[TxMt] Re: Conditional regex

[Michael Sheets]

On May 22, 2009, at 12:56 PM, Dijk van Marc wrote:

> 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.

Start a begin/end match with a look-ahead on ATOM|HETATM:

(?:ATOM|HETATM)

And an end of $ (end of line). Inside the pattern rules have a match  
for the GLN.

You could use a conditional regex to do this if it was before the  
ATOM, but you can't do look-behinds of an unspecified length.