I found this thread from October 2015 where Sjoerd concludes that back references replacement is not implemented in the MonetDB wrapper: https://www.monetdb.org/pipermail/users-list/2015-October/008492.html

Probably that is still the case?

On 20 April 2016 at 16:35, Roberto Cornacchia <roberto.cornacchia@gmail.com> wrote:
After declaring a wrapper for pcre.replace:

sql>create function pcre_replace(s string, pattern string, repl string, flags string) returns string external name pcre."replace";
operation successful (6.187ms)

I cannot get replacements with references to captures work (but the code in pcre.c seems to support it):

sql>select pcre_replace('Apple Pear', '^([a-zA-Z]+)\\s*([a-zA-Z]+)$', '\\1 \\2', '');
+---------------------------+
| pcre_replace_single_value |
+===========================+
| \1 \2                     |
+---------------------------+
1 tuple (0.309ms)

The pattern is matched, but the back references are replaced literally.
Am I not getting the syntax correct?
From http://www.pcre.org/original/doc/html/pcresyntax.html I see I could use any of \1, \g1, \g{1] for back references, but none seem to work.

Roberto