#APLX, 19 bytes

s[(∊(⎕a⎕A⎕D)[a])⍋s] 

⎕a⎕A⎕D lower upper digits

(…)[a] reorder according to array a

∊ flatten

(…)⍋s according to that "alphabet", give the indices that would sort string s

s[…] use that to reorder s

APLX, 19 bytes

s[(∊(⎕a⎕A⎕D)[a])⍋s] 

⎕a⎕A⎕D lower upper digits

(…)[a] reorder according to array a

∊ flatten

(…)⍋s according to that "alphabet", give the indices that would sort string s

s[…] use that to reorder s