In Codemirror, they have a method called markText() described here: http://codemirror.net/doc/manual.html#api_marker
One of the options for that method allows you to collapse the marked text and replace it with something else. So, for example I can find all instances of "foo" and replace them with say, an underscore, but the internal value for the text remains "foo". And when a user deletes the underscore, the entire token "foo" is deleted, under the hood.
I'd like to switch to Ace for a simple IDE-like interface I'm building but I pretty much need this feature. (Unfortunately Codemirror has other issues which are making me look elsewhere)
Is there any way to achieve this in Ace?
There is no builtin way of doing this yet. But there are two ways to implement it. If you need to replace only a few instances you could use folds (see session.addFold), but if you need to have many replacements and replacements are going to be inside a token, you can change values of highlighting tokens in session.bgTokenizer.lines. In any case you should create feature request for this at https://github.com/ajaxorg/ace/issues/new