利用正则式计算表达式的值

RT,最近又看了下JAVA的正则式,解决了之前对JAVA中正则式的一些疑问.
写了个利用正则式计算表达式值的代码.算法很简单,就是"先乘除,后加减,有括号先算括号里的"
ps:当然,计算表达式值最常用也最有效的方法是利用逆波兰式,这儿只是拿正则式来练练手.
程序没有经过严谨的测试,可能有bug.

java 代码

import  java.util.regex.*;
import  java.util.*;
/**
*利用正则式计算表达式的值
*@author: Eastsun
*@version: 0.5 07.2.26
*/
public   class  Calculate{
    /*
    正则式这块贴上来会影响代码结构,略之.
    详见打包后的代码
    */
     private   static   double  operator(Matcher m, int  index){
        index =index* 2 ;
         double  a,b;
         if (m.group( 1 )!= null ){
            a =Double.parseDouble(m.group( 1 ));
            b =Double.parseDouble(m.group( 2 ));
        }
         else {
            index ++;
            a =Double.parseDouble(m.group( 3 ));
            b =Double.parseDouble(m.group( 4 ));
        }
         double  r = 0.0 ;
         switch (index){
             case   0 :
                r =a+b;
                 break ;
             case   1 :
                r =a-b;
                 break ;
             case   2 :
                r =a*b;
                 break ;
             case   3 :
                r =a/b;
                 break ;
        }
         return  r;
    }
     public   static   double  eval(String str){
        StringBuilder sb = new  StringBuilder(str.replaceAll( "\\s+" , "" ));
         while ( true ){
            Matcher m =BRA.matcher(sb);
             if (m.find()){
                sb.replace(m.start(),m.end(),m.group( 1 ));
            }
             else {
                 int  index = 1 ;
                m =MUL_OR_DIV.matcher(sb);
                 if (!m.find()){
                    index --;
                    m =ADD_OR_SUB.matcher(sb);
                     if (!m.find())  break ;
                }
                sb.replace(m.start(),m.end(), "" +operator(m,index));
            }
        }
         return  Double.parseDouble(sb.toString());
    }
     public   static   void  main(String[] args){
        Scanner scan = new  Scanner(System.in);
         while ( true ){
            System.out.print( "\nEnter a expression :" );
            String str =scan.next();
             if (str.equals( "exit" ))  break ;
            System.out.print(str + " = " +eval(str));
        }
    }
}