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> One such engine is rust's https://github.com/rust-lang/regex https://github.com/rust-lang/regex It can process arbitrary deep nestings? I'm not familiar wit

> One such engine is rust's https://github.com/rust-lang/regex https://github.com/rust-lang/regex It can process arbitrary deep nestings? I'm not familiar with it, but I had a look and I can't see anything in there that would allow it to do it. Can you point to syntax that allows it? In general regex's that run in linear time are based on DFA's (as opposed to NFA's - which is what most use). A DFA is part of the Chomsky hierarchy of languages that has DFA's at the bottom (aka regex's, least powerful - can't handle recursive structures), a DFA coupled with an infinite stack for memory (aka LR parsers, can't handle context sensitive grammars, also runs in very close to linear time), and finally a DFA coupled with an infinite tape (aka Turing Machine, can do any sort of computation). In each of these cases the DFA (regex) is essentially a computer program and all they are doing is allowing it to store stuff on various kinds of memory (none, stack, tape with arbitrary movements). An interesting corollary of this is any computer program can be expressed as a DFA.

burntsushi 7y ago
> It can process arbitrary deep nestings? The GP is presumably citing it as an example of a linear time regex, and not as something that can parse arbitrarily deep nestings. Note that "linear time regex engines" is a characterization that holds the size of the regex constant, such that "linear time" is with respect to the input only. The actual worst case time complexity of such regex engines is O(m * n), where m ~ len(regex) and n ~ len(input). So even with a linear time regex engine, if you bloat the size of the regex, you'll make searching slower. But, it will still be done linearly with respect to the size of the input. > In general regex's that run in linear time are based on DFA's (as opposed to NFA's - which is what most use). No. Please stop spreading the misuse of these terms. DFAs and NFAs have very precise meanings. It's one thing to call things regexes that aren't actually regular, but DFAs and NFAs should retain their precise theoretical meaning. A linear time regex engine such as Rust's `regex` crate uses both DFAs and NFAs in its implementation. The Perl and PCRE folks (along with Jeffrey Friedl) have bastardized this terminology. PCRE for example claims to provide a "DFA" regex engine in its API, but it's actually an NFA. The main implementation inside PCRE (and Perl) is backtracking oriented, and carries with it much more power than an NFA. If it didn't, then the NFA could be executed in linear time, because NFAs and DFAs have equivalent power. > An interesting corollary of this is any computer program can be expressed as a DFA. No they can't. You're confusing terminology quite a bit here.

burntsushi 7y ago

> It can process arbitrary deep nestings? The GP is presumably citing it as an example of a linear time regex, and not as something that can parse arbitrarily deep nestings. Note that "linear time regex engines" is a characterization that holds the size of the regex constant, such that "linear time" is with respect to the input only. The actual worst case time complexity of such regex engines is O(m * n), where m ~ len(regex) and n ~ len(input). So even with a linear time regex engine, if you bloat the size of the regex, you'll make searching slower. But, it will still be done linearly with respect to the size of the input. > In general regex's that run in linear time are based on DFA's (as opposed to NFA's - which is what most use). No. Please stop spreading the misuse of these terms. DFAs and NFAs have very precise meanings. It's one thing to call things regexes that aren't actually regular, but DFAs and NFAs should retain their precise theoretical meaning. A linear time regex engine such as Rust's `regex` crate uses both DFAs and NFAs in its implementation. The Perl and PCRE folks (along with Jeffrey Friedl) have bastardized this terminology. PCRE for example claims to provide a "DFA" regex engine in its API, but it's actually an NFA. The main implementation inside PCRE (and Perl) is backtracking oriented, and carries with it much more power than an NFA. If it didn't, then the NFA could be executed in linear time, because NFAs and DFAs have equivalent power. > An interesting corollary of this is any computer program can be expressed as a DFA. No they can't. You're confusing terminology quite a bit here.