Pre-compile regular expressions as standard

[Jake Howard]

See also ​forum thread.

It's fairly well known that there's a performance boost pre-compiling regexes (either using re.compile or _lazy_re_compile, depending on popularity). However, some of Django's internals compile at time of use, which adds unnecessary extra work. Python's re module caches the most recent 512 patterns, however a reasonably sized Django project can easily go beyond that.

I suggest replacing patterns defined at runtime with pre-compiled patterns done at module import time (or _lazy_re_compile if the pattern isn't used much). This probably wants to be done as a single patch, rather than trying to separate per module or the ticket dragging on.

I also suggest reasonably rejecting any non pre-compiled regexes in the ​coding style.