請參考！

import java.io.*;
public class RegularJude {
public static void main(String[] args) throws IOException {
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
while(true) {
System.out.print("Please Input a String :");
String str = br.readLine();
if (!str.matches("[0-9a-zA-Z_]+")) {
System.out.println("Your Input is not legal !");
} else
System.out.println("God Job !!");
}


}
}

不要數字的話，0-9 拿掉！
!str.matches("[a-zA-Z_]+")