Why are groups in regex doesn't works correctly

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C#

Match match = Regex.Match(textBox1.Text, @"(\d)*(\d)");
         MessageBox.Show(match.Groups[1].Value.ToString());
             double x2 = Convert.ToDouble(match.Groups[1].Value.ToString());
             double x1 = Convert.ToInt32(match.Groups[2].Value.ToString());

             textBox1.Text = Regex.Replace(textBox1.Text, @"\d*\d", (x1 * x2).ToString());

What I have tried:

I have been trying to write a math parser and I catch a problem with regex groups for example

textBox.Text = 2*2

and match.Groups[1].Value is empty.

Posted 27-May-19 10:57am

Member 14145167

Updated 27-May-19 12:17pm

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Dave Kreskowiak 27-May-19 17:33pm

Whether this works or not depends on the complexity of the expressions you're planning on supporting. If it's nothing more than "some value * some value", then you could get away with a set of RegEx's to grab the numbers and do the math.

If you're planning on something more complex, RegEx's are not going to it for you.

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Patrice T · Answer 1 · 2019-05-27T12:18:00

Quote:
Why are groups in regex doesn't works correctly

Because * have a special meaning in RegEx, when you want to use it as literal. You need to escape * to \* .

Just a few interesting links to help building and debugging RegEx.
Here is a link to RegEx documentation:
perlre - perldoc.perl.org[^]
Here is links to tools to help build RegEx and debug them:
.NET Regex Tester - Regex Storm[^]
Expresso Regular Expression Tool[^]
RegExr: Learn, Build, & Test RegEx[^]
Online regex tester and debugger: PHP, PCRE, Python, Golang and JavaScript[^]
This one show you the RegEx as a nice graph which is really helpful to understand what is doing a RegEx: Debuggex: Online visual regex tester. JavaScript, Python, and PCRE.[^]
This site also show the Regex in a nice graph but can't test what match the RegEx: Regexper[^]

Mohibur Rashid · Answer 2 · 2019-05-27T11:32:00

For your better understanding

C#

using System;
using System.Text.RegularExpressions;

namespace myApp
{
	class Program
	{
		static void Main(string[] args)
		{
			String chk = "5*6";
			Match match = Regex.Match(chk, @"(\d)\*(\d)");
			Console.WriteLine(match.Groups[0]);
			Console.WriteLine(match.Groups[1]);
			Console.WriteLine(match.Groups[2]);
		}
	}
}

Result:

myApp$ dotnet run
5*6
5
6

Why are groups in regex doesn't works correctly

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