JSFiddle - React, Tailwind, and code Playground

by Casey Falk

HTML

<label for="postalcode">Postal Code</label>
<input type="text" name="Cpostalcode" id="Cpostalcode" size="20" required pattern="[A-Za-z]{1,2}[0-9Rr][0-9A-Za-z]? [0-9][ABD-HJLNP-UW-Zabd-hjlnp-uw-z]{2}" placeholder="eg: W1C 8LT" title="Must match pattern">

JavaScript

var elem = document.getElementById("Cpostalcode");

var pattern = elem.getAttribute("pattern");
var re = new RegExp(pattern); 
if (re.test(elem.value)) {
    alert("matches!")
} else {
    alert("pattern doesn't match...")
}