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Re: Exceptions in IPC::Open2
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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,
8listed from highest precedence to lowest. Note that all operators
9borrowed from C keep the same precedence relationship with each other,
10even where C's precedence is slightly screwy. (This makes learning
54310121 11Perl easier for C folks.) With very few exceptions, these all
c07a80fd 12operate on scalar values 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
cb1a09d0 41=head1 DESCRIPTION
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42
43=head2 Terms and List Operators (Leftward)
44
54310121 45A TERM has the highest precedence in Perl. They includes variables,
5f05dabc 46quote and quote-like operators, any expression in parentheses,
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47and any function whose arguments are parenthesized. Actually, there
48aren't really functions in this sense, just list operators and unary
49operators behaving as functions because you put parentheses around
50the arguments. These are all documented in L<perlfunc>.
51
52If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
53is followed by a left parenthesis as the next token, the operator and
54arguments within parentheses are taken to be of highest precedence,
55just like a normal function call.
56
57In the absence of parentheses, the precedence of list operators such as
58C<print>, C<sort>, or C<chmod> is either very high or very low depending on
54310121 59whether you are looking at the left side or the right side of the operator.
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60For example, in
61
62 @ary = (1, 3, sort 4, 2);
63 print @ary; # prints 1324
64
65the commas on the right of the sort are evaluated before the sort, but
66the commas on the left are evaluated after. In other words, list
67operators tend to gobble up all the arguments that follow them, and
68then act like a simple TERM with regard to the preceding expression.
5f05dabc 69Note that you have to be careful with parentheses:
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70
71 # These evaluate exit before doing the print:
72 print($foo, exit); # Obviously not what you want.
73 print $foo, exit; # Nor is this.
74
75 # These do the print before evaluating exit:
76 (print $foo), exit; # This is what you want.
77 print($foo), exit; # Or this.
78 print ($foo), exit; # Or even this.
79
80Also note that
81
82 print ($foo & 255) + 1, "\n";
83
54310121 84probably doesn't do what you expect at first glance. See
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85L<Named Unary Operators> for more discussion of this.
86
87Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
54310121 88well as subroutine and method calls, and the anonymous
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89constructors C<[]> and C<{}>.
90
2ae324a7 91See also L<Quote and Quote-like Operators> toward the end of this section,
c07a80fd 92as well as L<"I/O Operators">.
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93
94=head2 The Arrow Operator
95
96Just as in C and C++, "C<-E<gt>>" is an infix dereference operator. If the
97right side is either a C<[...]> or C<{...}> subscript, then the left side
98must be either a hard or symbolic reference to an array or hash (or
99a location capable of holding a hard reference, if it's an lvalue (assignable)).
100See L<perlref>.
101
102Otherwise, the right side is a method name or a simple scalar variable
103containing the method name, and the left side must either be an object
104(a blessed reference) or a class name (that is, a package name).
105See L<perlobj>.
106
5f05dabc 107=head2 Auto-increment and Auto-decrement
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108
109"++" and "--" work as in C. That is, if placed before a variable, they
110increment or decrement the variable before returning the value, and if
111placed after, increment or decrement the variable after returning the value.
112
54310121 113The auto-increment operator has a little extra builtin magic to it. If
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114you increment a variable that is numeric, or that has ever been used in
115a numeric context, you get a normal increment. If, however, the
5f05dabc 116variable has been used in only string contexts since it was set, and
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117has a value that is not null and matches the pattern
118C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each
119character within its range, with carry:
120
121 print ++($foo = '99'); # prints '100'
122 print ++($foo = 'a0'); # prints 'a1'
123 print ++($foo = 'Az'); # prints 'Ba'
124 print ++($foo = 'zz'); # prints 'aaa'
125
5f05dabc 126The auto-decrement operator is not magical.
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127
128=head2 Exponentiation
129
130Binary "**" is the exponentiation operator. Note that it binds even more
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131tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
132implemented using C's pow(3) function, which actually works on doubles
133internally.)
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134
135=head2 Symbolic Unary Operators
136
5f05dabc 137Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
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138precedence version of this.
139
140Unary "-" performs arithmetic negation if the operand is numeric. If
141the operand is an identifier, a string consisting of a minus sign
142concatenated with the identifier is returned. Otherwise, if the string
143starts with a plus or minus, a string starting with the opposite sign
144is returned. One effect of these rules is that C<-bareword> is equivalent
145to C<"-bareword">.
146
5f05dabc 147Unary "~" performs bitwise negation, i.e., 1's complement.
2c268ad5 148(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
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149
150Unary "+" has no effect whatsoever, even on strings. It is useful
151syntactically for separating a function name from a parenthesized expression
152that would otherwise be interpreted as the complete list of function
5ba421f6 153arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
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154
155Unary "\" creates a reference to whatever follows it. See L<perlref>.
156Do not confuse this behavior with the behavior of backslash within a
157string, although both forms do convey the notion of protecting the next
158thing from interpretation.
159
160=head2 Binding Operators
161
c07a80fd 162Binary "=~" binds a scalar expression to a pattern match. Certain operations
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163search or modify the string $_ by default. This operator makes that kind
164of operation work on some other string. The right argument is a search
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165pattern, substitution, or transliteration. The left argument is what is
166supposed to be searched, substituted, or transliterated instead of the default
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167$_. The return value indicates the success of the operation. (If the
168right argument is an expression rather than a search pattern,
2c268ad5 169substitution, or transliteration, it is interpreted as a search pattern at run
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170time. This can be is less efficient than an explicit search, because the
171pattern must be compiled every time the expression is evaluated.
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172
173Binary "!~" is just like "=~" except the return value is negated in
174the logical sense.
175
176=head2 Multiplicative Operators
177
178Binary "*" multiplies two numbers.
179
180Binary "/" divides two numbers.
181
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182Binary "%" computes the modulus of two numbers. Given integer
183operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
184C<$a> minus the largest multiple of C<$b> that is not greater than
185C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
186smallest multiple of C<$b> that is not less than C<$a> (i.e. the
187result will be less than or equal to zero).
a0d0e21e 188
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189Note than when C<use integer> is in scope "%" give you direct access
190to the modulus operator as implemented by your C compiler. This
191operator is not as well defined for negative operands, but it will
192execute faster.
193
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194Binary "x" is the repetition operator. In a scalar context, it
195returns a string consisting of the left operand repeated the number of
196times specified by the right operand. In a list context, if the left
5f05dabc 197operand is a list in parentheses, it repeats the list.
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198
199 print '-' x 80; # print row of dashes
200
201 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
202
203 @ones = (1) x 80; # a list of 80 1's
204 @ones = (5) x @ones; # set all elements to 5
205
206
207=head2 Additive Operators
208
209Binary "+" returns the sum of two numbers.
210
211Binary "-" returns the difference of two numbers.
212
213Binary "." concatenates two strings.
214
215=head2 Shift Operators
216
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217Binary "<<" returns the value of its left argument shifted left by the
218number of bits specified by the right argument. Arguments should be
219integers. (See also L<Integer Arithmetic>.)
a0d0e21e 220
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221Binary ">>" returns the value of its left argument shifted right by
222the number of bits specified by the right argument. Arguments should
223be integers. (See also L<Integer Arithmetic>.)
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224
225=head2 Named Unary Operators
226
227The various named unary operators are treated as functions with one
228argument, with optional parentheses. These include the filetest
229operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
230
231If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
232is followed by a left parenthesis as the next token, the operator and
233arguments within parentheses are taken to be of highest precedence,
234just like a normal function call. Examples:
235
236 chdir $foo || die; # (chdir $foo) || die
237 chdir($foo) || die; # (chdir $foo) || die
238 chdir ($foo) || die; # (chdir $foo) || die
239 chdir +($foo) || die; # (chdir $foo) || die
240
241but, because * is higher precedence than ||:
242
243 chdir $foo * 20; # chdir ($foo * 20)
244 chdir($foo) * 20; # (chdir $foo) * 20
245 chdir ($foo) * 20; # (chdir $foo) * 20
246 chdir +($foo) * 20; # chdir ($foo * 20)
247
248 rand 10 * 20; # rand (10 * 20)
249 rand(10) * 20; # (rand 10) * 20
250 rand (10) * 20; # (rand 10) * 20
251 rand +(10) * 20; # rand (10 * 20)
252
5ba421f6 253See also L<"Terms and List Operators (Leftward)">.
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254
255=head2 Relational Operators
256
6ee5d4e7 257Binary "E<lt>" returns true if the left argument is numerically less than
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258the right argument.
259
6ee5d4e7 260Binary "E<gt>" returns true if the left argument is numerically greater
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261than the right argument.
262
6ee5d4e7 263Binary "E<lt>=" returns true if the left argument is numerically less than
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264or equal to the right argument.
265
6ee5d4e7 266Binary "E<gt>=" returns true if the left argument is numerically greater
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267than or equal to the right argument.
268
269Binary "lt" returns true if the left argument is stringwise less than
270the right argument.
271
272Binary "gt" returns true if the left argument is stringwise greater
273than the right argument.
274
275Binary "le" returns true if the left argument is stringwise less than
276or equal to the right argument.
277
278Binary "ge" returns true if the left argument is stringwise greater
279than or equal to the right argument.
280
281=head2 Equality Operators
282
283Binary "==" returns true if the left argument is numerically equal to
284the right argument.
285
286Binary "!=" returns true if the left argument is numerically not equal
287to the right argument.
288
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289Binary "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left
290argument is numerically less than, equal to, or greater than the right
291argument.
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292
293Binary "eq" returns true if the left argument is stringwise equal to
294the right argument.
295
296Binary "ne" returns true if the left argument is stringwise not equal
297to the right argument.
298
299Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise
300less than, equal to, or greater than the right argument.
301
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302"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
303by the current locale if C<use locale> is in effect. See L<perllocale>.
304
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305=head2 Bitwise And
306
307Binary "&" returns its operators ANDed together bit by bit.
2c268ad5 308(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
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309
310=head2 Bitwise Or and Exclusive Or
311
312Binary "|" returns its operators ORed together bit by bit.
2c268ad5 313(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
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314
315Binary "^" returns its operators XORed together bit by bit.
2c268ad5 316(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
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317
318=head2 C-style Logical And
319
320Binary "&&" performs a short-circuit logical AND operation. That is,
321if the left operand is false, the right operand is not even evaluated.
322Scalar or list context propagates down to the right operand if it
323is evaluated.
324
325=head2 C-style Logical Or
326
327Binary "||" performs a short-circuit logical OR operation. That is,
328if the left operand is true, the right operand is not even evaluated.
329Scalar or list context propagates down to the right operand if it
330is evaluated.
331
332The C<||> and C<&&> operators differ from C's in that, rather than returning
3330 or 1, they return the last value evaluated. Thus, a reasonably portable
334way to find out the home directory (assuming it's not "0") might be:
335
336 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
337 (getpwuid($<))[7] || die "You're homeless!\n";
338
339As more readable alternatives to C<&&> and C<||>, Perl provides "and" and
340"or" operators (see below). The short-circuit behavior is identical. The
341precedence of "and" and "or" is much lower, however, so that you can
342safely use them after a list operator without the need for
343parentheses:
344
345 unlink "alpha", "beta", "gamma"
346 or gripe(), next LINE;
347
348With the C-style operators that would have been written like this:
349
350 unlink("alpha", "beta", "gamma")
351 || (gripe(), next LINE);
352
353=head2 Range Operator
354
355Binary ".." is the range operator, which is really two different
356operators depending on the context. In a list context, it returns an
357array of values counting (by ones) from the left value to the right
358value. This is useful for writing C<for (1..10)> loops and for doing
359slice operations on arrays. Be aware that under the current implementation,
54310121 360a temporary array is created, so you'll burn a lot of memory if you
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361write something like this:
362
363 for (1 .. 1_000_000) {
364 # code
54310121 365 }
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366
367In a scalar context, ".." returns a boolean value. The operator is
368bistable, like a flip-flop, and emulates the line-range (comma) operator
369of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
370own boolean state. It is false as long as its left operand is false.
371Once the left operand is true, the range operator stays true until the
372right operand is true, I<AFTER> which the range operator becomes false
373again. (It doesn't become false till the next time the range operator is
374evaluated. It can test the right operand and become false on the same
375evaluation it became true (as in B<awk>), but it still returns true once.
376If you don't want it to test the right operand till the next evaluation
377(as in B<sed>), use three dots ("...") instead of two.) The right
378operand is not evaluated while the operator is in the "false" state, and
379the left operand is not evaluated while the operator is in the "true"
380state. The precedence is a little lower than || and &&. The value
381returned is either the null string for false, or a sequence number
382(beginning with 1) for true. The sequence number is reset for each range
383encountered. The final sequence number in a range has the string "E0"
384appended to it, which doesn't affect its numeric value, but gives you
385something to search for if you want to exclude the endpoint. You can
386exclude the beginning point by waiting for the sequence number to be
387greater than 1. If either operand of scalar ".." is a numeric literal,
388that operand is implicitly compared to the C<$.> variable, the current
389line number. Examples:
390
391As a scalar operator:
392
393 if (101 .. 200) { print; } # print 2nd hundred lines
394 next line if (1 .. /^$/); # skip header lines
395 s/^/> / if (/^$/ .. eof()); # quote body
396
397As a list operator:
398
399 for (101 .. 200) { print; } # print $_ 100 times
3e3baf6d 400 @foo = @foo[0 .. $#foo]; # an expensive no-op
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401 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
402
403The range operator (in a list context) makes use of the magical
5f05dabc 404auto-increment algorithm if the operands are strings. You
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405can say
406
407 @alphabet = ('A' .. 'Z');
408
409to get all the letters of the alphabet, or
410
411 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
412
413to get a hexadecimal digit, or
414
415 @z2 = ('01' .. '31'); print $z2[$mday];
416
417to get dates with leading zeros. If the final value specified is not
418in the sequence that the magical increment would produce, the sequence
419goes until the next value would be longer than the final value
420specified.
421
422=head2 Conditional Operator
423
424Ternary "?:" is the conditional operator, just as in C. It works much
425like an if-then-else. If the argument before the ? is true, the
426argument before the : is returned, otherwise the argument after the :
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427is returned. For example:
428
54310121 429 printf "I have %d dog%s.\n", $n,
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430 ($n == 1) ? '' : "s";
431
432Scalar or list context propagates downward into the 2nd
54310121 433or 3rd argument, whichever is selected.
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434
435 $a = $ok ? $b : $c; # get a scalar
436 @a = $ok ? @b : @c; # get an array
437 $a = $ok ? @b : @c; # oops, that's just a count!
438
439The operator may be assigned to if both the 2nd and 3rd arguments are
440legal lvalues (meaning that you can assign to them):
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441
442 ($a_or_b ? $a : $b) = $c;
443
cb1a09d0 444This is not necessarily guaranteed to contribute to the readability of your program.
a0d0e21e 445
4633a7c4 446=head2 Assignment Operators
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447
448"=" is the ordinary assignment operator.
449
450Assignment operators work as in C. That is,
451
452 $a += 2;
453
454is equivalent to
455
456 $a = $a + 2;
457
458although without duplicating any side effects that dereferencing the lvalue
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459might trigger, such as from tie(). Other assignment operators work similarly.
460The following are recognized:
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461
462 **= += *= &= <<= &&=
463 -= /= |= >>= ||=
464 .= %= ^=
465 x=
466
467Note that while these are grouped by family, they all have the precedence
468of assignment.
469
470Unlike in C, the assignment operator produces a valid lvalue. Modifying
471an assignment is equivalent to doing the assignment and then modifying
472the variable that was assigned to. This is useful for modifying
473a copy of something, like this:
474
475 ($tmp = $global) =~ tr [A-Z] [a-z];
476
477Likewise,
478
479 ($a += 2) *= 3;
480
481is equivalent to
482
483 $a += 2;
484 $a *= 3;
485
748a9306 486=head2 Comma Operator
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487
488Binary "," is the comma operator. In a scalar context it evaluates
489its left argument, throws that value away, then evaluates its right
490argument and returns that value. This is just like C's comma operator.
491
492In a list context, it's just the list argument separator, and inserts
493both its arguments into the list.
494
6ee5d4e7 495The =E<gt> digraph is mostly just a synonym for the comma operator. It's useful for
cb1a09d0 496documenting arguments that come in pairs. As of release 5.001, it also forces
4633a7c4 497any word to the left of it to be interpreted as a string.
748a9306 498
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499=head2 List Operators (Rightward)
500
501On the right side of a list operator, it has very low precedence,
502such that it controls all comma-separated expressions found there.
503The only operators with lower precedence are the logical operators
504"and", "or", and "not", which may be used to evaluate calls to list
505operators without the need for extra parentheses:
506
507 open HANDLE, "filename"
508 or die "Can't open: $!\n";
509
5ba421f6 510See also discussion of list operators in L<Terms and List Operators (Leftward)>.
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511
512=head2 Logical Not
513
514Unary "not" returns the logical negation of the expression to its right.
515It's the equivalent of "!" except for the very low precedence.
516
517=head2 Logical And
518
519Binary "and" returns the logical conjunction of the two surrounding
520expressions. It's equivalent to && except for the very low
5f05dabc 521precedence. This means that it short-circuits: i.e., the right
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522expression is evaluated only if the left expression is true.
523
524=head2 Logical or and Exclusive Or
525
526Binary "or" returns the logical disjunction of the two surrounding
527expressions. It's equivalent to || except for the very low
5f05dabc 528precedence. This means that it short-circuits: i.e., the right
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529expression is evaluated only if the left expression is false.
530
531Binary "xor" returns the exclusive-OR of the two surrounding expressions.
532It cannot short circuit, of course.
533
534=head2 C Operators Missing From Perl
535
536Here is what C has that Perl doesn't:
537
538=over 8
539
540=item unary &
541
542Address-of operator. (But see the "\" operator for taking a reference.)
543
544=item unary *
545
54310121 546Dereference-address operator. (Perl's prefix dereferencing
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547operators are typed: $, @, %, and &.)
548
549=item (TYPE)
550
54310121 551Type casting operator.
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552
553=back
554
5f05dabc 555=head2 Quote and Quote-like Operators
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556
557While we usually think of quotes as literal values, in Perl they
558function as operators, providing various kinds of interpolating and
559pattern matching capabilities. Perl provides customary quote characters
560for these behaviors, but also provides a way for you to choose your
561quote character for any of them. In the following table, a C<{}> represents
562any pair of delimiters you choose. Non-bracketing delimiters use
54310121 563the same character fore and aft, but the 4 sorts of brackets
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564(round, angle, square, curly) will all nest.
565
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566 Customary Generic Meaning Interpolates
567 '' q{} Literal no
568 "" qq{} Literal yes
569 `` qx{} Command yes
570 qw{} Word list no
571 // m{} Pattern match yes
572 s{}{} Substitution yes
573 tr{}{} Transliteration no (but see below)
a0d0e21e 574
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575Note that there can be whitespace between the operator and the quoting
576characters, except when C<#> is being used as the quoting character.
a3cb178b 577C<q#foo#> is parsed as being the string C<foo>, while C<q #foo#> is the
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578operator C<q> followed by a comment. Its argument will be taken from the
579next line. This allows you to write:
580
581 s {foo} # Replace foo
582 {bar} # with bar.
583
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584For constructs that do interpolation, variables beginning with "C<$>"
585or "C<@>" are interpolated, as are the following sequences. Within
586a transliteration, the first ten of these sequences may be used.
a0d0e21e 587
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588 \t tab (HT, TAB)
589 \n newline (LF, NL)
590 \r return (CR)
591 \f form feed (FF)
592 \b backspace (BS)
593 \a alarm (bell) (BEL)
594 \e escape (ESC)
a0d0e21e
LW
595 \033 octal char
596 \x1b hex char
597 \c[ control char
2c268ad5 598
a0d0e21e
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599 \l lowercase next char
600 \u uppercase next char
601 \L lowercase till \E
602 \U uppercase till \E
603 \E end case modification
1d2dff63 604 \Q quote non-word characters till \E
a0d0e21e 605
a034a98d 606If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
7b8d334a 607and C<\U> is taken from the current locale. See L<perllocale>.
a034a98d 608
1d2dff63
GS
609You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
610An unescaped C<$> or C<@> interpolates the corresponding variable,
611while escaping will cause the literal string C<\$> to be inserted.
612You'll need to write something like C<m/\Quser\E\@\Qhost/>.
613
a0d0e21e
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614Patterns are subject to an additional level of interpretation as a
615regular expression. This is done as a second pass, after variables are
616interpolated, so that regular expressions may be incorporated into the
617pattern from the variables. If this is not what you want, use C<\Q> to
618interpolate a variable literally.
619
620Apart from the above, there are no multiple levels of interpolation. In
5f05dabc 621particular, contrary to the expectations of shell programmers, back-quotes
a0d0e21e
LW
622do I<NOT> interpolate within double quotes, nor do single quotes impede
623evaluation of variables when used within double quotes.
624
5f05dabc 625=head2 Regexp Quote-Like Operators
cb1a09d0 626
5f05dabc 627Here are the quote-like operators that apply to pattern
cb1a09d0
AD
628matching and related activities.
629
a0d0e21e
LW
630=over 8
631
632=item ?PATTERN?
633
634This is just like the C</pattern/> search, except that it matches only
635once between calls to the reset() operator. This is a useful
5f05dabc 636optimization when you want to see only the first occurrence of
a0d0e21e
LW
637something in each file of a set of files, for instance. Only C<??>
638patterns local to the current package are reset.
639
640This usage is vaguely deprecated, and may be removed in some future
641version of Perl.
642
fb73857a 643=item m/PATTERN/cgimosx
a0d0e21e 644
fb73857a 645=item /PATTERN/cgimosx
a0d0e21e
LW
646
647Searches a string for a pattern match, and in a scalar context returns
648true (1) or false (''). If no string is specified via the C<=~> or
649C<!~> operator, the $_ string is searched. (The string specified with
650C<=~> need not be an lvalue--it may be the result of an expression
651evaluation, but remember the C<=~> binds rather tightly.) See also
652L<perlre>.
a034a98d
DD
653See L<perllocale> for discussion of additional considerations which apply
654when C<use locale> is in effect.
a0d0e21e
LW
655
656Options are:
657
fb73857a 658 c Do not reset search position on a failed match when /g is in effect.
5f05dabc 659 g Match globally, i.e., find all occurrences.
a0d0e21e
LW
660 i Do case-insensitive pattern matching.
661 m Treat string as multiple lines.
5f05dabc 662 o Compile pattern only once.
a0d0e21e 663 s Treat string as single line.
48c036b1 664 t Taint $1 etc. if target string is tainted.
a0d0e21e
LW
665 x Use extended regular expressions.
666
667If "/" is the delimiter then the initial C<m> is optional. With the C<m>
668you can use any pair of non-alphanumeric, non-whitespace characters as
669delimiters. This is particularly useful for matching Unix path names
7bac28a0
PP
670that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
671the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
a0d0e21e
LW
672
673PATTERN may contain variables, which will be interpolated (and the
674pattern recompiled) every time the pattern search is evaluated. (Note
675that C<$)> and C<$|> might not be interpolated because they look like
676end-of-string tests.) If you want such a pattern to be compiled only
677once, add a C</o> after the trailing delimiter. This avoids expensive
678run-time recompilations, and is useful when the value you are
679interpolating won't change over the life of the script. However, mentioning
680C</o> constitutes a promise that you won't change the variables in the pattern.
681If you change them, Perl won't even notice.
682
4633a7c4 683If the PATTERN evaluates to a null string, the last
a3cb178b 684successfully matched regular expression is used instead.
a0d0e21e
LW
685
686If used in a context that requires a list value, a pattern match returns a
687list consisting of the subexpressions matched by the parentheses in the
5f05dabc 688pattern, i.e., (C<$1>, $2, $3...). (Note that here $1 etc. are also set, and
1d2dff63
GS
689that this differs from Perl 4's behavior.) If there are no parentheses,
690the return value is the list C<(1)> for success or C<('')> upon failure.
691With parentheses, C<()> is returned upon failure.
a0d0e21e
LW
692
693Examples:
694
695 open(TTY, '/dev/tty');
696 <TTY> =~ /^y/i && foo(); # do foo if desired
697
698 if (/Version: *([0-9.]*)/) { $version = $1; }
699
700 next if m#^/usr/spool/uucp#;
701
702 # poor man's grep
703 $arg = shift;
704 while (<>) {
705 print if /$arg/o; # compile only once
706 }
707
708 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
709
710This last example splits $foo into the first two words and the
5f05dabc
PP
711remainder of the line, and assigns those three fields to $F1, $F2, and
712$Etc. The conditional is true if any variables were assigned, i.e., if
a0d0e21e
LW
713the pattern matched.
714
715The C</g> modifier specifies global pattern matching--that is, matching
716as many times as possible within the string. How it behaves depends on
717the context. In a list context, it returns a list of all the
718substrings matched by all the parentheses in the regular expression.
719If there are no parentheses, it returns a list of all the matched
720strings, as if there were parentheses around the whole pattern.
721
722In a scalar context, C<m//g> iterates through the string, returning TRUE
c90c0ff4
PP
723each time it matches, and FALSE when it eventually runs out of matches.
724(In other words, it remembers where it left off last time and restarts
725the search at that point. You can actually find the current match
726position of a string or set it using the pos() function; see
727L<perlfunc/pos>.) A failed match normally resets the search position to
90248788 728the beginning of the string, but you can avoid that by adding the C</c>
c90c0ff4
PP
729modifier (e.g. C<m//gc>). Modifying the target string also resets the
730search position.
731
732You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
733zero-width assertion that matches the exact position where the previous
734C<m//g>, if any, left off. The C<\G> assertion is not supported without
735the C</g> modifier; currently, without C</g>, C<\G> behaves just like
736C<\A>, but that's accidental and may change in the future.
737
738Examples:
a0d0e21e
LW
739
740 # list context
741 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
742
743 # scalar context
5f05dabc 744 $/ = ""; $* = 1; # $* deprecated in modern perls
54310121 745 while (defined($paragraph = <>)) {
a0d0e21e
LW
746 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
747 $sentences++;
748 }
749 }
750 print "$sentences\n";
751
c90c0ff4 752 # using m//gc with \G
137443ea 753 $_ = "ppooqppqq";
44a8e56a
PP
754 while ($i++ < 2) {
755 print "1: '";
c90c0ff4 756 print $1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 757 print "2: '";
c90c0ff4 758 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 759 print "3: '";
c90c0ff4 760 print $1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a
PP
761 }
762
763The last example should print:
764
765 1: 'oo', pos=4
137443ea 766 2: 'q', pos=5
44a8e56a
PP
767 3: 'pp', pos=7
768 1: '', pos=7
137443ea
PP
769 2: 'q', pos=8
770 3: '', pos=8
44a8e56a 771
c90c0ff4 772A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 773combine several regexps like this to process a string part-by-part,
c90c0ff4
PP
774doing different actions depending on which regexp matched. Each
775regexp tries to match where the previous one leaves off.
e7ea3e70 776
3fe9a6f1 777 $_ = <<'EOL';
e7ea3e70 778 $url = new URI::URL "http://www/"; die if $url eq "xXx";
3fe9a6f1
PP
779 EOL
780 LOOP:
e7ea3e70 781 {
c90c0ff4
PP
782 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
783 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
784 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
785 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
786 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
787 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
788 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
e7ea3e70
IZ
789 print ". That's all!\n";
790 }
791
792Here is the output (split into several lines):
793
794 line-noise lowercase line-noise lowercase UPPERCASE line-noise
795 UPPERCASE line-noise lowercase line-noise lowercase line-noise
796 lowercase lowercase line-noise lowercase lowercase line-noise
797 MiXeD line-noise. That's all!
44a8e56a 798
a0d0e21e
LW
799=item q/STRING/
800
801=item C<'STRING'>
802
68dc0745
PP
803A single-quoted, literal string. A backslash represents a backslash
804unless followed by the delimiter or another backslash, in which case
805the delimiter or backslash is interpolated.
a0d0e21e
LW
806
807 $foo = q!I said, "You said, 'She said it.'"!;
808 $bar = q('This is it.');
68dc0745 809 $baz = '\n'; # a two-character string
a0d0e21e
LW
810
811=item qq/STRING/
812
813=item "STRING"
814
815A double-quoted, interpolated string.
816
817 $_ .= qq
818 (*** The previous line contains the naughty word "$1".\n)
819 if /(tcl|rexx|python)/; # :-)
68dc0745 820 $baz = "\n"; # a one-character string
a0d0e21e
LW
821
822=item qx/STRING/
823
824=item `STRING`
825
826A string which is interpolated and then executed as a system command.
827The collected standard output of the command is returned. In scalar
4a6725af 828context, it comes back as a single (potentially multi-line) string.
a0d0e21e
LW
829In list context, returns a list of lines (however you've defined lines
830with $/ or $INPUT_RECORD_SEPARATOR).
831
832 $today = qx{ date };
833
bb32b41a
GS
834Note that how the string gets evaluated is entirely subject to the
835command interpreter on your system. On most platforms, you will have
836to protect shell metacharacters if you want them treated literally.
837On some platforms (notably DOS-like ones), the shell may not be
838capable of dealing with multiline commands, so putting newlines in
839the string may not get you what you want. You may be able to evaluate
840multiple commands in a single line by separating them with the command
841separator character, if your shell supports that (e.g. C<;> on many Unix
842shells; C<&> on the Windows NT C<cmd> shell).
843
844Beware that some command shells may place restrictions on the length
845of the command line. You must ensure your strings don't exceed this
846limit after any necessary interpolations. See the platform-specific
847release notes for more details about your particular environment.
848
849Also realize that using this operator frequently leads to unportable
850programs.
851
dc848c6f 852See L<"I/O Operators"> for more discussion.
a0d0e21e
LW
853
854=item qw/STRING/
855
856Returns a list of the words extracted out of STRING, using embedded
857whitespace as the word delimiters. It is exactly equivalent to
858
859 split(' ', q/STRING/);
860
861Some frequently seen examples:
862
863 use POSIX qw( setlocale localeconv )
864 @EXPORT = qw( foo bar baz );
865
7bac28a0
PP
866A common mistake is to try to separate the words with comma or to put
867comments into a multi-line qw-string. For this reason the C<-w>
868switch produce warnings if the STRING contains the "," or the "#"
869character.
870
a0d0e21e
LW
871=item s/PATTERN/REPLACEMENT/egimosx
872
873Searches a string for a pattern, and if found, replaces that pattern
874with the replacement text and returns the number of substitutions
e37d713d 875made. Otherwise it returns false (specifically, the empty string).
a0d0e21e
LW
876
877If no string is specified via the C<=~> or C<!~> operator, the C<$_>
878variable is searched and modified. (The string specified with C<=~> must
879be a scalar variable, an array element, a hash element, or an assignment
5f05dabc 880to one of those, i.e., an lvalue.)
a0d0e21e
LW
881
882If the delimiter chosen is single quote, no variable interpolation is
883done on either the PATTERN or the REPLACEMENT. Otherwise, if the
884PATTERN contains a $ that looks like a variable rather than an
885end-of-string test, the variable will be interpolated into the pattern
5f05dabc 886at run-time. If you want the pattern compiled only once the first time
a0d0e21e 887the variable is interpolated, use the C</o> option. If the pattern
4633a7c4 888evaluates to a null string, the last successfully executed regular
a0d0e21e 889expression is used instead. See L<perlre> for further explanation on these.
a034a98d
DD
890See L<perllocale> for discussion of additional considerations which apply
891when C<use locale> is in effect.
a0d0e21e
LW
892
893Options are:
894
895 e Evaluate the right side as an expression.
5f05dabc 896 g Replace globally, i.e., all occurrences.
a0d0e21e
LW
897 i Do case-insensitive pattern matching.
898 m Treat string as multiple lines.
5f05dabc 899 o Compile pattern only once.
a0d0e21e
LW
900 s Treat string as single line.
901 x Use extended regular expressions.
902
903Any non-alphanumeric, non-whitespace delimiter may replace the
904slashes. If single quotes are used, no interpretation is done on the
e37d713d 905replacement string (the C</e> modifier overrides this, however). Unlike
54310121 906Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
e37d713d 907text is not evaluated as a command. If the
a0d0e21e 908PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
5f05dabc 909pair of quotes, which may or may not be bracketing quotes, e.g.,
a0d0e21e 910C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the
7b8d334a 911replacement portion to be interpreted as a full-fledged Perl expression
a0d0e21e
LW
912and eval()ed right then and there. It is, however, syntax checked at
913compile-time.
914
915Examples:
916
917 s/\bgreen\b/mauve/g; # don't change wintergreen
918
919 $path =~ s|/usr/bin|/usr/local/bin|;
920
921 s/Login: $foo/Login: $bar/; # run-time pattern
922
923 ($foo = $bar) =~ s/this/that/;
924
925 $count = ($paragraph =~ s/Mister\b/Mr./g);
926
927 $_ = 'abc123xyz';
928 s/\d+/$&*2/e; # yields 'abc246xyz'
929 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
930 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
931
932 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
933 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
934 s/^=(\w+)/&pod($1)/ge; # use function call
935
936 # /e's can even nest; this will expand
937 # simple embedded variables in $_
938 s/(\$\w+)/$1/eeg;
939
940 # Delete C comments.
941 $program =~ s {
4633a7c4
LW
942 /\* # Match the opening delimiter.
943 .*? # Match a minimal number of characters.
944 \*/ # Match the closing delimiter.
a0d0e21e
LW
945 } []gsx;
946
947 s/^\s*(.*?)\s*$/$1/; # trim white space
948
949 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
950
54310121 951Note the use of $ instead of \ in the last example. Unlike
5f05dabc 952B<sed>, we use the \E<lt>I<digit>E<gt> form in only the left hand side.
6ee5d4e7 953Anywhere else it's $E<lt>I<digit>E<gt>.
a0d0e21e 954
5f05dabc 955Occasionally, you can't use just a C</g> to get all the changes
a0d0e21e
LW
956to occur. Here are two common cases:
957
958 # put commas in the right places in an integer
959 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4
960 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5
961
962 # expand tabs to 8-column spacing
963 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
964
965
966=item tr/SEARCHLIST/REPLACEMENTLIST/cds
967
968=item y/SEARCHLIST/REPLACEMENTLIST/cds
969
2c268ad5 970Transliterates all occurrences of the characters found in the search list
a0d0e21e
LW
971with the corresponding character in the replacement list. It returns
972the number of characters replaced or deleted. If no string is
2c268ad5 973specified via the =~ or !~ operator, the $_ string is transliterated. (The
54310121
PP
974string specified with =~ must be a scalar variable, an array element, a
975hash element, or an assignment to one of those, i.e., an lvalue.)
2c268ad5
TP
976A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
977does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121
PP
978For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
979SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
980its own pair of quotes, which may or may not be bracketing quotes,
2c268ad5 981e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
a0d0e21e
LW
982
983Options:
984
985 c Complement the SEARCHLIST.
986 d Delete found but unreplaced characters.
987 s Squash duplicate replaced characters.
988
989If the C</c> modifier is specified, the SEARCHLIST character set is
990complemented. If the C</d> modifier is specified, any characters specified
991by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note
992that this is slightly more flexible than the behavior of some B<tr>
993programs, which delete anything they find in the SEARCHLIST, period.)
994If the C</s> modifier is specified, sequences of characters that were
2c268ad5 995transliterated to the same character are squashed down to a single instance of the
a0d0e21e
LW
996character.
997
998If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
999exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1000than the SEARCHLIST, the final character is replicated till it is long
1001enough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated.
1002This latter is useful for counting characters in a class or for
1003squashing character sequences in a class.
1004
1005Examples:
1006
1007 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1008
1009 $cnt = tr/*/*/; # count the stars in $_
1010
1011 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1012
1013 $cnt = tr/0-9//; # count the digits in $_
1014
1015 tr/a-zA-Z//s; # bookkeeper -> bokeper
1016
1017 ($HOST = $host) =~ tr/a-z/A-Z/;
1018
1019 tr/a-zA-Z/ /cs; # change non-alphas to single space
1020
1021 tr [\200-\377]
1022 [\000-\177]; # delete 8th bit
1023
2c268ad5 1024If multiple transliterations are given for a character, only the first one is used:
748a9306
LW
1025
1026 tr/AAA/XYZ/
1027
2c268ad5 1028will transliterate any A to X.
748a9306 1029
2c268ad5 1030Note that because the transliteration table is built at compile time, neither
a0d0e21e
LW
1031the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1032interpolation. That means that if you want to use variables, you must use
1033an eval():
1034
1035 eval "tr/$oldlist/$newlist/";
1036 die $@ if $@;
1037
1038 eval "tr/$oldlist/$newlist/, 1" or die $@;
1039
1040=back
1041
1042=head2 I/O Operators
1043
54310121 1044There are several I/O operators you should know about.
7b8d334a 1045A string enclosed by backticks (grave accents) first undergoes
a0d0e21e
LW
1046variable substitution just like a double quoted string. It is then
1047interpreted as a command, and the output of that command is the value
1048of the pseudo-literal, like in a shell. In a scalar context, a single
1049string consisting of all the output is returned. In a list context,
1050a list of values is returned, one for each line of output. (You can
1051set C<$/> to use a different line terminator.) The command is executed
1052each time the pseudo-literal is evaluated. The status value of the
1053command is returned in C<$?> (see L<perlvar> for the interpretation
1054of C<$?>). Unlike in B<csh>, no translation is done on the return
1055data--newlines remain newlines. Unlike in any of the shells, single
1056quotes do not hide variable names in the command from interpretation.
1057To pass a $ through to the shell you need to hide it with a backslash.
54310121
PP
1058The generalized form of backticks is C<qx//>. (Because backticks
1059always undergo shell expansion as well, see L<perlsec> for
cb1a09d0 1060security concerns.)
a0d0e21e
LW
1061
1062Evaluating a filehandle in angle brackets yields the next line from
aa689395
PP
1063that file (newline, if any, included), or C<undef> at end of file.
1064Ordinarily you must assign that value to a variable, but there is one
1065situation where an automatic assignment happens. I<If and ONLY if> the
1066input symbol is the only thing inside the conditional of a C<while> or
1067C<for(;;)> loop, the value is automatically assigned to the variable
7b8d334a
GS
1068C<$_>. In these loop constructs, the assigned value (whether assignment
1069is automatic or explcit) is then tested to see if it is defined.
1070The defined test avoids problems where line has a string value
1071that would be treated as false by perl e.g. "" or "0" with no trailing
1072newline. (This may seem like an odd thing to you, but you'll use the
1073construct in almost every Perl script you write.) Anyway, the following
1074lines are equivalent to each other:
a0d0e21e 1075
748a9306 1076 while (defined($_ = <STDIN>)) { print; }
7b8d334a 1077 while ($_ = <STDIN>) { print; }
a0d0e21e
LW
1078 while (<STDIN>) { print; }
1079 for (;<STDIN>;) { print; }
748a9306 1080 print while defined($_ = <STDIN>);
7b8d334a 1081 print while ($_ = <STDIN>);
a0d0e21e
LW
1082 print while <STDIN>;
1083
7b8d334a
GS
1084and this also behaves similarly, but avoids the use of $_ :
1085
1086 while (my $line = <STDIN>) { print $line }
1087
1088If you really mean such values to terminate the loop they should be
1089tested for explcitly:
1090
1091 while (($_ = <STDIN>) ne '0') { ... }
1092 while (<STDIN>) { last unless $_; ... }
1093
1094In other boolean contexts C<E<lt>I<filehandle>E<gt>> without explcit C<defined>
1095test or comparison will solicit a warning if C<-w> is in effect.
1096
5f05dabc
PP
1097The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1098filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in
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1099packages, where they would be interpreted as local identifiers rather
1100than global.) Additional filehandles may be created with the open()
cb1a09d0 1101function. See L<perlfunc/open()> for details on this.
a0d0e21e 1102
6ee5d4e7 1103If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a
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1104list consisting of all the input lines is returned, one line per list
1105element. It's easy to make a I<LARGE> data space this way, so use with
1106care.
1107
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PP
1108The null filehandle E<lt>E<gt> is special and can be used to emulate the
1109behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from
a0d0e21e 1110standard input, or from each file listed on the command line. Here's
d28ebecd 1111how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is
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LW
1112checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened
1113gives you standard input. The @ARGV array is then processed as a list
1114of filenames. The loop
1115
1116 while (<>) {
1117 ... # code for each line
1118 }
1119
1120is equivalent to the following Perl-like pseudo code:
1121
3e3baf6d 1122 unshift(@ARGV, '-') unless @ARGV;
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LW
1123 while ($ARGV = shift) {
1124 open(ARGV, $ARGV);
1125 while (<ARGV>) {
1126 ... # code for each line
1127 }
1128 }
1129
1130except that it isn't so cumbersome to say, and will actually work. It
1131really does shift array @ARGV and put the current filename into variable
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PP
1132$ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a
1133synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above
1134doesn't work because it treats E<lt>ARGVE<gt> as non-magical.)
a0d0e21e 1135
d28ebecd 1136You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up
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LW
1137containing the list of filenames you really want. Line numbers (C<$.>)
1138continue as if the input were one big happy file. (But see example
1139under eof() for how to reset line numbers on each file.)
1140
1141If you want to set @ARGV to your own list of files, go right ahead. If
54310121 1142you want to pass switches into your script, you can use one of the
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LW
1143Getopts modules or put a loop on the front like this:
1144
1145 while ($_ = $ARGV[0], /^-/) {
1146 shift;
1147 last if /^--$/;
1148 if (/^-D(.*)/) { $debug = $1 }
1149 if (/^-v/) { $verbose++ }
1150 ... # other switches
1151 }
1152 while (<>) {
1153 ... # code for each line
1154 }
1155
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GS
1156The E<lt>E<gt> symbol will return C<undef> for end-of-file only once.
1157If you call it again after this it will assume you are processing another
1158@ARGV list, and if you haven't set @ARGV, will input from STDIN.
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1159
1160If the string inside the angle brackets is a reference to a scalar
5f05dabc 1161variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the
cb1a09d0
AD
1162filehandle to input from, or a reference to the same. For example:
1163
1164 $fh = \*STDIN;
1165 $line = <$fh>;
a0d0e21e 1166
cb1a09d0
AD
1167If the string inside angle brackets is not a filehandle or a scalar
1168variable containing a filehandle name or reference, then it is interpreted
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LW
1169as a filename pattern to be globbed, and either a list of filenames or the
1170next filename in the list is returned, depending on context. One level of
1171$ interpretation is done first, but you can't say C<E<lt>$fooE<gt>>
1172because that's an indirect filehandle as explained in the previous
6ee5d4e7 1173paragraph. (In older versions of Perl, programmers would insert curly
4633a7c4 1174brackets to force interpretation as a filename glob: C<E<lt>${foo}E<gt>>.
d28ebecd 1175These days, it's considered cleaner to call the internal function directly
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LW
1176as C<glob($foo)>, which is probably the right way to have done it in the
1177first place.) Example:
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LW
1178
1179 while (<*.c>) {
1180 chmod 0644, $_;
1181 }
1182
1183is equivalent to
1184
1185 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1186 while (<FOO>) {
1187 chop;
1188 chmod 0644, $_;
1189 }
1190
1191In fact, it's currently implemented that way. (Which means it will not
1192work on filenames with spaces in them unless you have csh(1) on your
1193machine.) Of course, the shortest way to do the above is:
1194
1195 chmod 0644, <*.c>;
1196
1197Because globbing invokes a shell, it's often faster to call readdir() yourself
5f05dabc 1198and do your own grep() on the filenames. Furthermore, due to its current
54310121 1199implementation of using a shell, the glob() routine may get "Arg list too
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1200long" errors (unless you've installed tcsh(1L) as F</bin/csh>).
1201
5f05dabc 1202A glob evaluates its (embedded) argument only when it is starting a new
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LW
1203list. All values must be read before it will start over. In a list
1204context this isn't important, because you automatically get them all
1205anyway. In a scalar context, however, the operator returns the next value
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GS
1206each time it is called, or a C<undef> value if you've just run out. As
1207for filehandles an automatic C<defined> is generated when the glob
1208occurs in the test part of a C<while> or C<for> - because legal glob returns
1209(e.g. a file called F<0>) would otherwise terminate the loop.
1210Again, C<undef> is returned only once. So if you're expecting a single value
1211from a glob, it is much better to say
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LW
1212
1213 ($file) = <blurch*>;
1214
1215than
1216
1217 $file = <blurch*>;
1218
1219because the latter will alternate between returning a filename and
54310121 1220returning FALSE.
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LW
1221
1222It you're trying to do variable interpolation, it's definitely better
1223to use the glob() function, because the older notation can cause people
e37d713d 1224to become confused with the indirect filehandle notation.
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LW
1225
1226 @files = glob("$dir/*.[ch]");
1227 @files = glob($files[$i]);
1228
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1229=head2 Constant Folding
1230
1231Like C, Perl does a certain amount of expression evaluation at
1232compile time, whenever it determines that all of the arguments to an
1233operator are static and have no side effects. In particular, string
1234concatenation happens at compile time between literals that don't do
1235variable substitution. Backslash interpretation also happens at
1236compile time. You can say
1237
1238 'Now is the time for all' . "\n" .
1239 'good men to come to.'
1240
54310121 1241and this all reduces to one string internally. Likewise, if
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LW
1242you say
1243
1244 foreach $file (@filenames) {
1245 if (-s $file > 5 + 100 * 2**16) { ... }
54310121 1246 }
a0d0e21e 1247
54310121 1248the compiler will precompute the number that
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LW
1249expression represents so that the interpreter
1250won't have to.
1251
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TP
1252=head2 Bitwise String Operators
1253
1254Bitstrings of any size may be manipulated by the bitwise operators
1255(C<~ | & ^>).
1256
1257If the operands to a binary bitwise op are strings of different sizes,
1258B<or> and B<xor> ops will act as if the shorter operand had additional
1259zero bits on the right, while the B<and> op will act as if the longer
1260operand were truncated to the length of the shorter.
1261
1262 # ASCII-based examples
1263 print "j p \n" ^ " a h"; # prints "JAPH\n"
1264 print "JA" | " ph\n"; # prints "japh\n"
1265 print "japh\nJunk" & '_____'; # prints "JAPH\n";
1266 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
1267
1268If you are intending to manipulate bitstrings, you should be certain that
1269you're supplying bitstrings: If an operand is a number, that will imply
1270a B<numeric> bitwise operation. You may explicitly show which type of
1271operation you intend by using C<""> or C<0+>, as in the examples below.
1272
1273 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
1274 $foo = '150' | 105 ; # yields 255
1275 $foo = 150 | '105'; # yields 255
1276 $foo = '150' | '105'; # yields string '155' (under ASCII)
1277
1278 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
1279 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
a0d0e21e 1280
55497cff 1281=head2 Integer Arithmetic
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LW
1282
1283By default Perl assumes that it must do most of its arithmetic in
1284floating point. But by saying
1285
1286 use integer;
1287
1288you may tell the compiler that it's okay to use integer operations
1289from here to the end of the enclosing BLOCK. An inner BLOCK may
54310121 1290countermand this by saying
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LW
1291
1292 no integer;
1293
1294which lasts until the end of that BLOCK.
1295
55497cff 1296The bitwise operators ("&", "|", "^", "~", "<<", and ">>") always
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TP
1297produce integral results. (But see also L<Bitwise String Operators>.)
1298However, C<use integer> still has meaning
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PP
1299for them. By default, their results are interpreted as unsigned
1300integers. However, if C<use integer> is in effect, their results are
5f05dabc 1301interpreted as signed integers. For example, C<~0> usually evaluates
55497cff 1302to a large integral value. However, C<use integer; ~0> is -1.
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PP
1303
1304=head2 Floating-point Arithmetic
1305
1306While C<use integer> provides integer-only arithmetic, there is no
1307similar ways to provide rounding or truncation at a certain number of
1308decimal places. For rounding to a certain number of digits, sprintf()
1309or printf() is usually the easiest route.
1310
1311The POSIX module (part of the standard perl distribution) implements
1312ceil(), floor(), and a number of other mathematical and trigonometric
1313functions. The Math::Complex module (part of the standard perl
1314distribution) defines a number of mathematical functions that can also
1315work on real numbers. Math::Complex not as efficient as POSIX, but
1316POSIX can't work with complex numbers.
1317
1318Rounding in financial applications can have serious implications, and
1319the rounding method used should be specified precisely. In these
1320cases, it probably pays not to trust whichever system rounding is
1321being used by Perl, but to instead implement the rounding function you
1322need yourself.