The notation \G is used in a match or substitution in conjunction with the /g modifier to anchor the regular expression to the point just past where the last match occurred, i.e. the pos() point. A failed match resets the position of \G unless the /c modifier is in effect. \G can be used in a match without the /g modifier; it acts the same (i.e. still anchors at the pos() point) but of course only matches once and does not update pos(), as non-/g expressions never do. \G in an expression applied to a target string that has never been matched against a /g expression before or has had its pos() reset is functionally equivalent to \A, which matches at the beginning of the string.
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:
s/^(>+)/':' x length($1)/gem;Or, using \G, the much simpler (and faster):
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.)
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
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.