Extract Parser#range_lookup out of Expression.parse

lib/liquid/parser.rb

@@ -58,8 +58,10 @@ module Liquid
         string
       when :number
         number
+      when :open_round
+        range_lookup
       else
-        parse_expression(expression_string)
+        raise SyntaxError, "#{token} is not a valid expression"
       end
     end
 
@@ -88,6 +90,15 @@ module Liquid
       VariableLookup.new(name, lookups, command_flags)
     end
 
+    def range_lookup
+      consume(:open_round)
+      first = expression
+      consume(:dotdot)
+      last = expression
+      consume(:close_round)
+      RangeLookup.create(first, last)
+    end
+
     def expression_string
       token = @tokens[@p]
       case token[0]

lib/liquid/range_lookup.rb

@@ -2,13 +2,15 @@
 
 module Liquid
   class RangeLookup
-    def self.create(start_obj, end_obj, start_markup, end_markup)
+    def self.create(start_obj, end_obj, start_markup = nil, end_markup = nil)
       if start_obj.respond_to?(:evaluate) || end_obj.respond_to?(:evaluate)
         new(start_obj, end_obj)
       else
         begin
           start_obj.to_i..end_obj.to_i
         rescue NoMethodError
+          start_markup = start_obj.to_s unless start_markup
+          end_markup = end_obj.to_s unless end_markup
           invalid_expr = start_markup unless start_obj.respond_to?(:to_i)
           invalid_expr ||= end_markup unless end_obj.respond_to?(:to_i)
           if invalid_expr