regex for Err operator.

[Raga]

Hi, Iam trying to write a source filter using Filter::Simple to use the
Binary // (Err) operator in perl6. Iam having a tough time to construct
the regex for this. I do have a semi working regex to substitute

$foo = $bar // $baz ;

s/(.*\s*)(.*)\b\s*\/\/\s*(.*)\b/$1defined($2) ? $2 : $3/gm;

But I wanted to know if there was a better way of constructing the
regex

Thanks
Ragavendar