Regex Performance With and Without RegexOptions.Compiled Using .NET Framework 4.8 and .NET Core 3.1 (December 2019)

We’ve had some internal discussion around the usage of RegexOptions.Compiled in .NET — how it works and when it’s appropriate to use it. Despite being named Compiled is isn’t compiled at build time, it’s a runtime optimization for repeated use of a regex. And, it’s backed by a cache that has a default limit of 15. You can read more about this here: https://docs.microsoft.com/en-us/dotnet/standard/base-types/compilation-and-reuse-in-regular-expressions.

That said, what are the performance characteristics of normal and compiled regular expressions on .NET Framework 4.8 and .NET Core 3.1?

Benchmark results

https://github.com/kendaleiv/dotnet-regex-benchmarks#results

BenchmarkDotNet=v0.12.0, OS=Windows 10.0.18362
Intel Core i7-8550U CPU 1.80GHz (Kaby Lake R), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=3.1.100
  [Host]        : .NET Core 3.1.0 (CoreCLR 4.700.19.56402, CoreFX 4.700.19.56404), X64 RyuJIT
  .NET Core x64 : .NET Core 3.1.0 (CoreCLR 4.700.19.56402, CoreFX 4.700.19.56404), X64 RyuJIT
  LegacyJitX86  : .NET Framework 4.8 (4.8.4042.0), X86 LegacyJIT
  RyuJitX64     : .NET Framework 4.8 (4.8.4042.0), X64 RyuJIT

Single

{: .img-fluid .border }

1,000

{: .img-fluid .border }

100,000

{: .img-fluid .border }

1,000,000

{: .img-fluid .border }

Conclusion

Using a compiled regex for a single result is a performance burden. Using a compiled regex as a performance optimization doesn’t make sense until you’re matching it many times (at least on the hardware described by this benchmark). With performance optimizations ideally one measures to ensure performance is improved and by how much. And, don’t forget to consider the cache size if you’re using compiled regex.

The benchmark is at https://github.com/kendaleiv/dotnet-regex-benchmarks if you’d like to try it out yourself.