From: Brian Candler <B.Candler@...>
Date: 2003-05-23T17:05:24+09:00
Subject: Re: why is "does" missing from this sub!-stitution?

On Fri, May 23, 2003 at 08:44:16AM +0900, Dave Oshel wrote:
> Maybe so, but this works as expected:
> 
> `/sw/bin/fortune -m Nillity`.each { |x|
>    x.sub!( /(_+\010+)([a-zA-Z]+\b)/, "<u>" + '\2' + "</u>" )
>    print x
> }
> 
> Maybe it's a bug ... or a feature ... or a bug ... or a bug AND a 
> feature ...  ;-)

It's definitely a feature and not a bug. $2 behaves like a regular variable,
and as you've seen, you have to use normal string interpolation #{...} to
insert it. If $2=="foo" and you write the string "<u>#{$2}</u>" then it is
interpolated as "<u>foo</u>", and *then* the resulting string is what you
are passing as a parameter to 'sub'. If $2 is nil, then you pass "<u></u>".
So sub has no idea where to substitute any value.

However the string '\2' is treated as a special case by sub itself. When it
sees that sequence, instead of inserting \ and 2 into the result, it inserts
the value of $2.

It would be very strange if there were strings with some sort of 'deferred
interpolation', i.e. the string contained the sequence '#{expr}' but expr is
re-evaluated every time you read the string. Ruby is flexible enough that
you could implement that if you wanted though!

Regards,

Brian.