Trying to replace the header host value if it doesn't match 2 conditions

Thanks Vijay,

Let me know if I did something wrong, but I entered the rule like this:

when HTTP_REQUEST {

HOST = [string tolower [HTTP::host]]

if { (not ($HOST eq "abc.host.com")) or

   (not ($HOST eq "xyz.host.com")) } {

    HTTP::header replace Host "abc.host.com" 

  } 

 }

And it gave me this error: 01070151:3: Rule [/Common/rule_header_replace_abc] error: /Common/rule_header_replace_abc:2: error: [undefined procedure: HOST][HOST = [string tolower [HTTP::host]]]