From: Tammo Tjarks <tammo@...>
Date: 2006-11-04T23:25:16+09:00
Subject: behavior of substitutions with sub

Hello,
I have seen a somehow strange behavior with Strings and the sub-method.
Maybe you cen give me an idea if it is a bug and where it comes from. 
It have seen the problem when I wanted to make an substition with a 
sub(/<regexp>/,"") instead of (/<regexp/,'').

To get the idea a example in irb. I tried it also with version 1.8.5 but
have seen it first with 1.8.4:

====================================================
irb(main):012:0> tst = "Das war ja nix"
=> "Das war ja nix"
irb(main):013:0> tst2 =tst
=> "Das war ja nix"
irb(main):014:0> tst2.sub(/nix/,"was")
=> "Das war ja was"
irb(main):015:0>  tst2.sub(/(n)+ix/,"$1")
=> "Das war ja $1"
irb(main):016:0> tst2.sub(/(n)+ix/,"#{$1}")
=> "Das war ja n"
irb(main):017:0> tst2.sub(/(n)+(i)+(x)+/,"#{$3}#{$2}#{$1}")
=> "Das war ja n"
irb(main):018:0> tst2.sub(/(n)+(i)+(x)+/,'\3\2\1')
=> "Das war ja xin"
irb(main):019:0> tst2.sub(/(n)+(i)+(x)+/,"#{$3}#{$2}#{$1}")
=> "Das war ja xin"

==================================================================

What I do not understand is, that the substitution in the ''-expression with
\1, \2 and \3 work without trouble but when I use the normal ""-String and
the #{$1}, #{$2} and #{$3}-notation he seems alway to refer to the last
regular expression and not the one in sub-method. 
It is only an example. In the script I have seen it I cicrcumvent the
problem by adding an extra regular expression before without using the
result. But I think it is a strange behavior. 

Any comments about that. 

Regards,
Tammo