use warnings;
use strict;

use Config;
use Math::BigInt;

my $regex = do {
	my $max = eval $Config{nv_overflows_integers_at} or die;
	my $len1 = length($max) - 1;
	my $range = substr $max, 0, 1;
	$range = $range eq "1" ? "1" : "1-$range";
	qr{ \A (?: (?!0) [0-9]{1,$len1}
		| ( [$range] [0-9]{$len1} )
			(?(?{ Math::BigInt->new($^N)->bgt($max) })(*F))
	) \z }x }; 

use Test::More;

diag explain $regex;

unlike 0, $regex;
  like 1, $regex;
  like 3, $regex;
unlike "", $regex;
unlike "x", $regex;
unlike "123y", $regex;
unlike -1, $regex;
unlike "-9999999999999999999999999999999", $regex;

my $x = eval($Config{nv_overflows_integers_at})-1;
like "$x", $regex, "max-1 ($x)";
$x++;
like "$x", $regex, "max works ($x)";
$x++;
unlike "$x", $regex, "max+1 fails ($x)";
$x+=1_000;
unlike "$x", $regex;

unlike "999999999999999999999999999999999999", $regex;

done_testing;