regex HTTP::header replace "Host"

I am trying to create an irule that can use a regex like:

/^([\s\S]?)(domain.com)./g

So if a user types:

domainname.com

abc.domainname.com

abc.domainname.com\xyz

It will still work.

Current irule

when HTTP_REQUEST {
  if { [string tolower [HTTP::uri] ] contains "/?node=1" } {
    HTTP::header replace "Host" "dev.domainname.com"
    HTTP::path "/"
    node 192.168.70.57 443
  }
}

I have tried to put the irule in line but it keeps giving errors and google is no help at all.

when HTTP_REQUEST {
  if { [string tolower [HTTP::uri] ] contains "/?node=1" } {
    HTTP::header replace "Host" "/^([\s\S]*?)(domainname).*/g"
    HTTP::path "/"
    node 192.168.70.57 443
  }
}