Agent Skills: .NET Regex Source Generator

.NET Regex Source Generator

The regex source generator creates compile-time generated regex implementations that are:

• AOT-compatible: Works with Native AOT and trimming
• Debuggable: Step through the generated matching code
• Performant: No runtime compilation overhead

When to Use

This skill applies when the user:

• Wants to optimize regex performance
• Needs AOT-compatible regex patterns
• Asks about [GeneratedRegex] attribute
• Mentions converting new Regex(...) to source-generated
• Discusses regex compilation or startup performance

Workflow

1. Find regex usages in the codebase:

   • new Regex(...) constructor calls
   • Regex.IsMatch(), Regex.Match(), Regex.Replace() static method calls
   • Other static Regex.* methods with inline patterns

2. For each regex with a compile-time known pattern:

   • Ensure the containing class is partial
   • Create a partial method with [GeneratedRegex] attribute:

     [GeneratedRegex("pattern", RegexOptions.IgnoreCase)]
     private static partial Regex MyRegex();
     

   • Name the method descriptively based on what the pattern matches

3. Replace usages to call the generated method:

   // Before
   var regex = new Regex(@"\d+", RegexOptions.Compiled);
   if (regex.IsMatch(input)) { ... }
   
   // After
   if (MyNumberRegex().IsMatch(input)) { ... }
   
   [GeneratedRegex(@"\d+")]
   private static partial Regex MyNumberRegex();
   

4. Verify with dotnet build

5. If build fails, check:

   • Class is marked partial
   • Pattern is a compile-time constant
   • .NET version is 7 or higher

Key Notes

| Note | Detail | |------|--------| | RegexOptions.Compiled | Ignored by source gen - remove it | | .NET Version | Requires .NET 7+ | | Caching | Generated method caches singleton internally | | Timeout | Use [GeneratedRegex("pattern", RegexOptions.None, 1000)] for timeout (milliseconds) |

Pattern Conversion Examples

Instance with options:

// Before
private readonly Regex _emailRegex = new(@"^[\w-\.]+@[\w-]+\.\w+$", RegexOptions.Compiled | RegexOptions.IgnoreCase);

// After
[GeneratedRegex(@"^[\w-\.]+@[\w-]+\.\w+$", RegexOptions.IgnoreCase)]
private static partial Regex EmailRegex();

Static method call:

// Before
if (Regex.IsMatch(input, @"^\d{3}-\d{4}$"))

// After
if (PhoneNumberRegex().IsMatch(input))

[GeneratedRegex(@"^\d{3}-\d{4}$")]
private static partial Regex PhoneNumberRegex();

Error Handling

• If pattern is not constant: Cannot use source gen - leave as runtime regex
• If class is not partial: Add partial modifier to the class declaration
• If build fails after conversion: Check error messages for unsupported pattern features