Convert a lightweight text format to hypertext

convert.py

#!/usr/bin/env python

import sys, re

with open(sys.argv[1], 'rb') as f: 
   text = unicode(f.read(), 'utf-8')

r_paragraph = re.compile(ur'^|(?<=\n\n)(?=\S)')
r_link = re.compile(ur'(?s)(?<!\\)\*([^*]+)\*(\W?)\s+(\S*[./]\S+)')
r_em = re.compile(ur'(?<!\\)\*([^*]+)\*')

def s_link(match): 
   a, b, c = match.groups()
   if (not ':' in c) and (not c.startswith('.') or c.startswith('/')): 
      c = 'http://' + c
   if a.startswith('[') and a.endswith(']'): 
      return '<img src="%s" alt="%s">%s' % (c, a[1:-1], b)
   return '<a href="%s">%s</a>%s' % (c, a, b)

text = text.replace('<', '&lt;')
text = r_link.sub(s_link, text)
text = r_em.sub(r'<strong>\g<1></strong>', text)
text = r_paragraph.sub('<p>', text)
text = text.replace('\\*', '*')

print text