dotnet-source-gen-regex

Development

Converts Regex instances to use the compile-time source generator. Use when the user wants to optimize regex performance or enable AOT compatibility.

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.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

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
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

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();

Verify with dotnet build
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+quot;, RegexOptions.Compiled | RegexOptions.IgnoreCase);

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

Static method call:

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

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

[GeneratedRegex(@"^\d{3}-\d{4}quot;)]
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