java pattern regex accepting string or string followed by digit

Having trouble with this piece of code. It is suppose to accept the following inputs, U,R,E,X or +,-,*,/ followed by a space and digit

   public static void main(String[] args) {
   Scanner input = new Scanner( System.in );//scanner
   System.out.println("COMP10152 - Lab#5 - Calculator using Queues and Stacks ... by _______________ \n");
   System.out.println("Enter tokens. Legal tokens are integers, +, -, *, /, U[ndo], R[edo], E[valuate] and [e]X[it] ");
   while (!input.hasNext("[UREXurex]?[+-*/]{1}[0-9][1]")) {
System.out.println("Invalid Entry - Please enter operation (operator(+,-,*,/) value) or U,R,E,X");
input.next();
   }