title: Rgegexpression - select according to occurrences
body: <p>I have <br>
  [1] "43300" "22222" "22222" "22222" "22222" "22222" "44200" "32122" "22222" "22222" "55000" "22222" "55000" "22222" "33220" "22222" <br>
  [17] "22222" "22222" "22222" "22222" "22222" "22222" "32221" "22222" "55000" "22222" "22222" "22222" "22222" "22222" "33220" "33310" <br></p>
  
  <p>How do I select the ones (inside "") with</p>
  
  <p>a) no occurrence of 0 <br>
  b) exactly 1 occurrence of 0 <br>
  c) exactly 2 occurrences of 0 <br>
  d) exactly 3 occurrences of 0 etc. <br></p>
  
  <p>for a) ([1-5^0]+) didn't work</p>