_012_java_正则表达式实现的加减乘除四则运算的计算器

来自：https://blog.csdn.net/flying_fish_wj/article/details/52097903，感谢作者的无私分享。

其中Pattern.compile("\\(([^()]+)\\)")类型的编译模式中的matcher.start()指向的是“（”，而mathcer.end()指向的是“ ）”的下一位。

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1. import java.util.regex.Matcher;  
2. import java.util.regex.Pattern;  
3.   
4.   
5. public class Calculator {  
6.     public static void main(String[] args) {  
7.         String src = "(3 + (5 - 2) * 10 / 2 * 3 + 15) * (8 - 4)";  
8.         System.out.println(cal(src));  
9.     }  
10.     public static String cal(String src) {  
11.         StringBuilder builder = new StringBuilder();  
12.         if (src.contains("(")) {  
13.             Pattern pattern = Pattern.compile("\\(([^()]+)\\)");  
14.             Matcher matcher = pattern.matcher(src);  
15.             int lastEnd = 0;  
16.             while (matcher.find()) {  
17.                   
18.                 builder.append(src.substring(lastEnd, matcher.start()));  
19.                 System.out.println(builder.toString());  
20.                 builder.append(cal(matcher.group(1)));  
21.                 lastEnd = matcher.end();  
22.             }  
23.             builder.append(src.substring(lastEnd));  
24.         } else {  
25.             Pattern pattern = Pattern.compile("([\\d.]+)\\s*([*/])\\s*([\\d.]+)");  
26.             builder.append(src);  
27.             Matcher matcher = pattern.matcher(builder.toString());  
28.             while (matcher.find()){  
29.                 float f1 = Float.parseFloat(matcher.group(1));  
30.                 float f2 = Float.parseFloat(matcher.group(3));  
31.                 float result = 0;  
32.                 switch (matcher.group(2)){  
33.                     case "*":  
34.                         result = f1 * f2;  
35.                         break;  
36.                     case "/":  
37.                         result = f1 / f2;  
38.                         break;  
39.                 }  
40.                 builder.replace(matcher.start(), matcher.end(),  
41.                         String.valueOf(result));  
42.                 matcher.reset(builder.toString());  
43.             }  
44.             pattern = Pattern.compile("([\\d.]+)\\s*([+-])\\s*([\\d.]+)");  
45.             matcher = pattern.matcher(builder.toString());  
46.             while (matcher.find()){  
47.                 float f1 = Float.parseFloat(matcher.group(1));  
48.                 float f2 = Float.parseFloat(matcher.group(3));  
49.                 float result = 0;  
50.                 switch (matcher.group(2)){  
51.                     case "+":  
52.                         result = f1 + f2;  
53.                         break;  
54.                     case "-":  
55.                         result = f1 - f2;  
56.                         break;  
57.                 }  
58.                 builder.replace(matcher.start(), matcher.end(),  
59.                         String.valueOf(result));  
60.                 matcher.reset(builder.toString());  
61.             }  
62.             return builder.toString();  
63.         }  
64.         System.out.println(builder);  
65.         return cal(builder.toString());  
66.     }  
67. }