.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:
     csharp

     [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:

   csharp

   // 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

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