Regex not to allow double underscores

Question

Trying to apply regex for not allowing a string with double underscores

 [a-z][a-z0-9_-]+[^__]

but its failing in many cases like:

Answer 1

in perl it would be:

    if($a =~ /__/){
    } else{
    }

which means if string a contains "__" do something, if not do something else. Of course there is many ways how to beutify such code