Basically Ruby 1.8 fails to backtrack correctly a number of regular
expressions. The simplest one is probably:

irb(main):036:0>  /a(ba|.)*?a/.match('axba')
=> nil

There is more to it, and since I had to write it down for my students,
I'm just going to link to the gory details:
http://www.cs.umd.edu/~gaburici/rubyre.html

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