Testing how Apex instanceof handles different string inputs

instanceof_test_apex.cls

// Testing how instanceof handles different string inputs. 
// It returns false positives.
// for safer alternatives see: https://salesforce.stackexchange.com/questions/392364/apex-instanceof-id-sometimes-false-positive
@isTest static void test_instanceOf() {
    // String str = 'cam - 123';
    // String str = '701andIamNotARealId';
    // String str = '003BC00000ArsxPYAR'; // says this is an ID, but ID is from other org
    String str = '243;0919957138;z71';  // says this is an Id; it obviously isn't

    Boolean isId;

    Test.startTest();
    // Id idValue = (Id)str; // this is an alternative to `instanceof`
    isId = str instanceof Id;
    if (isId) {
        System.debug('string ' + str + ' is a valid Id');
    } else {
        System.debug('string ' + str + ' is NOT a valid Id');
    }
    Test.stopTest();

    System.assert(isId == false);

    // System.debug('idValue');
    // System.debug(idValue);

    // System.assertEquals(str, idValue);
}