rfc2822 April 2001

3.2.4. Atom

   Several productions in structured header field bodies are simply
   strings of certain basic characters.  Such productions are called
   atoms.

   Some of the structured header field bodies also allow the period
   character (".", ASCII value 46) within runs of atext.  An additional
   "dot-atom" token is defined for those purposes.

atext           =       ALPHA / DIGIT / ; Any character except controls,
              "!" / "#" /     ;  SP, and specials.
              "$" / "%" /     ;  Used for atoms
              "&" / "'" /
              "*" / "+" /
              "-" / "/" /
              "=" / "?" /
              "^" / "_" /
              "`" / "{" /
              "|" / "}" /
              "~"

atom            =       [CFWS] 1*atext [CFWS]

dot-atom        =       [CFWS] dot-atom-text [CFWS]

dot-atom-text   =       1*atext *("." 1*atext)

   Both atom and dot-atom are interpreted as a single unit, comprised of
   the string of characters that make it up.  Semantically, the optional
   comments and FWS surrounding the rest of the characters are not part
   of the atom; the atom is only the run of atext characters in an atom,
   or the atext and "." characters in a dot-atom.