Tammo Tjarks wrote:
> 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. 

That's easily explained: the string you hand off to sub is interpreted 
before the call happens.  But the grouping variables are only set after 
that, i.e. when sub executes.

The version with \1 etc. works, because method sub does the 
interpretation of the string and replaces \1 with contents of the first 
group etc:

irb(main):005:0> puts( "\\1" )
\1
=> nil
irb(main):006:0> puts( "#{$1}" )

=> nil

> 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. 

Not at all once you understand how this works.  It is the general 
mechanism of method arguments being evaluated before the method call 
starts.  It's basically the same as:

irb(main):001:0> def foo(x) puts x end
=> nil
irb(main):002:0> foo( 1 + 2 )
3
=> nil

Kind regards

	robert