The shortcut doesn't work when

 - cname is an empty string, numeric or any special character not
   allow in identifiers
 - the aname starts with a number. This is due to the fact, that the
   "." is both used as a decimal point in floating point numbers and
   as a separator between chain, residue number and atom name. When
   parsing a numerical token, we have deal with trailing "."
   appropriately and stop parsing when the next character is not
   numeric. However, when the character following the "." is numeric,
   we also treat it as part of the numeric value.