To check if a string begins with a cmene, extract as many cmene-syllaboids as you can from the left (possibly none), then you must find a consonant or cluster (details of which clusters are allowed are still to be worked out) followed by a pause. In that case, you have a cmene, otherwise the string does not begin with a cmene.
A cmene-syllaboid consists of an optional coda, any number of consonantal-syllables (possibly none), an onset and a nucleus, but it can't be one of doi, la, lai or la'i. It can also be a digit, which stands for the syllable corresponding to its name, as in la 2005nan., the year 2005. (The reason to include the coda and consonantal-syllables as part of the syllaboid is so that things like ndoi are admissible.)