[{_id:123,key2:["Mermaid / Fairy","Superhero"]},{_id:456,key2:["Slug"]},{_id:567,key2:["Slug","Grimace"]},{_id:678,key2:["Slug","Superhero"]},{_id:789,key2:["Mermaid / Fairy","Slug"]}]

db.collection.aggregate([{$match:{key2:{$regex:"Mermaid / Fairy|Superhero"}}}])