What is the use of ‘\G’ anchor in regex?

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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?

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