Rejects leading and trailing whitespace and admits internal whitespace only as single spaces; blank strings fail by
consequence rather than by a rule of their own. In multiline mode, single and double newlines are admitted as well,
and nothing else.
Parameters
multiline: boolean = false
Whether to admit newlines as paragraph and line separators
Constrains strings to normalised whitespace.
Rejects leading and trailing whitespace and admits internal whitespace only as single spaces; blank strings fail by consequence rather than by a rule of their own. In multiline mode, single and double newlines are admitted as well, and nothing else.