How to Use Regex Groups in C# for Pattern Matching

In C#, you use Regex groups by placing parts of your pattern inside parentheses (). You can then access these groups using the Groups property of a Match object to extract specific parts of the matched text.

Syntax

Regex groups are created by enclosing a part of the pattern in parentheses (). Each group is numbered starting from 1, while group 0 is the entire match. You use Regex.Match to find a match and then access groups via Match.Groups[index].

• (): Defines a capturing group.
• Match.Groups[0]: The full matched string.
• Match.Groups[1]: The first captured group.
• Match.Groups[n]: The nth captured group.

csharp

using System.Text.RegularExpressions;

string pattern = "(\w+)@(\w+)\.com";
string input = "[e-mail]";

Match match = Regex.Match(input, pattern);
if (match.Success) {
    string fullMatch = match.Groups[0].Value; // entire match
    string user = match.Groups[1].Value;      // first group
    string domain = match.Groups[2].Value;    // second group
}

Example

This example shows how to extract the username and domain from an email address using regex groups in C#.

csharp

using System;
using System.Text.RegularExpressions;

class Program {
    static void Main() {
        string pattern = "(\\w+)@(\\w+)\\.com";
        string input = "[e-mail]";

        Match match = Regex.Match(input, pattern);
        if (match.Success) {
            Console.WriteLine($"Full match: {match.Groups[0].Value}");
            Console.WriteLine($"Username: {match.Groups[1].Value}");
            Console.WriteLine($"Domain: {match.Groups[2].Value}");
        } else {
            Console.WriteLine("No match found.");
        }
    }
}

Output

Full match: [e-mail]
Username: contact
Domain: openai

Common Pitfalls

Common mistakes when using regex groups in C# include:

• Forgetting to escape special characters like \. in the pattern.
• Accessing group indexes that do not exist, causing runtime errors.
• Using non-capturing groups (?:...) when you want to capture data.
• Assuming group 0 is a captured group instead of the full match.

Always check Match.Success before accessing groups to avoid errors.

csharp

using System;
using System.Text.RegularExpressions;

class Program {
    static void Main() {
        string pattern = "(\\w+)@(\\w+)\.com"; // Correctly escaped dot
        string input = "[e-mail]";

        Match match = Regex.Match(input, pattern);
        if (match.Success) {
            // This will work as expected because dot is escaped
            Console.WriteLine(match.Groups[2].Value);
        }
    }
}

Quick Reference

• Group 0: Entire matched string.
• Group 1, 2, ...: Captured groups in order.
• Regex.Match(input, pattern): Finds first match.
• Match.Groups[index].Value: Gets group text.
• Escape special characters like \. in patterns.

Key Takeaways

Use parentheses () in your regex pattern to create capturing groups.

Access captured groups with Match.Groups[index], where 0 is the full match.

Always check Match.Success before reading groups to avoid errors.

Escape special characters like dot (.) with a backslash (\.) in patterns.

Non-capturing groups (?:...) do not store matched text for extraction.