[рони]

Teamur,

<div id="out"></div>
<script>
let s='a_4_8_15_16 , b_23_42'; // и т.д.
for(s of s.matchAll(/[^,\s]+/g)){
s = s[0].match(/[a-z]|_\d+/g)
out.append(JSON.stringify(s))
};

</script>