When using groups within a regex, it is usually possible to "back reference" to such a group via its number (or name), such as "\1" to reference to the first (capturing) group.

Am I right that this is not supported with SQL Anywhere?

Here's a sample:

-- search for a number followed by non-digits and a further number
select regexp_substr('abc-101-abd-102-abe', '(\\d+)\\D+\\d+');
-- -> returns "101-abd-102"

-- If I want the match to restrict to those cases
-- where the first and second number are identical,
-- that requires a backreference via \1, such as
select regexp_substr('abc-101-abd-102-abe', '(\\d+)\\D+\\1'); -- should not match
select regexp_substr('abc-101-abd-101-abe', '(\\d+)\\D+\\1'); -- should match

However, adding '\\1' to the regexp pattern leads to SQLCODE -1135 aka "Invalid regular expression: unsupported syntax in '(\d+)\D+\1'"

I'm using 16.0.0.2704 but I guess the regex support has been unchanged for quite a number of versions.