match function

catfish1116 has asked for the wisdom of the Perl Monks concerning the following question:

I am working on the match functionality and have come up with a problem. Below is code and error:

print "Enter a string: ";
   chomp(my $_ = <STDIN>);

   if ($_ = /([A|a])/) {
       my $afterstring  = $';
       print "This is the after string $afterstring\n";
      if( $afterstring = /([B|b)/ ) {
          print The string has an a  but also a following b\n";
      } else {
          print "The string has an a , but not a following b\n";
      }
    } else {
        print "String did not match\n";
    }
[download]

And here is the error:

Uncaught exception from user code
        Unmatched [ in regex; marked by <-- HERE in m/([ <-- HERE B|b)
+/ at Exercise_7_4.pl line 24.
[download]

TIA The Catfish

Comment on match function Select or Download Code

Replies are listed 'Best First'.
Re: match function by Corion (Patriarch) on Jun 09, 2020 at 19:01 UTC
What is your question? I think the error message is quite clear: `Unmatched [ in regex; marked by <-- HERE in m/([ <-- HERE B\|b)` [download] There is an unmatched opening square bracket in your regular expression. Your regular expression is `/([B\|b)/` [download] Maybe you wanted to write `[Bb]`, or maybe `([B\|b])`? But that's not what you wrote. Also, the following code will most certainly not do what you think it does: `if ($_ = /([A\|a])/) {` [download] Most likely, you want to use the regex binding operator `=~` and not the assignment operator `=`.	[reply] [d/l] [select]
Re: match function by choroba (Cardinal) on Jun 09, 2020 at 20:41 UTC
Note also the missing double quote here: `print The string has an a but also a following b\n"; # ^` [download] `map{substr$_->[0],$_->[1]\|\|0,1}[\\|\|{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^ARGV,3]`	[reply] [d/l] [select]
Re: match function by BillKSmith (Monsignor) on Jun 09, 2020 at 22:50 UTC
It is easier to test explicitly for what you want. `use strict; use warnings; print "Enter a string: "; chomp ($_ = <>); my $msg = !/a/i ? 'String did not match' : !/a.b/i ? 'The string has an a, but not a following b' : 'The string has an a, but also a following b' ; print $msg, "\n";` [download] UPDATE: Even better, use positive tests: `my $msg = /a.b/i ? 'The string has an a, and also a following b' : /a[^bB]*$/i ? 'The string has an a, but not a following b' : 'String did not match' ;` [download] Bill	[reply] [d/l] [select]
Re: match function by AnomalousMonk (Archbishop) on Jun 09, 2020 at 21:57 UTC
Just for the sake of completeness... I think Corion has already alluded to this, but the `[a\|A]` character class consists in the `'\|'` (pipe) and `'a'` and `'A'` characters. The `\|` regex alternation metacharacter is not meta inside a character class. See Bracketed Character Classes in perlrecharclass. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re: match function by Marshall (Canon) on Jun 10, 2020 at 20:13 UTC
I liked all of the comments in this thread. I would add a cautionary note about the use of $'. In Perl versions <5.20 just this statement `my $afterstring = $';` can cause a huge regex performance penalty on every regex in the entire script, not just limited to where this variable is actually used. https://perldoc.perl.org/perlvar.html gives the gory details. This thread shows a number of ways to avoid using this special variable and I recommend doing so. Perl versions can remain in use for a surprisingly long time because Perl is integral to the operation of many OS's and there will be a default "system Perl" that is only updated in a very conservative way. There are plenty of Perl 5.8 installations still out there. I don't see any compelling reason to use $' in this example and would not do so.	[reply] [d/l]
Re^2: match function by AnomalousMonk (Archbishop) on Jun 10, 2020 at 22:20 UTC
The "Performance issues" sub-section of perlvar that Marshall links to covers this of course, but I think it's worth emphasizing that the related variables $` $& $' all have the same problem prior to Perl 5.20. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]