Mongo regex index matching - multiple start strings - regex

Question

I want to match multiple start strings in mongo. explain() shows that it's using the indexedfield index for this query:
db.mycol.find({indexedfield:/^startstring/,nonindexedfield:/somesubstring/});
However, the following query for multiple start strings is really slow. When I run explain I get an error. Judging by the faults I can see in mongostat (7k a second) it's scanning the entire collection. It's also alternating between 0% locked and 90-95% locked every few seconds.
db.mycol.find({indexedfield:/^(startstring1|startstring2)/,nonindexedfield:/somesubstring/}).explain();
JavaScript execution failed: error: { "$err" : "assertion src/mongo/db/key.cpp:421" } at src/mongo/shell/query.js:L128
Can anyone shed some light on how I can do this or what is causing the explain error?
UPDATE - more info
Ok, so I managed to get explain to work on the more complex query by limiting the number of results. The difference is this:
For a single substring, "^/BA1/" (yes it's postcodes)
"cursor" : "BtreeCursor pc_1 multi",
"isMultiKey" : false,
"n" : 10,
"nscannedObjects" : 10,
"nscanned" : 10,
"nscannedObjectsAllPlans" : 19,
"nscannedAllPlans" : 19,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"indexedfield" : [
[
"BA1",
"BA2"
],
[
/^BA1/,
/^BA1/
]
]
}
For multiple substrings "^(BA1|BA2)/"
"cursor" : "BtreeCursor pc_1 multi",
"isMultiKey" : false,
"n" : 10,
"nscannedObjects" : 10,
"nscanned" : 1075276,
"nscannedObjectsAllPlans" : 1075285,
"nscannedAllPlans" : 2150551,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 5,
"nChunkSkips" : 0,
"millis" : 4596,
"indexBounds" : {
"indexedfield" : [
[
"",
{
}
],
[
/^(BA1|BA2)/,
/^(BA1|BA2)/
]
]
}
which doesn't look very good.

Answer 1

$or solves the problem in terms of using the indexes (thanks EddieJamsession). Queries are now lightening fast.
db.mycoll.find({$or: [{indexedfield:/^startstring1/},{indexedfield:/^startstring2/],nonindexedfield:/somesubstring/})
However, I would still like to do this with a regex if possible so I'm leaving the question open. Not least because I now have to refactor my application to take these types of queries into account.