Reverse grep?

JamesA has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: Reverse grep? by chromatic (Archbishop) on Mar 01, 2002 at 19:18 UTC
Look in perlfaq6, and specifically How do I efficiently match many regular expressions at once?. You can define them in a separate file, perhaps in a hash or an array. This file can be a module or a file that's required or done.	[reply]
Re: Reverse grep? by rbc (Curate) on Mar 01, 2002 at 19:09 UTC
If you mean grep -v behavoir you could try ... `my @results = grep { !/$pattern/ }, @array;` [download] ... I hope that works. If not let me know cause I use it in some of my scripts.	[reply] [d/l]
Re: Reverse grep? by Juerd (Abbot) on Mar 01, 2002 at 19:39 UTC
For information about Perl's `grep` command, use perldoc -f grep. To reverse the condition, negate the condition: `grep { !( expression ) } LIST` (Using a block is easier in many cases, and read perlop to find out when you can leave out the parens). To reverse grep's output, use reverse: `reverse grep BLOCK LIST` or `reverse grep EXPR, LIST` `Lbh ebgngrq guvf grkg naq abj lbh pna ernq vg. Fb jung? :) -- Whreq` [download]	[reply] [d/l] [select]
Re: Reverse grep? by chipmunk (Parson) on Mar 02, 2002 at 17:23 UTC
By "reverse grep", I take it that you mean you want to loop over a list of regexes with one string, rather than loop over a list of strings with one regex. As far as Perl is concerned, these are the same. Your difficulty comes from thinking of Perl's grep as being the same as the command line grep. This is not the case. In Perl's grep, the conditional is not necessarily a pattern match; it is any arbitrary Perl expression. For example, you could do something like this: `@results = grep { $_ & 1 } (1 .. 10)` to get a list of odd numbers between 1 and 10. Thus, you can do any filtering you want with grep, with the proper conditional expression. What you're looking for could be written something like this (with some regexes I made up): `my $url = 'http://www.perlmonks.org/'; my @regexes = ('/private(?:/\|$)', 'python'); if (grep { $url =~ $_ } @regexes) { # handle bad URL } else { # handle good URL }` [download] As suggested in another response, you'd probably want to precompile the regexes with qr// for efficiency.	[reply] [d/l] [select]
Re: Reverse grep? by JamesA (Acolyte) on Mar 02, 2002 at 00:45 UTC
I am trying to do something like the following but can't get it to work: `open FILE, "regex" or die "Couldn't open: $!"; while (<FILE>) { if ( $ENV{"REDIRECT_URL"} +~ /$_/ ) { print $_; last; } } close FILE;` [download] Where the file "regex" would contain all of the patterns I want to test against the URL.	[reply] [d/l]
Re: Re: Reverse grep? by dws (Chancellor) on Mar 02, 2002 at 00:47 UTC
You need to add this: `while (<FILE>) { + chomp; if ( $ENV{"REDIRECT_URL"} +~ /$_/ ) { print $_; last; }` [download] Otherwise, you have a trailing newline at the end of your pattern.	[reply] [d/l]
Re: Re: Reverse grep? by coolmichael (Deacon) on Mar 02, 2002 at 05:23 UTC
I think +~ might be a typo. Don't you want this instead? `if ( $ENV{"REDIRECT_URL"} =~ /$_/ ) { print $_; last; } ^` [download] Please correct me if I'm wrong.	[reply] [d/l]
(jeffa) 3Re: Reverse grep? by jeffa (Bishop) on Mar 02, 2002 at 18:35 UTC
Considering that one must hold down the shift key for the tilde character, coupled with the fact that the plus sign character shares the same key as the equal sign character ... yes ;) But, have you played with the results yet? `[prompt]$ perl -le 'print /RED/' [prompt]$ perl -le 'print ~/RED/' 4294967295 [prompt]$ perl -le 'print hex("F"x8)' 4294967295 [prompt]$ perl -le '$_ = "REDIRECT"; print /RED/' 1 [prompt]$ perl -le '$_ = "REDIRECT"; print ~/RED/' 4294967294 [prompt]$ perl -le '$_ = "REDIRECT"; print 5+~/RED/' 4294967299` [download] Gotta love Perl's flexibility! :D jeffa L-LL-L--L-LL-L--L-LL-L-- -R--R-RR-R--R-RR-R--R-RR B--B--B--B--B--B--B--B-- H---H---H---H---H---H--- (the triplet paradiddle with high-hat)	[reply] [d/l]


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