1 import java.util.regex.Matcher;
 2 import java.util.regex.Pattern;
 3 
 4 //已知一段英文，查找里面由三个字母构成的单词的个数
 5 public class redex1 {
 6     public static void main(String[] args) {
 7         String s="IT is within my memory that Melville Clarendon, a lad of sixteen years, was riding through Southern Minnesota, in company with his sister Dorothy, a sweet little miss not quite half his own age.";
 8         //定义正则表达式，即3个字母并且两边是边界
 9         String redex="\\b\\w{3}\\b";
10         Pattern p=Pattern.compile(redex);
11         Matcher m=p.matcher(s);
12         while (m.find()){
13             System.out.println(m.group());
14         }
15 
16     }
17 }

结果：