Use the $in Operator with a Regular Expression in Java

Question

I am trying to write $in query with $regex in mongo + java. It is working fine from mongo shell and I am getting the results:

db.getCollection('collection').find({ "_id" : { "$in" : [ /^1332444344/]}})

However, when I try to executed from my java app, I get no results:

DBCollection collection = db.getCollection("collection");
BasicDBObject query = new BasicDBObject();
inQuery.put("_id", new BasicDBObject("$in", input));
DBCursor cursor = collection.find(query);

// This returns no results
// Query: { "_id" : { "$in" : [ { "$regex" : "/^1332444344/"} ] }}
input = Lists.newArrayList(Pattern.compile("/^1332444344/"));

// The following as well returned nothing
// Query: { "_id" : { "$in" : [ "/^1332444344/" ] }}
input = Lists.newArrayList("/^1332444344/"));

The Java Code is adding the double quotes around regex and it seems this is causing the issue.

I also tried the following but got a json parse exception:

BasicQuery bq = new BasicQuery("{ \"_id\" : { \"$in\" : [ /^1332444344/ ]}}")

Is there any workaround for this issue?

Thanks

Answer 1

I think it's the / s that are really causing the issue. You can't have a / before the start of string ( ^ ).

Java doesn't use / to indicate the start and end of regexes as other languages do. It uses " to mark the start and end, since regexes are written like any other string.

Remove the / , at least at the start of the regex; you may well want to remove the one at the end as well (depends on what you're actually trying to match).