dotnet-source-gen-regex

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

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

1. 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

1. 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();

```

1. Verify with dotnet build

1. If build fails, check:

- Class is marked partial

- Pattern is a compile-time constant

- .NET version is 7 or higher

| 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) |

Instance with options:

```csharp

// 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:

```csharp

// Before

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

// After

if (PhoneNumberRegex().IsMatch(input))

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

private static partial Regex PhoneNumberRegex();

```

• 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