From pcre changelog, version 6.7:

18. A valid (though odd) pattern that looked like a POSIX character class but used an invalid character after [ (for example [[,abc,]]) caused pcrecompile() to give the error "Failed: internal error: code overflow" or in some cases to crash with a glibc free() error. This could even happen if the pattern terminated after [[ but there just happened to be a sequence of letters, a binary zero, and a closing ] in the memory that followed.

Acknowledgements:

Red Hat would like to thank Ludwig Nussel for reporting this issue.