Re: Pattern matching exlusion

[Loops]

You don't specify which characters are valid as part of the identifier, so the following assumes that any letter or number is okay. From your code snippet, it looked as if you wanted to allow either upper or lower case matching as well:

my $formget = qr(10-[KQ][A-Z0-9]*(?!/))i;
while (my $form_type = <DATA>) {
        if ($form_type =~ /^$formget/ ) {
                print "Valid: $form_type";
        } else {
                print "Invalid: $form_type";
        }
}

__DATA__
10-K
10-KSB
10-K405
10-ksb405
10-Q
10-K/A
10-Q/A

Output:

Valid: 10-K
Valid: 10-KSB
Valid: 10-K405
Valid: 10-ksb405
Valid: 10-Q
Invalid: 10-K/A
Invalid: 10-Q/A

Check out the section titled "Look-Around Assertions" in perlre about using the (?! ...) construct to specify what's called a negative lookahead. That is, something which must not follow the current pattern.

[AnomalousMonk]

my $formget = qr(10-[KQ][A-Z0-9]*(?!/))i;


The problem with this regex when used in a  /^$formget/ match is that it allows strings like  10-KA/ to match: if a  / is found at the end of  10-KA/ the regex can backtrack to  10-K and look ahead to  A which is not a  / character! BrowserUk's approach below gets around this by using an end-of-string anchor assertion to make sure that only non-/ characters follow the [KQ]. If anchoring were not possible, another way would be to use an 'atomic'  (?>pattern) group which will not allow backtracking into it:

c:\@Work\Perl\monks>perl -wMstrict -le
"my @tests = qw[
   10-K    10-KSB      10-K405    10-KSB405    10-Q
   10-K/B  10-KSB/ABC  10-K405/A  10-KSB405/A  10-Q/A  10-KA/
   ];
 ;;
 my $formget = qr{ (?i) 10- [KQ] [A-Z0-9]* (?! /) }xms;
 printf 'valid:  ';
 /^$formget/ and printf qq{'$_' } for @tests;
 print '';
 ;;
 my $formget2 = qr{ (?> (?i) 10- [KQ] [A-Z0-9]*) (?! /) }xms;
 printf 'valid2: ';
 /^$formget2/ and printf qq{'$_' } for @tests;
"
valid:  '10-K' '10-KSB' '10-K405' '10-KSB405' '10-Q' '10-KSB/ABC' '10-
+K405/A' '10-KSB405/A' '10-KA/'
valid2: '10-K' '10-KSB' '10-K405' '10-KSB405' '10-Q'

[wrkrbeee]

Thank you AnomalousMonk, that makes sense. I really appreciate your time and interest. Have a great weekend!

[wrkrbeee]

Hi Loops, Thank you for your help. Tell me if I am wrong concerning the interpretation of your expression: qr(10-KQA-Z0-9*(?!/))i; 1. You are saying find a string (QR) 2. which begins with 10-K or 10-Q KQ 3. followed by any letter from A-Z (A-Z] 4. followed by any number 0-9 [0-9} 5. excluding any string which terminates with slash *(?!/))i Is that right? If so, how would the expression change if I were looking for upper case letters only? Your insight is invaluable, and I am grateful! Rick

[Loops]

The [A-Z0-9] part of the expression matches any letter or any number. And it's followed by an asterisk "*", which means zero or more instances. So "1B3" and "K4R" both match at that point in the pattern.

To use upper case only, remove the trailing "i" from the initial $formget assignment.

[wrkrbeee]

Thank you Loops, again I am grateful for your help. Could not do it without your insight. Thanks, and Happy Holidays! Rick