private void button5_Click(object sender, EventArgs e)
    {
      const string matchNumerator = @"\((?<Numerator>[\d,]*(\.\d*))\)";
      const string matchDenominator = @"\((?<Denominator>[\d,]*(\.\d*))\)";
      const string matchBoth = matchNumerator + @"\/" + matchDenominator;
 
      const string txtBoxInput = @"(5.2)/(4.3)";
 
 
      System.Text.RegularExpressions.Match m = System.Text.RegularExpressions.Regex.Match(txtBoxInput, matchBoth);
 
      if (m.Success)
      {
        decimal dNumerator, dDenominator;
        if (decimal.TryParse(m.Groups["Numerator"].Value, out dNumerator) && decimal.TryParse(m.Groups["Denominator"].Value, out dDenominator))
        {
          string latex = @"\frac {" + dNumerator.ToString() + "}{" + dDenominator.ToString() + "}";
          decimal valueOut = dNumerator / dDenominator;
          System.Diagnostics.Debugger.Break();
        }
      }
 
    }

System.Diagnostics.Debugger.Break();

System.Diagnostics.Debugger.Break();