From: Felix Windt <fwmailinglists@...>
Date: 2007-10-08T21:54:20+09:00
Subject: Re: regexp question - look for parentheses then remove them

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> -----Original Message-----
> From: Jes=FAs Gabriel y Gal=E1n [mailto:jgabrielygalan@gmail.com]=20
> Sent: Monday, October 08, 2007 5:25 AM
> To: ruby-talk ML
> Subject: Re: regexp question - look for parentheses then remove them
>=20
> On 10/8/07, Max Williams <toastkid.williams@gmail.com> wrote:
> > I'm struggling with a regular expression problem, can anyone help?
> >
> > I want to take a string, look for anything in parentheses,=20
> and if i find
> > anything, put it into an array, minus the parentheses.
> >
> > currently i'm doing this:
> >
> > parentheses =3D /\(.*\)/
> > array =3D string.scan(parentheses)
> >
> > This gives me eg
> >
> > "3 * (1 + 2)" =3D> ["(1 + 2)"]
> >
> >  - but is there an easy way to strip the parentheses off=20
> before putting
> > it into the array?
> >
> > eg
> > "3 * (1 + 2)" =3D> ["1 + 2"]
> >
> > In addition, if i have nested parentheses inside the outer=20
> parentheses,
> > i want to keep them, eg
> >
> > "3 * (1 + (4 / 2))" =3D> ["1 + (4 / 2)"]
> >
> > can anyone show me how to do this?
>=20
> x =3D "3 * (1 + 2)".match(/\((.*)\)/)
> x.captures
> =3D> ["1 + 2"]
> x =3D "3 * (2 + (1 + 3))".match(/\((.*)\)/)
> x.captures
> =3D> ["2 + (1 + 3)"]
>=20
> Hope this helps,
>=20
> Jesus.
>=20

That can fail if you have more than one bracket pair on the lowest =
level:

irb(main):002:0> "3 * (2 + (1 + 3)) + (1 * 4)".match(/\((.*)\)/).to_a
=3D> ["(2 + (1 + 3)) + (1 * 4)", "2 + (1 + 3)) + (1 * 4"]

I'm not sure you can match more complex examples using a regular =
expression
- you may be able to pull something off with lookaheads, but I think =
it'd be
easier to just parse the string manually and count opened brackets:

$ cat para.rb
def para(str)
  open =3D 0
  matches =3D []
  current =3D ""
  str.split(/\s*/).each do |char|
    if char =3D=3D ")"
      open -=3D 1
      if open =3D=3D 0
        matches << current
        current =3D ""
      else
        current << char
      end
    elsif char =3D=3D "("
      open +=3D 1
      if open > 1
        current << char
      end
    elsif open > 0
      current << char
    end
  end
  matches
end


$ irb
irb(main):001:0> require 'para'
=3D> true
irb(main):002:0> para("1+2")
=3D> []
irb(main):003:0> para("(1+2)")
=3D> ["1+2"]
irb(main):004:0> para("(1+2)*3")
=3D> ["1+2"]
irb(main):005:0> para("((1+2)*3)")
=3D> ["(1+2)*3"]
irb(main):006:0> para("((1+2)*3)+(5*6)")
=3D> ["(1+2)*3", "5*6"]
irb(main):007:0> para("((1+2)*3)+(5*6*(1-3*(1-4)))")
=3D> ["(1+2)*3", "5*6*(1-3*(1-4))"]
irb(main):008:0>

There are probably far more elegant ways.

HTH,

Felix

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