Regular Expressions in C# with Examples: In Practice

Regular Expressions in C# with Examples: In Practice

Regular expressions (regex) are a powerful tool for searching, validating, and replacing text based on patterns. In C# (.NET), they are implemented through the System.Text.RegularExpressions.Regex class. In this article, we will explore the main capabilities using real-world examples: from simple email validation to extracting data from HTML. All examples are ready to copy and use in your projects.

Basics of Working with Regex in C#

The Regex class provides three key methods:

• IsMatch — checks if a string matches a pattern;
• Match — finds the first match;
• Matches — finds all matches.

A simple example — checking if a string contains digits:

using System;using System.Text.RegularExpressions;

public class RegexExample{    public static void Main()    {        string text = "My number: 12345";        string pattern = @"\\d+"; // one or more digits

        bool hasDigits = Regex.IsMatch(text, pattern);        Console.WriteLine(hasDigits); // True    }}

Note the @ before the string — this is a verbatim string literal, which disables backslash escaping. Without it, you would have to write "\\\\d+".

Email and Phone Number Validation

Email Validation

Email validation is a classic task. A simple but working pattern:

public static bool IsValidEmail(string email){    string pattern = @"^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}$";    return Regex.IsMatch(email, pattern);}

// UsageConsole.WriteLine(IsValidEmail("user@example.com")); // TrueConsole.WriteLine(IsValidEmail("user@.com"));        // False

Let's break down the pattern:

• ^ — start of string;
• [a-zA-Z0-9._%+-]+ — local part (letters, digits, and special characters);
• @ — the "at" symbol;
• [a-zA-Z0-9.-]+ — domain;
• \\. — dot (escaped);
• [a-zA-Z]{2,} — top-level domain (at least 2 letters);
• $ — end of string.

Phone Number Validation

Example for Russian format (+7 XXX XXX-XX-XX):

public static bool IsValidRussianPhone(string phone){    string pattern = @"^\\+7\\d{3}\\d{3}-\\d{2}-\\d{2}$";    // Or a more flexible variant:    // @"^\\+7[\\s-]?\\d{3}[\\s-]?\\d{3}[\\s-]?\\d{2}[\\s-]?\\d{2}$"    return Regex.IsMatch(phone, pattern);}

Extracting Data from Text Using Capture Groups

Capture groups (parentheses ()) allow you to extract parts of a match. For example, let's extract all emails from text:

string text = "Contacts: support@site.com, admin@test.org, info@company.net";string pattern = @"([a-zA-Z0-9._%+-]+)@([a-zA-Z0-9.-]+)\\.([a-zA-Z]{2,})";

MatchCollection matches = Regex.Matches(text, pattern);foreach (Match match in matches){    Console.WriteLine($"Full email: {match.Value}");    Console.WriteLine($"Local part: {match.Groups[1].Value}");    Console.WriteLine($"Domain: {match.Groups[2].Value}");    Console.WriteLine($"TLD: {match.Groups[3].Value}");    Console.WriteLine("---");}

Result:

Full email: support@site.comLocal part: supportDomain: siteTLD: com---

Groups are numbered starting from 1 (group 0 is the entire match). You can also name them:

string pattern = @"(?<user>[a-zA-Z0-9._%+-]+)@(?<domain>[a-zA-Z0-9.-]+)\\.(?<tld>[a-zA-Z]{2,})";// Access: match.Groups["user"].Value

Extracting Links from HTML

string html = @"<a href='https://example.com'>Examp