Python: Understand multiple parse mode flags specified in a regular expression string

[geoffw0]

Understand / correctly parse multiple parse mode flags specified in a regular expression string, for example i and s in:

(?is)abc.*

Previously we were only recognizing the first flag (i).

This is a backport of part of the Swift PR #13715 .

[yoff]

Very nice PR, good choice of tests. Also, thanks for doing this 🙏

[yoff]

The test failures can be explained by flag_group_start now only matching (? and not also the first mode character.
They do pose the question if mode characters should not be considered normal characters. They do not seem like good candidates for first and last items to be matched...

[yoff]

In summary, I think you can just fix the test expectations and then we can merge :-)

[geoffw0]

Sorry, my notification on this got lost somewhere. Thank you for reviewing.

There's some related investigation / discussion going on in the Java version of this PR (#13778), so I'll wait until everything's settled and port any fixes that arise back to here.

[geoffw0]

I've added a commit accepting the test changes. @yoff it's my understanding that #13975 will fix these?

[yoff]

I've added a commit accepting the test changes. @yoff it's my understanding that #13975 will fix these?

Yes. I will rebase, then we shall see :-)

[yoff]

I've added a commit accepting the test changes. @yoff it's my understanding that #13975 will fix these?

Yes. I will rebase, then we shall see :-)

Scratch that, no need to rebase. We have a PR on top where these tests pass without change.

[yoff]

Let us merge this now, and then let us merge #13975 soon.