From: james.d.masters@...
Date: 2007-06-07T14:30:18+09:00
Subject: Re: Error when substituting a backslash in a string

On Jun 6, 4:27 pm, "Federico Zagarzaz�" <fzagarz...@gmail.com> wrote:
> 1) puts 'fede'.sub('e', '\4')     # => fde
> 2) puts 'fede'.sub('e', '\\4')    # => fde
> 3) puts 'fede'.sub('e', '\\\4')   # => f\4de
> 4) puts 'fede'.sub('e', '\\\\4')  # => f\4de
> 5) puts 'fede'.sub('e', '\\\\\4') # => f\de
> 6) puts 'fede'.sub('e', '\\\\\\4') # => f\de
> 7) puts 'fede'.sub('e', '\\\\\\\4') # => f\\4de

You have to be careful with using backslashes with a number for a
replacement string.  In a replacement string, \# (where # is a number)
indicates that you want to insert something that was matched in the
original string - the matched things are in parentheses and occur in
regular expressions.  For example:

irb(main):001:0> a = "Page 3 of 86"
=> "Page 3 of 86"
irb(main):002:0> a.sub(/^Page (\d+) of (\d+)$/, 'You are on page \1
out of a total of \2 pages')
=> "You are on page 3 out of a total of 86 pages"

The PickAxe has an excellent subject on this and goes over your
question with all sorts of backslash permutations.  I guarantee that
it will clarify things further for you:

http://www.rubycentral.com/book/tut_stdtypes.html
(see "Backslash Sequences in the Substitution" section)