$in 操作符

【$in 操作符】

　　The $in operator selects the documents where the value of a field equals any value in the specified array.

　　$in 选择包含指定值之一的 doc。

　　

　　If the field holds an array, then the $in operator selects the documents whose field holds an array that contains at least one element that matches a value in the specified array

Use the $in Operator with a Regular Expression

The $in operator can specify matching values using regular expressions of the form /pattern/. Youcannot use $regex operator expressions inside an $in.

Consider the following example:

db.inventory.find( { tags: { $in: [ /^be/, /^st/ ] } } )

This query selects all documents in the inventory collection where the tags field holds an array that contains at least one element that starts with either be or st.

参考：https://docs.mongodb.org/manual/reference/operator/query/in/#op._S_in