🚀 C# Regular Expressions – Master Pattern Matching in Strings

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🧲 Introduction – Why Use Regular Expressions in C#

Regular expressions (regex) allow you to define powerful patterns for matching, searching, and manipulating text. In C#, the System.Text.RegularExpressions namespace provides full support for regex-based operations, making it a powerful tool for input validation, string parsing, and text extraction.

🎯 In this guide, you’ll learn:

• What regular expressions are and how to write them
• How to use the Regex class in C#
• Real-world use cases: email validation, text searching, replacements
• Common regex patterns and best practices

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🔍 Core Concept – What Are Regular Expressions?

A regular expression is a string pattern made up of characters and symbols that define a search or matching rule.

Example Pattern:

^\d{3}-\d{2}-\d{4}$

✅ Matches U.S. Social Security numbers like 123-45-6789

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📚 Key Regex Classes in C#

Class	Purpose
Regex	Core class to match/replace/search
Match	Represents a single match
MatchCollection	Represents all matches found
Group	Represents a subexpression match

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💻 Code Example – Regex Match

using System;
using System.Text.RegularExpressions;

class RegexExample
{
    static void Main()
    {
        string pattern = @"\d+";
        string input = "There are 123 apples and 45 oranges.";

        MatchCollection matches = Regex.Matches(input, pattern);
        
        foreach (Match match in matches)
        {
            Console.WriteLine(match.Value);
        }
    }
}

📤 Output:

123  
45

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🛠️ Common Regex Patterns

Pattern	Description	Example Match
\d	Digit	0-9
\w	Word character	a-z, A-Z, 0-9
\s	Whitespace	space, tab, newline
.	Any character (except newline)	a, b, 1, etc.
^	Start of string
$	End of string
*, +, ?	Zero/more, one/more, zero/one	\d+ matches 1+ digits
[]	Character set	[aeiou]
`	`	OR
()	Grouping	(abc)+

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🔁 Replace with Regex

string input = "Call me at 123-456-7890";
string masked = Regex.Replace(input, @"\d{3}-\d{3}-\d{4}", "***-***-****");

Console.WriteLine(masked);  // Output: Call me at ***-***-****

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✅ Use Case Examples

• Email validation: ^[\w\.-]+@[\w\.-]+\.\w+$
• Phone number masking
• Extracting numbers or words
• Parsing logs or structured text

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💡 Tips, Pitfalls & Best Practices

💡 Tip: Use regex101.com to test patterns interactively.

⚠️ Pitfall: Regex patterns can become hard to read. Always comment or document complex ones.

📘 Best Practice: Use named groups for clarity:

(?<area>\d{3})-(?<exchange>\d{3})-(?<line>\d{4})

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📌 Summary – Recap & Next Steps

C# regular expressions are essential for powerful text parsing and validation. With Regex, you can search, extract, and transform strings efficiently.

🔍 Key Takeaways:

• Use Regex.Match(), Regex.Matches(), and Regex.Replace() for powerful string operations
• Learn basic pattern symbols like \d, \w, ^, $
• Master use cases like input validation and data extraction

⚙️ Up next: Dive into 🚀 C# Preprocessor Directives to control compilation with #if, #define, and more.

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❓ FAQ – C# Regular Expressions

❓ What namespace contains Regex in C#?
✅ System.Text.RegularExpressions

❓ Can regex be case-insensitive?
✅ Yes. Use RegexOptions.IgnoreCase.

❓ What’s the difference between Match and Matches?
✅ Match finds the first match; Matches returns all matches as a collection.

❓ Are regex expressions compiled?
✅ You can use RegexOptions.Compiled for faster repeated execution.

❓ Can I validate email or phone numbers with regex?
✅ Yes. Regex is commonly used for format validation.

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