I need help with validating usernames using regular expressions.  I
only want to allow numbers, letters, and the underscore as valid
characters for the username.  I just cannot seem to find the solution,
but I believe I am close.  Any help provided is appreciated!  Here is
my code below which I am using to validate usernames.

public bool ValidateHandle(string handle)
{
     bool retVal = false;

     if(Regex.IsMatch(handle, "^[a-zA-Z0-9_]$"))
     {
         retVal = true;
     }

     return retVal;
}