Pattern matching fails because of special characters

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

Dear Monks, I am trying to grep for a chunk of lines in a text file but it does not seem to work:

FINAL PREDICTION
================
1: (out) 10-33    (10.82)

        10        20        30        40        50        60
SSSSSSSS-OOOOOOXXXXXXXXXXXXIIIIII+++++++++++++++++++++++++++
MSSPLRMDVTFLLAAIAVTWVCGLKIGFPGFSTPPRSFIQHPKRTLCPEDCDIASPFKCE

        70        80        90       100       110       120
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
ESPTCLRLFQVCNGRWDCEHGSDEDNALCAAVLRPLECMIWEFLEGQRDWILPNLFNDAN

       130       140       150       160       170       180
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
TDLVAHALHEAYSMGDLQSYLNLTDQNIENIRNSTRGAIVGDPRPLMALGMPDRAWPEVM

       190    
++++++++++++++++
YLLKELYNLGLDVWAE
[download]

and I only need to grep the lines that contain S,I,O,X,- and + (basically the middle lines in every 3 lines. I tried this:

    $output='';
    while($_=~/\A([S\-\+XIO]+)\z/g)
    {

        $part=$1;
        $output=$output.$part;
    }
    print "$output\n";
[download]

it does not work. The desired output would be:

SSSSSSSS-OOOOOOXXXXXXXXXXXXIIIIII+++++++++++++++++++++++++++++++++++++
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
[download]

Comment on Pattern matching fails because of special characters Select or Download Code

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Re: Pattern matching fails because of special characters by AnomalousMonk (Archbishop) on Jun 10, 2019 at 01:15 UTC
Where and with what is `$_` being assigned? If `$_` does not match the pattern, the `while` loop will never be entered. If `$_` does match, the `while` loop will be infinite. `c:\@Work\Perl\monks>perl -wMstrict -le "$_ = 'no joy'; ;; my $output = ''; while($_ =~ /\A([S\-\+XIO]+)\z/g) { my $part = $1; $output = $output . $part; } print qq{final output: '$output'}; " final output: ''` [download] Please see Short, Self-Contained, Correct Example.	[reply] [d/l] [select]
Re: Pattern matching fails because of special characters by LanX (Saint) on Jun 10, 2019 at 00:18 UTC
Hi > `while($_=~/\A([S\-\+XIO]+)\z/g)` ~~You can't escape with backslashes inside a character class.~~ ° Put the minus at the end and drop all backslashes, ~~unless you want to match '\'~~.° See also Re: regex question Underscores, lines and paratheses. HTH! :) Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice} °) thanks to AnomalousMonk++ for correcting me!	[reply] [d/l]
Re^2: Pattern matching fails because of special characters by AnomalousMonk (Archbishop) on Jun 10, 2019 at 01:02 UTC
You can't escape with backslashes inside a character class. Put the minus at the end and drop all backslashes, unless you want to match \ . A counterexample: `c:\@Work\Perl\monks>perl -wMstrict -le "my $s = 'abc\ZD+A-G\HJxyz'; print qq{'$s'}; ;; $s =~ m{ ([A\-Z\+GD]+) }xms; print qq{'$1'}; " 'abc\ZD+A-G\HJxyz' 'ZD+A-G'` [download] (But I agree that putting the - at the end and dropping the backslashes would have the same effect.) Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^3: Pattern matching fails because of special characters by LanX (Saint) on Jun 10, 2019 at 01:25 UTC
you are right perlrecharclass#Special-Characters-Inside-a-Bracketed-Character-Class `Characters that may carry a special meaning inside a character class are: \ , ^ , - , and , and are discussed below. They can be escaped with a backslash, although this is sometimes not needed, in which case the backslash may be omitted.` Mea culpa! (time to retire? :) Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice} PS: I'm afraid perldoc.perl.org "improved" to the worse.	[reply]
Re: Pattern matching fails because of special characters by stevieb (Canon) on Jun 09, 2019 at 23:31 UTC
It would be prudent and helpful if you could show us what you're currently getting back, and how it's wrong. Edit your Original Post with that information.	[reply]