Problem:    Possibly use of NULL pointer. (Coverity)
Solution:   Reset the re_in_use flag earlier.

Problem:    Executing regexp recursively fails with a crash.
Solution:   Move global variables into "rex".

Problem:    Cannot build without syntax highlighting.
Solution:   Add #ifdef around using reg_do_extmatch.

Problem:    Segfault when pattern with \z() is very slow.
Solution:   Check for NULL regprog.  Add "nfa_fail" to test_override() to be
            able to test this.  Fix that 'searchhl' resets called_emsg.

Problem:    Invalid memory acces with pattern using look-behind match.
            (Dominique Pelle)
Solution:   Get a pointer to the right line.

Problem:    Clearing a pointer takes two lines.
Solution:   Add VIM_CLEAR() and replace vim_clear(). (Hirohito Higashi,
            closes #2629)

Problem:    Undefined left shift in gethexchrs(). (geeknik)
Solution:   Use unsigned long. (idea by Christian Brabandt, closes #2255)

Problem:    Newer gcc warns for implicit fallthrough.
Solution:   Consistently use a FALLTHROUGH comment. (Christian Brabandt)

Problem:    Syntax highlighting can make cause a freeze.
Solution:   Apply 'redrawtime' to syntax highlighting, per window.

Problem:    The hlsearch test fails on fast systems.
Solution:   Make the search pattern slower.  Fix that the old regexp engine
            doesn't timeout properly.