Inputmask/micromasks/mm.js

(() => {
  window.mm = (i, mask) => {
    const d = { 9: "[0-9]", a: "[a-z]" },
      msk = mask.split("");
    i.addEventListener("input", handler);
    i.addEventListener("focus", handler);

    function handler(e) {
      if (e.type == "focus" && i.value !== "") return;
      const mskd = [];
      let s = i.selectionStart - 1;
      msk.forEach((el, n) => {
        if (d[el]) {
          const t = new RegExp(d[el], "i").test(i.value.charAt(n));
          mskd.push(t ? i.value.charAt(n) : "_");
          if (t && s === n && i.value.charAt(n) !== "_") {
            s++;
          }
        } else {
          mskd.push(el);
          if (s === n) s++;
        }
      });
      i.value = mskd.join("");
      i.selectionStart = i.selectionEnd = s < 0 ? 0 : s;
      setTimeout(function () {
        i.selectionStart = i.selectionEnd = s < 0 ? 0 : s;
      }, 0);
    }
  };
})();