$re = '/^(\S++).*\K(?:\R\1(?:\h.*|$))+/m';
$str = 'john jane
john 123
john jim jane
jane john
jane 123
jane 456
jim
jim 1';
$subst = "";
$result = preg_replace($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