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Home: Perl Programming Help: Frequently Asked Questions:
What good is \G in a regular expression?

 



Jasmine
Administrator

Mar 15, 2001, 6:09 AM

Post #1 of 1 (12539 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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