Re: git bug: Perl compatible regular expressions do not work as expected

[Junio C Hamano <gitster@pobox.com>]

René Scharfe <l.s.r@web.de> writes:

> 54463d32ef only affects basic regular expressions (BRE), but -G and -S
> use extended ones (ERE).  macOS allows both to be "enhanced".
>
> I have a hard time finding a readable reference to the documentation to
> brings us all onto the same page regarding what REG_ENHANCED actually
> does.  [1] is in raw troff format, [2] isn't very pretty and shows ads.

Wow, [2] is also misleading in that it loses the distinction between
'+' and '\+'; in [1] at least you can see the differences between \&+
and \e+ ;-)

> Anyway, the rather lengthy section "ENHANCED FEATURES" explains it.

I suspect that 54463d32ef was done in a conservative way to avoid
unintended side effects to make ERE "enhanced".  I am not 100%
certain, but after reading the documentation you pointed at, I do
not see a valid expression without ENHANCED flag starting to mean
totally different thing with it (well, an extra '?' turning a
pattern from greedy to minimal may count as such a change in
semantics, but I do not see anybody sensible adding an extra '?'
in a pattern in the first place).

Perhaps it is safe to go ahead and loosen what 54463d32ef did, with
something like this (this is not even compile tested, of course)?

 Makefile                  | 9 +++++++++
 compat/regcomp_enhanced.c | 2 ++
 git-compat-util.h         | 2 +-
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git c/Makefile w/Makefile
index 50ee51fde3..1fe0eb399e 100644
--- c/Makefile
+++ w/Makefile
@@ -293,6 +293,10 @@ include shared.mak
 # the flag REG_ENHANCED and you'd like to use it to enable enhanced basic
 # regular expressions.
 #
+# Define USE_ENHANCED_REGULAR_EXPRESSIONS if your C library provides
+# the flag REG_ENHANCED and you'd like to use it to enable enhanced
+# regular expressions for both BRE and ERE.
+#
 # Define HAVE_DEV_TTY if your system can open /dev/tty to interact with the
 # user.
 #
@@ -2041,11 +2045,16 @@ ifdef NO_REGEX
 	COMPAT_CFLAGS += -Icompat/regex
 	COMPAT_OBJS += compat/regex/regex.o
 else
+ifdef USE_ENHANCED_REGULAR_EXPRESSIONS
+	COMPAT_CFLAGS += -DUSE_ENHANCED_REGULAR_EXPRESSIONS
+	COMPAT_OBJS += compat/regcomp_enhanced.o
+else
 ifdef USE_ENHANCED_BASIC_REGULAR_EXPRESSIONS
 	COMPAT_CFLAGS += -DUSE_ENHANCED_BASIC_REGULAR_EXPRESSIONS
 	COMPAT_OBJS += compat/regcomp_enhanced.o
 endif
 endif
+endif
 ifdef NATIVE_CRLF
 	BASIC_CFLAGS += -DNATIVE_CRLF
 endif
diff --git c/compat/regcomp_enhanced.c w/compat/regcomp_enhanced.c
index 84193ce53b..aadbbac6d3 100644
--- c/compat/regcomp_enhanced.c
+++ w/compat/regcomp_enhanced.c
@@ -3,7 +3,9 @@
 
 int git_regcomp(regex_t *preg, const char *pattern, int cflags)
 {
+#ifndef USE_ENHANCED_REGULAR_EXPRESSIONS
 	if (!(cflags & REG_EXTENDED))
+#endif
 		cflags |= REG_ENHANCED;
 	return regcomp(preg, pattern, cflags);
 }
diff --git c/git-compat-util.h w/git-compat-util.h
index 1e6592624d..95d9b75a15 100644
--- c/git-compat-util.h
+++ w/git-compat-util.h
@@ -1377,7 +1377,7 @@ static inline int regexec_buf(const regex_t *preg, const char *buf, size_t size,
 	return regexec(preg, buf, nmatch, pmatch, eflags | REG_STARTEND);
 }
 
-#ifdef USE_ENHANCED_BASIC_REGULAR_EXPRESSIONS
+#if defined(USE_ENHANCED_BASIC_REGULAR_EXPRESSIONS) || defined(USE_ENHANCED_REGULAR_EXPRESSIONS)
 int git_regcomp(regex_t *preg, const char *pattern, int cflags);
 #define regcomp git_regcomp
 #endif