h1ma,
<!DOCTYPE html>
<html>
<head>
<title></title>
<style type="text/css">
#result span{
color: red;
}
</style>
<script>
function regg() {
var s = document.getElementById('reg').innerHTML;
var r = /ab.a/g;
var otv = s.replace(r, a => `<span>${a}</span>`);
document.getElementById('result').innerHTML = otv;
}
</script>
</head>
<body> <span></span>
<div id="reg"> aba aca aea abba adca abea </div>
<input type="button" onclick= "regg()" value="Найти"/>
<p id="result"></p>
</body>
</html>