It's not JavaScript bytecode, but regex bytecode. V8 parses regular expressions to regex ASTs, takes them through some more intermediate forms, then finally to a special regex bytecode. That bytecode is either interpreted, or JIT compiled (depending of some build options, I think).
You can look at a list of bytecodes in [1], and at the x64 JIT in [2].
I was thinking: but V8 doesn't have byte code.
Then opened the link:
"After optimization we generate native machine code which uses backtracking to try different alternatives."