def keep_file_regex
      %r!\A#{Regexp.quote(site.dest)}\/(#{Regexp.union(site.keep_files).source})!
    end
def keep_file_regex
      %r!\A#{Regexp.quote(site.dest)}\/(#{Regexp.union(site.keep_files).source})!
    end
Private: Creates a regular expression from the config's keep_files array

 Examples
   ['.git','.svn'] with site.dest "/myblog/_site" creates
   the following regex: /\A\/myblog\/_site\/(\.git|\/.svn)/

 Returns the regular expression