to the original URI in order to construct the next domain name
          to lookup. The grammar of the substitution expression is given
          in the next section.

   Replacement
          The next NAME to query for NAPTR, SRV, or A records depending
          on the value of the flags field. As mentioned above, this may
          be compressed.

Substitution Expression Grammar:
================================

   The content of the regexp field is a substitution expression. True
   sed(1) substitution expressions are not appropriate for use in this
   application for a variety of reasons, therefore the contents of the
   regexp field MUST follow the grammar below:

subst_expr   = delim-char  ere  delim-char  repl  delim-char  *flags
delim-char   = "/" / "!" / ... (Any non-digit or non-flag character other
               than backslash '\'. All occurances of a delim_char in a
               subst_expr must be the same character.)
ere          = POSIX Extended Regular Expression (see [13], section
               2.8.4)
repl         = dns_str /  backref / repl dns_str  / repl backref
dns_str      = 1*DNS_CHAR
backref      = "\" 1POS_DIGIT




 
RFC 2168            Resolution of URIs Using the DNS           June 1997


flags        = "i"
DNS_CHAR     = "-" / "0" / ... / "9" / "a" / ... / "z" / "A" / ... / "Z"
POS_DIGIT    = "1" / "2" / ... / "9"  ; 0 is not an allowed backref
value domain name (see RFC-1123 [14]).

   The result of applying the substitution expression to the original
   URI MUST result in a string that obeys the syntax for DNS host names
   [14]. Since it is possible for the regexp field to be improperly
   specified, such that a non-conforming host name can be constructed,
   client software SHOULD verify that the result is a legal host name
   before making queries on it.

   Backref expressions in the repl portion of the substitution
   expression are replaced by the (possibly empty) string of characters
   enclosed by '(' and ')' in the ERE portion of the substitution
   expression. N is a single digit from 1 through 9, inclusive. It
   specifies the N'th backref expression, the one that begins with the
   N'th '(' and continues to the matching ')'.  For example, the ERE
                      (A(B(C)DE)(F)G)
   has backref expressions:
                      \1  = ABCDEFG
                      \2  = BCDE
                      \3  = C
                      \4  = F
                 \5..\9  = error - no matching subexpression

   The "i" flag indicates that the ERE matching SHALL be performed in a
   case-insensitive fashion. Furthermore, any backref replacements MAY
   be normalized to lower case when the "i" flag is given.

   The first character in the substitution expression shall be used as
   the character that delimits the components of the substitution
   expression.  There must be exactly three non-escaped occurrences of
   the delimiter character in a substitution expression. Since escaped
   occurrences of the delimiter character will be interpreted as
   occurrences of that character, digits MUST NOT be used as delimiters.
   Backrefs would be confused with literal digits were this allowed.
   Similarly, if flags are specified in the substitution expression, the
   delimiter character must not also be a flag character.













 
RFC 2168            Resolution of URIs Using the DNS           June 1997


Advice to domain administrators:
================================

   Beware of regular expressions. Not only are they a pain to get
   correct on their own, but there is the previously mentioned
   interaction with DNS. Any backslashes in a regexp must be entered
   twice in a zone file in order to appear once in a query response.
   More seriously, the need for double backslashes has probably not been
   tested by all implementors of DNS servers. We anticipate that urn.net
   will be the heaviest user of regexps. Only when delegating portions