Any functionality in a MongoDB Aggregation like a simple "regex replace"?

Question

I'm trying to create a step in a MongoDB aggregation pipeline which will replace the value of a full Social Security Number (like "123-45-6789") with a masked value, preserving the last four digits (like "XXX-XX-6789").

I see $replaceAll and $replaceOne but it looks like those only accept string literals, not regex expressions. I see there is a regex find that looks like it can very clumsily fit into a replace stage:

"maskedSsn" : {
    "$replaceOne" : { 
        "input" : "$ssn",
        "find" : { 
            "$regexFind" : { 
                "input" : "$ssn", 
                "regex" : "\\d{3}-\\d{2}\\-"
            }
        },
        "replacement" : "XXX-XX-"
    }
}

But with this I get the error "$replaceOne requires that 'find' be a string, found: {match: \"000-00-\", idx: 0, captures: []} (the SSN is mocked, random data here). I'm not sure how to just get the "match" out of the resultant object as part of this step. I could add this regex match as a different field, then get the match in this stage of my aggregation for a simple string replacement, but I would prefer to keep all the logic in one step if possible.

It's hard for me to believe that this isn't possible in one step, with something like a regex replace (which is a feature of every language I've used). Is there a functionality that I'm missing?

Answer 1

There is no generalized regexReplace operator in MongoDB.

You could kind of get where you want using $regexFind, like

{$regexFind: {
   input:"$ssn",
   regex:"^\\d{3}-\\d{2}-(\\d{4})$"
}}

That would return an object similar to:

{ 
  match: "123-45-6789",
  idx: 0
  captures: ["6789"]
}

You could use $getField, $arrayElemAt and $concat to extract the captures part, then do what you need with that.

db.collection.aggregate([
  {$addFields: {
      maskedssn: {
        $concat: [
          "XXX-XX-",
          {$arrayElemAt: [
              {$getField: {
                  field: "captures",
                  input: {$regexFind: {
                      input: "$ssn",
                      regex: "^\\d{3}-\\d{2}-(\\d{4})$"
                    }}
              }},
              0
          ]}
        ]
      }
 }}
])

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