插入空白 第二字母前 (10401)

C2{=REPLACE(B2,LEN(B2)-MATCH(1=1,ISERR(-LEFT(RIGHT(B2,ROW($1:$9)))),)+1,," ")

另解=REPLACE(B2,LOOKUP(1,0/ISERR(-(0&MID(B2,ROW($1:$9),1))),ROW($1:$9)),," ")