JSFiddle - React, Tailwind, and code Playground

JavaScript

var a = 'F5';
var b = '5F';
var c = '555';
var d = '-5';

if (a.match(/^[0-9]\d*$/))
    console.log('a is valid');

if (b.match(/^[0-9]\d*$/))
    console.log('b is valid');

if (c.match(/^[0-9]\d*$/))
    console.log('c is valid');

if (d.match(/^[0-9]\d*$/))
    console.log('d is valid');