Developing a recursive command parser

Hi all, i'm trying to develop an application able to interpret commands loaded from a database or any other source similiar to this $strParam = 'title=%SESSION(sessionvar)' or a more complicated $strParam = 'title=%SESSION(sessionvar_%COOKIE(mycookievar))' What I need, is to find any special command starting width a configurable special char (% in this case), get the text inside it and elaborate it... The final result could be a simple string like these $strParam = 'title=My Session Title' and $strParam = 'title=My Session and Koookie Value' I'm starting to working on it, do you know if is there a way to accomplish to this using PCRE or if there is something just developed? Thanks, Straps