Question

I need to match a pattern that starts with a capital letter, is followed by a series of the same letter in lowercase, and then ends with the same letter capitalized again.

Should Match:

XxxxX
NnN
YyyyyyyY

Should Not Match:

XyyyX
XxxxB
XyyyZ

I know how to match the two capital letters using backreferences, but I’m not sure how to capture the middle portion where the letter needs to be the same but a different case.

Thanks.

Answer 1

You can’t do it all in a single regular expression. You can test most of it using a regular expression, but you need a separate test to see if the upper and lower case letters are the same:

Match match = Regex.Match(s, @"^([A-Z])([a-z])+\1$");
if (match.Success && match.Groups[1].Value.Equals(match.Groups[2].Value,
    StringComparison.InvariantCultureIgnoreCase)) {
    // OK
}

Note: Here I’m assuming that you only want A-Z.