Appends to sb two strings: the text from the append position up to the beginning of the
most recent match, and then the replacement with submatch groups substituted for references of
the form $n, where n is the group number in decimal.
Appends to sb two strings: the text from the append position up to the beginning of the
most recent match, and then the replacement with submatch groups substituted for references of
the form $n, where n is the group number in decimal.