Mastering Regular Expression. Part 4: Real-World Regex Examples in C#

This part contains step-by-step, real-world C# examples using Regular Expressions for:

1. Email and phone number validation
2. Password strength checking
3. Extracting URLs and domains
4. Parsing log files
5. Cleaning HTML tags
6. Renaming files in bulk
7. Web scraping-like text processing
8. Form input validation
9. File path parsing
10. Text analytics (word frequency, tokenization)

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4.1 Validate Email Addresses

string pattern = @"^[\w\.-]+@[\w\.-]+\.\w{2,}$";
string input = "john.doe@example.com";

bool valid = Regex.IsMatch(input, pattern);
Console.WriteLine(valid); // True

🧠 Explanation:

• ^[\w\.-]+ → Start with word characters, dot, or dash
• @ → Literal at-symbol
• [\w\.-]+ → Domain name
• \.\w{2,}$ → Dot + at least 2 letters for TLD

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4.2 Validate Phone Numbers (US)

string pattern = @"^\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}$";

string[] samples = {
    "123-456-7890", "(123) 456 7890", "123.456.7890", "1234567890"
};

foreach (var phone in samples)
    Console.WriteLine($"{phone}: {Regex.IsMatch(phone, pattern)}");

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4.3 Password Strength Checker

string pattern = @"^(?=.*[A-Z])(?=.*\d)(?=.*[!@#$%^&*()_+]).{8,}$";

string input = "Pa$$w0rd123";
bool strong = Regex.IsMatch(input, pattern);

Console.WriteLine(strong); // True

✅ Requires uppercase, digit, special character, and 8+ characters

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4.4 Extract URLs from Text

string pattern = @"https?://[^\s]+";
string text = "Visit https://openai.com or http://example.org for details.";

foreach (Match m in Regex.Matches(text, pattern))
    Console.WriteLine(m.Value);

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4.5 Parse Log Entries

Example log:

[2025-08-01 12:34:56] ERROR: Disk failure on drive C

string pattern = @"\[(?<timestamp>.+?)\]\s(?<level>[A-Z]+):\s(?<message>.+)";
string input = "[2025-08-01 12:34:56] ERROR: Disk failure on drive C";

Match m = Regex.Match(input, pattern);
if (m.Success) {
    Console.WriteLine($"Time: {m.Groups["timestamp"]}");
    Console.WriteLine($"Level: {m.Groups["level"]}");
    Console.WriteLine($"Message: {m.Groups["message"]}");
}

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4.6 Strip HTML Tags

string html = "<div><p>Hello <strong>world</strong></p></div>";
string noTags = Regex.Replace(html, "<.*?>", "");

Console.WriteLine(noTags); // Hello world

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4.7 Rename Files in Bulk

Imagine renaming file names like 2025_report_final.txt to report_2025.txt

string pattern = @"(\d{4})_(\w+)_final\.txt";
string input = "2025_report_final.txt";

string result = Regex.Replace(input, pattern, "$2_$1.txt");
Console.WriteLine(result); // report_2025.txt

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4.8 Validate User Input (Form Example)

Username Rules:

• Only letters/numbers
• Length: 4–12

string pattern = @"^[a-zA-Z0-9]{4,12}$";

string[] usernames = { "admin", "root123", "bad user", "too_long_username" };

foreach (string name in usernames)
    Console.WriteLine($"{name}: {Regex.IsMatch(name, pattern)}");

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4.9 Extract File Paths and Extensions

string pattern = @"(?<name>[\w\-]+)\.(?<ext>jpg|png|gif)";
string input = "img1.jpg, banner.png, doc.txt";

foreach (Match m in Regex.Matches(input, pattern)) {
    Console.WriteLine($"File: {m.Groups["name"]}, Ext: {m.Groups["ext"]}");
}

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4.10 Word Frequency Counter (Tokenization)

string text = "Regex is powerful. Regex is flexible.";

var matches = Regex.Matches(text.ToLower(), @"\w+");
var freq = new Dictionary<string, int>();

foreach (Match m in matches) {
    string word = m.Value;
    if (!freq.ContainsKey(word)) freq[word] = 0;
    freq[word]++;
}

foreach (var kv in freq)
    Console.WriteLine($"{kv.Key}: {kv.Value}");

Outputs:

regex: 2

is: 2

powerful: 1

flexible: 1

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4.11 Clean & Normalize Input

Remove multiple spaces, trim start/end:

string input = "   This    is    a    test.   ";
string cleaned = Regex.Replace(input.Trim(), @"\s+", " ");

Console.WriteLine(cleaned); // This is a test.

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4.12 Match Dates in Various Formats

string pattern = @"\b(\d{1,2})[/-](\d{1,2})[/-](\d{2,4})\b";
string input = "Today is 8/4/2025 or 04-08-25";

foreach (Match m in Regex.Matches(input, pattern)) {
    Console.WriteLine($"Date: {m.Value}");
}

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4.13 Advanced: Matching Balanced HTML Tags (Simplified)

string pattern = @"<(?<tag>\w+)>.*?</\k<tag>>";
string html = "<b>Hello</b><i>World</i><p>Ignore</div>";

foreach (Match m in Regex.Matches(html, pattern)) {
    Console.WriteLine(m.Value);
}

⚠️ Not for deeply nested tags — use parsers like HtmlAgilityPack for HTML trees

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4.15 Test-Driven Validation with Regex

Unit test email format with Regex:

Assert.IsTrue(Regex.IsMatch("test@openai.com", @"^[\w\.-]+@[\w\.-]+\.\w{2,}$"));

Integrate regex with unit tests in xUnit, NUnit, or MSTest for CI/CD.

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4.16 Summary

In this practical C#-focused chapter, we’ve used regex to:

✅ Validate and extract structured data
✅ Parse, clean, and transform strings
✅ Automate file handling
✅ Analyze text content

With regex, you can remove repetitive boilerplate and write more expressive, compact, and reliable code.

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🔜 Coming Next

In Part 5, we’ll cover:

Regex Performance and Optimization in C#, including:

• How regex engines work under the hood
• Avoiding catastrophic backtracking
• Compiled regex
• Benchmarking pattern efficiency
• Profiling & tuning patterns in .NET apps