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test1,test2,"test3",test4,"te,st5","te,st6"

soll in:

test1
test2
"test3"
test4
"te,st5"
"te,st6"					

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            List<string> lsResultStrings = new List<string>();
            string sInput = "test1,test2,\"test3\",test4,\"te,st5\",\"te,st6\"";
            string sPattern = "\"(.*?)\",?|,?(.*?),";
            
            Regex oRegularExpression = new Regex(sPattern);
            MatchCollection oMatches = oRegularExpression.Matches(sInput);

            foreach (Match oMatch in oMatches)
            {
                // check which group successed and add value to 
                // result list
                if (oMatch.Groups[1].Success)
                {
                    lsResultStrings.Add(oMatch.Groups[1].Value);
                }
                else
                {
                    lsResultStrings.Add(oMatch.Groups[2].Value);
                }
            }					

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var
  I: Integer;
begin
  with TStringList.Create do
    try
      Delimiter := ','; // is already default, shown just for completeness
      QuoteChar := '"'; // is already default, shown just for completeness
      StrictDelimiter := True; // optional: new line etc. should not be considered a delimiter
      DelimitedText := 'test1,test2,"test3",test4,"te,st5","te,st6"';
      for I := 0 to Count - 1 do  // Output
        ShowMessage(Strings[I]);
    finally
      Free;
    end;
end;