php's preg_match returning different number of matches for same pattern

Question

I'm trying out preg_match with a roman numeral to integer converter. The problem is, for certain inputs, preg_replace seems to be giving too few matches. The code:

function romanNumeralToInt($romanNumeral)
{   preg_match
    (   '/^(M?M?M?)'
        .'((CM)|(CD)|((D?)(C?C?C?)))'
        .'((XC)|(XL)|((L?)(X?X?X?)))'
        .'((IX)|(IV)|((V?)(I?I?I?)))$/', $romanNumeral, $match);
    print_r($match);

    $result=0;
    $result += 1000*strlen($match[1]);
    if(strlen($match[3]) != 0){$result += 900;}
    if(strlen($match[4]) != 0){$result += 400;}
    if(strlen($match[5]) != 0)
    {   $result += 100*strlen($match[7]) + 500*strlen($match[6]);
    }
    if(strlen($match[9]) != 0){$result += 90;}
    if(strlen($match[10]) != 0){$result += 40;}
    if(strlen($match[11]) != 0)
    {   $result += 10*strlen($match[13]) + 50*strlen($match[12]);
    }
    if(strlen($match[15]) != 0){$result += 9;}
    if(strlen($match[16]) != 0){$result += 4;}
    if(strlen($match[17]) != 0)
    {   $result += 1*strlen($match[19]) + 5*strlen($match[18]);
    }

    return $result;
}

echo romanNumeralToInt("XXVIII"); // gives correct results

But any roman numeral ending in "IV" will cut off the last 3 matches ($matches will only contain elements 0-16 rather than the full 0-19), and similarly any roman numeral ending in "IX" will cut off the last 4 matches.

Is this expected behavior, or is my PHP buggy?

Answer 1

I expect this to be expected behavior. =)

Regex tries to match OR groups from left to right, stopping as soon as it finds a match, so it will never try to match those last three (or four) groups if it finds a IV or IX.

Actually, I think that, if your expression contains a CM or XL or something like that, some of the other entries will be missing, too.

I find that using RegExr helps a lot with debugging regular expressions. Using this for your regex, some groups catch empty strings, and some groups contain NO MATCH.