Network Access Identifier

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Formal definition of the Network Access Identifier (NAI)

The grammar for the NAI is given below, described in augmented Backus-Naur form.

  nai         = username / ( username "@" realm )

  username    = dot-string

  realm       = realm "." label

  label       = let-dig * (ldh-str)

  ldh-str     = *( Alpha / Digit / "-" ) let-dig

  dot-string  = string / ( dot-string "." string )

  string      = char / ( string char )

  char        = c / ( "\" x )

  let-dig     = Alpha / Digit

  Alpha       = %x41-5A / %x61-7A   ; A-Z / a-z

  Digit       = %x30-39  ;0-9

  c           = < any one of the 128 ASCII characters, but
                 not any special or SP >

  x           = %x00-7F
                ; all 127 ASCII characters, no exception

  SP          = %x20 ; Space character

  special     = "<" / ">" / "(" / ")" / "[" / "]" / "\" / "."
                 / "," / ";" / ":" / "@" / %x22  / Ctl

  Ctl         = %x00-1F / %x7F
                ; the control characters (ASCII codes 0 through 31
                ; inclusive and 127)

  Examples of valid Network Access Identifiers include:

       fred@3com.com
       fred@foo-9.com
       fred_smith@big-co.com
       fred=?#$&*+-/^smith@bigco.com
       fred@bigco.com
       nancy@eng.bigu.edu
       eng!nancy@bigu.edu
       eng%nancy@bigu.edu

  Examples of invalid Network Access Identifiers include:

       fred@foo
       fred@foo_9.com
       @howard.edu
       fred@bigco.com@smallco.com
       eng:nancy@bigu.edu
       eng;nancy@bigu.edu
       <nancy>@bigu.edu