I would argue that calling "regular" warfare "regular" has always been a misnomer. Over the course of human history, what we now term irregular or asymmetric is in fact far and away the dominant form of conflict. That's a throw-away point in the context of this discussion though.

More importantly to my way of looking at things is the fact that our division of warfare into regular and irregular is itself entirely artificial and very, very modern. The idea that armies only fight armies within a limited sphere of engagement and that anything else constitutes an aberration only dates back a few centuries, and has only ever been applied selectively. Before that, the distinction between regular and irregular breaks down, and, more importantly, people didn't seem to have been thinking about warfare as being divisible in that way.

Strategikon XI is a great example of what I'm trying to say. Maurice contrasts there the fighting styles of the different races the Byzantine Empire was at the time facing. His section on the Slavs has decidedly "irregular" undertones. He speaks for example of campaigning in the winter when enemy food supplies will be at their lowest, of how to deal with Slavic guerrilla warfare, and of how to pacify villages (clear and hold, rather than sweeping, incidentally). Go back a couple pages though, and he's talking about how to come to grips with Persian archer superiority, or how to deal with Frankish infantry in close battle. What I'm trying to get at is that the hard and fast line between fighting armies and fighting peoples just wasn't there, even in the most professional of pre-modern armies.

Even getting into the modern era I don't think you see the same division that we've had over the last half century or so. The pacification of the Caribbean was a decidedly irregular affair, as was colonial expansion through the 18th and 19th centuries. The British colonial wars of the early half of the 20th century (Ireland and Palestine in particular come to mind) were considered to be at least partially military exercises, despite the lack of a regular opponent. We always considered the Marine Corps to be an expeditionary force, and used it very irregularly throughout Central and South America. Professional militaries don't seem to have fully shied away from the irregular until the last couple decades really, even if the irregular component of the military experience might not have been as heavily treated in doctrine as what we call the regular component.