assign_expression

# File lib/spec/story/step.rb, line 42
        def assign_expression(string_or_regexp)
          if String === string_or_regexp
            expression = string_or_regexp.dup
            expression.gsub! '(', '\('
            expression.gsub! ')', '\)'
          elsif Regexp === string_or_regexp
            expression = string_or_regexp.source
          end
          while expression =~ PARAM_PATTERN
            expression.gsub!($2, "(.*?)")
          end
          @expression = Regexp.new("^#{expression}$")
        end