This matcher accepts only ASCII whitespace characters,
and is always anchored, i.e. it will never match input
that starts with a non-ASCII whitespace character.
This is a combination, jack-of-all trades matcher, which is
executes a set of smaller matches to form a large one. Some
parts of the “pattern” are known statically, while others
rely on runtime variable bindings and expression evaluation,
hence “smart”. This is the default matcher that is used any
time match blocks or substitutions are present in a pattern.