[Exim] Grouping allowed in system filter pcre?

Hi folks,

The following system filter gives me an error during testing,
complaining that the opening paren ("(") is unexpected:

| # Exim filter
|
| if error_message then finish endif
|
| if $recipients matches \\b(sheldonh|root)@starjuice\\.net\\b
| then
| unseen deliver sheldonh-archive
| finish
| endif

Is grouping allowed in regular expressions in the system filter, and if
so what am I doing wrong?

I'm having this problem with both exim-4.10 and exim-3.36.

Ciao,
Sheldon.