Erlang has binary pattern matchers for writing binary protocol parsers. They blew my mind when I learned about them because I hadn't realized how simple binary protocol parsing could be given the right abstractions. The concept is extremely obvious in hindsight which is what makes it such a nice feature.
Another feature I miss is constraint solving from languages like Prolog and Picat. Some problems are extremely easy to express as a set of constraints and then solve with a constraint solver.
There's also the grammar facility of Perl 6 for writing down arbitrary language grammars. These are regular expressions on steroids. Most other languages don't come close. I don't necessarily miss it but souped up regexes can be handy every so often.
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