Thanks for you answer. That makes sense.

Regards,
Tammo

Robert Klemme wrote:

> 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