Pattern matching regex problem

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?

Comment on Pattern matching regex problem Select or Download Code

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]
Re^2: Pattern matching regex problem by Anonymous Monk on Nov 23, 2021 at 09:05 UTC
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]
Re^3: Pattern matching regex problem by hippo (Archbishop) on Nov 23, 2021 at 10:01 UTC
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^4: Pattern matching regex problem by bibliophile (Prior) on Nov 23, 2021 at 15:00 UTC
Re^5: Pattern matching regex problem by ikegami (Patriarch) on Nov 23, 2021 at 21:00 UTC
Re^4: Pattern matching regex problem by ikegami (Patriarch) on Nov 23, 2021 at 21:00 UTC
Re^5: Pattern matching regex problem (Abigail's Length Horror) by eyepopslikeamosquito (Archbishop) on Nov 24, 2021 at 10:45 UTC
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]