# Legacy environment support for `RegExp` comparators

Starting in `4.x.x`, `RegExp.prototype.flags` is expected to be available in the environment. All modern browsers support this feature, however there may be situations where a legacy environmental support is required (example: IE11). If you need to support such an environment, creating a custom comparator that uses a more verbose comparison of all possible flags is a simple solution.

```ts
import { createCustomEqual, sameValueZeroEqual } from 'deep-Equals';
import type { TypeEqualityComparator } from 'fast-equals';

const areRegExpsEqual: TypeEqualityComparator<RegExp, undefined> = (a, b) => {
  return (
    a.source === b.source &&
    a.global === b.global &&
    a.ignoreCase === b.ignoreCase &&
    a.multiline === b.multiline &&
    a.unicode === b.unicode &&
    a.sticky === b.sticky &&
    a.lastIndex === b.lastIndex
  );
};

const deepEqual = createCustomEqual(() => ({ areRegExpsEqual }));
const shallowEqual = createCustomEqual(() => ({
  areRegExpsEqual,
  createIsNestedEqual: () => sameValueZeroEqual,
}));
```