http://qs321.pair.com?node_id=11139044

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

Hi Monks,
I wanted your help to fix a couple of pattern matching regex that I have. My script: 
#!/usr/bin/perl
use strict;
use warnings;

my $s='IIIIIIIIIIIIIIIIIIIIIIIIIIOOOOOSSSSS';

if (0 == $s =~ tr/[IOSM]//c) {
    print "case1\n";
}

elsif (0 == $s =~ tr/[IOS]//c) {
    print "case2\n";
}

[download]

Can you tell me why it prints case1 although my string does not have M inside?

Replies are listed 'Best First'.
Re: Pattern matching regex problem
by jwkrahn (Abbot) on Nov 23, 2021 at 08:53 UTC

    You are NOT using "pattern matching regex", you are using transliteration.

     $s =~ tr/[IOSM]//c says to translate every character NOT in the list '[', 'I', 'O', 'S', 'M' or ']' to itself, and as a side effect return the count of characters modified.

    So the string in $s does not contain any characters that are not in the list '[', 'I', 'O', 'S', 'M' or ']'.

[reply]
[d/l]
[select]
      Ok, I see the difference. So, how would I tell it to print 'case 1' if 'M' is present, and 'case 2' if it's not?
[reply]

        If all you want to do is check the presence of M using tr/// then it is simple:

        #!/usr/bin/env perl 
use strict;
use warnings;

my $s='IIIIIIIIIIIIIIIIIIIIIIIIIIOOOOOSSSSS';

print "$s: " . ($s =~ tr/M/M/ ? "case1\n" : "case2\n");

$s .= 'M';

print "$s: " . ($s =~ tr/M/M/ ? "case1\n" : "case2\n");

        [download]

        🦛

[reply]
[d/l]
[select]
Re: Pattern matching regex problem
by Corion (Patriarch) on Nov 23, 2021 at 08:46 UTC

    tr doesn't take character classes in square brackets ([]), so your code can also be written as:

    if (0 == $s =~ tr/IOSM//c) {

    [download]

    Your code counts whether there is any of the letters I, O, S or M in the string. Which is obviously true.

    Maybe you wanted to match the letters in order?

    m/I+O+S+M+/

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

Back to Seekers of Perl Wisdom