Converts a subset of markdown into BBcode

markdown_to_bbcode.py

import re

def markdown_to_bbcode(s):
    links = {}
    codes = []
    def gather_link(m):
        links[m.group(1)]=m.group(2); return ""
    def replace_link(m):
        return "[url=%s]%s[/url]" % (links[m.group(2) or m.group(1)], m.group(1))
    def gather_code(m):
        codes.append(m.group(3)); return "[code=%d]" % len(codes)
    def replace_code(m):
        return "%s" % codes[int(m.group(1)) - 1]
    
    def translate(p="%s", g=1):
        def inline(m):
            s = m.group(g)
            s = re.sub(r"(`+)(\s*)(.*?)\2\1", gather_code, s)
            s = re.sub(r"\[(.*?)\]\[(.*?)\]", replace_link, s)
            s = re.sub(r"\[(.*?)\]\((.*?)\)", "[url=\\2]\\1[/url]", s)
            s = re.sub(r"<(https?:\S+)>", "[url=\\1]\\1[/url]", s)
            s = re.sub(r"\B([*_]{2})\b(.+?)\1\B", "[b]\\2[/b]", s)
            s = re.sub(r"\B([*_])\b(.+?)\1\B", "[i]\\2[/i]", s)
            return p % s
        return inline
    
    s = re.sub(r"(?m)^\[(.*?)]:\s*(\S+).*$", gather_link, s)
    s = re.sub(r"(?m)^    (.*)$", "~[code]\\1[/code]", s)
    s = re.sub(r"(?m)^(\S.*)\n=+\s*$", translate("~[size=200][b]%s[/b][/size]"), s)
    s = re.sub(r"(?m)^(\S.*)\n-+\s*$", translate("~[size=100][b]%s[/b][/size]"), s)
    s = re.sub(r"(?m)^#\s+(.*?)\s*#*$", translate("~[size=200][b]%s[/b][/size]"), s)
    s = re.sub(r"(?m)^##\s+(.*?)\s*#*$", translate("~[size=100][b]%s[/b][/size]"), s)
    s = re.sub(r"(?m)^###\s+(.*?)\s*#*$", translate("~[b]%s[/b]"), s)
    s = re.sub(r"(?m)^> (.*)$", translate("~[quote]%s[/quote]"), s)
    s = re.sub(r"(?m)^[-+*]\s+(.*)$", translate("~[list]\n[*]%s\n[/list]"), s)
    s = re.sub(r"(?m)^\d+\.\s+(.*)$", translate("~[list=1]\n[*]%s\n[/list]"), s)
    s = re.sub(r"(?m)^((?!~).*)$", translate(), s)
    s = re.sub(r"(?m)^~\[", "[", s)
    s = re.sub(r"\[/code]\n\[code(=.*?)?]", "\n", s)
    s = re.sub(r"\[/quote]\n\[quote]", "\n", s)
    s = re.sub(r"\[/list]\n\[list(=1)?]\n", "", s)
    s = re.sub(r"(?m)\[code=(\d+)]", replace_code, s)
    
    return s