String类的三个内建正则表达式工具：
1.matches()方法
示例：检查一个句子是否以大写字母开头，以句号结尾
1 public static boolean checkFormat(String sentence){
2 return sentence.matches("^[A-Z].+\\.$");
3 }
2.split()方法
示例：以空格分割knights字符串并以数组形式返回
1 public static void test(){
2 String knights =
3 "Then, when you have found the shrubbery,"
4 + "you must cut down the mightiest tree in the forest... "
5 + "with... a herring";
6 String newString = Arrays.toString(knights.split(" "));
7 System.out.println(newString);
8 }
3.replaceFirst()和replaceAll()方法
示例：替换knights字符串中所有元音字母为下划线
1 public static void test(){
2 String knights =
3 "Then, when you have found the shrubbery,"
4 + "you must cut down the mightiest tree in the forest... "
5 + "with... a herring";
6 String newKnights = knights.replaceAll("[AaEeIiOoUu]", "_");
7 System.out.println(newKnights);
8 }
replaceFirst()方法只在首次出现时替换，replaceAll则替换所有满足条件的部分