How to replace pairs tags with regexp

[Suliman via Digitalmars-d-learn]

On Friday, 21 July 2017 at 06:19:43 UTC, Suliman wrote:
> There reason of issue above is spaces before "#".

What wrong with next regex https://dpaste.dzfl.pl/024a47ed2a56

I expect that it will select:

#Header
my header text

##SubHeader
my sub header text

Because: ^#{3}