1.3 Regular Expressions
定义1.52(regular expression)
regular expression可以理解成language之间的操作。
比如 \
Say that is a regular expression if is
- ,for ,
- ,
- ,
- , where and are regular expressions,
- , where and are regular expressions, or
- , where is regular expressions,
虽然只有三个operation,但很多其他operations可以构造出来,比如,补、交
定理(L(RE) = L(FA))
A language is regular if and only if some regular expression describes it
证明方法:引入GNFA来证明充分性,构造法证明必要性;
FA和RE有相同的表达能力