Internet technical specifications often need to define a format
syntax and are free to employ whatever notation their authors deem
useful. Over the years, a modified version of Backus-Naur Form
(BNF), called Augmented BNF (ABNF), has been popular among many
Internet specifications. It balances compactness and simplicity,
with reasonable representational power. In the early days of the
Arpanet, each specification contained its own definition of ABNF.
This included the email specifications,
RFC733 and then
RFC822 which
have come to be the common citations for defining ABNF. The current
document separates out that definition, to permit selective
reference. Predictably, it also provides some modifications and
enhancements.
The differences between standard BNF and ABNF involve naming rules,
repetition, alternatives, order-independence, and value ranges.
Appendix A (Core) supplies rule definitions and encoding for a core
lexical analyzer of the type common to several Internet
specifications. It is provided as a convenience and is otherwise
separate from the meta language defined in the body of this document,
and separate from its formal status.
