Tried this a few times back and forth, but find it hard to make the Regex find only sentences without full stops. One commonly recommended method is using (?!) to match negative lookbehind, since regex doesn’t use “not” (there is only “or”, where you use the pipe |). But using negative lookbehind on full stops only yields strange results in InDesign, or maybe I just don’t master it. So, what’s left? Well, I would use brackets.
search for:
^\u[\u\d\ -,]+$
replace with:
$0.
works for me. It will match
^:from the beginning of the line
\u:one upper case character (as this was meant to match sentences, not just single words, like short captions, I prefer having that \u up front)
[\u\d\ -,]:any upper case, lower case, digit, space, comma or dash
+: repeatedly until
$: the end of the line (but not including the paragraph break, which is a good point, and it means we can ditch the parenthesises, thanks Peter Kahrel)
If you need to match other characters, like exclamation mark, add them (with a backslash before if necessary) inside the bracket.