I've never seen either of them in real-world code. Concatenation is something but I think it's a lot less flexible - if you have a parser for something and want to make a parser for a comma-separated list of that thing, with parser combinators that's one call, whereas with a regex I don't think the string-manipulation is straightforward, and would the named capture groups still work if they were now being hit multiple times?
You're definitely right that concatenation is less composable, and I agree that parser combinators are more powerful. I was just pointing out there are somethings you can do in regexes to make things more readable.
And about captures inside repetition: I don't see any reason they couldn't capture a list of strings instead of a string in dynamically typed languages but in all regex libraries I'm aware of they do something that seems useless to me: they only capture the last occurrence! (They probably just overwrite the capture on each repetition.)
I think many regex libraries have named captures and in most languages you can either concatenate regexes or build regexes from strings which in turn can be built from concatenation. Furthermore, I thought building regexes by concatenation of smaller pieces was a commonly recommended technique for improving readability (I have no data on whether the recommendation is commonly followed or not, of course).