I realise I didn't really answer the question of which release -E came in in those.

For FreeBSD and NetBSD, that would be their respective 5.0 version even if they're many years apart.

For MacOS/X I don't know exactly, but https://github.com/NetBSD/src/blob/trunk/share/misc/bsd-family-tree suggests that could be 10.3 which is meant to have synced with FreeBSD 5.1

DragonFly BSD from the start (2003), I've not checked any other FreeBSD derivative (https://en.wikipedia.org/wiki/Comparison_of_BSD_operating_systems#FreeBSD).

OpenBSD's find doesn't have -E, I've not checked any of its derivatives (like MirBSD).

Chronologically:

-regex added first to NetBSD (inspired from GNU's) in:

commit 5ddc160e898d16e1f981c5f3f7c8aa41db732ac4
Author: cgd <cgd@NetBSD.org>
Date:   Tue Jul 20 01:28:41 1999 +0000

    add -regex and -iregex primaries which, like GNU find's primaries of the
    same name, match files' entire paths against regular expressions.
    -regex is case sensitive, -iregex is case-insensitive.  Note that these
    primaries are not entirely compatible with the GNU find primaries,
    because their BREs appear to support alternation with \| whereas our BREs
    do not.  Also note there are no primaries which provide extended regular
    expressions matching, though if they are desired they would be trivial
    to implement.


FreeBSD added all of -E -regex -i{name,path,regex} at the same time in:

commit 7c1d4b3ae901ee2c7a3537b10db787464fb26df0
Author: Akinori MUSHA <knu@FreeBSD.org>
Date:   Fri Feb 23 16:20:55 2001 +0000

    Implement the following options and primaries:

         -E      Interpret regular expressions followed by -regex and -iregex op-
                 tions as extended (modern) regular expressions rather than basic
                 regular expressions (BRE's).  The re_format(7) manual page fully
                 describes both formats.

         -iname pattern
                 Like -name, but the match is case insensitive.

         -ipath pattern
                 Like -path, but the match is case insensitive.

         -regex pattern
                 True if the whole path of the file matches pattern using regular
                 expression.  To match a file named ``./foo/xyzzy'', you can use
                 the regular expression ``.*/[xyz]*'' or ``.*/foo/.*'', but not
                 ``xyzzy'' or ``/foo/''.

         -iregex pattern
                 Like -regex, but the match is case insensitive.

    These are meant to be compatible with other find(1) implementations
    such as GNU's or NetBSD's except regexp library differences.

    Reviewed by:    sobomax, dcs, and some other people on -current

This option is implemented by the attached patch.  I haven't pushed this patch yet.


(file #56126)

I'm happy to implement this option as long as we can nail down which regex dialect -E should select.   Is there any chance you could provide more detail on your answer to question 1, perhaps by performing some experiments?

This sounds useful.   I have two questions.   The first is about the intended behaviour of -E and the second is about the portability aspect.

1. The interpretation of posix-egrep is documented accurately in the Texinfo manual for find (accurate because the documentation is generated automatically from the controlling regex dialect bitmask value).    Could you check in detail to make sure that FreeBSD's interpretation of -E really does yield the same regex dialect?   I ask this because the only thing worse for portability than not offering the same option is to offer the same option with very subtle differences in behaviour...

2. If this flag is implemented you will have (at least) three versions of find to worry about: 1. FreeBSD grep, which supports -E.   2. New versions of NGU find, likewise.   3. Older versions of GNU find, which don't.   What is your plan for cross-platform searching in this situation?