replaceFirstMapped method
Replace the first occurrence of from in this string.
Returns a new string, which is this string
except that the first match of from, starting from startIndex,
is replaced by the result of calling replace with the match object.
The optional startIndex is by default set to 0. If provided, it must be
an integer in the range [0 .. len], where len is this string's length.
Implementation
String replaceFirstMapped(Pattern from, String replace(Match match),
    [int startIndex = 0]);