一个特殊字符串，用于描述匹配一个字符串集合的模式。可以用它来进行字符串的匹配、替换和拆分。
正则表达式语法用于匹配符合某一特征的字符串
| metacharacter | Description |
|---|---|
| | | Find a match for any one of the patterns separated by | as in: cat|dog|fish |
| . | Find just one instance of any character |
| ^ | Finds a match as the beginning of a string as in: ^Hello |
| $ | Finds a match at the end of the string as in: World$ |
| d | Find a digit |
| s | Find a whitespace character |
| b | Find a match at the beginning of a word like this: bWORD, or at the end of a word like this: WORDb |
| uxxxx | Find the Unicode character specified by the hexadecimal number xxxx |
用于搜索在某一范围内的字符
| expression | Description |
|---|---|
| [abc] | Find one character from the options between the brackets |
| [^abc] | Find one character NOT between the brackets |
| [0-9] | Find one character from the range 0 to 9 |
用于匹配字符出现的次数
| Quantifier | Description |
|---|---|
| n+ | Matches any string that contains at least one n |
| n* | Matches any string that contains zero or more occurrences of n |
| n? | Matches any string that contains zero or one occurrences of n |
| n{x} | Matches any string that contains a sequence of X n's |
| n{x,y} | Matches any string that contains a sequence of X to Y n's |
| n{x,} | Matches any string that contains a sequence of at least X n's |
使用String中的matches方法来匹配字符串，返回True或者False
替换和拆分字符串使用String中的replaceAll方法替换所有匹配的子字符串，类似的方法还有replaceFirst
使用split方法将一个字符串以匹配的分隔符拆分为子字符串
note正则表达式以元表达式的形式给出，Java中若要使用 要以转义字符 \ 表示
默认情况下，量词符都是贪婪的，如
System.out.println(“Jaaavaa".replaceFirst("a+","R"));
会匹配到 aaa ，可以通过在正则表达式后面加？使量词符变为惰性，如
System.out.println(“Jaaavaa".replaceFirst("a+？","R"));
会匹配到 a
