php - What is the use of 'G' anchor in regex?

Question

I'm having a difficulty with understanding how G anchor works in PHP flavor of regular expressions.

I'm inclined to think (even though I may be wrong) that G is used instead of ^ in situations when multiple matches of the same string are taking place.

Could someone please show an example of how Gshould be used, and explain how and why it works?

Answer 1

UPDATE

G forces the pattern to only return matches that are part of a continuous chain of matches. From the first match each subsequent match must be preceded by a match. If you break the chain the matches end.

<?php
$pattern = '#(match),#';
$subject = "match,match,match,match,not-match,match";

preg_match_all( $pattern, $subject, $matches );

//Will output match 5 times because it skips over not-match
foreach ( $matches[1] as $match ) {
    echo $match . '<br />';
}

echo '<br />';

$pattern = '#(Gmatch),#';
$subject = "match,match,match,match,not-match,match";

preg_match_all( $pattern, $subject, $matches );

//Will only output match 4 times because at not-match the chain is broken
foreach ( $matches[1] as $match ) {
    echo $match . '<br />';
}
?>

This is straight from the docs

The fourth use of backslash is for certain simple assertions. An assertion specifies a condition that has to be met at a particular point in a match, without consuming any characters from the subject string. The use of subpatterns for more complicated assertions is described below. The backslashed assertions are

 G
    first matching position in subject

The G assertion is true only when the current matching position is at the start point of the match, as specified by the offset argument of preg_match(). It differs from A when the value of offset is non-zero.

http://www.php.net/manual/en/regexp.reference.escape.php

You will have to scroll down that page a bit but there it is.

There is a really good example in ruby but it is the same in php.

How the Anchor z and G works in Ruby?