How do I replace only specific match in a match collection?

Question

I am writing a solve method that solves an equation. The method will be recursive; searching for all outer parenthesis and when found recall solve for values inside the parenthesis and return the value when no parenthesis are found.

The process should look like this

20 * (6+3) / ((4+6)*9)
20 * 9 / ((4+6)*9)
20 * 9 / (10*9)
20 * 9 / 90
2

As you see each match may have a different replacement value. I need to replace the parenthesis to what it evaluates to. Is there a way to do this. Here is what I have so far.

public int solve(string etq)
{
    Regex rgx = new Regex(@"\(([^()]|(?R))*\)");
    MatchCollection matches;

    matches = rgx.Matches(etq);
        
    foreach(Match m in matches)
    {
        //replace m in etq with unique value here
    }

    //calculations here
    return calculation
}

Regex.replace(...) replaces all occurrences of the pattern specified. I want to be able to match to multiple scenario and replace each scenario with different output

I don't think your code works in C#. Did you copy the code from PHP/Perl and hope that it would work in C#? C# doesn't support call routine syntax `(?R)`. It only supports balancing group. — nhahtdh, Jul 27 '15 at 16:01
Why don't you use a [proper solution](http://stackoverflow.com/a/29996191/3764814) instead? The regex way *will* be painful. — Lucas Trzesniewski, Jul 27 '15 at 19:14

Ulugbek Umirov · Accepted Answer · 2015-07-27T16:37:32.353

8

Simple solution:

string input = "20 * (6+3) / ((4+6)*9)";
Console.WriteLine(input);
DataTable dt = new DataTable();
Regex rx = new Regex(@"\([^()]*\)");
string expression = input;
while (rx.IsMatch(expression))
{
    expression = rx.Replace(expression, m => dt.Compute(m.Value, null).ToString(), 1);
    Console.WriteLine(expression);
}
Console.WriteLine(dt.Compute(expression, null));

https://dotnetfiddle.net/U6Hh1e

edited Jul 27 '15 at 16:37

answered Jul 27 '15 at 16:18

Ulugbek Umirov

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Nice one, I didn't know about `DataTable.Compute`. Now I wouldn't be surprised to find a `DataTable.MakeCoffee` in this class. :-) – Lucas Trzesniewski Jul 27 '15 at 19:18
As this works with the expression I gave, I will be adding other functions such as square root, reciprocal, modulo and others which Im not sure this would work for. – Benjamin Jul 27 '15 at 19:23
@Benjamin Well, the main question was about finding `(...)` and replacing it with calculated result. Calculation of the result by using DataTable is just a sample. If you need more powerful calculation engine, you can use NCalc - https://ncalc.codeplex.com/ – Ulugbek Umirov Jul 28 '15 at 04:19

score 2 · Answer 2 · answered Jul 06 '17 at 17:39

2

This would be an even simpler solution, using the match properties to replace using substring:

public static string Replace(this Match match, string source, string replacement) { return source.Substring(0, match.Index) + replacement + source.Substring(match.Index + match.Length); }

answered Jul 06 '17 at 17:39

bigtv

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Worth adding to this, if you wanted to replace a select number of matches, you can use this function, but you will need to add an index modifier, which you will change base on the string length change. Then use add the index modifier to the match index (start the modifier at 0). – markthewizard1234 Jan 23 '18 at 14:34

How do I replace only specific match in a match collection?

2 Answers2