Core server unit tests show that it's permissible to leave off the $options field, as requested in SERVER-275 :
https://github.com/mongodb/mongo/blob/8190d8db538c629973a3a581a97251b22b668c7a/jstests/regex2.js#L43-47
However, Java driver will error out if one doesn't supply the $options field with a $regex query form. The following line is caller's ticket to (iirc) a NullPointerException :
https://github.com/mongodb/mongo-java-driver/blob/master/src/main/com/mongodb/util/JSONCallback.java#L94