Looks like revision 47154 introduced a regexp that
hangs Python (Ctrl-C won't kill the process, CPU usage
sits near 100%) under some circumstances.  A test case
is attached (sgmllib.html and hang_sgmllib.py).

The problem isn't seen if you read the whole file (or
nearly the whole file) at once.  But that doesn't make
it a non-bug, AFAICS.

I'm not sure what the problem is, but presumably the
relevant part of the patch is this:

+starttag = re.compile(r'<[a-zA-Z][-_.:a-zA-Z0-9]*\s*('
+        r'\s*([a-zA-Z_][-:.a-zA-Z_0-9]*)(\s*=\s*'
+       
r'(\'[^\']*\'|"[^"]*"|[-a-zA-Z0-9./,:;+*%?!&$\(\)_#=~@]'
+       
r'[][\-a-zA-Z0-9./,:;+*%?!&$\(\)_#=~\'"@]*(?=[\s>/<])))?'
+    r')*\s*/?\s*(?=[<>])')


The patch attached to bug 1515142 (also from Sam Ruby
-- claims to fix a regression introduced by his recent
sgmllib patches, and has not yet been applied) does NOT
fix the problem.

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Slimmed down test case is attached.(The regex pattern in
question is used)

FYI, r47154 is backported to 2.4 branch(r47155).

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I've been testing quiver's test case:

- With Eclipse's QuickREx plugin: it hangs. It was
configured in PCRE mode (which uses Jakarta-ORO Perl 5
regular expressions implementation), and no additional options.

- With grep: grep exits with a fatal error and dumps a stack
trace. grep was run also in Perl mode, with the command:
grep -P -f regexp.txt test.txt

I can't find an explanation for this, but I don't know much
about regexps. I hope it has some utility for the resolution
of this bug nevertheless.

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Again FYI, here's the diff where presumably the bug was
introduced:
Lib/sgmllib.py?rev=47080&r1=46996&r2=47080">http://svn.python.org/view/python/trunk/Lib/sgmllib.py?rev=47080&r1=46996&r2=47080

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Sorry, correct URL is
Lib/sgmllib.py?rev=47154&r1=47080&r2=47154">http://svn.python.org/view/python/trunk/Lib/sgmllib.py?rev=47154&r1=47080&r2=47154

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I reverted the patch and added the test case for sgml so the
infinite loop doesn't recur.

Committed revision 51854. (head)
Committed revision 51850. (2.5)
Committed revision 51853. (2.4)

I will add the hang_re test cause to test_crashers or somewhere.