I recently tried to upgrade from 2.4.9 to 2.5.5 and I'm experiencing catastrophic regex query times with 2.5.5 so I had to downgrade again. My query simply is sth. like this:
{
"folder":/^weblogic/,
"type":"file"
}

2.4.9 gives me this query plan:
"cursor": "BtreeCursor type_folder multi",
"nscanned": NumberInt(2583),

2.5.5 gives this query plan:
"cursor": "Complex Plan",
"nscanned": NumberInt(8470527),

This plan is then running for literally ages
I wonder why 2.5.5 isn't even considering my "type_folder" index for this simple query.

Unfortunately I cannot provide test data because I could only reproduce the problem with our live database which data is quite large and non-disclosable. Nevertheless here is the full explain plan for 2.5.5 in case you can discern anything:

{
"cursor": "Complex Plan",
"n": NumberInt(2582),
"nscannedObjects": NumberInt(0),
"nscanned": NumberInt(8470527),
"nscannedObjectsAllPlans": NumberInt(0),
"nscannedAllPlans": NumberInt(8470527),
"nYields": NumberInt(66196),
"nChunkSkips": NumberInt(0),
"millis": NumberInt(4006),
"allPlans": [

{ "cursor": "Complex Plan", "n": NumberInt(2582), "nscannedObjects": NumberInt(0), "nscanned": NumberInt(8470527), "nChunkSkips": NumberInt(0) }

],
"server": "edison:27017",
"stats": {
"type": "SORT",
"works": NumberInt(8473112),
"yields": NumberInt(66196),
"unyields": NumberInt(66196),
"invalidates": NumberInt(0),
"advanced": NumberInt(2582),
"needTime": NumberInt(8470528),
"needFetch": NumberInt(0),
"isEOF": NumberInt(1),
"forcedFetches": NumberInt(0),
"children": [
{
"type": "FETCH",
"works": NumberInt(8470528),
"yields": NumberInt(66196),
"unyields": NumberInt(66196),
"invalidates": NumberInt(0),
"advanced": NumberInt(2582),
"needTime": NumberInt(8467945),
"needFetch": NumberInt(0),
"isEOF": NumberInt(1),
"alreadyHasObj": NumberInt(0),
"forcedFetches": NumberInt(0),
"matchTested": NumberInt(0),
"children": [
{
"type": "AND_HASH",
"works": NumberInt(8470527),
"yields": NumberInt(66196),
"unyields": NumberInt(66196),
"invalidates": NumberInt(0),
"advanced": NumberInt(2582),
"needTime": NumberInt(8467945),
"needFetch": NumberInt(0),
"isEOF": NumberInt(1),
"flaggedButPassed": NumberInt(0),
"flaggedInProgress": NumberInt(0),
"mapAfterChild_0": NumberInt(3459),
"children": [
{
"type": "IXSCAN",
"works": NumberInt(3460),
"yields": NumberInt(66196),
"unyields": NumberInt(66196),
"invalidates": NumberInt(0),
"advanced": NumberInt(3459),
"needTime": NumberInt(0),
"needFetch": NumberInt(0),
"isEOF": NumberInt(1),
"keyPattern": "

{ folder: 1 }

",
"bounds":

{ "folder": [ [ "weblogic", "weblogid" ], [ "/^weblogic/", "/^weblogic/" ] ] }

,
"isMultiKey": NumberInt(0),
"yieldMovedCursor": NumberInt(0),
"dupsTested": NumberInt(0),
"dupsDropped": NumberInt(0),
"seenInvalidated": NumberInt(0),
"matchTested": NumberInt(0),
"keysExamined": NumberInt(3460),
"children": [

]
},
{
"type": "IXSCAN",
"works": NumberInt(8467067),
"yields": NumberInt(66196),
"unyields": NumberInt(66196),
"invalidates": NumberInt(0),
"advanced": NumberInt(8467067),
"needTime": NumberInt(0),
"needFetch": NumberInt(0),
"isEOF": NumberInt(1),
"keyPattern": "

{ type: 1 }

",
"bounds":

{ "type": [ [ "file", "file" ] ] }

,
"isMultiKey": NumberInt(0),
"yieldMovedCursor": NumberInt(0),
"dupsTested": NumberInt(0),
"dupsDropped": NumberInt(0),
"seenInvalidated": NumberInt(0),
"matchTested": NumberInt(0),
"keysExamined": NumberInt(8467067),
"children": [

]
}
]
}
]
}
]
}
}