StringBuilder sb = new StringBuilder();
sb.AppendLine("<tr A>");
sb.AppendLine("<tr B>");
sb.AppendLine("<tr C>");
sb.AppendLine("<tr D>");
sb.AppendLine("<tr E>");
string result = Regex.Replace(sb.ToString(), @"(<tr )([A-Z])(>)", "$1$3");
Console.Write(result);
Console.Read();
替换后的结果为:
<tr>
<tr>
<tr>
<tr>
<tr>