From: Dondi <Donovan.Dillon@...>
Date: 2008-02-06T04:54:55+09:00
Subject: Re: Extracting vowels and consonants using regular expression

On Feb 3, 11:42 pm, Dondi <Donovan.Dil...@gmail.com> wrote:
> On Feb 2, 4:36 pm, Donovan Dillon <donovan.dil...@verizon.net> wrote:
>
>
>
> > Thanks Robert, Corey, Philip, Stephano, et. al, for all of the great
> > suggestions. However, they all seem to ignore the conditional nature of 'y'
> > as a vowel. I would like the regex to treat 'y' as a vowel when there is no
> > other vowel either before or after it. The string I used initially, was
> > drawn from the following perl code that accomplishes this (found athttp://www.perlmonks.org/?node_id=592867):
>
> > my @vowels = ( /[aeiou]|(?<![aeiou])y(?![aeiou])/gi );
>
> > The "(?<!...)" is a "zero-width negative-look-behind assertion".
>
> > The "(?!...)" is a "zero-width negative-look-ahead assertion".
>
> > Together, they match the condition of treating "y as a vowel only if there
> > is no other vowel before or after it."
>
> > This was my attempt at converting the Perl fragment to Ruby syntax:
>
> > scan(/[aeiou]|(?![aeiou])y(?![aeiou])/i)
>
> > I have since discovered that Ruby 1.8 lacks regex look-behind assertion --
> > which explains why the code was failing. As a result, I have fallen back to
> > the following which currently ignores the 'y':
>
> > class String
> >   def vowels
> >     scan(/[aeiou]/i)
> >   end
> >   def consonants
> >     scan(/[^aeiou]/i)
> >   end
> > end
>
> > Any ideas how to modify this to include the conditional treatment of "y as a
> > vowel only if there is no other vowel before or after it?"
>
> > (i.e., is there a way to simulate the perl "zero-width negative-look-behind"
> > and "zero-width negative-look-ahead" assertions for 'y' in Ruby 1.8?)
>
> > On 2/2/08 4:17 PM, in article
> > 335e48a90802021417n4ba96a11i746bd5118f184...@mail.gmail.com, "Robert Dober"
>
> > <robert.do...@gmail.com> wrote:
> > > Oh sorry using 1.9 excluseviley now...
> > > Robert
>
> Origumura is the default regex library for Ruby 1.9. It includes look-
> behind assertions (wohoo!) ... and ... It turns out that there is a
> gem available so you don't have to upgrade to Ruby 1.9 or monkey
> around with creating a custom Ruby 1.8.x build.
>
> The gem relies upon a c library that can be downloaded from here:http://www.geocities.jp/kosako3/oniguruma/
>
> After installing the library, origuruma installation is a breeze
> using:
>
> sudo gem install -r origuruma
>
> However, my progress has come to a screeching halt as I am now
> receiving the following error:
>
> ** Starting Mongrel listening at 0.0.0.0:3000
> ** Starting Rails with development environment...
> Exiting
> /usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in
> `gem_original_require': ./lib/string_extensions.rb:4: undefined
> (?...)
> sequence: /[aeiou]|(?<![aeiou])y(?![aeiou])/ (SyntaxError)
> ./lib/string_extensions.rb:8: undefined (?...) sequence: /![aeiou]|(?
> <=[aeiou])y(?=[aeiou])/      from /usr/local/lib/ruby/site_ruby/1.8/
> rubygems/custom_require.rb:27:in `require'
>
> It seems to be complaining about the look-behind and look-ahead
> assertions in the following code fragment (which origuruma is supposed
> to support):
>
> class String
>   def vowels
>     scan(/[aeiou]|(?<![aeiou])y(?![aeiou])/i)
>   end
>   def consonants
>     scan(/![aeiou]|(?<=[aeiou])y(?=[aeiou])/i)
>   end
> end
>
> According to this reference (http://www.geocities.jp/kosako3/
> oniguruma/
> doc/RE.txt), the look behind and look ahead syntax that I am using
> appears to be correct
> (ref section 7. Extended groups).
>
> This suggests either:
>
> A. Ruby may be using the default regexp library instead of the
> oniguruma regexp library,
>
> B. The oniguruma regexp library is not accessible via the 'scan'
> method, or
>
> C. Something else entirely
>
> ... hmmm ... <scratches head/>
>
> Any suggestions?


Thanks for all the help everyone. The problem was solved with the help
from pullmonkey on Rails Forum! Here is the solution:

Objective:

1. Extract vowels and consonants from a string
2. Handle the conditional treatment of 'y' as a vowel under the
following circumstances:
     - y is a vowel if it is surrounded by consonants
     - y is a consonant if it is adjacent to a vowel

Here is the code that works:

  def vowels(name_str)
    reg = Oniguruma::ORegexp.new('[aeiou]|(?<![aeiou])y(?![aeiou])')
    reg.match_all(name_str).to_s.scan(/./)
  end

  def consonants(name_str)
    reg = Oniguruma::ORegexp.new('[bcdfghjklmnpqrstvwx]|(?<=[aeiou])y|
y(?=[aeiou])')
    reg.match_all(name_str).to_s.scan(/./)
  end

(Note, the .scan(/./) can be eliminated to return an array)

The major problem was getting the code to accurately treat "y" as a
consonant. The key to solving this problem was to:

1. define unconditional consonants explicitly (i.e.,
[bcdfghjklmnpqrstvwx]) -- not as [^aeiou] which automatically includes
"y" thus OVER-RIDING any conditional reatment of "y" that follows

2. define conditional "y" regexp assertions independently, i.e., "| (?
<=[aeiou]) y | y (?=[aeiou])" -- not "|(?<=[aeiou]) y (?=[aeiou])"
which only matches "y" preceded AND followed by a vowel, not preceded
OR followed by a vowel

HTH.