The Subs command effects (single or) multiple substitutions in a string s as specified by its first argument. The precise way in which those substitutions take place depends upon the form of the first argument, which may be a single equation of the form L = R, or a set or list of such equations.

If the first argument is a single equation L = R, then Subs( L = R, s ) simply replaces occurrences of the substring L in s by the string R, by using the command StringTools:-SubstituteAll( s, L, R ).

When a list of substitutions is presented as the first argument, then the substitutions in the list are performed serially, in the order specified. This means that the first substitution L1 = R1 is applied to the string s, using the command StringTools:-SubstituteAll( s, L1, R1 ). Then, the substitution L2 = R2 is applied to the result; then the substitution L3 = R3 is applied to the result of that, and so on.

When the substitutions are given as a set, then parallel substitutions are effected, with the "longest-match rule" being used to resolve multiple matches in the input string s. This means that, if two of the left hand sides among L1, L2, ... match a substring of s at the same position, then the longest such match is used in the substitutions.

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.

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

$\mathrm{Subs}\left(''ab''=''XY''\,''uvabbaabcst''\right)$

$''uvXYbaXYcst''$

$\mathrm{Subs}\left(\left\{''ab''=''XY''\,''abc''=''QRST''\right\}\,''uvabbaabcst''\right)$

$''uvXYbaQRSTst''$

$\mathrm{Subs}\left(\left[''ab''=''XY''\,''abc''=''QRST''\right]\,''uvabbaabcst''\right)$

$''uvXYbaXYcst''$

$\mathrm{Subs}\left(\left[''Yc''=''FOO''\,''ab''=''XY''\,''abc''=''QRST''\right]\,''uvabbaabcst''\right)$

$''uvXYbaXYcst''$

$\mathrm{Subs}\left(\left[''ab''=''XY''\,''abc''=''QRST''\,''Yc''=''FOO''\right]\,''uvabbaabcst''\right)$

$''uvXYbaXFOOst''$

The StringTools[Subs] command was introduced in Maple 2017.

For more information on Maple 2017 changes, see Updates in Maple 2017.