Anyone who thinks that it is the Russian regular army that the Ukrainians are fighting, are deluded fools. If Putin was going to invade, he would blitzkrieg Ukraine and it would be over in less than 24 hours.
And he would instantly lose the popular support that he has been building for years. Loss of support = loss of money. That's why he's never going to invade the old fashioned way until or unless the other side is provoked into making an even bigger provocation.
Instead, you have an all-volunteer militia army that is comprised mostly of locals of Donbass, Donetsk, Lugansk with some international volunteers (there are some French battalion groups fighting with the rebels, among others) and some Russian volunteers. But they are just that - volunteers - they were not sent there with orders.
Yeah, volunteers. The worst kinds of idiots. Dumb enough to kill other people for some nationalist crap that they truly believe in. Bloody jihadists...