Emphasis edge cases

Four stars ** and three stars and five stars ** end. Bold with star inside ab and italic with inside ab* done. Adjacent onetwothree and *ab and abc patterns. Empty bold * and empty spaced and underscore. Unclosed bold to end of line here. Unclosed italic to end and _under to end. Mixed nested markers and _other way_ around. Star at start word and end word alone. Under at start word and end word alone. Bold under and a_b and ab here. Triple _three unders and a_ done. Wordboldword no spaces and wordem*word tight.