#16 regexp sometimes crashes Yap

In Yap 5.11:

?- use_module(library(regexp)).
% reconsulting /usr/share/Yap/regexp.yap...
% reconsulted /usr/share/Yap/regexp.yap in module regexp, 0 msec 4096 bytes
yes
?- regexp("^(.+(a))\\2$", "krowaa", [], [_, X, _]).
X = [107,114,111,119,97] ?
yes
?- regexp("^(.+(a))\\2$", "krowaa", [], [_, X]).
yap: regexec.c:1372: pop_fail_stack: Assertion `num >= 0' failed.

Aborted (core dumped)

I would rather expect that the predicate to fail instead of core dumping.

[Vitor Santos Costa]

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Sorry for the long delay, I didn't use to track this.
It looks like this is a problem with the regular expression that causes regexc to crash. regcomp should fail but it doesn't.