$ uptex testfiles/string-001.lvt
This is upTeX, Version 3.14159265-p3.6-u1.20 (utf8.uptex) (TeX Live 2015) (preloaded format=uptex)
restricted \write18 enabled.
(./testfiles/string-001.lvt
(/home/davidc/texmf/tex/latex/l3build/regression-test.tex
! Undefined control sequence.
l.50 \expandafter\edef\csname\detokenize