The DifferencePositions( s, t ) function computes the positions at which the strings s and t differ.  It returns the (ordered) expression sequence of these positions.  If s and t are equal, then NULL is returned. If s and t differ in length, then the positions up to the shorter of s and t are computed and returned.

The length of the returned expression sequence is equal to the Hamming distance between s and t, provided that s and t are the same length. (Note that the HammingDistance command computes the generalized Hamming distance for strings of unequal length.)

All of the StringTools package commands treat strings as (null-terminated) sequences of $8$-bit (ASCII) characters.  Thus, there is no support for multibyte character encodings, such as unicode encodings.

$\mathrm{with}\left(\mathrm{StringTools}\right)\:$

$\mathrm{DifferencePositions}\left(''abc''\,''abc''\right)$

$\mathrm{DifferencePositions}\left(''abcde''\,''axcye''\right)$

$2,4$

$\mathrm{HammingDistance}\left(''abcde''\,''axcye''\right)$

$2$

$\mathrm{DifferencePositions}\left(''abcde''\,''axcyefg''\right)$

$2,4$

$\mathrm{HammingDistance}\left(''abcde''\,''axcyefg''\right)$

$4$

$\mathrm{DifferencePositions}\left(''edit''\,''tide''\right)$

$1,2,3,4$

$\mathrm{IsDerangement}\left(''edit''\,''tide''\right)$

$\mathrm{true}$

The StringTools[DifferencePositions] command was introduced in Maple 18.

For more information on Maple 18 changes, see Updates in Maple 18.