3 perlop - Perl operators and precedence
7 =head2 Operator Precedence and Associativity
9 Operator precedence and associativity work in Perl more or less like
10 they do in mathematics.
12 I<Operator precedence> means some operators are evaluated before
13 others. For example, in C<2 + 4 * 5>, the multiplication has higher
14 precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
15 22> and not C<6 * 5 == 30>.
17 I<Operator associativity> defines what happens if a sequence of the
18 same operators is used one after another: whether the evaluator will
19 evaluate the left operations first or the right. For example, in C<8
20 - 4 - 2>, subtraction is left associative so Perl evaluates the
21 expression left to right. C<8 - 4> is evaluated first making the
22 expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
24 Perl operators have the following associativity and precedence,
25 listed from highest precedence to lowest. Operators borrowed from
26 C keep the same precedence relationship with each other, even where
27 C's precedence is slightly screwy. (This makes learning Perl easier
28 for C folks.) With very few exceptions, these all operate on scalar
29 values only, not array values.
31 left terms and list operators (leftward)
35 right ! ~ \ and unary + and -
40 nonassoc named unary operators
41 nonassoc < > <= >= lt gt le ge
42 nonassoc == != <=> eq ne cmp
51 nonassoc list operators (rightward)
56 In the following sections, these operators are covered in precedence order.
58 Many operators can be overloaded for objects. See L<overload>.
60 =head2 Terms and List Operators (Leftward)
62 A TERM has the highest precedence in Perl. They include variables,
63 quote and quote-like operators, any expression in parentheses,
64 and any function whose arguments are parenthesized. Actually, there
65 aren't really functions in this sense, just list operators and unary
66 operators behaving as functions because you put parentheses around
67 the arguments. These are all documented in L<perlfunc>.
69 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
70 is followed by a left parenthesis as the next token, the operator and
71 arguments within parentheses are taken to be of highest precedence,
72 just like a normal function call.
74 In the absence of parentheses, the precedence of list operators such as
75 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
76 whether you are looking at the left side or the right side of the operator.
79 @ary = (1, 3, sort 4, 2);
80 print @ary; # prints 1324
82 the commas on the right of the sort are evaluated before the sort,
83 but the commas on the left are evaluated after. In other words,
84 list operators tend to gobble up all arguments that follow, and
85 then act like a simple TERM with regard to the preceding expression.
86 Be careful with parentheses:
88 # These evaluate exit before doing the print:
89 print($foo, exit); # Obviously not what you want.
90 print $foo, exit; # Nor is this.
92 # These do the print before evaluating exit:
93 (print $foo), exit; # This is what you want.
94 print($foo), exit; # Or this.
95 print ($foo), exit; # Or even this.
99 print ($foo & 255) + 1, "\n";
101 probably doesn't do what you expect at first glance. The parentheses
102 enclose the argument list for C<print> which is evaluated (printing
103 the result of C<$foo & 255>). Then one is added to the return value
104 of C<print> (usually 1). The result is something like this:
106 1 + 1, "\n"; # Obviously not what you meant.
108 To do what you meant properly, you must write:
110 print(($foo & 255) + 1, "\n");
112 See L<Named Unary Operators> for more discussion of this.
114 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
115 well as subroutine and method calls, and the anonymous
116 constructors C<[]> and C<{}>.
118 See also L<Quote and Quote-like Operators> toward the end of this section,
119 as well as L<"I/O Operators">.
121 =head2 The Arrow Operator
123 "C<< -> >>" is an infix dereference operator, just as it is in C
124 and C++. If the right side is either a C<[...]>, C<{...}>, or a
125 C<(...)> subscript, then the left side must be either a hard or
126 symbolic reference to an array, a hash, or a subroutine respectively.
127 (Or technically speaking, a location capable of holding a hard
128 reference, if it's an array or hash reference being used for
129 assignment.) See L<perlreftut> and L<perlref>.
131 Otherwise, the right side is a method name or a simple scalar
132 variable containing either the method name or a subroutine reference,
133 and the left side must be either an object (a blessed reference)
134 or a class name (that is, a package name). See L<perlobj>.
136 =head2 Auto-increment and Auto-decrement
138 "++" and "--" work as in C. That is, if placed before a variable,
139 they increment or decrement the variable by one before returning the
140 value, and if placed after, increment or decrement after returning the
144 print $i++; # prints 0
145 print ++$j; # prints 1
147 Note that just as in C, Perl doesn't define B<when> the variable is
148 incremented or decremented. You just know it will be done sometime
149 before or after the value is returned. This also means that modifying
150 a variable twice in the same statement will lead to undefined behaviour.
151 Avoid statements like:
156 Perl will not guarantee what the result of the above statements is.
158 The auto-increment operator has a little extra builtin magic to it. If
159 you increment a variable that is numeric, or that has ever been used in
160 a numeric context, you get a normal increment. If, however, the
161 variable has been used in only string contexts since it was set, and
162 has a value that is not the empty string and matches the pattern
163 C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
164 character within its range, with carry:
166 print ++($foo = '99'); # prints '100'
167 print ++($foo = 'a0'); # prints 'a1'
168 print ++($foo = 'Az'); # prints 'Ba'
169 print ++($foo = 'zz'); # prints 'aaa'
171 C<undef> is always treated as numeric, and in particular is changed
172 to C<0> before incrementing (so that a post-increment of an undef value
173 will return C<0> rather than C<undef>).
175 The auto-decrement operator is not magical.
177 =head2 Exponentiation
179 Binary "**" is the exponentiation operator. It binds even more
180 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
181 implemented using C's pow(3) function, which actually works on doubles
184 =head2 Symbolic Unary Operators
186 Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
187 precedence version of this.
189 Unary "-" performs arithmetic negation if the operand is numeric. If
190 the operand is an identifier, a string consisting of a minus sign
191 concatenated with the identifier is returned. Otherwise, if the string
192 starts with a plus or minus, a string starting with the opposite sign
193 is returned. One effect of these rules is that -bareword is equivalent
194 to the string "-bareword".
196 Unary "~" performs bitwise negation, i.e., 1's complement. For
197 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
198 L<Bitwise String Operators>.) Note that the width of the result is
199 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
200 bits wide on a 64-bit platform, so if you are expecting a certain bit
201 width, remember to use the & operator to mask off the excess bits.
203 Unary "+" has no effect whatsoever, even on strings. It is useful
204 syntactically for separating a function name from a parenthesized expression
205 that would otherwise be interpreted as the complete list of function
206 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
208 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
209 and L<perlref>. Do not confuse this behavior with the behavior of
210 backslash within a string, although both forms do convey the notion
211 of protecting the next thing from interpolation.
213 =head2 Binding Operators
215 Binary "=~" binds a scalar expression to a pattern match. Certain operations
216 search or modify the string $_ by default. This operator makes that kind
217 of operation work on some other string. The right argument is a search
218 pattern, substitution, or transliteration. The left argument is what is
219 supposed to be searched, substituted, or transliterated instead of the default
220 $_. When used in scalar context, the return value generally indicates the
221 success of the operation. Behavior in list context depends on the particular
222 operator. See L</"Regexp Quote-Like Operators"> for details and
223 L<perlretut> for examples using these operators.
225 If the right argument is an expression rather than a search pattern,
226 substitution, or transliteration, it is interpreted as a search pattern at run
229 Binary "!~" is just like "=~" except the return value is negated in
232 =head2 Multiplicative Operators
234 Binary "*" multiplies two numbers.
236 Binary "/" divides two numbers.
238 Binary "%" computes the modulus of two numbers. Given integer
239 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
240 C<$a> minus the largest multiple of C<$b> that is not greater than
241 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
242 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
243 result will be less than or equal to zero).
244 Note that when C<use integer> is in scope, "%" gives you direct access
245 to the modulus operator as implemented by your C compiler. This
246 operator is not as well defined for negative operands, but it will
249 Binary "x" is the repetition operator. In scalar context or if the left
250 operand is not enclosed in parentheses, it returns a string consisting
251 of the left operand repeated the number of times specified by the right
252 operand. In list context, if the left operand is enclosed in
253 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
254 If the right operand is zero or negative, it returns an empty string
255 or an empty list, depending on the context.
257 print '-' x 80; # print row of dashes
259 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
261 @ones = (1) x 80; # a list of 80 1's
262 @ones = (5) x @ones; # set all elements to 5
265 =head2 Additive Operators
267 Binary "+" returns the sum of two numbers.
269 Binary "-" returns the difference of two numbers.
271 Binary "." concatenates two strings.
273 =head2 Shift Operators
275 Binary "<<" returns the value of its left argument shifted left by the
276 number of bits specified by the right argument. Arguments should be
277 integers. (See also L<Integer Arithmetic>.)
279 Binary ">>" returns the value of its left argument shifted right by
280 the number of bits specified by the right argument. Arguments should
281 be integers. (See also L<Integer Arithmetic>.)
283 Note that both "<<" and ">>" in Perl are implemented directly using
284 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
285 in force then signed C integers are used, else unsigned C integers are
286 used. Either way, the implementation isn't going to generate results
287 larger than the size of the integer type Perl was built with (32 bits
290 The result of overflowing the range of the integers is undefined
291 because it is undefined also in C. In other words, using 32-bit
292 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
293 of bits is also undefined.
295 =head2 Named Unary Operators
297 The various named unary operators are treated as functions with one
298 argument, with optional parentheses.
300 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
301 is followed by a left parenthesis as the next token, the operator and
302 arguments within parentheses are taken to be of highest precedence,
303 just like a normal function call. For example,
304 because named unary operators are higher precedence than ||:
306 chdir $foo || die; # (chdir $foo) || die
307 chdir($foo) || die; # (chdir $foo) || die
308 chdir ($foo) || die; # (chdir $foo) || die
309 chdir +($foo) || die; # (chdir $foo) || die
311 but, because * is higher precedence than named operators:
313 chdir $foo * 20; # chdir ($foo * 20)
314 chdir($foo) * 20; # (chdir $foo) * 20
315 chdir ($foo) * 20; # (chdir $foo) * 20
316 chdir +($foo) * 20; # chdir ($foo * 20)
318 rand 10 * 20; # rand (10 * 20)
319 rand(10) * 20; # (rand 10) * 20
320 rand (10) * 20; # (rand 10) * 20
321 rand +(10) * 20; # rand (10 * 20)
323 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
324 treated like named unary operators, but they don't follow this functional
325 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
326 equivalent to C<-f "$file.bak">.
328 See also L<"Terms and List Operators (Leftward)">.
330 =head2 Relational Operators
332 Binary "<" returns true if the left argument is numerically less than
335 Binary ">" returns true if the left argument is numerically greater
336 than the right argument.
338 Binary "<=" returns true if the left argument is numerically less than
339 or equal to the right argument.
341 Binary ">=" returns true if the left argument is numerically greater
342 than or equal to the right argument.
344 Binary "lt" returns true if the left argument is stringwise less than
347 Binary "gt" returns true if the left argument is stringwise greater
348 than the right argument.
350 Binary "le" returns true if the left argument is stringwise less than
351 or equal to the right argument.
353 Binary "ge" returns true if the left argument is stringwise greater
354 than or equal to the right argument.
356 =head2 Equality Operators
358 Binary "==" returns true if the left argument is numerically equal to
361 Binary "!=" returns true if the left argument is numerically not equal
362 to the right argument.
364 Binary "<=>" returns -1, 0, or 1 depending on whether the left
365 argument is numerically less than, equal to, or greater than the right
366 argument. If your platform supports NaNs (not-a-numbers) as numeric
367 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
368 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
369 returns true, as does NaN != anything else. If your platform doesn't
370 support NaNs then NaN is just a string with numeric value 0.
372 perl -le '$a = NaN; print "No NaN support here" if $a == $a'
373 perl -le '$a = NaN; print "NaN support here" if $a != $a'
375 Binary "eq" returns true if the left argument is stringwise equal to
378 Binary "ne" returns true if the left argument is stringwise not equal
379 to the right argument.
381 Binary "cmp" returns -1, 0, or 1 depending on whether the left
382 argument is stringwise less than, equal to, or greater than the right
385 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
386 by the current locale if C<use locale> is in effect. See L<perllocale>.
390 Binary "&" returns its operands ANDed together bit by bit.
391 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
393 Note that "&" has lower priority than relational operators, so for example
394 the brackets are essential in a test like
396 print "Even\n" if ($x & 1) == 0;
398 =head2 Bitwise Or and Exclusive Or
400 Binary "|" returns its operands ORed together bit by bit.
401 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
403 Binary "^" returns its operands XORed together bit by bit.
404 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
406 Note that "|" and "^" have lower priority than relational operators, so
407 for example the brackets are essential in a test like
409 print "false\n" if (8 | 2) != 10;
411 =head2 C-style Logical And
413 Binary "&&" performs a short-circuit logical AND operation. That is,
414 if the left operand is false, the right operand is not even evaluated.
415 Scalar or list context propagates down to the right operand if it
418 =head2 C-style Logical Or
420 Binary "||" performs a short-circuit logical OR operation. That is,
421 if the left operand is true, the right operand is not even evaluated.
422 Scalar or list context propagates down to the right operand if it
425 =head2 C-style Logical Defined-Or
427 Although it has no direct equivalent in C, Perl's C<//> operator is related
428 to its C-style or. In fact, it's exactly the same as C<||>, except that it
429 tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
430 is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
431 rather than the value of C<defined($a)>) and is exactly equivalent to
432 C<defined($a) ? $a : $b>. This is very useful for providing default values
433 for variables. If you actually want to test if at least one of C<$a> and
434 C<$b> is defined, use C<defined($a // $b)>.
436 The C<||>, C<//> and C<&&> operators return the last value evaluated
437 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
438 portable way to find out the home directory might be:
440 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
441 (getpwuid($<))[7] // die "You're homeless!\n";
443 In particular, this means that you shouldn't use this
444 for selecting between two aggregates for assignment:
446 @a = @b || @c; # this is wrong
447 @a = scalar(@b) || @c; # really meant this
448 @a = @b ? @b : @c; # this works fine, though
450 As more readable alternatives to C<&&>, C<//> and C<||> when used for
451 control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
452 The short-circuit behavior is identical. The precedence of "and", "err"
453 and "or" is much lower, however, so that you can safely use them after a
454 list operator without the need for parentheses:
456 unlink "alpha", "beta", "gamma"
457 or gripe(), next LINE;
459 With the C-style operators that would have been written like this:
461 unlink("alpha", "beta", "gamma")
462 || (gripe(), next LINE);
464 Using "or" for assignment is unlikely to do what you want; see below.
466 =head2 Range Operators
468 Binary ".." is the range operator, which is really two different
469 operators depending on the context. In list context, it returns a
470 list of values counting (up by ones) from the left value to the right
471 value. If the left value is greater than the right value then it
472 returns the empty list. The range operator is useful for writing
473 C<foreach (1..10)> loops and for doing slice operations on arrays. In
474 the current implementation, no temporary array is created when the
475 range operator is used as the expression in C<foreach> loops, but older
476 versions of Perl might burn a lot of memory when you write something
479 for (1 .. 1_000_000) {
483 The range operator also works on strings, using the magical auto-increment,
486 In scalar context, ".." returns a boolean value. The operator is
487 bistable, like a flip-flop, and emulates the line-range (comma) operator
488 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
489 own boolean state. It is false as long as its left operand is false.
490 Once the left operand is true, the range operator stays true until the
491 right operand is true, I<AFTER> which the range operator becomes false
492 again. It doesn't become false till the next time the range operator is
493 evaluated. It can test the right operand and become false on the same
494 evaluation it became true (as in B<awk>), but it still returns true once.
495 If you don't want it to test the right operand till the next
496 evaluation, as in B<sed>, just use three dots ("...") instead of
497 two. In all other regards, "..." behaves just like ".." does.
499 The right operand is not evaluated while the operator is in the
500 "false" state, and the left operand is not evaluated while the
501 operator is in the "true" state. The precedence is a little lower
502 than || and &&. The value returned is either the empty string for
503 false, or a sequence number (beginning with 1) for true. The
504 sequence number is reset for each range encountered. The final
505 sequence number in a range has the string "E0" appended to it, which
506 doesn't affect its numeric value, but gives you something to search
507 for if you want to exclude the endpoint. You can exclude the
508 beginning point by waiting for the sequence number to be greater
511 If either operand of scalar ".." is a constant expression,
512 that operand is considered true if it is equal (C<==>) to the current
513 input line number (the C<$.> variable).
515 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
516 but that is only an issue if you use a floating point expression; when
517 implicitly using C<$.> as described in the previous paragraph, the
518 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
519 is set to a floating point value and you are not reading from a file.
520 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
521 you want in scalar context because each of the operands are evaluated
522 using their integer representation.
526 As a scalar operator:
528 if (101 .. 200) { print; } # print 2nd hundred lines, short for
529 # if ($. == 101 .. $. == 200) ...
530 next line if (1 .. /^$/); # skip header lines, short for
531 # ... if ($. == 1 .. /^$/);
532 s/^/> / if (/^$/ .. eof()); # quote body
534 # parse mail messages
536 $in_header = 1 .. /^$/;
537 $in_body = /^$/ .. eof;
544 close ARGV if eof; # reset $. each file
547 Here's a simple example to illustrate the difference between
548 the two range operators:
563 This program will print only the line containing "Bar". If
564 the range operator is changed to C<...>, it will also print the
567 And now some examples as a list operator:
569 for (101 .. 200) { print; } # print $_ 100 times
570 @foo = @foo[0 .. $#foo]; # an expensive no-op
571 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
573 The range operator (in list context) makes use of the magical
574 auto-increment algorithm if the operands are strings. You
577 @alphabet = ('A' .. 'Z');
579 to get all normal letters of the English alphabet, or
581 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
583 to get a hexadecimal digit, or
585 @z2 = ('01' .. '31'); print $z2[$mday];
587 to get dates with leading zeros. If the final value specified is not
588 in the sequence that the magical increment would produce, the sequence
589 goes until the next value would be longer than the final value
592 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
593 return two elements in list context.
595 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
597 =head2 Conditional Operator
599 Ternary "?:" is the conditional operator, just as in C. It works much
600 like an if-then-else. If the argument before the ? is true, the
601 argument before the : is returned, otherwise the argument after the :
602 is returned. For example:
604 printf "I have %d dog%s.\n", $n,
605 ($n == 1) ? '' : "s";
607 Scalar or list context propagates downward into the 2nd
608 or 3rd argument, whichever is selected.
610 $a = $ok ? $b : $c; # get a scalar
611 @a = $ok ? @b : @c; # get an array
612 $a = $ok ? @b : @c; # oops, that's just a count!
614 The operator may be assigned to if both the 2nd and 3rd arguments are
615 legal lvalues (meaning that you can assign to them):
617 ($a_or_b ? $a : $b) = $c;
619 Because this operator produces an assignable result, using assignments
620 without parentheses will get you in trouble. For example, this:
622 $a % 2 ? $a += 10 : $a += 2
626 (($a % 2) ? ($a += 10) : $a) += 2
630 ($a % 2) ? ($a += 10) : ($a += 2)
632 That should probably be written more simply as:
634 $a += ($a % 2) ? 10 : 2;
636 =head2 Assignment Operators
638 "=" is the ordinary assignment operator.
640 Assignment operators work as in C. That is,
648 although without duplicating any side effects that dereferencing the lvalue
649 might trigger, such as from tie(). Other assignment operators work similarly.
650 The following are recognized:
657 Although these are grouped by family, they all have the precedence
660 Unlike in C, the scalar assignment operator produces a valid lvalue.
661 Modifying an assignment is equivalent to doing the assignment and
662 then modifying the variable that was assigned to. This is useful
663 for modifying a copy of something, like this:
665 ($tmp = $global) =~ tr [A-Z] [a-z];
676 Similarly, a list assignment in list context produces the list of
677 lvalues assigned to, and a list assignment in scalar context returns
678 the number of elements produced by the expression on the right hand
679 side of the assignment.
681 =head2 Comma Operator
683 Binary "," is the comma operator. In scalar context it evaluates
684 its left argument, throws that value away, then evaluates its right
685 argument and returns that value. This is just like C's comma operator.
687 In list context, it's just the list argument separator, and inserts
688 both its arguments into the list.
690 The C<< => >> operator is a synonym for the comma, but forces any word
691 (consisting entirely of word characters) to its left to be interpreted
692 as a string (as of 5.001). This includes words that might otherwise be
693 considered a constant or function call.
695 use constant FOO => "something";
697 my %h = ( FOO => 23 );
705 my %h = ("something", 23);
707 If the argument on the left is not a word, it is first interpreted as
708 an expression, and then the string value of that is used.
710 The C<< => >> operator is helpful in documenting the correspondence
711 between keys and values in hashes, and other paired elements in lists.
713 %hash = ( $key => $value );
714 login( $username => $password );
716 =head2 List Operators (Rightward)
718 On the right side of a list operator, it has very low precedence,
719 such that it controls all comma-separated expressions found there.
720 The only operators with lower precedence are the logical operators
721 "and", "or", and "not", which may be used to evaluate calls to list
722 operators without the need for extra parentheses:
724 open HANDLE, "filename"
725 or die "Can't open: $!\n";
727 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
731 Unary "not" returns the logical negation of the expression to its right.
732 It's the equivalent of "!" except for the very low precedence.
736 Binary "and" returns the logical conjunction of the two surrounding
737 expressions. It's equivalent to && except for the very low
738 precedence. This means that it short-circuits: i.e., the right
739 expression is evaluated only if the left expression is true.
741 =head2 Logical or, Defined or, and Exclusive Or
743 Binary "or" returns the logical disjunction of the two surrounding
744 expressions. It's equivalent to || except for the very low precedence.
745 This makes it useful for control flow
747 print FH $data or die "Can't write to FH: $!";
749 This means that it short-circuits: i.e., the right expression is evaluated
750 only if the left expression is false. Due to its precedence, you should
751 probably avoid using this for assignment, only for control flow.
753 $a = $b or $c; # bug: this is wrong
754 ($a = $b) or $c; # really means this
755 $a = $b || $c; # better written this way
757 However, when it's a list-context assignment and you're trying to use
758 "||" for control flow, you probably need "or" so that the assignment
759 takes higher precedence.
761 @info = stat($file) || die; # oops, scalar sense of stat!
762 @info = stat($file) or die; # better, now @info gets its due
764 Then again, you could always use parentheses.
766 Binary "err" is equivalent to C<//>--it's just like binary "or", except it tests
767 its left argument's definedness instead of its truth. There are two ways to
768 remember "err": either because many functions return C<undef> on an B<err>or,
769 or as a sort of correction: C<$a=($b err 'default')>
771 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
772 It cannot short circuit, of course.
774 =head2 C Operators Missing From Perl
776 Here is what C has that Perl doesn't:
782 Address-of operator. (But see the "\" operator for taking a reference.)
786 Dereference-address operator. (Perl's prefix dereferencing
787 operators are typed: $, @, %, and &.)
791 Type-casting operator.
795 =head2 Quote and Quote-like Operators
797 While we usually think of quotes as literal values, in Perl they
798 function as operators, providing various kinds of interpolating and
799 pattern matching capabilities. Perl provides customary quote characters
800 for these behaviors, but also provides a way for you to choose your
801 quote character for any of them. In the following table, a C<{}> represents
802 any pair of delimiters you choose.
804 Customary Generic Meaning Interpolates
809 // m{} Pattern match yes*
811 s{}{} Substitution yes*
812 tr{}{} Transliteration no (but see below)
815 * unless the delimiter is ''.
817 Non-bracketing delimiters use the same character fore and aft, but the four
818 sorts of brackets (round, angle, square, curly) will all nest, which means
827 Note, however, that this does not always work for quoting Perl code:
829 $s = q{ if($a eq "}") ... }; # WRONG
831 is a syntax error. The C<Text::Balanced> module (from CPAN, and
832 starting from Perl 5.8 part of the standard distribution) is able
835 There can be whitespace between the operator and the quoting
836 characters, except when C<#> is being used as the quoting character.
837 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
838 operator C<q> followed by a comment. Its argument will be taken
839 from the next line. This allows you to write:
841 s {foo} # Replace foo
844 The following escape sequences are available in constructs that interpolate
845 and in transliterations.
852 \a alarm (bell) (BEL)
854 \033 octal char (ESC)
856 \x{263a} wide hex char (SMILEY)
857 \c[ control char (ESC)
858 \N{name} named Unicode character
860 B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
861 the vertical tab (VT - ASCII 11).
863 The following escape sequences are available in constructs that interpolate
864 but not in transliterations.
866 \l lowercase next char
867 \u uppercase next char
870 \E end case modification
871 \Q quote non-word characters till \E
873 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
874 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
875 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
876 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
877 C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
880 All systems use the virtual C<"\n"> to represent a line terminator,
881 called a "newline". There is no such thing as an unvarying, physical
882 newline character. It is only an illusion that the operating system,
883 device drivers, C libraries, and Perl all conspire to preserve. Not all
884 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
885 on a Mac, these are reversed, and on systems without line terminator,
886 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
887 you mean a "newline" for your system, but use the literal ASCII when you
888 need an exact character. For example, most networking protocols expect
889 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
890 and although they often accept just C<"\012">, they seldom tolerate just
891 C<"\015">. If you get in the habit of using C<"\n"> for networking,
892 you may be burned some day.
894 For constructs that do interpolate, variables beginning with "C<$>"
895 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
896 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
897 But method calls such as C<< $obj->meth >> are not.
899 Interpolating an array or slice interpolates the elements in order,
900 separated by the value of C<$">, so is equivalent to interpolating
901 C<join $", @array>. "Punctuation" arrays such as C<@+> are only
902 interpolated if the name is enclosed in braces C<@{+}>.
904 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
905 An unescaped C<$> or C<@> interpolates the corresponding variable,
906 while escaping will cause the literal string C<\$> to be inserted.
907 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
909 Patterns are subject to an additional level of interpretation as a
910 regular expression. This is done as a second pass, after variables are
911 interpolated, so that regular expressions may be incorporated into the
912 pattern from the variables. If this is not what you want, use C<\Q> to
913 interpolate a variable literally.
915 Apart from the behavior described above, Perl does not expand
916 multiple levels of interpolation. In particular, contrary to the
917 expectations of shell programmers, back-quotes do I<NOT> interpolate
918 within double quotes, nor do single quotes impede evaluation of
919 variables when used within double quotes.
921 =head2 Regexp Quote-Like Operators
923 Here are the quote-like operators that apply to pattern
924 matching and related activities.
930 This is just like the C</pattern/> search, except that it matches only
931 once between calls to the reset() operator. This is a useful
932 optimization when you want to see only the first occurrence of
933 something in each file of a set of files, for instance. Only C<??>
934 patterns local to the current package are reset.
938 # blank line between header and body
941 reset if eof; # clear ?? status for next file
944 This usage is vaguely deprecated, which means it just might possibly
945 be removed in some distant future version of Perl, perhaps somewhere
946 around the year 2168.
948 =item m/PATTERN/cgimosx
950 =item /PATTERN/cgimosx
952 Searches a string for a pattern match, and in scalar context returns
953 true if it succeeds, false if it fails. If no string is specified
954 via the C<=~> or C<!~> operator, the $_ string is searched. (The
955 string specified with C<=~> need not be an lvalue--it may be the
956 result of an expression evaluation, but remember the C<=~> binds
957 rather tightly.) See also L<perlre>. See L<perllocale> for
958 discussion of additional considerations that apply when C<use locale>
963 c Do not reset search position on a failed match when /g is in effect.
964 g Match globally, i.e., find all occurrences.
965 i Do case-insensitive pattern matching.
966 m Treat string as multiple lines.
967 o Compile pattern only once.
968 s Treat string as single line.
969 x Use extended regular expressions.
971 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
972 you can use any pair of non-alphanumeric, non-whitespace characters
973 as delimiters. This is particularly useful for matching path names
974 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
975 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
976 If "'" is the delimiter, no interpolation is performed on the PATTERN.
978 PATTERN may contain variables, which will be interpolated (and the
979 pattern recompiled) every time the pattern search is evaluated, except
980 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
981 C<$|> are not interpolated because they look like end-of-string tests.)
982 If you want such a pattern to be compiled only once, add a C</o> after
983 the trailing delimiter. This avoids expensive run-time recompilations,
984 and is useful when the value you are interpolating won't change over
985 the life of the script. However, mentioning C</o> constitutes a promise
986 that you won't change the variables in the pattern. If you change them,
987 Perl won't even notice. See also L<"qr/STRING/imosx">.
989 If the PATTERN evaluates to the empty string, the last
990 I<successfully> matched regular expression is used instead. In this
991 case, only the C<g> and C<c> flags on the empty pattern is honoured -
992 the other flags are taken from the original pattern. If no match has
993 previously succeeded, this will (silently) act instead as a genuine
994 empty pattern (which will always match).
996 Note that it's possible to confuse Perl into thinking C<//> (the empty
997 regex) is really C<//> (the defined-or operator). Perl is usually pretty
998 good about this, but some pathological cases might trigger this, such as
999 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1000 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1001 will assume you meant defined-or. If you meant the empty regex, just
1002 use parentheses or spaces to disambiguate, or even prefix the empty
1003 regex with an C<m> (so C<//> becomes C<m//>).
1005 If the C</g> option is not used, C<m//> in list context returns a
1006 list consisting of the subexpressions matched by the parentheses in the
1007 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1008 also set, and that this differs from Perl 4's behavior.) When there are
1009 no parentheses in the pattern, the return value is the list C<(1)> for
1010 success. With or without parentheses, an empty list is returned upon
1015 open(TTY, '/dev/tty');
1016 <TTY> =~ /^y/i && foo(); # do foo if desired
1018 if (/Version: *([0-9.]*)/) { $version = $1; }
1020 next if m#^/usr/spool/uucp#;
1025 print if /$arg/o; # compile only once
1028 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1030 This last example splits $foo into the first two words and the
1031 remainder of the line, and assigns those three fields to $F1, $F2, and
1032 $Etc. The conditional is true if any variables were assigned, i.e., if
1033 the pattern matched.
1035 The C</g> modifier specifies global pattern matching--that is,
1036 matching as many times as possible within the string. How it behaves
1037 depends on the context. In list context, it returns a list of the
1038 substrings matched by any capturing parentheses in the regular
1039 expression. If there are no parentheses, it returns a list of all
1040 the matched strings, as if there were parentheses around the whole
1043 In scalar context, each execution of C<m//g> finds the next match,
1044 returning true if it matches, and false if there is no further match.
1045 The position after the last match can be read or set using the pos()
1046 function; see L<perlfunc/pos>. A failed match normally resets the
1047 search position to the beginning of the string, but you can avoid that
1048 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1049 string also resets the search position.
1051 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1052 zero-width assertion that matches the exact position where the previous
1053 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1054 still anchors at pos(), but the match is of course only attempted once.
1055 Using C<\G> without C</g> on a target string that has not previously had a
1056 C</g> match applied to it is the same as using the C<\A> assertion to match
1057 the beginning of the string. Note also that, currently, C<\G> is only
1058 properly supported when anchored at the very beginning of the pattern.
1063 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1067 while (defined($paragraph = <>)) {
1068 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1072 print "$sentences\n";
1074 # using m//gc with \G
1078 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1080 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1082 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1084 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1086 The last example should print:
1096 Notice that the final match matched C<q> instead of C<p>, which a match
1097 without the C<\G> anchor would have done. Also note that the final match
1098 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1099 final match did indeed match C<p>, it's a good bet that you're running an
1100 older (pre-5.6.0) Perl.
1102 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1103 combine several regexps like this to process a string part-by-part,
1104 doing different actions depending on which regexp matched. Each
1105 regexp tries to match where the previous one leaves off.
1108 $url = new URI::URL "http://www/"; die if $url eq "xXx";
1112 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1113 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1114 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1115 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1116 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1117 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1118 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1119 print ". That's all!\n";
1122 Here is the output (split into several lines):
1124 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1125 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1126 lowercase lowercase line-noise lowercase lowercase line-noise
1127 MiXeD line-noise. That's all!
1133 A single-quoted, literal string. A backslash represents a backslash
1134 unless followed by the delimiter or another backslash, in which case
1135 the delimiter or backslash is interpolated.
1137 $foo = q!I said, "You said, 'She said it.'"!;
1138 $bar = q('This is it.');
1139 $baz = '\n'; # a two-character string
1145 A double-quoted, interpolated string.
1148 (*** The previous line contains the naughty word "$1".\n)
1149 if /\b(tcl|java|python)\b/i; # :-)
1150 $baz = "\n"; # a one-character string
1152 =item qr/STRING/imosx
1154 This operator quotes (and possibly compiles) its I<STRING> as a regular
1155 expression. I<STRING> is interpolated the same way as I<PATTERN>
1156 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1157 is done. Returns a Perl value which may be used instead of the
1158 corresponding C</STRING/imosx> expression.
1162 $rex = qr/my.STRING/is;
1169 The result may be used as a subpattern in a match:
1172 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1173 $string =~ $re; # or used standalone
1174 $string =~ /$re/; # or this way
1176 Since Perl may compile the pattern at the moment of execution of qr()
1177 operator, using qr() may have speed advantages in some situations,
1178 notably if the result of qr() is used standalone:
1181 my $patterns = shift;
1182 my @compiled = map qr/$_/i, @$patterns;
1185 foreach my $pat (@compiled) {
1186 $success = 1, last if /$pat/;
1192 Precompilation of the pattern into an internal representation at
1193 the moment of qr() avoids a need to recompile the pattern every
1194 time a match C</$pat/> is attempted. (Perl has many other internal
1195 optimizations, but none would be triggered in the above example if
1196 we did not use qr() operator.)
1200 i Do case-insensitive pattern matching.
1201 m Treat string as multiple lines.
1202 o Compile pattern only once.
1203 s Treat string as single line.
1204 x Use extended regular expressions.
1206 See L<perlre> for additional information on valid syntax for STRING, and
1207 for a detailed look at the semantics of regular expressions.
1213 A string which is (possibly) interpolated and then executed as a
1214 system command with C</bin/sh> or its equivalent. Shell wildcards,
1215 pipes, and redirections will be honored. The collected standard
1216 output of the command is returned; standard error is unaffected. In
1217 scalar context, it comes back as a single (potentially multi-line)
1218 string, or undef if the command failed. In list context, returns a
1219 list of lines (however you've defined lines with $/ or
1220 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1222 Because backticks do not affect standard error, use shell file descriptor
1223 syntax (assuming the shell supports this) if you care to address this.
1224 To capture a command's STDERR and STDOUT together:
1226 $output = `cmd 2>&1`;
1228 To capture a command's STDOUT but discard its STDERR:
1230 $output = `cmd 2>/dev/null`;
1232 To capture a command's STDERR but discard its STDOUT (ordering is
1235 $output = `cmd 2>&1 1>/dev/null`;
1237 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1238 but leave its STDOUT to come out the old STDERR:
1240 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1242 To read both a command's STDOUT and its STDERR separately, it's easiest
1243 to redirect them separately to files, and then read from those files
1244 when the program is done:
1246 system("program args 1>program.stdout 2>program.stderr");
1248 Using single-quote as a delimiter protects the command from Perl's
1249 double-quote interpolation, passing it on to the shell instead:
1251 $perl_info = qx(ps $$); # that's Perl's $$
1252 $shell_info = qx'ps $$'; # that's the new shell's $$
1254 How that string gets evaluated is entirely subject to the command
1255 interpreter on your system. On most platforms, you will have to protect
1256 shell metacharacters if you want them treated literally. This is in
1257 practice difficult to do, as it's unclear how to escape which characters.
1258 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1259 to emulate backticks safely.
1261 On some platforms (notably DOS-like ones), the shell may not be
1262 capable of dealing with multiline commands, so putting newlines in
1263 the string may not get you what you want. You may be able to evaluate
1264 multiple commands in a single line by separating them with the command
1265 separator character, if your shell supports that (e.g. C<;> on many Unix
1266 shells; C<&> on the Windows NT C<cmd> shell).
1268 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1269 output before starting the child process, but this may not be supported
1270 on some platforms (see L<perlport>). To be safe, you may need to set
1271 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1272 C<IO::Handle> on any open handles.
1274 Beware that some command shells may place restrictions on the length
1275 of the command line. You must ensure your strings don't exceed this
1276 limit after any necessary interpolations. See the platform-specific
1277 release notes for more details about your particular environment.
1279 Using this operator can lead to programs that are difficult to port,
1280 because the shell commands called vary between systems, and may in
1281 fact not be present at all. As one example, the C<type> command under
1282 the POSIX shell is very different from the C<type> command under DOS.
1283 That doesn't mean you should go out of your way to avoid backticks
1284 when they're the right way to get something done. Perl was made to be
1285 a glue language, and one of the things it glues together is commands.
1286 Just understand what you're getting yourself into.
1288 See L<"I/O Operators"> for more discussion.
1292 Evaluates to a list of the words extracted out of STRING, using embedded
1293 whitespace as the word delimiters. It can be understood as being roughly
1296 split(' ', q/STRING/);
1298 the differences being that it generates a real list at compile time, and
1299 in scalar context it returns the last element in the list. So
1304 is semantically equivalent to the list:
1308 Some frequently seen examples:
1310 use POSIX qw( setlocale localeconv )
1311 @EXPORT = qw( foo bar baz );
1313 A common mistake is to try to separate the words with comma or to
1314 put comments into a multi-line C<qw>-string. For this reason, the
1315 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1316 produces warnings if the STRING contains the "," or the "#" character.
1318 =item s/PATTERN/REPLACEMENT/egimosx
1320 Searches a string for a pattern, and if found, replaces that pattern
1321 with the replacement text and returns the number of substitutions
1322 made. Otherwise it returns false (specifically, the empty string).
1324 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1325 variable is searched and modified. (The string specified with C<=~> must
1326 be scalar variable, an array element, a hash element, or an assignment
1327 to one of those, i.e., an lvalue.)
1329 If the delimiter chosen is a single quote, no interpolation is
1330 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1331 PATTERN contains a $ that looks like a variable rather than an
1332 end-of-string test, the variable will be interpolated into the pattern
1333 at run-time. If you want the pattern compiled only once the first time
1334 the variable is interpolated, use the C</o> option. If the pattern
1335 evaluates to the empty string, the last successfully executed regular
1336 expression is used instead. See L<perlre> for further explanation on these.
1337 See L<perllocale> for discussion of additional considerations that apply
1338 when C<use locale> is in effect.
1342 e Evaluate the right side as an expression.
1343 g Replace globally, i.e., all occurrences.
1344 i Do case-insensitive pattern matching.
1345 m Treat string as multiple lines.
1346 o Compile pattern only once.
1347 s Treat string as single line.
1348 x Use extended regular expressions.
1350 Any non-alphanumeric, non-whitespace delimiter may replace the
1351 slashes. If single quotes are used, no interpretation is done on the
1352 replacement string (the C</e> modifier overrides this, however). Unlike
1353 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1354 text is not evaluated as a command. If the
1355 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1356 pair of quotes, which may or may not be bracketing quotes, e.g.,
1357 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1358 replacement portion to be treated as a full-fledged Perl expression
1359 and evaluated right then and there. It is, however, syntax checked at
1360 compile-time. A second C<e> modifier will cause the replacement portion
1361 to be C<eval>ed before being run as a Perl expression.
1365 s/\bgreen\b/mauve/g; # don't change wintergreen
1367 $path =~ s|/usr/bin|/usr/local/bin|;
1369 s/Login: $foo/Login: $bar/; # run-time pattern
1371 ($foo = $bar) =~ s/this/that/; # copy first, then change
1373 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1376 s/\d+/$&*2/e; # yields 'abc246xyz'
1377 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1378 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1380 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1381 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1382 s/^=(\w+)/&pod($1)/ge; # use function call
1384 # expand variables in $_, but dynamics only, using
1385 # symbolic dereferencing
1388 # Add one to the value of any numbers in the string
1391 # This will expand any embedded scalar variable
1392 # (including lexicals) in $_ : First $1 is interpolated
1393 # to the variable name, and then evaluated
1396 # Delete (most) C comments.
1398 /\* # Match the opening delimiter.
1399 .*? # Match a minimal number of characters.
1400 \*/ # Match the closing delimiter.
1403 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1405 for ($variable) { # trim whitespace in $variable, cheap
1410 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1412 Note the use of $ instead of \ in the last example. Unlike
1413 B<sed>, we use the \<I<digit>> form in only the left hand side.
1414 Anywhere else it's $<I<digit>>.
1416 Occasionally, you can't use just a C</g> to get all the changes
1417 to occur that you might want. Here are two common cases:
1419 # put commas in the right places in an integer
1420 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1422 # expand tabs to 8-column spacing
1423 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1425 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1427 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1429 Transliterates all occurrences of the characters found in the search list
1430 with the corresponding character in the replacement list. It returns
1431 the number of characters replaced or deleted. If no string is
1432 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1433 string specified with =~ must be a scalar variable, an array element, a
1434 hash element, or an assignment to one of those, i.e., an lvalue.)
1436 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1437 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1438 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1439 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1440 its own pair of quotes, which may or may not be bracketing quotes,
1441 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1443 Note that C<tr> does B<not> do regular expression character classes
1444 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1445 the tr(1) utility. If you want to map strings between lower/upper
1446 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1447 using the C<s> operator if you need regular expressions.
1449 Note also that the whole range idea is rather unportable between
1450 character sets--and even within character sets they may cause results
1451 you probably didn't expect. A sound principle is to use only ranges
1452 that begin from and end at either alphabets of equal case (a-e, A-E),
1453 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1454 character sets in full.
1458 c Complement the SEARCHLIST.
1459 d Delete found but unreplaced characters.
1460 s Squash duplicate replaced characters.
1462 If the C</c> modifier is specified, the SEARCHLIST character set
1463 is complemented. If the C</d> modifier is specified, any characters
1464 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1465 (Note that this is slightly more flexible than the behavior of some
1466 B<tr> programs, which delete anything they find in the SEARCHLIST,
1467 period.) If the C</s> modifier is specified, sequences of characters
1468 that were transliterated to the same character are squashed down
1469 to a single instance of the character.
1471 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1472 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1473 than the SEARCHLIST, the final character is replicated till it is long
1474 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1475 This latter is useful for counting characters in a class or for
1476 squashing character sequences in a class.
1480 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1482 $cnt = tr/*/*/; # count the stars in $_
1484 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1486 $cnt = tr/0-9//; # count the digits in $_
1488 tr/a-zA-Z//s; # bookkeeper -> bokeper
1490 ($HOST = $host) =~ tr/a-z/A-Z/;
1492 tr/a-zA-Z/ /cs; # change non-alphas to single space
1495 [\000-\177]; # delete 8th bit
1497 If multiple transliterations are given for a character, only the
1502 will transliterate any A to X.
1504 Because the transliteration table is built at compile time, neither
1505 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1506 interpolation. That means that if you want to use variables, you
1509 eval "tr/$oldlist/$newlist/";
1512 eval "tr/$oldlist/$newlist/, 1" or die $@;
1516 A line-oriented form of quoting is based on the shell "here-document"
1517 syntax. Following a C<< << >> you specify a string to terminate
1518 the quoted material, and all lines following the current line down to
1519 the terminating string are the value of the item. The terminating
1520 string may be either an identifier (a word), or some quoted text. If
1521 quoted, the type of quotes you use determines the treatment of the
1522 text, just as in regular quoting. An unquoted identifier works like
1523 double quotes. There must be no space between the C<< << >> and
1524 the identifier, unless the identifier is quoted. (If you put a space it
1525 will be treated as a null identifier, which is valid, and matches the first
1526 empty line.) The terminating string must appear by itself (unquoted and
1527 with no surrounding whitespace) on the terminating line.
1530 The price is $Price.
1533 print << "EOF"; # same as above
1534 The price is $Price.
1537 print << `EOC`; # execute commands
1542 print <<"foo", <<"bar"; # you can stack them
1548 myfunc(<< "THIS", 23, <<'THAT');
1555 Just don't forget that you have to put a semicolon on the end
1556 to finish the statement, as Perl doesn't know you're not going to
1564 If you want your here-docs to be indented with the
1565 rest of the code, you'll need to remove leading whitespace
1566 from each line manually:
1568 ($quote = <<'FINIS') =~ s/^\s+//gm;
1569 The Road goes ever on and on,
1570 down from the door where it began.
1573 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1574 the quoted material must come on the lines following the final delimiter.
1589 If the terminating identifier is on the last line of the program, you
1590 must be sure there is a newline after it; otherwise, Perl will give the
1591 warning B<Can't find string terminator "END" anywhere before EOF...>.
1593 Additionally, the quoting rules for the identifier are not related to
1594 Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
1595 in place of C<''> and C<"">, and the only interpolation is for backslashing
1596 the quoting character:
1598 print << "abc\"def";
1602 Finally, quoted strings cannot span multiple lines. The general rule is
1603 that the identifier must be a string literal. Stick with that, and you
1608 =head2 Gory details of parsing quoted constructs
1610 When presented with something that might have several different
1611 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1612 principle to pick the most probable interpretation. This strategy
1613 is so successful that Perl programmers often do not suspect the
1614 ambivalence of what they write. But from time to time, Perl's
1615 notions differ substantially from what the author honestly meant.
1617 This section hopes to clarify how Perl handles quoted constructs.
1618 Although the most common reason to learn this is to unravel labyrinthine
1619 regular expressions, because the initial steps of parsing are the
1620 same for all quoting operators, they are all discussed together.
1622 The most important Perl parsing rule is the first one discussed
1623 below: when processing a quoted construct, Perl first finds the end
1624 of that construct, then interprets its contents. If you understand
1625 this rule, you may skip the rest of this section on the first
1626 reading. The other rules are likely to contradict the user's
1627 expectations much less frequently than this first one.
1629 Some passes discussed below are performed concurrently, but because
1630 their results are the same, we consider them individually. For different
1631 quoting constructs, Perl performs different numbers of passes, from
1632 one to five, but these passes are always performed in the same order.
1636 =item Finding the end
1638 The first pass is finding the end of the quoted construct, whether
1639 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1640 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1641 terminates C<qq[]> construct, or a C<< > >> which terminates a
1642 fileglob started with C<< < >>.
1644 When searching for single-character non-pairing delimiters, such
1645 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1646 when searching for single-character pairing delimiter like C<[>,
1647 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1648 C<[>, C<]> are skipped as well. When searching for multicharacter
1649 delimiters, nothing is skipped.
1651 For constructs with three-part delimiters (C<s///>, C<y///>, and
1652 C<tr///>), the search is repeated once more.
1654 During this search no attention is paid to the semantics of the construct.
1657 "$hash{"$foo/$bar"}"
1662 bar # NOT a comment, this slash / terminated m//!
1665 do not form legal quoted expressions. The quoted part ends on the
1666 first C<"> and C</>, and the rest happens to be a syntax error.
1667 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1668 the example above is not C<m//x>, but rather C<m//> with no C</x>
1669 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1671 Also no attention is paid to C<\c\> during this search.
1672 Thus the second C<\> in C<qq/\c\/> is interpreted as a part of C<\/>,
1673 and the following C</> is not recognized as a delimiter.
1674 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1676 =item Removal of backslashes before delimiters
1678 During the second pass, text between the starting and ending
1679 delimiters is copied to a safe location, and the C<\> is removed
1680 from combinations consisting of C<\> and delimiter--or delimiters,
1681 meaning both starting and ending delimiters will should these differ.
1682 This removal does not happen for multi-character delimiters.
1683 Note that the combination C<\\> is left intact, just as it was.
1685 Starting from this step no information about the delimiters is
1690 The next step is interpolation in the text obtained, which is now
1691 delimiter-independent. There are four different cases.
1695 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1697 No interpolation is performed.
1701 The only interpolation is removal of C<\> from pairs C<\\>.
1703 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1705 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1706 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1707 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1708 The other combinations are replaced with appropriate expansions.
1710 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1711 is interpolated in the usual way. Something like C<"\Q\\E"> has
1712 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1713 result is the same as for C<"\\\\E">. As a general rule, backslashes
1714 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1715 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1716 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1721 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1723 Interpolated scalars and arrays are converted internally to the C<join> and
1724 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1726 $foo . " XXX '" . (join $", @arr) . "'";
1728 All operations above are performed simultaneously, left to right.
1730 Because the result of C<"\Q STRING \E"> has all metacharacters
1731 quoted, there is no way to insert a literal C<$> or C<@> inside a
1732 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1733 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1736 Note also that the interpolation code needs to make a decision on
1737 where the interpolated scalar ends. For instance, whether
1738 C<< "a $b -> {c}" >> really means:
1740 "a " . $b . " -> {c}";
1746 Most of the time, the longest possible text that does not include
1747 spaces between components and which contains matching braces or
1748 brackets. because the outcome may be determined by voting based
1749 on heuristic estimators, the result is not strictly predictable.
1750 Fortunately, it's usually correct for ambiguous cases.
1752 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1754 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1755 happens (almost) as with C<qq//> constructs, but the substitution
1756 of C<\> followed by RE-special chars (including C<\>) is not
1757 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1758 a C<#>-comment in a C<//x>-regular expression, no processing is
1759 performed whatsoever. This is the first step at which the presence
1760 of the C<//x> modifier is relevant.
1762 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1763 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1764 different estimators) to be either an array element or C<$var>
1765 followed by an RE alternative. This is where the notation
1766 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1767 array element C<-9>, not as a regular expression from the variable
1768 C<$arr> followed by a digit, which would be the interpretation of
1769 C</$arr[0-9]/>. Since voting among different estimators may occur,
1770 the result is not predictable.
1772 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1773 the replacement text of C<s///> to correct the incorrigible
1774 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1775 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1776 (that is, the C<$^W> variable) was set.
1778 The lack of processing of C<\\> creates specific restrictions on
1779 the post-processed text. If the delimiter is C</>, one cannot get
1780 the combination C<\/> into the result of this step. C</> will
1781 finish the regular expression, C<\/> will be stripped to C</> on
1782 the previous step, and C<\\/> will be left as is. Because C</> is
1783 equivalent to C<\/> inside a regular expression, this does not
1784 matter unless the delimiter happens to be character special to the
1785 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1786 alphanumeric char, as in:
1790 In the RE above, which is intentionally obfuscated for illustration, the
1791 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1792 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
1793 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1794 non-whitespace choices.
1798 This step is the last one for all constructs except regular expressions,
1799 which are processed further.
1801 =item Interpolation of regular expressions
1803 Previous steps were performed during the compilation of Perl code,
1804 but this one happens at run time--although it may be optimized to
1805 be calculated at compile time if appropriate. After preprocessing
1806 described above, and possibly after evaluation if catenation,
1807 joining, casing translation, or metaquoting are involved, the
1808 resulting I<string> is passed to the RE engine for compilation.
1810 Whatever happens in the RE engine might be better discussed in L<perlre>,
1811 but for the sake of continuity, we shall do so here.
1813 This is another step where the presence of the C<//x> modifier is
1814 relevant. The RE engine scans the string from left to right and
1815 converts it to a finite automaton.
1817 Backslashed characters are either replaced with corresponding
1818 literal strings (as with C<\{>), or else they generate special nodes
1819 in the finite automaton (as with C<\b>). Characters special to the
1820 RE engine (such as C<|>) generate corresponding nodes or groups of
1821 nodes. C<(?#...)> comments are ignored. All the rest is either
1822 converted to literal strings to match, or else is ignored (as is
1823 whitespace and C<#>-style comments if C<//x> is present).
1825 Parsing of the bracketed character class construct, C<[...]>, is
1826 rather different than the rule used for the rest of the pattern.
1827 The terminator of this construct is found using the same rules as
1828 for finding the terminator of a C<{}>-delimited construct, the only
1829 exception being that C<]> immediately following C<[> is treated as
1830 though preceded by a backslash. Similarly, the terminator of
1831 C<(?{...})> is found using the same rules as for finding the
1832 terminator of a C<{}>-delimited construct.
1834 It is possible to inspect both the string given to RE engine and the
1835 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1836 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1837 switch documented in L<perlrun/"Command Switches">.
1839 =item Optimization of regular expressions
1841 This step is listed for completeness only. Since it does not change
1842 semantics, details of this step are not documented and are subject
1843 to change without notice. This step is performed over the finite
1844 automaton that was generated during the previous pass.
1846 It is at this stage that C<split()> silently optimizes C</^/> to
1851 =head2 I/O Operators
1853 There are several I/O operators you should know about.
1855 A string enclosed by backticks (grave accents) first undergoes
1856 double-quote interpolation. It is then interpreted as an external
1857 command, and the output of that command is the value of the
1858 backtick string, like in a shell. In scalar context, a single string
1859 consisting of all output is returned. In list context, a list of
1860 values is returned, one per line of output. (You can set C<$/> to use
1861 a different line terminator.) The command is executed each time the
1862 pseudo-literal is evaluated. The status value of the command is
1863 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
1864 Unlike in B<csh>, no translation is done on the return data--newlines
1865 remain newlines. Unlike in any of the shells, single quotes do not
1866 hide variable names in the command from interpretation. To pass a
1867 literal dollar-sign through to the shell you need to hide it with a
1868 backslash. The generalized form of backticks is C<qx//>. (Because
1869 backticks always undergo shell expansion as well, see L<perlsec> for
1872 In scalar context, evaluating a filehandle in angle brackets yields
1873 the next line from that file (the newline, if any, included), or
1874 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1875 (sometimes known as file-slurp mode) and the file is empty, it
1876 returns C<''> the first time, followed by C<undef> subsequently.
1878 Ordinarily you must assign the returned value to a variable, but
1879 there is one situation where an automatic assignment happens. If
1880 and only if the input symbol is the only thing inside the conditional
1881 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1882 the value is automatically assigned to the global variable $_,
1883 destroying whatever was there previously. (This may seem like an
1884 odd thing to you, but you'll use the construct in almost every Perl
1885 script you write.) The $_ variable is not implicitly localized.
1886 You'll have to put a C<local $_;> before the loop if you want that
1889 The following lines are equivalent:
1891 while (defined($_ = <STDIN>)) { print; }
1892 while ($_ = <STDIN>) { print; }
1893 while (<STDIN>) { print; }
1894 for (;<STDIN>;) { print; }
1895 print while defined($_ = <STDIN>);
1896 print while ($_ = <STDIN>);
1897 print while <STDIN>;
1899 This also behaves similarly, but avoids $_ :
1901 while (my $line = <STDIN>) { print $line }
1903 In these loop constructs, the assigned value (whether assignment
1904 is automatic or explicit) is then tested to see whether it is
1905 defined. The defined test avoids problems where line has a string
1906 value that would be treated as false by Perl, for example a "" or
1907 a "0" with no trailing newline. If you really mean for such values
1908 to terminate the loop, they should be tested for explicitly:
1910 while (($_ = <STDIN>) ne '0') { ... }
1911 while (<STDIN>) { last unless $_; ... }
1913 In other boolean contexts, C<< <I<filehandle>> >> without an
1914 explicit C<defined> test or comparison elicit a warning if the
1915 C<use warnings> pragma or the B<-w>
1916 command-line switch (the C<$^W> variable) is in effect.
1918 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1919 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
1920 in packages, where they would be interpreted as local identifiers
1921 rather than global.) Additional filehandles may be created with
1922 the open() function, amongst others. See L<perlopentut> and
1923 L<perlfunc/open> for details on this.
1925 If a <FILEHANDLE> is used in a context that is looking for
1926 a list, a list comprising all input lines is returned, one line per
1927 list element. It's easy to grow to a rather large data space this
1928 way, so use with care.
1930 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
1931 See L<perlfunc/readline>.
1933 The null filehandle <> is special: it can be used to emulate the
1934 behavior of B<sed> and B<awk>. Input from <> comes either from
1935 standard input, or from each file listed on the command line. Here's
1936 how it works: the first time <> is evaluated, the @ARGV array is
1937 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
1938 gives you standard input. The @ARGV array is then processed as a list
1939 of filenames. The loop
1942 ... # code for each line
1945 is equivalent to the following Perl-like pseudo code:
1947 unshift(@ARGV, '-') unless @ARGV;
1948 while ($ARGV = shift) {
1951 ... # code for each line
1955 except that it isn't so cumbersome to say, and will actually work.
1956 It really does shift the @ARGV array and put the current filename
1957 into the $ARGV variable. It also uses filehandle I<ARGV>
1958 internally--<> is just a synonym for <ARGV>, which
1959 is magical. (The pseudo code above doesn't work because it treats
1960 <ARGV> as non-magical.)
1962 You can modify @ARGV before the first <> as long as the array ends up
1963 containing the list of filenames you really want. Line numbers (C<$.>)
1964 continue as though the input were one big happy file. See the example
1965 in L<perlfunc/eof> for how to reset line numbers on each file.
1967 If you want to set @ARGV to your own list of files, go right ahead.
1968 This sets @ARGV to all plain text files if no @ARGV was given:
1970 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
1972 You can even set them to pipe commands. For example, this automatically
1973 filters compressed arguments through B<gzip>:
1975 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1977 If you want to pass switches into your script, you can use one of the
1978 Getopts modules or put a loop on the front like this:
1980 while ($_ = $ARGV[0], /^-/) {
1983 if (/^-D(.*)/) { $debug = $1 }
1984 if (/^-v/) { $verbose++ }
1985 # ... # other switches
1989 # ... # code for each line
1992 The <> symbol will return C<undef> for end-of-file only once.
1993 If you call it again after this, it will assume you are processing another
1994 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
1996 If what the angle brackets contain is a simple scalar variable (e.g.,
1997 <$foo>), then that variable contains the name of the
1998 filehandle to input from, or its typeglob, or a reference to the
2004 If what's within the angle brackets is neither a filehandle nor a simple
2005 scalar variable containing a filehandle name, typeglob, or typeglob
2006 reference, it is interpreted as a filename pattern to be globbed, and
2007 either a list of filenames or the next filename in the list is returned,
2008 depending on context. This distinction is determined on syntactic
2009 grounds alone. That means C<< <$x> >> is always a readline() from
2010 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2011 That's because $x is a simple scalar variable, but C<$hash{key}> is
2012 not--it's a hash element. Even C<< <$x > >> (note the extra space)
2013 is treated as C<glob("$x ")>, not C<readline($x)>.
2015 One level of double-quote interpretation is done first, but you can't
2016 say C<< <$foo> >> because that's an indirect filehandle as explained
2017 in the previous paragraph. (In older versions of Perl, programmers
2018 would insert curly brackets to force interpretation as a filename glob:
2019 C<< <${foo}> >>. These days, it's considered cleaner to call the
2020 internal function directly as C<glob($foo)>, which is probably the right
2021 way to have done it in the first place.) For example:
2027 is roughly equivalent to:
2029 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2035 except that the globbing is actually done internally using the standard
2036 C<File::Glob> extension. Of course, the shortest way to do the above is:
2040 A (file)glob evaluates its (embedded) argument only when it is
2041 starting a new list. All values must be read before it will start
2042 over. In list context, this isn't important because you automatically
2043 get them all anyway. However, in scalar context the operator returns
2044 the next value each time it's called, or C<undef> when the list has
2045 run out. As with filehandle reads, an automatic C<defined> is
2046 generated when the glob occurs in the test part of a C<while>,
2047 because legal glob returns (e.g. a file called F<0>) would otherwise
2048 terminate the loop. Again, C<undef> is returned only once. So if
2049 you're expecting a single value from a glob, it is much better to
2052 ($file) = <blurch*>;
2058 because the latter will alternate between returning a filename and
2061 If you're trying to do variable interpolation, it's definitely better
2062 to use the glob() function, because the older notation can cause people
2063 to become confused with the indirect filehandle notation.
2065 @files = glob("$dir/*.[ch]");
2066 @files = glob($files[$i]);
2068 =head2 Constant Folding
2070 Like C, Perl does a certain amount of expression evaluation at
2071 compile time whenever it determines that all arguments to an
2072 operator are static and have no side effects. In particular, string
2073 concatenation happens at compile time between literals that don't do
2074 variable substitution. Backslash interpolation also happens at
2075 compile time. You can say
2077 'Now is the time for all' . "\n" .
2078 'good men to come to.'
2080 and this all reduces to one string internally. Likewise, if
2083 foreach $file (@filenames) {
2084 if (-s $file > 5 + 100 * 2**16) { }
2087 the compiler will precompute the number which that expression
2088 represents so that the interpreter won't have to.
2092 Perl doesn't officially have a no-op operator, but the bare constants
2093 C<0> and C<1> are special-cased to not produce a warning in a void
2094 context, so you can for example safely do
2098 =head2 Bitwise String Operators
2100 Bitstrings of any size may be manipulated by the bitwise operators
2103 If the operands to a binary bitwise op are strings of different
2104 sizes, B<|> and B<^> ops act as though the shorter operand had
2105 additional zero bits on the right, while the B<&> op acts as though
2106 the longer operand were truncated to the length of the shorter.
2107 The granularity for such extension or truncation is one or more
2110 # ASCII-based examples
2111 print "j p \n" ^ " a h"; # prints "JAPH\n"
2112 print "JA" | " ph\n"; # prints "japh\n"
2113 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2114 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2116 If you are intending to manipulate bitstrings, be certain that
2117 you're supplying bitstrings: If an operand is a number, that will imply
2118 a B<numeric> bitwise operation. You may explicitly show which type of
2119 operation you intend by using C<""> or C<0+>, as in the examples below.
2121 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
2122 $foo = '150' | 105 ; # yields 255
2123 $foo = 150 | '105'; # yields 255
2124 $foo = '150' | '105'; # yields string '155' (under ASCII)
2126 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2127 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2129 See L<perlfunc/vec> for information on how to manipulate individual bits
2132 =head2 Integer Arithmetic
2134 By default, Perl assumes that it must do most of its arithmetic in
2135 floating point. But by saying
2139 you may tell the compiler that it's okay to use integer operations
2140 (if it feels like it) from here to the end of the enclosing BLOCK.
2141 An inner BLOCK may countermand this by saying
2145 which lasts until the end of that BLOCK. Note that this doesn't
2146 mean everything is only an integer, merely that Perl may use integer
2147 operations if it is so inclined. For example, even under C<use
2148 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2151 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2152 and ">>") always produce integral results. (But see also
2153 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2154 them. By default, their results are interpreted as unsigned integers, but
2155 if C<use integer> is in effect, their results are interpreted
2156 as signed integers. For example, C<~0> usually evaluates to a large
2157 integral value. However, C<use integer; ~0> is C<-1> on twos-complement
2160 =head2 Floating-point Arithmetic
2162 While C<use integer> provides integer-only arithmetic, there is no
2163 analogous mechanism to provide automatic rounding or truncation to a
2164 certain number of decimal places. For rounding to a certain number
2165 of digits, sprintf() or printf() is usually the easiest route.
2168 Floating-point numbers are only approximations to what a mathematician
2169 would call real numbers. There are infinitely more reals than floats,
2170 so some corners must be cut. For example:
2172 printf "%.20g\n", 123456789123456789;
2173 # produces 123456789123456784
2175 Testing for exact equality of floating-point equality or inequality is
2176 not a good idea. Here's a (relatively expensive) work-around to compare
2177 whether two floating-point numbers are equal to a particular number of
2178 decimal places. See Knuth, volume II, for a more robust treatment of
2182 my ($X, $Y, $POINTS) = @_;
2184 $tX = sprintf("%.${POINTS}g", $X);
2185 $tY = sprintf("%.${POINTS}g", $Y);
2189 The POSIX module (part of the standard perl distribution) implements
2190 ceil(), floor(), and other mathematical and trigonometric functions.
2191 The Math::Complex module (part of the standard perl distribution)
2192 defines mathematical functions that work on both the reals and the
2193 imaginary numbers. Math::Complex not as efficient as POSIX, but
2194 POSIX can't work with complex numbers.
2196 Rounding in financial applications can have serious implications, and
2197 the rounding method used should be specified precisely. In these
2198 cases, it probably pays not to trust whichever system rounding is
2199 being used by Perl, but to instead implement the rounding function you
2202 =head2 Bigger Numbers
2204 The standard Math::BigInt and Math::BigFloat modules provide
2205 variable-precision arithmetic and overloaded operators, although
2206 they're currently pretty slow. At the cost of some space and
2207 considerable speed, they avoid the normal pitfalls associated with
2208 limited-precision representations.
2211 $x = Math::BigInt->new('123456789123456789');
2214 # prints +15241578780673678515622620750190521
2216 There are several modules that let you calculate with (bound only by
2217 memory and cpu-time) unlimited or fixed precision. There are also
2218 some non-standard modules that provide faster implementations via
2219 external C libraries.
2221 Here is a short, but incomplete summary:
2223 Math::Fraction big, unlimited fractions like 9973 / 12967
2224 Math::String treat string sequences like numbers
2225 Math::FixedPrecision calculate with a fixed precision
2226 Math::Currency for currency calculations
2227 Bit::Vector manipulate bit vectors fast (uses C)
2228 Math::BigIntFast Bit::Vector wrapper for big numbers
2229 Math::Pari provides access to the Pari C library
2230 Math::BigInteger uses an external C library
2231 Math::Cephes uses external Cephes C library (no big numbers)
2232 Math::Cephes::Fraction fractions via the Cephes library
2233 Math::GMP another one using an external C library
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