Lessons · Regex · the pattern cache, and when it stops helping

Compiling, and what it is really for

The module-level calls look the compiled pattern up in a cache, so the usual advice to compile is about clarity, not about a large hidden cost.

Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.

What it is for

Knowing this stops you optimising the wrong thing. The case that genuinely needs compiling is the one where the pattern is built per row and the cache never hits.

How to think about it

Compile for a name and a place to put a comment. Compile for real when the patterns are made from data rather than written down.

Worked example

re.compile(r'\d+').findall('a1b22')

A pattern object, with the methods on it.

re.findall(r'\d+', 'a1b22')

The same work, through the cache.

re.compile(r'\d+').pattern

And the object remembers what it was made from.

Your turn

Build the pattern once and use it.

re.(r'\d+').findall('a1b22')

The trap

The engine is identical either way. If a regex is your bottleneck, the pattern is the thing to change, not where it was built.

Practise the pattern cache, and when it stops helping on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.