Which regulation are you talking about? Do you have a link? I'd be interested in reading it.
"I recused myself not because of any asserted wrongdoing on my part during the campaign," Sessions said. "But because a Department of Justice regulation, 28 CFR 45.2, required it.""That regulation states, in effect, that department employees should not participate in investigations of a campaign if they have served as a campaign advisor," said Sessions.
28 CFR 45.2 - Disqualification arising from personal or political relationship. <- Link
I think the regulation is cited in post 189. (28 CFR 45.2)