Forming Regular Expressions in C#

How to Form Regular Expressions in C#?

Regular Expressions (RegEx) are patterns used to match character combinations in strings. Understanding how to construct these patterns is essential for tasks such as validation, searching, and text extraction.

Basic Components of Regular Expressions

Regular expressions consist of different elements that help in pattern matching.

Expression	Description	Example Match
`.`	Matches any single character except newline	`a.b` → Matches acb, axb
`^`	Indicates the start of a string	`^hello` → Matches "hello world"
`$`	Indicates the end of a string	`world$` → Matches "hello world"
`\d`	Matches any digit (0-9)	`\d\d\d` → Matches "123"
`\w`	Matches any alphanumeric character (a-z, A-Z, 0-9, _)	`\w+` → Matches "hello123"
`\s`	Matches any whitespace character	`hello\sworld` → Matches "hello world"
`[abc]`	Matches any one of the specified characters	`b[aeiou]t` → Matches "bat", "bit", "but"
`[^abc]`	Matches any character except those in brackets	`[^0-9]` → Matches any non-digit
`(abc)`	Groups characters together	`(hello){2}` → Matches "hellohello"

Quantifiers in Regular Expressions

Quantifiers define how many times a character, group, or class must appear in the input.

Quantifier	Description	Example
`*`	Matches 0 or more occurrences	`a*` → Matches "", "a", "aaa"
`+`	Matches 1 or more occurrences	`a+` → Matches "a", "aaa" but not ""
`?`	Matches 0 or 1 occurrence	`a?` → Matches "", "a"
`{n}`	Matches exactly n occurrences	`\d{3}` → Matches "123"
`{n,}`	Matches at least n occurrences	`\d{3,}` → Matches "123", "12345"
`{n,m}`	Matches between n and m occurrences	`\d{2,4}` → Matches "12", "123", "1234"

Example: Extracting Email Addresses

The following example demonstrates how to extract email addresses from a text.

Example: Extracting Emails Using RegEx

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
        string text = "Contact me at example@domain.com or support@company.org.";
        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($"Found Email: {match.Value}");
        }
    }
}

// Output:
// Found Email: example@domain.com
// Found Email: support@company.org

The Regex.Matches() method extracts all email addresses from the given text.

When to Use Regular Expressions?

For input validation (e.g., emails, phone numbers, passwords).
To search and extract specific patterns from text.
For replacing text dynamically in large data sets.
To clean and format text efficiently.