Re: [Python-markdown-discuss] infinite loop in python-markdown

[Yuri T. <qar...@gm...>]

Aaron and Anand found a serious bug with one of the regular
expressions that causes python markdown to hang on certain.  This
email is to announce a tentative solution and to check if anyone has
better ideas.

The attached script.py is the test case that Aaron and Anand sent in;
script2.py is my variation on it just includes the relevant parts of
markdown.py.  The regular expression in question is supposed to catch
patterns like [title][link_id].  The catch is that it supposed to
allow for balanced parentheses in the title part, i.e., one should be
able to write [one [[[[[two]]]]] three][link_id]

To the best of my knowledge, python's regular expressions do not offer
support balanced parentheses, so markdown.py handles this somewhat
with a rather ugly regular expression (see script2.py) which catches
matching brackets up two 6 levels deep.  The resulting expressing is
quite big, so it is perhaps not too surprising that this causes
problems on long paragraph.   I found a fix for this particular
problem:

replace

        + (NOBRACKET + r'(\['+NOBRACKET)*6
        + (NOBRACKET+ r'\])*'+NOBRACKET)*6

with:

        + (NOBRACKET + r'(\[')*6
        + (NOBRACKET+ r'\])*')*6

in the defintion of "BRK".  Which as far as my test suite shows does
not causes any problems (and really shouldn't, since the second
NOBRACKET was redundant in both cases).

However, I thought I would use this as an opportunity to check if
anyone has any suggestions for a better way to handle this.

Does this warrant a new release?

  - yuri

-- 
Yuri Takhteyev
Ph.D. Candidate, UC Berkeley School of Information
http://takhteyev.org/, http://www.freewisdom.org/