src/regexp/exec.go - go - Git at Google

 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package regexp

 import (
 	"io"
 	"regexp/syntax"
 )

 // A queue is a 'sparse array' holding pending threads of execution.
 // See http://research.swtch.com/2008/03/using-uninitialized-memory-for-fun-and.html
 type queue struct {
 	sparse []uint32
 	dense  []entry
 }

 // A entry is an entry on a queue.
 // It holds both the instruction pc and the actual thread.
 // Some queue entries are just place holders so that the machine
 // knows it has considered that pc. Such entries have t == nil.
 type entry struct {
 	pc uint32
 	t  *thread
 }

 // A thread is the state of a single path through the machine:
 // an instruction and a corresponding capture array.
 // See http://swtch.com/~rsc/regexp/regexp2.html
 type thread struct {
 	inst *syntax.Inst
 	cap  []int
 }

 // A machine holds all the state during an NFA simulation for p.
 type machine struct {
 	re             *Regexp      // corresponding Regexp
 	p              *syntax.Prog // compiled program
 	op             *onePassProg // compiled onepass program, or notOnePass
 	maxBitStateLen int          // max length of string to search with bitstate
 	b              *bitState    // state for backtracker, allocated lazily
 	q0, q1         queue        // two queues for runq, nextq
 	pool           []*thread    // pool of available threads
 	matched        bool         // whether a match was found
 	matchcap       []int        // capture information for the match

 	// cached inputs, to avoid allocation
 	inputBytes  inputBytes
 	inputString inputString
 	inputReader inputReader
 }

 func (m *machine) newInputBytes(b []byte) input {
 	m.inputBytes.str = b
 	return &m.inputBytes
 }

 func (m *machine) newInputString(s string) input {
 	m.inputString.str = s
 	return &m.inputString
 }

 func (m *machine) newInputReader(r io.RuneReader) input {
 	m.inputReader.r = r
 	m.inputReader.atEOT = false
 	m.inputReader.pos = 0
 	return &m.inputReader
 }

 // progMachine returns a new machine running the prog p.
 func progMachine(p *syntax.Prog, op *onePassProg) *machine {
 	m := &machine{p: p, op: op}
 	n := len(m.p.Inst)
 	m.q0 = queue{make([]uint32, n), make([]entry, 0, n)}
 	m.q1 = queue{make([]uint32, n), make([]entry, 0, n)}
 	ncap := p.NumCap
 	if ncap < 2 {
 		ncap = 2
 	}
 	if op == notOnePass {
 		m.maxBitStateLen = maxBitStateLen(p)
 	}
 	m.matchcap = make([]int, ncap)
 	return m
 }

 func (m *machine) init(ncap int) {
 	for _, t := range m.pool {
 		t.cap = t.cap[:ncap]
 	}
 	m.matchcap = m.matchcap[:ncap]
 }

 // alloc allocates a new thread with the given instruction.
 // It uses the free pool if possible.
 func (m *machine) alloc(i *syntax.Inst) *thread {
 	var t *thread
 	if n := len(m.pool); n > 0 {
 		t = m.pool[n-1]
 		m.pool = m.pool[:n-1]
 	} else {
 		t = new(thread)
 		t.cap = make([]int, len(m.matchcap), cap(m.matchcap))
 	}
 	t.inst = i
 	return t
 }

 // match runs the machine over the input starting at pos.
 // It reports whether a match was found.
 // If so, m.matchcap holds the submatch information.
 func (m *machine) match(i input, pos int) bool {
 	startCond := m.re.cond
 	if startCond == ^syntax.EmptyOp(0) { // impossible
 		return false
 	}
 	m.matched = false
 	for i := range m.matchcap {
 		m.matchcap[i] = -1
 	}
 	runq, nextq := &m.q0, &m.q1
 	r, r1 := endOfText, endOfText
 	width, width1 := 0, 0
 	r, width = i.step(pos)
 	if r != endOfText {
 		r1, width1 = i.step(pos + width)
 	}
 	var flag syntax.EmptyOp
 	if pos == 0 {
 		flag = syntax.EmptyOpContext(-1, r)
 	} else {
 		flag = i.context(pos)
 	}
 	for {
 		if len(runq.dense) == 0 {
 			if startCond&syntax.EmptyBeginText != 0 && pos != 0 {
 				// 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)
 				r1, width1 = i.step(pos + width)
 			}
 		}
 		if !m.matched {
 			if len(m.matchcap) > 0 {
 				m.matchcap[0] = pos
 			}
 			m.add(runq, uint32(m.p.Start), pos, m.matchcap, flag, nil)
 		}
 		flag = syntax.EmptyOpContext(r, r1)
 		m.step(runq, nextq, pos, pos+width, r, flag)
 		if width == 0 {
 			break
 		}
 		if len(m.matchcap) == 0 && m.matched {
 			// Found a match and not paying attention
 			// to where it is, so any match will do.
 			break
 		}
 		pos += width
 		r, width = r1, width1
 		if r != endOfText {
 			r1, width1 = i.step(pos + width)
 		}
 		runq, nextq = nextq, runq
 	}
 	m.clear(nextq)
 	return m.matched
 }

 // clear frees all threads on the thread queue.
 func (m *machine) clear(q *queue) {
 	for _, d := range q.dense {
 		if d.t != nil {
 			m.pool = append(m.pool, d.t)
 		}
 	}
 	q.dense = q.dense[:0]
 }

 // step executes one step of the machine, running each of the threads
 // on runq and appending new threads to nextq.
 // The step processes the rune c (which may be endOfText),
 // which starts at position pos and ends at nextPos.
 // nextCond gives the setting for the empty-width flags after c.
 func (m *machine) step(runq, nextq *queue, pos, nextPos int, c rune, nextCond syntax.EmptyOp) {
 	longest := m.re.longest
 	for j := 0; j < len(runq.dense); j++ {
 		d := &runq.dense[j]
 		t := d.t
 		if t == nil {
 			continue
 		}
 		if longest && m.matched && len(t.cap) > 0 && m.matchcap[0] < t.cap[0] {
 			m.pool = append(m.pool, t)
 			continue
 		}
 		i := t.inst
 		add := false
 		switch i.Op {
 		default:
 			panic("bad inst")

 		case syntax.InstMatch:
 			if len(t.cap) > 0 && (!longest || !m.matched || m.matchcap[1] < pos) {
 				t.cap[1] = pos
 				copy(m.matchcap, t.cap)
 			}
 			if !longest {
 				// First-match mode: cut off all lower-priority threads.
 				for _, d := range runq.dense[j+1:] {
 					if d.t != nil {
 						m.pool = append(m.pool, d.t)
 					}
 				}
 				runq.dense = runq.dense[:0]
 			}
 			m.matched = true

 		case syntax.InstRune:
 			add = i.MatchRune(c)
 		case syntax.InstRune1:
 			add = c == i.Rune[0]
 		case syntax.InstRuneAny:
 			add = true
 		case syntax.InstRuneAnyNotNL:
 			add = c != '\n'
 		}
 		if add {
 			t = m.add(nextq, i.Out, nextPos, t.cap, nextCond, t)
 		}
 		if t != nil {
 			m.pool = append(m.pool, t)
 		}
 	}
 	runq.dense = runq.dense[:0]
 }

 // add adds an entry to q for pc, unless the q already has such an entry.
 // It also recursively adds an entry for all instructions reachable from pc by following
 // empty-width conditions satisfied by cond.  pos gives the current position
 // in the input.
 func (m *machine) add(q *queue, pc uint32, pos int, cap []int, cond syntax.EmptyOp, t *thread) *thread {
 	if pc == 0 {
 		return t
 	}
 	if j := q.sparse[pc]; j < uint32(len(q.dense)) && q.dense[j].pc == pc {
 		return t
 	}

 	j := len(q.dense)
 	q.dense = q.dense[:j+1]
 	d := &q.dense[j]
 	d.t = nil
 	d.pc = pc
 	q.sparse[pc] = uint32(j)

 	i := &m.p.Inst[pc]
 	switch i.Op {
 	default:
 		panic("unhandled")
 	case syntax.InstFail:
 		// nothing
 	case syntax.InstAlt, syntax.InstAltMatch:
 		t = m.add(q, i.Out, pos, cap, cond, t)
 		t = m.add(q, i.Arg, pos, cap, cond, t)
 	case syntax.InstEmptyWidth:
 		if syntax.EmptyOp(i.Arg)&^cond == 0 {
 			t = m.add(q, i.Out, pos, cap, cond, t)
 		}
 	case syntax.InstNop:
 		t = m.add(q, i.Out, pos, cap, cond, t)
 	case syntax.InstCapture:
 		if int(i.Arg) < len(cap) {
 			opos := cap[i.Arg]
 			cap[i.Arg] = pos
 			m.add(q, i.Out, pos, cap, cond, nil)
 			cap[i.Arg] = opos
 		} else {
 			t = m.add(q, i.Out, pos, cap, cond, t)
 		}
 	case syntax.InstMatch, syntax.InstRune, syntax.InstRune1, syntax.InstRuneAny, syntax.InstRuneAnyNotNL:
 		if t == nil {
 			t = m.alloc(i)
 		} else {
 			t.inst = i
 		}
 		if len(cap) > 0 && &t.cap[0] != &cap[0] {
 			copy(t.cap, cap)
 		}
 		d.t = t
 		t = nil
 	}
 	return t
 }

 // onepass runs the machine over the input starting at pos.
 // It reports whether a match was found.
 // If so, m.matchcap holds the submatch information.
 func (m *machine) onepass(i input, pos int) bool {
 	startCond := m.re.cond
 	if startCond == ^syntax.EmptyOp(0) { // impossible
 		return false
 	}
 	m.matched = false
 	for i := range m.matchcap {
 		m.matchcap[i] = -1
 	}
 	r, r1 := endOfText, endOfText
 	width, width1 := 0, 0
 	r, width = i.step(pos)
 	if r != endOfText {
 		r1, width1 = i.step(pos + width)
 	}
 	var flag syntax.EmptyOp
 	if pos == 0 {
 		flag = syntax.EmptyOpContext(-1, r)
 	} else {
 		flag = i.context(pos)
 	}
 	pc := m.op.Start
 	inst := m.op.Inst[pc]
 	// If there is a simple literal prefix, skip over it.
 	if pos == 0 && syntax.EmptyOp(inst.Arg)&^flag == 0 &&
 		len(m.re.prefix) > 0 && i.canCheckPrefix() {
 		// Match requires literal prefix; fast search for it.
 		if i.hasPrefix(m.re) {
 			pos += len(m.re.prefix)
 			r, width = i.step(pos)
 			r1, width1 = i.step(pos + width)
 			flag = i.context(pos)
 			pc = int(m.re.prefixEnd)
 		} else {
 			return m.matched
 		}
 	}
 	for {
 		inst = m.op.Inst[pc]
 		pc = int(inst.Out)
 		switch inst.Op {
 		default:
 			panic("bad inst")
 		case syntax.InstMatch:
 			m.matched = true
 			if len(m.matchcap) > 0 {
 				m.matchcap[0] = 0
 				m.matchcap[1] = pos
 			}
 			return m.matched
 		case syntax.InstRune:
 			if !inst.MatchRune(r) {
 				return m.matched
 			}
 		case syntax.InstRune1:
 			if r != inst.Rune[0] {
 				return m.matched
 			}
 		case syntax.InstRuneAny:
 			// Nothing
 		case syntax.InstRuneAnyNotNL:
 			if r == '\n' {
 				return m.matched
 			}
 		// peek at the input rune to see which branch of the Alt to take
 		case syntax.InstAlt, syntax.InstAltMatch:
 			pc = int(onePassNext(&inst, r))
 			continue
 		case syntax.InstFail:
 			return m.matched
 		case syntax.InstNop:
 			continue
 		case syntax.InstEmptyWidth:
 			if syntax.EmptyOp(inst.Arg)&^flag != 0 {
 				return m.matched
 			}
 			continue
 		case syntax.InstCapture:
 			if int(inst.Arg) < len(m.matchcap) {
 				m.matchcap[inst.Arg] = pos
 			}
 			continue
 		}
 		if width == 0 {
 			break
 		}
 		flag = syntax.EmptyOpContext(r, r1)
 		pos += width
 		r, width = r1, width1
 		if r != endOfText {
 			r1, width1 = i.step(pos + width)
 		}
 	}
 	return m.matched
 }

 // empty is a non-nil 0-element slice,
 // so doExecute can avoid an allocation
 // when 0 captures are requested from a successful match.
 var empty = make([]int, 0)

 // doExecute finds the leftmost match in the input and returns
 // the position of its subexpressions.
 func (re *Regexp) doExecute(r io.RuneReader, b []byte, s string, pos int, ncap int) []int {
 	m := re.get()
 	var i input
 	var size int
 	if r != nil {
 		i = m.newInputReader(r)
 	} else if b != nil {
 		i = m.newInputBytes(b)
 		size = len(b)
 	} else {
 		i = m.newInputString(s)
 		size = len(s)
 	}
 	if m.op != notOnePass {
 		if !m.onepass(i, pos) {
 			re.put(m)
 			return nil
 		}
 	} else if size < m.maxBitStateLen && r == nil {
 		if m.b == nil {
 			m.b = newBitState(m.p)
 		}
 		if !m.backtrack(i, pos, size, ncap) {
 			re.put(m)
 			return nil
 		}
 	} else {
 		m.init(ncap)
 		if !m.match(i, pos) {
 			re.put(m)
 			return nil
 		}
 	}
 	if ncap == 0 {
 		re.put(m)
 		return empty // empty but not nil
 	}
 	cap := make([]int, len(m.matchcap))
 	copy(cap, m.matchcap)
 	re.put(m)
 	return cap
 }
	// Copyright 2011 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package regexp

	import (
	"io"
	"regexp/syntax"
	)

	// A queue is a 'sparse array' holding pending threads of execution.
	// See http://research.swtch.com/2008/03/using-uninitialized-memory-for-fun-and.html
	type queue struct {
	sparse []uint32
	dense []entry
	}

	// A entry is an entry on a queue.
	// It holds both the instruction pc and the actual thread.
	// Some queue entries are just place holders so that the machine
	// knows it has considered that pc. Such entries have t == nil.
	type entry struct {
	pc uint32
	t *thread
	}

	// A thread is the state of a single path through the machine:
	// an instruction and a corresponding capture array.
	// See http://swtch.com/~rsc/regexp/regexp2.html
	type thread struct {
	inst *syntax.Inst
	cap []int
	}

	// A machine holds all the state during an NFA simulation for p.
	type machine struct {
	re *Regexp // corresponding Regexp
	p *syntax.Prog // compiled program
	op *onePassProg // compiled onepass program, or notOnePass
	maxBitStateLen int // max length of string to search with bitstate
	b *bitState // state for backtracker, allocated lazily
	q0, q1 queue // two queues for runq, nextq
	pool []*thread // pool of available threads
	matched bool // whether a match was found
	matchcap []int // capture information for the match

	// cached inputs, to avoid allocation
	inputBytes inputBytes
	inputString inputString
	inputReader inputReader
	}

	func (m *machine) newInputBytes(b []byte) input {
	m.inputBytes.str = b
	return &m.inputBytes
	}

	func (m *machine) newInputString(s string) input {
	m.inputString.str = s
	return &m.inputString
	}

	func (m *machine) newInputReader(r io.RuneReader) input {
	m.inputReader.r = r
	m.inputReader.atEOT = false
	m.inputReader.pos = 0
	return &m.inputReader
	}

	// progMachine returns a new machine running the prog p.
	func progMachine(p syntax.Prog, op onePassProg) *machine {
	m := &machine{p: p, op: op}
	n := len(m.p.Inst)
	m.q0 = queue{make([]uint32, n), make([]entry, 0, n)}
	m.q1 = queue{make([]uint32, n), make([]entry, 0, n)}
	ncap := p.NumCap
	if ncap < 2 {
	ncap = 2
	}
	if op == notOnePass {
	m.maxBitStateLen = maxBitStateLen(p)
	}
	m.matchcap = make([]int, ncap)
	return m
	}

	func (m *machine) init(ncap int) {
	for _, t := range m.pool {
	t.cap = t.cap[:ncap]
	}
	m.matchcap = m.matchcap[:ncap]
	}

	// alloc allocates a new thread with the given instruction.
	// It uses the free pool if possible.
	func (m machine) alloc(i syntax.Inst) *thread {
	var t *thread
	if n := len(m.pool); n > 0 {
	t = m.pool[n-1]
	m.pool = m.pool[:n-1]
	} else {
	t = new(thread)
	t.cap = make([]int, len(m.matchcap), cap(m.matchcap))
	}
	t.inst = i
	return t
	}

	// match runs the machine over the input starting at pos.
	// It reports whether a match was found.
	// If so, m.matchcap holds the submatch information.
	func (m *machine) match(i input, pos int) bool {
	startCond := m.re.cond
	if startCond == ^syntax.EmptyOp(0) { // impossible
	return false
	}
	m.matched = false
	for i := range m.matchcap {
	m.matchcap[i] = -1
	}
	runq, nextq := &m.q0, &m.q1
	r, r1 := endOfText, endOfText
	width, width1 := 0, 0
	r, width = i.step(pos)
	if r != endOfText {
	r1, width1 = i.step(pos + width)
	}
	var flag syntax.EmptyOp
	if pos == 0 {
	flag = syntax.EmptyOpContext(-1, r)
	} else {
	flag = i.context(pos)
	}
	for {
	if len(runq.dense) == 0 {
	if startCond&syntax.EmptyBeginText != 0 && pos != 0 {
	// 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)
	r1, width1 = i.step(pos + width)
	}
	}
	if !m.matched {
	if len(m.matchcap) > 0 {
	m.matchcap[0] = pos
	}
	m.add(runq, uint32(m.p.Start), pos, m.matchcap, flag, nil)
	}
	flag = syntax.EmptyOpContext(r, r1)
	m.step(runq, nextq, pos, pos+width, r, flag)
	if width == 0 {
	break
	}
	if len(m.matchcap) == 0 && m.matched {
	// Found a match and not paying attention
	// to where it is, so any match will do.
	break
	}
	pos += width
	r, width = r1, width1
	if r != endOfText {
	r1, width1 = i.step(pos + width)
	}
	runq, nextq = nextq, runq
	}
	m.clear(nextq)
	return m.matched
	}

	// clear frees all threads on the thread queue.
	func (m machine) clear(q queue) {
	for _, d := range q.dense {
	if d.t != nil {
	m.pool = append(m.pool, d.t)
	}
	}
	q.dense = q.dense[:0]
	}

	// step executes one step of the machine, running each of the threads
	// on runq and appending new threads to nextq.
	// The step processes the rune c (which may be endOfText),
	// which starts at position pos and ends at nextPos.
	// nextCond gives the setting for the empty-width flags after c.
	func (m machine) step(runq, nextq queue, pos, nextPos int, c rune, nextCond syntax.EmptyOp) {
	longest := m.re.longest
	for j := 0; j < len(runq.dense); j++ {
	d := &runq.dense[j]
	t := d.t
	if t == nil {
	continue
	}
	if longest && m.matched && len(t.cap) > 0 && m.matchcap[0] < t.cap[0] {
	m.pool = append(m.pool, t)
	continue
	}
	i := t.inst
	add := false
	switch i.Op {
	default:
	panic("bad inst")

	case syntax.InstMatch:
	if len(t.cap) > 0 && (!longest \|\| !m.matched \|\| m.matchcap[1] < pos) {
	t.cap[1] = pos
	copy(m.matchcap, t.cap)
	}
	if !longest {
	// First-match mode: cut off all lower-priority threads.
	for _, d := range runq.dense[j+1:] {
	if d.t != nil {
	m.pool = append(m.pool, d.t)
	}
	}
	runq.dense = runq.dense[:0]
	}
	m.matched = true

	case syntax.InstRune:
	add = i.MatchRune(c)
	case syntax.InstRune1:
	add = c == i.Rune[0]
	case syntax.InstRuneAny:
	add = true
	case syntax.InstRuneAnyNotNL:
	add = c != '\n'
	}
	if add {
	t = m.add(nextq, i.Out, nextPos, t.cap, nextCond, t)
	}
	if t != nil {
	m.pool = append(m.pool, t)
	}
	}
	runq.dense = runq.dense[:0]
	}

	// add adds an entry to q for pc, unless the q already has such an entry.
	// It also recursively adds an entry for all instructions reachable from pc by following
	// empty-width conditions satisfied by cond. pos gives the current position
	// in the input.
	func (m machine) add(q queue, pc uint32, pos int, cap []int, cond syntax.EmptyOp, t thread) thread {
	if pc == 0 {
	return t
	}
	if j := q.sparse[pc]; j < uint32(len(q.dense)) && q.dense[j].pc == pc {
	return t
	}

	j := len(q.dense)
	q.dense = q.dense[:j+1]
	d := &q.dense[j]
	d.t = nil
	d.pc = pc
	q.sparse[pc] = uint32(j)

	i := &m.p.Inst[pc]
	switch i.Op {
	default:
	panic("unhandled")
	case syntax.InstFail:
	// nothing
	case syntax.InstAlt, syntax.InstAltMatch:
	t = m.add(q, i.Out, pos, cap, cond, t)
	t = m.add(q, i.Arg, pos, cap, cond, t)
	case syntax.InstEmptyWidth:
	if syntax.EmptyOp(i.Arg)&^cond == 0 {
	t = m.add(q, i.Out, pos, cap, cond, t)
	}
	case syntax.InstNop:
	t = m.add(q, i.Out, pos, cap, cond, t)
	case syntax.InstCapture:
	if int(i.Arg) < len(cap) {
	opos := cap[i.Arg]
	cap[i.Arg] = pos
	m.add(q, i.Out, pos, cap, cond, nil)
	cap[i.Arg] = opos
	} else {
	t = m.add(q, i.Out, pos, cap, cond, t)
	}
	case syntax.InstMatch, syntax.InstRune, syntax.InstRune1, syntax.InstRuneAny, syntax.InstRuneAnyNotNL:
	if t == nil {
	t = m.alloc(i)
	} else {
	t.inst = i
	}
	if len(cap) > 0 && &t.cap[0] != &cap[0] {
	copy(t.cap, cap)
	}
	d.t = t
	t = nil
	}
	return t
	}

	// onepass runs the machine over the input starting at pos.
	// It reports whether a match was found.
	// If so, m.matchcap holds the submatch information.
	func (m *machine) onepass(i input, pos int) bool {
	startCond := m.re.cond
	if startCond == ^syntax.EmptyOp(0) { // impossible
	return false
	}
	m.matched = false
	for i := range m.matchcap {
	m.matchcap[i] = -1
	}
	r, r1 := endOfText, endOfText
	width, width1 := 0, 0
	r, width = i.step(pos)
	if r != endOfText {
	r1, width1 = i.step(pos + width)
	}
	var flag syntax.EmptyOp
	if pos == 0 {
	flag = syntax.EmptyOpContext(-1, r)
	} else {
	flag = i.context(pos)
	}
	pc := m.op.Start
	inst := m.op.Inst[pc]
	// If there is a simple literal prefix, skip over it.
	if pos == 0 && syntax.EmptyOp(inst.Arg)&^flag == 0 &&
	len(m.re.prefix) > 0 && i.canCheckPrefix() {
	// Match requires literal prefix; fast search for it.
	if i.hasPrefix(m.re) {
	pos += len(m.re.prefix)
	r, width = i.step(pos)
	r1, width1 = i.step(pos + width)
	flag = i.context(pos)
	pc = int(m.re.prefixEnd)
	} else {
	return m.matched
	}
	}
	for {
	inst = m.op.Inst[pc]
	pc = int(inst.Out)
	switch inst.Op {
	default:
	panic("bad inst")
	case syntax.InstMatch:
	m.matched = true
	if len(m.matchcap) > 0 {
	m.matchcap[0] = 0
	m.matchcap[1] = pos
	}
	return m.matched
	case syntax.InstRune:
	if !inst.MatchRune(r) {
	return m.matched
	}
	case syntax.InstRune1:
	if r != inst.Rune[0] {
	return m.matched
	}
	case syntax.InstRuneAny:
	// Nothing
	case syntax.InstRuneAnyNotNL:
	if r == '\n' {
	return m.matched
	}
	// peek at the input rune to see which branch of the Alt to take
	case syntax.InstAlt, syntax.InstAltMatch:
	pc = int(onePassNext(&inst, r))
	continue
	case syntax.InstFail:
	return m.matched
	case syntax.InstNop:
	continue
	case syntax.InstEmptyWidth:
	if syntax.EmptyOp(inst.Arg)&^flag != 0 {
	return m.matched
	}
	continue
	case syntax.InstCapture:
	if int(inst.Arg) < len(m.matchcap) {
	m.matchcap[inst.Arg] = pos
	}
	continue
	}
	if width == 0 {
	break
	}
	flag = syntax.EmptyOpContext(r, r1)
	pos += width
	r, width = r1, width1
	if r != endOfText {
	r1, width1 = i.step(pos + width)
	}
	}
	return m.matched
	}

	// empty is a non-nil 0-element slice,
	// so doExecute can avoid an allocation
	// when 0 captures are requested from a successful match.
	var empty = make([]int, 0)

	// doExecute finds the leftmost match in the input and returns
	// the position of its subexpressions.
	func (re *Regexp) doExecute(r io.RuneReader, b []byte, s string, pos int, ncap int) []int {
	m := re.get()
	var i input
	var size int
	if r != nil {
	i = m.newInputReader(r)
	} else if b != nil {
	i = m.newInputBytes(b)
	size = len(b)
	} else {
	i = m.newInputString(s)
	size = len(s)
	}
	if m.op != notOnePass {
	if !m.onepass(i, pos) {
	re.put(m)
	return nil
	}
	} else if size < m.maxBitStateLen && r == nil {
	if m.b == nil {
	m.b = newBitState(m.p)
	}
	if !m.backtrack(i, pos, size, ncap) {
	re.put(m)
	return nil
	}
	} else {
	m.init(ncap)
	if !m.match(i, pos) {
	re.put(m)
	return nil
	}
	}
	if ncap == 0 {
	re.put(m)
	return empty // empty but not nil
	}
	cap := make([]int, len(m.matchcap))
	copy(cap, m.matchcap)
	re.put(m)
	return cap
	}