Require a local-part to be a valid email address in AUTOMAIL_RE.

[nzlosh]

Valid email addresses require a local-part before the @ followed by domain. This PR updates the AUTOMAIL_RE to use the + quantifier to require 1 or more characters in the local-part instead of the * quantifier which allows 0 characters.

The current regex allows the invalid addresses in the form <@domain>. My particular use case is formatting messages in Slack, where mentioned users take the form <@Uxxxxx>. markdown is incorrectly processing the mention string form as an email address. I still want markdown's automail feature for processing email addressing in the text.

The below code demonstrates the current and new behaviour:

>>> AUTOMAIL_RE = r'<([^<> !]*@[^@<> ]*)>'
>>> print(re.match(AUTOMAIL_RE, "<@localhost>"))
<re.Match object; span=(0, 12), match='<@localhost>'>
>>> print(re.match(AUTOMAIL_RE, "<bob@localhost>"))
<re.Match object; span=(0, 15), match='<bob@localhost>'>

>>> AUTOMAIL_RE = r'<([^<> !]+@[^@<> ]*)>'
>>> print(re.match(AUTOMAIL_RE, "<@localhost>"))
None
>>> print(re.match(AUTOMAIL_RE, "<bob@localhost>"))
<re.Match object; span=(0, 15), match='<bob@localhost>'>

[mitya57]

Make it makes sense to also replace * with + in the domain part? With your fix it still allows for addresses like bob@:

>>> AUTOMAIL_RE = r'<([^<> !]+@[^@<> ]*)>'
>>> re.match(AUTOMAIL_RE, "<bob@>")
<re.Match object; span=(0, 6), match='<bob@>'>

[nzlosh]

I've updated the regex to require both local-part and domain.

[nzlosh]

I've added tests for the missing local-part and domain. I noticed that the <> characters are encoded in html form <> for the missing local-part, but not for the missing domain part. The tests mirror this behaviour, but I don't know if it's expected behaviour or not.

[waylan]

All new tests should use the new testing framework. It appears that no changes have been made to the autolinks since introducing the new framework so we don't have a file for that yet. One should be added at tests/test_syntax/inline/autolinks.py. You only need to include the tests for the behaviors relevant to this PR. However, you may add additional tests if you would like.

Regarding the differing behaviors, I see that as a problem. If any part of the autolink does not match, then the behavior should be the same.

[nzlosh]

Thanks for the pointers, I've updated the PR with the recommended changes.