From: Alex LeDonne <aledonne.listmail@...>
Date: 2007-05-31T00:28:17+09:00
Subject: Re: Substituting Variables

On 5/30/07, Andrew Stewart <boss@airbladesoftware.com> wrote:
> Hello,
>
> I am running into difficulties trying to use a variable in a
> substitution.  Here's an example:
>
> var = 'October'
> '30 May 2007'.sub( /(\d+)\s(\w+)\s(\d+)/, '\1 #{var} \3' )
>
> Here I would like to get '30 October 2007'.  The back-references,
> i.e. \1 and \3, only work within single quotation marks.  But the
> interpolation only works within double quotation marks.  What to do?
>
> Here's an example in IRB:
>
>  >> var = 'October'
>  >> re = /(\d+)\s(\w+)\s(\d+)/
>
>  >> '30 May 2007'.sub(re, '\1 #{var} \3')
> => "30 \#{var} 2007"
>
>  >> '30 May 2007'.sub(re, "\1 #{var} \3")
> => "\001 October \003"
>
>  >> '30 May 2007'.sub(re, "#{$1} #{var} #{$3}")
> => "30 October 2007"
>
> Actually that last one does what I want -- but according to the
> Pickaxe[1] $1 and friends aren't supposed to work and in fact it
> doesn't work in my Ruby class.  So I think IRB is flattering to deceive.
>
> [1] String.gsub: "If a string is used as the replacement, special
> variables from the match (such as $& and $1) cannot be substituted
> into it, as substitution into the string occurs before the pattern
> match starts.  However, the sequences \1, \2, and so on may be used
> to interpolated successive groups in the match."
>
> I have found a workaround but I'm hoping there's a better way:
>
>  >> '30 May 2007'.sub(re, '\1' + " #{var} " + '\3')
> => "30 October 2007"
>
> Any insight would be much appreciated.
>
> Thanks and regards,
> Andy Stewart

Same workaround, but easier to read (to me, anyway):
'30 May 2007'.sub(re, '\1 ' << var << ' \3')
Since you're assembling a string, there's no need to interpolate.

And, if you're not using the middle capture group, do away with it. I
also captured the spaces in the flanking groups.

re2 = /(\d+\s)\w+(\s\d+)/
'30 May 2007'.sub(re2, '\1' << var << '\2')

In 1.9, Oniguruma will give us zero-width lookahead and lookbehind.

-A