To check whether a string begins with a cmavo, first you have to check that it does not begin with a cmene or with a CVCy-lujvo.
A CVCy-lujvo consists of a CVC-rafsi, then a y, then any number of initial-rafsi (possibly none) and finally a brivla-core.
If the string does not begin with a cmene or with a CVCy-lujvo, then it begins with a cmavo if it begins with a cmavo-form and is followed by a pause or by a lojban-word.
A cmavo-form consists of a non-h, non-cluster onset, any number of nucleus (possibly none), and a final nucleus which is not stressed or, if stressed, it is not followed by a cluster. A cmavo-form can also consist of one or more y's in a row, or of a digit.