in reply to qr// for Expect.pm problem

Could you please enlighten me to what am I doing wrong?

When you interpolate the string $number_string into the regex qr/.*$card,$sim.*$number_string.*/ the backslash you originally had on the '+' sign is effectively 'used up'. So by the time the regex comes to be used, that '+' will be treated as a regex meta character, specifically a quantifier modifier, for the preceding space.

Often when interpolating a string into a regex, the way to ensure any meta-like characters within as treated as literals is to bracket the interpolated variable in \Q and \E.

eg. qr/.*$card,$sim.*\Q$number_string\E.*/.

But as the string actually contains meta-characters that you want recognised as such, the other option is to double the backslashes on the '+' so that one remains by the time the regex is used. Ie.

my $number_string = 'Your number is \\+([0-9]+)';

$exp->expect($timeout,
        [ qr/.*$card,$sim.*$number_string.*/,
        ...

[download]

Which should resolve this particular problem.

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Re^2: qr// for Expect.pm problem
by silentius (Monk) on Jun 25, 2011 at 20:08 UTC
    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

    [download]
    or this will turn each backslash into 3 backslashes instead of just 2? I am confused. Once again thank you!, you have already helped.
[reply]
[d/l]
[select]

      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'

      [download]

      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'

      [download]
[reply]
[d/l]
[select]

        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!

[reply]

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