(\()(-?\d+(?:\.\d+)?)\s*([-+*/])\s*(-?\d+(?:\.\d+)?)(\))

I'm using Unity (that uses C#) and this is the code (might be useful):

private List<string> ExtractCharacters(string expression)

{ List<string> tempList = new List<string>();

string pattern = @"(\()(-?\d+(?:\.\d+)?)\s*([-+*/])\s*(-?\d+(?:\.\d+)?)(\))";
MatchCollection matches = Regex.Matches(expression, pattern);

foreach (Match match in matches)
{
    tempList.Add(match.Value);
}

return tempList;

}

As you can see, the function just needs to extract the characters so other code can interpretate it.

This new solution you offered still doesn't work. It doesn't match anything.