From: Peter Hickman <peter@...>
Date: 2007-08-23T20:27:10+09:00
Subject: Re: Renaming Files

The problem here is the ! on the gsub. This is changing the contents of 
f *before* it gets passed to the function:

def show(a, b)
    puts "[#{a}] [#{b}]"
end

x = "Here is a string"

show(x, x.gsub!(/\s/,'_'))

results in

[Here_is_a_string] [Here_is_a_string]

But change the gsub! to a plain gsub and you get

[Here is a string] [Here_is_a_string]

because a plain gsub returns a copy of the modified string without 
modifying the original.

Only use the ! versions when you are absolutely sure you need to.