I'm pleased to report that my ANTLR grammar outperforms* the current mediawiki parser on the following pathological text: [[[[image:foo.jpg|thumb|[[[o]]][[foo||]]|[[image:bar.jpg|thumb|[[roo my doo|zoo|]]]]]]]]] It's really amazing what you discover about Wikitext when you sit down to analyse it like this. For example, a square bracket - [ - is: - the start of an external link, if the rest of it is present, and not in a context where external links are forbidden (notably, captions of internal links or other external links), and not inside a nowiki tag - part of the start of an internal link, as long as the rest is present, and it couldn't be interpreted as an internal link, and in an appropriate context - a literal otherwise - that is, in any non-linkable context, not followed by the appropriate tags to make it a link, or inside a nowiki A pipe - | - is: - an option separator for an image, provided that it's not within an embedded object such as internal link or another image, and provided that it's not within a nowiki - a link caption separator, provided that it's not in nowiki tags - any of a dozen other cases that I haven't dealt with yet, like tables, templates, parser functions, categories, ... - literal otherwise. It's fun! I think... Steve * The current parser gives up. ANTLR, after a monumental struggle involving 21 levels of method call and a bit of backtracking, parses it correctly.