TokenizeIdentifier.cs

// Identifier - starts with letter or underscore
if (char.IsLetter(_currentChar) || _currentChar == '_')
{
    var sb = new StringBuilder();

    // Accept letter, digit or underscore
    while (char.IsLetterOrDigit(_currentChar) || _currentChar == '_')
    {
        sb.Append(_currentChar);
        NextChar();
    }

    // Setup token
    _identifier = sb.ToString();
    _currentToken = Token.Identifier;
    return;
}