2.5.5 regression: strong emphasis adjacent to word/CJK chars fails when content starts or ends with punctuation #688

Yeah, this looks like it's down to a change in the rules as to what is a valid em/strong opening/closing delimiter run.

From the minimal repro examples:

1. Leftmost ** is not a valid opening delimiter run as it's followed by punctuation, but not preceded by punctuation/whitespace as well
2. Rightmost ** is not a valid closing run as it's preceeded by punctuation but not followed by punctioation/whitespace
3. Leftmost ** invalid for same reason as 1

For the GFM implementation we tried to follow GFM's spec, and their rules on what a left/right flanking delimiter run can consist of.

In theory you could do something like this to restore the old behaviour:

class ForceOldIAB(markdown2.ItalicAndBoldProcessor):
    name = 'force-old-iab'
    order = (markdown2.Stage.ITALIC_AND_BOLD,), (markdown2.Stage.ITALIC_AND_BOLD,)

    def sub(self, match):
        syntax = match.group(1)
        tag = 'strong' if len(syntax) == 2 else 'em'
        return f'<{tag}>{match.group(2)}</{tag}>'

ForceOldIAB.register()

markdown2.markdown(text, extras=['force-old-iab']

Although testing this, the old strong/em behaviour was changed in #644 before being overwritten by the GFM changes in the same release cycle. So the old behaviour still wouldn't work.

I guess, since the updated behaviour of the old implementation never actually got used, since the GFM changes superseded them and fixed all the issues from that PR anyway, we could revert that change for the old implementation so that users have an option to stick with the old behaviour.
@nicholasserra thoughts on this?

Created #690 just in case with the suggested change of reverting the legacy behaviour, although open to discussion

So basically, if we merge #690 and revert to the old behavior, they'd just need to add their own extra to disable the GFM default, and it would work as expected?

Yeah, pretty much