Create one RegEx to validate a username

Question

I wrote this code to validate a username meets the given conditions, does anyone see how I can conbine the 2 RegExs into one? Code is c#

    /// <summary>
    /// Determines whether the username meets conditions.
    /// Username conditions:
    /// Must be 1 to 24 character in length
    /// Must start with letter a-zA-Z
    /// May contain letters, numbers or '.','-' or '_'
    /// Must not end in '.','-','._' or '-_' 
    /// </summary>
    /// <param name="userName">proposed username</param>
    /// <returns>True if the username is valid</returns>
    private static Regex sUserNameAllowedRegEx = new Regex(@"^[a-zA-Z]{1}[a-zA-Z0-9\._\-]{0,23}[^.-]$", RegexOptions.Compiled);
    private static Regex sUserNameIllegalEndingRegEx = new Regex(@"(\.|\-|\._|\-_)$", RegexOptions.Compiled);
    public static bool IsUserNameAllowed(string userName)
    {
        if (string.IsNullOrEmpty(userName)
            || !sUserNameAllowedRegEx.IsMatch(userName)
            || sUserNameIllegalEndingRegEx.IsMatch(userName)
            || ProfanityFilter.IsOffensive(userName))
        {
            return false;
        }
        return true;
    }

Answer 1

If I understand your requirements correctly, the below should be what you want. The \w matches letters, digits, or _.

The negative lookbehind (the (?<![-.]) part) allows _ unless the preceding character was . or -.

@"^(?=[a-zA-Z])[-\w.]{0,23}([a-zA-Z\d]|(?<![-.])_)$"

Answer 2

Try adding a greedy + on the last character class and make the middle class non-greedy:

@"^[a-zA-Z][a-zA-Z0-9\._\-]{0,22}?[a-zA-Z0-9]{0,2}$"

this will disallow anything ending in any combination of ., -, or _. This isn't exactly what you have in your original regex, but I figure it's probably what you're going for.