On the number of spaces in horizontal rules: The spec is fuzzy: "If you wish, you may use spaces between the hyphens or asterisks." Markdown.pl 1.0.1's hr regexes limit the number of spaces between the hr chars to one or two. We'll reproduce that limit here.

no spaces

one space

one space with some leading space

two spaces

three spaces (these shouldn't be hr)

- - -

* * *

_ _ _

one or two spaces

longer