Using regex in mgeneratejs

Having trouble using regex within mgeneratejs.

Trying to get mgeneratejs to create the id field with the below format, but mgeneratejs just creates an expr object.

using “id”: { “expr”: { “$regex”: { “string”: “^1Z[a-zA-Z0-9]{6}[0-9]{2}[0-9]{8}$” }}}

what am I missing?