$re = '/(?|((?:
m(?=a)
|(?<=m)a
|a(?=x)
|(?<=a)x
|x(?=m)
|(?<=x)m
|m(?=u)
|(?<=m)u
|u(?=s)
|(?<=u)s
|s(?=t)
|(?<=s)t
|t(?=e)
|(?<=t)e
|e(?=r)
|(?<=e)r
)+)|([maxmuster]))/x';
$str = 'maxmx dfgdfg
Mymaxmusis
Lemdfgudf
muster
mustr
zxmus
stackoverflow';
preg_match_all($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