[TxMt] Conditional regex

Hello all,

I'm developing a bundle for handling biomolecular structure files (PDB files). I'm now working on getting the language description sorted out. In doing this I ran into a regex problem.

The situation: A PDB file is essentially a table. One line from such a table can be as follows:

ATOM 14 CA GLN A 2 -27.648 -9.581 30.325 1.00 10.00

In the language description I would like to use a regular expression construct that matched 'GLN' only if the line starts with either 'ATOM' or 'HETATM'. This seams like a conditional regular expression but my attempts to implement it at such have failed so far.

Can anyone help?

Thanks, Marc