This originally appeared in a test failure in python-bsonjs.

This is the failing test: https://github.com/mongodb-labs/python-bsonjs/blob/2947fc596c76fa82e47b903785bf1411f1c9be05/test/test_bsonjs.py#L146-L149. It fails with these logs:

ERROR: test_regex (test.test_bsonjs.TestBsonjs)
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Traceback (most recent call last):
File "/home/travis/build/mongodb-labs/python-bsonjs/test/test_bsonjs.py", line 147, in test_regex
res = bsonjs_loads('{"r": {"$regex": "a*b"}}')['r']
File "/home/travis/build/mongodb-labs/python-bsonjs/test/test_bsonjs.py", line 63, in bsonjs_loads
return to_object(bsonjs.loads(json_str))
ValueError: Missing "$options" after "$regex"

This specific test is loading legacy regex syntax without the $options field:

{"r": {"$regex": "a*b"}}

Though based on _bson_json_read_append_regex also rejects the canonical representation of a regex if the options field was missing, like:

{"r": {"$regularExpression": { "pattern": "a*b" }}

If libbson is passing the BSON corpus tests, I imagine it is not required by the spec that we permit regular expressions without options field. But if other tools may produce this (as the comment in test_bsonjs.py says), it seems worthwhile to not reject.

For more info about regex representation in JSON, see the extended JSON spec: https://github.com/mongodb/specifications/blob/master/source/extended-json.rst