ABNF
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... The grammar for the NAI is given below, described in Augmented
Backus-Naur Form (ABNF) as documented in [RFC4234]. The grammar for
the username ...
... username portion in an NAI MUST fulfill the ABNF in this
specification as well as the requirements specified in [RFC4013 ...
... RFC4013].
Ensuring that NAIs conform to their ABNF is not sufficient; it is
also necessary to ensure that they do not contain prohibited
output.
...
...
In this case, the part before the (non-escaped) '!' MUST be a realm
name as defined in the ABNF in Section 2.1. This realm name is an
"IDN-unaware domain name slot", just like the realm name after the
...
...
Note that the syntax described in this section is optional and is not
a part of the ABNF. The '!' character may appear in the username
portion of an NAI ...
... Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234draft, October 2005. ...
...
o Several clarifications and improvements have been incorporated
into the ABNF specification for NAIs.
...
... Hartman, and Richard Perlman provided many useful comments on this
document. The ABNF validator at http://www.apps.ietf.org/abnf.html
was used to verify the syntactic correctness of the ABNF in
...
... document. The ABNF validator at http://www.apps.ietf.org/abnf.html
was used to verify the syntactic correctness of the ABNF in
Section 2.1.
...