[SOLVED] Regex in Linux does not work

chrism01 · 11-11-2015, 09:49 PM

Hey syg00,
how are you ?

So you're saying the first '.*' effectively causes it to match the last occurrence?
I did a quick couple of tests and that seems to be the case.
How would you specify the first match then?
(this is worrying; I used to be ok at regexes...)

syg00 · 11-11-2015, 09:55 PM

Yep - it matches the entire text. Then the regex engine starts working backwards until the next regex element matches.
Rinse, shake, repeat.

Of course, regex ain't regex.

chrism01 · 11-11-2015, 09:58 PM

True; I need to re-read Friedl's book. Funnily enough I was planning to revise that today, hence the interest.
So (as per my last qn) how would you force it to match 1st occurrence instead?

EDIT: oh yeah; just started doing it in Perl : with the front & back wildcards you get the last one; without wildcards you get the first.

Still working on the sed version (I always was a bit iffy with sed...)

syg00 · 11-11-2015, 10:25 PM

Sorry - forgot that q.
You can't in sed. You need non-greedy quantifiers, which last I looked sed doesn't support them. perlre is simplest soln.

chrism01 · 11-11-2015, 11:10 PM

That explains why I'm banging my head on the desk.... its so nice to stop

pan64 · 11-12-2015, 02:01 AM

what about this?
sed -r -n 's/11:([0-9]+).*/11:\1/;s/.*11://p'

syg00 · 11-12-2015, 02:04 AM

You can't generalise it for an indeterminate number of occurrences of the required string (in sed)

pan64 · 11-12-2015, 02:23 AM

obviously not, but can find first occurence

rknichols · 11-12-2015, 10:36 AM

Quote:

Originally Posted by syg00

You can't generalise it for an indeterminate number of occurrences of the required string (in sed)

Never say, "It can't," with sed. It's language is, after all, Turing complete. Given enough time and resources, it can do anything any computer could ever do. Whether it is a practical candidate for a given application is a different question.

berndbausch · 11-12-2015, 11:38 AM

Quote:

Originally Posted by rknichols

Never say, "It can't," with sed. It's language is, after all, Turing complete. Given enough time and resources, it can do anything any computer could ever do. Whether it is a practical candidate for a given application is a different question.

Your assignment: Rewrite the Linux kernel in sed.

chrism01 · 11-12-2015, 05:27 PM

Now that would impress even Linus