while($string=~ m/reg(ex)/)
{
    $string=~ s/$1/ister/;
}

##</code><code>##

my $string = "Sometimes there are extra effects you didn't foresee when using a regex.";

while($string=~ m/reg(ex)/)
{
    $string=~ s/$1/ister/;
}

print $string;


##</code><code>##

Sometimes there are istertra effects you didn't foresee when using a register.