Regular-Irregular is the first studio album by South Korean boy band NCT 127, the second sub-unit of the South Korean boy band NCT. Described as a multi-genre concept album with a total of eleven tracks, the album was released by SM Entertainment on October 12, 2018, and distributed by IRIVER. This marks NCT 127's first Korean major release in a year and four months since their previous EP, Cherry Bomb, which was released in June 2017. It is also their first release as a ten-member group since the addition of Jungwoo in September 2018. It is also the group's last release to date to feature member Winwin, as he would focus his promotions on WayV following the album's release.

NCT 127 is the first fixed and second overall sub-unit of the South Korean boy band NCT, formed and managed by SM Entertainment. They originally debuted on July 7, 2016, with their debut extended play NCT #127, with an original lineup of seven members: Taeil, Taeyong, Yuta, Jaehyun, Winwin, Mark, and Haechan. Doyoung and Johnny joined the unit in December 2016, ahead of their second EP Limitless, while Jungwoo, the final member, was introduced in September 2018 ahead of their first studio album Regular-Irregular. They are known for their experimental hip hop-based music, which they refer to as "neo music", as well as their vocal and rap abilities, choreographies, and performances.