1 | =head1 NAME
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2 |
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3 | perlfaq6 - Regular Expressions ($Revision: 1.38 $, $Date: 2005/12/31 00:54:37 $)
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4 |
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5 | =head1 DESCRIPTION
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6 |
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7 | This section is surprisingly small because the rest of the FAQ is
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8 | littered with answers involving regular expressions. For example,
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9 | decoding a URL and checking whether something is a number are handled
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10 | with regular expressions, but those answers are found elsewhere in
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11 | this document (in L<perlfaq9>: "How do I decode or create those %-encodings
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12 | on the web" and L<perlfaq4>: "How do I determine whether a scalar is
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13 | a number/whole/integer/float", to be precise).
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14 |
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15 | =head2 How can I hope to use regular expressions without creating illegible and unmaintainable code?
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16 | X<regex, legibility> X<regexp, legibility>
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17 | X<regular expression, legibility> X</x>
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18 |
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19 | Three techniques can make regular expressions maintainable and
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20 | understandable.
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21 |
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22 | =over 4
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23 |
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24 | =item Comments Outside the Regex
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25 |
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26 | Describe what you're doing and how you're doing it, using normal Perl
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27 | comments.
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28 |
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29 | # turn the line into the first word, a colon, and the
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30 | # number of characters on the rest of the line
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31 | s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg;
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32 |
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33 | =item Comments Inside the Regex
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34 |
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35 | The C</x> modifier causes whitespace to be ignored in a regex pattern
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36 | (except in a character class), and also allows you to use normal
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37 | comments there, too. As you can imagine, whitespace and comments help
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38 | a lot.
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39 |
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40 | C</x> lets you turn this:
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41 |
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42 | s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs;
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43 |
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44 | into this:
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45 |
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46 | s{ < # opening angle bracket
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47 | (?: # Non-backreffing grouping paren
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48 | [^>'"] * # 0 or more things that are neither > nor ' nor "
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49 | | # or else
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50 | ".*?" # a section between double quotes (stingy match)
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51 | | # or else
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52 | '.*?' # a section between single quotes (stingy match)
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53 | ) + # all occurring one or more times
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54 | > # closing angle bracket
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55 | }{}gsx; # replace with nothing, i.e. delete
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56 |
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57 | It's still not quite so clear as prose, but it is very useful for
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58 | describing the meaning of each part of the pattern.
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59 |
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60 | =item Different Delimiters
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61 |
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62 | While we normally think of patterns as being delimited with C</>
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63 | characters, they can be delimited by almost any character. L<perlre>
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64 | describes this. For example, the C<s///> above uses braces as
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65 | delimiters. Selecting another delimiter can avoid quoting the
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66 | delimiter within the pattern:
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67 |
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68 | s/\/usr\/local/\/usr\/share/g; # bad delimiter choice
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69 | s#/usr/local#/usr/share#g; # better
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70 |
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71 | =back
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72 |
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73 | =head2 I'm having trouble matching over more than one line. What's wrong?
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74 | X<regex, multiline> X<regexp, multiline> X<regular expression, multiline>
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75 |
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76 | Either you don't have more than one line in the string you're looking
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77 | at (probably), or else you aren't using the correct modifier(s) on
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78 | your pattern (possibly).
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79 |
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80 | There are many ways to get multiline data into a string. If you want
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81 | it to happen automatically while reading input, you'll want to set $/
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82 | (probably to '' for paragraphs or C<undef> for the whole file) to
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83 | allow you to read more than one line at a time.
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84 |
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85 | Read L<perlre> to help you decide which of C</s> and C</m> (or both)
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86 | you might want to use: C</s> allows dot to include newline, and C</m>
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87 | allows caret and dollar to match next to a newline, not just at the
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88 | end of the string. You do need to make sure that you've actually
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89 | got a multiline string in there.
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90 |
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91 | For example, this program detects duplicate words, even when they span
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92 | line breaks (but not paragraph ones). For this example, we don't need
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93 | C</s> because we aren't using dot in a regular expression that we want
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94 | to cross line boundaries. Neither do we need C</m> because we aren't
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95 | wanting caret or dollar to match at any point inside the record next
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96 | to newlines. But it's imperative that $/ be set to something other
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97 | than the default, or else we won't actually ever have a multiline
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98 | record read in.
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99 |
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100 | $/ = ''; # read in more whole paragraph, not just one line
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101 | while ( <> ) {
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102 | while ( /\b([\w'-]+)(\s+\1)+\b/gi ) { # word starts alpha
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103 | print "Duplicate $1 at paragraph $.\n";
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104 | }
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105 | }
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106 |
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107 | Here's code that finds sentences that begin with "From " (which would
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108 | be mangled by many mailers):
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109 |
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110 | $/ = ''; # read in more whole paragraph, not just one line
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111 | while ( <> ) {
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112 | while ( /^From /gm ) { # /m makes ^ match next to \n
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113 | print "leading from in paragraph $.\n";
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114 | }
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115 | }
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116 |
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117 | Here's code that finds everything between START and END in a paragraph:
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118 |
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119 | undef $/; # read in whole file, not just one line or paragraph
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120 | while ( <> ) {
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121 | while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries
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122 | print "$1\n";
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123 | }
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124 | }
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125 |
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126 | =head2 How can I pull out lines between two patterns that are themselves on different lines?
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127 | X<..>
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128 |
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129 | You can use Perl's somewhat exotic C<..> operator (documented in
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130 | L<perlop>):
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131 |
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132 | perl -ne 'print if /START/ .. /END/' file1 file2 ...
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133 |
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134 | If you wanted text and not lines, you would use
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135 |
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136 | perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ...
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137 |
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138 | But if you want nested occurrences of C<START> through C<END>, you'll
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139 | run up against the problem described in the question in this section
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140 | on matching balanced text.
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141 |
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142 | Here's another example of using C<..>:
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143 |
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144 | while (<>) {
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145 | $in_header = 1 .. /^$/;
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146 | $in_body = /^$/ .. eof();
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147 | # now choose between them
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148 | } continue {
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149 | reset if eof(); # fix $.
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150 | }
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151 |
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152 | =head2 I put a regular expression into $/ but it didn't work. What's wrong?
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153 | X<$/, regexes in> X<$INPUT_RECORD_SEPARATOR, regexes in>
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154 | X<$RS, regexes in>
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155 |
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156 | Up to Perl 5.8.0, $/ has to be a string. This may change in 5.10,
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157 | but don't get your hopes up. Until then, you can use these examples
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158 | if you really need to do this.
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159 |
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160 | If you have File::Stream, this is easy.
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161 |
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162 | use File::Stream;
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163 | my $stream = File::Stream->new(
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164 | $filehandle,
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165 | separator => qr/\s*,\s*/,
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166 | );
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167 |
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168 | print "$_\n" while <$stream>;
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169 |
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170 | If you don't have File::Stream, you have to do a little more work.
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171 |
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172 | You can use the four argument form of sysread to continually add to
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173 | a buffer. After you add to the buffer, you check if you have a
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174 | complete line (using your regular expression).
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175 |
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176 | local $_ = "";
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177 | while( sysread FH, $_, 8192, length ) {
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178 | while( s/^((?s).*?)your_pattern/ ) {
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179 | my $record = $1;
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180 | # do stuff here.
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181 | }
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182 | }
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183 |
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184 | You can do the same thing with foreach and a match using the
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185 | c flag and the \G anchor, if you do not mind your entire file
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186 | being in memory at the end.
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187 |
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188 | local $_ = "";
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189 | while( sysread FH, $_, 8192, length ) {
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190 | foreach my $record ( m/\G((?s).*?)your_pattern/gc ) {
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191 | # do stuff here.
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192 | }
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193 | substr( $_, 0, pos ) = "" if pos;
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194 | }
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195 |
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196 |
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197 | =head2 How do I substitute case insensitively on the LHS while preserving case on the RHS?
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198 | X<replace, case preserving> X<substitute, case preserving>
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199 | X<substitution, case preserving> X<s, case preserving>
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200 |
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201 | Here's a lovely Perlish solution by Larry Rosler. It exploits
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202 | properties of bitwise xor on ASCII strings.
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203 |
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204 | $_= "this is a TEsT case";
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205 |
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206 | $old = 'test';
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207 | $new = 'success';
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208 |
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209 | s{(\Q$old\E)}
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210 | { uc $new | (uc $1 ^ $1) .
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211 | (uc(substr $1, -1) ^ substr $1, -1) x
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212 | (length($new) - length $1)
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213 | }egi;
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214 |
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215 | print;
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216 |
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217 | And here it is as a subroutine, modeled after the above:
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218 |
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219 | sub preserve_case($$) {
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220 | my ($old, $new) = @_;
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221 | my $mask = uc $old ^ $old;
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222 |
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223 | uc $new | $mask .
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224 | substr($mask, -1) x (length($new) - length($old))
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225 | }
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226 |
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227 | $a = "this is a TEsT case";
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228 | $a =~ s/(test)/preserve_case($1, "success")/egi;
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229 | print "$a\n";
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230 |
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231 | This prints:
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232 |
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233 | this is a SUcCESS case
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234 |
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235 | As an alternative, to keep the case of the replacement word if it is
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236 | longer than the original, you can use this code, by Jeff Pinyan:
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237 |
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238 | sub preserve_case {
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239 | my ($from, $to) = @_;
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240 | my ($lf, $lt) = map length, @_;
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241 |
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242 | if ($lt < $lf) { $from = substr $from, 0, $lt }
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243 | else { $from .= substr $to, $lf }
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244 |
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245 | return uc $to | ($from ^ uc $from);
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246 | }
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247 |
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248 | This changes the sentence to "this is a SUcCess case."
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249 |
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250 | Just to show that C programmers can write C in any programming language,
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251 | if you prefer a more C-like solution, the following script makes the
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252 | substitution have the same case, letter by letter, as the original.
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253 | (It also happens to run about 240% slower than the Perlish solution runs.)
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254 | If the substitution has more characters than the string being substituted,
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255 | the case of the last character is used for the rest of the substitution.
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256 |
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257 | # Original by Nathan Torkington, massaged by Jeffrey Friedl
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258 | #
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259 | sub preserve_case($$)
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260 | {
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261 | my ($old, $new) = @_;
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262 | my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc
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263 | my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new));
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264 | my ($len) = $oldlen < $newlen ? $oldlen : $newlen;
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265 |
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266 | for ($i = 0; $i < $len; $i++) {
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267 | if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) {
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268 | $state = 0;
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269 | } elsif (lc $c eq $c) {
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270 | substr($new, $i, 1) = lc(substr($new, $i, 1));
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271 | $state = 1;
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272 | } else {
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273 | substr($new, $i, 1) = uc(substr($new, $i, 1));
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274 | $state = 2;
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275 | }
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276 | }
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277 | # finish up with any remaining new (for when new is longer than old)
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278 | if ($newlen > $oldlen) {
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279 | if ($state == 1) {
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280 | substr($new, $oldlen) = lc(substr($new, $oldlen));
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281 | } elsif ($state == 2) {
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282 | substr($new, $oldlen) = uc(substr($new, $oldlen));
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283 | }
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284 | }
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285 | return $new;
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286 | }
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287 |
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288 | =head2 How can I make C<\w> match national character sets?
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289 | X<\w>
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290 |
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291 | Put C<use locale;> in your script. The \w character class is taken
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292 | from the current locale.
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293 |
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294 | See L<perllocale> for details.
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295 |
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296 | =head2 How can I match a locale-smart version of C</[a-zA-Z]/>?
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297 | X<alpha>
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298 |
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299 | You can use the POSIX character class syntax C</[[:alpha:]]/>
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300 | documented in L<perlre>.
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301 |
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302 | No matter which locale you are in, the alphabetic characters are
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303 | the characters in \w without the digits and the underscore.
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304 | As a regex, that looks like C</[^\W\d_]/>. Its complement,
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305 | the non-alphabetics, is then everything in \W along with
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306 | the digits and the underscore, or C</[\W\d_]/>.
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307 |
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308 | =head2 How can I quote a variable to use in a regex?
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309 | X<regex, escaping> X<regexp, escaping> X<regular expression, escaping>
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310 |
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311 | The Perl parser will expand $variable and @variable references in
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312 | regular expressions unless the delimiter is a single quote. Remember,
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313 | too, that the right-hand side of a C<s///> substitution is considered
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314 | a double-quoted string (see L<perlop> for more details). Remember
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315 | also that any regex special characters will be acted on unless you
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316 | precede the substitution with \Q. Here's an example:
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317 |
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318 | $string = "Placido P. Octopus";
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319 | $regex = "P.";
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320 |
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321 | $string =~ s/$regex/Polyp/;
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322 | # $string is now "Polypacido P. Octopus"
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323 |
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324 | Because C<.> is special in regular expressions, and can match any
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325 | single character, the regex C<P.> here has matched the <Pl> in the
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326 | original string.
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327 |
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328 | To escape the special meaning of C<.>, we use C<\Q>:
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329 |
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330 | $string = "Placido P. Octopus";
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331 | $regex = "P.";
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332 |
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333 | $string =~ s/\Q$regex/Polyp/;
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334 | # $string is now "Placido Polyp Octopus"
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335 |
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336 | The use of C<\Q> causes the <.> in the regex to be treated as a
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337 | regular character, so that C<P.> matches a C<P> followed by a dot.
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338 |
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339 | =head2 What is C</o> really for?
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340 | X</o>
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341 |
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342 | Using a variable in a regular expression match forces a re-evaluation
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343 | (and perhaps recompilation) each time the regular expression is
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344 | encountered. The C</o> modifier locks in the regex the first time
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345 | it's used. This always happens in a constant regular expression, and
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346 | in fact, the pattern was compiled into the internal format at the same
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347 | time your entire program was.
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348 |
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349 | Use of C</o> is irrelevant unless variable interpolation is used in
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350 | the pattern, and if so, the regex engine will neither know nor care
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351 | whether the variables change after the pattern is evaluated the I<very
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352 | first> time.
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353 |
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354 | C</o> is often used to gain an extra measure of efficiency by not
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355 | performing subsequent evaluations when you know it won't matter
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356 | (because you know the variables won't change), or more rarely, when
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357 | you don't want the regex to notice if they do.
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358 |
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359 | For example, here's a "paragrep" program:
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360 |
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361 | $/ = ''; # paragraph mode
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362 | $pat = shift;
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363 | while (<>) {
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364 | print if /$pat/o;
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365 | }
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366 |
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367 | =head2 How do I use a regular expression to strip C style comments from a file?
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368 |
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369 | While this actually can be done, it's much harder than you'd think.
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370 | For example, this one-liner
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371 |
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372 | perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c
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373 |
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374 | will work in many but not all cases. You see, it's too simple-minded for
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375 | certain kinds of C programs, in particular, those with what appear to be
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376 | comments in quoted strings. For that, you'd need something like this,
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377 | created by Jeffrey Friedl and later modified by Fred Curtis.
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378 |
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379 | $/ = undef;
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380 | $_ = <>;
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381 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse;
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382 | print;
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383 |
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384 | This could, of course, be more legibly written with the C</x> modifier, adding
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385 | whitespace and comments. Here it is expanded, courtesy of Fred Curtis.
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386 |
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387 | s{
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388 | /\* ## Start of /* ... */ comment
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389 | [^*]*\*+ ## Non-* followed by 1-or-more *'s
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390 | (
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391 | [^/*][^*]*\*+
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392 | )* ## 0-or-more things which don't start with /
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393 | ## but do end with '*'
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394 | / ## End of /* ... */ comment
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395 |
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396 | | ## OR various things which aren't comments:
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397 |
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398 | (
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399 | " ## Start of " ... " string
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400 | (
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401 | \\. ## Escaped char
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402 | | ## OR
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403 | [^"\\] ## Non "\
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404 | )*
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405 | " ## End of " ... " string
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406 |
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407 | | ## OR
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408 |
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409 | ' ## Start of ' ... ' string
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410 | (
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411 | \\. ## Escaped char
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412 | | ## OR
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413 | [^'\\] ## Non '\
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414 | )*
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415 | ' ## End of ' ... ' string
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416 |
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417 | | ## OR
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418 |
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419 | . ## Anything other char
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420 | [^/"'\\]* ## Chars which doesn't start a comment, string or escape
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421 | )
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422 | }{defined $2 ? $2 : ""}gxse;
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423 |
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424 | A slight modification also removes C++ comments:
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425 |
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426 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse;
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427 |
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428 | =head2 Can I use Perl regular expressions to match balanced text?
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429 | X<regex, matching balanced test> X<regexp, matching balanced test>
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430 | X<regular expression, matching balanced test>
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431 |
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432 | Historically, Perl regular expressions were not capable of matching
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433 | balanced text. As of more recent versions of perl including 5.6.1
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434 | experimental features have been added that make it possible to do this.
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435 | Look at the documentation for the (??{ }) construct in recent perlre manual
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436 | pages to see an example of matching balanced parentheses. Be sure to take
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437 | special notice of the warnings present in the manual before making use
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438 | of this feature.
|
---|
439 |
|
---|
440 | CPAN contains many modules that can be useful for matching text
|
---|
441 | depending on the context. Damian Conway provides some useful
|
---|
442 | patterns in Regexp::Common. The module Text::Balanced provides a
|
---|
443 | general solution to this problem.
|
---|
444 |
|
---|
445 | One of the common applications of balanced text matching is working
|
---|
446 | with XML and HTML. There are many modules available that support
|
---|
447 | these needs. Two examples are HTML::Parser and XML::Parser. There
|
---|
448 | are many others.
|
---|
449 |
|
---|
450 | An elaborate subroutine (for 7-bit ASCII only) to pull out balanced
|
---|
451 | and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>,
|
---|
452 | or C<(> and C<)> can be found in
|
---|
453 | http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz .
|
---|
454 |
|
---|
455 | The C::Scan module from CPAN also contains such subs for internal use,
|
---|
456 | but they are undocumented.
|
---|
457 |
|
---|
458 | =head2 What does it mean that regexes are greedy? How can I get around it?
|
---|
459 | X<greedy> X<greediness>
|
---|
460 |
|
---|
461 | Most people mean that greedy regexes match as much as they can.
|
---|
462 | Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>,
|
---|
463 | C<{}>) that are greedy rather than the whole pattern; Perl prefers local
|
---|
464 | greed and immediate gratification to overall greed. To get non-greedy
|
---|
465 | versions of the same quantifiers, use (C<??>, C<*?>, C<+?>, C<{}?>).
|
---|
466 |
|
---|
467 | An example:
|
---|
468 |
|
---|
469 | $s1 = $s2 = "I am very very cold";
|
---|
470 | $s1 =~ s/ve.*y //; # I am cold
|
---|
471 | $s2 =~ s/ve.*?y //; # I am very cold
|
---|
472 |
|
---|
473 | Notice how the second substitution stopped matching as soon as it
|
---|
474 | encountered "y ". The C<*?> quantifier effectively tells the regular
|
---|
475 | expression engine to find a match as quickly as possible and pass
|
---|
476 | control on to whatever is next in line, like you would if you were
|
---|
477 | playing hot potato.
|
---|
478 |
|
---|
479 | =head2 How do I process each word on each line?
|
---|
480 | X<word>
|
---|
481 |
|
---|
482 | Use the split function:
|
---|
483 |
|
---|
484 | while (<>) {
|
---|
485 | foreach $word ( split ) {
|
---|
486 | # do something with $word here
|
---|
487 | }
|
---|
488 | }
|
---|
489 |
|
---|
490 | Note that this isn't really a word in the English sense; it's just
|
---|
491 | chunks of consecutive non-whitespace characters.
|
---|
492 |
|
---|
493 | To work with only alphanumeric sequences (including underscores), you
|
---|
494 | might consider
|
---|
495 |
|
---|
496 | while (<>) {
|
---|
497 | foreach $word (m/(\w+)/g) {
|
---|
498 | # do something with $word here
|
---|
499 | }
|
---|
500 | }
|
---|
501 |
|
---|
502 | =head2 How can I print out a word-frequency or line-frequency summary?
|
---|
503 |
|
---|
504 | To do this, you have to parse out each word in the input stream. We'll
|
---|
505 | pretend that by word you mean chunk of alphabetics, hyphens, or
|
---|
506 | apostrophes, rather than the non-whitespace chunk idea of a word given
|
---|
507 | in the previous question:
|
---|
508 |
|
---|
509 | while (<>) {
|
---|
510 | while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'"
|
---|
511 | $seen{$1}++;
|
---|
512 | }
|
---|
513 | }
|
---|
514 | while ( ($word, $count) = each %seen ) {
|
---|
515 | print "$count $word\n";
|
---|
516 | }
|
---|
517 |
|
---|
518 | If you wanted to do the same thing for lines, you wouldn't need a
|
---|
519 | regular expression:
|
---|
520 |
|
---|
521 | while (<>) {
|
---|
522 | $seen{$_}++;
|
---|
523 | }
|
---|
524 | while ( ($line, $count) = each %seen ) {
|
---|
525 | print "$count $line";
|
---|
526 | }
|
---|
527 |
|
---|
528 | If you want these output in a sorted order, see L<perlfaq4>: "How do I
|
---|
529 | sort a hash (optionally by value instead of key)?".
|
---|
530 |
|
---|
531 | =head2 How can I do approximate matching?
|
---|
532 | X<match, approximate> X<matching, approximate>
|
---|
533 |
|
---|
534 | See the module String::Approx available from CPAN.
|
---|
535 |
|
---|
536 | =head2 How do I efficiently match many regular expressions at once?
|
---|
537 | X<regex, efficiency> X<regexp, efficiency>
|
---|
538 | X<regular expression, efficiency>
|
---|
539 |
|
---|
540 | ( contributed by brian d foy )
|
---|
541 |
|
---|
542 | Avoid asking Perl to compile a regular expression every time
|
---|
543 | you want to match it. In this example, perl must recompile
|
---|
544 | the regular expression for every iteration of the foreach()
|
---|
545 | loop since it has no way to know what $pattern will be.
|
---|
546 |
|
---|
547 | @patterns = qw( foo bar baz );
|
---|
548 |
|
---|
549 | LINE: while( <> )
|
---|
550 | {
|
---|
551 | foreach $pattern ( @patterns )
|
---|
552 | {
|
---|
553 | print if /\b$pattern\b/i;
|
---|
554 | next LINE;
|
---|
555 | }
|
---|
556 | }
|
---|
557 |
|
---|
558 | The qr// operator showed up in perl 5.005. It compiles a
|
---|
559 | regular expression, but doesn't apply it. When you use the
|
---|
560 | pre-compiled version of the regex, perl does less work. In
|
---|
561 | this example, I inserted a map() to turn each pattern into
|
---|
562 | its pre-compiled form. The rest of the script is the same,
|
---|
563 | but faster.
|
---|
564 |
|
---|
565 | @patterns = map { qr/\b$_\b/i } qw( foo bar baz );
|
---|
566 |
|
---|
567 | LINE: while( <> )
|
---|
568 | {
|
---|
569 | foreach $pattern ( @patterns )
|
---|
570 | {
|
---|
571 | print if /\b$pattern\b/i;
|
---|
572 | next LINE;
|
---|
573 | }
|
---|
574 | }
|
---|
575 |
|
---|
576 | In some cases, you may be able to make several patterns into
|
---|
577 | a single regular expression. Beware of situations that require
|
---|
578 | backtracking though.
|
---|
579 |
|
---|
580 | $regex = join '|', qw( foo bar baz );
|
---|
581 |
|
---|
582 | LINE: while( <> )
|
---|
583 | {
|
---|
584 | print if /\b(?:$regex)\b/i;
|
---|
585 | }
|
---|
586 |
|
---|
587 | For more details on regular expression efficiency, see Mastering
|
---|
588 | Regular Expressions by Jeffrey Freidl. He explains how regular
|
---|
589 | expressions engine work and why some patterns are surprisingly
|
---|
590 | inefficient. Once you understand how perl applies regular
|
---|
591 | expressions, you can tune them for individual situations.
|
---|
592 |
|
---|
593 | =head2 Why don't word-boundary searches with C<\b> work for me?
|
---|
594 | X<\b>
|
---|
595 |
|
---|
596 | (contributed by brian d foy)
|
---|
597 |
|
---|
598 | Ensure that you know what \b really does: it's the boundary between a
|
---|
599 | word character, \w, and something that isn't a word character. That
|
---|
600 | thing that isn't a word character might be \W, but it can also be the
|
---|
601 | start or end of the string.
|
---|
602 |
|
---|
603 | It's not (not!) the boundary between whitespace and non-whitespace,
|
---|
604 | and it's not the stuff between words we use to create sentences.
|
---|
605 |
|
---|
606 | In regex speak, a word boundary (\b) is a "zero width assertion",
|
---|
607 | meaning that it doesn't represent a character in the string, but a
|
---|
608 | condition at a certain position.
|
---|
609 |
|
---|
610 | For the regular expression, /\bPerl\b/, there has to be a word
|
---|
611 | boundary before the "P" and after the "l". As long as something other
|
---|
612 | than a word character precedes the "P" and succeeds the "l", the
|
---|
613 | pattern will match. These strings match /\bPerl\b/.
|
---|
614 |
|
---|
615 | "Perl" # no word char before P or after l
|
---|
616 | "Perl " # same as previous (space is not a word char)
|
---|
617 | "'Perl'" # the ' char is not a word char
|
---|
618 | "Perl's" # no word char before P, non-word char after "l"
|
---|
619 |
|
---|
620 | These strings do not match /\bPerl\b/.
|
---|
621 |
|
---|
622 | "Perl_" # _ is a word char!
|
---|
623 | "Perler" # no word char before P, but one after l
|
---|
624 |
|
---|
625 | You don't have to use \b to match words though. You can look for
|
---|
626 | non-word characters surrounded by word characters. These strings
|
---|
627 | match the pattern /\b'\b/.
|
---|
628 |
|
---|
629 | "don't" # the ' char is surrounded by "n" and "t"
|
---|
630 | "qep'a'" # the ' char is surrounded by "p" and "a"
|
---|
631 |
|
---|
632 | These strings do not match /\b'\b/.
|
---|
633 |
|
---|
634 | "foo'" # there is no word char after non-word '
|
---|
635 |
|
---|
636 | You can also use the complement of \b, \B, to specify that there
|
---|
637 | should not be a word boundary.
|
---|
638 |
|
---|
639 | In the pattern /\Bam\B/, there must be a word character before the "a"
|
---|
640 | and after the "m". These patterns match /\Bam\B/:
|
---|
641 |
|
---|
642 | "llama" # "am" surrounded by word chars
|
---|
643 | "Samuel" # same
|
---|
644 |
|
---|
645 | These strings do not match /\Bam\B/
|
---|
646 |
|
---|
647 | "Sam" # no word boundary before "a", but one after "m"
|
---|
648 | "I am Sam" # "am" surrounded by non-word chars
|
---|
649 |
|
---|
650 |
|
---|
651 | =head2 Why does using $&, $`, or $' slow my program down?
|
---|
652 | X<$MATCH> X<$&> X<$POSTMATCH> X<$'> X<$PREMATCH> X<$`>
|
---|
653 |
|
---|
654 | (contributed by Anno Siegel)
|
---|
655 |
|
---|
656 | Once Perl sees that you need one of these variables anywhere in the
|
---|
657 | program, it provides them on each and every pattern match. That means
|
---|
658 | that on every pattern match the entire string will be copied, part of it
|
---|
659 | to $`, part to $&, and part to $'. Thus the penalty is most severe with
|
---|
660 | long strings and patterns that match often. Avoid $&, $', and $` if you
|
---|
661 | can, but if you can't, once you've used them at all, use them at will
|
---|
662 | because you've already paid the price. Remember that some algorithms
|
---|
663 | really appreciate them. As of the 5.005 release, the $& variable is no
|
---|
664 | longer "expensive" the way the other two are.
|
---|
665 |
|
---|
666 | Since Perl 5.6.1 the special variables @- and @+ can functionally replace
|
---|
667 | $`, $& and $'. These arrays contain pointers to the beginning and end
|
---|
668 | of each match (see perlvar for the full story), so they give you
|
---|
669 | essentially the same information, but without the risk of excessive
|
---|
670 | string copying.
|
---|
671 |
|
---|
672 | =head2 What good is C<\G> in a regular expression?
|
---|
673 | X<\G>
|
---|
674 |
|
---|
675 | You use the C<\G> anchor to start the next match on the same
|
---|
676 | string where the last match left off. The regular
|
---|
677 | expression engine cannot skip over any characters to find
|
---|
678 | the next match with this anchor, so C<\G> is similar to the
|
---|
679 | beginning of string anchor, C<^>. The C<\G> anchor is typically
|
---|
680 | used with the C<g> flag. It uses the value of pos()
|
---|
681 | as the position to start the next match. As the match
|
---|
682 | operator makes successive matches, it updates pos() with the
|
---|
683 | position of the next character past the last match (or the
|
---|
684 | first character of the next match, depending on how you like
|
---|
685 | to look at it). Each string has its own pos() value.
|
---|
686 |
|
---|
687 | Suppose you want to match all of consective pairs of digits
|
---|
688 | in a string like "1122a44" and stop matching when you
|
---|
689 | encounter non-digits. You want to match C<11> and C<22> but
|
---|
690 | the letter <a> shows up between C<22> and C<44> and you want
|
---|
691 | to stop at C<a>. Simply matching pairs of digits skips over
|
---|
692 | the C<a> and still matches C<44>.
|
---|
693 |
|
---|
694 | $_ = "1122a44";
|
---|
695 | my @pairs = m/(\d\d)/g; # qw( 11 22 44 )
|
---|
696 |
|
---|
697 | If you use the \G anchor, you force the match after C<22> to
|
---|
698 | start with the C<a>. The regular expression cannot match
|
---|
699 | there since it does not find a digit, so the next match
|
---|
700 | fails and the match operator returns the pairs it already
|
---|
701 | found.
|
---|
702 |
|
---|
703 | $_ = "1122a44";
|
---|
704 | my @pairs = m/\G(\d\d)/g; # qw( 11 22 )
|
---|
705 |
|
---|
706 | You can also use the C<\G> anchor in scalar context. You
|
---|
707 | still need the C<g> flag.
|
---|
708 |
|
---|
709 | $_ = "1122a44";
|
---|
710 | while( m/\G(\d\d)/g )
|
---|
711 | {
|
---|
712 | print "Found $1\n";
|
---|
713 | }
|
---|
714 |
|
---|
715 | After the match fails at the letter C<a>, perl resets pos()
|
---|
716 | and the next match on the same string starts at the beginning.
|
---|
717 |
|
---|
718 | $_ = "1122a44";
|
---|
719 | while( m/\G(\d\d)/g )
|
---|
720 | {
|
---|
721 | print "Found $1\n";
|
---|
722 | }
|
---|
723 |
|
---|
724 | print "Found $1 after while" if m/(\d\d)/g; # finds "11"
|
---|
725 |
|
---|
726 | You can disable pos() resets on fail with the C<c> flag.
|
---|
727 | Subsequent matches start where the last successful match
|
---|
728 | ended (the value of pos()) even if a match on the same
|
---|
729 | string as failed in the meantime. In this case, the match
|
---|
730 | after the while() loop starts at the C<a> (where the last
|
---|
731 | match stopped), and since it does not use any anchor it can
|
---|
732 | skip over the C<a> to find "44".
|
---|
733 |
|
---|
734 | $_ = "1122a44";
|
---|
735 | while( m/\G(\d\d)/gc )
|
---|
736 | {
|
---|
737 | print "Found $1\n";
|
---|
738 | }
|
---|
739 |
|
---|
740 | print "Found $1 after while" if m/(\d\d)/g; # finds "44"
|
---|
741 |
|
---|
742 | Typically you use the C<\G> anchor with the C<c> flag
|
---|
743 | when you want to try a different match if one fails,
|
---|
744 | such as in a tokenizer. Jeffrey Friedl offers this example
|
---|
745 | which works in 5.004 or later.
|
---|
746 |
|
---|
747 | while (<>) {
|
---|
748 | chomp;
|
---|
749 | PARSER: {
|
---|
750 | m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; };
|
---|
751 | m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; };
|
---|
752 | m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; };
|
---|
753 | m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; };
|
---|
754 | }
|
---|
755 | }
|
---|
756 |
|
---|
757 | For each line, the PARSER loop first tries to match a series
|
---|
758 | of digits followed by a word boundary. This match has to
|
---|
759 | start at the place the last match left off (or the beginning
|
---|
760 | of the string on the first match). Since C<m/ \G( \d+\b
|
---|
761 | )/gcx> uses the C<c> flag, if the string does not match that
|
---|
762 | regular expression, perl does not reset pos() and the next
|
---|
763 | match starts at the same position to try a different
|
---|
764 | pattern.
|
---|
765 |
|
---|
766 | =head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
|
---|
767 | X<DFA> X<NFA> X<POSIX>
|
---|
768 |
|
---|
769 | While it's true that Perl's regular expressions resemble the DFAs
|
---|
770 | (deterministic finite automata) of the egrep(1) program, they are in
|
---|
771 | fact implemented as NFAs (non-deterministic finite automata) to allow
|
---|
772 | backtracking and backreferencing. And they aren't POSIX-style either,
|
---|
773 | because those guarantee worst-case behavior for all cases. (It seems
|
---|
774 | that some people prefer guarantees of consistency, even when what's
|
---|
775 | guaranteed is slowness.) See the book "Mastering Regular Expressions"
|
---|
776 | (from O'Reilly) by Jeffrey Friedl for all the details you could ever
|
---|
777 | hope to know on these matters (a full citation appears in
|
---|
778 | L<perlfaq2>).
|
---|
779 |
|
---|
780 | =head2 What's wrong with using grep in a void context?
|
---|
781 | X<grep>
|
---|
782 |
|
---|
783 | The problem is that grep builds a return list, regardless of the context.
|
---|
784 | This means you're making Perl go to the trouble of building a list that
|
---|
785 | you then just throw away. If the list is large, you waste both time and space.
|
---|
786 | If your intent is to iterate over the list, then use a for loop for this
|
---|
787 | purpose.
|
---|
788 |
|
---|
789 | In perls older than 5.8.1, map suffers from this problem as well.
|
---|
790 | But since 5.8.1, this has been fixed, and map is context aware - in void
|
---|
791 | context, no lists are constructed.
|
---|
792 |
|
---|
793 | =head2 How can I match strings with multibyte characters?
|
---|
794 | X<regex, and multibyte characters> X<regexp, and multibyte characters>
|
---|
795 | X<regular expression, and multibyte characters>
|
---|
796 |
|
---|
797 | Starting from Perl 5.6 Perl has had some level of multibyte character
|
---|
798 | support. Perl 5.8 or later is recommended. Supported multibyte
|
---|
799 | character repertoires include Unicode, and legacy encodings
|
---|
800 | through the Encode module. See L<perluniintro>, L<perlunicode>,
|
---|
801 | and L<Encode>.
|
---|
802 |
|
---|
803 | If you are stuck with older Perls, you can do Unicode with the
|
---|
804 | C<Unicode::String> module, and character conversions using the
|
---|
805 | C<Unicode::Map8> and C<Unicode::Map> modules. If you are using
|
---|
806 | Japanese encodings, you might try using the jperl 5.005_03.
|
---|
807 |
|
---|
808 | Finally, the following set of approaches was offered by Jeffrey
|
---|
809 | Friedl, whose article in issue #5 of The Perl Journal talks about
|
---|
810 | this very matter.
|
---|
811 |
|
---|
812 | Let's suppose you have some weird Martian encoding where pairs of
|
---|
813 | ASCII uppercase letters encode single Martian letters (i.e. the two
|
---|
814 | bytes "CV" make a single Martian letter, as do the two bytes "SG",
|
---|
815 | "VS", "XX", etc.). Other bytes represent single characters, just like
|
---|
816 | ASCII.
|
---|
817 |
|
---|
818 | So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the
|
---|
819 | nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'.
|
---|
820 |
|
---|
821 | Now, say you want to search for the single character C</GX/>. Perl
|
---|
822 | doesn't know about Martian, so it'll find the two bytes "GX" in the "I
|
---|
823 | am CVSGXX!" string, even though that character isn't there: it just
|
---|
824 | looks like it is because "SG" is next to "XX", but there's no real
|
---|
825 | "GX". This is a big problem.
|
---|
826 |
|
---|
827 | Here are a few ways, all painful, to deal with it:
|
---|
828 |
|
---|
829 | $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent "martian"
|
---|
830 | # bytes are no longer adjacent.
|
---|
831 | print "found GX!\n" if $martian =~ /GX/;
|
---|
832 |
|
---|
833 | Or like this:
|
---|
834 |
|
---|
835 | @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g;
|
---|
836 | # above is conceptually similar to: @chars = $text =~ m/(.)/g;
|
---|
837 | #
|
---|
838 | foreach $char (@chars) {
|
---|
839 | print "found GX!\n", last if $char eq 'GX';
|
---|
840 | }
|
---|
841 |
|
---|
842 | Or like this:
|
---|
843 |
|
---|
844 | while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded
|
---|
845 | print "found GX!\n", last if $1 eq 'GX';
|
---|
846 | }
|
---|
847 |
|
---|
848 | Here's another, slightly less painful, way to do it from Benjamin
|
---|
849 | Goldberg, who uses a zero-width negative look-behind assertion.
|
---|
850 |
|
---|
851 | print "found GX!\n" if $martian =~ m/
|
---|
852 | (?<![A-Z])
|
---|
853 | (?:[A-Z][A-Z])*?
|
---|
854 | GX
|
---|
855 | /x;
|
---|
856 |
|
---|
857 | This succeeds if the "martian" character GX is in the string, and fails
|
---|
858 | otherwise. If you don't like using (?<!), a zero-width negative
|
---|
859 | look-behind assertion, you can replace (?<![A-Z]) with (?:^|[^A-Z]).
|
---|
860 |
|
---|
861 | It does have the drawback of putting the wrong thing in $-[0] and $+[0],
|
---|
862 | but this usually can be worked around.
|
---|
863 |
|
---|
864 | =head2 How do I match a pattern that is supplied by the user?
|
---|
865 |
|
---|
866 | Well, if it's really a pattern, then just use
|
---|
867 |
|
---|
868 | chomp($pattern = <STDIN>);
|
---|
869 | if ($line =~ /$pattern/) { }
|
---|
870 |
|
---|
871 | Alternatively, since you have no guarantee that your user entered
|
---|
872 | a valid regular expression, trap the exception this way:
|
---|
873 |
|
---|
874 | if (eval { $line =~ /$pattern/ }) { }
|
---|
875 |
|
---|
876 | If all you really want is to search for a string, not a pattern,
|
---|
877 | then you should either use the index() function, which is made for
|
---|
878 | string searching, or, if you can't be disabused of using a pattern
|
---|
879 | match on a non-pattern, then be sure to use C<\Q>...C<\E>, documented
|
---|
880 | in L<perlre>.
|
---|
881 |
|
---|
882 | $pattern = <STDIN>;
|
---|
883 |
|
---|
884 | open (FILE, $input) or die "Couldn't open input $input: $!; aborting";
|
---|
885 | while (<FILE>) {
|
---|
886 | print if /\Q$pattern\E/;
|
---|
887 | }
|
---|
888 | close FILE;
|
---|
889 |
|
---|
890 | =head1 AUTHOR AND COPYRIGHT
|
---|
891 |
|
---|
892 | Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
|
---|
893 | other authors as noted. All rights reserved.
|
---|
894 |
|
---|
895 | This documentation is free; you can redistribute it and/or modify it
|
---|
896 | under the same terms as Perl itself.
|
---|
897 |
|
---|
898 | Irrespective of its distribution, all code examples in this file
|
---|
899 | are hereby placed into the public domain. You are permitted and
|
---|
900 | encouraged to use this code in your own programs for fun
|
---|
901 | or for profit as you see fit. A simple comment in the code giving
|
---|
902 | credit would be courteous but is not required.
|
---|