C# Regex Groups

Regex.Match returns a Match object. The Groups property on a Match gets the captured groups within the regular expression. It is useful for extracting a part of a string from a match. It can be used with multiple captured parts.

This C# Regex article shows how to use the Groups property on a Match result.

Example

To start, IndexOf and LastIndexOf are inflexible when compared to Regex.Match. The Regex type gives more control. It lets you specify substrings with a certain range of characters, such as A-Za-z0-9. This example has good control over what substring it matches.

Program that uses Match Groups [C#]

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
	// A
	// The input string we are using
	string input = "OneTwoThree";

	// B
	// The regular expression we use to match
	Regex r1 = new Regex(@"One([A-Za-z0-9\-]+)Three");

	// C
	// Match the input and write results
	Match match = r1.Match(input);
	if (match.Success)
	{
	    string v = match.Groups[1].Value;
	    Console.WriteLine("Between One and Three: {0}",
		v);
	}
    }
}

Output

Between One and Three: Two

Description. In part B, we see the verbatim string syntax, which uses the @ sign at the start. It escapes characters differently than normal string declarations. In part C, we call the Match method on the Regex we created. This returns a Match object. We can extract the capture from this object. It is important to use the Groups[1] syntax. The groups are indexed starting at 1, not 0.

Note: Some collections in the .NET Framework are indexed starting at 1 not 0. This trips up developers. There is no sense at all to this.

Example 2

Alternatively you can solve this problem by using the IndexOf and LastIndexOf methods. There are many small variations on this code pattern. It is more fragile. It normally requires more development effort.

Program that uses Index [C#]

using System;

class Program
{
    static void Main()
    {
	// A
	// The input string we are using
	string input = "OneTwoThree";

	// B
	// Find first instance of this string
	int i1 = input.IndexOf("One");
	if (i1 != -1)
	{
	    // C
	    // Find last instance of the last string
	    int i2 = input.LastIndexOf("Three");
	    if (i2 != -1)
	    {
		// D
		// Get the substring and print it.
		int start = i1 + "One".Length;
		int len = i2 - start;
		string bet = input.Substring(start, len);
		Console.WriteLine("Between One and Three: {0}",
		    bet);
	    }
	}
    }
}

Output

Between One and Three: Two

Description. This code uses char positions to find the first instance of the left side string. It then finds the last instance of the right side string. It doesn't restrict the character ranges to certain digits and letters like the first example does. It may fail in other cases. It is likely that this version is faster.

Note: Regex performance is a difficult subject. Often it doesn't matter that much.

Summary

You can find a string between two delimiters of multiple characters. The Split method doesn't give you as much control over delimiters. You can compile these two examples into C# console programs. Then you can see their output and modify as needed.