start := -1 // valid span start if >= 0
	for i := 0; i < len(s); {
		r, size := utf8.DecodeRune(s[i:])
		if f(r) {
			if start >= 0 {
				spans = append(spans, span{start, i})
				start = -1
			}
		} else {
			if start < 0 {
				start = i
			}
		}
		i += size
	}

	// Last field might end at EOF.
	if start >= 0 {
		spans = append(spans, span{start, len(s)})
	}