<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;
	}
}
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