$在java编程中有什么意思吗？
则么用它呢？

p=p.replaceAll(" +", " ");
        String reg1="(\\d)()([a-zA-Z])";
        String reg2="([a-zA-Z])()(\\d)";
        p=p.replaceAll(reg1,"$1_$3");
        p=p.replaceAll(reg2,"$1_$3");
中$怎么理解呢？