Thanks. I badly explained what I wanted, but this does what I asked. What I actually needed was a way of finding long quotes which didn't preclude long quotes containing an apostrophe s. This seems to work in the Regex demo, but doesn't in InDesign.
‘.{250,}(?=[.’])(?=[,’])(?=[?’])(?=[!’])
https://regex101.com/r/Um2Ylb/1
There are two long quotes (one with a possessive apostrophe s) and one short quote and it finds both long quotes. But in InDesign it just finds anything preceded by an apostrophe (it ignores the full point, question mark, comma, and exclamation mark). I think this is because it's only looking at a single character in the positive lookbehind. Is there a way of getting it to look for both characters?