From: Brian Candler <B.Candler@...>
Date: 2007-08-07T18:15:26+09:00
Subject: Re: Determining the common prefix for several strings

>   until prefix.empty? or list.all? { |str| str =~ /^#{prefix}/ }
>     prefix.chop!
>   end

The trouble with that is that it won't work properly if #{prefix} contains
regexp special characters, and it may even bomb out:

irb(main):001:0> prefix = "abc["
=> "abc["
irb(main):002:0> "abc[def" =~ /^#{prefix}/
RegexpError: invalid regular expression; '[' can't be the last character ie. can't start range at the end of pattern: /^abc[/
        from (irb):2
irb(main):003:0>

You can use Regexp::escape to fix that, but then you're still building a
regexp every time round the loop. That's why I think String#index is better
here.