From: Peter Vandenabeele <peter@...>
Date: 2012-01-14T03:01:33+09:00
Subject: Re: negative grep

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On Fri, Jan 13, 2012 at 3:18 PM, Robert Klemme
<shortcutter@googlemail.com>wrote:

> On Thu, Jan 12, 2012 at 10:22 PM, Peter Vandenabeele
> <peter@vandenabeele.com> wrote:
>
...

> >
> > Well, a 'v' option, equivalent to the UNIX grep seems quite
> straightforward
> > to conceive?
> >
> > peterv@ASUS:~$ ruby -v
> > ruby 1.9.3p0 (2011-10-30 revision 33570) [i686-linux]
> > peterv@ASUS:~$ echo -e "abc\ndef\nghi" | grep abc
> > abc
> > peterv@ASUS:~$ echo -e "abc\ndef\nghi" | grep -v abc
> > def
> > ghi
> > peterv@ASUS:~$ ruby -e 'puts %w{ abc def ghi}.grep(/abc/)'
> > abc
> > peterv@ASUS:~$ ruby -e 'puts %w{ abc def ghi}.grep(/abc/v)'
> > -e:1: unknown regexp option - v
> >
> > Having "def" and "ghi" as a result here would be nice :-)
>
> Pretty easy to do:
>
> module Enumerable
>  def grep_v(cond)
>    select {|x| not cond === x}
>  end
> end
>
> irb(main):006:0> a=10.times.map &:to_s
> => ["0", "1", "2", "3", "4", "5", "6", "7", "8", "9"]
> irb(main):007:0> a.grep /0/
> => ["0"]
> irb(main):008:0> a.grep_v /0/
> => ["1", "2", "3", "4", "5", "6", "7", "8", "9"]
>
>
Thank you.

It took me quite some time to grasp that the '-i' and '-v' options of
$ grep  -i
$ grep -v
do something very different ...

'-i' changes the behavior of the Regexp _itself_

'-v' changes the behavior of the filter on the lines, _after_ the regexp is
calculated
(changes it from "ruby select" to "ruby reject")

So, I believe now that my earlier proposal of a Ruby regexp like /.../v does
not make a lot of sense in this context.

Negation could be an option for the Enumerable grep command, but that
is probably more complex than just implementing the grep_v you suggested.

Thanks,

Peter

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