use m4 as a replacement for sed

Hi List,

i want to use m4 to replace certain substrings in a text file.

My file looks like this (a Nagios configuration file):

define service {
    register               0
    name                   app_mssql_DBSRV
    host_name              HOSTNAME
}

What i want to do is to replace DBSRV and HOSTNAME which is simple with sed:

$ cat sample.cfg | sed -e 's/DBSRV/CMQS/g' -e 's/HOSTNAME/dbsrv11/g'
define service {
    register               0
    name                   app_mssql_CMQS
    host_name              dbsrv11
}

Alas, i like the definition of macros on the command line with m4. I think
this would be easier to script.

$ cat sample.cfg | m4 -DDBSRV=CMQS -DHOSTNAME=dbsrv11
define service {
    register               0
    name                   app_mssql_DBSRV
    host_name              dbsrv11
}

Unfortunately i found no way to make the DBSRV-substring in app_mssql_DBSRV
be recognized as a macro.
I played with "app_mssql_(DBSRV)" and it worked, but the delimiters appeared
in the output so i had to add a sed-command which replaced (.*) with .* .The
other thing is...i don't want to add too much extra code to the input file. 
What i understand is, DBSRV as a substring is not a macro and will me
ignored by m4.
Maybe it's impossible at all but maybe someone knows a simple trick how to
detect a macro name embedded in another string.

Cheers,
Gerhard