String Editor - unprintable text

Hi,  When using the string editor (in the basic device driver). - what format codes etc. are allowed - is this documented anywhere?  I'm trying to figure out how to include unprintable chars ..  and to keep it readable and possible to format values in correctly was trying not to revert to just using hex.   .. in the example below I need the variable to be formatted in as a 2 digit hex representation of the integer..