Question

I’ve looked high and low for an example of how to implement a Regex global replace in C# where there are Groups involved, but I’ve come up empty. So I wrote my own. Can anyone suggest a better way to do this?

static void Main(string[] args)
{
    Regex re = new Regex(@"word(\d)-(\d)");
    string input = "start word1-2 filler word3-4 end";
    StringBuilder output = new StringBuilder();
    int beg = 0;
    Match match = re.Match(input);
    while (match.Success)
    {
        // get string before match
        output.Append(input.Substring(beg, match.Index - beg));

        // replace "wordX-Y" with "wdX-Y"
        string repl = "wd" + match.Groups[1].Value + "-" + match.Groups[2].Value;
        // get replacement string
        output.Append(re.Replace(input.Substring(match.Index, match.Length), repl));

        // get string after match
        Match nmatch = match.NextMatch();
        int end = (nmatch.Success) ? nmatch.Index : input.Length;
        output.Append(input.Substring(match.Index + match.Length, end - (match.Index + match.Length)));

        beg = end;
        match = nmatch;
    }
    if (beg == 0)
        output.Append(input);
}

Answer 1

You can pass Replace a MatchEvaluator. It’s a delegate that takes a Match and returns the string you want to replace it with.

e.g.

string output = re.Replace(
    input,
    m => "wd" + m.Groups[1].Value + "-" + m.Groups[2].Value);

Alternatively, and I’m less sure about this, you could use lookahead – “check that this text follows, but don’t include it in the match”. The syntax is (?=whatver) so I think you’d need something like word(?=\d-\d) and then just replace it with wd.