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##C, 112 101 96 98 104 bytes

Thanks to @MartinEnder for saving 5 3 bytes while fixing my code!

j,i,k,l;f(char*s){l=strlen(s);for(i=l,j=k=0;j<l;)k+=(s[j++]-48)*(i>1?i--:-1);return!(k%11)&&k&&(l^8)<2;}

Returns 0 if invalid, 1 if valid. Try it online!

C, 112 101 96 98 104 bytes

Thanks to @MartinEnder for saving 5 3 bytes while fixing my code!

j,i,k,l;f(char*s){l=strlen(s);for(i=l,j=k=0;j<l;)k+=(s[j++]-48)*(i>1?i--:-1);return!(k%11)&&k&&(l^8)<2;}

Returns 0 if invalid, 1 if valid. Try it online!

##C, 112 101 96 98 104 bytes

Thanks to @MartinEnder for saving 5 3 bytes while fixing my code!

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&&(l^8)<2;}

Returns 0 if invalid, 1 if valid. Try it online!

##C, 112 101 96 98 104 bytes

Thanks to @MartinEnder for saving 5 3 bytes while fixing my code!

j,i,k,l;f(char*s){l=strlen(s);for(i=l,j=k=0;j<l;)k+=(s[j++]-48)*(i>1?i--:-1);return!(k%11)&&k&&(l^8)<2;}

Returns 0 if invalid, 1 if valid. Try it online!

##C, 112 101 96 98 bytes

Thanks to @MartinEnder for saving 5 3 bytes while fixing my code!

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&&(l^8)<2;}

Returns 0 if invalid, 1 if valid. Try it online!

##C, 112 101 96 98 104 bytes

Thanks to @MartinEnder for saving 5 3 bytes while fixing my code!

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&&(l^8)<2;}

Returns 0 if invalid, 1 if valid. Try it online!