C# RegEx for nested function

Question

I am having trouble to evaluate the following expression using RegEx in C#.

add(1, 2, sub(4, add(1, 2)), div(4, 2))

which will be evaluated as below.

=> add(1, 2, sub(4, 3), 2)
=> add(1, 2, 1, 2)
=> 6

Functions are picked up from regex expression and parameters are of any numbers. Thanks in advance.

Here is what I am trying:

class Program
{
    static Regex extractFuncRegex = new Regex(@"(?<func>add|sub|div)\s*\((?<params>.*)\)$", RegexOptions.ExplicitCapture);
    static Regex extractArgsRegex = new Regex(@"([^,]+\(.+?\))|([^,]+)");


    static void Main(string[] args)
    {
        string test = @"add(1, 2, sub(4, add(1, 2)), div(4, 2))";
        Console.WriteLine(ParseFunction(test));
        Console.ReadLine();
    }

    static string ParseFunction(string expr)
    {
        expr = extractFuncRegex.Replace(expr, (m) =>
            {
                string func = m.Groups["func"].Value;
                string param = m.Groups["params"].Value;

                Console.WriteLine("Function: {0}", func);

                MatchCollection paramCollection = extractArgsRegex.Matches(param);
                List<string> pa = new List<string>();

                foreach (Match item in paramCollection)
                {
                    string p = item.Groups[0].Value.Trim();
                    Console.WriteLine("\tParameter: {0}", p);

                    if (extractFuncRegex.IsMatch(p))
                        p = ParseFunction(p);

                    pa.Add(p);
                }

                switch (func)
                {
                    case "add":
                        float a1 = 0;
                        foreach (string item in pa)
                            a1 += float.Parse(item);
                        return a1.ToString();

                    case "sub":
                        return (float.Parse(pa[0]) - float.Parse(pa[1])).ToString();

                    case "div":
                        return (float.Parse(pa[0]) / float.Parse(pa[1])).ToString();

                    default:
                        return expr;
                }
            });

        return expr;
    }
}

If you debug, you can see, there is a problem to parse

sub(4, add(1, 2))

Regular expressions are not well suited for evaluating recursive structures. The .NET regular expression engine has some extensions to help you deal with recursion, but these are not easy to work with and would likely make your code much more difficult to maintain. I'd strongly recommend writing a proper parser instead. — p.s.w.g, Jan 13 '15 at 10:39

score 4 · Accepted Answer · answered Jan 13 '15 at 11:49

You've clearly done good work on this so far so I'm not going to say "don't use regular expressions, throw it away and use something else" - I'm going to show how you can make your code work with minimal changes.

Firstly, change your extractFuncRegex to

@"(?<func>add|sub|div)\s*\((?<params>[^()]*)\)"

I have replaced the .* in the params group with [^()]*. this means that it will only only match a function call which doesn't contain any other function calls - because that's the only thing we can work on directly. I've also removed the trailing $ to make it work.

The trick now is to call either ParseFunction or extractFuncRegex.Replace until no replacements are made. For example, you could put the call to extractFuncRegex.Replace in a loop like so (untested):

bool doneWork = true;
while (doneWork)
{
    doneWork = false;
    expr = extractFuncRegex.Replace(expr, (m) =>
        {
            doneWork = true;
            ...
        });
}
...

Using this, you get a sequence of gradually-simplified expressions. At each stage, only the deepest function calls are replaced.

add(1, 2, sub(4, add(1, 2)), div(4, 2))
                 |--------   |--------
add(1, 2, sub(4, 3        ), 2        )
          |----------------
add(1, 2, 1                , 2        )
|--------------------------------------
6

C# RegEx for nested function

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