var input = @"add(mul(a,add(b,c)),d) + e - sub(f,g)";
var regex = new Regex(@"
\( # Match (
(
[^()]+ # all chars except ()
| (?<Level>\() # or if ( then Level += 1
| (?<-Level>\)) # or if ) then Level -= 1
)+ # Repeat (to go from inside to outside)
(?(Level)(?!)) # zero-width negative lookahead assertion
\) # Match )",
RegexOptions.IgnorePatternWhitespace);
foreach (Match c in regex.Matches(input))
{
Console.WriteLine(c.Value.Trim('(', ')'));
}