Previously, one of the syntaxes for italic was (greatly summarized) something
like this:
[^_]_[^_]+_[^_]
That is to say, the regex matched the blanks on both sides of the italic span,
as well as the actual span content. That means that if you had consecutive
italic words:
_some_ _italic_ _words_
...only the odd-numbered words would be highlighted: the space after "_some_"
was counted as part of that span, so it wasn't available as part of "_italic_"
and therefore "_italic_" wouldn't be highlighted. Likewise, if the first word
in a buffer was italic, it wouldn't be highlighted because the first underscore
was not preceded by a non-underscore character!
Now we use lookahead/lookbehind assertions, which don't count as part of the
matched span, so consecutive spans don't interfere with one another.
Fixes#2111.
9e142c6643