Role of the font in deciding the cluster boundaries

One quick feedback on the regular expression approach to decide a grapheme
cluster. In many Indic scripts, whether to display a sequence contianing
<..consonant, virama, consonant..> as a single cluster or to split them
with explicit visible virama is font dependent.

For example, in Malayalam, sequence S-KHA (സ്ഖ) would be displayed with
with explicit virama in a reformed script font and as a single unit in
traditional script font.