Date: Wed, 10 Feb 2010 12:24:57 +0100 From: =?utf-8?Q?Dag-Erling_Sm=C3=B8rgrav?= <des@des.no> To: Garrett Cooper <yanefbsd@gmail.com> Cc: Andrew Brampton <brampton+freebsd@gmail.com>, freebsd-hackers@freebsd.org Subject: Re: sysctl with regex? Message-ID: <86fx59jpti.fsf@ds4.des.no> In-Reply-To: <26049703-8844-4476-B277-776A4EFC0A53@gmail.com> (Garrett Cooper's message of "Tue, 9 Feb 2010 16:14:12 -0800") References: <d41814901002091308s7e894b55p880bde165bbbe703@mail.gmail.com> <86tytqvwky.fsf@ds4.des.no> <d41814901002091528i4884987cmb7347dfe4d50bdc5@mail.gmail.com> <26049703-8844-4476-B277-776A4EFC0A53@gmail.com>
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Garrett Cooper <yanefbsd@gmail.com> writes:
> C-shell globs as some programming languages referring to it as,
> i.e. perl (which this is a subset of the globs concept) allow for
> expansion via `*' to be `anything'. Regexp style globs for what you're
> looking for would be either .* (greedy) or .+ (non-greedy), with it
> being most likely the latter case.
Uh, not quite.
Formally, a regular expression is a textual representation of a finite
state machine that describes a context-free grammar.
A glob pattern can be trivially translated to a regular expression, but
not the other way around. Basically, * in a glob pattern corresponds to
[^/]*, ? corresponds to ., and [abcd] and [^abcd] have the same meaning
as in a regular expression. The glob pattern syntax has no equivalent
for +, ?, {m,n}, (foo|bar), etc.
Some shells implement something that resembles alternations, where
{foo,bar} corresponds to (foo|bar), but these are expanded before the
glob pattern. For instance, /tmp/{*,*} is expanded to /tmp/* /tmp/*,
which is then expanded to two complete copies of the list of files and
directories in /tmp.
There is no such thing as a "regexp style glob", and I have no idea what
you mean by "a subset of the globs concept" or where Perl fits into the
discussion.
Finally, .* and .+ are *both* greedy. Perl's regular expression syntax
includes non-greedy variants for both (.*? and .+? respectively).
Note that the [], +, ? and {m,n} notations are merely shorthand for
expressions which can be expressed using only concatenation, alternation
and the kleene star, which are the only operations available in formal
regular expressions.
> I'll see if I can whip up a quick patch in the next day or so -- but
> before I do that, does it make more sense to do globs or regular
> expressions? There are pluses and minuses to each version and would
> require some degree of parsing (and potentially escaping).
I think you'll find that, at least in this particular case, regular
expressions are an order of magnitude harder to implement than glob
patterns.
DES
--
Dag-Erling Smørgrav - des@des.no
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