紧急，怎样在数字前面加下划线呢

[supergirlbarbie]

class StrPart{
public static void main(String args[]){
String s="student teacher I Beijing123 shanghai nanjing You.";
String text="",word=null;
int i=0,start,end;
char ch;
start=0;
end=s.length();

s.trim();
while(start<end)
{ s=s.trim();
for(i=0;i<s.length();i++){
ch=s.charAt(i);
if(ch==' ')
break;

}
end=i;
word=s.substring(start,end);
//首字母变大写
/*String temp="";
int k;
for(k=0;k<word.length();k++)
{
ch=word.charAt(k);
if(ch>='a'&& ch<='z'&&k==0)
ch=(char)(ch-32);
temp+=ch;
}
//数字前插入下划线
???
text+=word+" ";
s=s.substring(end,s.length());
start=0;
end=s.length();
}
System.out.println(text);
}
}

[hybeureka]

估计得用正则...

[supergirlbarbie]

我刚入门，还不懂正则是什么