Proposed rules: encourage use of `re.compile` on constants in module scope, forbid it in function scope

I'd be interested in writing checks for this, if it would be acceptable. Please just tell me / close this issue if these rules would not be welcome. I often see `re` usage which would be made better by (1) using `re.compile` (rather than the free functions which take a string) or (2) moving `re.compile` from function scope out to module scope.[^1] [^1]: Yes, it _can_ be okay if the `re` cache covers your needs. But I've seen this in library code, where you cannot make any assumptions about how many regexes are being compiled and put in the cache. Of course, for these changes to be correct, the regex string needs to be constant, but that's the common case. The regex flags also need to be considered. Example 1, encourage moving away from the free functions when a pattern can be pre-compiled: ```python # okay def foo(x): return re.search(r"\w+", x) # better _WORD_PAT = re.compile(r"\w+") def foo(x): return _WORD_PAT.search(x) ``` Example 2, there's no real reason to do it this way: ```python # bad, why write this? compile out in module scope def foo(x): pat = re.compile(r"\w+") return pat.search(x) ``` (fix is the same as Example 1) Example 3, can we handle flags? What if they're ORed together? ```python def foo(x): pat = re.compile(r"[abc\n]+", flags=re.I | re.M) return pat.search(x) ``` --- I therefore suggest two rules, one on by default and one in the opinionated category: - Opinionated: You should use `re.compile()` instead of `{re.search, re.match, re.fullmatch, re.split, re.findall, re.finditer, re.sub, re.subn}` on a string - Non-opinionated: You should not call `re.compile()` on a constant in function scope -- do it in class or module scope I'd suggest starting with turning these rules on only where `flags` is omitted from the call, so the 1-argument form of `re.compile` and the 2-argument forms of the various functions. `flags` handling can be a later enhancement. --- Implementation should be simple. Detecting these names would be similar to `B017` -- just look for a node named `"re"` and don't worry about the name being rebound in funny ways.