Extracting a list of substrings from MongoDB using a Regular Expression

Question

I need to extract a part of a string that matches a regex and return it.

I have a set of documents such as:

{"_id" :12121, "fileName" : "apple.doc"}, 
{"_id" :12125, "fileName" : "rap.txt"},
{"_id" :12126, "fileName" : "tap.pdf"}, 
{"_id" :12126, "fileName" : "cricket.txt"}, 

I need to extract all file extensions and return {".doc", ".txt", ".pdf"}.

I am trying to use the $regex operator to find the sub strings and aggregate on the results but am unable to extract the required part and pass it down the pipeline.

I have tried something like this without success:

aggregate([
  { $match: { "name": { $regex: '/\.[0-9a-z]+$/i', "$options": "i" } } },
  { $group: { _id: null, tot: { $push: "$name" } } }
])

Answer 1

It will be possible to do this in the upcoming version of MongoDB(as the time of this writing) using the aggregation framework and the $indexOfCP operator. Until then, your best bet here is MapReduce.

var mapper = function() { 
    emit(this._id, this.fileName.substring(this.fileName.indexOf(".")))
};

db.coll.mapReduce(mapper, 
                  function(key, value) {}, 
                  { "out": { "inline": 1 }}
)["results"]

Which yields:

[
    {
        "_id" : 12121,
        "value" : ".doc"
    },
    {
        "_id" : 12125,
        "value" : ".txt"
    },
    {
        "_id" : 12126,
        "value" : ".pdf"
    },
    {
        "_id" : 12127,
        "value" : ".txt"
    }
]

---

For completeness here is the solution using the aggregation framework^*

db.coll.aggregate(
    [
        { "$match": { "name": /\.[0-9a-z]+$/i } },
        { "$group": { 
            "_id": null,
            "extension":  { 
                "$push": {
                    "$substr": [ 
                        "$fileName", 
                        { "$indexOfCP": [ "$fileName", "." ] }, 
                        -1 
                    ]
                }
            }
        }}
    ])

which produces:

{ 
    "_id" : null, 
    "extensions" : [ ".doc", ".txt", ".pdf", ".txt" ] 
}

---

_{*current development version of MongoDB (as the time of this writing).}