A program I'm writing requires that the Boost Regex library was compiled with ICU support.

How can I determine at configure/compile time whether the Regex library has ICU support?

I'd like to prevent the user from shooting him/herself in the foot.

Um, you can't detect it at compile time except that, if ICU is not in the users include path then boost/regex/icu.hpp won't compile. You will also get linker errors if you attempt to use ICU support and the library was built without it. So.... an AC_TRY_LINK in your configure script along with a minimal test program that invokes a u32regex would do the trick. HTH, John.