An operation on formal languages that gives for any language L the language L*, defined by
where Λ is the empty word. Thus a word
w is in L* if and only if it has the form
with each
wi in
L, i.e. is a concatenation of words in
L.
The Kleene-plus (L+) of L, is defined by
Thus
L+ comprises the nonempty strings of
L*.