How to use a Registry Variable in UpdateAttribute

[James McMahon]

I have an incoming filename value of LarryCurlyMoe_ABCDstooges.
I want to apply an expression in an UpdateAttribute processor that performs
a substringAfter with a Registry variable reference.
My Registry variable myTokens has value ABCD.
I'd like to apply
${filename:toUpper():substringAfter(toUpper(${myTokens}))} to give me a new
filename of STOOGES.
The expression above throws an error. How do I craft that expression to
reference the value in Registry variable myTokens?
Thanks in advance.