Head to tail transistor pair, not a Darlington pair, not a Szlikai Darlington
What I see in the OP's circuit diagram is a reference voltage source (implemented by R2 and D1), emitter follower (Q1) and a current-limiting circuit (Q2 and R1).
The load current &quot;creates&quot; a voltage drop VR1 across R1. At normal conditions, it is less than 0.7 V (the voltage threshold of the protection circuit), and Q2 is cut-off. The output voltage Vreg follows the unregulated input voltage V1.
When the voltage drop exceeds the voltage threshold (approximately 0.7 V), Q2 turns on and begins diverting Q1's base current. Thus the load current is limited.
This circuit solution is widely used in the output stages of power and op-amp amplifiers (see, for example, the 741 output stage).