To emulate look-behind assertion, it runs the main regexp
automaton and some look-behind automata in parallel, and
an automaton refers look-behind automaton state when
the automaton encounts look-behind assertion.
Suprising point is reading input string only once even if
regexp cotains look-behind. It is unique feature.

It does not support captures in look-behind
because the meaning of captures in look-behind is unknown.
(and implementing is so hard, hehe ;)

I believe additional cost of matching the regexp without look-behind
is little.

Because it cannot get correct fork size for now, but Inst size is too
large for this.