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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 | $_. The return value indicates the success of the operation. (If the |
168 | right argument is an expression rather than a search pattern, |
169 | substitution, or translation, it is interpreted as a search pattern at run |
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170 | time. This can be is less efficient than an explicit search, because the |
171 | pattern must be compiled every time the expression is evaluated. |
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172 | |
173 | Binary "!~" is just like "=~" except the return value is negated in |
174 | the logical sense. |
175 | |
176 | =head2 Multiplicative Operators |
177 | |
178 | Binary "*" multiplies two numbers. |
179 | |
180 | Binary "/" divides two numbers. |
181 | |
182 | Binary "%" computes the modulus of the two numbers. |
183 | |
184 | Binary "x" is the repetition operator. In a scalar context, it |
185 | returns a string consisting of the left operand repeated the number of |
186 | times specified by the right operand. In a list context, if the left |
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187 | operand is a list in parentheses, it repeats the list. |
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188 | |
189 | print '-' x 80; # print row of dashes |
190 | |
191 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over |
192 | |
193 | @ones = (1) x 80; # a list of 80 1's |
194 | @ones = (5) x @ones; # set all elements to 5 |
195 | |
196 | |
197 | =head2 Additive Operators |
198 | |
199 | Binary "+" returns the sum of two numbers. |
200 | |
201 | Binary "-" returns the difference of two numbers. |
202 | |
203 | Binary "." concatenates two strings. |
204 | |
205 | =head2 Shift Operators |
206 | |
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207 | Binary "<<" returns the value of its left argument shifted left by the |
208 | number of bits specified by the right argument. Arguments should be |
209 | integers. (See also L<Integer Arithmetic>.) |
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210 | |
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211 | Binary ">>" returns the value of its left argument shifted right by |
212 | the number of bits specified by the right argument. Arguments should |
213 | be integers. (See also L<Integer Arithmetic>.) |
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214 | |
215 | =head2 Named Unary Operators |
216 | |
217 | The various named unary operators are treated as functions with one |
218 | argument, with optional parentheses. These include the filetest |
219 | operators, like C<-f>, C<-M>, etc. See L<perlfunc>. |
220 | |
221 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) |
222 | is followed by a left parenthesis as the next token, the operator and |
223 | arguments within parentheses are taken to be of highest precedence, |
224 | just like a normal function call. Examples: |
225 | |
226 | chdir $foo || die; # (chdir $foo) || die |
227 | chdir($foo) || die; # (chdir $foo) || die |
228 | chdir ($foo) || die; # (chdir $foo) || die |
229 | chdir +($foo) || die; # (chdir $foo) || die |
230 | |
231 | but, because * is higher precedence than ||: |
232 | |
233 | chdir $foo * 20; # chdir ($foo * 20) |
234 | chdir($foo) * 20; # (chdir $foo) * 20 |
235 | chdir ($foo) * 20; # (chdir $foo) * 20 |
236 | chdir +($foo) * 20; # chdir ($foo * 20) |
237 | |
238 | rand 10 * 20; # rand (10 * 20) |
239 | rand(10) * 20; # (rand 10) * 20 |
240 | rand (10) * 20; # (rand 10) * 20 |
241 | rand +(10) * 20; # rand (10 * 20) |
242 | |
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243 | See also L<"Terms and List Operators (Leftward)">. |
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244 | |
245 | =head2 Relational Operators |
246 | |
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247 | Binary "E<lt>" returns true if the left argument is numerically less than |
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248 | the right argument. |
249 | |
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250 | Binary "E<gt>" returns true if the left argument is numerically greater |
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251 | than the right argument. |
252 | |
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253 | Binary "E<lt>=" returns true if the left argument is numerically less than |
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254 | or equal to the right argument. |
255 | |
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256 | Binary "E<gt>=" returns true if the left argument is numerically greater |
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257 | than or equal to the right argument. |
258 | |
259 | Binary "lt" returns true if the left argument is stringwise less than |
260 | the right argument. |
261 | |
262 | Binary "gt" returns true if the left argument is stringwise greater |
263 | than the right argument. |
264 | |
265 | Binary "le" returns true if the left argument is stringwise less than |
266 | or equal to the right argument. |
267 | |
268 | Binary "ge" returns true if the left argument is stringwise greater |
269 | than or equal to the right argument. |
270 | |
271 | =head2 Equality Operators |
272 | |
273 | Binary "==" returns true if the left argument is numerically equal to |
274 | the right argument. |
275 | |
276 | Binary "!=" returns true if the left argument is numerically not equal |
277 | to the right argument. |
278 | |
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279 | Binary "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left |
280 | argument is numerically less than, equal to, or greater than the right |
281 | argument. |
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282 | |
283 | Binary "eq" returns true if the left argument is stringwise equal to |
284 | the right argument. |
285 | |
286 | Binary "ne" returns true if the left argument is stringwise not equal |
287 | to the right argument. |
288 | |
289 | Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise |
290 | less than, equal to, or greater than the right argument. |
291 | |
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292 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
293 | by the current locale if C<use locale> is in effect. See L<perllocale>. |
294 | |
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295 | =head2 Bitwise And |
296 | |
297 | Binary "&" returns its operators ANDed together bit by bit. |
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298 | (See also L<Integer Arithmetic>.) |
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299 | |
300 | =head2 Bitwise Or and Exclusive Or |
301 | |
302 | Binary "|" returns its operators ORed together bit by bit. |
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303 | (See also L<Integer Arithmetic>.) |
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304 | |
305 | Binary "^" returns its operators XORed together bit by bit. |
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306 | (See also L<Integer Arithmetic>.) |
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307 | |
308 | =head2 C-style Logical And |
309 | |
310 | Binary "&&" performs a short-circuit logical AND operation. That is, |
311 | if the left operand is false, the right operand is not even evaluated. |
312 | Scalar or list context propagates down to the right operand if it |
313 | is evaluated. |
314 | |
315 | =head2 C-style Logical Or |
316 | |
317 | Binary "||" performs a short-circuit logical OR operation. That is, |
318 | if the left operand is true, the right operand is not even evaluated. |
319 | Scalar or list context propagates down to the right operand if it |
320 | is evaluated. |
321 | |
322 | The C<||> and C<&&> operators differ from C's in that, rather than returning |
323 | 0 or 1, they return the last value evaluated. Thus, a reasonably portable |
324 | way to find out the home directory (assuming it's not "0") might be: |
325 | |
326 | $home = $ENV{'HOME'} || $ENV{'LOGDIR'} || |
327 | (getpwuid($<))[7] || die "You're homeless!\n"; |
328 | |
329 | As more readable alternatives to C<&&> and C<||>, Perl provides "and" and |
330 | "or" operators (see below). The short-circuit behavior is identical. The |
331 | precedence of "and" and "or" is much lower, however, so that you can |
332 | safely use them after a list operator without the need for |
333 | parentheses: |
334 | |
335 | unlink "alpha", "beta", "gamma" |
336 | or gripe(), next LINE; |
337 | |
338 | With the C-style operators that would have been written like this: |
339 | |
340 | unlink("alpha", "beta", "gamma") |
341 | || (gripe(), next LINE); |
342 | |
343 | =head2 Range Operator |
344 | |
345 | Binary ".." is the range operator, which is really two different |
346 | operators depending on the context. In a list context, it returns an |
347 | array of values counting (by ones) from the left value to the right |
348 | value. This is useful for writing C<for (1..10)> loops and for doing |
349 | slice operations on arrays. Be aware that under the current implementation, |
350 | a temporary array is created, so you'll burn a lot of memory if you |
351 | write something like this: |
352 | |
353 | for (1 .. 1_000_000) { |
354 | # code |
355 | } |
356 | |
357 | In a scalar context, ".." returns a boolean value. The operator is |
358 | bistable, like a flip-flop, and emulates the line-range (comma) operator |
359 | of B<sed>, B<awk>, and various editors. Each ".." operator maintains its |
360 | own boolean state. It is false as long as its left operand is false. |
361 | Once the left operand is true, the range operator stays true until the |
362 | right operand is true, I<AFTER> which the range operator becomes false |
363 | again. (It doesn't become false till the next time the range operator is |
364 | evaluated. It can test the right operand and become false on the same |
365 | evaluation it became true (as in B<awk>), but it still returns true once. |
366 | If you don't want it to test the right operand till the next evaluation |
367 | (as in B<sed>), use three dots ("...") instead of two.) The right |
368 | operand is not evaluated while the operator is in the "false" state, and |
369 | the left operand is not evaluated while the operator is in the "true" |
370 | state. The precedence is a little lower than || and &&. The value |
371 | returned is either the null string for false, or a sequence number |
372 | (beginning with 1) for true. The sequence number is reset for each range |
373 | encountered. The final sequence number in a range has the string "E0" |
374 | appended to it, which doesn't affect its numeric value, but gives you |
375 | something to search for if you want to exclude the endpoint. You can |
376 | exclude the beginning point by waiting for the sequence number to be |
377 | greater than 1. If either operand of scalar ".." is a numeric literal, |
378 | that operand is implicitly compared to the C<$.> variable, the current |
379 | line number. Examples: |
380 | |
381 | As a scalar operator: |
382 | |
383 | if (101 .. 200) { print; } # print 2nd hundred lines |
384 | next line if (1 .. /^$/); # skip header lines |
385 | s/^/> / if (/^$/ .. eof()); # quote body |
386 | |
387 | As a list operator: |
388 | |
389 | for (101 .. 200) { print; } # print $_ 100 times |
390 | @foo = @foo[$[ .. $#foo]; # an expensive no-op |
391 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items |
392 | |
393 | The range operator (in a list context) makes use of the magical |
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394 | auto-increment algorithm if the operands are strings. You |
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395 | can say |
396 | |
397 | @alphabet = ('A' .. 'Z'); |
398 | |
399 | to get all the letters of the alphabet, or |
400 | |
401 | $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; |
402 | |
403 | to get a hexadecimal digit, or |
404 | |
405 | @z2 = ('01' .. '31'); print $z2[$mday]; |
406 | |
407 | to get dates with leading zeros. If the final value specified is not |
408 | in the sequence that the magical increment would produce, the sequence |
409 | goes until the next value would be longer than the final value |
410 | specified. |
411 | |
412 | =head2 Conditional Operator |
413 | |
414 | Ternary "?:" is the conditional operator, just as in C. It works much |
415 | like an if-then-else. If the argument before the ? is true, the |
416 | argument before the : is returned, otherwise the argument after the : |
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417 | is returned. For example: |
418 | |
419 | printf "I have %d dog%s.\n", $n, |
420 | ($n == 1) ? '' : "s"; |
421 | |
422 | Scalar or list context propagates downward into the 2nd |
423 | or 3rd argument, whichever is selected. |
424 | |
425 | $a = $ok ? $b : $c; # get a scalar |
426 | @a = $ok ? @b : @c; # get an array |
427 | $a = $ok ? @b : @c; # oops, that's just a count! |
428 | |
429 | The operator may be assigned to if both the 2nd and 3rd arguments are |
430 | legal lvalues (meaning that you can assign to them): |
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431 | |
432 | ($a_or_b ? $a : $b) = $c; |
433 | |
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434 | This is not necessarily guaranteed to contribute to the readability of your program. |
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435 | |
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436 | =head2 Assignment Operators |
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437 | |
438 | "=" is the ordinary assignment operator. |
439 | |
440 | Assignment operators work as in C. That is, |
441 | |
442 | $a += 2; |
443 | |
444 | is equivalent to |
445 | |
446 | $a = $a + 2; |
447 | |
448 | although without duplicating any side effects that dereferencing the lvalue |
449 | might trigger, such as from tie(). Other assignment operators work similarly. |
450 | The following are recognized: |
451 | |
452 | **= += *= &= <<= &&= |
453 | -= /= |= >>= ||= |
454 | .= %= ^= |
455 | x= |
456 | |
457 | Note that while these are grouped by family, they all have the precedence |
458 | of assignment. |
459 | |
460 | Unlike in C, the assignment operator produces a valid lvalue. Modifying |
461 | an assignment is equivalent to doing the assignment and then modifying |
462 | the variable that was assigned to. This is useful for modifying |
463 | a copy of something, like this: |
464 | |
465 | ($tmp = $global) =~ tr [A-Z] [a-z]; |
466 | |
467 | Likewise, |
468 | |
469 | ($a += 2) *= 3; |
470 | |
471 | is equivalent to |
472 | |
473 | $a += 2; |
474 | $a *= 3; |
475 | |
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476 | =head2 Comma Operator |
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477 | |
478 | Binary "," is the comma operator. In a scalar context it evaluates |
479 | its left argument, throws that value away, then evaluates its right |
480 | argument and returns that value. This is just like C's comma operator. |
481 | |
482 | In a list context, it's just the list argument separator, and inserts |
483 | both its arguments into the list. |
484 | |
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485 | The =E<gt> digraph is mostly just a synonym for the comma operator. It's useful for |
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486 | documenting arguments that come in pairs. As of release 5.001, it also forces |
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487 | any word to the left of it to be interpreted as a string. |
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488 | |
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489 | =head2 List Operators (Rightward) |
490 | |
491 | On the right side of a list operator, it has very low precedence, |
492 | such that it controls all comma-separated expressions found there. |
493 | The only operators with lower precedence are the logical operators |
494 | "and", "or", and "not", which may be used to evaluate calls to list |
495 | operators without the need for extra parentheses: |
496 | |
497 | open HANDLE, "filename" |
498 | or die "Can't open: $!\n"; |
499 | |
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500 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
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501 | |
502 | =head2 Logical Not |
503 | |
504 | Unary "not" returns the logical negation of the expression to its right. |
505 | It's the equivalent of "!" except for the very low precedence. |
506 | |
507 | =head2 Logical And |
508 | |
509 | Binary "and" returns the logical conjunction of the two surrounding |
510 | expressions. It's equivalent to && except for the very low |
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511 | precedence. This means that it short-circuits: i.e., the right |
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512 | expression is evaluated only if the left expression is true. |
513 | |
514 | =head2 Logical or and Exclusive Or |
515 | |
516 | Binary "or" returns the logical disjunction of the two surrounding |
517 | expressions. It's equivalent to || except for the very low |
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518 | precedence. This means that it short-circuits: i.e., the right |
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519 | expression is evaluated only if the left expression is false. |
520 | |
521 | Binary "xor" returns the exclusive-OR of the two surrounding expressions. |
522 | It cannot short circuit, of course. |
523 | |
524 | =head2 C Operators Missing From Perl |
525 | |
526 | Here is what C has that Perl doesn't: |
527 | |
528 | =over 8 |
529 | |
530 | =item unary & |
531 | |
532 | Address-of operator. (But see the "\" operator for taking a reference.) |
533 | |
534 | =item unary * |
535 | |
536 | Dereference-address operator. (Perl's prefix dereferencing |
537 | operators are typed: $, @, %, and &.) |
538 | |
539 | =item (TYPE) |
540 | |
541 | Type casting operator. |
542 | |
543 | =back |
544 | |
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545 | =head2 Quote and Quote-like Operators |
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546 | |
547 | While we usually think of quotes as literal values, in Perl they |
548 | function as operators, providing various kinds of interpolating and |
549 | pattern matching capabilities. Perl provides customary quote characters |
550 | for these behaviors, but also provides a way for you to choose your |
551 | quote character for any of them. In the following table, a C<{}> represents |
552 | any pair of delimiters you choose. Non-bracketing delimiters use |
553 | the same character fore and aft, but the 4 sorts of brackets |
554 | (round, angle, square, curly) will all nest. |
555 | |
556 | Customary Generic Meaning Interpolates |
557 | '' q{} Literal no |
558 | "" qq{} Literal yes |
559 | `` qx{} Command yes |
560 | qw{} Word list no |
561 | // m{} Pattern match yes |
562 | s{}{} Substitution yes |
563 | tr{}{} Translation no |
564 | |
cb1a09d0 |
565 | For constructs that do interpolation, variables beginning with "C<$>" or "C<@>" |
a0d0e21e |
566 | are interpolated, as are the following sequences: |
567 | |
6ee5d4e7 |
568 | \t tab (HT, TAB) |
569 | \n newline (LF, NL) |
570 | \r return (CR) |
571 | \f form feed (FF) |
572 | \b backspace (BS) |
573 | \a alarm (bell) (BEL) |
574 | \e escape (ESC) |
a0d0e21e |
575 | \033 octal char |
576 | \x1b hex char |
577 | \c[ control char |
578 | \l lowercase next char |
579 | \u uppercase next char |
580 | \L lowercase till \E |
581 | \U uppercase till \E |
582 | \E end case modification |
583 | \Q quote regexp metacharacters till \E |
584 | |
a034a98d |
585 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> |
586 | and <\U> is taken from the current locale. See L<perllocale>. |
587 | |
a0d0e21e |
588 | Patterns are subject to an additional level of interpretation as a |
589 | regular expression. This is done as a second pass, after variables are |
590 | interpolated, so that regular expressions may be incorporated into the |
591 | pattern from the variables. If this is not what you want, use C<\Q> to |
592 | interpolate a variable literally. |
593 | |
594 | Apart from the above, there are no multiple levels of interpolation. In |
5f05dabc |
595 | particular, contrary to the expectations of shell programmers, back-quotes |
a0d0e21e |
596 | do I<NOT> interpolate within double quotes, nor do single quotes impede |
597 | evaluation of variables when used within double quotes. |
598 | |
5f05dabc |
599 | =head2 Regexp Quote-Like Operators |
cb1a09d0 |
600 | |
5f05dabc |
601 | Here are the quote-like operators that apply to pattern |
cb1a09d0 |
602 | matching and related activities. |
603 | |
a0d0e21e |
604 | =over 8 |
605 | |
606 | =item ?PATTERN? |
607 | |
608 | This is just like the C</pattern/> search, except that it matches only |
609 | once between calls to the reset() operator. This is a useful |
5f05dabc |
610 | optimization when you want to see only the first occurrence of |
a0d0e21e |
611 | something in each file of a set of files, for instance. Only C<??> |
612 | patterns local to the current package are reset. |
613 | |
614 | This usage is vaguely deprecated, and may be removed in some future |
615 | version of Perl. |
616 | |
617 | =item m/PATTERN/gimosx |
618 | |
619 | =item /PATTERN/gimosx |
620 | |
621 | Searches a string for a pattern match, and in a scalar context returns |
622 | true (1) or false (''). If no string is specified via the C<=~> or |
623 | C<!~> operator, the $_ string is searched. (The string specified with |
624 | C<=~> need not be an lvalue--it may be the result of an expression |
625 | evaluation, but remember the C<=~> binds rather tightly.) See also |
626 | L<perlre>. |
a034a98d |
627 | See L<perllocale> for discussion of additional considerations which apply |
628 | when C<use locale> is in effect. |
a0d0e21e |
629 | |
630 | Options are: |
631 | |
5f05dabc |
632 | g Match globally, i.e., find all occurrences. |
a0d0e21e |
633 | i Do case-insensitive pattern matching. |
634 | m Treat string as multiple lines. |
5f05dabc |
635 | o Compile pattern only once. |
a0d0e21e |
636 | s Treat string as single line. |
637 | x Use extended regular expressions. |
638 | |
639 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> |
640 | you can use any pair of non-alphanumeric, non-whitespace characters as |
641 | delimiters. This is particularly useful for matching Unix path names |
642 | that contain "/", to avoid LTS (leaning toothpick syndrome). |
643 | |
644 | PATTERN may contain variables, which will be interpolated (and the |
645 | pattern recompiled) every time the pattern search is evaluated. (Note |
646 | that C<$)> and C<$|> might not be interpolated because they look like |
647 | end-of-string tests.) If you want such a pattern to be compiled only |
648 | once, add a C</o> after the trailing delimiter. This avoids expensive |
649 | run-time recompilations, and is useful when the value you are |
650 | interpolating won't change over the life of the script. However, mentioning |
651 | C</o> constitutes a promise that you won't change the variables in the pattern. |
652 | If you change them, Perl won't even notice. |
653 | |
4633a7c4 |
654 | If the PATTERN evaluates to a null string, the last |
655 | successfully executed regular expression is used instead. |
a0d0e21e |
656 | |
657 | If used in a context that requires a list value, a pattern match returns a |
658 | list consisting of the subexpressions matched by the parentheses in the |
5f05dabc |
659 | pattern, i.e., (C<$1>, $2, $3...). (Note that here $1 etc. are also set, and |
a0d0e21e |
660 | that this differs from Perl 4's behavior.) If the match fails, a null |
661 | array is returned. If the match succeeds, but there were no parentheses, |
662 | a list value of (1) is returned. |
663 | |
664 | Examples: |
665 | |
666 | open(TTY, '/dev/tty'); |
667 | <TTY> =~ /^y/i && foo(); # do foo if desired |
668 | |
669 | if (/Version: *([0-9.]*)/) { $version = $1; } |
670 | |
671 | next if m#^/usr/spool/uucp#; |
672 | |
673 | # poor man's grep |
674 | $arg = shift; |
675 | while (<>) { |
676 | print if /$arg/o; # compile only once |
677 | } |
678 | |
679 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) |
680 | |
681 | This last example splits $foo into the first two words and the |
5f05dabc |
682 | remainder of the line, and assigns those three fields to $F1, $F2, and |
683 | $Etc. The conditional is true if any variables were assigned, i.e., if |
a0d0e21e |
684 | the pattern matched. |
685 | |
686 | The C</g> modifier specifies global pattern matching--that is, matching |
687 | as many times as possible within the string. How it behaves depends on |
688 | the context. In a list context, it returns a list of all the |
689 | substrings matched by all the parentheses in the regular expression. |
690 | If there are no parentheses, it returns a list of all the matched |
691 | strings, as if there were parentheses around the whole pattern. |
692 | |
693 | In a scalar context, C<m//g> iterates through the string, returning TRUE |
694 | each time it matches, and FALSE when it eventually runs out of |
695 | matches. (In other words, it remembers where it left off last time and |
696 | restarts the search at that point. You can actually find the current |
44a8e56a |
697 | match position of a string or set it using the pos() function--see |
698 | L<perlfunc/pos>.) Note that you can use this feature to stack C<m//g> |
699 | matches or intermix C<m//g> matches with C<m/\G.../>. |
700 | |
a0d0e21e |
701 | If you modify the string in any way, the match position is reset to the |
702 | beginning. Examples: |
703 | |
704 | # list context |
705 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); |
706 | |
707 | # scalar context |
5f05dabc |
708 | $/ = ""; $* = 1; # $* deprecated in modern perls |
a0d0e21e |
709 | while ($paragraph = <>) { |
710 | while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { |
711 | $sentences++; |
712 | } |
713 | } |
714 | print "$sentences\n"; |
715 | |
44a8e56a |
716 | # using m//g with \G |
717 | $_ = "ppooqppq"; |
718 | while ($i++ < 2) { |
719 | print "1: '"; |
720 | print $1 while /(o)/g; print "', pos=", pos, "\n"; |
721 | print "2: '"; |
722 | print $1 if /\G(q)/; print "', pos=", pos, "\n"; |
723 | print "3: '"; |
724 | print $1 while /(p)/g; print "', pos=", pos, "\n"; |
725 | } |
726 | |
727 | The last example should print: |
728 | |
729 | 1: 'oo', pos=4 |
730 | 2: 'q', pos=4 |
731 | 3: 'pp', pos=7 |
732 | 1: '', pos=7 |
733 | 2: 'q', pos=7 |
734 | 3: '', pos=7 |
735 | |
736 | Note how C<m//g> matches change the value reported by C<pos()>, but the |
737 | non-global match doesn't. |
738 | |
e7ea3e70 |
739 | A useful idiom for C<lex>-like scanners is C</\G.../g>. You can |
740 | combine several regexps like this to process a string part-by-part, |
741 | doing different actions depending on which regexp matched. The next |
742 | regexp would step in at the place the previous one left off. |
743 | |
744 | $_ = <<'EOL'; |
745 | $url = new URI::URL "http://www/"; die if $url eq "xXx"; |
746 | EOL |
747 | LOOP: |
748 | { |
749 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/g; |
750 | print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/g; |
751 | print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/g; |
752 | print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/g; |
753 | print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/g; |
754 | print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/g; |
755 | print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/g; |
756 | print ". That's all!\n"; |
757 | } |
758 | |
759 | Here is the output (split into several lines): |
760 | |
761 | line-noise lowercase line-noise lowercase UPPERCASE line-noise |
762 | UPPERCASE line-noise lowercase line-noise lowercase line-noise |
763 | lowercase lowercase line-noise lowercase lowercase line-noise |
764 | MiXeD line-noise. That's all! |
44a8e56a |
765 | |
a0d0e21e |
766 | =item q/STRING/ |
767 | |
768 | =item C<'STRING'> |
769 | |
770 | A single-quoted, literal string. Backslashes are ignored, unless |
771 | followed by the delimiter or another backslash, in which case the |
772 | delimiter or backslash is interpolated. |
773 | |
774 | $foo = q!I said, "You said, 'She said it.'"!; |
775 | $bar = q('This is it.'); |
776 | |
777 | =item qq/STRING/ |
778 | |
779 | =item "STRING" |
780 | |
781 | A double-quoted, interpolated string. |
782 | |
783 | $_ .= qq |
784 | (*** The previous line contains the naughty word "$1".\n) |
785 | if /(tcl|rexx|python)/; # :-) |
786 | |
787 | =item qx/STRING/ |
788 | |
789 | =item `STRING` |
790 | |
791 | A string which is interpolated and then executed as a system command. |
792 | The collected standard output of the command is returned. In scalar |
793 | context, it comes back as a single (potentially multi-line) string. |
794 | In list context, returns a list of lines (however you've defined lines |
795 | with $/ or $INPUT_RECORD_SEPARATOR). |
796 | |
797 | $today = qx{ date }; |
798 | |
799 | See L<I/O Operators> for more discussion. |
800 | |
801 | =item qw/STRING/ |
802 | |
803 | Returns a list of the words extracted out of STRING, using embedded |
804 | whitespace as the word delimiters. It is exactly equivalent to |
805 | |
806 | split(' ', q/STRING/); |
807 | |
808 | Some frequently seen examples: |
809 | |
810 | use POSIX qw( setlocale localeconv ) |
811 | @EXPORT = qw( foo bar baz ); |
812 | |
813 | =item s/PATTERN/REPLACEMENT/egimosx |
814 | |
815 | Searches a string for a pattern, and if found, replaces that pattern |
816 | with the replacement text and returns the number of substitutions |
e37d713d |
817 | made. Otherwise it returns false (specifically, the empty string). |
a0d0e21e |
818 | |
819 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> |
820 | variable is searched and modified. (The string specified with C<=~> must |
821 | be a scalar variable, an array element, a hash element, or an assignment |
5f05dabc |
822 | to one of those, i.e., an lvalue.) |
a0d0e21e |
823 | |
824 | If the delimiter chosen is single quote, no variable interpolation is |
825 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the |
826 | PATTERN contains a $ that looks like a variable rather than an |
827 | end-of-string test, the variable will be interpolated into the pattern |
5f05dabc |
828 | at run-time. If you want the pattern compiled only once the first time |
a0d0e21e |
829 | the variable is interpolated, use the C</o> option. If the pattern |
4633a7c4 |
830 | evaluates to a null string, the last successfully executed regular |
a0d0e21e |
831 | expression is used instead. See L<perlre> for further explanation on these. |
a034a98d |
832 | See L<perllocale> for discussion of additional considerations which apply |
833 | when C<use locale> is in effect. |
a0d0e21e |
834 | |
835 | Options are: |
836 | |
837 | e Evaluate the right side as an expression. |
5f05dabc |
838 | g Replace globally, i.e., all occurrences. |
a0d0e21e |
839 | i Do case-insensitive pattern matching. |
840 | m Treat string as multiple lines. |
5f05dabc |
841 | o Compile pattern only once. |
a0d0e21e |
842 | s Treat string as single line. |
843 | x Use extended regular expressions. |
844 | |
845 | Any non-alphanumeric, non-whitespace delimiter may replace the |
846 | slashes. If single quotes are used, no interpretation is done on the |
e37d713d |
847 | replacement string (the C</e> modifier overrides this, however). Unlike |
5f05dabc |
848 | Perl 4, Perl 5 treats back-ticks as normal delimiters; the replacement |
e37d713d |
849 | text is not evaluated as a command. If the |
a0d0e21e |
850 | PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own |
5f05dabc |
851 | pair of quotes, which may or may not be bracketing quotes, e.g., |
a0d0e21e |
852 | C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the |
853 | replacement portion to be interpreter as a full-fledged Perl expression |
854 | and eval()ed right then and there. It is, however, syntax checked at |
855 | compile-time. |
856 | |
857 | Examples: |
858 | |
859 | s/\bgreen\b/mauve/g; # don't change wintergreen |
860 | |
861 | $path =~ s|/usr/bin|/usr/local/bin|; |
862 | |
863 | s/Login: $foo/Login: $bar/; # run-time pattern |
864 | |
865 | ($foo = $bar) =~ s/this/that/; |
866 | |
867 | $count = ($paragraph =~ s/Mister\b/Mr./g); |
868 | |
869 | $_ = 'abc123xyz'; |
870 | s/\d+/$&*2/e; # yields 'abc246xyz' |
871 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' |
872 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' |
873 | |
874 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e |
875 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e |
876 | s/^=(\w+)/&pod($1)/ge; # use function call |
877 | |
878 | # /e's can even nest; this will expand |
879 | # simple embedded variables in $_ |
880 | s/(\$\w+)/$1/eeg; |
881 | |
882 | # Delete C comments. |
883 | $program =~ s { |
4633a7c4 |
884 | /\* # Match the opening delimiter. |
885 | .*? # Match a minimal number of characters. |
886 | \*/ # Match the closing delimiter. |
a0d0e21e |
887 | } []gsx; |
888 | |
889 | s/^\s*(.*?)\s*$/$1/; # trim white space |
890 | |
891 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields |
892 | |
893 | Note the use of $ instead of \ in the last example. Unlike |
5f05dabc |
894 | B<sed>, we use the \E<lt>I<digit>E<gt> form in only the left hand side. |
6ee5d4e7 |
895 | Anywhere else it's $E<lt>I<digit>E<gt>. |
a0d0e21e |
896 | |
5f05dabc |
897 | Occasionally, you can't use just a C</g> to get all the changes |
a0d0e21e |
898 | to occur. Here are two common cases: |
899 | |
900 | # put commas in the right places in an integer |
901 | 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4 |
902 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5 |
903 | |
904 | # expand tabs to 8-column spacing |
905 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; |
906 | |
907 | |
908 | =item tr/SEARCHLIST/REPLACEMENTLIST/cds |
909 | |
910 | =item y/SEARCHLIST/REPLACEMENTLIST/cds |
911 | |
912 | Translates all occurrences of the characters found in the search list |
913 | with the corresponding character in the replacement list. It returns |
914 | the number of characters replaced or deleted. If no string is |
915 | specified via the =~ or !~ operator, the $_ string is translated. (The |
916 | string specified with =~ must be a scalar variable, an array element, |
5f05dabc |
917 | or an assignment to one of those, i.e., an lvalue.) For B<sed> devotees, |
a0d0e21e |
918 | C<y> is provided as a synonym for C<tr>. If the SEARCHLIST is |
919 | delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of |
5f05dabc |
920 | quotes, which may or may not be bracketing quotes, e.g., C<tr[A-Z][a-z]> |
a0d0e21e |
921 | or C<tr(+-*/)/ABCD/>. |
922 | |
923 | Options: |
924 | |
925 | c Complement the SEARCHLIST. |
926 | d Delete found but unreplaced characters. |
927 | s Squash duplicate replaced characters. |
928 | |
929 | If the C</c> modifier is specified, the SEARCHLIST character set is |
930 | complemented. If the C</d> modifier is specified, any characters specified |
931 | by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note |
932 | that this is slightly more flexible than the behavior of some B<tr> |
933 | programs, which delete anything they find in the SEARCHLIST, period.) |
934 | If the C</s> modifier is specified, sequences of characters that were |
935 | translated to the same character are squashed down to a single instance of the |
936 | character. |
937 | |
938 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted |
939 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter |
940 | than the SEARCHLIST, the final character is replicated till it is long |
941 | enough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated. |
942 | This latter is useful for counting characters in a class or for |
943 | squashing character sequences in a class. |
944 | |
945 | Examples: |
946 | |
947 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case |
948 | |
949 | $cnt = tr/*/*/; # count the stars in $_ |
950 | |
951 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky |
952 | |
953 | $cnt = tr/0-9//; # count the digits in $_ |
954 | |
955 | tr/a-zA-Z//s; # bookkeeper -> bokeper |
956 | |
957 | ($HOST = $host) =~ tr/a-z/A-Z/; |
958 | |
959 | tr/a-zA-Z/ /cs; # change non-alphas to single space |
960 | |
961 | tr [\200-\377] |
962 | [\000-\177]; # delete 8th bit |
963 | |
748a9306 |
964 | If multiple translations are given for a character, only the first one is used: |
965 | |
966 | tr/AAA/XYZ/ |
967 | |
968 | will translate any A to X. |
969 | |
a0d0e21e |
970 | Note that because the translation table is built at compile time, neither |
971 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote |
972 | interpolation. That means that if you want to use variables, you must use |
973 | an eval(): |
974 | |
975 | eval "tr/$oldlist/$newlist/"; |
976 | die $@ if $@; |
977 | |
978 | eval "tr/$oldlist/$newlist/, 1" or die $@; |
979 | |
980 | =back |
981 | |
982 | =head2 I/O Operators |
983 | |
984 | There are several I/O operators you should know about. |
5f05dabc |
985 | A string is enclosed by back-ticks (grave accents) first undergoes |
a0d0e21e |
986 | variable substitution just like a double quoted string. It is then |
987 | interpreted as a command, and the output of that command is the value |
988 | of the pseudo-literal, like in a shell. In a scalar context, a single |
989 | string consisting of all the output is returned. In a list context, |
990 | a list of values is returned, one for each line of output. (You can |
991 | set C<$/> to use a different line terminator.) The command is executed |
992 | each time the pseudo-literal is evaluated. The status value of the |
993 | command is returned in C<$?> (see L<perlvar> for the interpretation |
994 | of C<$?>). Unlike in B<csh>, no translation is done on the return |
995 | data--newlines remain newlines. Unlike in any of the shells, single |
996 | quotes do not hide variable names in the command from interpretation. |
997 | To pass a $ through to the shell you need to hide it with a backslash. |
5f05dabc |
998 | The generalized form of back-ticks is C<qx//>. (Because back-ticks |
cb1a09d0 |
999 | always undergo shell expansion as well, see L<perlsec> for |
1000 | security concerns.) |
a0d0e21e |
1001 | |
1002 | Evaluating a filehandle in angle brackets yields the next line from |
aa689395 |
1003 | that file (newline, if any, included), or C<undef> at end of file. |
1004 | Ordinarily you must assign that value to a variable, but there is one |
1005 | situation where an automatic assignment happens. I<If and ONLY if> the |
1006 | input symbol is the only thing inside the conditional of a C<while> or |
1007 | C<for(;;)> loop, the value is automatically assigned to the variable |
1008 | C<$_>. The assigned value is then tested to see if it is defined. |
1009 | (This may seem like an odd thing to you, but you'll use the construct |
1010 | in almost every Perl script you write.) Anyway, the following lines |
1011 | are equivalent to each other: |
a0d0e21e |
1012 | |
748a9306 |
1013 | while (defined($_ = <STDIN>)) { print; } |
a0d0e21e |
1014 | while (<STDIN>) { print; } |
1015 | for (;<STDIN>;) { print; } |
748a9306 |
1016 | print while defined($_ = <STDIN>); |
a0d0e21e |
1017 | print while <STDIN>; |
1018 | |
5f05dabc |
1019 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
1020 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in |
a0d0e21e |
1021 | packages, where they would be interpreted as local identifiers rather |
1022 | than global.) Additional filehandles may be created with the open() |
cb1a09d0 |
1023 | function. See L<perlfunc/open()> for details on this. |
a0d0e21e |
1024 | |
6ee5d4e7 |
1025 | If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a |
a0d0e21e |
1026 | list consisting of all the input lines is returned, one line per list |
1027 | element. It's easy to make a I<LARGE> data space this way, so use with |
1028 | care. |
1029 | |
d28ebecd |
1030 | The null filehandle E<lt>E<gt> is special and can be used to emulate the |
1031 | behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from |
a0d0e21e |
1032 | standard input, or from each file listed on the command line. Here's |
d28ebecd |
1033 | how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is |
a0d0e21e |
1034 | checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened |
1035 | gives you standard input. The @ARGV array is then processed as a list |
1036 | of filenames. The loop |
1037 | |
1038 | while (<>) { |
1039 | ... # code for each line |
1040 | } |
1041 | |
1042 | is equivalent to the following Perl-like pseudo code: |
1043 | |
1044 | unshift(@ARGV, '-') if $#ARGV < $[; |
1045 | while ($ARGV = shift) { |
1046 | open(ARGV, $ARGV); |
1047 | while (<ARGV>) { |
1048 | ... # code for each line |
1049 | } |
1050 | } |
1051 | |
1052 | except that it isn't so cumbersome to say, and will actually work. It |
1053 | really does shift array @ARGV and put the current filename into variable |
5f05dabc |
1054 | $ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a |
1055 | synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above |
1056 | doesn't work because it treats E<lt>ARGVE<gt> as non-magical.) |
a0d0e21e |
1057 | |
d28ebecd |
1058 | You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up |
a0d0e21e |
1059 | containing the list of filenames you really want. Line numbers (C<$.>) |
1060 | continue as if the input were one big happy file. (But see example |
1061 | under eof() for how to reset line numbers on each file.) |
1062 | |
1063 | If you want to set @ARGV to your own list of files, go right ahead. If |
1064 | you want to pass switches into your script, you can use one of the |
1065 | Getopts modules or put a loop on the front like this: |
1066 | |
1067 | while ($_ = $ARGV[0], /^-/) { |
1068 | shift; |
1069 | last if /^--$/; |
1070 | if (/^-D(.*)/) { $debug = $1 } |
1071 | if (/^-v/) { $verbose++ } |
1072 | ... # other switches |
1073 | } |
1074 | while (<>) { |
1075 | ... # code for each line |
1076 | } |
1077 | |
d28ebecd |
1078 | The E<lt>E<gt> symbol will return FALSE only once. If you call it again after |
a0d0e21e |
1079 | this it will assume you are processing another @ARGV list, and if you |
1080 | haven't set @ARGV, will input from STDIN. |
1081 | |
1082 | If the string inside the angle brackets is a reference to a scalar |
5f05dabc |
1083 | variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the |
cb1a09d0 |
1084 | filehandle to input from, or a reference to the same. For example: |
1085 | |
1086 | $fh = \*STDIN; |
1087 | $line = <$fh>; |
a0d0e21e |
1088 | |
cb1a09d0 |
1089 | If the string inside angle brackets is not a filehandle or a scalar |
1090 | variable containing a filehandle name or reference, then it is interpreted |
4633a7c4 |
1091 | as a filename pattern to be globbed, and either a list of filenames or the |
1092 | next filename in the list is returned, depending on context. One level of |
1093 | $ interpretation is done first, but you can't say C<E<lt>$fooE<gt>> |
1094 | because that's an indirect filehandle as explained in the previous |
6ee5d4e7 |
1095 | paragraph. (In older versions of Perl, programmers would insert curly |
4633a7c4 |
1096 | brackets to force interpretation as a filename glob: C<E<lt>${foo}E<gt>>. |
d28ebecd |
1097 | These days, it's considered cleaner to call the internal function directly |
4633a7c4 |
1098 | as C<glob($foo)>, which is probably the right way to have done it in the |
1099 | first place.) Example: |
a0d0e21e |
1100 | |
1101 | while (<*.c>) { |
1102 | chmod 0644, $_; |
1103 | } |
1104 | |
1105 | is equivalent to |
1106 | |
1107 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); |
1108 | while (<FOO>) { |
1109 | chop; |
1110 | chmod 0644, $_; |
1111 | } |
1112 | |
1113 | In fact, it's currently implemented that way. (Which means it will not |
1114 | work on filenames with spaces in them unless you have csh(1) on your |
1115 | machine.) Of course, the shortest way to do the above is: |
1116 | |
1117 | chmod 0644, <*.c>; |
1118 | |
1119 | Because globbing invokes a shell, it's often faster to call readdir() yourself |
5f05dabc |
1120 | and do your own grep() on the filenames. Furthermore, due to its current |
a0d0e21e |
1121 | implementation of using a shell, the glob() routine may get "Arg list too |
1122 | long" errors (unless you've installed tcsh(1L) as F</bin/csh>). |
1123 | |
5f05dabc |
1124 | A glob evaluates its (embedded) argument only when it is starting a new |
4633a7c4 |
1125 | list. All values must be read before it will start over. In a list |
1126 | context this isn't important, because you automatically get them all |
1127 | anyway. In a scalar context, however, the operator returns the next value |
1128 | each time it is called, or a FALSE value if you've just run out. Again, |
1129 | FALSE is returned only once. So if you're expecting a single value from |
1130 | a glob, it is much better to say |
1131 | |
1132 | ($file) = <blurch*>; |
1133 | |
1134 | than |
1135 | |
1136 | $file = <blurch*>; |
1137 | |
1138 | because the latter will alternate between returning a filename and |
1139 | returning FALSE. |
1140 | |
1141 | It you're trying to do variable interpolation, it's definitely better |
1142 | to use the glob() function, because the older notation can cause people |
e37d713d |
1143 | to become confused with the indirect filehandle notation. |
4633a7c4 |
1144 | |
1145 | @files = glob("$dir/*.[ch]"); |
1146 | @files = glob($files[$i]); |
1147 | |
a0d0e21e |
1148 | =head2 Constant Folding |
1149 | |
1150 | Like C, Perl does a certain amount of expression evaluation at |
1151 | compile time, whenever it determines that all of the arguments to an |
1152 | operator are static and have no side effects. In particular, string |
1153 | concatenation happens at compile time between literals that don't do |
1154 | variable substitution. Backslash interpretation also happens at |
1155 | compile time. You can say |
1156 | |
1157 | 'Now is the time for all' . "\n" . |
1158 | 'good men to come to.' |
1159 | |
1160 | and this all reduces to one string internally. Likewise, if |
1161 | you say |
1162 | |
1163 | foreach $file (@filenames) { |
1164 | if (-s $file > 5 + 100 * 2**16) { ... } |
1165 | } |
1166 | |
1167 | the compiler will pre-compute the number that |
1168 | expression represents so that the interpreter |
1169 | won't have to. |
1170 | |
1171 | |
55497cff |
1172 | =head2 Integer Arithmetic |
a0d0e21e |
1173 | |
1174 | By default Perl assumes that it must do most of its arithmetic in |
1175 | floating point. But by saying |
1176 | |
1177 | use integer; |
1178 | |
1179 | you may tell the compiler that it's okay to use integer operations |
1180 | from here to the end of the enclosing BLOCK. An inner BLOCK may |
1181 | countermand this by saying |
1182 | |
1183 | no integer; |
1184 | |
1185 | which lasts until the end of that BLOCK. |
1186 | |
55497cff |
1187 | The bitwise operators ("&", "|", "^", "~", "<<", and ">>") always |
1188 | produce integral results. However, C<use integer> still has meaning |
1189 | for them. By default, their results are interpreted as unsigned |
1190 | integers. However, if C<use integer> is in effect, their results are |
5f05dabc |
1191 | interpreted as signed integers. For example, C<~0> usually evaluates |
55497cff |
1192 | to a large integral value. However, C<use integer; ~0> is -1. |