JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" name="size" onkeyup="test(this)">
    <div name="length" id="length"></div>
    <div name="width" id="width"></div>

JavaScript

function test(obj) {
    var len = document.getElementById('length'),
        wid = document.getElementById('width');
        match = '';
    if( match = /^(\d{2,3}|1000)x(\d{2,3}|1000)$/.exec(obj.value)) {
        len.innerHTML = match[1];
        wid.innerHTML = match[2];
    } else {
        len.innerHTML = wid.innerHTML = '';  
    }
}