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

Problem:    When 'hlsearch' is set and matching with the last search pattern
            is very slow, Vim becomes unusable.  Cannot quit search by
            pressing CTRL-C.
Solution:   When the search times out set a flag and don't try again.  Check
            for timeout and CTRL-C in NFA loop that adds states.

Problem:    Modelines in source code are inconsistant.
Solution:   Use the same line in most files.  Add 'noet'.  (Naruhiko Nishino)

Problem:    With some regexp patterns the NFA engine uses many states and
            becomes very slow.  To the user it looks like Vim freezes.
Solution:   When the number of states reaches a limit fall back to the old
            engine. (Christian Brabandt)

Problem:    Duplicate code in regexec().
Solution:   Add line_lbr flag to regexec_nl().

Problem:    HP-UX compiler can't handle zero size array. (Charles Cooper)
Solution:   Make the array one item big.

Problem:    New regexp engine: Matching plain text could be faster.
Solution:   Detect a plain text match and handle it specifically.  Add
            vim_regfree().

Problem:    New regexp engine: negated flag is hardly used.
Solution:   Add separate _NEG states, remove negated flag.

Problem:    New regexp engine is a bit slow.
Solution:   Skip ahead to a character that must match.  Don't try matching a
            "^" patter past the start of line.

Problem:    New regexp engine: saving and restoring lastlist in the states
            takes a lot of time.
Solution:   Use a second lastlist value for the first recursive call.