csharp-regex

name

csharp-regex

description

Use the [GeneratedRegex] source generator for ALL regex in .NET 7+ C#. Produces AOT-safe, trimming-friendly, compile-time regex with zero startup cost and better throughput than RegexOptions.Compiled. Trigger whenever the user writes, reviews, or asks about regex in C# — including new Regex(...), Regex.IsMatch, pattern matching, string validation, or text parsing — even if they don't mention source generation. Never suggest new Regex(...) with RegexOptions.Compiled when [GeneratedRegex] is available.

C# Source-Generated Regex Skill

Use [GeneratedRegex] on a partial method or property so the compiler emits a fully-optimised, AOT-safe Regex implementation at build time. This delivers the throughput of RegexOptions.Compiled with near-zero startup cost, and the output is ordinary C# that you can read and debug.

Why this matters

new Regex(pattern, RegexOptions.Compiled) pays a heavy startup cost (reflection-emit, JIT compilation) and is not AOT-safe. [GeneratedRegex] compiles the pattern into readable C# during the build, giving you:

• Better throughput than RegexOptions.Compiled (and more, in some benchmarks)
• Near-zero startup cost — no JIT compile at runtime
• AOT / trimming safe — no Reflection.Emit
• Debuggable — step through the generated C# in any debugger
• Automatic singleton caching — no static readonly field needed

Diagnostic SYSLIB1045 fires when the compiler detects a place that could use [GeneratedRegex] but doesn't.

Quick reference

Concept	See
Constraints, method vs property, minimal examples	basics.md
RegexOptions, timeouts, culture	options.md
Where to declare regex, file organisation	patterns.md
Common mistakes and compiler errors	pitfalls.md

Minimal examples

Method (≥ .NET 7):

public static partial class Patterns
{
    [GeneratedRegex(@"\d+")]
    public static partial Regex Digits();
}

Property (≥ .NET 9, C# 13):

public static partial class Patterns
{
    [GeneratedRegex(@"\d+")]
    public static partial Regex Digits { get; }
}

Essential rules at a glance

• The containing class must be partial.
• Method form: must be static, partial, parameterless, non-generic, returning Regex.
• Property form: must be static, partial, getter-only, returning Regex (C# 13 / .NET 9+).
• Do not add RegexOptions.Compiled — it is ignored by the generator.
• The generator handles caching; no static readonly field is needed.

What to read next

• First time using [GeneratedRegex] → basics.md
• Need IgnoreCase, timeouts, or culture → options.md
• Deciding where to put the declaration → patterns.md
• Getting a compiler error → pitfalls.md