(function () {
      const boxes = Array.from(document.querySelectorAll('.captcha'));
      const form  = boxes[0]?.form || null;

      boxes.forEach((inp, i) => {
        inp.maxLength = 1;

        // Only allow A-Z0-9 (case-insensitive). Change to /^\d$/ for digits only.
        inp.addEventListener('beforeinput', (e) => {
          if (e.inputType === 'insertText' && e.data && !/^[A-Za-z0-9]$/.test(e.data)) {
            e.preventDefault();
          }
        });

        // Handle typing & paste
        inp.addEventListener('input', (e) => {
          let v = inp.value.toUpperCase();

          // If user pasted multiple chars, spread them across the following inputs
          if (v.length > 1) {
            const chars = v.replace(/\s+/g, '').split('');
            // Put first char here
            inp.value = chars.shift() || '';
            // Fill subsequent boxes
            let j = i + 1;
            while (chars.length && j < boxes.length) {
              boxes[j++].value = chars.shift();
            }
            // Focus the next empty box (or last one)
            const nextEmpty = boxes.find((b) => b.value === '');
            (nextEmpty || boxes[boxes.length - 1]).focus();
          } else {
            // Normal single char
            inp.value = v;
            if (v && i < boxes.length - 1) boxes[i + 1].focus();
          }

        });

        inp.addEventListener('keydown', (e) => {
          if (e.key === 'Backspace' && !inp.value && i > 0) {
            e.preventDefault();
            boxes[i - 1].focus();
            boxes[i - 1].value = '';
          } else if (e.key === 'ArrowLeft' && i > 0) {
            e.preventDefault(); boxes[i - 1].focus();
          } else if (e.key === 'ArrowRight' && i < boxes.length - 1) {
            e.preventDefault(); boxes[i + 1].focus();
          }
        });

        inp.addEventListener('focus', () => inp.select());
      });
    })();