Alphanumeric input is often required in many computer applications. Virtual reality (VR) applications are no exception. For instance, enabling the alphanumeric input can augment a typical VR based walkthrough system so that the user can make notes and annotations. However, methods for efficient alphanumeric input for HMD-based VR systems have only been treated in a limited way. A most typical approach is to use a virtual keyboard so that the users can leverage upon one's typing skill. However, such an approach requires intricate and difficult finger tracking and poses usability problems. Thus, we propose to use a real keyboard as a tangible interface to the virtual keyboard and provide natural haptic/tactile feedback. Instead of tracking the fingers with cumbersome devices, this paper demonstrates that finger positions can be approximated at sufficient accuracy for the typing task by only tracking the two hands. The concept of continuous calibration is introduced to compensate for the dynamically changing errors from the tracking devices, sensor positions, keyboard positions and assumed default hand sizes. The proposed alphanumeric input system is empirically compared to that of the real through an experiment, and the results show that users can indeed obtain near-real performance out of such a system.