"values are integer numerals, optionally followed by an insertion code of the form ^X where X is a letter. in regex terms: '[0-9+](^[A-Z])?"^^ . . . "relates a residue in a PDB structure to the coordinate given\nto it in its ATOM records in the traditional PDB format"^^ . . "e.g. the 1st ATOM record for the C chain of 1JWS starts 'ATOM 3015 N PRO C 306 ...'; hence pdb_coord '306'"^^ . "has coordinate from PDB record"^^ .