$_ = join '', <DATA>;
s/(<a href="https?:\/\/(?:(?!target=).)*?)>/$1 target="_blank">/igs;
print;

__DATA__
<a href="dsfsdf">sdfdsf</a>
<a href="http://dsfsdf">sdfdsf</a>
<a href="https://dsfsdf">sdfdsf</a>
<a href="http://dsfsdf" target="_blank">sdfdsf</a>
<a href="http://dsfsdf"
>sdfdsf</a>