Compilation phases, interpretation versus JIT, memory layout, and garbage-collection theory.
3 questions in CS Fundamentals.
Regular expressions in the formal sense describe exactly the regular languages, the ones a finite automaton recognises with fixed memory, so they cannot match balanced parentheses or anything needing an unbounded count. Backreferences take practical regex past that class and cost the linear-time matching guarantee.
Source is tokenised, parsed, checked, lowered to an intermediate representation, optimised and emitted - either ahead of time to machine code or to bytecode executed by a runtime. A JIT compiles hot bytecode at run time using profiles, which is why managed programs get faster after warm-up. Use this compilers answer to show the decision, trade-off, and evidence rather than a memorised definition.
A JIT compiles against a profile gathered earlier, so its fast code rests on assumptions that can be invalidated: a new receiver type at a call site, a branch taken for the first time, a class loaded that gives an interface a second implementation. The method deoptimises and recompiles from a worse profile. It also connects deoptimisation to the point an interviewer is testing.