Regex Groups

In C# Regex.Match returns a Match object. The Groups property on a Match gets the captured groups within the regular expression.

This property is useful for extracting a part of a string from a match. It can be used with multiple captured parts. Groups can be accessed with an int or string.

An example

Regular expressions are more powerful than most string methods. Regex lets you specify substrings with a certain range of characters, such as "A-Za-z0-9."

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
        // Part 1: the input string we are using.
        string input = "a45";

        // Part 2: the regular expression we use to match.
        Regex regex = new Regex(@"a(\d)(\d)");

        // Part 3: match the input and write results.
        Match match = regex.Match(input);
        if (match.Success)
        {
            string value1 = match.Groups[1].Value;
            string value2 = match.Groups[2].Value;
            Console.WriteLine("GROUP 1 and 2: {0}, {1}", value1, value2);
        }
    }
}
GROUP 1 and 2: 4, 5

Named group

Here we use a named group in a regular expression. We then access the value of the string that matches that group with the Groups property. We use a string index key.

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
        // ... Input string.
        string input = "Left12345Right";

        // ... Use named group in regular expression.
        Regex expression = new Regex(@"Left(?<middle>\d+)Right");

        // ... See if we matched.
        Match match = expression.Match(input);
        if (match.Success)
        {
            // ... Get group by name.
            string result = match.Groups["middle"].Value;
            Console.WriteLine("Middle: {0}", result);
        }
    }
}
Middle: 12345

Between, before, after

Regular expressions are not needed for simple parsing tasks. Consider a specialized method that can access strings between, before and after other strings.

Summary

You can find a string between two delimiters of multiple characters. With Groups, we can access "captured" values with an int or string.