The supported changes to the string are concatenating two strings, or cutting a string into parts. This allows for adding/removing characters from a string.
Best use for this (imo) is in doing a regular expression over a large user entered document. As the user types, we can give regular expression updates in real time as we do not need to recompute everything.
Limits: "we only have character classes, braces and the operators +, *, ?, |", so only basic expressions.