"Terms and List Operators (Leftward)">.
358 =head2 Relational Operators
359 X<relational operator> X<operator, relational>
361 Binary "<" returns true if the left argument is numerically less than
365 Binary ">" returns true if the left argument is numerically greater
366 than the right argument.
369 Binary "<=" returns true if the left argument is numerically less than
370 or equal to the right argument.
373 Binary ">=" returns true if the left argument is numerically greater
374 than or equal to the right argument.
377 Binary "lt" returns true if the left argument is stringwise less than
381 Binary "gt" returns true if the left argument is stringwise greater
382 than the right argument.
385 Binary "le" returns true if the left argument is stringwise less than
386 or equal to the right argument.
389 Binary "ge" returns true if the left argument is stringwise greater
390 than or equal to the right argument.
393 =head2 Equality Operators
394 X<equality> X<equal> X<equals> X<operator, equality>
396 Binary "==" returns true if the left argument is numerically equal to
400 Binary "!=" returns true if the left argument is numerically not equal
401 to the right argument.
404 Binary "<=>" returns -1, 0, or 1 depending on whether the left
405 argument is numerically less than, equal to, or greater than the right
406 argument. If your platform supports NaNs (not-a-numbers) as numeric
407 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
408 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
409 returns true, as does NaN != anything else. If your platform doesn't
410 support NaNs then NaN is just a string with numeric value 0.
411 X<< <=> >> X<spaceship>
413 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
414 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
416 Binary "eq" returns true if the left argument is stringwise equal to
420 Binary "ne" returns true if the left argument is stringwise not equal
421 to the right argument.
424 Binary "cmp" returns -1, 0, or 1 depending on whether the left
425 argument is stringwise less than, equal to, or greater than the right
429 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
430 by the current locale if C<use locale> is in effect. See L<perllocale>.
433 X<operator, bitwise, and> X<bitwise and> X<&>
435 Binary "&" returns its operands ANDed together bit by bit.
436 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
438 Note that "&" has lower priority than relational operators, so for example
439 the brackets are essential in a test like
441 print "Even\n" if ($x & 1) == 0;
443 =head2 Bitwise Or and Exclusive Or
444 X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
447 Binary "|" returns its operands ORed together bit by bit.
448 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
450 Binary "^" returns its operands XORed together bit by bit.
451 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
453 Note that "|" and "^" have lower priority than relational operators, so
454 for example the brackets are essential in a test like
456 print "false\n" if (8 | 2) != 10;
458 =head2 C-style Logical And
459 X<&&> X<logical and> X<operator, logical, and>
461 Binary "&&" performs a short-circuit logical AND operation. That is,
462 if the left operand is false, the right operand is not even evaluated.
463 Scalar or list context propagates down to the right operand if it
466 =head2 C-style Logical Or
467 X<||> X<operator, logical, or>
469 Binary "||" performs a short-circuit logical OR operation. That is,
470 if the left operand is true, the right operand is not even evaluated.
471 Scalar or list context propagates down to the right operand if it
474 =head2 C-style Logical Defined-Or
475 X<//> X<operator, logical, defined-or>
477 Although it has no direct equivalent in C, Perl's C<//> operator is related
478 to its C-style or. In fact, it's exactly the same as C<||>, except that it
479 tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
480 is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
481 rather than the value of C<defined($a)>) and is exactly equivalent to
482 C<defined($a) ? $a : $b>. This is very useful for providing default values
483 for variables. If you actually want to test if at least one of C<$a> and
484 C<$b> is defined, use C<defined($a // $b)>.
486 The C<||>, C<//> and C<&&> operators return the last value evaluated
487 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
488 portable way to find out the home directory might be:
490 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
491 (getpwuid($<))[7] // die "You're homeless!\n";
493 In particular, this means that you shouldn't use this
494 for selecting between two aggregates for assignment:
496 @a = @b || @c; # this is wrong
497 @a = scalar(@b) || @c; # really meant this
498 @a = @b ? @b : @c; # this works fine, though
500 As more readable alternatives to C<&&>, C<//> and C<||> when used for
501 control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
502 The short-circuit behavior is identical. The precedence of "and", "err"
503 and "or" is much lower, however, so that you can safely use them after a
504 list operator without the need for parentheses:
506 unlink "alpha", "beta", "gamma"
507 or gripe(), next LINE;
509 With the C-style operators that would have been written like this:
511 unlink("alpha", "beta", "gamma")
512 || (gripe(), next LINE);
514 Using "or" for assignment is unlikely to do what you want; see below.
516 =head2 Range Operators
517 X<operator, range> X<range> X<..> X<...>
519 Binary ".." is the range operator, which is really two different
520 operators depending on the context. In list context, it returns a
521 list of values counting (up by ones) from the left value to the right
522 value. If the left value is greater than the right value then it
523 returns the empty list. The range operator is useful for writing
524 C<foreach (1..10)> loops and for doing slice operations on arrays. In
525 the current implementation, no temporary array is created when the
526 range operator is used as the expression in C<foreach> loops, but older
527 versions of Perl might burn a lot of memory when you write something
530 for (1 .. 1_000_000) {
534 The range operator also works on strings, using the magical auto-increment,
537 In scalar context, ".." returns a boolean value. The operator is
538 bistable, like a flip-flop, and emulates the line-range (comma) operator
539 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
540 own boolean state. It is false as long as its left operand is false.
541 Once the left operand is true, the range operator stays true until the
542 right operand is true, I<AFTER> which the range operator becomes false
543 again. It doesn't become false till the next time the range operator is
544 evaluated. It can test the right operand and become false on the same
545 evaluation it became true (as in B<awk>), but it still returns true once.
546 If you don't want it to test the right operand till the next
547 evaluation, as in B<sed>, just use three dots ("...") instead of
548 two. In all other regards, "..." behaves just like ".." does.
550 The right operand is not evaluated while the operator is in the
551 "false" state, and the left operand is not evaluated while the
552 operator is in the "true" state. The precedence is a little lower
553 than || and &&. The value returned is either the empty string for
554 false, or a sequence number (beginning with 1) for true. The
555 sequence number is reset for each range encountered. The final
556 sequence number in a range has the string "E0" appended to it, which
557 doesn't affect its numeric value, but gives you something to search
558 for if you want to exclude the endpoint. You can exclude the
559 beginning point by waiting for the sequence number to be greater
562 If either operand of scalar ".." is a constant expression,
563 that operand is considered true if it is equal (C<==>) to the current
564 input line number (the C<$.> variable).
566 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
567 but that is only an issue if you use a floating point expression; when
568 implicitly using C<$.> as described in the previous paragraph, the
569 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
570 is set to a floating point value and you are not reading from a file.
571 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
572 you want in scalar context because each of the operands are evaluated
573 using their integer representation.
577 As a scalar operator:
579 if (101 .. 200) { print; } # print 2nd hundred lines, short for
580 # if ($. == 101 .. $. == 200) ...
581 next line if (1 .. /^$/); # skip header lines, short for
582 # ... if ($. == 1 .. /^$/);
583 s/^/> / if (/^$/ .. eof()); # quote body
585 # parse mail messages
587 $in_header = 1 .. /^$/;
588 $in_body = /^$/ .. eof;
595 close ARGV if eof; # reset $. each file
598 Here's a simple example to illustrate the difference between
599 the two range operators:
614 This program will print only the line containing "Bar". If
615 the range operator is changed to C<...>, it will also print the
618 And now some examples as a list operator:
620 for (101 .. 200) { print; } # print $_ 100 times
621 @foo = @foo[0 .. $#foo]; # an expensive no-op
622 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
624 The range operator (in list context) makes use of the magical
625 auto-increment algorithm if the operands are strings. You
628 @alphabet = ('A' .. 'Z');
630 to get all normal letters of the English alphabet, or
632 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
634 to get a hexadecimal digit, or
636 @z2 = ('01' .. '31'); print $z2[$mday];
638 to get dates with leading zeros. If the final value specified is not
639 in the sequence that the magical increment would produce, the sequence
640 goes until the next value would be longer than the final value
643 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
644 return two elements in list context.
646 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
648 =head2 Conditional Operator
649 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
651 Ternary "?:" is the conditional operator, just as in C. It works much
652 like an if-then-else. If the argument before the ? is true, the
653 argument before the : is returned, otherwise the argument after the :
654 is returned. For example:
656 printf "I have %d dog%s.\n", $n,
657 ($n == 1) ? '' : "s";
659 Scalar or list context propagates downward into the 2nd
660 or 3rd argument, whichever is selected.
662 $a = $ok ? $b : $c; # get a scalar
663 @a = $ok ? @b : @c; # get an array
664 $a = $ok ? @b : @c; # oops, that's just a count!
666 The operator may be assigned to if both the 2nd and 3rd arguments are
667 legal lvalues (meaning that you can assign to them):
669 ($a_or_b ? $a : $b) = $c;
671 Because this operator produces an assignable result, using assignments
672 without parentheses will get you in trouble. For example, this:
674 $a % 2 ? $a += 10 : $a += 2
678 (($a % 2) ? ($a += 10) : $a) += 2
682 ($a % 2) ? ($a += 10) : ($a += 2)
684 That should probably be written more simply as:
686 $a += ($a % 2) ? 10 : 2;
688 =head2 Assignment Operators
689 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
690 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<.=>
693 "=" is the ordinary assignment operator.
695 Assignment operators work as in C. That is,
703 although without duplicating any side effects that dereferencing the lvalue
704 might trigger, such as from tie(). Other assignment operators work similarly.
705 The following are recognized:
712 Although these are grouped by family, they all have the precedence
715 Unlike in C, the scalar assignment operator produces a valid lvalue.
716 Modifying an assignment is equivalent to doing the assignment and
717 then modifying the variable that was assigned to. This is useful
718 for modifying a copy of something, like this:
720 ($tmp = $global) =~ tr [A-Z] [a-z];
731 Similarly, a list assignment in list context produces the list of
732 lvalues assigned to, and a list assignment in scalar context returns
733 the number of elements produced by the expression on the right hand
734 side of the assignment.
736 =head2 Comma Operator
737 X<comma> X<operator, comma> X<,>
739 Binary "," is the comma operator. In scalar context it evaluates
740 its left argument, throws that value away, then evaluates its right
741 argument and returns that value. This is just like C's comma operator.
743 In list context, it's just the list argument separator, and inserts
744 both its arguments into the list.
746 The C<< => >> operator is a synonym for the comma, but forces any word
747 (consisting entirely of word characters) to its left to be interpreted
748 as a string (as of 5.001). This includes words that might otherwise be
749 considered a constant or function call.
751 use constant FOO => "something";
753 my %h = ( FOO => 23 );
761 my %h = ("something", 23);
763 If the argument on the left is not a word, it is first interpreted as
764 an expression, and then the string value of that is used.
766 The C<< => >> operator is helpful in documenting the correspondence
767 between keys and values in hashes, and other paired elements in lists.
769 %hash = ( $key => $value );
770 login( $username => $password );
772 =head2 List Operators (Rightward)
773 X<operator, list, rightward> X<list operator>
775 On the right side of a list operator, it has very low precedence,
776 such that it controls all comma-separated expressions found there.
777 The only operators with lower precedence are the logical operators
778 "and", "or", and "not", which may be used to evaluate calls to list
779 operators without the need for extra parentheses:
781 open HANDLE, "filename"
782 or die "Can't open: $!\n";
784 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
787 X<operator, logical, not> X<not>
789 Unary "not" returns the logical negation of the expression to its right.
790 It's the equivalent of "!" except for the very low precedence.
793 X<operator, logical, and> X<and>
795 Binary "and" returns the logical conjunction of the two surrounding
796 expressions. It's equivalent to && except for the very low
797 precedence. This means that it short-circuits: i.e., the right
798 expression is evaluated only if the left expression is true.
800 =head2 Logical or, Defined or, and Exclusive Or
801 X<operator, logical, or> X<operator, logical, xor> X<operator, logical, err>
802 X<operator, logical, defined or> X<operator, logical, exclusive or>
805 Binary "or" returns the logical disjunction of the two surrounding
806 expressions. It's equivalent to || except for the very low precedence.
807 This makes it useful for control flow
809 print FH $data or die "Can't write to FH: $!";
811 This means that it short-circuits: i.e., the right expression is evaluated
812 only if the left expression is false. Due to its precedence, you should
813 probably avoid using this for assignment, only for control flow.
815 $a = $b or $c; # bug: this is wrong
816 ($a = $b) or $c; # really means this
817 $a = $b || $c; # better written this way
819 However, when it's a list-context assignment and you're trying to use
820 "||" for control flow, you probably need "or" so that the assignment
821 takes higher precedence.
823 @info = stat($file) || die; # oops, scalar sense of stat!
824 @info = stat($file) or die; # better, now @info gets its due
826 Then again, you could always use parentheses.
828 Binary "err" is equivalent to C<//>--it's just like binary "or", except it tests
829 its left argument's definedness instead of its truth. There are two ways to
830 remember "err": either because many functions return C<undef> on an B<err>or,
831 or as a sort of correction: C<$a=($b err 'default')>
833 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
834 It cannot short circuit, of course.
836 =head2 C Operators Missing From Perl
837 X<operator, missing from perl> X<&> X<*>
838 X<typecasting> X<(TYPE)>
840 Here is what C has that Perl doesn't:
846 Address-of operator. (But see the "\" operator for taking a reference.)
850 Dereference-address operator. (Perl's prefix dereferencing
851 operators are typed: $, @, %, and &.)
855 Type-casting operator.
859 =head2 Quote and Quote-like Operators
860 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
861 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
862 X<escape sequence> X<escape>
865 While we usually think of quotes as literal values, in Perl they
866 function as operators, providing various kinds of interpolating and
867 pattern matching capabilities. Perl provides customary quote characters
868 for these behaviors, but also provides a way for you to choose your
869 quote character for any of them. In the following table, a C<{}> represents
870 any pair of delimiters you choose.
872 Customary Generic Meaning Interpolates
877 // m{} Pattern match yes*
879 s{}{} Substitution yes*
880 tr{}{} Transliteration no (but see below)
883 * unless the delimiter is ''.
885 Non-bracketing delimiters use the same character fore and aft, but the four
886 sorts of brackets (round, angle, square, curly) will all nest, which means
895 Note, however, that this does not always work for quoting Perl code:
897 $s = q{ if($a eq "}") ... }; # WRONG
899 is a syntax error. The C<Text::Balanced> module (from CPAN, and
900 starting from Perl 5.8 part of the standard distribution) is able
903 There can be whitespace between the operator and the quoting
904 characters, except when C<#> is being used as the quoting character.
905 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
906 operator C<q> followed by a comment. Its argument will be taken
907 from the next line. This allows you to write:
909 s {foo} # Replace foo
912 The following escape sequences are available in constructs that interpolate
913 and in transliterations.
914 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N>
921 \a alarm (bell) (BEL)
923 \033 octal char (ESC)
925 \x{263a} wide hex char (SMILEY)
926 \c[ control char (ESC)
927 \N{name} named Unicode character
929 B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
930 the vertical tab (VT - ASCII 11).
932 The following escape sequences are available in constructs that interpolate
933 but not in transliterations.
934 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
936 \l lowercase next char
937 \u uppercase next char
940 \E end case modification
941 \Q quote non-word characters till \E
943 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
944 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
945 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
946 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
947 C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
950 All systems use the virtual C<"\n"> to represent a line terminator,
951 called a "newline". There is no such thing as an unvarying, physical
952 newline character. It is only an illusion that the operating system,
953 device drivers, C libraries, and Perl all conspire to preserve. Not all
954 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
955 on a Mac, these are reversed, and on systems without line terminator,
956 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
957 you mean a "newline" for your system, but use the literal ASCII when you
958 need an exact character. For example, most networking protocols expect
959 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
960 and although they often accept just C<"\012">, they seldom tolerate just
961 C<"\015">. If you get in the habit of using C<"\n"> for networking,
962 you may be burned some day.
963 X<newline> X<line terminator> X<eol> X<end of line>
966 For constructs that do interpolate, variables beginning with "C<$>"
967 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
968 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
969 But method calls such as C<< $obj->meth >> are not.
971 Interpolating an array or slice interpolates the elements in order,
972 separated by the value of C<$">, so is equivalent to interpolating
973 C<join $", @array>. "Punctuation" arrays such as C<@+> are only
974 interpolated if the name is enclosed in braces C<@{+}>.
976 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
977 An unescaped C<$> or C<@> interpolates the corresponding variable,
978 while escaping will cause the literal string C<\$> to be inserted.
979 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
981 Patterns are subject to an additional level of interpretation as a
982 regular expression. This is done as a second pass, after variables are
983 interpolated, so that regular expressions may be incorporated into the
984 pattern from the variables. If this is not what you want, use C<\Q> to
985 interpolate a variable literally.
987 Apart from the behavior described above, Perl does not expand
988 multiple levels of interpolation. In particular, contrary to the
989 expectations of shell programmers, back-quotes do I<NOT> interpolate
990 within double quotes, nor do single quotes impede evaluation of
991 variables when used within double quotes.
993 =head2 Regexp Quote-Like Operators
996 Here are the quote-like operators that apply to pattern
997 matching and related activities.
1004 This is just like the C</pattern/> search, except that it matches only
1005 once between calls to the reset() operator. This is a useful
1006 optimization when you want to see only the first occurrence of
1007 something in each file of a set of files, for instance. Only C<??>
1008 patterns local to the current package are reset.
1012 # blank line between header and body
1015 reset if eof; # clear ?? status for next file
1018 This usage is vaguely deprecated, which means it just might possibly
1019 be removed in some distant future version of Perl, perhaps somewhere
1020 around the year 2168.
1022 =item m/PATTERN/cgimosx
1023 X<m> X<operator, match>
1024 X<regexp, options> X<regexp> X<regex, options> X<regex>
1025 X</c> X</i> X</m> X</o> X</s> X</x>
1027 =item /PATTERN/cgimosx
1029 Searches a string for a pattern match, and in scalar context returns
1030 true if it succeeds, false if it fails. If no string is specified
1031 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1032 string specified with C<=~> need not be an lvalue--it may be the
1033 result of an expression evaluation, but remember the C<=~> binds
1034 rather tightly.) See also L<perlre>. See L<perllocale> for
1035 discussion of additional considerations that apply when C<use locale>
1040 c Do not reset search position on a failed match when /g is in effect.
1041 g Match globally, i.e., find all occurrences.
1042 i Do case-insensitive pattern matching.
1043 m Treat string as multiple lines.
1044 o Compile pattern only once.
1045 s Treat string as single line.
1046 x Use extended regular expressions.
1048 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1049 you can use any pair of non-alphanumeric, non-whitespace characters
1050 as delimiters. This is particularly useful for matching path names
1051 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1052 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
1053 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1055 PATTERN may contain variables, which will be interpolated (and the
1056 pattern recompiled) every time the pattern search is evaluated, except
1057 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1058 C<$|> are not interpolated because they look like end-of-string tests.)
1059 If you want such a pattern to be compiled only once, add a C</o> after
1060 the trailing delimiter. This avoids expensive run-time recompilations,
1061 and is useful when the value you are interpolating won't change over
1062 the life of the script. However, mentioning C</o> constitutes a promise
1063 that you won't change the variables in the pattern. If you change them,
1064 Perl won't even notice. See also L<"qr/STRING/imosx">.
1066 If the PATTERN evaluates to the empty string, the last
1067 I<successfully> matched regular expression is used instead. In this
1068 case, only the C<g> and C<c> flags on the empty pattern is honoured -
1069 the other flags are taken from the original pattern. If no match has
1070 previously succeeded, this will (silently) act instead as a genuine
1071 empty pattern (which will always match).
1073 Note that it's possible to confuse Perl into thinking C<//> (the empty
1074 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1075 good about this, but some pathological cases might trigger this, such as
1076 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1077 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1078 will assume you meant defined-or. If you meant the empty regex, just
1079 use parentheses or spaces to disambiguate, or even prefix the empty
1080 regex with an C<m> (so C<//> becomes C<m//>).
1082 If the C</g> option is not used, C<m//> in list context returns a
1083 list consisting of the subexpressions matched by the parentheses in the
1084 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1085 also set, and that this differs from Perl 4's behavior.) When there are
1086 no parentheses in the pattern, the return value is the list C<(1)> for
1087 success. With or without parentheses, an empty list is returned upon
1092 open(TTY, '/dev/tty');
1093 <TTY> =~ /^y/i && foo(); # do foo if desired
1095 if (/Version: *([0-9.]*)/) { $version = $1; }
1097 next if m#^/usr/spool/uucp#;
1102 print if /$arg/o; # compile only once
1105 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1107 This last example splits $foo into the first two words and the
1108 remainder of the line, and assigns those three fields to $F1, $F2, and
1109 $Etc. The conditional is true if any variables were assigned, i.e., if
1110 the pattern matched.
1112 The C</g> modifier specifies global pattern matching--that is,
1113 matching as many times as possible within the string. How it behaves
1114 depends on the context. In list context, it returns a list of the
1115 substrings matched by any capturing parentheses in the regular
1116 expression. If there are no parentheses, it returns a list of all
1117 the matched strings, as if there were parentheses around the whole
1120 In scalar context, each execution of C<m//g> finds the next match,
1121 returning true if it matches, and false if there is no further match.
1122 The position after the last match can be read or set using the pos()
1123 function; see L<perlfunc/pos>. A failed match normally resets the
1124 search position to the beginning of the string, but you can avoid that
1125 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1126 string also resets the search position.
1128 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1129 zero-width assertion that matches the exact position where the previous
1130 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1131 still anchors at pos(), but the match is of course only attempted once.
1132 Using C<\G> without C</g> on a target string that has not previously had a
1133 C</g> match applied to it is the same as using the C<\A> assertion to match
1134 the beginning of the string. Note also that, currently, C<\G> is only
1135 properly supported when anchored at the very beginning of the pattern.
1140 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1144 while (defined($paragraph = <>)) {
1145 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1149 print "$sentences\n";
1151 # using m//gc with \G
1155 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1157 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1159 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1161 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1163 The last example should print:
1173 Notice that the final match matched C<q> instead of C<p>, which a match
1174 without the C<\G> anchor would have done. Also note that the final match
1175 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1176 final match did indeed match C<p>, it's a good bet that you're running an
1177 older (pre-5.6.0) Perl.
1179 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1180 combine several regexps like this to process a string part-by-part,
1181 doing different actions depending on which regexp matched. Each
1182 regexp tries to match where the previous one leaves off.
1185 $url = new URI::URL "http://www/"; die if $url eq "xXx";
1189 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1190 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1191 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1192 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1193 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1194 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1195 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1196 print ". That's all!\n";
1199 Here is the output (split into several lines):
1201 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1202 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1203 lowercase lowercase line-noise lowercase lowercase line-noise
1204 MiXeD line-noise. That's all!
1207 X<q> X<quote, double> X<'> X<''>
1211 A single-quoted, literal string. A backslash represents a backslash
1212 unless followed by the delimiter or another backslash, in which case
1213 the delimiter or backslash is interpolated.
1215 $foo = q!I said, "You said, 'She said it.'"!;
1216 $bar = q('This is it.');
1217 $baz = '\n'; # a two-character string
1220 X<qq> X<quote, double> X<"> X<"">
1224 A double-quoted, interpolated string.
1227 (*** The previous line contains the naughty word "$1".\n)
1228 if /\b(tcl|java|python)\b/i; # :-)
1229 $baz = "\n"; # a one-character string
1231 =item qr/STRING/imosx
1232 X<qr> X</i> X</m> X</o> X</s> X</x>
1234 This operator quotes (and possibly compiles) its I<STRING> as a regular
1235 expression. I<STRING> is interpolated the same way as I<PATTERN>
1236 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1237 is done. Returns a Perl value which may be used instead of the
1238 corresponding C</STRING/imosx> expression.
1242 $rex = qr/my.STRING/is;
1249 The result may be used as a subpattern in a match:
1252 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1253 $string =~ $re; # or used standalone
1254 $string =~ /$re/; # or this way
1256 Since Perl may compile the pattern at the moment of execution of qr()
1257 operator, using qr() may have speed advantages in some situations,
1258 notably if the result of qr() is used standalone:
1261 my $patterns = shift;
1262 my @compiled = map qr/$_/i, @$patterns;
1265 foreach my $pat (@compiled) {
1266 $success = 1, last if /$pat/;
1272 Precompilation of the pattern into an internal representation at
1273 the moment of qr() avoids a need to recompile the pattern every
1274 time a match C</$pat/> is attempted. (Perl has many other internal
1275 optimizations, but none would be triggered in the above example if
1276 we did not use qr() operator.)
1280 i Do case-insensitive pattern matching.
1281 m Treat string as multiple lines.
1282 o Compile pattern only once.
1283 s Treat string as single line.
1284 x Use extended regular expressions.
1286 See L<perlre> for additional information on valid syntax for STRING, and
1287 for a detailed look at the semantics of regular expressions.
1290 X<qx> X<`> X<``> X<backtick>
1294 A string which is (possibly) interpolated and then executed as a
1295 system command with C</bin/sh> or its equivalent. Shell wildcards,
1296 pipes, and redirections will be honored. The collected standard
1297 output of the command is returned; standard error is unaffected. In
1298 scalar context, it comes back as a single (potentially multi-line)
1299 string, or undef if the command failed. In list context, returns a
1300 list of lines (however you've defined lines with $/ or
1301 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1303 Because backticks do not affect standard error, use shell file descriptor
1304 syntax (assuming the shell supports this) if you care to address this.
1305 To capture a command's STDERR and STDOUT together:
1307 $output = `cmd 2>&1`;
1309 To capture a command's STDOUT but discard its STDERR:
1311 $output = `cmd 2>/dev/null`;
1313 To capture a command's STDERR but discard its STDOUT (ordering is
1316 $output = `cmd 2>&1 1>/dev/null`;
1318 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1319 but leave its STDOUT to come out the old STDERR:
1321 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1323 To read both a command's STDOUT and its STDERR separately, it's easiest
1324 to redirect them separately to files, and then read from those files
1325 when the program is done:
1327 system("program args 1>program.stdout 2>program.stderr");
1329 Using single-quote as a delimiter protects the command from Perl's
1330 double-quote interpolation, passing it on to the shell instead:
1332 $perl_info = qx(ps $$); # that's Perl's $$
1333 $shell_info = qx'ps $$'; # that's the new shell's $$
1335 How that string gets evaluated is entirely subject to the command
1336 interpreter on your system. On most platforms, you will have to protect
1337 shell metacharacters if you want them treated literally. This is in
1338 practice difficult to do, as it's unclear how to escape which characters.
1339 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1340 to emulate backticks safely.
1342 On some platforms (notably DOS-like ones), the shell may not be
1343 capable of dealing with multiline commands, so putting newlines in
1344 the string may not get you what you want. You may be able to evaluate
1345 multiple commands in a single line by separating them with the command
1346 separator character, if your shell supports that (e.g. C<;> on many Unix
1347 shells; C<&> on the Windows NT C<cmd> shell).
1349 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1350 output before starting the child process, but this may not be supported
1351 on some platforms (see L<perlport>). To be safe, you may need to set
1352 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1353 C<IO::Handle> on any open handles.
1355 Beware that some command shells may place restrictions on the length
1356 of the command line. You must ensure your strings don't exceed this
1357 limit after any necessary interpolations. See the platform-specific
1358 release notes for more details about your particular environment.
1360 Using this operator can lead to programs that are difficult to port,
1361 because the shell commands called vary between systems, and may in
1362 fact not be present at all. As one example, the C<type> command under
1363 the POSIX shell is very different from the C<type> command under DOS.
1364 That doesn't mean you should go out of your way to avoid backticks
1365 when they're the right way to get something done. Perl was made to be
1366 a glue language, and one of the things it glues together is commands.
1367 Just understand what you're getting yourself into.
1369 See L<"I/O Operators"> for more discussion.
1372 X<qw> X<quote, list> X<quote, words>
1374 Evaluates to a list of the words extracted out of STRING, using embedded
1375 whitespace as the word delimiters. It can be understood as being roughly
1378 split(' ', q/STRING/);
1380 the differences being that it generates a real list at compile time, and
1381 in scalar context it returns the last element in the list. So
1386 is semantically equivalent to the list:
1390 Some frequently seen examples:
1392 use POSIX qw( setlocale localeconv )
1393 @EXPORT = qw( foo bar baz );
1395 A common mistake is to try to separate the words with comma or to
1396 put comments into a multi-line C<qw>-string. For this reason, the
1397 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1398 produces warnings if the STRING contains the "," or the "#" character.
1400 =item s/PATTERN/REPLACEMENT/egimosx
1401 X<substitute> X<substitution> X<replace> X<regexp, replace>
1402 X<regexp, substitute> X</e> X</g> X</i> X</m> X</o> X</s> X</x>
1404 Searches a string for a pattern, and if found, replaces that pattern
1405 with the replacement text and returns the number of substitutions
1406 made. Otherwise it returns false (specifically, the empty string).
1408 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1409 variable is searched and modified. (The string specified with C<=~> must
1410 be scalar variable, an array element, a hash element, or an assignment
1411 to one of those, i.e., an lvalue.)
1413 If the delimiter chosen is a single quote, no interpolation is
1414 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1415 PATTERN contains a $ that looks like a variable rather than an
1416 end-of-string test, the variable will be interpolated into the pattern
1417 at run-time. If you want the pattern compiled only once the first time
1418 the variable is interpolated, use the C</o> option. If the pattern
1419 evaluates to the empty string, the last successfully executed regular
1420 expression is used instead. See L<perlre> for further explanation on these.
1421 See L<perllocale> for discussion of additional considerations that apply
1422 when C<use locale> is in effect.
1426 e Evaluate the right side as an expression.
1427 g Replace globally, i.e., all occurrences.
1428 i Do case-insensitive pattern matching.
1429 m Treat string as multiple lines.
1430 o Compile pattern only once.
1431 s Treat string as single line.
1432 x Use extended regular expressions.
1434 Any non-alphanumeric, non-whitespace delimiter may replace the
1435 slashes. If single quotes are used, no interpretation is done on the
1436 replacement string (the C</e> modifier overrides this, however). Unlike
1437 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1438 text is not evaluated as a command. If the
1439 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1440 pair of quotes, which may or may not be bracketing quotes, e.g.,
1441 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1442 replacement portion to be treated as a full-fledged Perl expression
1443 and evaluated right then and there. It is, however, syntax checked at
1444 compile-time. A second C<e> modifier will cause the replacement portion
1445 to be C<eval>ed before being run as a Perl expression.
1449 s/\bgreen\b/mauve/g; # don't change wintergreen
1451 $path =~ s|/usr/bin|/usr/local/bin|;
1453 s/Login: $foo/Login: $bar/; # run-time pattern
1455 ($foo = $bar) =~ s/this/that/; # copy first, then change
1457 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1460 s/\d+/$&*2/e; # yields 'abc246xyz'
1461 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1462 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1464 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1465 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1466 s/^=(\w+)/&pod($1)/ge; # use function call
1468 # expand variables in $_, but dynamics only, using
1469 # symbolic dereferencing
1472 # Add one to the value of any numbers in the string
1475 # This will expand any embedded scalar variable
1476 # (including lexicals) in $_ : First $1 is interpolated
1477 # to the variable name, and then evaluated
1480 # Delete (most) C comments.
1482 /\* # Match the opening delimiter.
1483 .*? # Match a minimal number of characters.
1484 \*/ # Match the closing delimiter.
1487 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1489 for ($variable) { # trim whitespace in $variable, cheap
1494 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1496 Note the use of $ instead of \ in the last example. Unlike
1497 B<sed>, we use the \<I<digit>> form in only the left hand side.
1498 Anywhere else it's $<I<digit>>.
1500 Occasionally, you can't use just a C</g> to get all the changes
1501 to occur that you might want. Here are two common cases:
1503 # put commas in the right places in an integer
1504 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1506 # expand tabs to 8-column spacing
1507 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1509 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1510 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
1512 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1514 Transliterates all occurrences of the characters found in the search list
1515 with the corresponding character in the replacement list. It returns
1516 the number of characters replaced or deleted. If no string is
1517 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1518 string specified with =~ must be a scalar variable, an array element, a
1519 hash element, or an assignment to one of those, i.e., an lvalue.)
1521 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1522 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1523 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1524 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1525 its own pair of quotes, which may or may not be bracketing quotes,
1526 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1528 Note that C<tr> does B<not> do regular expression character classes
1529 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1530 the tr(1) utility. If you want to map strings between lower/upper
1531 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1532 using the C<s> operator if you need regular expressions.
1534 Note also that the whole range idea is rather unportable between
1535 character sets--and even within character sets they may cause results
1536 you probably didn't expect. A sound principle is to use only ranges
1537 that begin from and end at either alphabets of equal case (a-e, A-E),
1538 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1539 character sets in full.
1543 c Complement the SEARCHLIST.
1544 d Delete found but unreplaced characters.
1545 s Squash duplicate replaced characters.
1547 If the C</c> modifier is specified, the SEARCHLIST character set
1548 is complemented. If the C</d> modifier is specified, any characters
1549 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1550 (Note that this is slightly more flexible than the behavior of some
1551 B<tr> programs, which delete anything they find in the SEARCHLIST,
1552 period.) If the C</s> modifier is specified, sequences of characters
1553 that were transliterated to the same character are squashed down
1554 to a single instance of the character.
1556 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1557 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1558 than the SEARCHLIST, the final character is replicated till it is long
1559 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1560 This latter is useful for counting characters in a class or for
1561 squashing character sequences in a class.
1565 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1567 $cnt = tr/*/*/; # count the stars in $_
1569 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1571 $cnt = tr/0-9//; # count the digits in $_
1573 tr/a-zA-Z//s; # bookkeeper -> bokeper
1575 ($HOST = $host) =~ tr/a-z/A-Z/;
1577 tr/a-zA-Z/ /cs; # change non-alphas to single space
1580 [\000-\177]; # delete 8th bit
1582 If multiple transliterations are given for a character, only the
1587 will transliterate any A to X.
1589 Because the transliteration table is built at compile time, neither
1590 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1591 interpolation. That means that if you want to use variables, you
1594 eval "tr/$oldlist/$newlist/";
1597 eval "tr/$oldlist/$newlist/, 1" or die $@;
1600 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
1602 A line-oriented form of quoting is based on the shell "here-document"
1603 syntax. Following a C<< << >> you specify a string to terminate
1604 the quoted material, and all lines following the current line down to
1605 the terminating string are the value of the item. The terminating
1606 string may be either an identifier (a word), or some quoted text. If
1607 quoted, the type of quotes you use determines the treatment of the
1608 text, just as in regular quoting. An unquoted identifier works like
1609 double quotes. There must be no space between the C<< << >> and
1610 the identifier, unless the identifier is quoted. (If you put a space it
1611 will be treated as a null identifier, which is valid, and matches the first
1612 empty line.) The terminating string must appear by itself (unquoted and
1613 with no surrounding whitespace) on the terminating line.
1616 The price is $Price.
1619 print << "EOF"; # same as above
1620 The price is $Price.
1623 print << `EOC`; # execute commands
1628 print <<"foo", <<"bar"; # you can stack them
1634 myfunc(<< "THIS", 23, <<'THAT');
1641 Just don't forget that you have to put a semicolon on the end
1642 to finish the statement, as Perl doesn't know you're not going to
1650 If you want your here-docs to be indented with the
1651 rest of the code, you'll need to remove leading whitespace
1652 from each line manually:
1654 ($quote = <<'FINIS') =~ s/^\s+//gm;
1655 The Road goes ever on and on,
1656 down from the door where it began.
1659 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1660 the quoted material must come on the lines following the final delimiter.
1675 If the terminating identifier is on the last line of the program, you
1676 must be sure there is a newline after it; otherwise, Perl will give the
1677 warning B<Can't find string terminator "END" anywhere before EOF...>.
1679 Additionally, the quoting rules for the identifier are not related to
1680 Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
1681 in place of C<''> and C<"">, and the only interpolation is for backslashing
1682 the quoting character:
1684 print << "abc\"def";
1688 Finally, quoted strings cannot span multiple lines. The general rule is
1689 that the identifier must be a string literal. Stick with that, and you
1694 =head2 Gory details of parsing quoted constructs
1695 X<quote, gory details>
1697 When presented with something that might have several different
1698 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1699 principle to pick the most probable interpretation. This strategy
1700 is so successful that Perl programmers often do not suspect the
1701 ambivalence of what they write. But from time to time, Perl's
1702 notions differ substantially from what the author honestly meant.
1704 This section hopes to clarify how Perl handles quoted constructs.
1705 Although the most common reason to learn this is to unravel labyrinthine
1706 regular expressions, because the initial steps of parsing are the
1707 same for all quoting operators, they are all discussed together.
1709 The most important Perl parsing rule is the first one discussed
1710 below: when processing a quoted construct, Perl first finds the end
1711 of that construct, then interprets its contents. If you understand
1712 this rule, you may skip the rest of this section on the first
1713 reading. The other rules are likely to contradict the user's
1714 expectations much less frequently than this first one.
1716 Some passes discussed below are performed concurrently, but because
1717 their results are the same, we consider them individually. For different
1718 quoting constructs, Perl performs different numbers of passes, from
1719 one to five, but these passes are always performed in the same order.
1723 =item Finding the end
1725 The first pass is finding the end of the quoted construct, whether
1726 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1727 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1728 terminates C<qq[]> construct, or a C<< > >> which terminates a
1729 fileglob started with C<< < >>.
1731 When searching for single-character non-pairing delimiters, such
1732 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1733 when searching for single-character pairing delimiter like C<[>,
1734 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1735 C<[>, C<]> are skipped as well. When searching for multicharacter
1736 delimiters, nothing is skipped.
1738 For constructs with three-part delimiters (C<s///>, C<y///>, and
1739 C<tr///>), the search is repeated once more.
1741 During this search no attention is paid to the semantics of the construct.
1744 "$hash{"$foo/$bar"}"
1749 bar # NOT a comment, this slash / terminated m//!
1752 do not form legal quoted expressions. The quoted part ends on the
1753 first C<"> and C</>, and the rest happens to be a syntax error.
1754 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1755 the example above is not C<m//x>, but rather C<m//> with no C</x>
1756 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1758 Also no attention is paid to C<\c\> during this search.
1759 Thus the second C<\> in C<qq/\c\/> is interpreted as a part of C<\/>,
1760 and the following C</> is not recognized as a delimiter.
1761 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1763 =item Removal of backslashes before delimiters
1765 During the second pass, text between the starting and ending
1766 delimiters is copied to a safe location, and the C<\> is removed
1767 from combinations consisting of C<\> and delimiter--or delimiters,
1768 meaning both starting and ending delimiters will should these differ.
1769 This removal does not happen for multi-character delimiters.
1770 Note that the combination C<\\> is left intact, just as it was.
1772 Starting from this step no information about the delimiters is
1778 The next step is interpolation in the text obtained, which is now
1779 delimiter-independent. There are four different cases.
1783 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1785 No interpolation is performed.
1789 The only interpolation is removal of C<\> from pairs C<\\>.
1791 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1793 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1794 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1795 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1796 The other combinations are replaced with appropriate expansions.
1798 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1799 is interpolated in the usual way. Something like C<"\Q\\E"> has
1800 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1801 result is the same as for C<"\\\\E">. As a general rule, backslashes
1802 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1803 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1804 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1809 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1811 Interpolated scalars and arrays are converted internally to the C<join> and
1812 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1814 $foo . " XXX '" . (join $", @arr) . "'";
1816 All operations above are performed simultaneously, left to right.
1818 Because the result of C<"\Q STRING \E"> has all metacharacters
1819 quoted, there is no way to insert a literal C<$> or C<@> inside a
1820 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1821 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1824 Note also that the interpolation code needs to make a decision on
1825 where the interpolated scalar ends. For instance, whether
1826 C<< "a $b -> {c}" >> really means:
1828 "a " . $b . " -> {c}";
1834 Most of the time, the longest possible text that does not include
1835 spaces between components and which contains matching braces or
1836 brackets. because the outcome may be determined by voting based
1837 on heuristic estimators, the result is not strictly predictable.
1838 Fortunately, it's usually correct for ambiguous cases.
1840 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1842 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1843 happens (almost) as with C<qq//> constructs, but the substitution
1844 of C<\> followed by RE-special chars (including C<\>) is not
1845 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1846 a C<#>-comment in a C<//x>-regular expression, no processing is
1847 performed whatsoever. This is the first step at which the presence
1848 of the C<//x> modifier is relevant.
1850 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1851 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1852 different estimators) to be either an array element or C<$var>
1853 followed by an RE alternative. This is where the notation
1854 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1855 array element C<-9>, not as a regular expression from the variable
1856 C<$arr> followed by a digit, which would be the interpretation of
1857 C</$arr[0-9]/>. Since voting among different estimators may occur,
1858 the result is not predictable.
1860 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1861 the replacement text of C<s///> to correct the incorrigible
1862 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1863 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1864 (that is, the C<$^W> variable) was set.
1866 The lack of processing of C<\\> creates specific restrictions on
1867 the post-processed text. If the delimiter is C</>, one cannot get
1868 the combination C<\/> into the result of this step. C</> will
1869 finish the regular expression, C<\/> will be stripped to C</> on
1870 the previous step, and C<\\/> will be left as is. Because C</> is
1871 equivalent to C<\/> inside a regular expression, this does not
1872 matter unless the delimiter happens to be character special to the
1873 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1874 alphanumeric char, as in:
1878 In the RE above, which is intentionally obfuscated for illustration, the
1879 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1880 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
1881 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1882 non-whitespace choices.
1886 This step is the last one for all constructs except regular expressions,
1887 which are processed further.
1889 =item Interpolation of regular expressions
1890 X<regexp, interpolation>
1892 Previous steps were performed during the compilation of Perl code,
1893 but this one happens at run time--although it may be optimized to
1894 be calculated at compile time if appropriate. After preprocessing
1895 described above, and possibly after evaluation if catenation,
1896 joining, casing translation, or metaquoting are involved, the
1897 resulting I<string> is passed to the RE engine for compilation.
1899 Whatever happens in the RE engine might be better discussed in L<perlre>,
1900 but for the sake of continuity, we shall do so here.
1902 This is another step where the presence of the C<//x> modifier is
1903 relevant. The RE engine scans the string from left to right and
1904 converts it to a finite automaton.
1906 Backslashed characters are either replaced with corresponding
1907 literal strings (as with C<\{>), or else they generate special nodes
1908 in the finite automaton (as with C<\b>). Characters special to the
1909 RE engine (such as C<|>) generate corresponding nodes or groups of
1910 nodes. C<(?#...)> comments are ignored. All the rest is either
1911 converted to literal strings to match, or else is ignored (as is
1912 whitespace and C<#>-style comments if C<//x> is present).
1914 Parsing of the bracketed character class construct, C<[...]>, is
1915 rather different than the rule used for the rest of the pattern.
1916 The terminator of this construct is found using the same rules as
1917 for finding the terminator of a C<{}>-delimited construct, the only
1918 exception being that C<]> immediately following C<[> is treated as
1919 though preceded by a backslash. Similarly, the terminator of
1920 C<(?{...})> is found using the same rules as for finding the
1921 terminator of a C<{}>-delimited construct.
1923 It is possible to inspect both the string given to RE engine and the
1924 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1925 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1926 switch documented in L<perlrun/"Command Switches">.
1928 =item Optimization of regular expressions
1929 X<regexp, optimization>
1931 This step is listed for completeness only. Since it does not change
1932 semantics, details of this step are not documented and are subject
1933 to change without notice. This step is performed over the finite
1934 automaton that was generated during the previous pass.
1936 It is at this stage that C<split()> silently optimizes C</^/> to
1941 =head2 I/O Operators
1942 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
1945 There are several I/O operators you should know about.
1947 A string enclosed by backticks (grave accents) first undergoes
1948 double-quote interpolation. It is then interpreted as an external
1949 command, and the output of that command is the value of the
1950 backtick string, like in a shell. In scalar context, a single string
1951 consisting of all output is returned. In list context, a list of
1952 values is returned, one per line of output. (You can set C<$/> to use
1953 a different line terminator.) The command is executed each time the
1954 pseudo-literal is evaluated. The status value of the command is
1955 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
1956 Unlike in B<csh>, no translation is done on the return data--newlines
1957 remain newlines. Unlike in any of the shells, single quotes do not
1958 hide variable names in the command from interpretation. To pass a
1959 literal dollar-sign through to the shell you need to hide it with a
1960 backslash. The generalized form of backticks is C<qx//>. (Because
1961 backticks always undergo shell expansion as well, see L<perlsec> for
1963 X<qx> X<`> X<``> X<backtick> X<glob>
1965 In scalar context, evaluating a filehandle in angle brackets yields
1966 the next line from that file (the newline, if any, included), or
1967 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1968 (sometimes known as file-slurp mode) and the file is empty, it
1969 returns C<''> the first time, followed by C<undef> subsequently.
1971 Ordinarily you must assign the returned value to a variable, but
1972 there is one situation where an automatic assignment happens. If
1973 and only if the input symbol is the only thing inside the conditional
1974 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1975 the value is automatically assigned to the global variable $_,
1976 destroying whatever was there previously. (This may seem like an
1977 odd thing to you, but you'll use the construct in almost every Perl
1978 script you write.) The $_ variable is not implicitly localized.
1979 You'll have to put a C<local $_;> before the loop if you want that
1982 The following lines are equivalent:
1984 while (defined($_ = <STDIN>)) { print; }
1985 while ($_ = <STDIN>) { print; }
1986 while (<STDIN>) { print; }
1987 for (;<STDIN>;) { print; }
1988 print while defined($_ = <STDIN>);
1989 print while ($_ = <STDIN>);
1990 print while <STDIN>;
1992 This also behaves similarly, but avoids $_ :
1994 while (my $line = <STDIN>) { print $line }
1996 In these loop constructs, the assigned value (whether assignment
1997 is automatic or explicit) is then tested to see whether it is
1998 defined. The defined test avoids problems where line has a string
1999 value that would be treated as false by Perl, for example a "" or
2000 a "0" with no trailing newline. If you really mean for such values
2001 to terminate the loop, they should be tested for explicitly:
2003 while (($_ = <STDIN>) ne '0') { ... }
2004 while (<STDIN>) { last unless $_; ... }
2006 In other boolean contexts, C<< <I<filehandle>> >> without an
2007 explicit C<defined> test or comparison elicit a warning if the
2008 C<use warnings> pragma or the B<-w>
2009 command-line switch (the C<$^W> variable) is in effect.
2011 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2012 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2013 in packages, where they would be interpreted as local identifiers
2014 rather than global.) Additional filehandles may be created with
2015 the open() function, amongst others. See L<perlopentut> and
2016 L<perlfunc/open> for details on this.
2017 X<stdin> X<stdout> X<sterr>
2019 If a <FILEHANDLE> is used in a context that is looking for
2020 a list, a list comprising all input lines is returned, one line per
2021 list element. It's easy to grow to a rather large data space this
2022 way, so use with care.
2024 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2025 See L<perlfunc/readline>.
2027 The null filehandle <> is special: it can be used to emulate the
2028 behavior of B<sed> and B<awk>. Input from <> comes either from
2029 standard input, or from each file listed on the command line. Here's
2030 how it works: the first time <> is evaluated, the @ARGV array is
2031 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2032 gives you standard input. The @ARGV array is then processed as a list
2033 of filenames. The loop
2036 ... # code for each line
2039 is equivalent to the following Perl-like pseudo code:
2041 unshift(@ARGV, '-') unless @ARGV;
2042 while ($ARGV = shift) {
2045 ... # code for each line
2049 except that it isn't so cumbersome to say, and will actually work.
2050 It really does shift the @ARGV array and put the current filename
2051 into the $ARGV variable. It also uses filehandle I<ARGV>
2052 internally--<> is just a synonym for <ARGV>, which
2053 is magical. (The pseudo code above doesn't work because it treats
2054 <ARGV> as non-magical.)
2056 You can modify @ARGV before the first <> as long as the array ends up
2057 containing the list of filenames you really want. Line numbers (C<$.>)
2058 continue as though the input were one big happy file. See the example
2059 in L<perlfunc/eof> for how to reset line numbers on each file.
2061 If you want to set @ARGV to your own list of files, go right ahead.
2062 This sets @ARGV to all plain text files if no @ARGV was given:
2064 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2066 You can even set them to pipe commands. For example, this automatically
2067 filters compressed arguments through B<gzip>:
2069 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2071 If you want to pass switches into your script, you can use one of the
2072 Getopts modules or put a loop on the front like this:
2074 while ($_ = $ARGV[0], /^-/) {
2077 if (/^-D(.*)/) { $debug = $1 }
2078 if (/^-v/) { $verbose++ }
2079 # ... # other switches
2083 # ... # code for each line
2086 The <> symbol will return C<undef> for end-of-file only once.
2087 If you call it again after this, it will assume you are processing another
2088 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2090 If what the angle brackets contain is a simple scalar variable (e.g.,
2091 <$foo>), then that variable contains the name of the
2092 filehandle to input from, or its typeglob, or a reference to the
2098 If what's within the angle brackets is neither a filehandle nor a simple
2099 scalar variable containing a filehandle name, typeglob, or typeglob
2100 reference, it is interpreted as a filename pattern to be globbed, and
2101 either a list of filenames or the next filename in the list is returned,
2102 depending on context. This distinction is determined on syntactic
2103 grounds alone. That means C<< <$x> >> is always a readline() from
2104 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2105 That's because $x is a simple scalar variable, but C<$hash{key}> is
2106 not--it's a hash element. Even C<< <$x > >> (note the extra space)
2107 is treated as C<glob("$x ")>, not C<readline($x)>.
2109 One level of double-quote interpretation is done first, but you can't
2110 say C<< <$foo> >> because that's an indirect filehandle as explained
2111 in the previous paragraph. (In older versions of Perl, programmers
2112 would insert curly brackets to force interpretation as a filename glob:
2113 C<< <${foo}> >>. These days, it's considered cleaner to call the
2114 internal function directly as C<glob($foo)>, which is probably the right
2115 way to have done it in the first place.) For example:
2121 is roughly equivalent to:
2123 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2129 except that the globbing is actually done internally using the standard
2130 C<File::Glob> extension. Of course, the shortest way to do the above is:
2134 A (file)glob evaluates its (embedded) argument only when it is
2135 starting a new list. All values must be read before it will start
2136 over. In list context, this isn't important because you automatically
2137 get them all anyway. However, in scalar context the operator returns
2138 the next value each time it's called, or C<undef> when the list has
2139 run out. As with filehandle reads, an automatic C<defined> is
2140 generated when the glob occurs in the test part of a C<while>,
2141 because legal glob returns (e.g. a file called F<0>) would otherwise
2142 terminate the loop. Again, C<undef> is returned only once. So if
2143 you're expecting a single value from a glob, it is much better to
2146 ($file) = <blurch*>;
2152 because the latter will alternate between returning a filename and
2155 If you're trying to do variable interpolation, it's definitely better
2156 to use the glob() function, because the older notation can cause people
2157 to become confused with the indirect filehandle notation.
2159 @files = glob("$dir/*.[ch]");
2160 @files = glob($files[$i]);
2162 =head2 Constant Folding
2163 X<constant folding> X<folding>
2165 Like C, Perl does a certain amount of expression evaluation at
2166 compile time whenever it determines that all arguments to an
2167 operator are static and have no side effects. In particular, string
2168 concatenation happens at compile time between literals that don't do
2169 variable substitution. Backslash interpolation also happens at
2170 compile time. You can say
2172 'Now is the time for all' . "\n" .
2173 'good men to come to.'
2175 and this all reduces to one string internally. Likewise, if
2178 foreach $file (@filenames) {
2179 if (-s $file > 5 + 100 * 2**16) { }
2182 the compiler will precompute the number which that expression
2183 represents so that the interpreter won't have to.
2188 Perl doesn't officially have a no-op operator, but the bare constants
2189 C<0> and C<1> are special-cased to not produce a warning in a void
2190 context, so you can for example safely do
2194 =head2 Bitwise String Operators
2195 X<operator, bitwise, string>
2197 Bitstrings of any size may be manipulated by the bitwise operators
2200 If the operands to a binary bitwise op are strings of different
2201 sizes, B<|> and B<^> ops act as though the shorter operand had
2202 additional zero bits on the right, while the B<&> op acts as though
2203 the longer operand were truncated to the length of the shorter.
2204 The granularity for such extension or truncation is one or more
2207 # ASCII-based examples
2208 print "j p \n" ^ " a h"; # prints "JAPH\n"
2209 print "JA" | " ph\n"; # prints "japh\n"
2210 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2211 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2213 If you are intending to manipulate bitstrings, be certain that
2214 you're supplying bitstrings: If an operand is a number, that will imply
2215 a B<numeric> bitwise operation. You may explicitly show which type of
2216 operation you intend by using C<""> or C<0+>, as in the examples below.
2218 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2219 $foo = '150' | 105; # yields 255
2220 $foo = 150 | '105'; # yields 255
2221 $foo = '150' | '105'; # yields string '155' (under ASCII)
2223 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2224 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2226 See L<perlfunc/vec> for information on how to manipulate individual bits
2229 =head2 Integer Arithmetic
2232 By default, Perl assumes that it must do most of its arithmetic in
2233 floating point. But by saying
2237 you may tell the compiler that it's okay to use integer operations
2238 (if it feels like it) from here to the end of the enclosing BLOCK.
2239 An inner BLOCK may countermand this by saying
2243 which lasts until the end of that BLOCK. Note that this doesn't
2244 mean everything is only an integer, merely that Perl may use integer
2245 operations if it is so inclined. For example, even under C<use
2246 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2249 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2250 and ">>") always produce integral results. (But see also
2251 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2252 them. By default, their results are interpreted as unsigned integers, but
2253 if C<use integer> is in effect, their results are interpreted
2254 as signed integers. For example, C<~0> usually evaluates to a large
2255 integral value. However, C<use integer; ~0> is C<-1> on twos-complement
2258 =head2 Floating-point Arithmetic
2259 X<floating-point> X<floating point> X<float> X<real>
2261 While C<use integer> provides integer-only arithmetic, there is no
2262 analogous mechanism to provide automatic rounding or truncation to a
2263 certain number of decimal places. For rounding to a certain number
2264 of digits, sprintf() or printf() is usually the easiest route.
2267 Floating-point numbers are only approximations to what a mathematician
2268 would call real numbers. There are infinitely more reals than floats,
2269 so some corners must be cut. For example:
2271 printf "%.20g\n", 123456789123456789;
2272 # produces 123456789123456784
2274 Testing for exact equality of floating-point equality or inequality is
2275 not a good idea. Here's a (relatively expensive) work-around to compare
2276 whether two floating-point numbers are equal to a particular number of
2277 decimal places. See Knuth, volume II, for a more robust treatment of
2281 my ($X, $Y, $POINTS) = @_;
2283 $tX = sprintf("%.${POINTS}g", $X);
2284 $tY = sprintf("%.${POINTS}g", $Y);
2288 The POSIX module (part of the standard perl distribution) implements
2289 ceil(), floor(), and other mathematical and trigonometric functions.
2290 The Math::Complex module (part of the standard perl distribution)
2291 defines mathematical functions that work on both the reals and the
2292 imaginary numbers. Math::Complex not as efficient as POSIX, but
2293 POSIX can't work with complex numbers.
2295 Rounding in financial applications can have serious implications, and
2296 the rounding method used should be specified precisely. In these
2297 cases, it probably pays not to trust whichever system rounding is
2298 being used by Perl, but to instead implement the rounding function you
2301 =head2 Bigger Numbers
2302 X<number, arbitrary precision>
2304 The standard Math::BigInt and Math::BigFloat modules provide
2305 variable-precision arithmetic and overloaded operators, although
2306 they're currently pretty slow. At the cost of some space and
2307 considerable speed, they avoid the normal pitfalls associated with
2308 limited-precision representations.
2311 $x = Math::BigInt->new('123456789123456789');
2314 # prints +15241578780673678515622620750190521
2316 There are several modules that let you calculate with (bound only by
2317 memory and cpu-time) unlimited or fixed precision. There are also
2318 some non-standard modules that provide faster implementations via
2319 external C libraries.
2321 Here is a short, but incomplete summary:
2323 Math::Fraction big, unlimited fractions like 9973 / 12967
2324 Math::String treat string sequences like numbers
2325 Math::FixedPrecision calculate with a fixed precision
2326 Math::Currency for currency calculations
2327 Bit::Vector manipulate bit vectors fast (uses C)
2328 Math::BigIntFast Bit::Vector wrapper for big numbers
2329 Math::Pari provides access to the Pari C library
2330 Math::BigInteger uses an external C library
2331 Math::Cephes uses external Cephes C library (no big numbers)
2332 Math::Cephes::Fraction fractions via the Cephes library
2333 Math::GMP another one using an external C library
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