Using regex to add leading zeroes

Question

I would like to add a certain number of leading zeroes (say up to 3) to all numbers of a string. For example:

Input: /2009/5/song 01 of 12

Outpu

Answer 1

C# version

        string input = "/2009/5/song 01 of 12";
        string regExPattern = @"(\/\d{4}\/)(\d+)(\/song\s+)(\d+)(\s+of\s+)(\d+)";
        string output = Regex.Replace(input, regExPattern, callback =>
        {
            string yearPrefix = callback.Groups[1].Value;
            string digit1 = int.Parse(callback.Groups[2].Value).ToString("0000");
            string songText = callback.Groups[3].Value;
            string digit2 = int.Parse(callback.Groups[4].Value).ToString("0000");
            string ofText = callback.Groups[5].Value;
            string digit3 = int.Parse(callback.Groups[6].Value).ToString("0000");
            return $"{yearPrefix}{digit1}{songText}{digit2}{ofText}{digit3}";
        });