// Copyright 2009 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 implements a simple regular expression library.
//
// The syntax of the regular expressions accepted is:
//
//	regexp:
//		concatenation { '|' concatenation }
//	concatenation:
//		{ closure }
//	closure:
//		term [ '*' | '+' | '?' ]
//	term:
//		'^'
//		'$'
//		'.'
//		character
//		'[' [ '^' ] character-ranges ']'
//		'(' regexp ')'
//
package regexp

import (
	"bytes"
	"container/vector"
	"io"
	"os"
	"strings"
	"utf8"
)

var debug = false

// Error codes returned by failures to parse an expression.
var (
	ErrInternal            = os.NewError("internal error")
	ErrUnmatchedLpar       = os.NewError("unmatched '('")
	ErrUnmatchedRpar       = os.NewError("unmatched ')'")
	ErrUnmatchedLbkt       = os.NewError("unmatched '['")
	ErrUnmatchedRbkt       = os.NewError("unmatched ']'")
	ErrBadRange            = os.NewError("bad range in character class")
	ErrExtraneousBackslash = os.NewError("extraneous backslash")
	ErrBadClosure          = os.NewError("repeated closure (**, ++, etc.)")
	ErrBareClosure         = os.NewError("closure applies to nothing")
	ErrBadBackslash        = os.NewError("illegal backslash escape")
)

// An instruction executed by the NFA
type instr interface {
	kind() int   // the type of this instruction: _CHAR, _ANY, etc.
	next() instr // the instruction to execute after this one
	setNext(i instr)
	index() int
	setIndex(i int)
	print()
}

// Fields and methods common to all instructions
type common struct {
	_next  instr
	_index int
}

func (c *common) next() instr     { return c._next }
func (c *common) setNext(i instr) { c._next = i }
func (c *common) index() int      { return c._index }
func (c *common) setIndex(i int)  { c._index = i }

// Regexp is the representation of a compiled regular expression.
// The public interface is entirely through methods.
type Regexp struct {
	expr        string // the original expression
	prefix      string // initial plain text string
	prefixBytes []byte // initial plain text bytes
	inst        *vector.Vector
	start       instr // first instruction of machine
	prefixStart instr // where to start if there is a prefix
	nbra        int   // number of brackets in expression, for subexpressions
}

const (
	_START     = iota // beginning of program
	_END              // end of program: success
	_BOT              // '^' beginning of text
	_EOT              // '$' end of text
	_CHAR             // 'a' regular character
	_CHARCLASS        // [a-z] character class
	_ANY              // '.' any character including newline
	_NOTNL            // [^\n] special case: any character but newline
	_BRA              // '(' parenthesized expression
	_EBRA             // ')'; end of '(' parenthesized expression
	_ALT              // '|' alternation
	_NOP              // do nothing; makes it easy to link without patching
)

// --- START start of program
type _Start struct {
	common
}

func (start *_Start) kind() int { return _START }
func (start *_Start) print()    { print("start") }

// --- END end of program
type _End struct {
	common
}

func (end *_End) kind() int { return _END }
func (end *_End) print()    { print("end") }

// --- BOT beginning of text
type _Bot struct {
	common
}

func (bot *_Bot) kind() int { return _BOT }
func (bot *_Bot) print()    { print("bot") }

// --- EOT end of text
type _Eot struct {
	common
}

func (eot *_Eot) kind() int { return _EOT }
func (eot *_Eot) print()    { print("eot") }

// --- CHAR a regular character
type _Char struct {
	common
	char int
}

func (char *_Char) kind() int { return _CHAR }
func (char *_Char) print()    { print("char ", string(char.char)) }

func newChar(char int) *_Char {
	c := new(_Char)
	c.char = char
	return c
}

// --- CHARCLASS [a-z]

type _CharClass struct {
	common
	char   int
	negate bool // is character class negated? ([^a-z])
	// vector of int, stored pairwise: [a-z] is (a,z); x is (x,x):
	ranges *vector.IntVector
}

func (cclass *_CharClass) kind() int { return _CHARCLASS }

func (cclass *_CharClass) print() {
	print("charclass")
	if cclass.negate {
		print(" (negated)")
	}
	for i := 0; i < cclass.ranges.Len(); i += 2 {
		l := cclass.ranges.At(i)
		r := cclass.ranges.At(i + 1)
		if l == r {
			print(" [", string(l), "]")
		} else {
			print(" [", string(l), "-", string(r), "]")
		}
	}
}

func (cclass *_CharClass) addRange(a, b int) {
	// range is a through b inclusive
	cclass.ranges.Push(a)
	cclass.ranges.Push(b)
}

func (cclass *_CharClass) matches(c int) bool {
	for i := 0; i < cclass.ranges.Len(); i = i + 2 {
		min := cclass.ranges.At(i)
		max := cclass.ranges.At(i + 1)
		if min <= c && c <= max {
			return !cclass.negate
		}
	}
	return cclass.negate
}

func newCharClass() *_CharClass {
	c := new(_CharClass)
	c.ranges = new(vector.IntVector)
	return c
}

// --- ANY any character
type _Any struct {
	common
}

func (any *_Any) kind() int { return _ANY }
func (any *_Any) print()    { print("any") }

// --- NOTNL any character but newline
type _NotNl struct {
	common
}

func (notnl *_NotNl) kind() int { return _NOTNL }
func (notnl *_NotNl) print()    { print("notnl") }

// --- BRA parenthesized expression
type _Bra struct {
	common
	n int // subexpression number
}

func (bra *_Bra) kind() int { return _BRA }
func (bra *_Bra) print()    { print("bra", bra.n) }

// --- EBRA end of parenthesized expression
type _Ebra struct {
	common
	n int // subexpression number
}

func (ebra *_Ebra) kind() int { return _EBRA }
func (ebra *_Ebra) print()    { print("ebra ", ebra.n) }

// --- ALT alternation
type _Alt struct {
	common
	left instr // other branch
}

func (alt *_Alt) kind() int { return _ALT }
func (alt *_Alt) print()    { print("alt(", alt.left.index(), ")") }

// --- NOP no operation
type _Nop struct {
	common
}

func (nop *_Nop) kind() int { return _NOP }
func (nop *_Nop) print()    { print("nop") }

func (re *Regexp) add(i instr) instr {
	i.setIndex(re.inst.Len())
	re.inst.Push(i)
	return i
}

type parser struct {
	re    *Regexp
	error os.Error
	nlpar int // number of unclosed lpars
	pos   int
	ch    int
}

const endOfFile = -1

func (p *parser) c() int { return p.ch }

func (p *parser) nextc() int {
	if p.pos >= len(p.re.expr) {
		p.ch = endOfFile
	} else {
		c, w := utf8.DecodeRuneInString(p.re.expr[p.pos:])
		p.ch = c
		p.pos += w
	}
	return p.ch
}

func newParser(re *Regexp) *parser {
	p := new(parser)
	p.re = re
	p.nextc() // load p.ch
	return p
}

func special(c int) bool {
	for _, r := range `\.+*?()|[]^$` {
		if c == r {
			return true
		}
	}
	return false
}

func specialcclass(c int) bool {
	for _, r := range `\-[]` {
		if c == r {
			return true
		}
	}
	return false
}

func (p *parser) charClass() instr {
	cc := newCharClass()
	if p.c() == '^' {
		cc.negate = true
		p.nextc()
	}
	left := -1
	for {
		switch c := p.c(); c {
		case ']', endOfFile:
			if left >= 0 {
				p.error = ErrBadRange
				return nil
			}
			// Is it [^\n]?
			if cc.negate && cc.ranges.Len() == 2 &&
				cc.ranges.At(0) == '\n' && cc.ranges.At(1) == '\n' {
				nl := new(_NotNl)
				p.re.add(nl)
				return nl
			}
			// Special common case: "[a]" -> "a"
			if !cc.negate && cc.ranges.Len() == 2 && cc.ranges.At(0) == cc.ranges.At(1) {
				c := newChar(cc.ranges.At(0))
				p.re.add(c)
				return c
			}
			p.re.add(cc)
			return cc
		case '-': // do this before backslash processing
			p.error = ErrBadRange
			return nil
		case '\\':
			c = p.nextc()
			switch {
			case c == endOfFile:
				p.error = ErrExtraneousBackslash
				return nil
			case c == 'n':
				c = '\n'
			case specialcclass(c):
				// c is as delivered
			default:
				p.error = ErrBadBackslash
				return nil
			}
			fallthrough
		default:
			p.nextc()
			switch {
			case left < 0: // first of pair
				if p.c() == '-' { // range
					p.nextc()
					left = c
				} else { // single char
					cc.addRange(c, c)
				}
			case left <= c: // second of pair
				cc.addRange(left, c)
				left = -1
			default:
				p.error = ErrBadRange
				return nil
			}
		}
	}
	return nil
}

func (p *parser) term() (start, end instr) {
	// term() is the leaf of the recursion, so it's sufficient to pick off the
	// error state here for early exit.
	// The other functions (closure(), concatenation() etc.) assume
	// it's safe to recur to here.
	if p.error != nil {
		return
	}
	switch c := p.c(); c {
	case '|', endOfFile:
		return nil, nil
	case '*', '+':
		p.error = ErrBareClosure
		return
	case ')':
		if p.nlpar == 0 {
			p.error = ErrUnmatchedRpar
			return
		}
		return nil, nil
	case ']':
		p.error = ErrUnmatchedRbkt
		return
	case '^':
		p.nextc()
		start = p.re.add(new(_Bot))
		return start, start
	case '$':
		p.nextc()
		start = p.re.add(new(_Eot))
		return start, start
	case '.':
		p.nextc()
		start = p.re.add(new(_Any))
		return start, start
	case '[':
		p.nextc()
		start = p.charClass()
		if p.error != nil {
			return
		}
		if p.c() != ']' {
			p.error = ErrUnmatchedLbkt
			return
		}
		p.nextc()
		return start, start
	case '(':
		p.nextc()
		p.nlpar++
		p.re.nbra++ // increment first so first subexpr is \1
		nbra := p.re.nbra
		start, end = p.regexp()
		if p.c() != ')' {
			p.error = ErrUnmatchedLpar
			return
		}
		p.nlpar--
		p.nextc()
		bra := new(_Bra)
		p.re.add(bra)
		ebra := new(_Ebra)
		p.re.add(ebra)
		bra.n = nbra
		ebra.n = nbra
		if start == nil {
			if end == nil {
				p.error = ErrInternal
				return
			}
			start = ebra
		} else {
			end.setNext(ebra)
		}
		bra.setNext(start)
		return bra, ebra
	case '\\':
		c = p.nextc()
		switch {
		case c == endOfFile:
			p.error = ErrExtraneousBackslash
			return
		case c == 'n':
			c = '\n'
		case special(c):
			// c is as delivered
		default:
			p.error = ErrBadBackslash
			return
		}
		fallthrough
	default:
		p.nextc()
		start = newChar(c)
		p.re.add(start)
		return start, start
	}
	panic("unreachable")
}

func (p *parser) closure() (start, end instr) {
	start, end = p.term()
	if start == nil || p.error != nil {
		return
	}
	switch p.c() {
	case '*':
		// (start,end)*:
		alt := new(_Alt)
		p.re.add(alt)
		end.setNext(alt) // after end, do alt
		alt.left = start // alternate brach: return to start
		start = alt      // alt becomes new (start, end)
		end = alt
	case '+':
		// (start,end)+:
		alt := new(_Alt)
		p.re.add(alt)
		end.setNext(alt) // after end, do alt
		alt.left = start // alternate brach: return to start
		end = alt        // start is unchanged; end is alt
	case '?':
		// (start,end)?:
		alt := new(_Alt)
		p.re.add(alt)
		nop := new(_Nop)
		p.re.add(nop)
		alt.left = start // alternate branch is start
		alt.setNext(nop) // follow on to nop
		end.setNext(nop) // after end, go to nop
		start = alt      // start is now alt
		end = nop        // end is nop pointed to by both branches
	default:
		return
	}
	switch p.nextc() {
	case '*', '+', '?':
		p.error = ErrBadClosure
	}
	return
}

func (p *parser) concatenation() (start, end instr) {
	for {
		nstart, nend := p.closure()
		if p.error != nil {
			return
		}
		switch {
		case nstart == nil: // end of this concatenation
			if start == nil { // this is the empty string
				nop := p.re.add(new(_Nop))
				return nop, nop
			}
			return
		case start == nil: // this is first element of concatenation
			start, end = nstart, nend
		default:
			end.setNext(nstart)
			end = nend
		}
	}
	panic("unreachable")
}

func (p *parser) regexp() (start, end instr) {
	start, end = p.concatenation()
	if p.error != nil {
		return
	}
	for {
		switch p.c() {
		default:
			return
		case '|':
			p.nextc()
			nstart, nend := p.concatenation()
			if p.error != nil {
				return
			}
			alt := new(_Alt)
			p.re.add(alt)
			alt.left = start
			alt.setNext(nstart)
			nop := new(_Nop)
			p.re.add(nop)
			end.setNext(nop)
			nend.setNext(nop)
			start, end = alt, nop
		}
	}
	panic("unreachable")
}

func unNop(i instr) instr {
	for i.kind() == _NOP {
		i = i.next()
	}
	return i
}

func (re *Regexp) eliminateNops() {
	for i := 0; i < re.inst.Len(); i++ {
		inst := re.inst.At(i).(instr)
		if inst.kind() == _END {
			continue
		}
		inst.setNext(unNop(inst.next()))
		if inst.kind() == _ALT {
			alt := inst.(*_Alt)
			alt.left = unNop(alt.left)
		}
	}
}

func (re *Regexp) dump() {
	print("prefix <", re.prefix, ">\n")
	for i := 0; i < re.inst.Len(); i++ {
		inst := re.inst.At(i).(instr)
		print(inst.index(), ": ")
		inst.print()
		if inst.kind() != _END {
			print(" -> ", inst.next().index())
		}
		print("\n")
	}
}

func (re *Regexp) doParse() os.Error {
	p := newParser(re)
	start := new(_Start)
	re.add(start)
	s, e := p.regexp()
	if p.error != nil {
		return p.error
	}
	start.setNext(s)
	re.start = start
	e.setNext(re.add(new(_End)))

	if debug {
		re.dump()
		println()
	}

	re.eliminateNops()
	if debug {
		re.dump()
		println()
	}
	if p.error == nil {
		re.setPrefix()
		if debug {
			re.dump()
			println()
		}
	}
	return p.error
}

// Extract regular text from the beginning of the pattern.
// That text can be used by doExecute to speed up matching.
func (re *Regexp) setPrefix() {
	var b []byte
	var utf = make([]byte, utf8.UTFMax)
	// First instruction is start; skip that.
	i := re.inst.At(0).(instr).next().index()
Loop:
	for i < re.inst.Len() {
		inst := re.inst.At(i).(instr)
		// stop if this is not a char
		if inst.kind() != _CHAR {
			break
		}
		// stop if this char can be followed by a match for an empty string,
		// which includes closures, ^, and $.
		switch re.inst.At(inst.next().index()).(instr).kind() {
		case _BOT, _EOT, _ALT:
			break Loop
		}
		n := utf8.EncodeRune(inst.(*_Char).char, utf)
		b = bytes.Add(b, utf[0:n])
		i = inst.next().index()
	}
	// point prefixStart instruction to first non-CHAR after prefix
	re.prefixStart = re.inst.At(i).(instr)
	re.prefixBytes = b
	re.prefix = string(b)
}

// Compile parses a regular expression and returns, if successful, a Regexp
// object that can be used to match against text.
func Compile(str string) (regexp *Regexp, error os.Error) {
	regexp = new(Regexp)
	regexp.expr = str
	regexp.inst = new(vector.Vector)
	error = regexp.doParse()
	return
}

// MustCompile is like Compile but panics if the expression cannot be parsed.
// It simplifies safe initialization of global variables holding compiled regular
// expressions.
func MustCompile(str string) *Regexp {
	regexp, error := Compile(str)
	if error != nil {
		panicln(`regexp: compiling "`, str, `": `, error.String())
	}
	return regexp
}

// NumSubexp returns the number of parenthesized subexpressions in this Regexp.
func (re *Regexp) NumSubexp() int { return re.nbra }

// The match arena allows us to reduce the garbage generated by tossing
// match vectors away as we execute.  Matches are ref counted and returned
// to a free list when no longer active.  Increases a simple benchmark by 22X.
type matchArena struct {
	head *matchVec
	len  int // length of match vector
}

type matchVec struct {
	m    []int // pairs of bracketing submatches. 0th is start,end
	ref  int
	next *matchVec
}

func (a *matchArena) new() *matchVec {
	if a.head == nil {
		const N = 10
		block := make([]matchVec, N)
		for i := 0; i < N; i++ {
			b := &block[i]
			b.next = a.head
			a.head = b
		}
	}
	m := a.head
	a.head = m.next
	m.ref = 0
	if m.m == nil {
		m.m = make([]int, a.len)
	}
	return m
}

func (a *matchArena) free(m *matchVec) {
	m.ref--
	if m.ref == 0 {
		m.next = a.head
		a.head = m
	}
}

func (a *matchArena) copy(m *matchVec) *matchVec {
	m1 := a.new()
	copy(m1.m, m.m)
	return m1
}

func (a *matchArena) noMatch() *matchVec {
	m := a.new()
	for i := range m.m {
		m.m[i] = -1 // no match seen; catches cases like "a(b)?c" on "ac"
	}
	m.ref = 1
	return m
}

type state struct {
	inst  instr // next instruction to execute
	match *matchVec
}

// Append new state to to-do list.  Leftmost-longest wins so avoid
// adding a state that's already active.  The matchVec will be inc-ref'ed
// if it is assigned to a state.
func (a *matchArena) addState(s []state, inst instr, match *matchVec, pos, end int) []state {
	switch inst.kind() {
	case _BOT:
		if pos == 0 {
			s = a.addState(s, inst.next(), match, pos, end)
		}
		return s
	case _EOT:
		if pos == end {
			s = a.addState(s, inst.next(), match, pos, end)
		}
		return s
	case _BRA:
		n := inst.(*_Bra).n
		match.m[2*n] = pos
		s = a.addState(s, inst.next(), match, pos, end)
		return s
	case _EBRA:
		n := inst.(*_Ebra).n
		match.m[2*n+1] = pos
		s = a.addState(s, inst.next(), match, pos, end)
		return s
	}
	index := inst.index()
	l := len(s)
	// States are inserted in order so it's sufficient to see if we have the same
	// instruction; no need to see if existing match is earlier (it is).
	for i := 0; i < l; i++ {
		if s[i].inst.index() == index {
			return s
		}
	}
	if l == cap(s) {
		s1 := make([]state, 2*l)[0:l]
		copy(s1, s)
		s = s1
	}
	s = s[0 : l+1]
	s[l].inst = inst
	s[l].match = match
	match.ref++
	if inst.kind() == _ALT {
		s = a.addState(s, inst.(*_Alt).left, a.copy(match), pos, end)
		// give other branch a copy of this match vector
		s = a.addState(s, inst.next(), a.copy(match), pos, end)
	}
	return s
}

// Accepts either string or bytes - the logic is identical either way.
// If bytes == nil, scan str.
func (re *Regexp) doExecute(str string, bytestr []byte, pos int) []int {
	var s [2][]state
	s[0] = make([]state, 10)[0:0]
	s[1] = make([]state, 10)[0:0]
	in, out := 0, 1
	var final state
	found := false
	end := len(str)
	if bytestr != nil {
		end = len(bytestr)
	}
	// fast check for initial plain substring
	prefixed := false // has this iteration begun by skipping a prefix?
	if re.prefix != "" {
		var advance int
		if bytestr == nil {
			advance = strings.Index(str[pos:], re.prefix)
		} else {
			advance = bytes.Index(bytestr[pos:], re.prefixBytes)
		}
		if advance == -1 {
			return []int{}
		}
		pos += advance + len(re.prefix)
		prefixed = true
	}
	arena := &matchArena{nil, 2 * (re.nbra + 1)}
	for pos <= end {
		if !found {
			// prime the pump if we haven't seen a match yet
			match := arena.noMatch()
			match.m[0] = pos
			if prefixed {
				s[out] = arena.addState(s[out], re.prefixStart, match, pos, end)
				prefixed = false // next iteration should start at beginning of machine.
			} else {
				s[out] = arena.addState(s[out], re.start.next(), match, pos, end)
			}
			arena.free(match) // if addState saved it, ref was incremented
		}
		in, out = out, in // old out state is new in state
		// clear out old state
		old := s[out]
		for _, state := range old {
			arena.free(state.match)
		}
		s[out] = old[0:0] // truncate state vector
		if found && len(s[in]) == 0 {
			// machine has completed
			break
		}
		charwidth := 1
		c := endOfFile
		if pos < end {
			if bytestr == nil {
				c, charwidth = utf8.DecodeRuneInString(str[pos:end])
			} else {
				c, charwidth = utf8.DecodeRune(bytestr[pos:end])
			}
		}
		pos += charwidth
		for _, st := range s[in] {
			switch st.inst.kind() {
			case _BOT:
			case _EOT:
			case _CHAR:
				if c == st.inst.(*_Char).char {
					s[out] = arena.addState(s[out], st.inst.next(), st.match, pos, end)
				}
			case _CHARCLASS:
				if st.inst.(*_CharClass).matches(c) {
					s[out] = arena.addState(s[out], st.inst.next(), st.match, pos, end)
				}
			case _ANY:
				if c != endOfFile {
					s[out] = arena.addState(s[out], st.inst.next(), st.match, pos, end)
				}
			case _NOTNL:
				if c != endOfFile && c != '\n' {
					s[out] = arena.addState(s[out], st.inst.next(), st.match, pos, end)
				}
			case _BRA:
			case _EBRA:
			case _ALT:
			case _END:
				// choose leftmost longest
				if !found || // first
					st.match.m[0] < final.match.m[0] || // leftmost
					(st.match.m[0] == final.match.m[0] && pos-charwidth > final.match.m[1]) { // longest
					if final.match != nil {
						arena.free(final.match)
					}
					final = st
					final.match.ref++
					final.match.m[1] = pos - charwidth
				}
				found = true
			default:
				st.inst.print()
				panic("unknown instruction in execute")
			}
		}
	}
	if final.match == nil {
		return nil
	}
	// if match found, back up start of match by width of prefix.
	if re.prefix != "" && len(final.match.m) > 0 {
		final.match.m[0] -= len(re.prefix)
	}
	return final.match.m
}


// ExecuteString matches the Regexp against the string s.
// The return value is an array of integers, in pairs, identifying the positions of
// substrings matched by the expression.
//    s[a[0]:a[1]] is the substring matched by the entire expression.
//    s[a[2*i]:a[2*i+1]] for i > 0 is the substring matched by the ith parenthesized subexpression.
// A negative value means the subexpression did not match any element of the string.
// An empty array means "no match".
func (re *Regexp) ExecuteString(s string) (a []int) {
	return re.doExecute(s, nil, 0)
}


// Execute matches the Regexp against the byte slice b.
// The return value is an array of integers, in pairs, identifying the positions of
// subslices matched by the expression.
//    b[a[0]:a[1]] is the subslice matched by the entire expression.
//    b[a[2*i]:a[2*i+1]] for i > 0 is the subslice matched by the ith parenthesized subexpression.
// A negative value means the subexpression did not match any element of the slice.
// An empty array means "no match".
func (re *Regexp) Execute(b []byte) (a []int) { return re.doExecute("", b, 0) }


// MatchString returns whether the Regexp matches the string s.
// The return value is a boolean: true for match, false for no match.
func (re *Regexp) MatchString(s string) bool { return len(re.doExecute(s, nil, 0)) > 0 }


// Match returns whether the Regexp matches the byte slice b.
// The return value is a boolean: true for match, false for no match.
func (re *Regexp) Match(b []byte) bool { return len(re.doExecute("", b, 0)) > 0 }


// MatchStrings matches the Regexp against the string s.
// The return value is an array of strings matched by the expression.
//    a[0] is the substring matched by the entire expression.
//    a[i] for i > 0 is the substring matched by the ith parenthesized subexpression.
// An empty array means ``no match''.
func (re *Regexp) MatchStrings(s string) (a []string) {
	r := re.doExecute(s, nil, 0)
	if r == nil {
		return nil
	}
	a = make([]string, len(r)/2)
	for i := 0; i < len(r); i += 2 {
		if r[i] != -1 { // -1 means no match for this subexpression
			a[i/2] = s[r[i]:r[i+1]]
		}
	}
	return
}

// MatchSlices matches the Regexp against the byte slice b.
// The return value is an array of subslices matched by the expression.
//    a[0] is the subslice matched by the entire expression.
//    a[i] for i > 0 is the subslice matched by the ith parenthesized subexpression.
// An empty array means ``no match''.
func (re *Regexp) MatchSlices(b []byte) (a [][]byte) {
	r := re.doExecute("", b, 0)
	if r == nil {
		return nil
	}
	a = make([][]byte, len(r)/2)
	for i := 0; i < len(r); i += 2 {
		if r[i] != -1 { // -1 means no match for this subexpression
			a[i/2] = b[r[i]:r[i+1]]
		}
	}
	return
}

// MatchString checks whether a textual regular expression
// matches a string.  More complicated queries need
// to use Compile and the full Regexp interface.
func MatchString(pattern string, s string) (matched bool, error os.Error) {
	re, err := Compile(pattern)
	if err != nil {
		return false, err
	}
	return re.MatchString(s), nil
}

// Match checks whether a textual regular expression
// matches a byte slice.  More complicated queries need
// to use Compile and the full Regexp interface.
func Match(pattern string, b []byte) (matched bool, error os.Error) {
	re, err := Compile(pattern)
	if err != nil {
		return false, err
	}
	return re.Match(b), nil
}

// ReplaceAllString returns a copy of src in which all matches for the Regexp
// have been replaced by repl.  No support is provided for expressions
// (e.g. \1 or $1) in the replacement string.
func (re *Regexp) ReplaceAllString(src, repl string) string {
	return re.ReplaceAllStringFunc(src, func(string) string { return repl })
}

// ReplaceAllStringFunc returns a copy of src in which all matches for the
// Regexp have been replaced by the return value of of function repl (whose
// first argument is the matched string).  No support is provided for
// expressions (e.g. \1 or $1) in the replacement string.
func (re *Regexp) ReplaceAllStringFunc(src string, repl func(string) string) string {
	lastMatchEnd := 0 // end position of the most recent match
	searchPos := 0    // position where we next look for a match
	buf := new(bytes.Buffer)
	for searchPos <= len(src) {
		a := re.doExecute(src, nil, searchPos)
		if len(a) == 0 {
			break // no more matches
		}

		// Copy the unmatched characters before this match.
		io.WriteString(buf, src[lastMatchEnd:a[0]])

		// Now insert a copy of the replacement string, but not for a
		// match of the empty string immediately after another match.
		// (Otherwise, we get double replacement for patterns that
		// match both empty and nonempty strings.)
		if a[1] > lastMatchEnd || a[0] == 0 {
			io.WriteString(buf, repl(src[a[0]:a[1]]))
		}
		lastMatchEnd = a[1]

		// Advance past this match; always advance at least one character.
		_, width := utf8.DecodeRuneInString(src[searchPos:])
		if searchPos+width > a[1] {
			searchPos += width
		} else if searchPos+1 > a[1] {
			// This clause is only needed at the end of the input
			// string.  In that case, DecodeRuneInString returns width=0.
			searchPos++
		} else {
			searchPos = a[1]
		}
	}

	// Copy the unmatched characters after the last match.
	io.WriteString(buf, src[lastMatchEnd:])

	return buf.String()
}

// ReplaceAll returns a copy of src in which all matches for the Regexp
// have been replaced by repl.  No support is provided for expressions
// (e.g. \1 or $1) in the replacement text.
func (re *Regexp) ReplaceAll(src, repl []byte) []byte {
	return re.ReplaceAllFunc(src, func([]byte) []byte { return repl })
}

// ReplaceAllFunc returns a copy of src in which all matches for the
// Regexp have been replaced by the return value of of function repl (whose
// first argument is the matched []byte).  No support is provided for
// expressions (e.g. \1 or $1) in the replacement string.
func (re *Regexp) ReplaceAllFunc(src []byte, repl func([]byte) []byte) []byte {
	lastMatchEnd := 0 // end position of the most recent match
	searchPos := 0    // position where we next look for a match
	buf := new(bytes.Buffer)
	for searchPos <= len(src) {
		a := re.doExecute("", src, searchPos)
		if len(a) == 0 {
			break // no more matches
		}

		// Copy the unmatched characters before this match.
		buf.Write(src[lastMatchEnd:a[0]])

		// Now insert a copy of the replacement string, but not for a
		// match of the empty string immediately after another match.
		// (Otherwise, we get double replacement for patterns that
		// match both empty and nonempty strings.)
		if a[1] > lastMatchEnd || a[0] == 0 {
			buf.Write(repl(src[a[0]:a[1]]))
		}
		lastMatchEnd = a[1]

		// Advance past this match; always advance at least one character.
		_, width := utf8.DecodeRune(src[searchPos:])
		if searchPos+width > a[1] {
			searchPos += width
		} else if searchPos+1 > a[1] {
			// This clause is only needed at the end of the input
			// string.  In that case, DecodeRuneInString returns width=0.
			searchPos++
		} else {
			searchPos = a[1]
		}
	}

	// Copy the unmatched characters after the last match.
	buf.Write(src[lastMatchEnd:])

	return buf.Bytes()
}

// QuoteMeta returns a string that quotes all regular expression metacharacters
// inside the argument text; the returned string is a regular expression matching
// the literal text.  For example, QuoteMeta(`[foo]`) returns `\[foo\]`.
func QuoteMeta(s string) string {
	b := make([]byte, 2*len(s))

	// A byte loop is correct because all metacharacters are ASCII.
	j := 0
	for i := 0; i < len(s); i++ {
		if special(int(s[i])) {
			b[j] = '\\'
			j++
		}
		b[j] = s[i]
		j++
	}
	return string(b[0:j])
}

// Find matches in slice b if b is non-nil, otherwise find matches in string s.
func (re *Regexp) allMatches(s string, b []byte, n int, deliver func(int, int)) {
	var end int
	if b == nil {
		end = len(s)
	} else {
		end = len(b)
	}

	for pos, i, prevMatchEnd := 0, 0, -1; i < n && pos <= end; {
		matches := re.doExecute(s, b, pos)
		if len(matches) == 0 {
			break
		}

		accept := true
		if matches[1] == pos {
			// We've found an empty match.
			if matches[0] == prevMatchEnd {
				// We don't allow an empty match right
				// after a previous match, so ignore it.
				accept = false
			}
			var width int
			if b == nil {
				_, width = utf8.DecodeRuneInString(s[pos:end])
			} else {
				_, width = utf8.DecodeRune(b[pos:end])
			}
			if width > 0 {
				pos += width
			} else {
				pos = end + 1
			}
		} else {
			pos = matches[1]
		}
		prevMatchEnd = matches[1]

		if accept {
			deliver(matches[0], matches[1])
			i++
		}
	}
}

// AllMatches slices the byte slice b into substrings that are successive
// matches of the Regexp within b. If n > 0, the function returns at most n
// matches. Text that does not match the expression will be skipped. Empty
// matches abutting a preceding match are ignored. The function returns a slice
// containing the matching substrings.
func (re *Regexp) AllMatches(b []byte, n int) [][]byte {
	if n <= 0 {
		n = len(b) + 1
	}
	result := make([][]byte, n)
	i := 0
	re.allMatches("", b, n, func(start, end int) {
		result[i] = b[start:end]
		i++
	})
	return result[0:i]
}

// AllMatchesString slices the string s into substrings that are successive
// matches of the Regexp within s. If n > 0, the function returns at most n
// matches. Text that does not match the expression will be skipped. Empty
// matches abutting a preceding match are ignored. The function returns a slice
// containing the matching substrings.
func (re *Regexp) AllMatchesString(s string, n int) []string {
	if n <= 0 {
		n = len(s) + 1
	}
	result := make([]string, n)
	i := 0
	re.allMatches(s, nil, n, func(start, end int) {
		result[i] = s[start:end]
		i++
	})
	return result[0:i]
}

// AllMatchesIter slices the byte slice b into substrings that are successive
// matches of the Regexp within b. If n > 0, the function returns at most n
// matches. Text that does not match the expression will be skipped. Empty
// matches abutting a preceding match are ignored. The function returns a
// channel that iterates over the matching substrings.
func (re *Regexp) AllMatchesIter(b []byte, n int) <-chan []byte {
	if n <= 0 {
		n = len(b) + 1
	}
	c := make(chan []byte, 10)
	go func() {
		re.allMatches("", b, n, func(start, end int) { c <- b[start:end] })
		close(c)
	}()
	return c
}

// AllMatchesStringIter slices the string s into substrings that are successive
// matches of the Regexp within s. If n > 0, the function returns at most n
// matches. Text that does not match the expression will be skipped. Empty
// matches abutting a preceding match are ignored. The function returns a
// channel that iterates over the matching substrings.
func (re *Regexp) AllMatchesStringIter(s string, n int) <-chan string {
	if n <= 0 {
		n = len(s) + 1
	}
	c := make(chan string, 10)
	go func() {
		re.allMatches(s, nil, n, func(start, end int) { c <- s[start:end] })
		close(c)
	}()
	return c
}