Regular expressions (regex) in C# provide a powerful way to match patterns in strings. They are useful for tasks such as validating input, searching for specific patterns, and manipulating strings. C# provides the System.Text.RegularExpressions namespace, which contains classes for working with regular expressions.

Key Classes in C#

• Regex: The main class for working with regular expressions.
• Match: Represents the result of a single regular expression match.
• MatchCollection: Represents a collection of Match objects.
• Group: Represents a capturing group in the regular expression.
• GroupCollection: Represents a collection of Group objects.

Basic Syntax

Here are some common regex patterns and their meanings:

Example 1: Basic Regex Matching

In this example, we will use regex to check if a string is a valid email address.

csharp
1using System;
2using System.Text.RegularExpressions;
3
4class Program
5{
6    static void Main()
7    {
8        string emailPattern = @"^[^@\s]+@[^@\s]+\.[^@\s]+$";
9        string[] testEmails = { "test@example.com", "invalid-email", "user@domain", "user@domain.com" };
10
11        foreach (var email in testEmails)
12        {
13            if (Regex.IsMatch(email, emailPattern))
14            {
15                Console.WriteLine($"{email} is a valid email address.");
16            }
17            else
18            {
19                Console.WriteLine($"{email} is NOT a valid email address.");
20            }
21        }
22    }
23}

Explanation

• Pattern: The regex pattern ^[^@\s]+@[^@\s]+\.[^@\s]+$ checks for a valid email format.
  • ^: Start of the string.
  • [^@\s]+: One or more characters that are not @ or whitespace.
  • @: The @ symbol.
  • [^@\s]+: One or more characters that are not @ or whitespace (domain).
  • \.: A literal dot.
  • [^@\s]+: One or more characters that are not @ or whitespace (top-level domain).
  • $: End of the string.

Example 2: Extracting Matches

In this example, we will extract all the numbers from a string.

csharp
1using System;
2using System.Text.RegularExpressions;
3
4class Program
5{
6    static void Main()
7    {
8        string input = "There are 3 apples, 5 bananas, and 12 oranges.";
9        string numberPattern = @"\d+";
10
11        MatchCollection matches = Regex.Matches(input, numberPattern);
12
13        Console.WriteLine("Numbers found:");
14        foreach (Match match in matches)
15        {
16            Console.WriteLine(match.Value);
17        }
18    }
19}

Explanation

• Pattern: The regex pattern \d+ matches one or more digits.
• Regex.Matches: This method returns a collection of all matches found in the input string.
• Output: The program prints all the numbers found in the input string.

Example 3: Replacing Text

In this example, we will replace all occurrences of a specific word in a string.

csharp
1using System;
2using System.Text.RegularExpressions;
3
4class Program
5{
6    static void Main()
7    {
8        string input = "The quick brown fox jumps over the lazy dog.";
9        string pattern = @"\blazy\b"; // Match the word "lazy" exactly
10        string replacement = "energetic";
11
12        string result = Regex.Replace(input, pattern, replacement);
13
14        Console.WriteLine("Original: " + input);
15        Console.WriteLine("Modified: " + result);
16    }
17}