Using regular expressions to compare numbers

Question

So this is an obvious case of \"you\'re doing it wrong\". I don\'t actually intend on doing this, but a conversation at work spurred this question:

C

Answer 1

This is quite easy.

#!/usr/bin/env perl
use strict;
use warnings;
use Regexp::Assemble;

for my $n (@ARGV)  {
    my $asm = new Regexp::Assemble;
    for (1 .. $n) { $asm->add($_) }
    for ($asm->re){
        s/\)$/\$/;
        s/^[^:]*:/^/;
        print "$n => /$_/\n";
    }
}

Now run it to find the pattern that matches integers between 1 and that number:

$ perl /tmp/ra 5 15 153 401 1144
5 => /^[12345]$/
15 => /^(?:[23456789]|1[012345]?)$/
153 => /^(?:1(?:[6789]|5[0123]?|0\d?|1\d?|2\d?|3\d?|4\d?)?|2\d?|3\d?|4\d?|5\d?|6\d?|7\d?|8\d?|9\d?)$/
401 => /^(?:1(?:0\d?|1\d?|2\d?|3\d?|4\d?|5\d?|6\d?|7\d?|8\d?|9\d?)?|2(?:0\d?|1\d?|2\d?|3\d?|4\d?|5\d?|6\d?|7\d?|8\d?|9\d?)?|3(?:0\d?|1\d?|2\d?|3\d?|4\d?|5\d?|6\d?|7\d?|8\d?|9\d?)?|4(?:[123456789]|0[01]?)?|5\d?|6\d?|7\d?|8\d?|9\d?)$/
1144 => /^(?:1(?:0(?:0\d?|1\d?|2\d?|3\d?|4\d?|5\d?|6\d?|7\d?|8\d?|9\d?)?|1(?:[56789]|4[01234]?|0\d?|1\d?|2\d?|3\d?)?|2\d?|3\d?|4\d?|5\d?|6\d?|7\d?|8\d?|9\d?)?|2(?:0\d?|1\d?|2\d?|3\d?|4\d?|5\d?|6\d?|7\d?|8\d?|9\d?)?|3(?:0\d?|1\d?|2\d?|3\d?|4\d?|5\d?|6\d?|7\d?|8\d?|9\d?)?|4(?:0\d?|1\d?|2\d?|3\d?|4\d?|5\d?|6\d?|7\d?|8\d?|9\d?)?|5(?:0\d?|1\d?|2\d?|3\d?|4\d?|5\d?|6\d?|7\d?|8\d?|9\d?)?|6(?:0\d?|1\d?|2\d?|3\d?|4\d?|5\d?|6\d?|7\d?|8\d?|9\d?)?|7(?:0\d?|1\d?|2\d?|3\d?|4\d?|5\d?|6\d?|7\d?|8\d?|9\d?)?|8(?:0\d?|1\d?|2\d?|3\d?|4\d?|5\d?|6\d?|7\d?|8\d?|9\d?)?|9(?:0\d?|1\d?|2\d?|3\d?|4\d?|5\d?|6\d?|7\d?|8\d?|9\d?)?)$/