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Don't you think people would have figured it out by themselves the moment programmers started writing parsers? I'm not sure his contribution was particularly ne

by ogogmad 10mo ago

Don't you think people would have figured it out by themselves the moment programmers started writing parsers? I'm not sure his contribution was particularly needed.

eru 10mo ago
Lots of things get invented / discovered multiple times when it's in the air. But just because Newton (or Leibnitz) existed, doesn't mean Leibnitz (or Newton) were any less visionary. For your very specific question: have a look at the sorry state of what's called 'regular expressions' many programming languages and libraries to see what programmers left loose can do. (Most of these 'regular expressions' add things like back-references etc that make matching their franken-'xpressions take exponential time in the worst case; but they neglect to put in stuff like intersection or complement of expressions, which are matchable in linear time.
- ogogmad 10mo ago
  Just checked after reading your comment. Surprisingly to me, AFAs (Alternating Finite Automatons) do let you introduce the Complement operation into Regex while preserving the O(mn) complexity of running NFAs. That's really subtle, because deciding Regex universality (i.e. whether a regex accepts every input) is PSPACE-COMPLETE. And since NFAs make it efficient to decide whether a regex matches NO inputs, any attempts to combine NFAs with regex Complement would trip on a massive landmine.
  - ogogmad 10mo ago
    Actually, I've now checked more thoroughly, and RE+complement matching is O(n^2 m), which is not that good.
    - eru 10mo ago
      See https://en.wikipedia.org/wiki/Regular_language#Closure_properties https://en.wikipedia.org/wiki/Regular_language#Closure_prope... The complement of a regular language is a regular language, and for any given regular language we can check whether a string is a member of that language in O(length of the string) time. Yes, depending on how you represent your regular language, the complement operator might not work play nicely with that representation. But eg it's fairly trivial for finite state machines or when matching via Brzozowski derivatives. See https://en.wikipedia.org/wiki/Brzozowski_derivative https://en.wikipedia.org/wiki/Brzozowski_derivative See also https://github.com/google/redgrep https://github.com/google/redgrep Regular languages have a bunch of closure properties. In practice, intersection and complement are really useful. So you can say things like: [regular_expression_for_matching_email_addresses] & ![my_list_of_naughty_words] Exercise for the reader: expand the expression above to allow the town of 'Scunthorpe'.

by ogogmad 10mo ago

Don't you think people would have figured it out by themselves the moment programmers started writing parsers? I'm not sure his contribution was particularly needed.

eru 10mo ago

Lots of things get invented / discovered multiple times when it's in the air. But just because Newton (or Leibnitz) existed, doesn't mean Leibnitz (or Newton) were any less visionary. For your very specific question: have a look at the sorry state of what's called 'regular expressions' many programming languages and libraries to see what programmers left loose can do. (Most of these 'regular expressions' add things like back-references etc that make matching their franken-'xpressions take exponential time in the worst case; but they neglect to put in stuff like intersection or complement of expressions, which are matchable in linear time.

ogogmad 10mo ago
Just checked after reading your comment. Surprisingly to me, AFAs (Alternating Finite Automatons) do let you introduce the Complement operation into Regex while preserving the O(mn) complexity of running NFAs. That's really subtle, because deciding Regex universality (i.e. whether a regex accepts every input) is PSPACE-COMPLETE. And since NFAs make it efficient to decide whether a regex matches NO inputs, any attempts to combine NFAs with regex Complement would trip on a massive landmine.
- ogogmad 10mo ago
  Actually, I've now checked more thoroughly, and RE+complement matching is O(n^2 m), which is not that good.
  - eru 10mo ago
    See https://en.wikipedia.org/wiki/Regular_language#Closure_properties https://en.wikipedia.org/wiki/Regular_language#Closure_prope... The complement of a regular language is a regular language, and for any given regular language we can check whether a string is a member of that language in O(length of the string) time. Yes, depending on how you represent your regular language, the complement operator might not work play nicely with that representation. But eg it's fairly trivial for finite state machines or when matching via Brzozowski derivatives. See https://en.wikipedia.org/wiki/Brzozowski_derivative https://en.wikipedia.org/wiki/Brzozowski_derivative See also https://github.com/google/redgrep https://github.com/google/redgrep Regular languages have a bunch of closure properties. In practice, intersection and complement are really useful. So you can say things like: [regular_expression_for_matching_email_addresses] & ![my_list_of_naughty_words] Exercise for the reader: expand the expression above to allow the town of 'Scunthorpe'.