> From: DNSOP [mailto:dnsop-boun...@ietf.org] On Behalf Of Brian Dickson
>
> > Apologies but I did not hear the full question regarding BULK RR’s
> > and the perl like back-references. If you could please repeat
> > the question we would be happy to comment.
> >
> >
> > Thanks,
> > John
> >
>
> Sorry for the delayed response.
>
Hi Brian,
Thank you so much for following up!
>
> The question was:
>
> Given the use of perl-style back-references ( like "${1}" ) - would
> it not make sense to use the same perl syntax to provide
> well-defined targets for those back-references?
>
>
> E.g. "my-special-(<regex-thing>)-(<regex-thing>)-with-suffix" and
> then there is no ambiguity as to what ${1} and ${2} point at?
>
> Perl regex rules are IMHO very clean and clever, and avoid ambiguity
> very nicely.
> (Also historical shout-out to Henry Spencer, whose regex mods served
> as the source for perl's.)
>
Full disclosure:
I especially love the perl-regex rule engine and can't imagine doing
even half the things I have to do every day without it.
I echo your thanks to Mr. Spencer and include the authors of libpcre
who's dedication has allowed me to easily use the perl-regex ruleset
for my every whim.
In fact, when we first set out to solve our problem, regex was our
instinctual go-to. Unfortunately, it soon became clear regex is
simply _too_ complicated for the task at hand.
First, in our case the patterns will _always_ be numeric ranges
(or sets of ranges) and regex, no matter the flavor, simply brings
too much baggage (i.e. overhead) to the party.
Second, regex was designed for string/ substring pattern matching
and is simply horrible at multi-digit numeric ranges
(the one thing we need it for).
Try out this example:
We want to match a numeric range from 0-255.
With PCRE, this becomes:
(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)
With BULK-RR, this becomes:
[0-255]
Now, lets take a slightly more complicated example. Say we have
an IPv4 /16 broken into 4 /18's.
Depending on which /18 is being targeted, the third octet can be
within the ranges: 0-63, 64-127, 128-191 or 192-255.
[0-63] vs. (0|[1-5][0-9]|6[0-3])
[64-127] vs. (6[4-9]|[789][0-9]|1[01][0-9]|12[0-7])
[128-191] vs. ???
[192-255] vs. ???
Even if you are an absolute regex genius and this poses zero
challenge, in situation like this I often ask myself:
- Which would you rather figure out in the middle of the night?
Or
- Would I like to be woken up for this or should it be easier
for 'someone else'?
>
> Things inside parentheses are the target of back-refs.
> In Perl, there is no need to escape the parentheses within regexes.
>
The square brackets can only contain numeric ranges (or sets of ranges)
so no need to escape within them either.
Thanks again,
John
NOTE: The regex pattern for 0-255 was borrowed from a quick web
search (link below).
https://chrisjwarwick.wordpress.com/2012/09/16/
more-regular-expressions-regex-for-ip-v4-addresses/
>
> Brian
>
>
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