gistfile1.rb

REGEXP_PAIRS = Hash[*%w[{ } ( ) [ ] < >]]
REGEXP_PAIRS.default_proc = proc { |h, k| k }
   
def on_regexp_beg(content)
  @regexp_stack << content
end
    
def on_regexp_end(content)
  start = @regexp_stack.pop[-1]
  stop  = Regexp.escape(REGEXP_PAIRS[start])
  content =~ /#{stop}([^#{stop}])$/
  extra, options = $`, $1 
end