These seem mostly harmless since Perl will just truncate the
match and start a new one on a newline boundary in our case.
The only downside is we'd end up with redundant <span> tags in
HTML.
Limiting the number of line matched ourselves with `{1,$NUM}'
doesn't seem prudent since lines vary in length, so we continue
to defer the job of limiting matches to the Perl regexp engine.
I've noticed this warning in practice on 100K+ line patches to
locale data.