Regular Expressions in C# with Examples for Beginners

Regular Expressions in C#: A Complete Guide for Beginners

Regular expressions (regex) are a powerful tool for searching, replacing, and validating strings based on a given pattern. In C# (.NET), working with regular expressions is implemented through the System.Text.RegularExpressions namespace. This article will help you master the basics: from creating simple patterns to using all the features of the Regex class.

1. Basics of Working with Regex in C#

First, include the namespace:

using System.Text.RegularExpressions;

The Regex class provides static and instance methods. The simplest way to check if a string contains a substring matching a pattern is the IsMatch method:

string text = "Hello, my email: test@example.com";string pattern = @"\\w+@\\w+\\.\\w+"; // simple email pattern

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

Pattern breakdown:

• \\w+ — one or more letters, digits, or underscores
• @ — the "at" symbol
• \\. — escaped dot (without escaping, a dot means "any character")

For repeated calls, it is better to create a Regex object to avoid recompiling the pattern:

Regex emailRegex = new Regex(@"\\w+@\\w+\\.\\w+");Console.WriteLine(emailRegex.IsMatch("admin@site.ru")); // True

2. Finding All Matches: Match and MatchCollection

The Match method returns the first match, while Matches returns all matches as a MatchCollection. Each Match object contains information about the found substring, its position, and length.

Example: extracting all numbers from a string

string data = "In 2023 there were 15 events, in 2024 — already 23.";string numberPattern = @"\\d+"; // one or more digits

MatchCollection matches = Regex.Matches(data, numberPattern);

foreach (Match match in matches){    Console.WriteLine($"Found: {match.Value} at position {match.Index}");}// Output:// Found: 2023 at position 3// Found: 15 at position 22// Found: 2024 at position 37// Found: 23 at position 51

Working with Groups. Groups allow you to extract parts of a match. For example, when searching for a date in the format "dd.mm.yyyy", you can separately get the day, month, and year:

string input = "Today is 25.12.2024";string datePattern = @"(\\d{2})\\.(\\d{2})\\.(\\d{4})";

Match match = Regex.Match(input, datePattern);if (match.Success){    Console.WriteLine($"Day: {match.Groups[1].Value}");   // 25    Console.WriteLine($"Month: {match.Groups[2].Value}");  // 12    Console.WriteLine($"Year: {match.Groups[3].Value}");    // 2024}

Groups are numbered starting from 1 (group 0 is the entire match). Named groups are more readable:

string namedPattern = @"(?<day>\\d{2})\\.(?<month>\\d{2})\\.(?<year>\\d{4})";match = Regex.Match(input, namedPattern);Console.WriteLine(match.Groups["day"].Value);   // 25

3. Advanced Features: Replacements and Options

Text replacement with Regex.Replace. It allows not only to remove or replace a substring but also to use groups in the replacement string:

string phone = "+7 (123) 456-78-90";// Remove everything except digitsstring clean = Regex.Replace(phone, @"[^\\d]", "");Console.WriteLine(clean); // 71234567890

// Change format: (123) 456-78-90 → 123-456-7890string formatted = Regex.Replace(phone,     @"\\((\\d{3})\\)\\s*(\\d{3})-(\\d{2})-(\\d{2})",    "$1-$2-$3$4");Console.WriteLine(formatted); // 123-456-7890

Important options (RegexOptions):