That’s a good solution – have you tested it? Coming from a Java background it doesn’t immediately make sense to me that String instanceof Id == true, but in the Apex world I don’t see a reason why it wouldn’t, and I would think it’s more efficient.

J

How about using instanceof keyword to check this String value is an Id. Then check the sObject Type matches what you are looking for.

public static Boolean isValid(String stringValue, Schema.SObjectType sObjectType) {
    Id sObjectId;
    if(isId(stringValue)) sObjectId = (Id)stringValue;
    return isValid(sObjectId, sObjectType);
}

public static Boolean isValid(Id sObjectId, Schema.SObjectType sObjectType) {
    return !(sObjectId==null || sObjectId.getSObjectType()!=sObjectType);
}

public static Boolean isId(String stringValue) {
    return stringValue InstanceOf Id;
}