Counting Strings in Regexs After A Conditional in Perl

Question

Storing all matching strings in an array in Perl does not seem to work correctly if I do it after a simple conditional involving the query string. Does anyone know why this is? I have some code below that illustrates this:

#!/usr/local/bin/perl -w

use warnings;
use strict;

my  ($string,    
    @instance_array,   
    );

$string = 'ABCDCECFCG';
print "String: $string\n\n";

# PRINT INSTANCE ARRAY BEFORE CONDITIONAL
@instance_array = $string =~ m/C./g;
print "BEFORE CONDITIONAL\n@instance_array\n"; print scalar @instance_array; print "\n\n";
undef @instance_array;

##############################################

#if (@instance_array = $string =~ m/C./g)
if ($string =~ m/C./g)
    {
    # PRINT INSTANCE ARRAY INSIDE CONDITIONAL
    print "INSIDE CONDITIONAL\n@instance_array\n"; print scalar @instance_array; print "\n\n";        
    };

print "String: $string\n\n";    
undef @instance_array;

############################################

# PRINT INSTANCE ARRAY AFTER CONDITIONAL
@instance_array = $string =~ m/C./g;
print "AFTER CONDITIONAL\n@instance_array\n"; print scalar @instance_array; print "\n\n";
undef @instance_array;

# PRINT FINAL INSTANCE ARRAY
@instance_array = $string =~ m/C./g;
print "AFTER CONDITIONAL\n@instance_array\n"; print scalar @instance_array; print "\n\n";
undef @instance_array;

exit;

Notice that the "BEFORE CONDITIONAL" @instance_array output is correct (i.e., there are four matches for the regex), the one inside the conditional is undefined (as it should be), but the "AFTER CONDITIONAL" @instance_array is missing the "CD" match. Also notice that the "FINAL" @instance_array is also correct, even though it is the exact same code as that used to print the "AFTER CONDITIONAL" @instance_array.

According to the $string print statement after the conditional, the conditional does not appear change the query $string in any way.

Another interesting thing to note: If I send all the matches to the @instance_array in the simple conditional (this line is commented out above the simple conditional), then all four matches are correctly placed in the array.

Does anyone know why the initial match ("CD") is not entering the @instance_array? Perhaps it is storing the "CD" match in an internal variable (like $1, even though I know this is not quite correct) and not properly reinitializing it after the conditional?

Thank you for your help everyone.

You're using `m//g` in scalar context, which steps through the string match-by-match. — melpomene, Oct 20 '16 at 18:29
Don't use `-w` (it's been obsolete since 2000, when `use warnings;` was introduced). — melpomene, Oct 20 '16 at 18:30
Thank you for telling me about the `-w`. I did not know that. You're right, the `m//g` is in scalar context, but the @instance_array should convert that into list context. The command seems to work fine without the conditional, I just don't know what happened to the "CD" when the conditional is added. — sfoy, Oct 20 '16 at 19:00
http://stackoverflow.com/questions/1849329/is-there-a-perl-shortcut-to-count-the-number-of-matches-in-a-string] Discusses using this technique to count the number of matches in a query string. I know others have used the technique; I just do not know why it is not including "CD" when counting. — sfoy, Oct 20 '16 at 19:16

score 2 · Accepted Answer · answered Oct 20 '16 at 20:03

2

Just remove /g in your condition i.e.:

if ($string =~ m/C./g)

For the if condition you shouldn't use /g. This is because it's changing the pos of the regex. You can use it, though, e.g. in while loop.

To see how it's being changed you can print it after that if statement:

print pos $string;

You will see that it's changed to 4. To reset the pos:

pos($string) = 0;

However, it's not recommended. Just don't use /g in the if.

answered Oct 20 '16 at 20:03

Dave Grabowski

1,619
10
19

Thank you Davvid. I tested the `pos` command in various places throughout my code and I understand what is going on now. Thank you so much for your help and the excellent explanation. – sfoy Oct 20 '16 at 20:40

score 0 · Answer 2 · edited Oct 21 '16 at 06:02

0

try this:

my @matches = ($string =~ /(regex)/g);

edited Oct 21 '16 at 06:02

Buddy

10,874
5
41
58

answered Oct 21 '16 at 05:59

EvilArtist

1

score 0 · Answer 3 · answered Oct 21 '16 at 13:09

About your regex "/C./"

This certainly will be a problem because you're not restricting the regex criteria. The character '.' as you're using means "literally" anything to Perl regex engine. Now, supposing that you're trying to find the character '.', you must escape the "." character inside your regex statement like this:

/^C\.$/

If you want to capture the "matches", you must mark what you want to bring from each match with groups between parenthesis. Something like that:

/(whatever)(foo)(bar)/

Matches results in Perl are stored in special variables and are submited to scope rules. Some variables are $_, $1, $2,..., "$n" when '$n' is the nth group defined inside the regular expression, and is just "illustrative" and doesn't exists as special variable in fact. Considering the example above:

my $string = "Perl regex rocks!";
my ($match1, $match2, $match3) = ($1,$2,$3) if $string =~ /^(\w+)\s(\w+)\s(\w+)!$/;

#$match1 = "Perl"
#$match2 = "regex"
#$match3 = "rocks"

Links of Perl regular expression:

http://perldoc.perl.org/perlre.html

http://perldoc.perl.org/perlreref.html

Beautiful book about Regular expressions(tons of examples in Perl)

http://shop.oreilly.com/product/9780596528126.do

Counting Strings in Regexs After A Conditional in Perl

3 Answers3