ChatGPT wrote some unit tests and found a small bug.

When asked to fix the bug, it came up with a different regex, which
would completely change what's valid and what's not, so that's totally
wrong:

    /^\s*(-)?([0-9]+)(?:[,.]([0-9]{1,2}))?\s*$/

When asked to fix it in another way, without changing the regex, it
suggested stripping the sign completely, which is even more wrong.

So I fixed it myself :)

lib/RevBank/Amount.pm

@@ -81,6 +81,8 @@ sub parse_string($class, $str) {
         = $str =~ /^\s*(?:\+|(-)?)([0-9]+)?(?:[,.]([0-9]{1,2}))?\s*$/
         or return undef;
 
+    defined $int or defined $cents or return undef;
+
     $int //= 0;
     $cents //= 0;
     $cents *= 10 if length($cents) == 1;  # 4.2 -> 4.20

t/amount2.t

@@ -0,0 +1,115 @@
+# These tests are generated using ChatGPT.
+# The tests that ChatGPT got wrong, or that throw (intended) warnings, are commented out.
+
+use strict;
+use warnings;
+use Test::More;
+
+use_ok 'RevBank::Amount';
+
+my $a = RevBank::Amount->new(500);
+my $b = RevBank::Amount->new(200);
+my $c = RevBank::Amount->new(-100);
+
+# Test basic arithmetic operations
+is($a + $b, RevBank::Amount->new(700), "Addition");
+is($a - $b, RevBank::Amount->new(300), "Subtraction");
+is($a * 2, RevBank::Amount->new(1000), "Multiplication");
+#is($a / 2, RevBank::Amount->new(250), "Division"); # Throws a warning (intended)
+is($c * -1, RevBank::Amount->new(100), "Unary minus");
+
+# Test comparison operators
+ok($a > $b, "Greater than");
+ok($c < $b, "Less than");
+ok($a == RevBank::Amount->new(500), "Equality");
+
+# Test stringification
+is($a->string, "5.00", "Stringification");
+is($a->string_flipped, "+ 5.00", "Stringification with flipped sign");
+
+# Test parsing from string
+is(RevBank::Amount->parse_string("5"), $a, "Parsing from string");
+
+# Test floating point arithmetic
+my $d = RevBank::Amount->new_from_float(4.56);
+is($d->float, 4.56, "Conversion to floating point");
+# ChatGPT got this wrong:
+#is($d * 2.5, RevBank::Amount->new(11.4), "Multiplication with float");
+is($d * 2.5, RevBank::Amount->new(1140), "Multiplication with float");
+
+# ChatGPT got this wrong:
+#is($d / 2, RevBank::Amount->new(2.28), "Division with float");
+# But the correct one throws a warning (as intended):
+#is($d / 2, RevBank::Amount->new(228), "Division with float");
+
+
+# Test constructor with floating point input
+is(RevBank::Amount->new_from_float(123.456), RevBank::Amount->new(12346), "Float constructor");
+
+# Test constructor with rounding to nearest integer
+is(RevBank::Amount->new_from_float(123.495), RevBank::Amount->new(12350), "Float constructor with rounding");
+
+# These make no sense:
+### Test constructor with rounding down
+##is(RevBank::Amount->new_from_float(123.449), RevBank::Amount->new(12344), "Float constructor with rounding down");
+##
+### Test constructor with rounding up
+##is(RevBank::Amount->new_from_float(123.455), RevBank::Amount->new(12346), "Float constructor with rounding up");
+
+# Test parsing of various input formats
+is(RevBank::Amount->parse_string("+1234"), RevBank::Amount->new(123400), "Parse positive integer");
+is(RevBank::Amount->parse_string("-1234"), RevBank::Amount->new(-123400), "Parse negative integer");
+is(RevBank::Amount->parse_string("1.23"), RevBank::Amount->new(123), "Parse positive float");
+is(RevBank::Amount->parse_string("-1.23"), RevBank::Amount->new(-123), "Parse negative float");
+is(RevBank::Amount->parse_string("-1.234"), undef, "Parse invalid float");
+
+# Test overloading of comparison operators
+ok(RevBank::Amount->new(100) > RevBank::Amount->new(50), "Overloaded greater than");
+ok(RevBank::Amount->new(50) < RevBank::Amount->new(100), "Overloaded less than");
+ok(RevBank::Amount->new(50) == RevBank::Amount->new(50), "Overloaded equal");
+
+
+my $C = "RevBank::Amount";
+
+subtest 'string parsing' => sub {
+
+    # valid input tests
+
+    is($C->parse_string("0"),         $C->new(0),     'parsing zero');
+    is($C->parse_string("0.0"),       $C->new(0),     'parsing zero with decimal point');
+    is($C->parse_string("100"),       $C->new(10000), 'parsing integer value');
+    is($C->parse_string("-100"),      $C->new(-10000), 'parsing negative integer value');
+    is($C->parse_string("100.00"),    $C->new(10000), 'parsing integer value with decimal point');
+    is($C->parse_string("100.25"),    $C->new(10025), 'parsing decimal value with two decimal places');
+    is($C->parse_string("100.2"),     $C->new(10020), 'parsing decimal value with one decimal place');
+    #is($C->parse_string("100.257"),   $C->new(10026), 'parsing decimal value with three decimal places (rounded up)');  # is not valid input
+    #is($C->parse_string("100.254"),   $C->new(10025), 'parsing decimal value with three decimal places (rounded down)');  # is not valid input
+    is($C->parse_string("+100.25"),   $C->new(10025), 'parsing positive decimal value with sign');
+    is($C->parse_string("-100.25"),   $C->new(-10025), 'parsing negative decimal value with sign');
+    is($C->parse_string("  100.25  "), $C->new(10025), 'parsing decimal value with leading and trailing whitespace');
+    is($C->parse_string("4.2"),       $C->new(420),   'parsing decimal value with one decimal place (no trailing zero)');
+
+    # invalid input tests
+
+    is($C->parse_string(""),             undef,         'empty input');
+    is($C->parse_string("   "),          undef,         'whitespace only');
+    is($C->parse_string("."),            undef,         'single decimal point');
+    is($C->parse_string("-"),            undef,         'single hyphen');
+    is($C->parse_string("+"),            undef,         'single plus sign');
+    is($C->parse_string("-100.256"),     undef,         'value with too many decimal places');
+    is($C->parse_string("-100.2567"),    undef,         'value with too many decimal places');
+    is($C->parse_string("foo"),          undef,         'non-numeric input');
+    is($C->parse_string("100foo"),       undef,         'non-numeric input after numeric value');
+    #is($C->parse_string("100,00"),       undef,         'comma as decimal separator');  # is valid input
+    is($C->parse_string("100.00.00"),    undef,         'multiple decimal points');
+    is($C->parse_string("-100.2.5"),     undef,         'multiple decimal points');
+};
+
+subtest 'parse_string' => sub {
+    my @invalid = qw(foobar -1.2.3 +1.2.3 123.1234 123.123456);
+    for my $input (@invalid) {
+        is(RevBank::Amount->parse_string($input), undef, "invalid input '$input' returns undef");
+    }
+};
+
+done_testing();