RE: [bug #22100] Enhancement request:

[Ed Avis]

Tony Abou-Assaleh wrote:

>> % grep -E --context=10 oranges --and '[0-9]'
>
>You can do this both in grep (using -z) and in awk (by 
>changing the record separator) and then matching for \n the 
>desired number of times.
>
>The and operator can be done as follows:
>
>grep -E '(re1.*re2)|(re2.*re1)'

That doesn't work for all cases, for example (with -E)

    re1=^hello
    re2=^\w+

But if you stick to plain string-generating expressions, without anchors, 
lookbehind or other exotic features then it does work.

I wonder if perl5 so-called regular expressions are 'closed under and' in this 
way.  Or if a perl5 regexp can be used to give the conjunction of two plain 
grep regexps, but not necessarily of two perl5 regexps.

As for --not, perl5 regexps do support negation, I think: the pattern ^(?!x)$ 
matches all lines except those matching ^x$.

>I haven't formally tested this in grep (only in Perl), but I 
>think chaining grep expressions is faster in most cases than 
>using a single complex expression.

Does grep's optimizer not have some rules it uses to turn a single expression 
into a conjunction or disjunction of smaller expressions?  Certainly it would 
be more parallelizable.

>Making grep do more with less is on my radar, but it is not a 
>priority at the moment. There are some serious bugs that need 
>to be fixed first.

Understood.

-- 
Ed Avis <address@hidden> 

>-----Original Message-----
>From: Tony Abou-Assaleh [mailto:address@hidden 
>Sent: Tuesday, February 12, 2008 5:56 AM
>To: Ed Avis
>Cc: address@hidden
>Subject: Re: [bug #22100] Enhancement request: 
>
>> IMHO, context matching is a good reason why it is more powerful (and 
>> not just syntactic icing or making the search faster) to support 
>> matching two patterns at once.  I don't think there is a 
>> straightforward awk equivalent, or grep-pipeline equivalent, to
>>
>
>In terms of "power", grep regexp can do a lot. Sure it gets 
>complicated, but the usage is less common, and for the rare 
>cases it can be done.
>
>I haven't formally tested this in grep (only in Perl), but I 
>think chaining grep expressions is faster in most cases than 
>using a single complex expression.
>
>Making grep do more with less is on my radar, but it is not a 
>priority at the moment. There are some serious bugs that need 
>to be fixed first.
>
>Cheers,
>
>TAA
>
>--
>Tony Abou-Assaleh
>Email:    address@hidden
>Web site: http://tony.abou-assaleh.net
>
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