Mastering Regular Expression. Part 10: Custom Regex Classes and Engine Internals in C#

C# goes beyond standard regex usage — it gives you tools to compile and cache patterns into strongly-typed classes using RegexCompilationInfo, which boosts performance and lets you bundle regex logic in a reusable way.

In this part, you’ll learn:

• How the .NET regex engine works under the hood
• How to create precompiled regex classes using RegexCompilationInfo
• Caching strategies
• Performance considerations
• When to compile vs. interpret

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10.1 How the .NET Regex Engine Works

Interpreted vs Compiled Mode

• Interpreted: default; parses and evaluates the regex every time.
• Compiled: transforms regex into IL (intermediate language) code.

Key Options:

RegexOptions.Compiled
RegexOptions.IgnoreCase
RegexOptions.Singleline
RegexOptions.ExplicitCapture

Compiled trades memory for speed — ideal for regexes used frequently.

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10.2 Regex Compilation via RegexCompilationInfo

You can precompile regex into standalone C# classes and generate a DLL that’s fast and reusable.

🔧 Step 1: Define Your Patterns

var regexes = new RegexCompilationInfo[]
{
    new RegexCompilationInfo(
        @"^\d{4}-\d{2}-\d{2}$",         // pattern
        RegexOptions.None,              // options
        "DateRegex",                    // class name
        "MyRegexLib",                   // namespace
        true                            // isPublic
    ),
    new RegexCompilationInfo(
        @"^[a-zA-Z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,}$",
        RegexOptions.IgnoreCase,
        "EmailRegex",
        "MyRegexLib",
        true
    )
};

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🔧 Step 2: Generate Assembly

RegexCompilationInfo[] patterns = { /* as above */ };

var assemblyName = new AssemblyName("MyRegexLibrary");
Regex.CompileToAssembly(patterns, assemblyName);

This generates a DLL (MyRegexLibrary.dll) with your compiled regexes.

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🔧 Step 3: Use Precompiled Regex

using MyRegexLib;

bool isDate = DateRegex.IsMatch("2025-08-04");
bool isEmail = EmailRegex.IsMatch("john@example.com");

✔️ No runtime parsing or interpretation. This is blazing fast.

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10.3 Benefits of Precompiling Regex

Feature	Benefit
✅ Speed	Skips parsing regex at runtime
✅ Reusability	Centralizes regex logic
✅ Compile-time checks	Errors during build, not runtime
✅ Cleaner Code	Strongly-typed matchers

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10.4 Caching Regex for Repeated Use

Even without precompiling, regex caching is essential for performance.

❌ Bad (compiled every time):

bool match = Regex.IsMatch(text, pattern);

✅ Good (reuse a static Regex):

static Regex phoneRegex = new Regex(@"\d{3}-\d{3}-\d{4}", RegexOptions.Compiled);

bool match = phoneRegex.IsMatch(input);

Avoid creating regex instances in loops.

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10.5 Regex Class Generator Template

Let’s build a C# Regex Factory class:

public static class RegexFactory
{
    public static readonly Regex Email = new(
        @"^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$",
        RegexOptions.Compiled | RegexOptions.IgnoreCase
    );

    public static readonly Regex Date = new(
        @"^\d{4}-\d{2}-\d{2}$",
        RegexOptions.Compiled
    );

    public static readonly Regex IPv4 = new(
        @"^((25[0-5]|2[0-4]\d|1\d{2}|[1-9]?\d)\.){3}(25[0-5]|2[0-4]\d|1\d{2}|[1-9]?\d)$",
        RegexOptions.Compiled
    );
}

Usage:

if (RegexFactory.Email.IsMatch(userInput)) { ... }

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10.6 Regex Compilation: When to Use It

Scenario	Recommendation
Regex runs 1000s+ times/sec	✅ Use Compiled
Runs occasionally	❌ Skip Compiled
Regex logic reused across apps	✅ Precompile to DLL
Regex built from user input	❌ Avoid compiling

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10.7 Benchmarking Regex Performance

Use Stopwatch to measure regex speed:

var sw = Stopwatch.StartNew();
for (int i = 0; i < 100000; i++)
{
    Regex.IsMatch("test@example.com", pattern);
}
sw.Stop();
Console.WriteLine(sw.ElapsedMilliseconds);

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10.8 Limitations of Regex Compilation

• Slower startup time if too many compiled regexes
• Can't compile dynamic patterns
• Not portable between platforms without same .NET version

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10.9 Organizing Regex Libraries in C#

Structure your regex codebase like this:

/RegexLib/
    EmailRegex.cs
    DateRegex.cs
    InputValidators.cs
    RegexFactory.cs

Keep patterns:

• Well-named
• Tested
• Version-controlled

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Summary

By precompiling regex:

• You gain performance
• You reduce runtime bugs
• You encapsulate and organize regex logic cleanly

You now know how to:

✅ Use RegexCompilationInfo
✅ Generate regex DLLs
✅ Cache regex safely
✅ Write reusable regex classes

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Coming Up Next

In Part 11, we’ll wrap up the series with:

“Regex Testing, Debugging, and Tooling for C# Developers”

We’ll explore:

• Online testers
• Regex debuggers
• Visualizers
• Unit test strategies
• Common libraries and extensions