# File formvalidator.rb, line 561
    def apply_regexp_constraint(key, constraint)
      # FIXME: multiple elements
      m = constraint.match(@form[key].to_s)
      if m
        if untaint?(key)
          @form[key] = m[0]
          @form[key].untaint
        end
      else
        @form.delete(key)
        @invalid_fields[key] ||= []
        unless @invalid_fields.include?(key)
          @invalid_fields[key].push(constraint.inspect)
        end
        nil
      end
    end