This is a PHP code. It is a project of roman converter

Can someone explain this statement to me? What does it mean?

$romanValid = "/^M{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$/";

and in the second function, what does the first&neighbor stand for? can you explain the logic of this function for me, please?? thank you

function validRoman($str) $romanValid = "/"M{0,4}(CM CD D2C{0,37) (XC XL L?X{0,33) (IX IVV?10,3)/": return (preg_match($romanValid, $str)); function RomanToNum($str) $first = 0; $neighbor = 0; $result = 0; $len = strlen($str); for ($i = 0; $i "I", 5 => "V", 10 => "X", 50 => "L", 100 => "C", 500 => "D", 1000 => "M"); return (array_search($val, $arr))