Overview
Princess_Bubblegum, or princess_bubblegum, is a English written tag for Princess Bubblegum. Due to pixiv tags not having spaces it can be hard to write something in English. Some users have resorted to using a underscore as spaces.
Since PrincessBubblegum is not case-sensitive and is not separated by underscores, partial match search is recommended in pixiv search.
For more details refer to the article Princess Bubblegum.