* this is almost a complete re-write of the answer.
+ the original usenet message example was a bit complicated
+ the code did not actually do what it said it did anyway
+ it was not clear what it should do
my answer is more elementary.
+ call the \G anchor an "anchor" to build on reader's knowledge
of other anchors (^, $, \b, \B)
+ mentions pos() s readers have another place to look for info
+ uses a small string in examples that are ready to run
+ adds a small example of the c flag before Jeffery's big example with it
+ adds an explantion of Jeffery's example
Index: perlfaq6.pod
===================================================================
RCS file: /cvs/public/perlfaq/perlfaq6.pod,v
retrieving revision 1.17
diff -r1.17 perlfaq6.pod
566,592c566,636
< The notation C<\G> is used in a match or substitution in conjunction with
< the C</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 C<\G> unless the C</c> modifier is in effect. C<\G> can be
< used in a match without the C</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-C</g> expressions never do. C<\G> in an expression
< applied to a target string that has never been matched against a C</g>
< expression before or has had its pos() reset is functionally equivalent to
< C<\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 C<< > >> characters), and
< you want change each leading C<< > >> into a corresponding C<:>. You
< could do so in this way:
<
< s/^(>+)/':' x length($1)/gem;
<
< Or, using C<\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</c>, which prevents a failed match with C</g> from
< resetting the search position back to the beginning of the string.)
---
> You use the C<\G> anchor to start the next match on the same
> string where the last match left off. The regular
> expression engine cannot skip over any characters to find
> the next match with this anchor, so C<\G> is similar to the
> beginning of string anchor, C<^>. The C<\G> anchor only
> works if you use the C<g> flag. It uses the value of pos()
> as the position to start the next match. As the match
> operator makes successive matches, it updates pos() with the
> position of the next character past the last match (or the
> first character of the next match, depending on how you like
> to look at it). Each string has its own pos() value.
>
> Suppose you want to match all of consective pairs of digits
> in a string like "1122a44" and stop matching when you
> encounter non-digits. You want to match C<11> and C<22> but
> the letter <a> shows up between C<22> and C<44> and you want
> to stop at C<a>. Simply matching pairs of digits skips over
> the C<a> and still matches C<44>.
>
> $_ = "1122a44";
> my @pairs = m/(\d\d)/g; # qw( 11 22 44 )
>
> If you use the \G anchor, you force the match after C<22> to
> start with the C<a>. The regular expression cannot match
> there since it does not find a digit, so the next match
> fails and the match operator returns the pairs it already
> found.
>
> $_ = "1122a44";
> my @pairs = m/\G(\d\d)/g; # qw( 11 22 )
>
> You can also use the C<\G> anchor in scalar context. You
> still need the C<g> flag.
>
> $_ = "1122a44";
> while( m/\G(\d\d)/g )
> {
> print "Found $1\n";
> }
>
> After the match fails at the letter C<a>, perl resets pos()
> and the next match on the same string starts at the beginning.
>
> $_ = "1122a44";
> while( m/\G(\d\d)/g )
> {
> print "Found $1\n";
> }
>
> print "Found $1 after while" if m/(\d\d)/g; # finds "11"
>
> You can disable pos() resets on fail with the C<c> flag.
> Subsequent matches start where the last successful match
> ended (the value of pos()) even if a match on the same
> string as failed in the meantime. In this case, the match
> after the while() loop starts at the C<a> (where the last
> match stopped), and since it does not use any anchor it can
> skip over the C<a> to find "44".
>
> $_ = "1122a44";
> while( m/\G(\d\d)/gc )
> {
> print "Found $1\n";
> }
>
> print "Found $1 after while" if m/(\d\d)/g; # finds "44"
>
> Typically you use the C<\G> anchor with the C<c> flag
> when you want to try a different match if one fails,
> such as in a tokenizer. Jeffrey Friedl offers this example
> which works in 5.004 or later.
597,600c641,644
< 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; };
---
> 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; };
604,628c648,655
< 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.
---
> For each line, the PARSER loop first tries to match a series
> of digits followed by a word boundary. This match has to
> start at the place the last match left off (or the beginning
> of the string on the first match). Since C<m/ \G( \d+\b
> )/gcx> uses the C<c> flag, if the string does not match that
> regular expression, perl does not reset pos() and the next
> match starts at the same position to try a different
> pattern.
