Before commit d904cc83 regex syntax flags used in etags were

   RE_CHAR_CLASSES | RE_INTERVALS | RE_SHY_GROUPS | RE_FRUGAL

   that got changed to 0. Including a patch that restores
   RE_CHAR_CLASSES and RE_INTERVALS and adds check for these features
   to the test. As far I can understand from the bug 74861, the
   absence of shy groups and non-greedy matching won't be fixed
   until Gnulib supports them