How do I replace a specific occurrence of a string in a string?

Question

I have a string which may contain \"title1\" twice in it.

e.g.

server/api/shows?title1=its always sunny in philadelphia&title1=breaking ba

Answer 1

You can use the regex replace MatchEvaluator and give it a "state":

string callingURL = @"server/api/shows?title1=its always sunny in philadelphia&title1=breaking bad";

int found = -1;
string callingUrl2 = Regex.Replace(callingURL, "title1=", x =>
{
    found++;
    return found == 1 ? "title2=" : x.Value;
});

The replace can be one-lined by using the postfixed ++ operator (quite unreadable).

string callingUrl2 = Regex.Replace(callingURL, "title1=", x => found++ == 1 ? "title2=" : x.Value);