All recursion is iteration. Sometimes you have a stack to help you, and other times you get tail recursion optimization, but it's always a loop.

What makes you think so?

Please have a look at the formal definition of regular expressions at https://en.wikipedia.org/wiki/Regular_expression#Formal_defi... on Wikipedia, and let me know where the stack and the iterations are. I can't find them.

https://en.wikipedia.org/wiki/Recursive_definition#Well_form... is also a good example.

Regular expressions are a particularly interesting example because you brought up 'loops', so I'm assuming you are interested in how you can implement some of these recursive definitions on a computer.

So for regular expressions a common technique is to compile them to a finite state machine. You can model that in your computer as one block of eg assembly instruction per machine state. To move to a different state, you just 'jmp' to the corresponding code's address.

That's pretty fun to work out, but I still don't see anything I would describe as a 'loop' here; but the recursive analysis still makes perfect sense.

Yes, some programming languages have special purpose looping constructs. But they are of limited usefulness, and not particularly fundamental: they usually get compiled away.

all iterations are recursion but not the other way around.