how to catch db error

i have insert new row to table in same time  ( 10 users )

my primary key is determined from user input with in 3 groups

Blue
Yellow
Green

if user select Blue ID must be B001, B002
Yellow Y001, Y002


i have check to get last Id using query but sometimes getting error duplicate key
i want to catch db error so i can change ID and try to insert again

    try{
      $result = $this->db->insert($table,$data);
      echo $result;
      print  $result;
    }
    catch(Exception $e)
    {
       echo "Test Error"
        //get new ID
        // Insert data with new ID
    }

but now working