// Anchored match, past beginning of text.
				break
			}
			if m.matched {
				// Have match; finished exploring alternatives.
				break
			}
			if len(m.re.prefix) > 0 && r1 != m.re.prefixRune && i.canCheckPrefix() {
				// Match requires literal prefix; fast search for it.
				advance := i.index(m.re, pos)
				if advance < 0 {
					break
				}
				pos += advance
				r, width = i.step(pos)

const endOfText rune = -1

// input abstracts different representations of the input text. It provides
// one-character lookahead.
type input interface {
	step(pos int) (r rune, width int) // advance one rune
	canCheckPrefix() bool             // can we look ahead without losing info?
	hasPrefix(re *Regexp) bool
	index(re *Regexp, pos int) int
	context(pos int) lazyFlag
}

// inputString scans a string.