^[^\s]*$        matches exactly 1-word keyword
^[^\s]*\s[^\s]*$    matches exactly 2-word keyword
^[^\s]*\s[^\s]*     matches keywords of at least 2 words (2 and more)
^([^\s]*\s){2}[^\s]*$    matches exactly 3-word keyword
^([^\s]*\s){4}[^\s]*$    matches 5-words-and-more keywords (longtail)