Use of a regex stored inside a YAML file
If you want to use a regexp in a yaml file you need to use !ruby/regexp

#config/settings.yml
defaults: &amp;defaults

  email_regex: !ruby/regexp '/^([\w\.%\+\-]+)@([\w\-]+\.)+([\w]{2,})$/i'


Edit:
 The solution proposed by Mike Bethany is very similar to the yaml implementation.

You can take a look to what is used in Ruby 1.9.2 here (search for "!ruby/regexp"):
https://github.com/tenderlove/psych/blob/master/lib/psych/visitors/to_ruby.rb

PS (and OT): I think, like Mike Bethany, that this basic functionality belong to the Regexp class not yaml, and need to be moved to a Regexp method. What do you think?