a stretch of whitespace, either between two words,
or (possibly zero-width) at the end of the input string.
As with most of the functions in this module,
parsing starts at the character pointed to by parameter <i>src</i>;
the return value is either a pointer to the position in the input string
that immediately follows the match
(in this case, to the first non-whitespace character encountered;
this <i>may</i> be the NUL character denoting end-of-string),
or NULL, meaning no match was found
(in this case, if 