$re = '/\b(?<whole_word>[a-z])\k{whole_word}++\b(*SKIP)(*FAIL)|(?<not_whole_word>[a-z])\k{not_whole_word}++/m';
$str = 'test ajjjo new
here bbb cca
here abbbbbc x
test jjka';
$subst = "$not_whole_word";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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