Issue #5588 has been updated by Suraj Kurapati.

File 5588_negative_lookahead.patch added

Alright, I have decided that partly negated regexps should behave like Tanaka's 2007 solution.  They are easier to reason about (consistently) in that form:

/"(?v:ruby)+"/ =~ %q("ruby") # yields nil
/"(?v:ruby)+"/ =~ %q("rubyperl") # yields nil
/"(?v:ruby)+"/ =~ %q("perlruby") # yields nil
/"(?v:ruby)+"/ =~ %q(abc"perlru-by"xyz) # yields 3 and $& is %q("perlru-by")

Please review my simplified patch (attached) that no longer uses OP_NEGATE.

Thanks for your consideration.
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Feature #5588: add negation flag (v) to Regexp
http://redmine.ruby-lang.org/issues/5588

Author: Suraj Kurapati
Status: Assigned
Priority: Normal
Assignee: Yui NARUSE
Category: core
Target version: 2.0.0


Please add a negation flag (v) to regexps which inverts them:

  "ruby" =~ /perl/v       #=> true   (turn on negation)
  "ruby" !~ /perl/v       #=> false  (turn on negation)
  "ruby" =~ /(?v:perl)/   #=> true   (turn on negation)
  "ruby" !~ /(?v:perl)/   #=> false  (turn on negation)
  "ruby" =~ /(?-v:perl)/  #=> false  (turn off negation)
  "ruby" !~ /(?-v:perl)/  #=> true   (turn off negation)

The flag name "v" comes from the ex(1) command of the same name.

This has beneficial applications where it is sometimes difficult
to construct what you want to match but much easier to construct
what you *do not* want to match.  Having this negation built in
the regexp itself would remove the need for us to change our 
Ruby code to process a regexp in a different way.  

For example, suppose that you are passing a regexp to the `--name`
command-line option of MiniTest.  This regexp tells MiniTest to
only run those tests whose names match.  If Ruby had a negation
flag on its regexps, then it would be suddenly trivial to make
MiniTest skip certain tests by negating the regexp we pass in.

In this manner, we get a beneficial feature without ever changing
our Ruby code (the codebase of MiniTest in this example).  :-)

Thanks for your consideration.


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