Hi I'll illustrate the problem I have seen in the past with an example. It may or may not be directly or indirectly linked with the seg fault issue you are seeing. Although mine is not a seg fault, it does demonstrate very different behaviour between gcc on Centos (4.1.2) and gcc on Ubuntu (4.5.2) Consider the following 3 source files: main.cpp -> ---------------------------------- #include<iostream> #include"my_exception.h" using namespace std; void foo(); int main() {        try        {                foo();        }        catch(const my_exception &err)        {                cout << err.what();        }        //catch(const exception &err)        //{                //cout << "OOPs unknown error ";        //}        cout <<"\n"; } ---------------------------------- dll.cpp -> ---------------------------------- #include"my_exception.h" void  __attribute__ ((visibility("default"))) foo() {   throw my_exception(); } ---------------------------------- my_exception.h -> ---------------------------------- #include<exception> class  my_exception: public std::exception { public:        virtual const char* what() const throw()        {                return "my_exception";        }        my_exception()throw(){}        virtual ~my_exception()throw(){} }; ---------------------------------- and the following makefile makefile -> ---------------------------------- CXX=g++ all:        $(CXX) dll.cpp   -fvisibility=hidden  -fPIC -shared -o libdll.so        $(CXX) main.cpp   -fPIC -L./ -ldll   -o main clean:        rm -fr lib*.so main ---------------------------------- Compiled on Centos 5.6 with gcc with the following specs: Using built-in specs. Target: x86_64-redhat-linux Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-libgcj-multifile --enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk --disable-dssi --disable-plugin --with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre --with-cpu=generic --host=x86_64-redhat-linux Thread model: posix gcc version 4.1.2 20080704 (Red Hat 4.1.2-50) The output is : terminate called after throwing an instance of 'my_exception'  what():  my_exception Aborted However compiled  with gcc on Ubuntu 11.04 with specs: Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.5.2-8ubuntu4' --with-bugurl=file:///usr/share/doc/gcc-4.5/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.5 --enable-shared --enable-multiarch --with-multiarch-defaults=x86_64-linux-gnu --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib/x86_64-linux-gnu --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.5 --libdir=/usr/lib/x86_64-linux-gnu --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-plugin --enable-gold --enable-ld=default --with-plugin-ld=ld.gold --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.5.2 (Ubuntu/Linaro 4.5.2-8ubuntu4) The output is : my_exception If I execute the Ubuntu compiled binary on Centos I still get the same 'Aborted' output. If I execute the Centos compiled binary on Ubuntu I get the expected output of the string 'my_exception' This suggests that the 'difference' is not coming from the compiler but from the runtime. As stated before this may not be related to your seg fault issue but the authors of regex could most likely tell straight away. Best Wishes N