match partial dhcp-client-identifier option

Hi all!

I'm working with development on diskless clients and these PXE-boot off of a SLES server. The thing is that it's not always the same clients booting and to complicate matters more there are two different hardware versions right now, so I want one version to boot one linux and the other to boot another linux. All this must be controlled by the dhcp client-identifier option which, in the new version is unique for every client, except for the 4 first bytes.

example:
client 1 with HW A sends CID "fooA bootme"
client 2 with HW A sends CID "fooA biteme1"

I've tried:
if substring (option dhcp-client-identifier, 1, 4) = "fooA"
and
if substring (option dhcp-client-identifier, 0, 4) = 66:6F:6F:41
with no success.

how can I match the "fooA" part with dhcp-client-identifier option to make client 1 and 2 boot the same linux?

Please help!

Solution:

dhcpd.conf:

option domain-name "exapmple.com";
ddns-update-style none;
default-lease-time 14400;
option local-option97 code 97 = string;

group {
#----HWA----
if substring (option local-option97, 1, 4) = "fooA" {
filename "fooA/pxelinux.0";
next-server <IP>;
}

# ----HWB----
if substring (option local-option97, 0, 5) = 00:66:6f:6f:42 {
filename "fooB/pxelinux.0";
next-server <IP>;
}

subnet <IP> netmask <mask> {
range <IP> <IP>;
max-lease-time 172800;
default-lease-time 14400;
}
}