Question

I need one string to be checked and modified many times (searching and replacing different seqences) but it is not working well. I guess its because of the immutability.

private string DoRegexCheck(string line)
{
  string pattern;

  foreach (string re in this.regexPatterns.Items)
  {
    pattern = re;
    Regex r = new Regex(pattern, RegexOptions.IgnoreCase);
    line=r.Replace(line, ""); //the line should be updated and the check should continue with updated line 
  }                            

  return line;
}

Answer 1

“it is not working well” is somewhat ambiguous.

If you mean “it isn’t changing line – then that code is fine. Immutability is not an issue at all, since we are changing to a new string each Replace (the code as shown does not try to edit an existing string). If it isn’t updating as expected, then your Regex patterns are simply incorrect.

---

If you mean performance: you can’t change the way Regex works on strings; I would, however, suggest caching the various regex using the Compiled option, so that you have an array or dictionary of pre-compiled Regex that you re-use. This is especially important if applying this for thousands of lines.