Check signal error.

Hello, I want to write code to check signal error. error when bit~=0&&not alternative pre bit; except for 1 0 0 1

 for j=1:length(sig)
    if sig(j)~=0;
        break;
    end
end
prv=sig(j);
i=j+1;
while i<=length(sig)
    if sig(i)~=0
        if (i>=4)&&sig(i)==prv&&sig(i)==sig(i-3)
            pre=sig(i);
        elseif (i>=5)&&sig(i)==prv&&sig(i)==sig(i-4)
            pre=sig(i);
        elseif sig(i)==prv
            msgbox('luan phien hoac B00V hoac 000V');
            return;
        end
        i=i+1;

    elseif sig(i)==0
        i=i+1;
    end