$re = '/\b
(?# capture each letter up to N of them)
(\w)
(?:(\w)
(?:(\w)
(?:(\w)
(?:(\w)
(?:(\w)
)?
)?
)?
)?
)?
\b
(?# stuff between the first and second words)
\s+
\b
(?# Assert that the same letters appear if we found one before)
(?(1)(?=\w*\1))
(?(2)(?=\w*\2))
(?(3)(?=\w*\3))
(?(4)(?=\w*\4))
(?(5)(?=\w*\5))
(?(6)(?=\w*\6))
(?# assert that the length matches)
(?(6)(\w{6})
|(?(5)(\w{5})
|(?(4)(\w{4})
|(?(3)(\w{3})
|(?(2)(\w{2})
)
)
)
)
)
\b
/mx';
$str = '
fourth thourf very veery vry very veryy rsun urns a a this is not pann pout toop topo now we go with smart trams maps amps because declamations anecdotalism reoccupation cornucopiate
';
preg_match_all($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