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 }; 

##</code><code>##

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;

##</code><code>##

#!/usr/bin/env perl
use warnings;
use strict;
use Benchmark qw/cmpthese/;

use feature 'state';
use Carp;
use Math::BigInt;
use Config;
use Regexp::Common qw/number/;

sub validate_int {
	my $str = shift;
	state $max = Math::BigInt->new(
		eval $Config{nv_overflows_integers_at} );
	croak "not an integer"
		unless defined $str && $str=~/\A$RE{num}{int}\z/;
	my $num = Math::BigInt->new($str);
	croak "integer to small" if $num < 0;
	croak "integer too big" if $num > $max;
	return $num->numify;
}

sub validate_int2 {
	my $str = shift;
	state $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 };
	croak "bad integer" unless $str =~ $regex;
	return 0+$str;
}

my $max = eval($Config{nv_overflows_integers_at});

cmpthese(-1, {
	first => sub {
		validate_int(1)==1 or die;
		validate_int(999)==999 or die;
		validate_int($max-1)==$max-1 or die;
		validate_int($max)==$max or die;
		defined eval { validate_int($max+1) } and die;
	},
	second => sub {
		validate_int2(1)==1 or die;
		validate_int2(999)==999 or die;
		validate_int2($max-1)==$max-1 or die;
		validate_int2($max)==$max or die;
		defined eval { validate_int2($max+1) } and die;
	},
});

__END__

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