This is regarding the change to re.sub() between 3.6 and 3.7 that results in different behavior even for simple cases like the following:

re.sub('a*','b', 'a') returns 'b' in 3.6 and 'bb' in 3.7

This change is well documented here:
https://docs.python.org/3/whatsnew/3.7.html#changes-in-the-python-api

However, it is not well documented here:
https://docs.python.org/3.7/library/re.html

The latter document does actually contain the appropriate text: "Empty matches for the pattern are replaced when adjacent to a previous non-empty match."

However, the formatting makes this text look like it was always there, and is not part of the 3.7 changes announcement.

That is how I interpreted it, leading to some lost productivity.

After so many years, people don't expect the regex engine to change like this, and that only makes it easier to misinterpret that text as always have been there vs. being new to 3.7.

Related:
https://bugs.python.org/issue32308

I believe the second note under "Changed in 3.7" is intended to address this:

> Empty matches for the pattern are replaced when adjacent to a previous non-empty match.

Obviously not as detailed as the What's New entry, but it's there.

You have not understood my message. I know that text is already there.

My point is that because it's in a separate paragraph, it looks like it's not part of Changed in 3.7.

There is nowhere else on the page where a change description is in a separate paragraph from the "Changed in version X.X:" text.

When I came across this I thought, this probably goes with the paragraph before it, right? In other words, it's probably another change from 3.7, just in a different paragraph?

But then I thought, no, they probably wouldn't change something like that (i.e. basic regex stuff) after so many years.

So it's not that obvious.

@mollison: would you like to open a PR w/ how you would expect it to be formatted?

@brett.cannon: Yes, I will submit a PR. I have the code ready to go already. I just submitted the Python contributor agreement. I think it will take the system a day or two to register that. Then, I will submit the PR.

@brett.cannon: PR is at https://github.com/python/cpython/pull/12879

New changeset 5ebfa840a1c9967da299356733da41b532688988 by Berker Peksag (mollison) in branch 'master':
bpo-36645: Fix ambiguous formatting in re.sub() documentation (GH-12879)
https://github.com/python/cpython/commit/5ebfa840a1c9967da299356733da41b532688988

New changeset 71b88827f6ad368eafa17983bd979175d24da888 by Miss Islington (bot) in branch '3.7':
bpo-36645: Fix ambiguous formatting in re.sub() documentation (GH-12879)
https://github.com/python/cpython/commit/71b88827f6ad368eafa17983bd979175d24da888

Thank you!