From: Ken Bloom <kbloom@...>
Date: 2009-03-19T00:37:30+09:00
Subject: Re: Interpolating $1, etc., from within a gsub block

On Tue, 17 Mar 2009 13:59:24 -0500, Lloyd Zusman wrote:

> I'm trying to use the block form of gsub in order to do substitution
> involving an interpolation stored within a variable, but I can't figure
> out how to get it to work.  Here's a simplified case of what I'm trying
> to do:
> 
> $fromre = Regexp.compile(ARGV[0])
> $repl   = ARGV[1]
> $replcount = 0
> 
> string = "... something arbitrary ..."
> 
> string.gsub!($fromre) {
>   |x|
>   $replcount += 1
>   $repl
> }
> 
> As you can see, the regexp comes in on the command line, as well as the
> replacement. I want to do this substution using the block form so I can
> count the number of replacements that were made.
> 
> This works as long as $repl doesn't contain any references to matched
> patterns. However, if this program is called 'myprog' and I do the
> following, the replacement of $1 with the indicated subexpression
> doesn't occur:
> 
> myprog 'abc(\S+)def' 'NEW-$1-MATCH'
> 
> In other words, if the string being substituted is this, "abcFOOBARdef",
> the result is not "NEW-FOOBAR-MATCH" as I would like it to be, but
> rather, "NEW-$1-MATCH".
> 
> It's clear why this doesn't work, but I can't figure out what construct
> to use within the gsub block or on the command line to make sure that it
> _does_ work.
> 
> I also tried this, and not surprisingly, it didn't work, either:
> 
> myprog 'abc(\S+)def' 'NEW-#{$1}-MATCH'
> 
> Can anyone suggest how I can accomplish this?
> 
> Thanks in advance.

Variable interpolation only works for string literals inside ruby code. 
If you bring in the string from anywhere else, you'll can use eval

eval '"'+string+'"' and hope there are no injection attacks in the string.

Alternatively, you can roll your own substitution:

string.gsub!($fromre) do |match|
  matchdata=Regexp.last_match
  $repl.gsub(/\$\d+/) do |subst|
     matchdata[subst[1..-1].to_i]
  end
end

--Ken

-- 
Chanoch (Ken) Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/