[pjknowles]

Problem

CMakeLists.txt takes finding regex.h to mean that regcomp() and friends can be linked. That is true where they are in libc (glibc, macOS, the BSDs), but not where regex.h belongs to a separate library. On MSYS2 (MinGW-w64, e.g. UCRT64) regex.h comes from libsystre (built on TRE). GKlib builds, but anything linked with it fails:

ld.exe: .../lib/libGKlib.a(string.c.obj):string.c:(.text+0x151): undefined reference to `regcomp'
ld.exe: .../lib/libGKlib.a(string.c.obj):string.c:(.text+0x1d4): undefined reference to `regexec'
ld.exe: .../lib/libGKlib.a(string.c.obj):string.c:(.text+0x35c): undefined reference to `regfree'

Fix

• Compile and link a small program that calls regcomp(), regexec(), regerror() and regfree().
• If that fails with the C library alone, try systre, tre, gnurx and regex in turn. Add the first one that works to GKlib's PUBLIC link libraries, so it is exported to find_package(GKlib) users. This is the same approach already used for libexecinfo.
• If none works, use GKlib's own implementation (gkregex) rather than producing a library that cannot be linked.
• check_symbol_exists() is deliberately not used: it accepts a macro, and TRE's regex.h defines regcomp etc. as macros for tre_regcomp etc., so it would wrongly report success.
• The GKREGEX option (make config gkregex=set) was declared but had no effect. It now selects gkregex unconditionally, which gives users a way to avoid the system regex.

Testing

MSYS2 UCRT64, GCC 16.2.0, CMake 4.4.4, libsystre 1.0.2. Each case builds and installs GKlib (GKLIB_BUILD_APPS=OFF unless stated), then builds a small find_package(GKlib) program that calls gk_strstr_replace() and runs it:

Case	Before	After
default	GKlib builds; test program fails to link (above)	systre detected and exported; program runs correctly
default, GKLIB_BUILD_APPS=ON	–	builds; program runs correctly
no usable regex library (library checks forced off), CMAKE_C_STANDARD=17	–	falls back to gkregex; program runs correctly
same, default C standard (C23)	–	gkregex.c fails to compile, as fixed by #66; with #66 applied, program runs correctly
GKREGEX=ON, CMAKE_C_STANDARD=17	–	uses gkregex; program runs correctly

macOS (Apple clang). The library check passes against libc, nothing extra is linked and the program runs correctly. I also ran GKREGEX=ON, plus a TRE-style regex.h that maps regcomp to tre_regcomp. With no library providing tre_regcomp, GKlib falls back to gkregex. When a libtre is available, tre is detected, exported and linked. In both cases the test program runs correctly. Without this change, that setup fails to link (_tre_regcomp undefined).

Not tested: Linux (the libc check should pass exactly as on macOS) and native MSVC (no regex.h, so behaviour is unchanged).

Known limitation: a fully static link (-static) on MSYS2 still needs -ltre -lintl -liconv added by the user. libsystre.a depends on these, and the check only sees the shared import library. Setting GKREGEX=ON avoids this.

Reported by Claude Code on behalf of @pjknowles, following a request from @ajmay81.

🤖 Generated with Claude Code

https://claude.ai/code/session_01HhHuoyV4eedKKTaFRt9Wfi