Poor terminology on my part. The user account was deleted, and his posts in phpBB2 continued to be listed with his name and the “Guest” status. In phpBB3 his posts still have his name next to the post, but with no “Guest” status. I just had a look at the phpbb_posts table (in the phpBB3 version of the database) and found that the post_username field contains his old username. For posts that are linked to users (who exist), this field is blank. The ID of the post is 1, which is the anonymous user.
Could you escape number+dot combinations that appear at start of a line? E.g.
1\.
This would be safer than converting the number to an ordered list item. BBCode ordered lists could still be converted to Markdown ordered lists.