As a sane person, I run my Perl programs with -w. So today I stumbled across an interesting problem: 

# This works fine
$a = "abc";
$a =~ s/^(?:a(\w))?(\w)$/$1:$2/; # works fine

# This produces the warning
# "Use of uninitialized value in concatenation (.)
$b = "c";
$b =~ s/^(?:a(\w))?(\w)$/$1:$2/;

[download]

I do not mind $1 to be empty; in fact, I want it to be empty if ^a\w is not there. But I only want to put the second character in $1 if the first character is "a", so 

$b =~ s/^a?(\w?)(\w)$/$1:$2/; # no warning here.

[download]

won't do. The $64,000 question is, can I accomplish this in one line and not produce a warning?

In reply to Uninitialized $1 in s/// by dmitri

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