[docutils:patches] #206 Improve SmartQuote performance

Thank you for the patch.

I wonder, why there is a considerable performance hit  despite the documentation saying that
>The compiled versions of the most recent patterns passed to re.compile() and the module-level matching functions are cached, so programs that use only a few regular expressions at a time needn’t worry about compiling regular expressions.

and Docutils uses far less than `re._MAXCACHE == 512` regular expressions.
Maybe "re.sub" is no *module-level matching function*? OTOH, the doc says:
>Pattern.sub(repl, string, count=0)
>     Identical to the sub() function, using the compiled pattern.


The 22% refer to the parse (or more precise parse+transform) time rather than the build time in a real-world use case (as the dummy builder is more efficient than a HTML builder, say). Still, 22% of the time to create a document tree is impressive.

Could you test the attached  simplified version of your patch. 
(If the unconditional pre-compilation is considered too wasteful in case "smartquotes" are switched off, I'd rather consider a conditional import of the "smartquotes" module.)

Another improvement may be achieved by simplifying the regexps themselves: 
The current version is taken from the "SmartyPants" module that also cares for HTML input and checks for character entities like  `–` or ` `.


Attachments:

- [pre-compile-regexes-simplified.patch](https://sourceforge.net/p/docutils/patches/_discuss/thread/2d7c8b7ca3/9c1b/attachment/pre-compile-regexes-simplified.patch) (7.4 kB; text/x-patch)


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**[patches:#206] Improve SmartQuote performance**

**Status:** open
**Group:** None
**Created:** Wed Aug 16, 2023 04:37 PM UTC by Chris Sewell
**Last Updated:** Wed Aug 16, 2023 04:37 PM UTC
**Owner:** nobody
**Attachments:**

- [0001-Pre-compile-smartquote-regexes.patch](https://sourceforge.net/p/docutils/patches/206/attachment/0001-Pre-compile-smartquote-regexes.patch) (8.3 kB; application/octet-stream)
- [sphinx-build-after.svg](https://sourceforge.net/p/docutils/patches/206/attachment/sphinx-build-after.svg) (199.1 kB; image/svg+xml)
- [sphinx-build-before.svg](https://sourceforge.net/p/docutils/patches/206/attachment/sphinx-build-before.svg) (282.1 kB; image/svg+xml)


Performing a representative sphinx-build (10 x docutils/docs/ref/rst/restructuredtext.txt, dummy builder), and analysing with py-spy, you can see from the attached flamegraph that the smartquote transform accouts for over 22% of the build time!

This PR attempts to improve that situation (at least down to 18%) by caching regex compilation 


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