Merge branch 'rs/use-enhanced-bre-on-macos' into maint-2.39

Newer regex library macOS stopped enabling GNU-like enhanced BRE,
where '\(A\|B\)' works as alternation, unless explicitly asked with
the REG_ENHANCED flag.  "git grep" now can be compiled to do so, to
retain the old behaviour.

* rs/use-enhanced-bre-on-macos:
  use enhanced basic regular expressions on macOS

Makefile

@@ -289,6 +289,10 @@ include shared.mak
 # Define NO_REGEX if your C library lacks regex support with REG_STARTEND
 # feature.
 #
+# Define USE_ENHANCED_BASIC_REGULAR_EXPRESSIONS if your C library provides
+# the flag REG_ENHANCED and you'd like to use it to enable enhanced basic
+# regular expressions.
+#
 # Define HAVE_DEV_TTY if your system can open /dev/tty to interact with the
 # user.
 #
@@ -2040,6 +2044,11 @@ endif
 ifdef NO_REGEX
 	COMPAT_CFLAGS += -Icompat/regex
 	COMPAT_OBJS += compat/regex/regex.o
+else
+ifdef USE_ENHANCED_BASIC_REGULAR_EXPRESSIONS
+	COMPAT_CFLAGS += -DUSE_ENHANCED_BASIC_REGULAR_EXPRESSIONS
+	COMPAT_OBJS += compat/regcomp_enhanced.o
+endif
 endif
 ifdef NATIVE_CRLF
 	BASIC_CFLAGS += -DNATIVE_CRLF

compat/regcomp_enhanced.c

@@ -0,0 +1,9 @@
+#include "../git-compat-util.h"
+#undef regcomp
+
+int git_regcomp(regex_t *preg, const char *pattern, int cflags)
+{
+	if (!(cflags & REG_EXTENDED))
+		cflags |= REG_ENHANCED;
+	return regcomp(preg, pattern, cflags);
+}

config.mak.uname

@@ -147,6 +147,7 @@ ifeq ($(uname_S),Darwin)
 	FREAD_READS_DIRECTORIES = UnfortunatelyYes
 	HAVE_NS_GET_EXECUTABLE_PATH = YesPlease
 	CSPRNG_METHOD = arc4random
+	USE_ENHANCED_BASIC_REGULAR_EXPRESSIONS = YesPlease
 
 	# Workaround for `gettext` being keg-only and not even being linked via
 	# `brew link --force gettext`, should be obsolete as of

git-compat-util.h

@@ -1346,6 +1346,11 @@ 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
+int git_regcomp(regex_t *preg, const char *pattern, int cflags);
+#define regcomp git_regcomp
+#endif
+
 #ifndef DIR_HAS_BSD_GROUP_SEMANTICS
 # define FORCE_DIR_SET_GID S_ISGID
 #else