Scanning
Deterministic finite automation(DFA)
Graph：不需要标识状态、不表示没有的转移。
Example：
1. 接受一个b
2. 最多接受一个b
3. nat = digit +
4. signedNat = (+|-)? Nat
5. Number = singedNat(“.”nat)?(E signedNat)?
6. comments(easily than write down a regular expression)
Lookahead ,backtracking and nondeterministic automata
Lookahead
finite automation for an identifier with delimiter and return value