Just cast the `string` to `bytes`

     
       function testStr(string str) constant returns (bool){
            bytes memory b = bytes(str);
            if(b.length != 7) return false;
            for(uint i; i<7; i++){
                if(i==1){
                    if(b[i] != 45) return false;
                }
                else if (i==4){
                    if(b[i] != 58) return false;
                }
                else if(b[i] < 48 || b[i] > 57) return false;
                
            }
        
            return true;
        }