I would like to know a bit more about the "ねば" grammatical construction. Every now and then, I hear people say sentences like
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)