underscore SPIP, asterisk SPIP, one two SPIP, three four SPIP, a SPIP, b SPIP

strong SPIP and em SPIP and strong SPIP and em SPIP

line SPIP line SPIP line

this_is_not_an_emphasis SPIP

an empty emphasis __ ** is not an emphasis SPIP

*mixed *double and single SPIP asterisk** spans