Replaces all occurrences in <subject> of <what> with <with>, performing string substitution. The occurrences are found from left to right and all substitutions happen simultaneously. By default, occurrences of <what> are searched for while ignoring the upper/lower case distinction. If <case-matters> is provided and true, then case is treated as significant in all comparisons.
strsub("%n is a fink.", "%n", "Fred") => "Fred is a fink."
strsub("foobar", "OB", "b") => "fobar"
strsub("foobar", "OB", "b", 1) => "foobar"