BBEdit/TextWrangler is a text editor for the Mac platform that has extensibility for custom languages. A few years ago Chris Campbell posted to a DevCentral thread that he had created a plugin for BBEdit (paid version) and TextWrangler (free version.) Both support syntax highlighting, but the paid version also supports command completion. This is for offline editing, there is no direct capability to connect to a BIG-IP for add/modify/delete or syntax validation, though someone with mad applescript skills might be able to integrate some iControl scripts to make it happen. Anyway, here’s a quick screen shot of the command completion in BBEdit:
And here is a screenshot of the syntax highlighting for a complete iRule:
Chris wrote a blog a few years back with the file contents and system location. I’ve updated the file and it’s available here in the codeshare.