Are Perl regexes turing complete?

Question

I\'ve seen Ruby and Perl programmers do some complicated code challenges entirely with regexes. The lookahead and lookbehind capabilities in Perl regexes make them more powe

Answer 1

For regexes in Perl there are two cases:

1. With embedded code: They are of course Turing-complete.
2. Without embedded code: They always halt so they are not general Turing machines.

Every regular language can be accepted by a finite automaton. Its input must be a finite string.

[...] a deterministic finite automaton (DFA)—also known as deterministic finite state machine—is a finite state machine that accepts/rejects finite strings of symbols [...].

The same goes for Turing machines: The formal definition does not even have input. It must be encoded in the finite number of states.

Alternative (equivalent) definitions include input, but it must be finite.