JSFiddle - React, Tailwind, and code Playground
JavaScript
var str='<img class="avatar avatar-small 012345" src="/img/staff_avatar_profile.jpg" /> olah blah';
var div = document.createElement('div');
div.innerHTML = str;
var numbers = div.getElementsByTagName('img')[0].className.match(/ (\d+)/)[1];
alert(numbers);