regex - MongoDB, performance of query by regular expression on indexed fields

Question

I want to find an account by name (in a MongoDB collection of 50K accounts)

In the usual way: we find with string

db.accounts.find({ name: 'Jon Skeet' })  // indexes help improve performance!

How about with regular expression? Is it an expensive operation?

db.accounts.find( { name: /Jon Skeet/ }) // worry! how indexes work with regex?

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Edit:

According to WiredPrairie:
MongoDB use prefix of RegEx to lookup indexes (ex: /^prefix.*/):

db.accounts.find( { name: /^Jon Skeet/ })  // indexes will help!'

MongoDB $regex

Answer 1

Actually according to the documentation,

If an index exists for the field, then MongoDB matches the regular expression against the values in the index, which can be faster than a collection scan. Further optimization can occur if the regular expression is a “prefix expression”, which means that all potential matches start with the same string. This allows MongoDB to construct a “range” from that prefix and only match against those values from the index that fall within that range.

http://docs.mongodb.org/manual/reference/operator/query/regex/#index-use

In other words:

For /Jon Skeet/ regex ,mongo will full scan the keys in the index then will fetch the matched documents, which can be faster than collection scan.

For /^Jon Skeet/ regex ,mongo will scan only the range that start with the regex in the index, which will be faster.