$_ = "a b=c";
/(a)(.*)(?:b=(\w))?/;

printf "\$2 is [%s]\n", defined $2 ? $2 : "undefined";
printf "\$3 is [%s]\n", defined $3 ? $3 : "undefined";

##</code><code>##

$2 is [ b=c]
$3 is [undefined]

##</code><code>##

$_ = "a b=c";
/(a)(.*?)(?:b=(\w))?/;

printf "\$2 is [%s]\n", defined $2 ? $2 : "undefined";
printf "\$3 is [%s]\n", defined $3 ? $3 : "undefined";

##</code><code>##

$2 is []
$3 is [undefined]