var mystr = "產品有不同的型號，型號比如有ab1,ab2,b1,b2,b3，等等.現在假設有a1,a2,a3,a4";
var matches = Regex.Matches(mystr, @"([a-z]+)\d+\s*[,，]\s*(\1(\d+)(?:\s*[,，]\s*)*)*");
foreach (Match match in matches)
{
　　if (string.IsNullOrEmpty(match.Groups[3].Value))
　　{
　　　　continue;
　　}
　　var str = match.Value;
　　var chart = match.Groups[1].Value;

　　var count = Convert.ToInt32(match.Groups[3].Value);
　　if (count>1)
　　{
　　　　mystr = mystr.Replace(str, chart + "1~" + chart + count+",");
　　}
}

Console.WriteLine(mystr);