Error extracting username when using the | rex field= statement

[cantrellr]

I have a user field where the name may or may not be prefixed with DOMAIN\ as shown below:

DOMAIN\CWIX-USER-SC-4
a.roset.nor
b.cwix.usa
DOMAIN\b-cwix-usa
b.mccartney.pld
c.merri.bel

I used regex.com PCRE (PHP) to craft the following expression:

(\S+\\)?(?P<username>[(\S+|\S+)]+)

However, when I use that expression in my search query, I'm getting the following error:

Error in 'rex' command: Encountered the following error while compiling the regex '((\S+\)?(?P<username>[(\S+|\S+)]+))': Regex: missing closing parenthesis.

Here is the line in the search query:

| rex field=user "((\S+\\)?(?P<username>[(\S+|\S+)]+))"

I have used the rex field statement many times in previous searches so I'm kind of lost at what is going on here. It's been a long week crafting dashboards and an extra set of eyes would be appreciated.

[cantrellr]

Unfortunately, your suggested rex did not produce any results. I ended up using the mvindex(split(field, "\\"), -1) statement instead. Thanks for your help.

[kennetkline]

Took me a minute;  I posted it wrong the first time
this works;    

if forgot to match "^"  (basically  \ OR ^ begins with)  

| rex field=user "(\w+\\|^)(?<username>.*)"

https://regex101.com/  (saves my bacon)