How does RegexOptions.Compiled work?

This entry in the BCL Team Blog gives a nice overview: "Regular Expression performance".

In short, there are three types of regex (each executing faster than the previous one):

1. interpreted

   fast to create on the fly, slow to execute

2. compiled (the one you seem to ask about)

   slower to create on the fly, fast to execute (good for execution in loops)

3. pre-compiled

   create at compile time of your app (no run-time creation penalty), fast to execute

So, if you intend to execute the regex only once, or in a non-performance-critical section of your app (i.e. user input validation), you are fine with option 1.

If you intend to run the regex in a loop (i.e. line-by-line parsing of file), you should go with option 2.

If you have many regexes that will never change for your app and are used intensely, you could go with option 3.

It should be noted that the performance of regular expressions since .NET 2.0 has been improved with an MRU cache of uncompiled regular expressions. The Regex library code no longer reinterprets the same un-compiled regular expression every time.

So there is potentially a bigger performance penalty with a compiled and on the fly regular expression. In addition to slower load times, the system also uses more memory to compile the regular expression to opcodes.

Essentially, the current advice is either do not compile a regular expression, or compile them in advance to a separate assembly.

Ref: BCL Team Blog Regular Expression performance [David Gutierrez]

1) Base Class Library Team on compiled regex

2) Coding Horror, referencing #1 with some good points on the tradeoffs

This does not answer the question but I recommend doing this:

[RegexGenerator($@"MyPatter")]
public partial Regex Regex_SomeRegex();

That way you get best of both worlds. It will be fast at initialization because its created at compile time. And it will also be fast when using it.