Regular Expressions in C# with Examples: In Practice

Regular Expressions in C# with Examples: From Theory to Practice

Regular expressions (regex) are a powerful tool for searching, extracting, and replacing text based on patterns. In C# (.NET), working with regex is implemented through the System.Text.RegularExpressions namespace and the Regex class. In this article, we will cover the main features with real examples: from simple search to email and phone validation.

1. Basics: The Regex Class and Its Methods

The main class for working with regex is Regex. It provides static and instance methods. Here are the main ones:

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• IsMatch — checks if the string contains a match for the pattern.
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• Match — finds the first match.
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• Matches — finds all matches.
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• Replace — replaces matches with a specified string.
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• Split — splits the string based on a pattern.
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Example 1. Simple Check

using System;\nusing System.Text.RegularExpressions;\n\nclass Program\n{\n    static void Main()\n    {\n        string text = "Hello, my number: 123-456-7890";\n        string pattern = @"\\d{3}-\\d{3}-\\d{4}";\n\n        if (Regex.IsMatch(text, pattern))\n        {\n            Console.WriteLine("Number found!");\n        }\n    }\n}\n

In this example, \\d{3} means "exactly three digits". The backslash is escaped using @ (verbatim string).

2. Capture Groups and Data Extraction

Capture groups allow you to extract parts of a match. They are defined with parentheses (). Each group gets a sequential number (starting from 1).

Example 2. Extracting Username and Domain from an Email

string email = "user@example.com";\nstring pattern = @"^(\\w+)@(\\w+\\.\\w+)$";\n\nMatch match = Regex.Match(email, pattern);\n\nif (match.Success)\n{\n    Console.WriteLine($"Username: {match.Groups[1].Value}");   // user\n    Console.WriteLine($"Domain: {match.Groups[2].Value}");   // example.com\n}\n

Here, (\\w+) captures one or more alphanumeric characters. ^ and $ are anchors for the start and end of the string.

Example 3. Parsing a URL

string url = "https://pythonlib.ru/page?id=42";\nstring pattern = @"^(https?)://([^/]+)(/.*)?$";\n\nMatch m = Regex.Match(url, pattern);\nif (m.Success)\n{\n    Console.WriteLine($"Protocol: {m.Groups[1].Value}"); // https\n    Console.WriteLine($"Domain: {m.Groups[2].Value}");    // pythonlib.ru\n    Console.WriteLine($"Path: {m.Groups[3].Value}");     // /page?id=42\n}\n

3. Text Replacement and Removal

The Replace method allows you to replace found fragments. You can use backreferences like $1, $2 to insert groups.

Example 4. Formatting a Phone Number

string raw = "89161234567";\nstring pattern = @"(\\d{1})(\\d{3})(\\d{3})(\\d{2})(\\d{2})";\nstring replacement = "+7 ($2) $3-$4-$5";\n\nstring formatted = Regex.Replace(raw, pattern, replacement);\nConsole.WriteLine(formatted); // +7 (916) 123-45-67\n

Example 5. Removing Extra Spaces

string text = "This    text   has    extra   spaces.";\nstring cleaned = Regex.Replace(text, @"\\s+", " ");\nConsole.WriteLine(cleaned); // "This text has extra spaces."\n

\\s+ — one or more whitespace characters. We replace them with a single space.

4. Real-World Cases: Email and Phone Validation

In practice, regex is most often used for validating user input. Here are two typical examples.

Email Validation (Simplified)

public static bool IsValidEmail(string email)\n{\n    if (string.IsNullOrWhiteSpace(email))\n        return false;\