add warning for regular expression with [/\]

[asottile]

pre-commit normalizes slashes to forward slashes since 2.5.0 (and further back to 1.19.0 for per-hook configuration)

there's already a similar warning for /* so this should be a fairly beginner-friendly changeset

[dogukanteber]

Can you open up the issue a little more, please? I'd like to fix this issue but I could not understand the problem exactly.

[asottile]

right now, one may write something like:

    files: ^somedir[/\].*\.py$
    # or
    exclude: ^somedir[\/]otherdir[\/].*\.py$

expecting that they need to match both posix paths somedir/foo/bar.py and windows paths somedir\foo\bar.py -- however this is unnecessary because pre-commit normalizes all slashes to forward slashes

there's a similar regular expression warning for doing something like files: whatever/*.py where someone assumes that it is a glob and not a regex

[asottile]

via #2053