##C, 112 bytes
j,i,k,l;f(char*s){l=strlen(s);for(i=l;j<l;)k+=(s[j++]-48)*(i>1?i--:-1);return!(k%11)&&(k!=0)&&((l==9)||(l==8));}
Returns 0 if invalid, 1 if valid. Try it online!
betseg
- 8.7k
- 3
- 28
- 60