Issue #7787 has been reported by calfeld (Christopher Alfeld).

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Bug #7787: ruby 1.9 regexp quantifiers behave unpredictably when stacked
https://bugs.ruby-lang.org/issues/7787

Author: calfeld (Christopher Alfeld)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 
ruby -v: ruby 1.9.3p327 (2012-11-10 revision 37606) [x86_64-darwin12]


Ruby (1.8 and 1.9) allows for stacked quantifiers such as /x{2}{5}/ and appears to treat them more or less as expected, i.e., modulo capturing, /x{2}{5}/ is equivalent to /(x{2}){5}/ is equivalent to /x{10}/.

However, in Ruby 1.9, such stacking quantifiers can lead to extreme search time and in ways that are difficult to predict.

    "x"*1000 =~ /x{2}{5}{8}{3}/     # runs instantly
    "x"*1000 =~ /x{2}{5}+{8}{3}/  # runs for an unknown but long time
    "x"*1000 =~ /x{2}{5}+{8}/      # runs instantly
    "x"*1000 =~ /x{2}{5}+{7}{3}/  # runs instantly
    "x"*1000 =~ /x{2}{5}+{7}{3} / # runs for an unknown but long time

All of the above run instantly in Ruby 1.8.  Note that adding parenthesis does not change the runtime.

I realize this might not be considered a bug and will take no offense if it as closed as not-a-bug.  But it is surprising, and I hope is of interest.


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