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1=head1 NAME
2
3perlop - Perl operators and precedence
4
5=head1 SYNOPSIS
6
7Perl operators have the following associativity and precedence,
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8listed from highest precedence to lowest. Operators borrowed from
9C keep the same precedence relationship with each other, even where
10C's precedence is slightly screwy. (This makes learning Perl easier
11for C folks.) With very few exceptions, these all operate on scalar
12values only, not array values.
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13
14 left terms and list operators (leftward)
15 left ->
16 nonassoc ++ --
17 right **
18 right ! ~ \ and unary + and -
54310121 19 left =~ !~
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20 left * / % x
21 left + - .
22 left << >>
23 nonassoc named unary operators
24 nonassoc < > <= >= lt gt le ge
25 nonassoc == != <=> eq ne cmp
26 left &
27 left | ^
28 left &&
29 left ||
137443ea 30 nonassoc .. ...
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31 right ?:
32 right = += -= *= etc.
33 left , =>
34 nonassoc list operators (rightward)
a5f75d66 35 right not
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36 left and
37 left or xor
38
39In the following sections, these operators are covered in precedence order.
40
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41Many operators can be overloaded for objects. See L<overload>.
42
cb1a09d0 43=head1 DESCRIPTION
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44
45=head2 Terms and List Operators (Leftward)
46
62c18ce2 47A TERM has the highest precedence in Perl. They include variables,
5f05dabc 48quote and quote-like operators, any expression in parentheses,
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49and any function whose arguments are parenthesized. Actually, there
50aren't really functions in this sense, just list operators and unary
51operators behaving as functions because you put parentheses around
52the arguments. These are all documented in L<perlfunc>.
53
54If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
55is followed by a left parenthesis as the next token, the operator and
56arguments within parentheses are taken to be of highest precedence,
57just like a normal function call.
58
59In the absence of parentheses, the precedence of list operators such as
60C<print>, C<sort>, or C<chmod> is either very high or very low depending on
54310121 61whether you are looking at the left side or the right side of the operator.
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62For example, in
63
64 @ary = (1, 3, sort 4, 2);
65 print @ary; # prints 1324
66
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67the commas on the right of the sort are evaluated before the sort,
68but the commas on the left are evaluated after. In other words,
69list operators tend to gobble up all arguments that follow, and
a0d0e21e 70then act like a simple TERM with regard to the preceding expression.
19799a22 71Be careful with parentheses:
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72
73 # These evaluate exit before doing the print:
74 print($foo, exit); # Obviously not what you want.
75 print $foo, exit; # Nor is this.
76
77 # These do the print before evaluating exit:
78 (print $foo), exit; # This is what you want.
79 print($foo), exit; # Or this.
80 print ($foo), exit; # Or even this.
81
82Also note that
83
84 print ($foo & 255) + 1, "\n";
85
54310121 86probably doesn't do what you expect at first glance. See
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87L<Named Unary Operators> for more discussion of this.
88
89Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
54310121 90well as subroutine and method calls, and the anonymous
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91constructors C<[]> and C<{}>.
92
2ae324a7 93See also L<Quote and Quote-like Operators> toward the end of this section,
c07a80fd 94as well as L<"I/O Operators">.
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95
96=head2 The Arrow Operator
97
35f2feb0 98"C<< -> >>" is an infix dereference operator, just as it is in C
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99and C++. If the right side is either a C<[...]>, C<{...}>, or a
100C<(...)> subscript, then the left side must be either a hard or
101symbolic reference to an array, a hash, or a subroutine respectively.
102(Or technically speaking, a location capable of holding a hard
103reference, if it's an array or hash reference being used for
104assignment.) See L<perlreftut> and L<perlref>.
a0d0e21e 105
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106Otherwise, the right side is a method name or a simple scalar
107variable containing either the method name or a subroutine reference,
108and the left side must be either an object (a blessed reference)
109or a class name (that is, a package name). See L<perlobj>.
a0d0e21e 110
5f05dabc 111=head2 Auto-increment and Auto-decrement
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112
113"++" and "--" work as in C. That is, if placed before a variable, they
114increment or decrement the variable before returning the value, and if
115placed after, increment or decrement the variable after returning the value.
116
54310121 117The auto-increment operator has a little extra builtin magic to it. If
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118you increment a variable that is numeric, or that has ever been used in
119a numeric context, you get a normal increment. If, however, the
5f05dabc 120variable has been used in only string contexts since it was set, and
5a964f20 121has a value that is not the empty string and matches the pattern
9c0670e1 122C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
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123character within its range, with carry:
124
125 print ++($foo = '99'); # prints '100'
126 print ++($foo = 'a0'); # prints 'a1'
127 print ++($foo = 'Az'); # prints 'Ba'
128 print ++($foo = 'zz'); # prints 'aaa'
129
5f05dabc 130The auto-decrement operator is not magical.
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131
132=head2 Exponentiation
133
19799a22 134Binary "**" is the exponentiation operator. It binds even more
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135tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
136implemented using C's pow(3) function, which actually works on doubles
137internally.)
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138
139=head2 Symbolic Unary Operators
140
5f05dabc 141Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
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142precedence version of this.
143
144Unary "-" performs arithmetic negation if the operand is numeric. If
145the operand is an identifier, a string consisting of a minus sign
146concatenated with the identifier is returned. Otherwise, if the string
147starts with a plus or minus, a string starting with the opposite sign
148is returned. One effect of these rules is that C<-bareword> is equivalent
149to C<"-bareword">.
150
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151Unary "~" performs bitwise negation, i.e., 1's complement. For
152example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
153L<Bitwise String Operators>.) Note that the width of the result is
154platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
155bits wide on a 64-bit platform, so if you are expecting a certain bit
156width, remember use the & operator to mask off the excess bits.
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157
158Unary "+" has no effect whatsoever, even on strings. It is useful
159syntactically for separating a function name from a parenthesized expression
160that would otherwise be interpreted as the complete list of function
5ba421f6 161arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
a0d0e21e 162
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163Unary "\" creates a reference to whatever follows it. See L<perlreftut>
164and L<perlref>. Do not confuse this behavior with the behavior of
165backslash within a string, although both forms do convey the notion
166of protecting the next thing from interpolation.
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167
168=head2 Binding Operators
169
c07a80fd 170Binary "=~" binds a scalar expression to a pattern match. Certain operations
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171search or modify the string $_ by default. This operator makes that kind
172of operation work on some other string. The right argument is a search
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173pattern, substitution, or transliteration. The left argument is what is
174supposed to be searched, substituted, or transliterated instead of the default
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175$_. When used in scalar context, the return value generally indicates the
176success of the operation. Behavior in list context depends on the particular
177operator. See L</"Regexp Quote-Like Operators"> for details.
178
179If the right argument is an expression rather than a search pattern,
2c268ad5 180substitution, or transliteration, it is interpreted as a search pattern at run
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181time. This can be less efficient than an explicit search, because the
182pattern must be compiled every time the expression is evaluated.
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183
184Binary "!~" is just like "=~" except the return value is negated in
185the logical sense.
186
187=head2 Multiplicative Operators
188
189Binary "*" multiplies two numbers.
190
191Binary "/" divides two numbers.
192
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193Binary "%" computes the modulus of two numbers. Given integer
194operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
195C<$a> minus the largest multiple of C<$b> that is not greater than
196C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
197smallest multiple of C<$b> that is not less than C<$a> (i.e. the
6bb4e6d4 198result will be less than or equal to zero).
f3798619 199Note than when C<use integer> is in scope, "%" gives you direct access
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200to the modulus operator as implemented by your C compiler. This
201operator is not as well defined for negative operands, but it will
202execute faster.
203
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204Binary "x" is the repetition operator. In scalar context or if the left
205operand is not enclosed in parentheses, it returns a string consisting
206of the left operand repeated the number of times specified by the right
207operand. In list context, if the left operand is enclosed in
208parentheses, it repeats the list.
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209
210 print '-' x 80; # print row of dashes
211
212 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
213
214 @ones = (1) x 80; # a list of 80 1's
215 @ones = (5) x @ones; # set all elements to 5
216
217
218=head2 Additive Operators
219
220Binary "+" returns the sum of two numbers.
221
222Binary "-" returns the difference of two numbers.
223
224Binary "." concatenates two strings.
225
226=head2 Shift Operators
227
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228Binary "<<" returns the value of its left argument shifted left by the
229number of bits specified by the right argument. Arguments should be
982ce180 230integers. (See also L<Integer Arithmetic>.)
a0d0e21e 231
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232Binary ">>" returns the value of its left argument shifted right by
233the number of bits specified by the right argument. Arguments should
982ce180 234be integers. (See also L<Integer Arithmetic>.)
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235
236=head2 Named Unary Operators
237
238The various named unary operators are treated as functions with one
239argument, with optional parentheses. These include the filetest
240operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
241
242If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
243is followed by a left parenthesis as the next token, the operator and
244arguments within parentheses are taken to be of highest precedence,
245just like a normal function call. Examples:
246
247 chdir $foo || die; # (chdir $foo) || die
248 chdir($foo) || die; # (chdir $foo) || die
249 chdir ($foo) || die; # (chdir $foo) || die
250 chdir +($foo) || die; # (chdir $foo) || die
251
252but, because * is higher precedence than ||:
253
254 chdir $foo * 20; # chdir ($foo * 20)
255 chdir($foo) * 20; # (chdir $foo) * 20
256 chdir ($foo) * 20; # (chdir $foo) * 20
257 chdir +($foo) * 20; # chdir ($foo * 20)
258
259 rand 10 * 20; # rand (10 * 20)
260 rand(10) * 20; # (rand 10) * 20
261 rand (10) * 20; # (rand 10) * 20
262 rand +(10) * 20; # rand (10 * 20)
263
5ba421f6 264See also L<"Terms and List Operators (Leftward)">.
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265
266=head2 Relational Operators
267
35f2feb0 268Binary "<" returns true if the left argument is numerically less than
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269the right argument.
270
35f2feb0 271Binary ">" returns true if the left argument is numerically greater
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272than the right argument.
273
35f2feb0 274Binary "<=" returns true if the left argument is numerically less than
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275or equal to the right argument.
276
35f2feb0 277Binary ">=" returns true if the left argument is numerically greater
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278than or equal to the right argument.
279
280Binary "lt" returns true if the left argument is stringwise less than
281the right argument.
282
283Binary "gt" returns true if the left argument is stringwise greater
284than the right argument.
285
286Binary "le" returns true if the left argument is stringwise less than
287or equal to the right argument.
288
289Binary "ge" returns true if the left argument is stringwise greater
290than or equal to the right argument.
291
292=head2 Equality Operators
293
294Binary "==" returns true if the left argument is numerically equal to
295the right argument.
296
297Binary "!=" returns true if the left argument is numerically not equal
298to the right argument.
299
35f2feb0 300Binary "<=>" returns -1, 0, or 1 depending on whether the left
6ee5d4e7 301argument is numerically less than, equal to, or greater than the right
d4ad863d 302argument. If your platform supports NaNs (not-a-numbers) as numeric
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303values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
304"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
305returns true, as does NaN != anything else. If your platform doesn't
306support NaNs then NaN is just a string with numeric value 0.
307
308 perl -le '$a = NaN; print "No NaN support here" if $a == $a'
309 perl -le '$a = NaN; print "NaN support here" if $a != $a'
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310
311Binary "eq" returns true if the left argument is stringwise equal to
312the right argument.
313
314Binary "ne" returns true if the left argument is stringwise not equal
315to the right argument.
316
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317Binary "cmp" returns -1, 0, or 1 depending on whether the left
318argument is stringwise less than, equal to, or greater than the right
319argument.
a0d0e21e 320
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321"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
322by the current locale if C<use locale> is in effect. See L<perllocale>.
323
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324=head2 Bitwise And
325
326Binary "&" returns its operators ANDed together bit by bit.
2c268ad5 327(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
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328
329=head2 Bitwise Or and Exclusive Or
330
331Binary "|" returns its operators ORed together bit by bit.
2c268ad5 332(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
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333
334Binary "^" returns its operators XORed together bit by bit.
2c268ad5 335(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
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336
337=head2 C-style Logical And
338
339Binary "&&" performs a short-circuit logical AND operation. That is,
340if the left operand is false, the right operand is not even evaluated.
341Scalar or list context propagates down to the right operand if it
342is evaluated.
343
344=head2 C-style Logical Or
345
346Binary "||" performs a short-circuit logical OR operation. That is,
347if the left operand is true, the right operand is not even evaluated.
348Scalar or list context propagates down to the right operand if it
349is evaluated.
350
351The C<||> and C<&&> operators differ from C's in that, rather than returning
3520 or 1, they return the last value evaluated. Thus, a reasonably portable
353way to find out the home directory (assuming it's not "0") might be:
354
355 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
356 (getpwuid($<))[7] || die "You're homeless!\n";
357
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358In particular, this means that you shouldn't use this
359for selecting between two aggregates for assignment:
360
361 @a = @b || @c; # this is wrong
362 @a = scalar(@b) || @c; # really meant this
363 @a = @b ? @b : @c; # this works fine, though
364
365As more readable alternatives to C<&&> and C<||> when used for
366control flow, Perl provides C<and> and C<or> operators (see below).
367The short-circuit behavior is identical. The precedence of "and" and
368"or" is much lower, however, so that you can safely use them after a
369list operator without the need for parentheses:
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370
371 unlink "alpha", "beta", "gamma"
372 or gripe(), next LINE;
373
374With the C-style operators that would have been written like this:
375
376 unlink("alpha", "beta", "gamma")
377 || (gripe(), next LINE);
378
eeb6a2c9 379Using "or" for assignment is unlikely to do what you want; see below.
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380
381=head2 Range Operators
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382
383Binary ".." is the range operator, which is really two different
5a964f20 384operators depending on the context. In list context, it returns an
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385array of values counting (up by ones) from the left value to the right
386value. If the left value is greater than the right value then it
387returns the empty array. The range operator is useful for writing
388C<foreach (1..10)> loops and for doing slice operations on arrays. In
389the current implementation, no temporary array is created when the
390range operator is used as the expression in C<foreach> loops, but older
391versions of Perl might burn a lot of memory when you write something
392like this:
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393
394 for (1 .. 1_000_000) {
395 # code
54310121 396 }
a0d0e21e 397
5a964f20 398In scalar context, ".." returns a boolean value. The operator is
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399bistable, like a flip-flop, and emulates the line-range (comma) operator
400of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
401own boolean state. It is false as long as its left operand is false.
402Once the left operand is true, the range operator stays true until the
403right operand is true, I<AFTER> which the range operator becomes false
19799a22 404again. It doesn't become false till the next time the range operator is
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405evaluated. It can test the right operand and become false on the same
406evaluation it became true (as in B<awk>), but it still returns true once.
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407If you don't want it to test the right operand till the next
408evaluation, as in B<sed>, just use three dots ("...") instead of
409two. In all other regards, "..." behaves just like ".." does.
410
411The right operand is not evaluated while the operator is in the
412"false" state, and the left operand is not evaluated while the
413operator is in the "true" state. The precedence is a little lower
414than || and &&. The value returned is either the empty string for
415false, or a sequence number (beginning with 1) for true. The
416sequence number is reset for each range encountered. The final
417sequence number in a range has the string "E0" appended to it, which
418doesn't affect its numeric value, but gives you something to search
419for if you want to exclude the endpoint. You can exclude the
420beginning point by waiting for the sequence number to be greater
421than 1. If either operand of scalar ".." is a constant expression,
422that operand is implicitly compared to the C<$.> variable, the
423current line number. Examples:
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424
425As a scalar operator:
426
427 if (101 .. 200) { print; } # print 2nd hundred lines
428 next line if (1 .. /^$/); # skip header lines
429 s/^/> / if (/^$/ .. eof()); # quote body
430
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431 # parse mail messages
432 while (<>) {
433 $in_header = 1 .. /^$/;
434 $in_body = /^$/ .. eof();
435 # do something based on those
436 } continue {
437 close ARGV if eof; # reset $. each file
438 }
439
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440As a list operator:
441
442 for (101 .. 200) { print; } # print $_ 100 times
3e3baf6d 443 @foo = @foo[0 .. $#foo]; # an expensive no-op
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444 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
445
5a964f20 446The range operator (in list context) makes use of the magical
5f05dabc 447auto-increment algorithm if the operands are strings. You
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448can say
449
450 @alphabet = ('A' .. 'Z');
451
19799a22 452to get all normal letters of the alphabet, or
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453
454 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
455
456to get a hexadecimal digit, or
457
458 @z2 = ('01' .. '31'); print $z2[$mday];
459
460to get dates with leading zeros. If the final value specified is not
461in the sequence that the magical increment would produce, the sequence
462goes until the next value would be longer than the final value
463specified.
464
465=head2 Conditional Operator
466
467Ternary "?:" is the conditional operator, just as in C. It works much
468like an if-then-else. If the argument before the ? is true, the
469argument before the : is returned, otherwise the argument after the :
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470is returned. For example:
471
54310121 472 printf "I have %d dog%s.\n", $n,
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473 ($n == 1) ? '' : "s";
474
475Scalar or list context propagates downward into the 2nd
54310121 476or 3rd argument, whichever is selected.
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477
478 $a = $ok ? $b : $c; # get a scalar
479 @a = $ok ? @b : @c; # get an array
480 $a = $ok ? @b : @c; # oops, that's just a count!
481
482The operator may be assigned to if both the 2nd and 3rd arguments are
483legal lvalues (meaning that you can assign to them):
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484
485 ($a_or_b ? $a : $b) = $c;
486
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487Because this operator produces an assignable result, using assignments
488without parentheses will get you in trouble. For example, this:
489
490 $a % 2 ? $a += 10 : $a += 2
491
492Really means this:
493
494 (($a % 2) ? ($a += 10) : $a) += 2
495
496Rather than this:
497
498 ($a % 2) ? ($a += 10) : ($a += 2)
499
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500That should probably be written more simply as:
501
502 $a += ($a % 2) ? 10 : 2;
503
4633a7c4 504=head2 Assignment Operators
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505
506"=" is the ordinary assignment operator.
507
508Assignment operators work as in C. That is,
509
510 $a += 2;
511
512is equivalent to
513
514 $a = $a + 2;
515
516although without duplicating any side effects that dereferencing the lvalue
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517might trigger, such as from tie(). Other assignment operators work similarly.
518The following are recognized:
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519
520 **= += *= &= <<= &&=
521 -= /= |= >>= ||=
522 .= %= ^=
523 x=
524
19799a22 525Although these are grouped by family, they all have the precedence
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526of assignment.
527
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528Unlike in C, the scalar assignment operator produces a valid lvalue.
529Modifying an assignment is equivalent to doing the assignment and
530then modifying the variable that was assigned to. This is useful
531for modifying a copy of something, like this:
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532
533 ($tmp = $global) =~ tr [A-Z] [a-z];
534
535Likewise,
536
537 ($a += 2) *= 3;
538
539is equivalent to
540
541 $a += 2;
542 $a *= 3;
543
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544Similarly, a list assignment in list context produces the list of
545lvalues assigned to, and a list assignment in scalar context returns
546the number of elements produced by the expression on the right hand
547side of the assignment.
548
748a9306 549=head2 Comma Operator
a0d0e21e 550
5a964f20 551Binary "," is the comma operator. In scalar context it evaluates
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552its left argument, throws that value away, then evaluates its right
553argument and returns that value. This is just like C's comma operator.
554
5a964f20 555In list context, it's just the list argument separator, and inserts
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556both its arguments into the list.
557
35f2feb0 558The => digraph is mostly just a synonym for the comma operator. It's useful for
cb1a09d0 559documenting arguments that come in pairs. As of release 5.001, it also forces
4633a7c4 560any word to the left of it to be interpreted as a string.
748a9306 561
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562=head2 List Operators (Rightward)
563
564On the right side of a list operator, it has very low precedence,
565such that it controls all comma-separated expressions found there.
566The only operators with lower precedence are the logical operators
567"and", "or", and "not", which may be used to evaluate calls to list
568operators without the need for extra parentheses:
569
570 open HANDLE, "filename"
571 or die "Can't open: $!\n";
572
5ba421f6 573See also discussion of list operators in L<Terms and List Operators (Leftward)>.
a0d0e21e
LW
574
575=head2 Logical Not
576
577Unary "not" returns the logical negation of the expression to its right.
578It's the equivalent of "!" except for the very low precedence.
579
580=head2 Logical And
581
582Binary "and" returns the logical conjunction of the two surrounding
583expressions. It's equivalent to && except for the very low
5f05dabc 584precedence. This means that it short-circuits: i.e., the right
a0d0e21e
LW
585expression is evaluated only if the left expression is true.
586
587=head2 Logical or and Exclusive Or
588
589Binary "or" returns the logical disjunction of the two surrounding
5a964f20
TC
590expressions. It's equivalent to || except for the very low precedence.
591This makes it useful for control flow
592
593 print FH $data or die "Can't write to FH: $!";
594
595This means that it short-circuits: i.e., the right expression is evaluated
596only if the left expression is false. Due to its precedence, you should
597probably avoid using this for assignment, only for control flow.
598
599 $a = $b or $c; # bug: this is wrong
600 ($a = $b) or $c; # really means this
601 $a = $b || $c; # better written this way
602
19799a22 603However, when it's a list-context assignment and you're trying to use
5a964f20
TC
604"||" for control flow, you probably need "or" so that the assignment
605takes higher precedence.
606
607 @info = stat($file) || die; # oops, scalar sense of stat!
608 @info = stat($file) or die; # better, now @info gets its due
609
19799a22 610Then again, you could always use parentheses.
a0d0e21e
LW
611
612Binary "xor" returns the exclusive-OR of the two surrounding expressions.
613It cannot short circuit, of course.
614
615=head2 C Operators Missing From Perl
616
617Here is what C has that Perl doesn't:
618
619=over 8
620
621=item unary &
622
623Address-of operator. (But see the "\" operator for taking a reference.)
624
625=item unary *
626
54310121 627Dereference-address operator. (Perl's prefix dereferencing
a0d0e21e
LW
628operators are typed: $, @, %, and &.)
629
630=item (TYPE)
631
19799a22 632Type-casting operator.
a0d0e21e
LW
633
634=back
635
5f05dabc 636=head2 Quote and Quote-like Operators
a0d0e21e
LW
637
638While we usually think of quotes as literal values, in Perl they
639function as operators, providing various kinds of interpolating and
640pattern matching capabilities. Perl provides customary quote characters
641for these behaviors, but also provides a way for you to choose your
642quote character for any of them. In the following table, a C<{}> represents
87275199 643any pair of delimiters you choose.
a0d0e21e 644
2c268ad5
TP
645 Customary Generic Meaning Interpolates
646 '' q{} Literal no
647 "" qq{} Literal yes
01ae956f 648 `` qx{} Command yes (unless '' is delimiter)
2c268ad5 649 qw{} Word list no
f70b4f9c
AB
650 // m{} Pattern match yes (unless '' is delimiter)
651 qr{} Pattern yes (unless '' is delimiter)
652 s{}{} Substitution yes (unless '' is delimiter)
2c268ad5 653 tr{}{} Transliteration no (but see below)
a0d0e21e 654
87275199
GS
655Non-bracketing delimiters use the same character fore and aft, but the four
656sorts of brackets (round, angle, square, curly) will all nest, which means
657that
658
659 q{foo{bar}baz}
35f2feb0 660
87275199
GS
661is the same as
662
663 'foo{bar}baz'
664
665Note, however, that this does not always work for quoting Perl code:
666
667 $s = q{ if($a eq "}") ... }; # WRONG
668
669is a syntax error. The C<Text::Balanced> module on CPAN is able to do this
670properly.
671
19799a22 672There can be whitespace between the operator and the quoting
fb73857a 673characters, except when C<#> is being used as the quoting character.
19799a22
GS
674C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
675operator C<q> followed by a comment. Its argument will be taken
676from the next line. This allows you to write:
fb73857a
PP
677
678 s {foo} # Replace foo
679 {bar} # with bar.
680
19799a22
GS
681For constructs that do interpolate, variables beginning with "C<$>"
682or "C<@>" are interpolated, as are the following escape sequences. Within
a0ed51b3 683a transliteration, the first eleven of these sequences may be used.
a0d0e21e 684
6ee5d4e7 685 \t tab (HT, TAB)
5a964f20 686 \n newline (NL)
6ee5d4e7
PP
687 \r return (CR)
688 \f form feed (FF)
689 \b backspace (BS)
690 \a alarm (bell) (BEL)
691 \e escape (ESC)
a0ed51b3
LW
692 \033 octal char (ESC)
693 \x1b hex char (ESC)
694 \x{263a} wide hex char (SMILEY)
19799a22 695 \c[ control char (ESC)
4a2d328f 696 \N{name} named char
2c268ad5 697
a0d0e21e
LW
698 \l lowercase next char
699 \u uppercase next char
700 \L lowercase till \E
701 \U uppercase till \E
702 \E end case modification
1d2dff63 703 \Q quote non-word characters till \E
a0d0e21e 704
a034a98d 705If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
423cee85 706and C<\U> is taken from the current locale. See L<perllocale>. For
4a2d328f 707documentation of C<\N{name}>, see L<charnames>.
a034a98d 708
5a964f20
TC
709All systems use the virtual C<"\n"> to represent a line terminator,
710called a "newline". There is no such thing as an unvarying, physical
19799a22 711newline character. It is only an illusion that the operating system,
5a964f20
TC
712device drivers, C libraries, and Perl all conspire to preserve. Not all
713systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
714on a Mac, these are reversed, and on systems without line terminator,
715printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
716you mean a "newline" for your system, but use the literal ASCII when you
717need an exact character. For example, most networking protocols expect
2a380090 718and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
5a964f20
TC
719and although they often accept just C<"\012">, they seldom tolerate just
720C<"\015">. If you get in the habit of using C<"\n"> for networking,
721you may be burned some day.
722
1d2dff63
GS
723You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
724An unescaped C<$> or C<@> interpolates the corresponding variable,
725while escaping will cause the literal string C<\$> to be inserted.
726You'll need to write something like C<m/\Quser\E\@\Qhost/>.
727
a0d0e21e
LW
728Patterns are subject to an additional level of interpretation as a
729regular expression. This is done as a second pass, after variables are
730interpolated, so that regular expressions may be incorporated into the
731pattern from the variables. If this is not what you want, use C<\Q> to
732interpolate a variable literally.
733
19799a22
GS
734Apart from the behavior described above, Perl does not expand
735multiple levels of interpolation. In particular, contrary to the
736expectations of shell programmers, back-quotes do I<NOT> interpolate
737within double quotes, nor do single quotes impede evaluation of
738variables when used within double quotes.
a0d0e21e 739
5f05dabc 740=head2 Regexp Quote-Like Operators
cb1a09d0 741
5f05dabc 742Here are the quote-like operators that apply to pattern
cb1a09d0
AD
743matching and related activities.
744
a0d0e21e
LW
745=over 8
746
747=item ?PATTERN?
748
749This is just like the C</pattern/> search, except that it matches only
750once between calls to the reset() operator. This is a useful
5f05dabc 751optimization when you want to see only the first occurrence of
a0d0e21e
LW
752something in each file of a set of files, for instance. Only C<??>
753patterns local to the current package are reset.
754
5a964f20
TC
755 while (<>) {
756 if (?^$?) {
757 # blank line between header and body
758 }
759 } continue {
760 reset if eof; # clear ?? status for next file
761 }
762
19799a22
GS
763This usage is vaguely depreciated, which means it just might possibly
764be removed in some distant future version of Perl, perhaps somewhere
765around the year 2168.
a0d0e21e 766
fb73857a 767=item m/PATTERN/cgimosx
a0d0e21e 768
fb73857a 769=item /PATTERN/cgimosx
a0d0e21e 770
5a964f20 771Searches a string for a pattern match, and in scalar context returns
19799a22
GS
772true if it succeeds, false if it fails. If no string is specified
773via the C<=~> or C<!~> operator, the $_ string is searched. (The
774string specified with C<=~> need not be an lvalue--it may be the
775result of an expression evaluation, but remember the C<=~> binds
776rather tightly.) See also L<perlre>. See L<perllocale> for
777discussion of additional considerations that apply when C<use locale>
778is in effect.
a0d0e21e
LW
779
780Options are:
781
fb73857a 782 c Do not reset search position on a failed match when /g is in effect.
5f05dabc 783 g Match globally, i.e., find all occurrences.
a0d0e21e
LW
784 i Do case-insensitive pattern matching.
785 m Treat string as multiple lines.
5f05dabc 786 o Compile pattern only once.
a0d0e21e
LW
787 s Treat string as single line.
788 x Use extended regular expressions.
789
790If "/" is the delimiter then the initial C<m> is optional. With the C<m>
01ae956f 791you can use any pair of non-alphanumeric, non-whitespace characters
19799a22
GS
792as delimiters. This is particularly useful for matching path names
793that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
7bac28a0 794the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
19799a22 795If "'" is the delimiter, no interpolation is performed on the PATTERN.
a0d0e21e
LW
796
797PATTERN may contain variables, which will be interpolated (and the
f70b4f9c 798pattern recompiled) every time the pattern search is evaluated, except
1f247705
GS
799for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
800C<$|> are not interpolated because they look like end-of-string tests.)
f70b4f9c
AB
801If you want such a pattern to be compiled only once, add a C</o> after
802the trailing delimiter. This avoids expensive run-time recompilations,
803and is useful when the value you are interpolating won't change over
804the life of the script. However, mentioning C</o> constitutes a promise
805that you won't change the variables in the pattern. If you change them,
13a2d996 806Perl won't even notice. See also L<"qr/STRING/imosx">.
a0d0e21e 807
5a964f20
TC
808If the PATTERN evaluates to the empty string, the last
809I<successfully> matched regular expression is used instead.
a0d0e21e 810
19799a22 811If the C</g> option is not used, C<m//> in list context returns a
a0d0e21e 812list consisting of the subexpressions matched by the parentheses in the
f7e33566
GS
813pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
814also set, and that this differs from Perl 4's behavior.) When there are
815no parentheses in the pattern, the return value is the list C<(1)> for
816success. With or without parentheses, an empty list is returned upon
817failure.
a0d0e21e
LW
818
819Examples:
820
821 open(TTY, '/dev/tty');
822 <TTY> =~ /^y/i && foo(); # do foo if desired
823
824 if (/Version: *([0-9.]*)/) { $version = $1; }
825
826 next if m#^/usr/spool/uucp#;
827
828 # poor man's grep
829 $arg = shift;
830 while (<>) {
831 print if /$arg/o; # compile only once
832 }
833
834 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
835
836This last example splits $foo into the first two words and the
5f05dabc
PP
837remainder of the line, and assigns those three fields to $F1, $F2, and
838$Etc. The conditional is true if any variables were assigned, i.e., if
a0d0e21e
LW
839the pattern matched.
840
19799a22
GS
841The C</g> modifier specifies global pattern matching--that is,
842matching as many times as possible within the string. How it behaves
843depends on the context. In list context, it returns a list of the
844substrings matched by any capturing parentheses in the regular
845expression. If there are no parentheses, it returns a list of all
846the matched strings, as if there were parentheses around the whole
847pattern.
a0d0e21e 848
7e86de3e 849In scalar context, each execution of C<m//g> finds the next match,
19799a22 850returning true if it matches, and false if there is no further match.
7e86de3e
G
851The position after the last match can be read or set using the pos()
852function; see L<perlfunc/pos>. A failed match normally resets the
853search position to the beginning of the string, but you can avoid that
854by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
855string also resets the search position.
c90c0ff4
PP
856
857You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
858zero-width assertion that matches the exact position where the previous
5d43e42d
DC
859C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
860still anchors at pos(), but the match is of course only attempted once.
861Using C<\G> without C</g> on a target string that has not previously had a
862C</g> match applied to it is the same as using the C<\A> assertion to match
863the beginning of the string.
c90c0ff4
PP
864
865Examples:
a0d0e21e
LW
866
867 # list context
868 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
869
870 # scalar context
5d43e42d 871 $/ = "";
19799a22
GS
872 while (defined($paragraph = <>)) {
873 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
874 $sentences++;
a0d0e21e
LW
875 }
876 }
877 print "$sentences\n";
878
c90c0ff4 879 # using m//gc with \G
137443ea 880 $_ = "ppooqppqq";
44a8e56a
PP
881 while ($i++ < 2) {
882 print "1: '";
c90c0ff4 883 print $1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 884 print "2: '";
c90c0ff4 885 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 886 print "3: '";
c90c0ff4 887 print $1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a 888 }
5d43e42d 889 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
44a8e56a
PP
890
891The last example should print:
892
893 1: 'oo', pos=4
137443ea 894 2: 'q', pos=5
44a8e56a
PP
895 3: 'pp', pos=7
896 1: '', pos=7
137443ea
PP
897 2: 'q', pos=8
898 3: '', pos=8
5d43e42d
DC
899 Final: 'q', pos=8
900
901Notice that the final match matched C<q> instead of C<p>, which a match
902without the C<\G> anchor would have done. Also note that the final match
903did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
904final match did indeed match C<p>, it's a good bet that you're running an
905older (pre-5.6.0) Perl.
44a8e56a 906
c90c0ff4 907A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 908combine several regexps like this to process a string part-by-part,
c90c0ff4
PP
909doing different actions depending on which regexp matched. Each
910regexp tries to match where the previous one leaves off.
e7ea3e70 911
3fe9a6f1 912 $_ = <<'EOL';
e7ea3e70 913 $url = new URI::URL "http://www/"; die if $url eq "xXx";
3fe9a6f1
PP
914 EOL
915 LOOP:
e7ea3e70 916 {
c90c0ff4
PP
917 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
918 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
919 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
920 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
921 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
922 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
923 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
e7ea3e70
IZ
924 print ". That's all!\n";
925 }
926
927Here is the output (split into several lines):
928
929 line-noise lowercase line-noise lowercase UPPERCASE line-noise
930 UPPERCASE line-noise lowercase line-noise lowercase line-noise
931 lowercase lowercase line-noise lowercase lowercase line-noise
932 MiXeD line-noise. That's all!
44a8e56a 933
a0d0e21e
LW
934=item q/STRING/
935
936=item C<'STRING'>
937
19799a22 938A single-quoted, literal string. A backslash represents a backslash
68dc0745
PP
939unless followed by the delimiter or another backslash, in which case
940the delimiter or backslash is interpolated.
a0d0e21e
LW
941
942 $foo = q!I said, "You said, 'She said it.'"!;
943 $bar = q('This is it.');
68dc0745 944 $baz = '\n'; # a two-character string
a0d0e21e
LW
945
946=item qq/STRING/
947
948=item "STRING"
949
950A double-quoted, interpolated string.
951
952 $_ .= qq
953 (*** The previous line contains the naughty word "$1".\n)
19799a22 954 if /\b(tcl|java|python)\b/i; # :-)
68dc0745 955 $baz = "\n"; # a one-character string
a0d0e21e 956
eec2d3df
GS
957=item qr/STRING/imosx
958
322edccd 959This operator quotes (and possibly compiles) its I<STRING> as a regular
19799a22
GS
960expression. I<STRING> is interpolated the same way as I<PATTERN>
961in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
962is done. Returns a Perl value which may be used instead of the
963corresponding C</STRING/imosx> expression.
4b6a7270
IZ
964
965For example,
966
967 $rex = qr/my.STRING/is;
968 s/$rex/foo/;
969
970is equivalent to
971
972 s/my.STRING/foo/is;
973
974The result may be used as a subpattern in a match:
eec2d3df
GS
975
976 $re = qr/$pattern/;
0a92e3a8
GS
977 $string =~ /foo${re}bar/; # can be interpolated in other patterns
978 $string =~ $re; # or used standalone
4b6a7270
IZ
979 $string =~ /$re/; # or this way
980
981Since Perl may compile the pattern at the moment of execution of qr()
19799a22 982operator, using qr() may have speed advantages in some situations,
4b6a7270
IZ
983notably if the result of qr() is used standalone:
984
985 sub match {
986 my $patterns = shift;
987 my @compiled = map qr/$_/i, @$patterns;
988 grep {
989 my $success = 0;
a7665c5e 990 foreach my $pat (@compiled) {
4b6a7270
IZ
991 $success = 1, last if /$pat/;
992 }
993 $success;
994 } @_;
995 }
996
19799a22
GS
997Precompilation of the pattern into an internal representation at
998the moment of qr() avoids a need to recompile the pattern every
999time a match C</$pat/> is attempted. (Perl has many other internal
1000optimizations, but none would be triggered in the above example if
1001we did not use qr() operator.)
eec2d3df
GS
1002
1003Options are:
1004
1005 i Do case-insensitive pattern matching.
1006 m Treat string as multiple lines.
1007 o Compile pattern only once.
1008 s Treat string as single line.
1009 x Use extended regular expressions.
1010
0a92e3a8
GS
1011See L<perlre> for additional information on valid syntax for STRING, and
1012for a detailed look at the semantics of regular expressions.
1013
a0d0e21e
LW
1014=item qx/STRING/
1015
1016=item `STRING`
1017
43dd4d21
JH
1018A string which is (possibly) interpolated and then executed as a
1019system command with C</bin/sh> or its equivalent. Shell wildcards,
1020pipes, and redirections will be honored. The collected standard
1021output of the command is returned; standard error is unaffected. In
1022scalar context, it comes back as a single (potentially multi-line)
1023string, or undef if the command failed. In list context, returns a
1024list of lines (however you've defined lines with $/ or
1025$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
5a964f20
TC
1026
1027Because backticks do not affect standard error, use shell file descriptor
1028syntax (assuming the shell supports this) if you care to address this.
1029To capture a command's STDERR and STDOUT together:
a0d0e21e 1030
5a964f20
TC
1031 $output = `cmd 2>&1`;
1032
1033To capture a command's STDOUT but discard its STDERR:
1034
1035 $output = `cmd 2>/dev/null`;
1036
1037To capture a command's STDERR but discard its STDOUT (ordering is
1038important here):
1039
1040 $output = `cmd 2>&1 1>/dev/null`;
1041
1042To exchange a command's STDOUT and STDERR in order to capture the STDERR
1043but leave its STDOUT to come out the old STDERR:
1044
1045 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1046
1047To read both a command's STDOUT and its STDERR separately, it's easiest
1048and safest to redirect them separately to files, and then read from those
1049files when the program is done:
1050
1051 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
1052
1053Using single-quote as a delimiter protects the command from Perl's
1054double-quote interpolation, passing it on to the shell instead:
1055
1056 $perl_info = qx(ps $$); # that's Perl's $$
1057 $shell_info = qx'ps $$'; # that's the new shell's $$
1058
19799a22 1059How that string gets evaluated is entirely subject to the command
5a964f20
TC
1060interpreter on your system. On most platforms, you will have to protect
1061shell metacharacters if you want them treated literally. This is in
1062practice difficult to do, as it's unclear how to escape which characters.
1063See L<perlsec> for a clean and safe example of a manual fork() and exec()
1064to emulate backticks safely.
a0d0e21e 1065
bb32b41a
GS
1066On some platforms (notably DOS-like ones), the shell may not be
1067capable of dealing with multiline commands, so putting newlines in
1068the string may not get you what you want. You may be able to evaluate
1069multiple commands in a single line by separating them with the command
1070separator character, if your shell supports that (e.g. C<;> on many Unix
1071shells; C<&> on the Windows NT C<cmd> shell).
1072
0f897271
GS
1073Beginning with v5.6.0, Perl will attempt to flush all files opened for
1074output before starting the child process, but this may not be supported
1075on some platforms (see L<perlport>). To be safe, you may need to set
1076C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1077C<IO::Handle> on any open handles.
1078
bb32b41a
GS
1079Beware that some command shells may place restrictions on the length
1080of the command line. You must ensure your strings don't exceed this
1081limit after any necessary interpolations. See the platform-specific
1082release notes for more details about your particular environment.
1083
5a964f20
TC
1084Using this operator can lead to programs that are difficult to port,
1085because the shell commands called vary between systems, and may in
1086fact not be present at all. As one example, the C<type> command under
1087the POSIX shell is very different from the C<type> command under DOS.
1088That doesn't mean you should go out of your way to avoid backticks
1089when they're the right way to get something done. Perl was made to be
1090a glue language, and one of the things it glues together is commands.
1091Just understand what you're getting yourself into.
bb32b41a 1092
dc848c6f 1093See L<"I/O Operators"> for more discussion.
a0d0e21e
LW
1094
1095=item qw/STRING/
1096
8127e0e3
GS
1097Evaluates to a list of the words extracted out of STRING, using embedded
1098whitespace as the word delimiters. It can be understood as being roughly
1099equivalent to:
a0d0e21e
LW
1100
1101 split(' ', q/STRING/);
1102
26ef7447
GS
1103the difference being that it generates a real list at compile time. So
1104this expression:
1105
1106 qw(foo bar baz)
1107
c0c5a66b 1108is semantically equivalent to the list:
26ef7447 1109
c0c5a66b 1110 'foo', 'bar', 'baz'
5a964f20 1111
a0d0e21e
LW
1112Some frequently seen examples:
1113
1114 use POSIX qw( setlocale localeconv )
1115 @EXPORT = qw( foo bar baz );
1116
19799a22
GS
1117A common mistake is to try to separate the words with comma or to
1118put comments into a multi-line C<qw>-string. For this reason, the
9f1b1f2d
GS
1119C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1120produces warnings if the STRING contains the "," or the "#" character.
7bac28a0 1121
a0d0e21e
LW
1122=item s/PATTERN/REPLACEMENT/egimosx
1123
1124Searches a string for a pattern, and if found, replaces that pattern
1125with the replacement text and returns the number of substitutions
e37d713d 1126made. Otherwise it returns false (specifically, the empty string).
a0d0e21e
LW
1127
1128If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1129variable is searched and modified. (The string specified with C<=~> must
5a964f20 1130be scalar variable, an array element, a hash element, or an assignment
5f05dabc 1131to one of those, i.e., an lvalue.)
a0d0e21e 1132
19799a22 1133If the delimiter chosen is a single quote, no interpolation is
a0d0e21e
LW
1134done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1135PATTERN contains a $ that looks like a variable rather than an
1136end-of-string test, the variable will be interpolated into the pattern
5f05dabc 1137at run-time. If you want the pattern compiled only once the first time
a0d0e21e 1138the variable is interpolated, use the C</o> option. If the pattern
5a964f20 1139evaluates to the empty string, the last successfully executed regular
a0d0e21e 1140expression is used instead. See L<perlre> for further explanation on these.
5a964f20 1141See L<perllocale> for discussion of additional considerations that apply
a034a98d 1142when C<use locale> is in effect.
a0d0e21e
LW
1143
1144Options are:
1145
1146 e Evaluate the right side as an expression.
5f05dabc 1147 g Replace globally, i.e., all occurrences.
a0d0e21e
LW
1148 i Do case-insensitive pattern matching.
1149 m Treat string as multiple lines.
5f05dabc 1150 o Compile pattern only once.
a0d0e21e
LW
1151 s Treat string as single line.
1152 x Use extended regular expressions.
1153
1154Any non-alphanumeric, non-whitespace delimiter may replace the
1155slashes. If single quotes are used, no interpretation is done on the
e37d713d 1156replacement string (the C</e> modifier overrides this, however). Unlike
54310121 1157Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
e37d713d 1158text is not evaluated as a command. If the
a0d0e21e 1159PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
5f05dabc 1160pair of quotes, which may or may not be bracketing quotes, e.g.,
35f2feb0 1161C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
cec88af6
GS
1162replacement portion to be treated as a full-fledged Perl expression
1163and evaluated right then and there. It is, however, syntax checked at
1164compile-time. A second C<e> modifier will cause the replacement portion
1165to be C<eval>ed before being run as a Perl expression.
a0d0e21e
LW
1166
1167Examples:
1168
1169 s/\bgreen\b/mauve/g; # don't change wintergreen
1170
1171 $path =~ s|/usr/bin|/usr/local/bin|;
1172
1173 s/Login: $foo/Login: $bar/; # run-time pattern
1174
5a964f20 1175 ($foo = $bar) =~ s/this/that/; # copy first, then change
a0d0e21e 1176
5a964f20 1177 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
a0d0e21e
LW
1178
1179 $_ = 'abc123xyz';
1180 s/\d+/$&*2/e; # yields 'abc246xyz'
1181 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1182 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1183
1184 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1185 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1186 s/^=(\w+)/&pod($1)/ge; # use function call
1187
5a964f20
TC
1188 # expand variables in $_, but dynamics only, using
1189 # symbolic dereferencing
1190 s/\$(\w+)/${$1}/g;
1191
cec88af6
GS
1192 # Add one to the value of any numbers in the string
1193 s/(\d+)/1 + $1/eg;
1194
1195 # This will expand any embedded scalar variable
1196 # (including lexicals) in $_ : First $1 is interpolated
1197 # to the variable name, and then evaluated
a0d0e21e
LW
1198 s/(\$\w+)/$1/eeg;
1199
5a964f20 1200 # Delete (most) C comments.
a0d0e21e 1201 $program =~ s {
4633a7c4
LW
1202 /\* # Match the opening delimiter.
1203 .*? # Match a minimal number of characters.
1204 \*/ # Match the closing delimiter.
a0d0e21e
LW
1205 } []gsx;
1206
5a964f20
TC
1207 s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
1208
1209 for ($variable) { # trim white space in $variable, cheap
1210 s/^\s+//;
1211 s/\s+$//;
1212 }
a0d0e21e
LW
1213
1214 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1215
54310121 1216Note the use of $ instead of \ in the last example. Unlike
35f2feb0
GS
1217B<sed>, we use the \<I<digit>> form in only the left hand side.
1218Anywhere else it's $<I<digit>>.
a0d0e21e 1219
5f05dabc 1220Occasionally, you can't use just a C</g> to get all the changes
19799a22 1221to occur that you might want. Here are two common cases:
a0d0e21e
LW
1222
1223 # put commas in the right places in an integer
19799a22 1224 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
a0d0e21e
LW
1225
1226 # expand tabs to 8-column spacing
1227 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1228
6940069f 1229=item tr/SEARCHLIST/REPLACEMENTLIST/cds
a0d0e21e 1230
6940069f 1231=item y/SEARCHLIST/REPLACEMENTLIST/cds
a0d0e21e 1232
2c268ad5 1233Transliterates all occurrences of the characters found in the search list
a0d0e21e
LW
1234with the corresponding character in the replacement list. It returns
1235the number of characters replaced or deleted. If no string is
2c268ad5 1236specified via the =~ or !~ operator, the $_ string is transliterated. (The
54310121
PP
1237string specified with =~ must be a scalar variable, an array element, a
1238hash element, or an assignment to one of those, i.e., an lvalue.)
8ada0baa 1239
2c268ad5
TP
1240A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1241does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121
PP
1242For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1243SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1244its own pair of quotes, which may or may not be bracketing quotes,
2c268ad5 1245e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
a0d0e21e 1246
cc255d5f
JH
1247Note that C<tr> does B<not> do regular expression character classes
1248such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1249the tr(1) utility. If you want to map strings between lower/upper
1250cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1251using the C<s> operator if you need regular expressions.
1252
8ada0baa
JH
1253Note also that the whole range idea is rather unportable between
1254character sets--and even within character sets they may cause results
1255you probably didn't expect. A sound principle is to use only ranges
1256that begin from and end at either alphabets of equal case (a-e, A-E),
1257or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1258character sets in full.
1259
a0d0e21e
LW
1260Options:
1261
1262 c Complement the SEARCHLIST.
1263 d Delete found but unreplaced characters.
1264 s Squash duplicate replaced characters.
1265
19799a22
GS
1266If the C</c> modifier is specified, the SEARCHLIST character set
1267is complemented. If the C</d> modifier is specified, any characters
1268specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1269(Note that this is slightly more flexible than the behavior of some
1270B<tr> programs, which delete anything they find in the SEARCHLIST,
1271period.) If the C</s> modifier is specified, sequences of characters
1272that were transliterated to the same character are squashed down
1273to a single instance of the character.
a0d0e21e
LW
1274
1275If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1276exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1277than the SEARCHLIST, the final character is replicated till it is long
5a964f20 1278enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
a0d0e21e
LW
1279This latter is useful for counting characters in a class or for
1280squashing character sequences in a class.
1281
1282Examples:
1283
1284 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1285
1286 $cnt = tr/*/*/; # count the stars in $_
1287
1288 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1289
1290 $cnt = tr/0-9//; # count the digits in $_
1291
1292 tr/a-zA-Z//s; # bookkeeper -> bokeper
1293
1294 ($HOST = $host) =~ tr/a-z/A-Z/;
1295
1296 tr/a-zA-Z/ /cs; # change non-alphas to single space
1297
1298 tr [\200-\377]
1299 [\000-\177]; # delete 8th bit
1300
19799a22
GS
1301If multiple transliterations are given for a character, only the
1302first one is used:
748a9306
LW
1303
1304 tr/AAA/XYZ/
1305
2c268ad5 1306will transliterate any A to X.
748a9306 1307
19799a22 1308Because the transliteration table is built at compile time, neither
a0d0e21e 1309the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
19799a22
GS
1310interpolation. That means that if you want to use variables, you
1311must use an eval():
a0d0e21e
LW
1312
1313 eval "tr/$oldlist/$newlist/";
1314 die $@ if $@;
1315
1316 eval "tr/$oldlist/$newlist/, 1" or die $@;
1317
1318=back
1319
75e14d17
IZ
1320=head2 Gory details of parsing quoted constructs
1321
19799a22
GS
1322When presented with something that might have several different
1323interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1324principle to pick the most probable interpretation. This strategy
1325is so successful that Perl programmers often do not suspect the
1326ambivalence of what they write. But from time to time, Perl's
1327notions differ substantially from what the author honestly meant.
1328
1329This section hopes to clarify how Perl handles quoted constructs.
1330Although the most common reason to learn this is to unravel labyrinthine
1331regular expressions, because the initial steps of parsing are the
1332same for all quoting operators, they are all discussed together.
1333
1334The most important Perl parsing rule is the first one discussed
1335below: when processing a quoted construct, Perl first finds the end
1336of that construct, then interprets its contents. If you understand
1337this rule, you may skip the rest of this section on the first
1338reading. The other rules are likely to contradict the user's
1339expectations much less frequently than this first one.
1340
1341Some passes discussed below are performed concurrently, but because
1342their results are the same, we consider them individually. For different
1343quoting constructs, Perl performs different numbers of passes, from
1344one to five, but these passes are always performed in the same order.
75e14d17 1345
13a2d996 1346=over 4
75e14d17
IZ
1347
1348=item Finding the end
1349
19799a22
GS
1350The first pass is finding the end of the quoted construct, whether
1351it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1352construct, a C</> that terminates a C<qq//> construct, a C<]> which
35f2feb0
GS
1353terminates C<qq[]> construct, or a C<< > >> which terminates a
1354fileglob started with C<< < >>.
75e14d17 1355
19799a22
GS
1356When searching for single-character non-pairing delimiters, such
1357as C</>, combinations of C<\\> and C<\/> are skipped. However,
1358when searching for single-character pairing delimiter like C<[>,
1359combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1360C<[>, C<]> are skipped as well. When searching for multicharacter
1361delimiters, nothing is skipped.
75e14d17 1362
19799a22
GS
1363For constructs with three-part delimiters (C<s///>, C<y///>, and
1364C<tr///>), the search is repeated once more.
75e14d17 1365
19799a22
GS
1366During this search no attention is paid to the semantics of the construct.
1367Thus:
75e14d17
IZ
1368
1369 "$hash{"$foo/$bar"}"
1370
2a94b7ce 1371or:
75e14d17
IZ
1372
1373 m/
2a94b7ce 1374 bar # NOT a comment, this slash / terminated m//!
75e14d17
IZ
1375 /x
1376
19799a22
GS
1377do not form legal quoted expressions. The quoted part ends on the
1378first C<"> and C</>, and the rest happens to be a syntax error.
1379Because the slash that terminated C<m//> was followed by a C<SPACE>,
1380the example above is not C<m//x>, but rather C<m//> with no C</x>
1381modifier. So the embedded C<#> is interpreted as a literal C<#>.
75e14d17
IZ
1382
1383=item Removal of backslashes before delimiters
1384
19799a22
GS
1385During the second pass, text between the starting and ending
1386delimiters is copied to a safe location, and the C<\> is removed
1387from combinations consisting of C<\> and delimiter--or delimiters,
1388meaning both starting and ending delimiters will should these differ.
1389This removal does not happen for multi-character delimiters.
1390Note that the combination C<\\> is left intact, just as it was.
75e14d17 1391
19799a22
GS
1392Starting from this step no information about the delimiters is
1393used in parsing.
75e14d17
IZ
1394
1395=item Interpolation
1396
19799a22
GS
1397The next step is interpolation in the text obtained, which is now
1398delimiter-independent. There are four different cases.
75e14d17 1399
13a2d996 1400=over 4
75e14d17
IZ
1401
1402=item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1403
1404No interpolation is performed.
1405
1406=item C<''>, C<q//>
1407
1408The only interpolation is removal of C<\> from pairs C<\\>.
1409
35f2feb0 1410=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
75e14d17 1411
19799a22
GS
1412C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1413converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1414is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1415The other combinations are replaced with appropriate expansions.
2a94b7ce 1416
19799a22
GS
1417Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1418is interpolated in the usual way. Something like C<"\Q\\E"> has
1419no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1420result is the same as for C<"\\\\E">. As a general rule, backslashes
1421between C<\Q> and C<\E> may lead to counterintuitive results. So,
1422C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1423as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2a94b7ce
IZ
1424
1425 $str = '\t';
1426 return "\Q$str";
1427
1428may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1429
19799a22 1430Interpolated scalars and arrays are converted internally to the C<join> and
92d29cee 1431C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
75e14d17 1432
19799a22 1433 $foo . " XXX '" . (join $", @arr) . "'";
75e14d17 1434
19799a22 1435All operations above are performed simultaneously, left to right.
75e14d17 1436
19799a22
GS
1437Because the result of C<"\Q STRING \E"> has all metacharacters
1438quoted, there is no way to insert a literal C<$> or C<@> inside a
1439C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1440C<"\\\$">; if not, it is interpreted as the start of an interpolated
1441scalar.
75e14d17 1442
19799a22
GS
1443Note also that the interpolation code needs to make a decision on
1444where the interpolated scalar ends. For instance, whether
35f2feb0 1445C<< "a $b -> {c}" >> really means:
75e14d17
IZ
1446
1447 "a " . $b . " -> {c}";
1448
2a94b7ce 1449or:
75e14d17
IZ
1450
1451 "a " . $b -> {c};
1452
19799a22
GS
1453Most of the time, the longest possible text that does not include
1454spaces between components and which contains matching braces or
1455brackets. because the outcome may be determined by voting based
1456on heuristic estimators, the result is not strictly predictable.
1457Fortunately, it's usually correct for ambiguous cases.
75e14d17
IZ
1458
1459=item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1460
19799a22
GS
1461Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1462happens (almost) as with C<qq//> constructs, but the substitution
1463of C<\> followed by RE-special chars (including C<\>) is not
1464performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1465a C<#>-comment in a C<//x>-regular expression, no processing is
1466performed whatsoever. This is the first step at which the presence
1467of the C<//x> modifier is relevant.
1468
1469Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1470interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1471different estimators) to be either an array element or C<$var>
1472followed by an RE alternative. This is where the notation
1473C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1474array element C<-9>, not as a regular expression from the variable
1475C<$arr> followed by a digit, which would be the interpretation of
1476C</$arr[0-9]/>. Since voting among different estimators may occur,
1477the result is not predictable.
1478
1479It is at this step that C<\1> is begrudgingly converted to C<$1> in
1480the replacement text of C<s///> to correct the incorrigible
1481I<sed> hackers who haven't picked up the saner idiom yet. A warning
9f1b1f2d
GS
1482is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1483(that is, the C<$^W> variable) was set.
19799a22
GS
1484
1485The lack of processing of C<\\> creates specific restrictions on
1486the post-processed text. If the delimiter is C</>, one cannot get
1487the combination C<\/> into the result of this step. C</> will
1488finish the regular expression, C<\/> will be stripped to C</> on
1489the previous step, and C<\\/> will be left as is. Because C</> is
1490equivalent to C<\/> inside a regular expression, this does not
1491matter unless the delimiter happens to be character special to the
1492RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1493alphanumeric char, as in:
2a94b7ce
IZ
1494
1495 m m ^ a \s* b mmx;
1496
19799a22 1497In the RE above, which is intentionally obfuscated for illustration, the
2a94b7ce 1498delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
19799a22
GS
1499RE is the same as for C<m/ ^ a s* b /mx>). There's more than one
1500reason you're encouraged to restrict your delimiters to non-alphanumeric,
1501non-whitespace choices.
75e14d17
IZ
1502
1503=back
1504
19799a22 1505This step is the last one for all constructs except regular expressions,
75e14d17
IZ
1506which are processed further.
1507
1508=item Interpolation of regular expressions
1509
19799a22
GS
1510Previous steps were performed during the compilation of Perl code,
1511but this one happens at run time--although it may be optimized to
1512be calculated at compile time if appropriate. After preprocessing
1513described above, and possibly after evaluation if catenation,
1514joining, casing translation, or metaquoting are involved, the
1515resulting I<string> is passed to the RE engine for compilation.
1516
1517Whatever happens in the RE engine might be better discussed in L<perlre>,
1518but for the sake of continuity, we shall do so here.
1519
1520This is another step where the presence of the C<//x> modifier is
1521relevant. The RE engine scans the string from left to right and
1522converts it to a finite automaton.
1523
1524Backslashed characters are either replaced with corresponding
1525literal strings (as with C<\{>), or else they generate special nodes
1526in the finite automaton (as with C<\b>). Characters special to the
1527RE engine (such as C<|>) generate corresponding nodes or groups of
1528nodes. C<(?#...)> comments are ignored. All the rest is either
1529converted to literal strings to match, or else is ignored (as is
1530whitespace and C<#>-style comments if C<//x> is present).
1531
1532Parsing of the bracketed character class construct, C<[...]>, is
1533rather different than the rule used for the rest of the pattern.
1534The terminator of this construct is found using the same rules as
1535for finding the terminator of a C<{}>-delimited construct, the only
1536exception being that C<]> immediately following C<[> is treated as
1537though preceded by a backslash. Similarly, the terminator of
1538C<(?{...})> is found using the same rules as for finding the
1539terminator of a C<{}>-delimited construct.
1540
1541It is possible to inspect both the string given to RE engine and the
1542resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1543in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
4a4eefd0 1544switch documented in L<perlrun/"Command Switches">.
75e14d17
IZ
1545
1546=item Optimization of regular expressions
1547
7522fed5 1548This step is listed for completeness only. Since it does not change
75e14d17 1549semantics, details of this step are not documented and are subject
19799a22
GS
1550to change without notice. This step is performed over the finite
1551automaton that was generated during the previous pass.
2a94b7ce 1552
19799a22
GS
1553It is at this stage that C<split()> silently optimizes C</^/> to
1554mean C</^/m>.
75e14d17
IZ
1555
1556=back
1557
a0d0e21e
LW
1558=head2 I/O Operators
1559
54310121 1560There are several I/O operators you should know about.
fbad3eb5 1561
7b8d334a 1562A string enclosed by backticks (grave accents) first undergoes
19799a22
GS
1563double-quote interpolation. It is then interpreted as an external
1564command, and the output of that command is the value of the
1565pseudo-literal, j
1566string consisting of all output is returned. In list context, a
1567list of values is returned, one per line of output. (You can set
1568C<$/> to use a different line terminator.) The command is executed
a0d0e21e
LW
1569each time the pseudo-literal is evaluated. The status value of the
1570command is returned in C<$?> (see L<perlvar> for the interpretation
1571of C<$?>). Unlike in B<csh>, no translation is done on the return
1572data--newlines remain newlines. Unlike in any of the shells, single
1573quotes do not hide variable names in the command from interpretation.
19799a22
GS
1574To pass a literal dollar-sign through to the shell you need to hide
1575it with a backslash. The generalized form of backticks is C<qx//>.
1576(Because backticks always undergo shell expansion as well, see
1577L<perlsec> for security concerns.)
1578
1579In scalar context, evaluating a filehandle in angle brackets yields
1580the next line from that file (the newline, if any, included), or
1581C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1582(sometimes known as file-slurp mode) and the file is empty, it
1583returns C<''> the first time, followed by C<undef> subsequently.
1584
1585Ordinarily you must assign the returned value to a variable, but
1586there is one situation where an automatic assignment happens. If
1587and only if the input symbol is the only thing inside the conditional
1588of a C<while> statement (even if disguised as a C<for(;;)> loop),
1589the value is automatically assigned to the global variable $_,
1590destroying whatever was there previously. (This may seem like an
1591odd thing to you, but you'll use the construct in almost every Perl
1592script you write.) The $_ variables is not implicitly localized.
1593You'll have to put a C<local $_;> before the loop if you want that
1594to happen.
1595
1596The following lines are equivalent:
a0d0e21e 1597
748a9306 1598 while (defined($_ = <STDIN>)) { print; }
7b8d334a 1599 while ($_ = <STDIN>) { print; }
a0d0e21e
LW
1600 while (<STDIN>) { print; }
1601 for (;<STDIN>;) { print; }
748a9306 1602 print while defined($_ = <STDIN>);
7b8d334a 1603 print while ($_ = <STDIN>);
a0d0e21e
LW
1604 print while <STDIN>;
1605
19799a22 1606This also behaves similarly, but avoids $_ :
7b8d334a
GS
1607
1608 while (my $line = <STDIN>) { print $line }
1609
19799a22
GS
1610In these loop constructs, the assigned value (whether assignment
1611is automatic or explicit) is then tested to see whether it is
1612defined. The defined test avoids problems where line has a string
1613value that would be treated as false by Perl, for example a "" or
1614a "0" with no trailing newline. If you really mean for such values
1615to terminate the loop, they should be tested for explicitly:
7b8d334a
GS
1616
1617 while (($_ = <STDIN>) ne '0') { ... }
1618 while (<STDIN>) { last unless $_; ... }
1619
35f2feb0 1620In other boolean contexts, C<< <I<filehandle>> >> without an
9f1b1f2d
GS
1621explicit C<defined> test or comparison elicit a warning if the
1622C<use warnings> pragma or the B<-w>
19799a22 1623command-line switch (the C<$^W> variable) is in effect.
7b8d334a 1624
5f05dabc 1625The filehandles STDIN, STDOUT, and STDERR are predefined. (The
19799a22
GS
1626filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
1627in packages, where they would be interpreted as local identifiers
1628rather than global.) Additional filehandles may be created with
1629the open() function, amongst others. See L<perlopentut> and
1630L<perlfunc/open> for details on this.
a0d0e21e 1631
35f2feb0 1632If a <FILEHANDLE> is used in a context that is looking for
19799a22
GS
1633a list, a list comprising all input lines is returned, one line per
1634list element. It's easy to grow to a rather large data space this
1635way, so use with care.
a0d0e21e 1636
35f2feb0 1637<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
19799a22 1638See L<perlfunc/readline>.
fbad3eb5 1639
35f2feb0
GS
1640The null filehandle <> is special: it can be used to emulate the
1641behavior of B<sed> and B<awk>. Input from <> comes either from
a0d0e21e 1642standard input, or from each file listed on the command line. Here's
35f2feb0 1643how it works: the first time <> is evaluated, the @ARGV array is
5a964f20 1644checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
a0d0e21e
LW
1645gives you standard input. The @ARGV array is then processed as a list
1646of filenames. The loop
1647
1648 while (<>) {
1649 ... # code for each line
1650 }
1651
1652is equivalent to the following Perl-like pseudo code:
1653
3e3baf6d 1654 unshift(@ARGV, '-') unless @ARGV;
a0d0e21e
LW
1655 while ($ARGV = shift) {
1656 open(ARGV, $ARGV);
1657 while (<ARGV>) {
1658 ... # code for each line
1659 }
1660 }
1661
19799a22
GS
1662except that it isn't so cumbersome to say, and will actually work.
1663It really does shift the @ARGV array and put the current filename
1664into the $ARGV variable. It also uses filehandle I<ARGV>
35f2feb0 1665internally--<> is just a synonym for <ARGV>, which
19799a22 1666is magical. (The pseudo code above doesn't work because it treats
35f2feb0 1667<ARGV> as non-magical.)
a0d0e21e 1668
35f2feb0 1669You can modify @ARGV before the first <> as long as the array ends up
a0d0e21e 1670containing the list of filenames you really want. Line numbers (C<$.>)
19799a22
GS
1671continue as though the input were one big happy file. See the example
1672in L<perlfunc/eof> for how to reset line numbers on each file.
5a964f20
TC
1673
1674If you want to set @ARGV to your own list of files, go right ahead.
1675This sets @ARGV to all plain text files if no @ARGV was given:
1676
1677 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
a0d0e21e 1678
5a964f20
TC
1679You can even set them to pipe commands. For example, this automatically
1680filters compressed arguments through B<gzip>:
1681
1682 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1683
1684If you want to pass switches into your script, you can use one of the
a0d0e21e
LW
1685Getopts modules or put a loop on the front like this:
1686
1687 while ($_ = $ARGV[0], /^-/) {
1688 shift;
1689 last if /^--$/;
1690 if (/^-D(.*)/) { $debug = $1 }
1691 if (/^-v/) { $verbose++ }
5a964f20 1692 # ... # other switches
a0d0e21e 1693 }
5a964f20 1694
a0d0e21e 1695 while (<>) {
5a964f20 1696 # ... # code for each line
a0d0e21e
LW
1697 }
1698
35f2feb0 1699The <> symbol will return C<undef> for end-of-file only once.
19799a22
GS
1700If you call it again after this, it will assume you are processing another
1701@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
a0d0e21e 1702
19799a22 1703If angle brackets contain is a simple scalar variable (e.g.,
35f2feb0 1704<$foo>), then that variable contains the name of the
19799a22
GS
1705filehandle to input from, or its typeglob, or a reference to the
1706same. For example:
cb1a09d0
AD
1707
1708 $fh = \*STDIN;
1709 $line = <$fh>;
a0d0e21e 1710
5a964f20
TC
1711If what's within the angle brackets is neither a filehandle nor a simple
1712scalar variable containing a filehandle name, typeglob, or typeglob
1713reference, it is interpreted as a filename pattern to be globbed, and
1714either a list of filenames or the next filename in the list is returned,
19799a22 1715depending on context. This distinction is determined on syntactic
35f2feb0
GS
1716grounds alone. That means C<< <$x> >> is always a readline() from
1717an indirect handle, but C<< <$hash{key}> >> is always a glob().
5a964f20
TC
1718That's because $x is a simple scalar variable, but C<$hash{key}> is
1719not--it's a hash element.
1720
1721One level of double-quote interpretation is done first, but you can't
35f2feb0 1722say C<< <$foo> >> because that's an indirect filehandle as explained
5a964f20
TC
1723in the previous paragraph. (In older versions of Perl, programmers
1724would insert curly brackets to force interpretation as a filename glob:
35f2feb0 1725C<< <${foo}> >>. These days, it's considered cleaner to call the
5a964f20 1726internal function directly as C<glob($foo)>, which is probably the right
19799a22 1727way to have done it in the first place.) For example:
a0d0e21e
LW
1728
1729 while (<*.c>) {
1730 chmod 0644, $_;
1731 }
1732
3a4b19e4 1733is roughly equivalent to:
a0d0e21e
LW
1734
1735 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1736 while (<FOO>) {
1737 chop;
1738 chmod 0644, $_;
1739 }
1740
3a4b19e4
GS
1741except that the globbing is actually done internally using the standard
1742C<File::Glob> extension. Of course, the shortest way to do the above is:
a0d0e21e
LW
1743
1744 chmod 0644, <*.c>;
1745
19799a22
GS
1746A (file)glob evaluates its (embedded) argument only when it is
1747starting a new list. All values must be read before it will start
1748over. In list context, this isn't important because you automatically
1749get them all anyway. However, in scalar context the operator returns
069e01df 1750the next value each time it's called, or C<undef> when the list has
19799a22
GS
1751run out. As with filehandle reads, an automatic C<defined> is
1752generated when the glob occurs in the test part of a C<while>,
1753because legal glob returns (e.g. a file called F<0>) would otherwise
1754terminate the loop. Again, C<undef> is returned only once. So if
1755you're expecting a single value from a glob, it is much better to
1756say
4633a7c4
LW
1757
1758 ($file) = <blurch*>;
1759
1760than
1761
1762 $file = <blurch*>;
1763
1764because the latter will alternate between returning a filename and
19799a22 1765returning false.
4633a7c4
LW
1766
1767It you're trying to do variable interpolation, it's definitely better
1768to use the glob() function, because the older notation can cause people
e37d713d 1769to become confused with the indirect filehandle notation.
4633a7c4
LW
1770
1771 @files = glob("$dir/*.[ch]");
1772 @files = glob($files[$i]);
1773
a0d0e21e
LW
1774=head2 Constant Folding
1775
1776Like C, Perl does a certain amount of expression evaluation at
19799a22 1777compile time whenever it determines that all arguments to an
a0d0e21e
LW
1778operator are static and have no side effects. In particular, string
1779concatenation happens at compile time between literals that don't do
19799a22 1780variable substitution. Backslash interpolation also happens at
a0d0e21e
LW
1781compile time. You can say
1782
1783 'Now is the time for all' . "\n" .
1784 'good men to come to.'
1785
54310121 1786and this all reduces to one string internally. Likewise, if
a0d0e21e
LW
1787you say
1788
1789 foreach $file (@filenames) {
5a964f20 1790 if (-s $file > 5 + 100 * 2**16) { }
54310121 1791 }
a0d0e21e 1792
19799a22
GS
1793the compiler will precompute the number which that expression
1794represents so that the interpreter won't have to.
a0d0e21e 1795
2c268ad5
TP
1796=head2 Bitwise String Operators
1797
1798Bitstrings of any size may be manipulated by the bitwise operators
1799(C<~ | & ^>).
1800
19799a22
GS
1801If the operands to a binary bitwise op are strings of different
1802sizes, B<|> and B<^> ops act as though the shorter operand had
1803additional zero bits on the right, while the B<&> op acts as though
1804the longer operand were truncated to the length of the shorter.
1805The granularity for such extension or truncation is one or more
1806bytes.
2c268ad5
TP
1807
1808 # ASCII-based examples
1809 print "j p \n" ^ " a h"; # prints "JAPH\n"
1810 print "JA" | " ph\n"; # prints "japh\n"
1811 print "japh\nJunk" & '_____'; # prints "JAPH\n";
1812 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
1813
19799a22 1814If you are intending to manipulate bitstrings, be certain that
2c268ad5 1815you're supplying bitstrings: If an operand is a number, that will imply
19799a22 1816a B<numeric> bitwise operation. You may explicitly show which type of
2c268ad5
TP
1817operation you intend by using C<""> or C<0+>, as in the examples below.
1818
1819 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
1820 $foo = '150' | 105 ; # yields 255
1821 $foo = 150 | '105'; # yields 255
1822 $foo = '150' | '105'; # yields string '155' (under ASCII)
1823
1824 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
1825 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
a0d0e21e 1826
1ae175c8
GS
1827See L<perlfunc/vec> for information on how to manipulate individual bits
1828in a bit vector.
1829
55497cff 1830=head2 Integer Arithmetic
a0d0e21e 1831
19799a22 1832By default, Perl assumes that it must do most of its arithmetic in
a0d0e21e
LW
1833floating point. But by saying
1834
1835 use integer;
1836
1837you may tell the compiler that it's okay to use integer operations
19799a22
GS
1838(if it feels like it) from here to the end of the enclosing BLOCK.
1839An inner BLOCK may countermand this by saying
a0d0e21e
LW
1840
1841 no integer;
1842
19799a22
GS
1843which lasts until the end of that BLOCK. Note that this doesn't
1844mean everything is only an integer, merely that Perl may use integer
1845operations if it is so inclined. For example, even under C<use
1846integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
1847or so.
1848
1849Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
13a2d996
SP
1850and ">>") always produce integral results. (But see also
1851L<Bitwise String Operators>.) However, C<use integer> still has meaning for
19799a22
GS
1852them. By default, their results are interpreted as unsigned integers, but
1853if C<use integer> is in effect, their results are interpreted
1854as signed integers. For example, C<~0> usually evaluates to a large
1855integral value. However, C<use integer; ~0> is C<-1> on twos-complement
1856machines.
68dc0745
PP
1857
1858=head2 Floating-point Arithmetic
1859
1860While C<use integer> provides integer-only arithmetic, there is no
19799a22
GS
1861analogous mechanism to provide automatic rounding or truncation to a
1862certain number of decimal places. For rounding to a certain number
1863of digits, sprintf() or printf() is usually the easiest route.
1864See L<perlfaq4>.
68dc0745 1865
5a964f20
TC
1866Floating-point numbers are only approximations to what a mathematician
1867would call real numbers. There are infinitely more reals than floats,
1868so some corners must be cut. For example:
1869
1870 printf "%.20g\n", 123456789123456789;
1871 # produces 123456789123456784
1872
1873Testing for exact equality of floating-point equality or inequality is
1874not a good idea. Here's a (relatively expensive) work-around to compare
1875whether two floating-point numbers are equal to a particular number of
1876decimal places. See Knuth, volume II, for a more robust treatment of
1877this topic.
1878
1879 sub fp_equal {
1880 my ($X, $Y, $POINTS) = @_;
1881 my ($tX, $tY);
1882 $tX = sprintf("%.${POINTS}g", $X);
1883 $tY = sprintf("%.${POINTS}g", $Y);
1884 return $tX eq $tY;
1885 }
1886
68dc0745 1887The POSIX module (part of the standard perl distribution) implements
19799a22
GS
1888ceil(), floor(), and other mathematical and trigonometric functions.
1889The Math::Complex module (part of the standard perl distribution)
1890defines mathematical functions that work on both the reals and the
1891imaginary numbers. Math::Complex not as efficient as POSIX, but
68dc0745
PP
1892POSIX can't work with complex numbers.
1893
1894Rounding in financial applications can have serious implications, and
1895the rounding method used should be specified precisely. In these
1896cases, it probably pays not to trust whichever system rounding is
1897being used by Perl, but to instead implement the rounding function you
1898need yourself.
5a964f20
TC
1899
1900=head2 Bigger Numbers
1901
1902The standard Math::BigInt and Math::BigFloat modules provide
19799a22
GS
1903variable-precision arithmetic and overloaded operators, although
1904they're currently pretty slow. At the cost of some space and
1905considerable speed, they avoid the normal pitfalls associated with
1906limited-precision representations.
5a964f20
TC
1907
1908 use Math::BigInt;
1909 $x = Math::BigInt->new('123456789123456789');
1910 print $x * $x;
1911
1912 # prints +15241578780673678515622620750190521
19799a22
GS
1913
1914The non-standard modules SSLeay::BN and Math::Pari provide
1915equivalent functionality (and much more) with a substantial
1916performance savings.
16070b82
GS
1917
1918=cut