Too many blank lines after a foot note freezes Kramdown binary

[aliok]

When I process following content with Kramdown, it freezes.

# Something
something[^note1].

# Footnotes
[^note1]:
    A note













































# Test

Here is my command:

kramdown the-file.markdown

Interestingly, it doesn't freeze when I reduce the number of blank lines.
Following input works just fine, although it takes more time than normal.

# Something
something[^note1].

# Footnotes
[^note1]:
    A note
























# Test

My environment:

ruby --version
> ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-darwin14]
kramdown --version
> 1.7.0
gem list

*** LOCAL GEMS ***
bigdecimal (1.2.6)
blankslate (2.1.2.4)
bundler (1.9.9)
celluloid (0.16.0)
classifier-reborn (2.0.3)
coffee-script (2.4.1)
coffee-script-source (1.9.1.1)
colorator (0.1)
execjs (2.5.2)
fast-stemmer (1.0.2)
ffi (1.9.8)
hitimes (1.2.2)
io-console (0.4.3)
jekyll (2.5.3)
jekyll-coffeescript (1.0.1)
jekyll-gist (1.2.1)
jekyll-paginate (1.1.0)
jekyll-sass-converter (1.3.0)
jekyll-watch (1.2.1)
json (1.8.1)
kramdown (1.7.0)
liquid (2.6.2)
listen (2.10.0)
mercenary (0.3.5)
minitest (5.4.3)
parslet (1.5.0)
posix-spawn (0.3.11)
power_assert (0.2.2)
psych (2.0.8)
pygments.rb (0.6.3)
rake (10.4.2)
rb-fsevent (0.9.5)
rb-inotify (0.9.5)
rdoc (4.2.0)
redcarpet (3.2.3)
safe_yaml (1.0.4)
sass (3.4.13)
test-unit (3.0.8)
timers (4.0.1)
toml (0.1.2)
yajl-ruby (1.2.1)

[gettalong]

Thanks for the bug report! I will look into it!

[gettalong]

The problem comes from the regular expressions used in kramdown. Since Ruby uses a backtracking Regexp engine some regular expressions can take more time than expected.

I have changed a regular expression to use an atomic group to avoid needless backtracking and this has fixed the bug and the huge amount of time it took to process your test case. The fix also passes all of kramdown's test cases 😄.

In addition normal code block matching should also be faster in pathological cases.