Split string: keep split chars separators C#

I'm kind of surprised that this isn't an option with string.split. I'm not very good with regex, but I know that you can do this sort of thing with Regex.Split. You would have to include System.Text.RegularExpressions, and then do something like this:

 string userString = "This is my first sentence. This is my second sentence!";
 string pattern = "(?<=[.!?])";
 string[] sentencesList = Regex.Split(userString, pattern);

In this case, every sentence beyond the first one will have at least one preceding space that you will want to remove. There is probably a way to do this with Regex, but I don't know how. You could also go through the resulting sentenceList array and call trim() on each string.

 for(int i = 0; i < sentenceList.Length; i++)
 {
     sentenceList[i] = sentenceList[i].Trim();
 }

Performance-wise, this is not very good, but if you aren't doing it often, it probably doesn't matter. If you don't want to use Regex, you can use a custom split method. Maybe something like this.

 public  string[] SplitString(string stringToSplit, char[] delimiters, bool includeDelimiter, bool trimEnds)
         {
             int index = -1;
             List<string> stringList = new List<string>();
             StringBuilder sBuilder = new StringBuilder();
 
             while(++index < stringToSplit.Length)
             {
                 sBuilder.Append(stringToSplit[index]);
 
                 foreach(char c in delimiters)
                 {
                     if(stringToSplit[index] == c)
                     {
                         if(!includeDelimiter)
                         {
                             sBuilder.Remove(sBuilder.Length -1, 1);
                         }
                         if(trimEnds)
                         {
                             stringList.Add(sBuilder.ToString().Trim());
                         }
                         else
                         {
                             stringList.Add(sBuilder.ToString());
                         }
                         sBuilder.Clear();
                         break;
                     }
                 }
             }
             return stringList.ToArray();
         }
     }

This isn't very pretty, but it will probably work for what you want to do. Hopefully someone else can respond with a better way of doing this.