= RegEx =

the program uses the pcre-library for regular expressions in ascii matching
rules (in config file). 

If you use a regex similar  to "^[:print:]*$" it does not match on
* all strings containing characters outside of printable ascii range (see 'man
pcre2pattern' for details)
* an empty string "" will not matched, because we defined PCRE_NOTEMPTY. 
  An empty string is not a valid match, this behaviour is a design choice.
* Unicode is not set, because TIFF specification is in general ASCII based
  (PCRE_UTF8 not set)