taking value and append to uri while masking?

Simpler code :

when HTTP_REQUEST { 
     Check for a non-null length host header 
    if {[string match {[a-zA-Z]} [HTTP::host]]} {
        HTTP::host "www.test123.com"
        HTTP::uri /[getfield [HTTP::host] . 1][HTTP::uri]
    }
}

I was able to get it to work using the following syntax:

when HTTP_REQUEST { Check for a non-null length host header if {[string match {[a-zA-Z]} [HTTP::host]]}{

   Use string commands to parse the subdomain
  set subdomain [string range [HTTP::host] 0 [expr {[string first . [HTTP::host]] -1}]]

   Use the getfield command to parse the subdomain
  set subdomain [getfield [HTTP::host] . 1]

   Use scan to parse the subdomain
  scan [HTTP::host] {%[^.]} subdomain

    HTTP::header replace Host "www.test123.com"
    HTTP::uri /$subdomain[HTTP::uri]

} }

I was able to find the first part on DevCentral forum of which it takes the value before the domain and adds as uri for testing purposes, however I get the following error when I try to save it.

01070151:3: Rule [/Common/ddc-wpchange1.com_irule] error: /Common/ddc-wpchange1.com_irule:8: error: [parse error: PARSE syntax 289 {syntax error in expression " [string tolower [HTTP::host]] ends with $::domain ": extra tokens at end of expression}][{ [string tolower [HTTP::host]] ends with $::domain }] /Common/ddc-wpchange1.com_irule:21: error: [wrong args][HTTP::host [string range $::domain 1 end]]

when RULE_INIT { Set the value for the domain with a leading period We'll check the host header value for any string the comes before this string and prepend it to the URI set ::domain ".test123.com" } when HTTP_REQUEST { if { [string tolower [HTTP::host]] ends with $::domain }{

   Get any part of the host that comes before the domain.
     Split the URI using the domain string.  Grab the first string that comes before the domain
  set subdomain [getfield [HTTP::host] $::domain 1]

   If subdomain isn't null, prepend it to the URI 
     You could also add additional checks here like subdomain doesn't equal       "www"
  if { not ($subdomain == "") }{

 HTTP::uri [concat "/$subdomain[HTTP::uri]"]

      Set the HTTP host to the domain
     HTTP::host [string range $::domain 1 end]
  }

} }