PHP regex do not match group

I am creating a wiki engin开发者_如何学编程e. The syntax for images is [Image:filename.jpg caption="This is my [[link]]."] [[link]] is the syntax for a link.

Whenever it parses the file, it parses as [Image:filename.jpg caption="This is my [[link]. The rest is not parsed. I tried the look-behind deal, but it still did not parse correctly. I there a regex where it can only match when there is one lone closing brace, ]?