regex_constants::match_partial docs indecipherable

[cppljevans]

the docs
say:

match_partial

    Specifies that if no match can be found, then it is acceptable to return a match [from, last) where from != last, if there exists some sequence of characters [from,to) of which [from,last) is a prefix, and which would result in a full match. 

with no explanation of what from, last and to are. One would guess that [from,last] is the string to be searched; however,
if that's so, then there cannot be a [from,to] where [from,last] is a prefix, because that would require last < to, which can't
be since the search string cannot extend past last.

Please clarify.