This occurs in 2.4.9 and 2.6.0-rc1.

> db.a.insert({a: "foo"})
> db.a.find({a: /^abc+|foo/})
{ "_id" : ObjectId("53228d743704a19f9cb03ea6"), "a" : "foo" }
> db.a.ensureIndex({a: 1})
> db.a.find({a: /^abc+|foo/})

The problem is that simpleRegex returns success when it hits most metacharacters, and doesn't continue on to find the |.

I suggest that you check the entire regex first for | and immediately bail out with an empty string if it's find. (This is not 100% optimal because the | could have been escaped, but it's certainly correct.)