regular expression
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... delegated over time. The result was a Resource Record that included
a regular expression that would be used by a client program to
rewrite a string into a domain name ...
... client program to
rewrite a string into a domain name. Regular expressions were chosen
for their compactness to expressivity ratio allowing for a great deal
of information to be encoded in a rather small DNS packet ...
... expression is given in the next section.
The regular expressions MUST NOT be used in a cumulative fashion,
that is, they should only be applied to the original string held
by the client ...
... in a subst_expr must be the same character.>
ere = POSIX Extended Regular Expression
repl = 1 * ( OCTET / backref )
backref = "\" 1POS_DIGIT ...
... target string. If not, the process halts. The domain that
results from the regular expression is then used as the domain of the
next loop through the NAPTR ...
... literal occurances of a backslash must be escaped by
another backslash. For the case of the cid.urn.arpa record above,
the regular expression entered into the master file should be
"/urn:cid:.+@([^\\.]+\\.)(.*)$/\\2/i". When the client code actually
...
... REPLACEMENT A <domain-name> which specifies the new value in the
case where the regular expression is a simple replacement
operation.
...
... can contain numerous backslashes and thus should be treated with
care. See Section 10 for how to correctly enter and escape the
regular expression.
...
...
Beware of regular expressions. Not only are they difficult to get
correct on their own, but there is the previously mentioned
interaction with DNS ...
... requires %encoding characters outside a limited repertoire. The
regular expressions MUST be written to operate on that canonical
form. Since international character sets will end up with extensive
...
... canonical
form. Since international character sets will end up with extensive
use of %encoded characters, regular expressions operating on them
will be essentially impossible to read or write by hand.
...
...
Regular expressions should be checked for sanity, not blindly passed
to something like PERL.
...