Regular Expressions/DFA/NFA
Regular Expressions (RE) 正規式
它的規則：
https://www.runoob.com/regexp/regexp-metachar.html
(測試網站）：
https://tool.oschina.net/regex/
Finite State Machine:(有限狀態機）
Finite Automaton：（有窮自動機）
DFA/NFA：確定的又窮自動機/非確定的有窮自動機
Two ways to define a set of strings (language)：
Regular expressions (REs)
Deterministic finite automatia (DFAs)
conclusion：
- Remarkable fact. DFAs and REs are equivalent (等價）
- In short, there is no formal language that can be recognized by some NFA and not by any DFA
- NFAs and REs are equivalent
- Not all formal languages are regular
Turing Machines
DFAs 與 TMs的比較
Universality
All computational devices are equivalent
Turing as a definition of what we can compute
Q&A ： 圖靈機,Java,mobile phone,are universality
Computability
根據前面的理論如果問題不能計算那麼所有的計算機都不能計算因爲
All computational devices are equivalent
The halting problem is undecidable: (圖靈機停機問題）https://zhidao.baidu.com/question/22004860.html