bug(csplit): some regexes, leads to a successfull match and valid regexes treated as invalid regexes

Hi, uutils mainteners

to reproduce that, we need first have an a file by example with that content

relunsec@relunsec:~/software/coreutils/target/debug$ cat a
hello
relunsec@relunsec:~/software/coreutils/target/debug$ 

it can be whatever, but just contains the h ascii char and then

we did

relunsec@relunsec:~/software/coreutils/target/debug$ sudo gnucsplit -b '%.d' a '/^^h/'
gnucsplit: ‘/^^h/’: match not found
6
relunsec@relunsec:~/software/coreutils/target/debug$ ls xx*
ls: cannot access 'xx*': No such file or directory
relunsec@relunsec:~/software/coreutils/target/debug$ sudo ./csplit -b '%.d' a '/^^h/'
0
6
relunsec@relunsec:~/software/coreutils/target/debug$ ls xx*
xx0  xx1

the gnu csplit say '/^^h/' no match however, the uu csplit one sees a match

it also with

relunsec@relunsec:~/software/coreutils/target/debug$ sudo ./csplit -b '%.d' a '/^?[a-z]/'
0
6
relunsec@relunsec:~/software/coreutils/target/debug$ sudo gnucsplit -b '%.d' a '/^?[a-z]/'
gnucsplit: ‘/^?[a-z]/’: match not found
6
relunsec@relunsec:~/software/coreutils/target/debug$ 

and valid regexes by uu treated as invalid, there a completly valid in standard

relunsec@relunsec:~/software/coreutils/target/debug$ cat a
[]
relunsec@relunsec:~/software/coreutils/target/debug$ sudo ./csplit -b '%.d' a '/^[[]]/'
csplit: '/^[[]]/': invalid pattern
relunsec@relunsec:~/software/coreutils/target/debug$ sudo gnucsplit -b '%.d' a '/^[[]]/'
0
3
relunsec@relunsec:~/software/coreutils/target/debug$ 

or just with

relunsec@relunsec:~/software/coreutils/target/debug$ sudo ./csplit -b '%.d' a '/^+/'
0
6
relunsec@relunsec:~/software/coreutils/target/debug$ sudo gnucsplit -b '%.d' a '/^+/'
gnucsplit: ‘/^+/’: match not found
6
relunsec@relunsec:~/software/coreutils/target/debug$ 

Start by running the reported csplit commands against the sample files and compare their results with GNU csplit. Trace the csplit implementation and its regular-expression handling, then ensure the shown patterns produce the same match or invalid-pattern behavior as GNU csplit and rerun all three reproductions.