Forum: RForum Regexp'n bbcode

I'm scratching my head over the goal of converting bbcode to markdown.
The gist of the problem boils down to handling regexp matches inside
regexp matches.

Example: bbcode:
[quote:4d59322f12="foo"][quote="bar"]baz[/quote]diu[/quote]
which I'd like to convert to:
> foo:
>> bar:
>> baz
> diu

What I've got so far is something like this:

  START = '\[quote'
  IDENT = '(?::\w+)?'
  USER = '(?:="([\w -_]+)")?'
  STOP = "\\[\\/quote#{IDENT}\\]"
  QUOTE = "#{START+IDENT+USER}\\]"
  INSIDEQUOTE = /#{QUOTE}(.+?)(?!#{START})#{STOP}/m

def bbcode2markdown(txt)
  @quotedusers = {}
    txt.scan(/#{START}/).each_with_index { |match,index|
      txt.gsub!(INSIDEQUOTE) { |str| $2.gsub(/^(.)/, '>\1').gsub(/(.)$/,
"\n\\1\n") }
      depth = '>' * (index + 1)
      if $1
        @quotedusers[depth] = $1
      end
    }

    @quotedusers.each { |depth,user|
      txt.sub!(/^#{depth}(\w)/, "#{depth}**#{user}**:  \\1")
    }
end

But I am missing some cases for some reason, and my brain is melting
merely at looking at that code after a couple of months after writing
it.
I'm sure there must be a straight-forward way to get this done, but my
melted brain just can't figure out how.