How to find a substring in a field in Mongodb

How can I find all the objects in a database with where a field of a object contains a substring?

If the field is A in an object of a collection with a string value:

I want to find all the objects in the db "database" where A contains a substring say "abc def".

I tried:

db.database.find({A: {$regex: '/^*(abc def)*$/''}})

but didn't work

UPDATE

A real string (in unicode):

Sujet  Commentaire sur  Star Wars  Episode III - La Revanche des Sith 1

Need to search for all entries with Star Wars

db.test.find({A: {$regex: '^*(star wars)*$''}}) not wokring