Don’t clobber the value of $MyString (line 3); instead, look at it. What is the Tinderbox putting in $MyString?
My guess is that either $MyString is being set to “” — which would be consistent with your report —or that $SearchDevonthink is not an attribute: remember that attributes are case-sensitive. (The casing of DEVONthink has driven me up the wall for a decade.)
My $Authors attribute is a system attribute of the Reference prototype. But your reply made me check my RegEx again and I found that I made mistake in action code syntax and regex itself. It’s working now, thanks!