my $digit = qr/^\d+$/;
my $not_digit = reverse_regex_assertion( $digit );

print 0+(1234 =~ $digit),
      0+(1234 =~ $not_digit);

sub reverse_regex_assertion {
    my $re = shift;
    return qr/(?(?{$re})(?!)|(?=))/;
}

# That expression broken into pieces
#qr/(? # Conditional (?(expr) (true) | (false) )
#      (?{$re}) # Test $re expression
#      (?!) # Assert false
#   |  # OR
#      (?=) # Assert true
#   )/x