regular expression test set
Lines are at least three fields, separated by one or more tabs.  "" stands
for an empty field.  First field is an RE.  Second field is flags.  If
C flag given, regcomp() is expected to fail, and the third field is the
error name (minus the leading REG_).

Otherwise it is expected to succeed, and the third field is the string to
try matching it against.  If there is no fourth field, the match is
expected to fail.  If there is a fourth field, it is the substring that
the RE is expected to match.  If there is a fifth field, it is a comma-
separated list of what the subexpressions should match, with - indicating
no match for that one.  In both the fourth and fifth fields, a (sub)field
starting with @ indicates that the (sub)expression is expected to match
a null string followed by the stuff after the @; this provides a way to
test where null strings match.  The character `N' in REs and strings
is newline, `S' is space, `T' is tab, `Z' is NUL.

The full list of flags:
  -	placeholder, does nothing
  b	RE is a BRE, not an ERE
  &	try it as both an ERE and a BRE
  C	regcomp() error expected, third field is error name
  i	REG_ICASE
  m	("mundane") REG_NOSPEC
  s	REG_NOSUB (not really testable)
  n	REG_NEWLINE
  ^	REG_NOTBOL
  $	REG_NOTEOL
  #	REG_STARTEND (see below)
  p	REG_PEND
  P	REG_POSIX

For REG_STARTEND, the start/end offsets are those of the substring
enclosed in ().