The concept of regular expressions
Regular expressions (English: Regular Expression , the code is often abbreviated as regex ).
Regular expression is a character string, a single character string will be described, is used to define the matching rules matches a set of syntactic rules matches a string. In development, regular expressions are typically used to retrieve, replace the text in line with those of a rule.
* A: 正则表达式的概念和作用
* a: 正则表达式的概述
* 正则表达式也是一个字符串，用来定义匹配规则，在Pattern类中有简单的规则定义。
可以结合字符串类的方法使用。
* 简单记：正则表达式是具有特殊含义的字符串。
* b: 正则表达式的作用
* 比如注册邮箱,邮箱有用户名和密码,一般会对其限制长度,这个限制长度的事情就是正则表达式做的
