$re = '/^ # beginning of string
(?:
. # match a single character
(?=
.*?
# if we already have captured the end of the string
(?(1)
# make sure we do not go past the correct position
(?= .\1$ )
)
# capture the end of the string +1 character, adding to \1 every iteration
( .\1?$ )
)
)* # repeat
# the middle character follows, capture it
(.)/xm';
$str = '1
123
12345
1234567
123456789
12345678901
1234567890123
123456789012345
';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php