$re = '/(L3(?:(?!L4).)*)OK/';
$str = '\'L1\' => (\'Vanessa\', \'Prague\', \'OK\'), \'L2\' => (\'Alex\', \'Paris\', \'OK\'), \'L3\' => (\'Paul\', \'Paris\', \'OK\'), \'L4\' => (\'John\', \'Madrid\', \'OK\')';
$subst = "REPLACED";
$result = preg_replace($re, $subst, $str, 1);
echo "The result of the substitution is ".$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