[syslog-ng] patterndb ESTRING delimiter char

I need to find a valid char to use as the ESTRING delimiter that won't appear in a URI. I had been using comma, which usually works, but lots of sites have commas in the URI which messes up the parsing of these logs. Tab is outlawed as per the documentation or I'd have used that. I've seen pipe used in URI's, so that's no better. Keep in mind that what I'm parsing has been URI unescaped. Is there a way to use a null byte or something? Thanks, Martin