Apex instanceof Id sometimes false positive

Question

I recently found that some strings, which are very much NOT Salesforce Ids, will cause instanceof Id to return a false positive.

Example:

String extId = '243209199570387970';
Boolean isId = extId instanceof Id;
System.debug(isId); // ouputs TRUE but should be false

Is this just simply a bug or is there some weird intended behavior in this? Are there any alternatives to checking if a String is a Salesforce ID?

Answer 1

Their check on instance of Id just checks if it is instance of String and the length of the String is either 15 or 18 characters. It also allows to type in any symbol that cannot be used in ID, such as ;, for example. It is not a bug, but rather not expected behavior, as you can run this code without errors, too:

Id extId = '243209199570387970';
System.debug(extId); 

As a workaround you can try this:

String extId = '243209199570387970';
Boolean isId = true;
try {
    ((Id) extId).getSobjectType();
} catch (Exception e) {
    isId = false;
}
System.debug(isId);