How to use regular expressions in MongoDB?

Regular expressions in MongoDB are used to filter documents in a collection and are used with the `$regex` operator.

Here’s an example of how to use regular expressions (regex) in MongoDB:

```
db.collection.find( { field: { $regex: /pattern/, $options: ‘’ } } )
```

Here’s an explanation:
- `field`: the field in the document to search.
- `/pattern/`: the pattern to search. This can be any regex pattern.
- ``: Options to modify the search. Here are some options:
- `i`: Case-insensitive search.
- `m`: Multi-line search.
- `x`: Ignore whitespace.
- `s`: Allows `.` to match newline characters.

Here’s an example of how to find all documents in a `users` collection with a `name` field that starts with the letter “A” (case-insensitive):

```
db.users.find( { name: { $regex: /^A/, $options: ‘i’ } } )
```

Do note, regular expressions can sometimes lead to performance issues. So use them judiciously and always try to optimize your regular expressions for best performance.