suspect error in regex.c

[Francesco Potorti`]

I don't know how regex.c works, but this looks strange to me (I
discovered it because of a warning issued by MSVC on a report I got
about etags, which uses regex.c):

== src/regex.c ==

1837:typedef unsigned regnum_t;

in regex_compile:
2245:  regnum_t regnum = 0;
2847:           COMPILE_STACK_TOP.regnum = shy ? -regnum : regnum;

Notice that an unsigned value is negated, then assigned to an unsigned
field.

later in regex_compile:
2897:           regnum_t this_group_regnum;
2906:           this_group_regnum = COMPILE_STACK_TOP.regnum;
2914:           if (this_group_regnum <= MAX_REGNUM && this_group_regnum > 0)

That is, the same unsigned value is compared against MAX_REGNUM and 0.
In principle, the code could be correct, but it does sound strange to me
that such a particular programming practice is not even commented.
Looks suspiciously like a bug to me.