Substrings with unlang

[Mikhail Sutormin]

Greetings,

We are running a VM with FR3 in Azure which authenticates users with their
O365 credentials against Azure ADDS. The problem is a legacy 20 characters
limitation of SamAccountName attribute. I strip "@domain.com" part off O365
username before passing it to ntlm_auth but some users have very long names
so "sampleverylongusername" is truncated to "sampleverylonguserna" in DC.
How do I right a simple check to truncate it in FR? I'm not skilled in
regular expressions.

So I've tried

        if (Stripped-User-Name =~ /(.{0,19})$/) {
                        update request {
                                Stripped-User-Name := "%{1}"
                        }
                }

but that cuts off first 20 characters and leaves the rest.