in reply to Re: qr// for Expect.pm problem
in thread qr// for Expect.pm problem

Thank you! This particular problem has a bit of twists... I get the $number_string string from a db and it will most of the times contain meta-like characters that need to taken literally but I also must grab a certain part of the matched line. So I made: my $quoted_number_string = quotemeta $number_string; then I did: $quoted_number_string =~ s/\\\@P\\\@/([0-9]+)/; (where @P@ in the original string marks the place where I must look for a sequence of digits with ([0-9]+)) and these are meta characters that must remain unescaped (so that I can store the matched sequence of digits). Then I finally do: qr/.*$card,$sim.*$quoted_number_string.*/i Now I am in doubt: should aply quotemeta twice like this? 
my $quoted_number_string = quotemeta $number_string;
$quoted_number_string = quotemeta $quoted_number_string;
$quoted_number_string    =~ s/\\\@P\\\@/([0-9]+)/;
...
qr/.*$card,$sim.*$quoted_number_string.*/i

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or this will turn each backslash into 3 backslashes instead of just 2? I am confused. Once again thank you!, you have already helped.

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Re^3: qr// for Expect.pm problem
by AnomalousMonk (Archbishop) on Jun 25, 2011 at 21:17 UTC

    Go step by step. (And, of course, understand each step.)

    AFAIU, the original source string is something like '+fo\o*@P@*b\ar+', in which all metacharacters must be metaquoted, but in which there is a '@P@' sub-string that must also be converted into an actual regex pattern. Studying and experimenting with double- and single-quoting rules will be very helpful. 

    >perl -wMstrict -le
"my $raw = '+fo\o*@P@*b\ar+';
 print qq{'$raw'};
 ;;
 my $mq = quotemeta $raw;
 print qq{'$mq'};
 ;;
 $mq =~ s{ \\ \@ P \\ \@ }'\+([0-9]+)'xmsg;
 print qq{'$mq'};
 ;;
 my $mq_rx = qr{$mq}xms;
 print $mq_rx;
 ;;
 my $s = '123 +fo\o*+987*b\ar+ 456';
 print qq{'$s'};
 $s =~ $mq_rx;
 print qq{'$1'};
"
'+fo\o*@P@*b\ar+'
'\+fo\\o\*\@P\@\*b\\ar\+'
'\+fo\\o\*\+([0-9]+)\*b\\ar\+'
(?msx-i:\+fo\\o\*\+([0-9]+)\*b\\ar\+)
'123 +fo\o*+987*b\ar+ 456'
'987'

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    It's also worth remembering that while the  q{} single-quote operator doesn't interpolate, it does do a bit of escaping WRT the \ backslash character, e.g.: 

    >perl -wMstrict -le
"my $sq = 'start \ \\ \' stop';
 print qq{'$sq'};
"
'start \ \ ' stop'

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      Thank you!

      Without your examples I wouldn't even know what functions and operators I had at my disposal to experiment with.

      Thank you so much once again!

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