[SOLVED] Regex: word^ matches what?

lesca · 02-04-2011, 08:37 PM

This regular expression may be the key, when I try to understand

Code:

sudo apt-get install lamp-server^

The manual says

Quote:

... anchor the regular expression with a '^' or '$' character, or create a more specific regular expression.

But there is no example how it works.

Any tip is welcome! Thanks!

acid_kewpie · 02-05-2011, 01:09 AM

well ^ means that *start* of the line, and $ means the end. so it'd be ^lamp-server to match strings starting with that phrase. I've never seen regexes used with apt-get though, is that supposed to work?

Snark1994 · 02-05-2011, 05:23 PM

@acid_kewpie, from the man-page:

Quote:

If no package matches the given expression and the expression contains one of '.', '?' or '*' then it is assumed to be a POSIX regular expression, and it is applied to all package names in the database. Any matches are then installed (or removed). Note that matching is done by substring so 'lo.*' matches 'how-lo' and 'lowest'. If this is undesired, anchor the regular expression with a '^' or '$' character, or create a more specific regular expression.

@lesca:
You might be interested in some basic regular expression stuff - they're quite useful

this is, as the link's title says, a very brief introduction to them...

lesca · 02-06-2011, 02:26 AM

But I think "^" should be at the start of the expression. Why not?

syg00 · 02-06-2011, 02:49 AM

Expression matching can be whatever Humpty Dumpty chooses it to mean.
Don't confuse it with regex - whatever that means ...

MTK358 · 02-06-2011, 08:34 AM

Quote:

Originally Posted by lesca

But I think "^" should be at the start of the expression. Why not?

Who said it shouldn't?

The apt-get man page assumes you are familiar with regular expressions, and if you were, you would know what ^ and $ mean.

^ means the beginning of the string, $ means the end.

For example, the regex "es" will match the word "test", but "^es" will not, because the string doesn't start with "es".

ta0kira · 02-06-2011, 11:03 PM

Quote:

Originally Posted by lesca

Code:

sudo apt-get install lamp-server^

I suppose better questions might be where did you see this and how do you know it's not an error? It sounds like apt-get uses "inspired by" POSIX regular expressions. It certainly wouldn't match anything as an extended POSIX regular expression; as a non-extended regular expression "^" is interpreted literally, but I doubt "^" is part of a normal package name.
Kevin Barry

lesca · 02-07-2011, 02:18 AM

Thank you for your answer.
I am sure it's right, because I followed those steps here

Kenhelm · 02-07-2011, 04:16 AM

According to this page
https://help.ubuntu.com/community/Tasksel
the '^' makes
sudo apt-get install lamp-server^
behave as an alternative to
sudo tasksel install lamp-server

Snark1994 · 02-07-2011, 03:29 PM

Quote:

Originally Posted by ta0kira

It sounds like apt-get uses "inspired by" POSIX regular expressions.

I dunno, the man-pages said it was treated as a POSIX regular expression. Did you have a particular reason to believe it doesn't behave the same way?