Date: Wed, 10 Feb 2010 09:23:01 -0800 From: Garrett Cooper <yanefbsd@gmail.com> To: =?ISO-8859-1?Q?Dag=2DErling_Sm=F8rgrav?= <des@des.no> Cc: Andrew Brampton <brampton+freebsd@gmail.com>, freebsd-hackers@freebsd.org Subject: Re: sysctl with regex? Message-ID: <7d6fde3d1002100923i6bbc24a7ocaf408f4d78ec59f@mail.gmail.com> In-Reply-To: <86fx59jpti.fsf@ds4.des.no> References: <d41814901002091308s7e894b55p880bde165bbbe703@mail.gmail.com> <86tytqvwky.fsf@ds4.des.no> <d41814901002091528i4884987cmb7347dfe4d50bdc5@mail.gmail.com> <26049703-8844-4476-B277-776A4EFC0A53@gmail.com> <86fx59jpti.fsf@ds4.des.no>
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2010/2/10 Dag-Erling Sm=F8rgrav <des@des.no>:
> 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. =A0Basically, * in a glob pattern corresponds t=
o
> [^/]*, ? corresponds to ., and [abcd] and [^abcd] have the same meaning
^^^^ ???? ^^^^
The former is a positive assertion, where the latter is a negative
assertion -- how can they have the same meaning?
> as in a regular expression. =A0The glob pattern syntax has no equivalent
> for +, ?, {m,n}, (foo|bar), etc.
+, {}, and () -- no... that's typically an extension to shell expanded
values (IIRC). ? however, is a supported glob quantifier [from
glob(3)]:
The argument pattern is a pointer to a pathname pattern to be expanded=
.
The glob() argument matches all accessible pathnames against the patte=
rn
and creates a list of the pathnames that match. In order to have acce=
ss
to a pathname, glob() requires search permission on every component of=
a
path except the last and read permission on each directory of any file=
-
name component of pattern that contains any of the special characters
`*', `?' or `['.
> Some shells implement something that resembles alternations, where
> {foo,bar} corresponds to (foo|bar), but these are expanded before the
> glob pattern. =A0For 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.
This is what I'm referring to:
http://perldoc.perl.org/functions/glob.html . Semantically I was wrong
in areas in my original statement, but I was trying to hand wave from
a basic `I don't know how globs vs regexps work', without the
technical lexigram discussion.
> Finally, .* and .+ are *both* greedy. =A0Perl's regular expression syntax
> includes non-greedy variants for both (.*? and .+? respectively).
Yes, but I didn't explicitly note those forms.
> 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.
Yes... I wholeheartedly agree...
Thanks :),
-Garrett
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