"Optimizing" concurrent regexes

I am looking into some regex work, and I ran into a performance problem. I need to run a particular regex over a large number (millions) of strings. That caused my spidey sense to… tingle. The code in question looked something like this:long matches = CountMatchingEntries(new Regex(@"\s+user id\s+"));Creating a regex for each invocation is… expensive. That is why we have the RegexOptions.Compiledflag, after all. And the Regex class is thread-safe, so I did the equivalent of this code:// class level static Regex s_regex = new Regex(@"\s+user id\s+", RegexOptions.Compiled); // inside a method long matches = CountMatchingEntries(s_regex);The performance of the system immediately took a big, stinking performance regression all over my benchmarks. At first, I was sure that I wasn’t doing something properly, so I re-wrote this using the modern approach, with source generators, like this:public partial class MyClass { <span class="token punctuation">[</span><span class="token…