JSFiddle - React, Tailwind, and code Playground

HTML

<input type='text' id="TWidth" value="4-1/2" onKeyUp="valid_Dimensions_Width(this);" />
<div id="TWidthError" class="error"></div>

CSS

.error { color: red }

JavaScript

function valid_Dimensions_Width(fieldObj) {
    // regular expression
    var error = ""
    var rgexp = /^([0-9]+|[0-9]+-[0-9]+\/[0-9]+|[0-9]+\/[0-9]+)$/;
    var input = fieldObj.value;
    if (!input.match(rgexp)) error = "The width field format is incorrect!";
    document.getElementById("TWidthError").innerHTML = error;
}