This post explains the problem well. Hash tags like "end" and "class" in the new 1.9 Hash literal format are hightlighted incorrectly. The post contains a fix that works perfectly, namely to edits the Ruby language file by moving the block of code at lines 201-208 to line 1570.

The API command view.replace always adds to the undo stack. However, it is useful for certain plugins to do a replace without actually adding to the undo stack. This will primarily be useful for collaborative editing plugins, where usually the other person's changes need to be replicated in your view, but each person keeps a separate undo stack (like in Google Docs).