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What good is \G in a regular expression?

Jasmine
Administrator

Mar 15, 2001, 6:09 AM

Post #1 of 1 (31639 views)

What good is \G in a regular expression?	Can't Post

What good is \G in a regular expression?

The notation \G is used in a match or substitution in conjunction the /g modifier (and ignored if there's no /g) to anchor the regular expression to the point just past where the last match occurred, i.e. the pos() point.

For example, suppose you had a line of text quoted in standard mail and Usenet notation, (that is, with leading > characters), and you want change each leading > into a corresponding :. You could do so in this way:

Code

     s/^(>+)/':' x length($1)/gem;

Or, using \G, the much simpler (and faster):

Code

    s/\G>/:/g;

A more sophisticated use might involve a tokenizer. The following lex-like example is courtesy of Jeffrey Friedl. It did not work in 5.003 due to bugs in that release, but does work in 5.004 or better. (Note the use of /c, which prevents a failed match with /g from resetting the search position back to the beginning of the string.)

Code

    while (<>) { 
      chomp; 
      PARSER: { 
           m/ \G( \d+\b    )/gcx    && do { print "number: $1\n";  redo; }; 
           m/ \G( \w+      )/gcx    && do { print "word:   $1\n";  redo; }; 
           m/ \G( \s+      )/gcx    && do { print "space:  $1\n";  redo; }; 
           m/ \G( [^\w\d]+ )/gcx    && do { print "other:  $1\n";  redo; }; 
      } 
    }

Of course, that could have been written as

Code

    while (<>) { 
      chomp; 
      PARSER: { 
           if ( /\G( \d+\b    )/gcx  { 
                print "number: $1\n"; 
                redo PARSER; 
           } 
           if ( /\G( \w+      )/gcx  { 
                print "word: $1\n"; 
                redo PARSER; 
           } 
           if ( /\G( \s+      )/gcx  { 
                print "space: $1\n"; 
                redo PARSER; 
           } 
           if ( /\G( [^\w\d]+ )/gcx  { 
                print "other: $1\n"; 
                redo PARSER; 
           } 
      } 
    }

But then you lose the vertical alignment of the regular expressions.

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