Disappearance of 'a' in compounds

Dear Friends,

I am still plugging away at a Javascript Pali Reader, and I have hit upon the problem of recognizing words like
"aacariyantevaasika" and "dosantara".  I am not sure about the second, but in the first, it looks like the aa has become
a.  I wonder if anyone has any information on when this occurs.  Are there any rules (eg before "nt")?  The program
currently will recognize "aacariya-antevaasika", but not "aacariya-ntevaasika".

Best wishes,

Yuttadhammo