===== ISTEM =====
Solve the following programming task.

TASK: Deep Clone an Object

Write a function 'deepClone(obj)' that deeply copies a given JavaScript object or array (including nested objects, arrays and primitives) without sharing references. It must also clone Date and RegExp objects.

STARTER CODE (javascript):
function deepClone(obj) {
  // Write your code here
}

module.exports = { deepClone };


RULES:
- Keep the function name and signature EXACTLY as given.
- Return working code only. No explanations.
- Put the code in a single ``` block.
- Try to solve it first. If you genuinely cannot, write only this single
  line instead of producing faulty code: CANNOT_SOLVE

===== HAM YANIT =====
```javascript
function deepClone(obj, seen = new WeakMap()) {
  // Primitives and functions are returned as-is
  if (obj === null || typeof obj !== 'object') {
    return obj;
  }

  // Handle circular references
  if (seen.has(obj)) {
    return seen.get(obj);
  }

  // Date
  if (obj instanceof Date) {
    return new Date(obj.getTime());
  }

  // RegExp
  if (obj instanceof RegExp) {
    const clone = new RegExp(obj.source, obj.flags);
    clone.lastIndex = obj.lastIndex;
    return clone;
  }

  // Array
  if (Array.isArray(obj)) {
    const clone = [];
    seen.set(obj, clone);
    for (let i = 0; i < obj.length; i++) {
      clone[i] = deepClone(obj[i], seen);
    }
    return clone;
  }

  // Plain object (preserve prototype)
  const clone = Object.create(Object.getPrototypeOf(obj));
  seen.set(obj, clone);
  for (const key of Object.keys(obj)) {
    clone[key] = deepClone(obj[key], seen);
  }
  return clone;
}

module.exports = { deepClone };
```