accountnujen,
введите образец в input .
<body>
<input id="tx" name="">
<output id="out">
<script>
tx.oninput = function() {
let match = this.value.match(/([^\x5c\x2F]+)[\x5c\x2F]([^\x5c\x2F]+[\x5c\x2F][^\x5c\x2F]+)/)
if(match) out.textContent = match.splice(1);
}
</script>
</body>