RxCache - Regular Expression Cache
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  Written 1998 by Lars Duening.
  Share and Enjoy!@

The rxcache acts as a wrapper around the regcomp() function
in the regexp module. It keeps track of the recently compiled
regular expression and stores both the expression string and
regexp's internal representation. If an expression is found
among the stored ones, the rxcache returns the stored internal
representation compiled earlier.

The interface is through the function regcomp_cache() which
takes the same parameters as regcomp(). However, since the
compiled expression is stored and managed by rxcache, the
caller of regcomp_cache() must not deallocate the compiled
expression after use.

The expressions are stored in a fixed size table, with entries
indexed by a hash over the pattern string. The table holds
RXCACHE_TABLE entries, and every entry can hold RXCACHE_CHAIN
expressions. Upon a hash collision, one of the stored expressions
is removed (and deallocated) using a round-robin selection.
This behaviour may not be optimal, but on the other hand
eventually deallocates expressions which are no longer used.
The two RXCACHE_ values are defined in config.h.

Initial tests showed an acceleration of the regexp _setup_
phase by factor 4. However, since the actual matching is as
fast as before, the net savings will be lower.

To install the rxcache, apply the diff file (use parameter
'-p1' with patch) and reconfigure and recompile the driver.

 -- Lars Duening; 17. Apr 1998