This phrase was used to describe Donald Trump in an article in Salon magazine.
Donald Trump is a better politician than a lot of people give him credit for. Granted, he often has no idea what he’s talking about, he contradicts himself on the regular, he has a repellent personality, he’s a misogynist and a bigot, and it feels like he stumbled ass-backwards into the Republican presidential nomination.
According to multiple slang dictionaries, including Online Slang Dictionary, "on the regular" is well-established slang for "often," "frequently," or "routinely." It is mainstream enough to be found in reputable print journalism (see below for that).
regularly; frequently. Also on the reg.
Ex: I hit up that spot on the regular.
- Online Slang Dictionary
Searching Ngram isn't very useful for this question because the phrase can be part of typically grammatical sentences, like "It wasn't on the regular menu."
Most of the anecdotal uses that I've been able to find have been within the last year or two, but I expect that it must have been a slang expression for longer than that in order to show up in the likes of Esquire and Rolling Stone.
When did this phrase come into vogue?
Is it uniquely American slang, and did it originate with a specific region or culture?
To vouch for the legitimacy of this phrase and prove that this is a serious question, here are few uses from major print publications:
Although it's damn near impossible to wake up flawless like Beyoncé on the regular, it is possible to get pro concealing tips from her makeup artist
- Cosmopolitan (2017)
It doesn't matter whether you and your buddy from work go out for drinks on the regular and play on the same intramural co-ed after-work bowling team: You still shouldn't trust them to keep your secrets.
- Esquire (2017)
Understanding West Virginia has taken on a newfound relevance since the aftermath of the most recent Presidential election. The state has occupied an unlikely place at the center of American politics so far this century, inspiring thinkpieces on the regular every four years.
- Rolling Stone (2017)