Thank you Nick,
In fact what I was looking for was not the Scintilla lexer (not at this point though), but the TagManager parser which definitions are stored into tagmanager. I modified the markdown.c according to this, and it's working fine, so I can try to adapt it in a real definition for txt2tags now, according to the HACKING file.
I'll tell you later when I get something usable for Geany.
>
> > for example in markdown for a header you can surround
> it by # like this : ## header 2 ##
> >
> > On txt2tags it's == header 2 ==. So I replaced this in
> the source code of LexMarkdown.cxx and recompiled geany, but
> it's not working when I change the source to markdown (I
> just made this quick test by keeping the markdown name):
>
> I don't know much about Scintilla lexers myself, but the
> MarkDown one seems to have other handling for the '=' and
> '#' chars
> which may need changing. I would cut out all the code you
> don't need
> for your lexer.
>
> > Where could I start for getting txt2tags working with
> Geany?
>
> Try the HACKING file for how to add a filetype.
>
> Regards,
> Nick
>