I've reproduced the problem, and it appears to be a change in the
newlib regular expression implementation, for a case that POSIX
leaves undefined.
I have bash 5.3 built from source, which apparently is a bit more
verbose than the default /usr/bin/bash.exe.
Here's a simpler example:
$ [[ x =~ ^(foo|)$ ]] ; echo $?
bash: [[: invalid regular expression `^(foo|)$': empty (sub)expression
2
$ [[ '' =~ ^(foo|)$ ]] ; echo $?
bash: [[: invalid regular expression `^(foo|)$': empty (sub)expression
2
$·
The expected result is 1 (no match) for the first and 0 (match) for
the second. A result of 2 indicates an invalid regular expression.
The error message is from newlib/libc/posix/regerror.c or
winsup/cygwin/regex/regerror.c (I presume the latter is used
in Cygwin).
The behavior is documented in the regex(3) man page:
‘|’ cannot appear first or last in a (sub)expression or
after another ‘|’, i.e., an operand of ‘|’ cannot be
an empty subexpression. An empty parenthesized subexpression,
‘()’, is legal and matches an empty (sub)string. An empty
string is not a legal RE.
So a workaround is to change the regular expression from
'^(foo|)$' to '^(foo|())$'.
"grep -E" accepts the empty subexpression without complaint.
On Ubuntu, both bash and "grep -E" accept the regular expression
with an empty subexpression is accepted without complaint.
POSIX leaves this explicitly undefined.
https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap09.html#tag_09_04_03
The <vertical-line> is special except when used in a bracket
expression (see 9.3.5 RE Bracket Expression). A <vertical-line>
appearing first or last in an ERE, or immediately following
a <vertical-line> or a <left-parenthesis>, or immediately
preceding a <right-parenthesis>, produces undefined results.
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