From: Thomas Hurst <tom.hurst@...>
Date: 2002-03-21T02:06:33+09:00
Subject: Re: Regex backreference in method "sub"

* Kirk Lowery (klowery@wts.edu) wrote:

> line.sub!(/(str)ing/,"#{$1}")

> Apparently, $1 never gets set to the matched group until *after*
> exiting the sub method. Why not? How do I do this kind of substution?

Look at the order of evaluation:

line.sub!(/(str)ing/,"#{$1}")

-> /(str)ing/ becomes a Regexp object instance
-> "#{$1}" becomes a String object instance
-> both are passed to line#sub!()

So it's equivilent to:

a = Regexp.new('(str)ing')
b = String.new("#{$1}")
line.sub!(a, b)

A different ordering is meaningless (although you can swap a and b).

The correct way to talk about backreferences is the traditional \
operator:

line.sub!(/(str)ing/,'\1')

Since this is evaluated by the regexp engine rather than the string
object.

Remember you need to escape it in double quotes, i.e. "\\1".

-- 
Thomas 'Freaky' Hurst  -  freaky@aagh.net  -  http://www.aagh.net/
-
The fact that boys are allowed to exist at all is evidence of a
remarkable Christian forbearance among men.
		-- Ambrose Bierce