ねば in 食べねば; relation between ねば and なければ?
I would like to know a bit more about the "ねば" grammatical construction. Every now and then, I hear people say sentences like > 東京に行かねばならぬ > 粘々丼を食べねば倒れる > なんとかせねばいい It's quite clear that ねば means the same as なければ, but - Is it a shorten form of なければ, or a different construction? - Is it an old form, or something that has been around for 20 years or so? - Is its usage limited to some regions? ages? social classes? - How (in)formal is it? - Is it like なきゃ and なくちゃ? (as far as usage is concerned, not nuances)