You can do this with a regular expression:
on $*:text:/(?<=is).{33}.$/iS:#ax1-admin:{
msg Caveman Test
}
This will check, for instance,
43a78c0fac6f1f23b57559f6ba45051e.
to see if it's 33 characters long. If it's exactly 33 chars, it will message Caveman 'test'; if not, nothing is messaged.