string source = "12ab(34cd)56e(fg78)h";
source = Regex.Replace(source, @"(\([^\)]*\))", "");

foreach (Match vMatch in Regex.Matches(source, "[a-z]"))
{
    Console.WriteLine(vMatch.Value);
}

------解决方案--------------------

(?<T1>[^\(\)]*)(?:(?=\()|(?<=\)))(?<T2>[^\(\)]*)
Class中再动态的加入自己的匹配字符

------解决方案--------------------
C# code
(?<!\([^\)]*?)[a-z]+

------解决方案--------------------
(?<Result>\w*)([\(]\w*[\)])*
string source = txtSource.Text;
           MatchCollection mc = null;
           Regex re = null;
           re = new Regex(@"(?<Result>\w*)([\(]\w*[\)])*", RegexOptions.IgnoreCase);
           mc = re.Matches(source);
           string r = String.Empty;
           foreach (Match m in mc)
           {
               r += m.Groups["Numbers"].Value;
           }
           txtResult.Text = r;
直接测试就可以了