These appear to be settings internal to the PCRE engine in order to limit the maximum amount of memory/time spent on trying to match some text to a pattern. The
pcreapi manpage does little to explain it in layman's terms:
The match_limit field provides a means of preventing PCRE from using
up a vast amount of resources when running patterns that are not going
to match, but which have a very large number of possibilities in their
search trees. The classic example is the use of nested unlimited
Internally, PCRE uses a function called match() which it
calls repeatedly (sometimes recursively). The limit set by match_limit
is imposed on the number of times this function is called during a
match, which has the effect of limiting the amount of backtracking
that can take place. For patterns that are not anchored, the count
restarts from zero for each position in the subject string.
default value for the limit can be set when PCRE is built; the default
default is 10 million, which handles all but the most extreme cases.
You can override the default by suppling pcre_exec() with a pcre_extra
block in which match_limit is set, and PCRE_EXTRA_MATCH_LIMIT is set
in the flags field. If the limit is exceeded, pcre_exec() returns
The match_limit_recursion field is similar to
match_limit, but instead of limiting the total number of times that
match() is called, it limits the depth of recursion. The recursion
depth is a smaller number than the total number of calls, because not
all calls to match() are recursive. This limit is of use only if it is
set smaller than match_limit.
Since the PCRE library built-in default is 10000000, my guess is that the lower setting is suggested for mod_security in order to prevent requests from being held up for a long time.