Hello,

Our UC-KLEE tool found a char-sign-extension bug in isc_regex_validate(), which may cause it to improperly accept or reject a regular expression depending on whether the compiler treated chars as signed or unsigned (either is allowed under the C specification). 

The offending code is in isc_regex_validate (lib/isc/regex.c) on line 258 in the latest git v9_9 HEAD:
  if (range == 2 && *c < range_start)

Some example inputs where the above condition may evaluate to true or false depending on the compiler are:
  \x5c-\x80
  \x83-\x02

The simplest fix is probably to change 'c' to an unsigned char, and possibly range_start to an unsigned int.

Thanks,
-David