[SERVER-30986] Server should error on invalid regex match expressions - MongoDB Jira

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The server accepts invalid regular expressions in the match language. It appears that such invalid regular expressions will match nothing:

> db.c.drop();

true

> db.c.insert({a: 1, b: "["});

WriteResult({ "nInserted" : 1 })

> db.c.find({a: 1, b: {$regex: "["}});

// Empty result set.

In order to fix this, the MatchExpression library should call pcrecpp::RE::error() and check whether the resulting error string is non-empty:

This bug appears to affect all versions of the server since at least 2.6.

causes

SERVER-34933 pcre verb support

related to

SERVER-32356 Use of options:x and comments with $regex search including \n can lead to incorrect documents being returned

SERVER-39697 Regex MatchExpression should error at parse time if the regex is not valid