Hi, cabal install regex-posix runs without issue. added it to cabal dependencies: name:                data-regex version:             0.1.0.0 -- synopsis: -- description: homepage:            https://github.com/githubuser/data-regex#readme license:             BSD3 license-file:        LICENSE author:              Alexander Chen maintainer:          example@example.com copyright:           2020 Alexander Chen category:            regex build-type:          Simple cabal-version:       >=1.10 extra-source-files:  README.md executable data-regex   hs-source-dirs:      src   main-is:             Main.hs   default-language:    Haskell2010   build-depends:       base >= 4.7 && < 5, regex-posix  Then I do: stack install regex-posix error> WARNING: Ignoring regex-posix's bounds on base (<0 && >=4.3 && <4.15); using base-4.13.0.0. Reason: trusting snapshot over cabal file dependency information. regex-posix> configure regex-posix> Configuring regex-posix-0.96.0.0... regex-posix> build regex-posix> Preprocessing library for regex-posix-0.96.0.0.. regex-posix> C:\Users\achen\AppData\Local\Temp\stack-e2b99a8a9eb45aea\regex-posix-0.96.0.0\Wrap.hsc:96:10: fatal error: regex.h: No such file or directory regex-posix> compilation terminated. regex-posix> compiling .stack-work\dist\29cc6475\build\Text\Regex\Posix\Wrap_hsc_make.c failed (exit code 1) regex-posix> command was: C:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\ghc-8.8.3\lib\../mingw/bin\gcc.exe -c .stack-work\dist\29cc6475\build\Text\Regex\Posix\Wrap_hsc_make.c -o .stack-work\dist\29cc6475\build\Text\Regex\Posix\Wrap_hsc_make.o -D__GLASGOW_HASKELL__=808 -Dmingw32_BUILD_OS=1 -Dx86_64_BUILD_ARCH=1 -Dmingw32_HOST_OS=1 -Dx86_64_HOST_ARCH=1 -Icbits -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\msys2-20180531\mingw64\include -I.stack-work\dist\29cc6475\build\cbits -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\msys2-20180531\mingw64\include -I.stack-work\dist\29cc6475\build\autogen -I.stack-work\dist\29cc6475\build\global-autogen -include .stack-work\dist\29cc6475\build\autogen\cabal_macros.h -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\msys2-20180531\mingw64\include -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\ghc-8.8.3\lib\bytestring-0.10.10.0\include -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\ghc-8.8.3\lib\base-4.13.0.0\include -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\ghc-8.8.3\lib\integer-gmp-1.0.2.0\include -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\ghc-8.8.3\lib/include -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\ghc-8.8.3/lib/include/ --  While building package regex-posix-0.96.0.0 using:       C:\sr\setup-exe-cache\x86_64-windows\Cabal-simple_Z6RU0evB_3.0.1.0_ghc-8.8.3.exe --builddir=.stack-work\dist\29cc6475 build --ghc-options " -fdiagnostics-color=always"           Process exited with code: ExitFailure 1 Progress 1/2 ========================================================================================================================================================================================= I also get an error in my main(but seems related): module Main where import Text.Regex.Posix main :: IO () main = do   let str1 = "one fish two fish red fish blue fish"   let str2 = "The quick brown fox jumps over the lazy dog."   print str1 =~ "one" ::Bool What am I doing wrong? best,