and indicating repetition and "local" alternatives.

     2.1.  RULE NAMING

          Angle brackets ("<", ">") are not  used,  in  general.   The
     name  of  a rule is simply the name itself, rather than "".
     Quotation-marks enclose literal text (which may be  upper  and/or
     lower  case).   Certain  basic  rules  are  in uppercase, such as
     SPACE, TAB, CRLF, DIGIT, ALPHA, etc.  Angle brackets are used  in
     rule  definitions,  and  in  the rest of this  document, whenever
     their presence will facilitate discerning the use of rule names.

     2.2.  RULE1 / RULE2:  ALTERNATIVES

          Elements separated by slash ("/") are alternatives.   There-
     fore "foo / bar" will accept foo or bar.

     2.3.  (RULE1 RULE2):  LOCAL ALTERNATIVES

          Elements enclosed in parentheses are  treated  as  a  single
     element.   Thus,  "(elem  (foo  /  bar)  elem)"  allows the token
     sequences "elem foo elem" and "elem bar elem".

     2.4.  *RULE:  REPETITION

          The character "*" preceding an element indicates repetition.
     The full form is:

                              *element

     indicating at least  and at most  occurrences  of  element.
     Default values are 0 and infinity so that "*(element)" allows any
     number, including zero; "1*element" requires at  least  one;  and
     "1*2element" allows one or two.

     2.5.  [RULE]:  OPTIONAL

          Square brackets enclose optional elements; "[foo  bar]"   is
     equivalent to "*1(foo bar)".

     2.6.  NRULE:  SPECIFIC REPETITION

          "(element)" is equivalent to "*(element)"; that is,
     exactly    occurrences  of (element). Thus 2DIGIT is a 2-digit
     number, and 3ALPHA is a string of three alphabetic characters.


     August 13, 1982               - 3 -                      RFC #822
 

 
     Standard for ARPA Internet Text Messages


     2.7.  #RULE:  LISTS

          A construct "#" is defined, similar to "*", as follows:

                              #element

     indicating at least  and at most  elements, each  separated
     by  one  or more commas (","). This makes the usual form of lists
     very easy; a rule such as '(element *("," element))' can be shown
     as  "1#element".   Wherever this construct is used, null elements
     are allowed, but do not  contribute  to  the  count  of  elements
     present.   That  is,  "(element),,(element)"  is  permitted,  but
     counts as only two elements.  Therefore, where at least one  ele-
     ment  is required, at least one non-null element must be present.
     Default values are 0 and infinity so that "#(element)" allows any
     number,  including  zero;  "1#element" requires at least one; and
     "1#2element" allows one or two.

     2.8.  ; COMMENTS

          A semi-colon, set off some distance to  the  right  of  rule
     text,  starts  a comment that continues to the end of line.  This
     is a simple way of including useful notes in  parallel  with  the
     specifications.