Behavior of isolated capture groups with alternation

[Thom1729]

The following two regexps seem to be equivalent:

a(?i)b|c
a(?i:b|c)

I expected the following to be equivalent instead:

a(?i)b|c
(?:a(?i)b)|c

The documentation mentions something like this, but it isn't completely clear to me. Is this behavior correct? If so, it might be helpful to add a test somewhere around here.

[kkos]

/a(?i)b|c/ is interpreted as /a(?i)(?:b|c)/ or /a(?i:b|c)/.
It is intended to match with Perl 5.

#!/usr/bin/perl

if ("C" =~ /(?i)b|c/) {
    print "OK\n";
}
else {
    print "FAIL\n";
}

if ("aC" =~ /a(?i)(?:b|c)/) {
    print "OK\n";
}
else {
    print "FAIL\n";
}

if ("C" =~ /(?:a(?i)b)|c/) {
    print "OK\n";
}
else {
    print "FAIL\n";
}

OK
OK
FAIL

But I just found /a(?i)b|c/ is not interpreted as /a(?i)(?:b|c)/ in Perl 5.

if ("C" =~ /a(?i)b|c/) {
    print "OK\n";
}
else {
    print "FAIL\n";
}

OK

There are different scopes for alternatives and option.
It is strange behavior for me, and I don't want to follow it.