5 ms·

You can say that again. Here's a Perl Regex to validate according to RFC 822: http://ex-parrot.com/~pdw/Mail-RFC822-Address.html http://ex-parrot.com/~pdw/Mail-

by smanek 18y ago

You can say that again. Here's a Perl Regex to validate according to RFC 822: http://ex-parrot.com/~pdw/Mail-RFC822-Address.html http://ex-parrot.com/~pdw/Mail-RFC822-Address.html. It's well over a page long. The problem is that email validation is really better suited to being done by a push-down automata (rather than a finite state machine, which is what true regexes are). For better or for worse, perl regular expressions aren't strictly finite state machines though - so you can kind of extend them beyond what is sane. P.S. Here's a ridiculously cool regex that can be used to determine if a number is prime: /^1?$|^(11+?)\1+$/ If you've got a few hours (a few seconds for cperciva ;-)) to waste, try and figure it out :-D

Anon84 18y ago
I'm not sure how this ( /^1?$|^(11+?)\1+$/ ) would match 3, 5, 7, etc... I'm no regex expert, maybe I'm missing something?
- deleted 18y ago
  [deleted]
- smanek 18y ago
  well, you're right - there are some more details. it needs a bit of wrapper code around it. It actually tests if n is composite, when presented with a pattern of n ones. E.g., '111' doesn't match so it is prime, and '1111' does match so it's composite.
  - rplevy 18y ago
    Wow, this is really amazing (to me at least because I had not seen or thought of this before). [ Warning: Spoiler! :) ] It's in base 1 (tally system). The first part up to the "|" matches the case where number 1 is the whole string, covering the fact of 1 not being considered prime. It also matches no string, I guess to mean 0. Next part matches a captured string consisting of 2 or more ones, and the captured string must be repeated at least one time. That is, 2+ repetitions of chunks of 2+ ones. This much is obvious, but it wasn't until I wrote it out that I realized what it is doing: multiples of 2 11+11 or 11+11+11 or 11+11+11+11 or ... = 4, 6, 8, 10, 12, ... multiples of 3 111+111 or 111+111+111 or 111+111+111+111 or ... = 6, 9, 12, ... multiples of 4 1111+1111 or 1111+1111+1111 or ... = 8, 12, 16, 20, ... multiples of 5 11111+11111 or 11111+11111+11111 or ... = 10, 15, 20, 25, ... multiples of n which leaves only primes remaining. Side note: In Perl, executing it this way: $nstr = 1 x $ARGV[0]; print ($nstr =~ m/^1?$|^(11+?)\1+$/ ? "composite\n" : "prime\n"); The largest prime I can find without a seg fault is 37397 And the largest composite I can find without a seg fault is 37399
- tlrobinson 18y ago
  Hint: don't express the number in base 10
gojomo 18y ago
You can watch that prime-testing regex in action. Here it is matching 49, showing it is not-prime: http://regex.powertoy.org/?pat=/%5E1%3F%24%7C%5E%2811+%3F%29%5C1+%24/&syn=perlSyntax&anim=1&rep=&in=1111111111111111111111111111111111111111111111111 http://regex.powertoy.org/?pat=/%5E1%3F%24%7C%5E%2811+%3F%29... And here failing against 47, showing it is prime: http://regex.powertoy.org/?pat=/%5E1%3F%24%7C%5E%2811+%3F%29%5C1+%24/&syn=perlSyntax&anim=1&rep=&in=11111111111111111111111111111111111111111111111 http://regex.powertoy.org/?pat=/%5E1%3F%24%7C%5E%2811+%3F%29...

by smanek 18y ago

Anon84 18y ago

I'm not sure how this ( /^1?$|^(11+?)\1+$/ ) would match 3, 5, 7, etc... I'm no regex expert, maybe I'm missing something?

deleted 18y ago
[deleted]
smanek 18y ago
well, you're right - there are some more details. it needs a bit of wrapper code around it. It actually tests if n is composite, when presented with a pattern of n ones. E.g., '111' doesn't match so it is prime, and '1111' does match so it's composite.
- rplevy 18y ago
  Wow, this is really amazing (to me at least because I had not seen or thought of this before). [ Warning: Spoiler! :) ] It's in base 1 (tally system). The first part up to the "|" matches the case where number 1 is the whole string, covering the fact of 1 not being considered prime. It also matches no string, I guess to mean 0. Next part matches a captured string consisting of 2 or more ones, and the captured string must be repeated at least one time. That is, 2+ repetitions of chunks of 2+ ones. This much is obvious, but it wasn't until I wrote it out that I realized what it is doing: multiples of 2 11+11 or 11+11+11 or 11+11+11+11 or ... = 4, 6, 8, 10, 12, ... multiples of 3 111+111 or 111+111+111 or 111+111+111+111 or ... = 6, 9, 12, ... multiples of 4 1111+1111 or 1111+1111+1111 or ... = 8, 12, 16, 20, ... multiples of 5 11111+11111 or 11111+11111+11111 or ... = 10, 15, 20, 25, ... multiples of n which leaves only primes remaining. Side note: In Perl, executing it this way: $nstr = 1 x $ARGV[0]; print ($nstr =~ m/^1?$|^(11+?)\1+$/ ? "composite\n" : "prime\n"); The largest prime I can find without a seg fault is 37397 And the largest composite I can find without a seg fault is 37399
tlrobinson 18y ago
Hint: don't express the number in base 10

gojomo 18y ago

You can watch that prime-testing regex in action. Here it is matching 49, showing it is not-prime: http://regex.powertoy.org/?pat=/%5E1%3F%24%7C%5E%2811+%3F%29%5C1+%24/&syn=perlSyntax&anim=1&rep=&in=1111111111111111111111111111111111111111111111111 http://regex.powertoy.org/?pat=/%5E1%3F%24%7C%5E%2811+%3F%29... And here failing against 47, showing it is prime: http://regex.powertoy.org/?pat=/%5E1%3F%24%7C%5E%2811+%3F%29%5C1+%24/&syn=perlSyntax&anim=1&rep=&in=11111111111111111111111111111111111111111111111 http://regex.powertoy.org/?pat=/%5E1%3F%24%7C%5E%2811+%3F%29...