<input maxlength='7' value='0123456'/>

$char-w: 1ch;
$gap: .5*$char-w;
$n-char: 7;
$in-w: $n-char*($char-w + $gap);

input {
  display: block;
  margin: 2em auto;
  border: none;
  padding: 0;
  width: $in-w;
  background: repeating-linear-gradient(90deg, 
    dimgrey 0, dimgrey $char-w, 
    transparent 0, transparent $char-w + $gap) 
    0 100%/ #{$in-w - $gap} 2px no-repeat;
  font: 5ch droid sans mono, consolas, monospace;
  letter-spacing: $gap;
  
  &:focus {
    outline: none;
    color: dodgerblue;
  }
}