Re: How To Remove Warnings From Regex

[Athanasius]

Hello roho,

Change $1 to \1 within the regex:

use strict;
use warnings;

my @txt = ('cc', 'cz');

for (@txt)
{
    if (m/^(.)\1$/)
    {
        print "True   \$_ = |$_|   \$1 = |$1|\n";
    }
    else
    {
        print "False  \$_ = |$_|   \$1 = |$1|\n";
    }
}

Output:

14:13 >perl 2092.SoPW.pl
True   $_ = |cc|   $1 = |c|
False  $_ = |cz|   $1 = |c|

14:14 >

As you can see:

• warnings are gone
• the closing $ is OK now
• $1 is no longer the empty string on match fail, but is rather the captured character “c”.

See e.g. Capture groups.

Hope that helps,

[roho]

Thank you Athanasius! That works perfectly. For as long as I can remember, I've avoided the backslash numbers in favor of the dollar-sign numbers, but I see in this case that is exactly what is called for. Thanks again.

[choroba]

\1 works inside the regex where the capture group was defined. $1 works everywhere else, i.e. in a replacement of a substitution. $1 in a regex refers to a previously matched regex (which explains the uninitialized warnings).

[roho]

Thanks for the explanation choroba. It always helps to know the "why" behind the way things work. It makes it easier to remember in the future when a similar situation arises. Perl Monks are the best!