https://codepen.io/littlelyon.

regular string substitution ~ number with a tag < span style=" color:red; "> 123

is not proficient in regularization, but it is also implemented:

        <style>
        .onlyone{
            color: red;
        }
    </style>
    <div id="div">
        123456789
    </div>
    
    <script>
        var ins = document.getElementById('div');
        var arr = ins.innerText.match(/\d{1,}/g);
        ins.innerHTML = ins.innerText.replace(/\d{1,}/g,'<span class="onlyone"></span>')
        var el = document.querySelectorAll('.onlyone');
        el.forEach(function(item,i){
            item.innerText = arr[i]
        })
    </script>

HTML

<p id="demo">
  123456789

STYLE

<style type="text/css">
    -sharpdemo span {
        color: red;
    }
</style>

JS

let text = document.getElementById("demo").innerText;
let html = text.replace(/(\d+)/g, `<span>$1</span>`);
document.getElementById("demo").innerHTML = html;