[Bug libstdc++/124000] std::regex::extended is too strict about escaped chars

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124000

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---

There are similar changes for BREs in the current POSIX spec:
https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap09.html#tag_09_03_02

The new wording defines the following additional escapes for BREs:

  - The ']' character; "\]" shall match a ']' character

  - The '?', '+', and '|' characters; it is implementation-defined whether
"\?", "\+",
    and "\|" each match the literal character '?', '+', or '|', respectively,
or behave
    as described for the ERE special characters '?', '+', and '|', respectively
(see
    9.4.3 ERE Special Characters).

    Note:
       A future version of this standard may require "\?", "\+", and "\|" to
behave as
       described for the ERE special characters '?', '+', and '|',
respectively.