A Simple Analogy

Regex is like a search template. Instead of searching for exact "john@example.com", you search for "any text @ any text . com" to find all email addresses. Regex describes patterns instead of specific strings.

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What Is Regex?

Regular expressions (regex) are patterns for matching and manipulating text. They let you search, validate, and extract data using symbolic patterns instead of literal strings.

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Why Use Regex?

• Pattern matching: Find text matching criteria
• Validation: Check email, phone, password format
• Extraction: Pull data from unstructured text
• Replacement: Find and replace complex patterns
• Parsing: Extract fields from logs or documents

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Basic Patterns

| Pattern | Matches | |---------|---------| | . | Any single character | | * | Zero or more of previous | | + | One or more of previous | | ? | Zero or one of previous | | [abc] | Any of: a, b, or c | | [^abc] | Not a, b, or c | | \d | Any digit (0-9) | | \w | Word character (a-z, A-Z, 0-9, _) | | \s | Whitespace | | ^ | Start of string | | $ | End of string |

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Common Examples

using System.Text.RegularExpressions;

// Email validation
var emailPattern = @"^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$";
bool isEmail = Regex.IsMatch("user@example.com", emailPattern);

// Phone validation (10 digits)
var phonePattern = @"^\d{3}-\d{3}-\d{4}$";
bool isPhone = Regex.IsMatch("123-456-7890", phonePattern);

// Extract numbers from text
var numbers = Regex.Matches("Price: $19.99, Quantity: 5", @"\d+");
// Result: ["19", "99", "5"]

// Replace pattern
string text = "2025-02-20";
string formatted = Regex.Replace(text, @"(\d{4})-(\d{2})-(\d{2})", "$3/$2/$1");
// Result: "20/02/2025"

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Common Validations

// Password: min 8 chars, uppercase, lowercase, digit, special char
var passwordPattern = @"^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$";

// URL
var urlPattern = @"^https?://[^\s/$.?#].[^\s]*$";

// Credit card (basic)
var cardPattern = @"^\d{4}[\s-]?\d{4}[\s-]?\d{4}[\s-]?\d{4}$";

// IP address
var ipPattern = @"^(\d{1,3}\.){3}\d{1,3}$";

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Practical Example

public class TextProcessor
{
    // Extract email addresses
    public static List<string> ExtractEmails(string text)
    {
        var pattern = @"[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}";
        var matches = Regex.Matches(text, pattern);
        return matches.Cast<Match>().Select(m => m.Value).ToList();
    }
    
    // Validate password strength
    public static bool IsStrongPassword(string password)
    {
        var pattern = @"^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&]).{8,}$";
        return Regex.IsMatch(password, pattern);
    }
    
    // Clean whitespace
    public static string NormalizeWhitespace(string text)
    {
        return Regex.Replace(text, @"\s+", " ").Trim();
    }
}

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Real-World Use Cases

• Log parsing: Extract errors from application logs
• CSV extraction: Pull data from unstructured CSVs
• HTML scraping: Extract links and text from HTML
• Data validation: Ensure format compliance
• Data transformation: Convert between formats

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Related Concepts to Explore

• Lookahead and lookbehind assertions
• Backreferences in replacements
• Named groups for clarity
• Performance optimization
• Regex in different languages

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Summary

Regular expressions provide powerful pattern matching for text processing. Master basic patterns to validate input, extract data, and transform strings efficiently in any application.