Reply To: how to grep ID5

But there is such a website! One page, for example, is this: FindBetween: A Useful GREP String

For your purposes, you need to adjust the example to the following:

(?<=[).*?(?=])

and leave the Change To field blank, applying Italics in the Change To format box. This does not address your specification

The text is within a paragraph and never a standalone paragraph. The text can be from 3 words on up in length.

— but is that really important? Because that's still possible using GREP, but the expression will go from “Anne-Marie can explain it” to “Jongware'll need to scratch his noodle to make it do exactly that, no more and no less”.

(Even better: do not apply 'italic' as a font override, as it will give the paragraph the nasty '+' “there is something going on here” mark, but create a Character Style for your italics and apply that instead.)