From: dblack@...
Date: 2003-05-14T03:56:23+09:00
Subject: Re: Regexp: why does (re)* return only last repetition?

Hi --

On Wed, 14 May 2003, Hal E. Fulton wrote:

> ----- Original Message -----
> From: <dblack@superlink.net>
> To: "ruby-talk ML" <ruby-talk@ruby-lang.org>
> Sent: Tuesday, May 13, 2003 1:21 PM
> Subject: Re: Regexp: why does (re)* return only last repetition?
>
>
> > I'm not sure what you mean by "interpolate again" (or reinterpolate).
> > I think everything is happening just once: you've created a string
> > ([a-z\]), and you're interpolating it into a regex.
> >
> > > Frankly, /#{a}/ for the above string should be no different than
> > > /[a-z\\]/.
> >
> > Except... a is a 6-char string ([a-z\]), and the contents of the regex
> > there is 7 characters :-)  There's no way for Ruby to backtrack and
> > know that, when you created the string, you typed \ twice.  You might
> > have produced the string this way:
> >
> >   a = [91, 97, 45, 122, 92, 93].map {|c| c.chr}.join   # [a-z\]
>
> I think what he wants is reasonable in a way...
> it would be nice if there were a way to specify
> a string (or a regex) that did not expand
> backslashes. If there were, that would solve his
> minor dilemma.
>
> "Interpolate" is not the right word here... but
> there are definitely two levels of processing
> going on. For example: The sequence of characters
> "\\\\n" gets mapped internally to "\\n" and if that
> in turn were used in a regex, it would be collpsed
> again into \n. Correct?

I don't think so:

  irb(main):016:0> a = "\\\\n"
  => "\\\\n"
  irb(main):017:0> puts a
  \\n
  => nil
  irb(main):018:0> /#{a}/
  => /\\n/
  irb(main):019:0> /#{a}/.match("\n")
  => nil

Once you create the string (in this case the 3-char string \\n), it
just gets dropped into the #{} expression as is.  (If you eval'd it
you could force an extra level of processing.)

> What about a naive solution like this?
>
>   class String
>     def raw
>       self.inspect[1..-2]
>     end
>   end
>
> And then /#{myvar.raw}/ or some such. Am I way
> off base here? This is untested. :)

Tricky :-) You might run aground on interpolating things other than
strings; you'd have to define Kernel#raw (as to_s.raw or maybe
to_str.raw).  And even then, well, I'm not sure... :-)


David

-- 
David Alan Black
home: dblack@superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav