Being a Regular Expressions guru, the user can customize the document source syntax, changing the txt2tags default marks to some he find more comfortable.
For example, a leading TAB is the Quotation mark. If the user doesn't like it, or his text editor has some strange relationship with TABs, he can define a new mark for Quoted text. Say a leading ">>> " was his choice. Then he will do this simple filter:
%!preproc: '^>>> ' '\t'
And on the document source, the quoted text will be something like:
>>> This is a quoted text. >>> The user defined this strange mark. >>> But they will be converted to TABs by PreProc.
Before the parsing begins, the strange ">>> " will be converted to TABs and txt2tags will recognize the Quote mark.
BEWARE! Extreme PreProc rules could eventually change the entire marks syntax, even generating conflicts between marks. Be really really careful when doing this.