This is a sub problem related to this problem:

http://www.mathworks.com/matlabcentral/cody/problems/621-cryptomath-addition

If you have a string

'cab' 

and you have a mapping where you want:

'abc' to map to the digits [906]

then you will get this:

% 'cab' , 'abc', [906] --> [6 9 0];

Another example is:

% 'feed', 'def', [541] --> [1 4 4 5];

Your inputs will be of this form:

out = stringSubstitute(string, letters, digits)

>> stringSubstitute('feed','def',[5 4 1])

ans =

       1     4     4     5