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1 | =head1 NAME |
2 | |
3 | perlop - Perl operators and precedence |
4 | |
5 | =head1 SYNOPSIS |
6 | |
7 | Perl operators have the following associativity and precedence, |
8 | listed from highest precedence to lowest. Note that all operators |
9 | borrowed from C keep the same precedence relationship with each other, |
10 | even where C's precedence is slightly screwy. (This makes learning |
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11 | Perl easier for C folks.) With very few exceptions, these all |
12 | operate 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 - |
19 | left =~ !~ |
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 || |
30 | nonassoc .. |
31 | right ?: |
32 | right = += -= *= etc. |
33 | left , => |
34 | nonassoc list operators (rightward) |
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35 | right not |
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36 | left and |
37 | left or xor |
38 | |
39 | In the following sections, these operators are covered in precedence order. |
40 | |
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41 | =head1 DESCRIPTION |
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42 | |
43 | =head2 Terms and List Operators (Leftward) |
44 | |
45 | Any TERM is of highest precedence of Perl. These includes variables, |
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46 | quote and quote-like operators, any expression in parentheses, |
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47 | and any function whose arguments are parenthesized. Actually, there |
48 | aren't really functions in this sense, just list operators and unary |
49 | operators behaving as functions because you put parentheses around |
50 | the arguments. These are all documented in L<perlfunc>. |
51 | |
52 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) |
53 | is followed by a left parenthesis as the next token, the operator and |
54 | arguments within parentheses are taken to be of highest precedence, |
55 | just like a normal function call. |
56 | |
57 | In the absence of parentheses, the precedence of list operators such as |
58 | C<print>, C<sort>, or C<chmod> is either very high or very low depending on |
59 | whether you look at the left side of operator or the right side of it. |
60 | For example, in |
61 | |
62 | @ary = (1, 3, sort 4, 2); |
63 | print @ary; # prints 1324 |
64 | |
65 | the commas on the right of the sort are evaluated before the sort, but |
66 | the commas on the left are evaluated after. In other words, list |
67 | operators tend to gobble up all the arguments that follow them, and |
68 | then act like a simple TERM with regard to the preceding expression. |
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69 | Note 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 ($foo), exit; # Or even this. |
79 | |
80 | Also note that |
81 | |
82 | print ($foo & 255) + 1, "\n"; |
83 | |
84 | probably doesn't do what you expect at first glance. See |
85 | L<Named Unary Operators> for more discussion of this. |
86 | |
87 | Also parsed as terms are the C<do {}> and C<eval {}> constructs, as |
88 | well as subroutine and method calls, and the anonymous |
89 | constructors C<[]> and C<{}>. |
90 | |
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91 | See also L<Quote and Quote-Like Operators> toward the end of this section, |
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92 | as well as L<"I/O Operators">. |
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93 | |
94 | =head2 The Arrow Operator |
95 | |
96 | Just as in C and C++, "C<-E<gt>>" is an infix dereference operator. If the |
97 | right side is either a C<[...]> or C<{...}> subscript, then the left side |
98 | must be either a hard or symbolic reference to an array or hash (or |
99 | a location capable of holding a hard reference, if it's an lvalue (assignable)). |
100 | See L<perlref>. |
101 | |
102 | Otherwise, the right side is a method name or a simple scalar variable |
103 | containing 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). |
105 | See L<perlobj>. |
106 | |
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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 |
110 | increment or decrement the variable before returning the value, and if |
111 | placed after, increment or decrement the variable after returning the value. |
112 | |
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113 | The auto-increment operator has a little extra built-in magic to it. If |
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114 | you increment a variable that is numeric, or that has ever been used in |
115 | a numeric context, you get a normal increment. If, however, the |
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116 | variable has been used in only string contexts since it was set, and |
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117 | has a value that is not null and matches the pattern |
118 | C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each |
119 | character within its range, with carry: |
120 | |
121 | print ++($foo = '99'); # prints '100' |
122 | print ++($foo = 'a0'); # prints 'a1' |
123 | print ++($foo = 'Az'); # prints 'Ba' |
124 | print ++($foo = 'zz'); # prints 'aaa' |
125 | |
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126 | The auto-decrement operator is not magical. |
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127 | |
128 | =head2 Exponentiation |
129 | |
130 | Binary "**" is the exponentiation operator. Note that it binds even more |
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131 | tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is |
132 | implemented using C's pow(3) function, which actually works on doubles |
133 | internally.) |
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134 | |
135 | =head2 Symbolic Unary Operators |
136 | |
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137 | Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower |
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138 | precedence version of this. |
139 | |
140 | Unary "-" performs arithmetic negation if the operand is numeric. If |
141 | the operand is an identifier, a string consisting of a minus sign |
142 | concatenated with the identifier is returned. Otherwise, if the string |
143 | starts with a plus or minus, a string starting with the opposite sign |
144 | is returned. One effect of these rules is that C<-bareword> is equivalent |
145 | to C<"-bareword">. |
146 | |
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147 | Unary "~" performs bitwise negation, i.e., 1's complement. |
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148 | (See also L<Integer Arithmetic>.) |
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149 | |
150 | Unary "+" has no effect whatsoever, even on strings. It is useful |
151 | syntactically for separating a function name from a parenthesized expression |
152 | that would otherwise be interpreted as the complete list of function |
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153 | arguments. (See examples above under L<Terms and List Operators (Leftward)>.) |
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154 | |
155 | Unary "\" creates a reference to whatever follows it. See L<perlref>. |
156 | Do not confuse this behavior with the behavior of backslash within a |
157 | string, although both forms do convey the notion of protecting the next |
158 | thing from interpretation. |
159 | |
160 | =head2 Binding Operators |
161 | |
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162 | Binary "=~" binds a scalar expression to a pattern match. Certain operations |
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163 | search or modify the string $_ by default. This operator makes that kind |
164 | of operation work on some other string. The right argument is a search |
165 | pattern, substitution, or translation. The left argument is what is |
166 | supposed to be searched, substituted, or translated instead of the default |
167 | $_. The return value indicates the success of the operation. (If the |
168 | right argument is an expression rather than a search pattern, |
169 | substitution, or translation, it is interpreted as a search pattern at run |
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170 | time. This is less efficient than an explicit search, because the pattern |
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171 | must be compiled every time the expression is evaluated--unless you've |
172 | used C</o>.) |
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173 | |
174 | Binary "!~" is just like "=~" except the return value is negated in |
175 | the logical sense. |
176 | |
177 | =head2 Multiplicative Operators |
178 | |
179 | Binary "*" multiplies two numbers. |
180 | |
181 | Binary "/" divides two numbers. |
182 | |
183 | Binary "%" computes the modulus of the two numbers. |
184 | |
185 | Binary "x" is the repetition operator. In a scalar context, it |
186 | returns a string consisting of the left operand repeated the number of |
187 | times specified by the right operand. In a list context, if the left |
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188 | operand is a list in parentheses, it repeats the list. |
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189 | |
190 | print '-' x 80; # print row of dashes |
191 | |
192 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over |
193 | |
194 | @ones = (1) x 80; # a list of 80 1's |
195 | @ones = (5) x @ones; # set all elements to 5 |
196 | |
197 | |
198 | =head2 Additive Operators |
199 | |
200 | Binary "+" returns the sum of two numbers. |
201 | |
202 | Binary "-" returns the difference of two numbers. |
203 | |
204 | Binary "." concatenates two strings. |
205 | |
206 | =head2 Shift Operators |
207 | |
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208 | Binary "<<" returns the value of its left argument shifted left by the |
209 | number of bits specified by the right argument. Arguments should be |
210 | integers. (See also L<Integer Arithmetic>.) |
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211 | |
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212 | Binary ">>" returns the value of its left argument shifted right by |
213 | the number of bits specified by the right argument. Arguments should |
214 | be integers. (See also L<Integer Arithmetic>.) |
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215 | |
216 | =head2 Named Unary Operators |
217 | |
218 | The various named unary operators are treated as functions with one |
219 | argument, with optional parentheses. These include the filetest |
220 | operators, like C<-f>, C<-M>, etc. See L<perlfunc>. |
221 | |
222 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) |
223 | is followed by a left parenthesis as the next token, the operator and |
224 | arguments within parentheses are taken to be of highest precedence, |
225 | just like a normal function call. Examples: |
226 | |
227 | chdir $foo || die; # (chdir $foo) || die |
228 | chdir($foo) || die; # (chdir $foo) || die |
229 | chdir ($foo) || die; # (chdir $foo) || die |
230 | chdir +($foo) || die; # (chdir $foo) || die |
231 | |
232 | but, because * is higher precedence than ||: |
233 | |
234 | chdir $foo * 20; # chdir ($foo * 20) |
235 | chdir($foo) * 20; # (chdir $foo) * 20 |
236 | chdir ($foo) * 20; # (chdir $foo) * 20 |
237 | chdir +($foo) * 20; # chdir ($foo * 20) |
238 | |
239 | rand 10 * 20; # rand (10 * 20) |
240 | rand(10) * 20; # (rand 10) * 20 |
241 | rand (10) * 20; # (rand 10) * 20 |
242 | rand +(10) * 20; # rand (10 * 20) |
243 | |
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244 | See also L<"Terms and List Operators (Leftward)">. |
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245 | |
246 | =head2 Relational Operators |
247 | |
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248 | Binary "E<lt>" returns true if the left argument is numerically less than |
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249 | the right argument. |
250 | |
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251 | Binary "E<gt>" returns true if the left argument is numerically greater |
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252 | than the right argument. |
253 | |
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254 | Binary "E<lt>=" returns true if the left argument is numerically less than |
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255 | or equal to the right argument. |
256 | |
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257 | Binary "E<gt>=" returns true if the left argument is numerically greater |
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258 | than or equal to the right argument. |
259 | |
260 | Binary "lt" returns true if the left argument is stringwise less than |
261 | the right argument. |
262 | |
263 | Binary "gt" returns true if the left argument is stringwise greater |
264 | than the right argument. |
265 | |
266 | Binary "le" returns true if the left argument is stringwise less than |
267 | or equal to the right argument. |
268 | |
269 | Binary "ge" returns true if the left argument is stringwise greater |
270 | than or equal to the right argument. |
271 | |
272 | =head2 Equality Operators |
273 | |
274 | Binary "==" returns true if the left argument is numerically equal to |
275 | the right argument. |
276 | |
277 | Binary "!=" returns true if the left argument is numerically not equal |
278 | to the right argument. |
279 | |
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280 | Binary "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left |
281 | argument is numerically less than, equal to, or greater than the right |
282 | argument. |
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283 | |
284 | Binary "eq" returns true if the left argument is stringwise equal to |
285 | the right argument. |
286 | |
287 | Binary "ne" returns true if the left argument is stringwise not equal |
288 | to the right argument. |
289 | |
290 | Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise |
291 | less than, equal to, or greater than the right argument. |
292 | |
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293 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
294 | by the current locale if C<use locale> is in effect. See L<perllocale>. |
295 | |
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296 | =head2 Bitwise And |
297 | |
298 | Binary "&" returns its operators ANDed together bit by bit. |
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299 | (See also L<Integer Arithmetic>.) |
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300 | |
301 | =head2 Bitwise Or and Exclusive Or |
302 | |
303 | Binary "|" returns its operators ORed together bit by bit. |
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304 | (See also L<Integer Arithmetic>.) |
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305 | |
306 | Binary "^" returns its operators XORed together bit by bit. |
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307 | (See also L<Integer Arithmetic>.) |
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308 | |
309 | =head2 C-style Logical And |
310 | |
311 | Binary "&&" performs a short-circuit logical AND operation. That is, |
312 | if the left operand is false, the right operand is not even evaluated. |
313 | Scalar or list context propagates down to the right operand if it |
314 | is evaluated. |
315 | |
316 | =head2 C-style Logical Or |
317 | |
318 | Binary "||" performs a short-circuit logical OR operation. That is, |
319 | if the left operand is true, the right operand is not even evaluated. |
320 | Scalar or list context propagates down to the right operand if it |
321 | is evaluated. |
322 | |
323 | The C<||> and C<&&> operators differ from C's in that, rather than returning |
324 | 0 or 1, they return the last value evaluated. Thus, a reasonably portable |
325 | way to find out the home directory (assuming it's not "0") might be: |
326 | |
327 | $home = $ENV{'HOME'} || $ENV{'LOGDIR'} || |
328 | (getpwuid($<))[7] || die "You're homeless!\n"; |
329 | |
330 | As more readable alternatives to C<&&> and C<||>, Perl provides "and" and |
331 | "or" operators (see below). The short-circuit behavior is identical. The |
332 | precedence of "and" and "or" is much lower, however, so that you can |
333 | safely use them after a list operator without the need for |
334 | parentheses: |
335 | |
336 | unlink "alpha", "beta", "gamma" |
337 | or gripe(), next LINE; |
338 | |
339 | With the C-style operators that would have been written like this: |
340 | |
341 | unlink("alpha", "beta", "gamma") |
342 | || (gripe(), next LINE); |
343 | |
344 | =head2 Range Operator |
345 | |
346 | Binary ".." is the range operator, which is really two different |
347 | operators depending on the context. In a list context, it returns an |
348 | array of values counting (by ones) from the left value to the right |
349 | value. This is useful for writing C<for (1..10)> loops and for doing |
350 | slice operations on arrays. Be aware that under the current implementation, |
351 | a temporary array is created, so you'll burn a lot of memory if you |
352 | write something like this: |
353 | |
354 | for (1 .. 1_000_000) { |
355 | # code |
356 | } |
357 | |
358 | In a scalar context, ".." returns a boolean value. The operator is |
359 | bistable, like a flip-flop, and emulates the line-range (comma) operator |
360 | of B<sed>, B<awk>, and various editors. Each ".." operator maintains its |
361 | own boolean state. It is false as long as its left operand is false. |
362 | Once the left operand is true, the range operator stays true until the |
363 | right operand is true, I<AFTER> which the range operator becomes false |
364 | again. (It doesn't become false till the next time the range operator is |
365 | evaluated. It can test the right operand and become false on the same |
366 | evaluation it became true (as in B<awk>), but it still returns true once. |
367 | If you don't want it to test the right operand till the next evaluation |
368 | (as in B<sed>), use three dots ("...") instead of two.) The right |
369 | operand is not evaluated while the operator is in the "false" state, and |
370 | the left operand is not evaluated while the operator is in the "true" |
371 | state. The precedence is a little lower than || and &&. The value |
372 | returned is either the null string for false, or a sequence number |
373 | (beginning with 1) for true. The sequence number is reset for each range |
374 | encountered. The final sequence number in a range has the string "E0" |
375 | appended to it, which doesn't affect its numeric value, but gives you |
376 | something to search for if you want to exclude the endpoint. You can |
377 | exclude the beginning point by waiting for the sequence number to be |
378 | greater than 1. If either operand of scalar ".." is a numeric literal, |
379 | that operand is implicitly compared to the C<$.> variable, the current |
380 | line number. Examples: |
381 | |
382 | As a scalar operator: |
383 | |
384 | if (101 .. 200) { print; } # print 2nd hundred lines |
385 | next line if (1 .. /^$/); # skip header lines |
386 | s/^/> / if (/^$/ .. eof()); # quote body |
387 | |
388 | As a list operator: |
389 | |
390 | for (101 .. 200) { print; } # print $_ 100 times |
391 | @foo = @foo[$[ .. $#foo]; # an expensive no-op |
392 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items |
393 | |
394 | The range operator (in a list context) makes use of the magical |
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395 | auto-increment algorithm if the operands are strings. You |
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396 | can say |
397 | |
398 | @alphabet = ('A' .. 'Z'); |
399 | |
400 | to get all the letters of the alphabet, or |
401 | |
402 | $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; |
403 | |
404 | to get a hexadecimal digit, or |
405 | |
406 | @z2 = ('01' .. '31'); print $z2[$mday]; |
407 | |
408 | to get dates with leading zeros. If the final value specified is not |
409 | in the sequence that the magical increment would produce, the sequence |
410 | goes until the next value would be longer than the final value |
411 | specified. |
412 | |
413 | =head2 Conditional Operator |
414 | |
415 | Ternary "?:" is the conditional operator, just as in C. It works much |
416 | like an if-then-else. If the argument before the ? is true, the |
417 | argument before the : is returned, otherwise the argument after the : |
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418 | is returned. For example: |
419 | |
420 | printf "I have %d dog%s.\n", $n, |
421 | ($n == 1) ? '' : "s"; |
422 | |
423 | Scalar or list context propagates downward into the 2nd |
424 | or 3rd argument, whichever is selected. |
425 | |
426 | $a = $ok ? $b : $c; # get a scalar |
427 | @a = $ok ? @b : @c; # get an array |
428 | $a = $ok ? @b : @c; # oops, that's just a count! |
429 | |
430 | The operator may be assigned to if both the 2nd and 3rd arguments are |
431 | legal lvalues (meaning that you can assign to them): |
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432 | |
433 | ($a_or_b ? $a : $b) = $c; |
434 | |
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435 | This is not necessarily guaranteed to contribute to the readability of your program. |
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436 | |
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437 | =head2 Assignment Operators |
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438 | |
439 | "=" is the ordinary assignment operator. |
440 | |
441 | Assignment operators work as in C. That is, |
442 | |
443 | $a += 2; |
444 | |
445 | is equivalent to |
446 | |
447 | $a = $a + 2; |
448 | |
449 | although without duplicating any side effects that dereferencing the lvalue |
450 | might trigger, such as from tie(). Other assignment operators work similarly. |
451 | The following are recognized: |
452 | |
453 | **= += *= &= <<= &&= |
454 | -= /= |= >>= ||= |
455 | .= %= ^= |
456 | x= |
457 | |
458 | Note that while these are grouped by family, they all have the precedence |
459 | of assignment. |
460 | |
461 | Unlike in C, the assignment operator produces a valid lvalue. Modifying |
462 | an assignment is equivalent to doing the assignment and then modifying |
463 | the variable that was assigned to. This is useful for modifying |
464 | a copy of something, like this: |
465 | |
466 | ($tmp = $global) =~ tr [A-Z] [a-z]; |
467 | |
468 | Likewise, |
469 | |
470 | ($a += 2) *= 3; |
471 | |
472 | is equivalent to |
473 | |
474 | $a += 2; |
475 | $a *= 3; |
476 | |
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477 | =head2 Comma Operator |
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478 | |
479 | Binary "," is the comma operator. In a scalar context it evaluates |
480 | its left argument, throws that value away, then evaluates its right |
481 | argument and returns that value. This is just like C's comma operator. |
482 | |
483 | In a list context, it's just the list argument separator, and inserts |
484 | both its arguments into the list. |
485 | |
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486 | The =E<gt> digraph is mostly just a synonym for the comma operator. It's useful for |
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487 | documenting arguments that come in pairs. As of release 5.001, it also forces |
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488 | any word to the left of it to be interpreted as a string. |
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489 | |
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490 | =head2 List Operators (Rightward) |
491 | |
492 | On the right side of a list operator, it has very low precedence, |
493 | such that it controls all comma-separated expressions found there. |
494 | The only operators with lower precedence are the logical operators |
495 | "and", "or", and "not", which may be used to evaluate calls to list |
496 | operators without the need for extra parentheses: |
497 | |
498 | open HANDLE, "filename" |
499 | or die "Can't open: $!\n"; |
500 | |
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501 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
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502 | |
503 | =head2 Logical Not |
504 | |
505 | Unary "not" returns the logical negation of the expression to its right. |
506 | It's the equivalent of "!" except for the very low precedence. |
507 | |
508 | =head2 Logical And |
509 | |
510 | Binary "and" returns the logical conjunction of the two surrounding |
511 | expressions. It's equivalent to && except for the very low |
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512 | precedence. This means that it short-circuits: i.e., the right |
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513 | expression is evaluated only if the left expression is true. |
514 | |
515 | =head2 Logical or and Exclusive Or |
516 | |
517 | Binary "or" returns the logical disjunction of the two surrounding |
518 | expressions. It's equivalent to || except for the very low |
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519 | precedence. This means that it short-circuits: i.e., the right |
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520 | expression is evaluated only if the left expression is false. |
521 | |
522 | Binary "xor" returns the exclusive-OR of the two surrounding expressions. |
523 | It cannot short circuit, of course. |
524 | |
525 | =head2 C Operators Missing From Perl |
526 | |
527 | Here is what C has that Perl doesn't: |
528 | |
529 | =over 8 |
530 | |
531 | =item unary & |
532 | |
533 | Address-of operator. (But see the "\" operator for taking a reference.) |
534 | |
535 | =item unary * |
536 | |
537 | Dereference-address operator. (Perl's prefix dereferencing |
538 | operators are typed: $, @, %, and &.) |
539 | |
540 | =item (TYPE) |
541 | |
542 | Type casting operator. |
543 | |
544 | =back |
545 | |
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546 | =head2 Quote and Quote-like Operators |
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547 | |
548 | While we usually think of quotes as literal values, in Perl they |
549 | function as operators, providing various kinds of interpolating and |
550 | pattern matching capabilities. Perl provides customary quote characters |
551 | for these behaviors, but also provides a way for you to choose your |
552 | quote character for any of them. In the following table, a C<{}> represents |
553 | any pair of delimiters you choose. Non-bracketing delimiters use |
554 | the same character fore and aft, but the 4 sorts of brackets |
555 | (round, angle, square, curly) will all nest. |
556 | |
557 | Customary Generic Meaning Interpolates |
558 | '' q{} Literal no |
559 | "" qq{} Literal yes |
560 | `` qx{} Command yes |
561 | qw{} Word list no |
562 | // m{} Pattern match yes |
563 | s{}{} Substitution yes |
564 | tr{}{} Translation no |
565 | |
cb1a09d0 |
566 | For constructs that do interpolation, variables beginning with "C<$>" or "C<@>" |
a0d0e21e |
567 | are interpolated, as are the following sequences: |
568 | |
6ee5d4e7 |
569 | \t tab (HT, TAB) |
570 | \n newline (LF, NL) |
571 | \r return (CR) |
572 | \f form feed (FF) |
573 | \b backspace (BS) |
574 | \a alarm (bell) (BEL) |
575 | \e escape (ESC) |
a0d0e21e |
576 | \033 octal char |
577 | \x1b hex char |
578 | \c[ control char |
579 | \l lowercase next char |
580 | \u uppercase next char |
581 | \L lowercase till \E |
582 | \U uppercase till \E |
583 | \E end case modification |
584 | \Q quote regexp metacharacters till \E |
585 | |
a034a98d |
586 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> |
587 | and <\U> is taken from the current locale. See L<perllocale>. |
588 | |
a0d0e21e |
589 | Patterns are subject to an additional level of interpretation as a |
590 | regular expression. This is done as a second pass, after variables are |
591 | interpolated, so that regular expressions may be incorporated into the |
592 | pattern from the variables. If this is not what you want, use C<\Q> to |
593 | interpolate a variable literally. |
594 | |
595 | Apart from the above, there are no multiple levels of interpolation. In |
5f05dabc |
596 | particular, contrary to the expectations of shell programmers, back-quotes |
a0d0e21e |
597 | do I<NOT> interpolate within double quotes, nor do single quotes impede |
598 | evaluation of variables when used within double quotes. |
599 | |
5f05dabc |
600 | =head2 Regexp Quote-Like Operators |
cb1a09d0 |
601 | |
5f05dabc |
602 | Here are the quote-like operators that apply to pattern |
cb1a09d0 |
603 | matching and related activities. |
604 | |
a0d0e21e |
605 | =over 8 |
606 | |
607 | =item ?PATTERN? |
608 | |
609 | This is just like the C</pattern/> search, except that it matches only |
610 | once between calls to the reset() operator. This is a useful |
5f05dabc |
611 | optimization when you want to see only the first occurrence of |
a0d0e21e |
612 | something in each file of a set of files, for instance. Only C<??> |
613 | patterns local to the current package are reset. |
614 | |
615 | This usage is vaguely deprecated, and may be removed in some future |
616 | version of Perl. |
617 | |
618 | =item m/PATTERN/gimosx |
619 | |
620 | =item /PATTERN/gimosx |
621 | |
622 | Searches a string for a pattern match, and in a scalar context returns |
623 | true (1) or false (''). If no string is specified via the C<=~> or |
624 | C<!~> operator, the $_ string is searched. (The string specified with |
625 | C<=~> need not be an lvalue--it may be the result of an expression |
626 | evaluation, but remember the C<=~> binds rather tightly.) See also |
627 | L<perlre>. |
a034a98d |
628 | See L<perllocale> for discussion of additional considerations which apply |
629 | when C<use locale> is in effect. |
a0d0e21e |
630 | |
631 | Options are: |
632 | |
5f05dabc |
633 | g Match globally, i.e., find all occurrences. |
a0d0e21e |
634 | i Do case-insensitive pattern matching. |
635 | m Treat string as multiple lines. |
5f05dabc |
636 | o Compile pattern only once. |
a0d0e21e |
637 | s Treat string as single line. |
638 | x Use extended regular expressions. |
639 | |
640 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> |
641 | you can use any pair of non-alphanumeric, non-whitespace characters as |
642 | delimiters. This is particularly useful for matching Unix path names |
643 | that contain "/", to avoid LTS (leaning toothpick syndrome). |
644 | |
645 | PATTERN may contain variables, which will be interpolated (and the |
646 | pattern recompiled) every time the pattern search is evaluated. (Note |
647 | that C<$)> and C<$|> might not be interpolated because they look like |
648 | end-of-string tests.) If you want such a pattern to be compiled only |
649 | once, add a C</o> after the trailing delimiter. This avoids expensive |
650 | run-time recompilations, and is useful when the value you are |
651 | interpolating won't change over the life of the script. However, mentioning |
652 | C</o> constitutes a promise that you won't change the variables in the pattern. |
653 | If you change them, Perl won't even notice. |
654 | |
4633a7c4 |
655 | If the PATTERN evaluates to a null string, the last |
656 | successfully executed regular expression is used instead. |
a0d0e21e |
657 | |
658 | If used in a context that requires a list value, a pattern match returns a |
659 | list consisting of the subexpressions matched by the parentheses in the |
5f05dabc |
660 | pattern, i.e., (C<$1>, $2, $3...). (Note that here $1 etc. are also set, and |
a0d0e21e |
661 | that this differs from Perl 4's behavior.) If the match fails, a null |
662 | array is returned. If the match succeeds, but there were no parentheses, |
663 | a list value of (1) is returned. |
664 | |
665 | Examples: |
666 | |
667 | open(TTY, '/dev/tty'); |
668 | <TTY> =~ /^y/i && foo(); # do foo if desired |
669 | |
670 | if (/Version: *([0-9.]*)/) { $version = $1; } |
671 | |
672 | next if m#^/usr/spool/uucp#; |
673 | |
674 | # poor man's grep |
675 | $arg = shift; |
676 | while (<>) { |
677 | print if /$arg/o; # compile only once |
678 | } |
679 | |
680 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) |
681 | |
682 | This last example splits $foo into the first two words and the |
5f05dabc |
683 | remainder of the line, and assigns those three fields to $F1, $F2, and |
684 | $Etc. The conditional is true if any variables were assigned, i.e., if |
a0d0e21e |
685 | the pattern matched. |
686 | |
687 | The C</g> modifier specifies global pattern matching--that is, matching |
688 | as many times as possible within the string. How it behaves depends on |
689 | the context. In a list context, it returns a list of all the |
690 | substrings matched by all the parentheses in the regular expression. |
691 | If there are no parentheses, it returns a list of all the matched |
692 | strings, as if there were parentheses around the whole pattern. |
693 | |
694 | In a scalar context, C<m//g> iterates through the string, returning TRUE |
695 | each time it matches, and FALSE when it eventually runs out of |
696 | matches. (In other words, it remembers where it left off last time and |
697 | restarts the search at that point. You can actually find the current |
44a8e56a |
698 | match position of a string or set it using the pos() function--see |
699 | L<perlfunc/pos>.) Note that you can use this feature to stack C<m//g> |
700 | matches or intermix C<m//g> matches with C<m/\G.../>. |
701 | |
a0d0e21e |
702 | If you modify the string in any way, the match position is reset to the |
703 | beginning. Examples: |
704 | |
705 | # list context |
706 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); |
707 | |
708 | # scalar context |
5f05dabc |
709 | $/ = ""; $* = 1; # $* deprecated in modern perls |
a0d0e21e |
710 | while ($paragraph = <>) { |
711 | while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { |
712 | $sentences++; |
713 | } |
714 | } |
715 | print "$sentences\n"; |
716 | |
44a8e56a |
717 | # using m//g with \G |
718 | $_ = "ppooqppq"; |
719 | while ($i++ < 2) { |
720 | print "1: '"; |
721 | print $1 while /(o)/g; print "', pos=", pos, "\n"; |
722 | print "2: '"; |
723 | print $1 if /\G(q)/; print "', pos=", pos, "\n"; |
724 | print "3: '"; |
725 | print $1 while /(p)/g; print "', pos=", pos, "\n"; |
726 | } |
727 | |
728 | The last example should print: |
729 | |
730 | 1: 'oo', pos=4 |
731 | 2: 'q', pos=4 |
732 | 3: 'pp', pos=7 |
733 | 1: '', pos=7 |
734 | 2: 'q', pos=7 |
735 | 3: '', pos=7 |
736 | |
737 | Note how C<m//g> matches change the value reported by C<pos()>, but the |
738 | non-global match doesn't. |
739 | |
e7ea3e70 |
740 | A useful idiom for C<lex>-like scanners is C</\G.../g>. You can |
741 | combine several regexps like this to process a string part-by-part, |
742 | doing different actions depending on which regexp matched. The next |
743 | regexp would step in at the place the previous one left off. |
744 | |
745 | $_ = <<'EOL'; |
746 | $url = new URI::URL "http://www/"; die if $url eq "xXx"; |
747 | EOL |
748 | LOOP: |
749 | { |
750 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/g; |
751 | print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/g; |
752 | print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/g; |
753 | print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/g; |
754 | print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/g; |
755 | print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/g; |
756 | print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/g; |
757 | print ". That's all!\n"; |
758 | } |
759 | |
760 | Here is the output (split into several lines): |
761 | |
762 | line-noise lowercase line-noise lowercase UPPERCASE line-noise |
763 | UPPERCASE line-noise lowercase line-noise lowercase line-noise |
764 | lowercase lowercase line-noise lowercase lowercase line-noise |
765 | MiXeD line-noise. That's all! |
44a8e56a |
766 | |
a0d0e21e |
767 | =item q/STRING/ |
768 | |
769 | =item C<'STRING'> |
770 | |
771 | A single-quoted, literal string. Backslashes are ignored, unless |
772 | followed by the delimiter or another backslash, in which case the |
773 | delimiter or backslash is interpolated. |
774 | |
775 | $foo = q!I said, "You said, 'She said it.'"!; |
776 | $bar = q('This is it.'); |
777 | |
778 | =item qq/STRING/ |
779 | |
780 | =item "STRING" |
781 | |
782 | A double-quoted, interpolated string. |
783 | |
784 | $_ .= qq |
785 | (*** The previous line contains the naughty word "$1".\n) |
786 | if /(tcl|rexx|python)/; # :-) |
787 | |
788 | =item qx/STRING/ |
789 | |
790 | =item `STRING` |
791 | |
792 | A string which is interpolated and then executed as a system command. |
793 | The collected standard output of the command is returned. In scalar |
794 | context, it comes back as a single (potentially multi-line) string. |
795 | In list context, returns a list of lines (however you've defined lines |
796 | with $/ or $INPUT_RECORD_SEPARATOR). |
797 | |
798 | $today = qx{ date }; |
799 | |
800 | See L<I/O Operators> for more discussion. |
801 | |
802 | =item qw/STRING/ |
803 | |
804 | Returns a list of the words extracted out of STRING, using embedded |
805 | whitespace as the word delimiters. It is exactly equivalent to |
806 | |
807 | split(' ', q/STRING/); |
808 | |
809 | Some frequently seen examples: |
810 | |
811 | use POSIX qw( setlocale localeconv ) |
812 | @EXPORT = qw( foo bar baz ); |
813 | |
814 | =item s/PATTERN/REPLACEMENT/egimosx |
815 | |
816 | Searches a string for a pattern, and if found, replaces that pattern |
817 | with the replacement text and returns the number of substitutions |
e37d713d |
818 | made. Otherwise it returns false (specifically, the empty string). |
a0d0e21e |
819 | |
820 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> |
821 | variable is searched and modified. (The string specified with C<=~> must |
822 | be a scalar variable, an array element, a hash element, or an assignment |
5f05dabc |
823 | to one of those, i.e., an lvalue.) |
a0d0e21e |
824 | |
825 | If the delimiter chosen is single quote, no variable interpolation is |
826 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the |
827 | PATTERN contains a $ that looks like a variable rather than an |
828 | end-of-string test, the variable will be interpolated into the pattern |
5f05dabc |
829 | at run-time. If you want the pattern compiled only once the first time |
a0d0e21e |
830 | the variable is interpolated, use the C</o> option. If the pattern |
4633a7c4 |
831 | evaluates to a null string, the last successfully executed regular |
a0d0e21e |
832 | expression is used instead. See L<perlre> for further explanation on these. |
a034a98d |
833 | See L<perllocale> for discussion of additional considerations which apply |
834 | when C<use locale> is in effect. |
a0d0e21e |
835 | |
836 | Options are: |
837 | |
838 | e Evaluate the right side as an expression. |
5f05dabc |
839 | g Replace globally, i.e., all occurrences. |
a0d0e21e |
840 | i Do case-insensitive pattern matching. |
841 | m Treat string as multiple lines. |
5f05dabc |
842 | o Compile pattern only once. |
a0d0e21e |
843 | s Treat string as single line. |
844 | x Use extended regular expressions. |
845 | |
846 | Any non-alphanumeric, non-whitespace delimiter may replace the |
847 | slashes. If single quotes are used, no interpretation is done on the |
e37d713d |
848 | replacement string (the C</e> modifier overrides this, however). Unlike |
5f05dabc |
849 | Perl 4, Perl 5 treats back-ticks as normal delimiters; the replacement |
e37d713d |
850 | text is not evaluated as a command. If the |
a0d0e21e |
851 | PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own |
5f05dabc |
852 | pair of quotes, which may or may not be bracketing quotes, e.g., |
a0d0e21e |
853 | C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the |
854 | replacement portion to be interpreter as a full-fledged Perl expression |
855 | and eval()ed right then and there. It is, however, syntax checked at |
856 | compile-time. |
857 | |
858 | Examples: |
859 | |
860 | s/\bgreen\b/mauve/g; # don't change wintergreen |
861 | |
862 | $path =~ s|/usr/bin|/usr/local/bin|; |
863 | |
864 | s/Login: $foo/Login: $bar/; # run-time pattern |
865 | |
866 | ($foo = $bar) =~ s/this/that/; |
867 | |
868 | $count = ($paragraph =~ s/Mister\b/Mr./g); |
869 | |
870 | $_ = 'abc123xyz'; |
871 | s/\d+/$&*2/e; # yields 'abc246xyz' |
872 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' |
873 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' |
874 | |
875 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e |
876 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e |
877 | s/^=(\w+)/&pod($1)/ge; # use function call |
878 | |
879 | # /e's can even nest; this will expand |
880 | # simple embedded variables in $_ |
881 | s/(\$\w+)/$1/eeg; |
882 | |
883 | # Delete C comments. |
884 | $program =~ s { |
4633a7c4 |
885 | /\* # Match the opening delimiter. |
886 | .*? # Match a minimal number of characters. |
887 | \*/ # Match the closing delimiter. |
a0d0e21e |
888 | } []gsx; |
889 | |
890 | s/^\s*(.*?)\s*$/$1/; # trim white space |
891 | |
892 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields |
893 | |
894 | Note the use of $ instead of \ in the last example. Unlike |
5f05dabc |
895 | B<sed>, we use the \E<lt>I<digit>E<gt> form in only the left hand side. |
6ee5d4e7 |
896 | Anywhere else it's $E<lt>I<digit>E<gt>. |
a0d0e21e |
897 | |
5f05dabc |
898 | Occasionally, you can't use just a C</g> to get all the changes |
a0d0e21e |
899 | to occur. Here are two common cases: |
900 | |
901 | # put commas in the right places in an integer |
902 | 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4 |
903 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5 |
904 | |
905 | # expand tabs to 8-column spacing |
906 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; |
907 | |
908 | |
909 | =item tr/SEARCHLIST/REPLACEMENTLIST/cds |
910 | |
911 | =item y/SEARCHLIST/REPLACEMENTLIST/cds |
912 | |
913 | Translates all occurrences of the characters found in the search list |
914 | with the corresponding character in the replacement list. It returns |
915 | the number of characters replaced or deleted. If no string is |
916 | specified via the =~ or !~ operator, the $_ string is translated. (The |
917 | string specified with =~ must be a scalar variable, an array element, |
5f05dabc |
918 | or an assignment to one of those, i.e., an lvalue.) For B<sed> devotees, |
a0d0e21e |
919 | C<y> is provided as a synonym for C<tr>. If the SEARCHLIST is |
920 | delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of |
5f05dabc |
921 | quotes, which may or may not be bracketing quotes, e.g., C<tr[A-Z][a-z]> |
a0d0e21e |
922 | or C<tr(+-*/)/ABCD/>. |
923 | |
924 | Options: |
925 | |
926 | c Complement the SEARCHLIST. |
927 | d Delete found but unreplaced characters. |
928 | s Squash duplicate replaced characters. |
929 | |
930 | If the C</c> modifier is specified, the SEARCHLIST character set is |
931 | complemented. If the C</d> modifier is specified, any characters specified |
932 | by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note |
933 | that this is slightly more flexible than the behavior of some B<tr> |
934 | programs, which delete anything they find in the SEARCHLIST, period.) |
935 | If the C</s> modifier is specified, sequences of characters that were |
936 | translated to the same character are squashed down to a single instance of the |
937 | character. |
938 | |
939 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted |
940 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter |
941 | than the SEARCHLIST, the final character is replicated till it is long |
942 | enough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated. |
943 | This latter is useful for counting characters in a class or for |
944 | squashing character sequences in a class. |
945 | |
946 | Examples: |
947 | |
948 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case |
949 | |
950 | $cnt = tr/*/*/; # count the stars in $_ |
951 | |
952 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky |
953 | |
954 | $cnt = tr/0-9//; # count the digits in $_ |
955 | |
956 | tr/a-zA-Z//s; # bookkeeper -> bokeper |
957 | |
958 | ($HOST = $host) =~ tr/a-z/A-Z/; |
959 | |
960 | tr/a-zA-Z/ /cs; # change non-alphas to single space |
961 | |
962 | tr [\200-\377] |
963 | [\000-\177]; # delete 8th bit |
964 | |
748a9306 |
965 | If multiple translations are given for a character, only the first one is used: |
966 | |
967 | tr/AAA/XYZ/ |
968 | |
969 | will translate any A to X. |
970 | |
a0d0e21e |
971 | Note that because the translation table is built at compile time, neither |
972 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote |
973 | interpolation. That means that if you want to use variables, you must use |
974 | an eval(): |
975 | |
976 | eval "tr/$oldlist/$newlist/"; |
977 | die $@ if $@; |
978 | |
979 | eval "tr/$oldlist/$newlist/, 1" or die $@; |
980 | |
981 | =back |
982 | |
983 | =head2 I/O Operators |
984 | |
985 | There are several I/O operators you should know about. |
5f05dabc |
986 | A string is enclosed by back-ticks (grave accents) first undergoes |
a0d0e21e |
987 | variable substitution just like a double quoted string. It is then |
988 | interpreted as a command, and the output of that command is the value |
989 | of the pseudo-literal, like in a shell. In a scalar context, a single |
990 | string consisting of all the output is returned. In a list context, |
991 | a list of values is returned, one for each line of output. (You can |
992 | set C<$/> to use a different line terminator.) The command is executed |
993 | each time the pseudo-literal is evaluated. The status value of the |
994 | command is returned in C<$?> (see L<perlvar> for the interpretation |
995 | of C<$?>). Unlike in B<csh>, no translation is done on the return |
996 | data--newlines remain newlines. Unlike in any of the shells, single |
997 | quotes do not hide variable names in the command from interpretation. |
998 | To pass a $ through to the shell you need to hide it with a backslash. |
5f05dabc |
999 | The generalized form of back-ticks is C<qx//>. (Because back-ticks |
cb1a09d0 |
1000 | always undergo shell expansion as well, see L<perlsec> for |
1001 | security concerns.) |
a0d0e21e |
1002 | |
1003 | Evaluating a filehandle in angle brackets yields the next line from |
748a9306 |
1004 | that file (newline included, so it's never false until end of file, at |
1005 | which time an undefined value is returned). Ordinarily you must assign |
1006 | that value to a variable, but there is one situation where an automatic |
a0d0e21e |
1007 | assignment happens. I<If and ONLY if> the input symbol is the only |
1008 | thing inside the conditional of a C<while> loop, the value is |
748a9306 |
1009 | automatically assigned to the variable C<$_>. The assigned value is |
1010 | then tested to see if it is defined. (This may seem like an odd thing |
1011 | to you, but you'll use the construct in almost every Perl script you |
1012 | write.) Anyway, the following lines are equivalent to each other: |
a0d0e21e |
1013 | |
748a9306 |
1014 | while (defined($_ = <STDIN>)) { print; } |
a0d0e21e |
1015 | while (<STDIN>) { print; } |
1016 | for (;<STDIN>;) { print; } |
748a9306 |
1017 | print while defined($_ = <STDIN>); |
a0d0e21e |
1018 | print while <STDIN>; |
1019 | |
5f05dabc |
1020 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
1021 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in |
a0d0e21e |
1022 | packages, where they would be interpreted as local identifiers rather |
1023 | than global.) Additional filehandles may be created with the open() |
cb1a09d0 |
1024 | function. See L<perlfunc/open()> for details on this. |
a0d0e21e |
1025 | |
6ee5d4e7 |
1026 | If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a |
a0d0e21e |
1027 | list consisting of all the input lines is returned, one line per list |
1028 | element. It's easy to make a I<LARGE> data space this way, so use with |
1029 | care. |
1030 | |
d28ebecd |
1031 | The null filehandle E<lt>E<gt> is special and can be used to emulate the |
1032 | behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from |
a0d0e21e |
1033 | standard input, or from each file listed on the command line. Here's |
d28ebecd |
1034 | how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is |
a0d0e21e |
1035 | checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened |
1036 | gives you standard input. The @ARGV array is then processed as a list |
1037 | of filenames. The loop |
1038 | |
1039 | while (<>) { |
1040 | ... # code for each line |
1041 | } |
1042 | |
1043 | is equivalent to the following Perl-like pseudo code: |
1044 | |
1045 | unshift(@ARGV, '-') if $#ARGV < $[; |
1046 | while ($ARGV = shift) { |
1047 | open(ARGV, $ARGV); |
1048 | while (<ARGV>) { |
1049 | ... # code for each line |
1050 | } |
1051 | } |
1052 | |
1053 | except that it isn't so cumbersome to say, and will actually work. It |
1054 | really does shift array @ARGV and put the current filename into variable |
5f05dabc |
1055 | $ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a |
1056 | synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above |
1057 | doesn't work because it treats E<lt>ARGVE<gt> as non-magical.) |
a0d0e21e |
1058 | |
d28ebecd |
1059 | You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up |
a0d0e21e |
1060 | containing the list of filenames you really want. Line numbers (C<$.>) |
1061 | continue as if the input were one big happy file. (But see example |
1062 | under eof() for how to reset line numbers on each file.) |
1063 | |
1064 | If you want to set @ARGV to your own list of files, go right ahead. If |
1065 | you want to pass switches into your script, you can use one of the |
1066 | Getopts modules or put a loop on the front like this: |
1067 | |
1068 | while ($_ = $ARGV[0], /^-/) { |
1069 | shift; |
1070 | last if /^--$/; |
1071 | if (/^-D(.*)/) { $debug = $1 } |
1072 | if (/^-v/) { $verbose++ } |
1073 | ... # other switches |
1074 | } |
1075 | while (<>) { |
1076 | ... # code for each line |
1077 | } |
1078 | |
d28ebecd |
1079 | The E<lt>E<gt> symbol will return FALSE only once. If you call it again after |
a0d0e21e |
1080 | this it will assume you are processing another @ARGV list, and if you |
1081 | haven't set @ARGV, will input from STDIN. |
1082 | |
1083 | If the string inside the angle brackets is a reference to a scalar |
5f05dabc |
1084 | variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the |
cb1a09d0 |
1085 | filehandle to input from, or a reference to the same. For example: |
1086 | |
1087 | $fh = \*STDIN; |
1088 | $line = <$fh>; |
a0d0e21e |
1089 | |
cb1a09d0 |
1090 | If the string inside angle brackets is not a filehandle or a scalar |
1091 | variable containing a filehandle name or reference, then it is interpreted |
4633a7c4 |
1092 | as a filename pattern to be globbed, and either a list of filenames or the |
1093 | next filename in the list is returned, depending on context. One level of |
1094 | $ interpretation is done first, but you can't say C<E<lt>$fooE<gt>> |
1095 | because that's an indirect filehandle as explained in the previous |
6ee5d4e7 |
1096 | paragraph. (In older versions of Perl, programmers would insert curly |
4633a7c4 |
1097 | brackets to force interpretation as a filename glob: C<E<lt>${foo}E<gt>>. |
d28ebecd |
1098 | These days, it's considered cleaner to call the internal function directly |
4633a7c4 |
1099 | as C<glob($foo)>, which is probably the right way to have done it in the |
1100 | first place.) Example: |
a0d0e21e |
1101 | |
1102 | while (<*.c>) { |
1103 | chmod 0644, $_; |
1104 | } |
1105 | |
1106 | is equivalent to |
1107 | |
1108 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); |
1109 | while (<FOO>) { |
1110 | chop; |
1111 | chmod 0644, $_; |
1112 | } |
1113 | |
1114 | In fact, it's currently implemented that way. (Which means it will not |
1115 | work on filenames with spaces in them unless you have csh(1) on your |
1116 | machine.) Of course, the shortest way to do the above is: |
1117 | |
1118 | chmod 0644, <*.c>; |
1119 | |
1120 | Because globbing invokes a shell, it's often faster to call readdir() yourself |
5f05dabc |
1121 | and do your own grep() on the filenames. Furthermore, due to its current |
a0d0e21e |
1122 | implementation of using a shell, the glob() routine may get "Arg list too |
1123 | long" errors (unless you've installed tcsh(1L) as F</bin/csh>). |
1124 | |
5f05dabc |
1125 | A glob evaluates its (embedded) argument only when it is starting a new |
4633a7c4 |
1126 | list. All values must be read before it will start over. In a list |
1127 | context this isn't important, because you automatically get them all |
1128 | anyway. In a scalar context, however, the operator returns the next value |
1129 | each time it is called, or a FALSE value if you've just run out. Again, |
1130 | FALSE is returned only once. So if you're expecting a single value from |
1131 | a glob, it is much better to say |
1132 | |
1133 | ($file) = <blurch*>; |
1134 | |
1135 | than |
1136 | |
1137 | $file = <blurch*>; |
1138 | |
1139 | because the latter will alternate between returning a filename and |
1140 | returning FALSE. |
1141 | |
1142 | It you're trying to do variable interpolation, it's definitely better |
1143 | to use the glob() function, because the older notation can cause people |
e37d713d |
1144 | to become confused with the indirect filehandle notation. |
4633a7c4 |
1145 | |
1146 | @files = glob("$dir/*.[ch]"); |
1147 | @files = glob($files[$i]); |
1148 | |
a0d0e21e |
1149 | =head2 Constant Folding |
1150 | |
1151 | Like C, Perl does a certain amount of expression evaluation at |
1152 | compile time, whenever it determines that all of the arguments to an |
1153 | operator are static and have no side effects. In particular, string |
1154 | concatenation happens at compile time between literals that don't do |
1155 | variable substitution. Backslash interpretation also happens at |
1156 | compile time. You can say |
1157 | |
1158 | 'Now is the time for all' . "\n" . |
1159 | 'good men to come to.' |
1160 | |
1161 | and this all reduces to one string internally. Likewise, if |
1162 | you say |
1163 | |
1164 | foreach $file (@filenames) { |
1165 | if (-s $file > 5 + 100 * 2**16) { ... } |
1166 | } |
1167 | |
1168 | the compiler will pre-compute the number that |
1169 | expression represents so that the interpreter |
1170 | won't have to. |
1171 | |
1172 | |
55497cff |
1173 | =head2 Integer Arithmetic |
a0d0e21e |
1174 | |
1175 | By default Perl assumes that it must do most of its arithmetic in |
1176 | floating point. But by saying |
1177 | |
1178 | use integer; |
1179 | |
1180 | you may tell the compiler that it's okay to use integer operations |
1181 | from here to the end of the enclosing BLOCK. An inner BLOCK may |
1182 | countermand this by saying |
1183 | |
1184 | no integer; |
1185 | |
1186 | which lasts until the end of that BLOCK. |
1187 | |
55497cff |
1188 | The bitwise operators ("&", "|", "^", "~", "<<", and ">>") always |
1189 | produce integral results. However, C<use integer> still has meaning |
1190 | for them. By default, their results are interpreted as unsigned |
1191 | integers. However, if C<use integer> is in effect, their results are |
5f05dabc |
1192 | interpreted as signed integers. For example, C<~0> usually evaluates |
55497cff |
1193 | to a large integral value. However, C<use integer; ~0> is -1. |