Single-word quantifiers for "zero or more"-like cardinalities
English isn't particularly well provided with quantifiers, and as you have noted, the ones it has are imprecise. A more natural way of expressing your example in English is to phrase it in terms of possibility or permission: “the input letters may be separated by spaces”.
But more generally, if you need to express precise bounds for a quantity you ought to use numeric qualifiers even in informal speech. I've occasionally been surprised by people who understood “a couple of x” to mean “at least one, but possibly three or more”, when in fact I meant “exactly two”.