I don't see any need for regular expressions here. The MongoDB query syntax allows you to do this kind of query directly e.g.
check out these examples, including this one (example 2.3 on the linked page):
Fetches documents (records) where the "number" field is > 2 and < 5.
If you're unsure of how to query stuff on MongoDB, maybe take some time to practice queries via
the MongoDB shell, which provides a
JSON API for queries. As an old RDBMS developer, I really like MongoDB's query tools, including the powerful and flexible
aggregation framework. But just like with an RDBMS, it's usually better to build your query via the DB's shell (SQL for an RDBMS, or MongoDB shell here) first, to make sure the query's right before you mix it up with all the
Java stuff.