It was initially added to support atomic groups, but that
support was never fully implemented, and CALL was only left
in the compiler, but not interpreter and parser.
ATOMIC_GROUP is now used to support atomic groups.
Limit the maximum capturing group to 2**30-1 on 64-bit platforms
(it was 2**31-1). No change on 32-bit platforms (2**28-1).
It allows to reduce the size of SRE(match_context):
- On 32 bit platform: 36 bytes, no change. (msvc2022)
- On 64 bit platform: 72 bytes -> 56 bytes. (msvc2022/gcc9.4)
which leads to increasing the depth of backtracking.
Serhiy Storchaka