libgo: update to go1.9

[official-gcc.git] / libgo / go / regexp / regexp.go

// 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 regular expression search.
//
// The syntax of the regular expressions accepted is the same
// general syntax used by Perl, Python, and other languages.
// More precisely, it is the syntax accepted by RE2 and described at
// https://golang.org/s/re2syntax, except for \C.
// For an overview of the syntax, run
//   go doc regexp/syntax
//
// The regexp implementation provided by this package is
// guaranteed to run in time linear in the size of the input.
// (This is a property not guaranteed by most open source
// implementations of regular expressions.) For more information
// about this property, see
//      http://swtch.com/~rsc/regexp/regexp1.html
// or any book about automata theory.
//
// All characters are UTF-8-encoded code points.
//
// There are 16 methods of Regexp that match a regular expression and identify
// the matched text. Their names are matched by this regular expression:
//
//      Find(All)?(String)?(Submatch)?(Index)?
//
// If 'All' is present, the routine matches successive non-overlapping
// matches of the entire expression. Empty matches abutting a preceding
// match are ignored. The return value is a slice containing the successive
// return values of the corresponding non-'All' routine. These routines take
// an extra integer argument, n; if n >= 0, the function returns at most n
// matches/submatches.
//
// If 'String' is present, the argument is a string; otherwise it is a slice
// of bytes; return values are adjusted as appropriate.
//
// If 'Submatch' is present, the return value is a slice identifying the
// successive submatches of the expression. Submatches are matches of
// parenthesized subexpressions (also known as capturing groups) within the
// regular expression, numbered from left to right in order of opening
// parenthesis. Submatch 0 is the match of the entire expression, submatch 1
// the match of the first parenthesized subexpression, and so on.
//
// If 'Index' is present, matches and submatches are identified by byte index
// pairs within the input string: result[2*n:2*n+1] identifies the indexes of
// the nth submatch. The pair for n==0 identifies the match of the entire
// expression. If 'Index' is not present, the match is identified by the
// text of the match/submatch. If an index is negative, it means that
// subexpression did not match any string in the input.
//
// There is also a subset of the methods that can be applied to text read
// from a RuneReader:
//
//      MatchReader, FindReaderIndex, FindReaderSubmatchIndex
//
// This set may grow. Note that regular expression matches may need to
// examine text beyond the text returned by a match, so the methods that
// match text from a RuneReader may read arbitrarily far into the input
// before returning.
//
// (There are a few other methods that do not match this pattern.)
//
package regexp

import (
        "bytes"
        "io"
        "regexp/syntax"
        "strconv"
        "strings"
        "sync"
        "unicode"
        "unicode/utf8"
)

// Regexp is the representation of a compiled regular expression.
// A Regexp is safe for concurrent use by multiple goroutines,
// except for configuration methods, such as Longest.
type Regexp struct {
        // read-only after Compile
        regexpRO

        // cache of machines for running regexp
        mu      sync.Mutex
        machine []*machine
}

type regexpRO struct {
        expr           string         // as passed to Compile
        prog           *syntax.Prog   // compiled program
        onepass        *onePassProg   // onepass program or nil
        prefix         string         // required prefix in unanchored matches
        prefixBytes    []byte         // prefix, as a []byte
        prefixComplete bool           // prefix is the entire regexp
        prefixRune     rune           // first rune in prefix
        prefixEnd      uint32         // pc for last rune in prefix
        cond           syntax.EmptyOp // empty-width conditions required at start of match
        numSubexp      int
        subexpNames    []string
        longest        bool
}

// String returns the source text used to compile the regular expression.
func (re *Regexp) String() string {
        return re.expr
}

// Copy returns a new Regexp object copied from re.
//
// When using a Regexp in multiple goroutines, giving each goroutine
// its own copy helps to avoid lock contention.
func (re *Regexp) Copy() *Regexp {
        // It is not safe to copy Regexp by value
        // since it contains a sync.Mutex.
        return &Regexp{
                regexpRO: re.regexpRO,
        }
}

// Compile parses a regular expression and returns, if successful,
// a Regexp object that can be used to match against text.
//
// When matching against text, the regexp returns a match that
// begins as early as possible in the input (leftmost), and among those
// it chooses the one that a backtracking search would have found first.
// This so-called leftmost-first matching is the same semantics
// that Perl, Python, and other implementations use, although this
// package implements it without the expense of backtracking.
// For POSIX leftmost-longest matching, see CompilePOSIX.
func Compile(expr string) (*Regexp, error) {
        return compile(expr, syntax.Perl, false)
}

// CompilePOSIX is like Compile but restricts the regular expression
// to POSIX ERE (egrep) syntax and changes the match semantics to
// leftmost-longest.
//
// That is, when matching against text, the regexp returns a match that
// begins as early as possible in the input (leftmost), and among those
// it chooses a match that is as long as possible.
// This so-called leftmost-longest matching is the same semantics
// that early regular expression implementations used and that POSIX
// specifies.
//
// However, there can be multiple leftmost-longest matches, with different
// submatch choices, and here this package diverges from POSIX.
// Among the possible leftmost-longest matches, this package chooses
// the one that a backtracking search would have found first, while POSIX
// specifies that the match be chosen to maximize the length of the first
// subexpression, then the second, and so on from left to right.
// The POSIX rule is computationally prohibitive and not even well-defined.
// See http://swtch.com/~rsc/regexp/regexp2.html#posix for details.
func CompilePOSIX(expr string) (*Regexp, error) {
        return compile(expr, syntax.POSIX, true)
}

// Longest makes future searches prefer the leftmost-longest match.
// That is, when matching against text, the regexp returns a match that
// begins as early as possible in the input (leftmost), and among those
// it chooses a match that is as long as possible.
// This method modifies the Regexp and may not be called concurrently
// with any other methods.
func (re *Regexp) Longest() {
        re.longest = true
}

func compile(expr string, mode syntax.Flags, longest bool) (*Regexp, error) {
        re, err := syntax.Parse(expr, mode)
        if err != nil {
                return nil, err
        }
        maxCap := re.MaxCap()
        capNames := re.CapNames()

        re = re.Simplify()
        prog, err := syntax.Compile(re)
        if err != nil {
                return nil, err
        }
        regexp := &Regexp{
                regexpRO: regexpRO{
                        expr:        expr,
                        prog:        prog,
                        onepass:     compileOnePass(prog),
                        numSubexp:   maxCap,
                        subexpNames: capNames,
                        cond:        prog.StartCond(),
                        longest:     longest,
                },
        }
        if regexp.onepass == notOnePass {
                regexp.prefix, regexp.prefixComplete = prog.Prefix()
        } else {
                regexp.prefix, regexp.prefixComplete, regexp.prefixEnd = onePassPrefix(prog)
        }
        if regexp.prefix != "" {
                // TODO(rsc): Remove this allocation by adding
                // IndexString to package bytes.
                regexp.prefixBytes = []byte(regexp.prefix)
                regexp.prefixRune, _ = utf8.DecodeRuneInString(regexp.prefix)
        }
        return regexp, nil
}

// get returns a machine to use for matching re.
// It uses the re's machine cache if possible, to avoid
// unnecessary allocation.
func (re *Regexp) get() *machine {
        re.mu.Lock()
        if n := len(re.machine); n > 0 {
                z := re.machine[n-1]
                re.machine = re.machine[:n-1]
                re.mu.Unlock()
                return z
        }
        re.mu.Unlock()
        z := progMachine(re.prog, re.onepass)
        z.re = re
        return z
}

// put returns a machine to the re's machine cache.
// There is no attempt to limit the size of the cache, so it will
// grow to the maximum number of simultaneous matches
// run using re.  (The cache empties when re gets garbage collected.)
func (re *Regexp) put(z *machine) {
        re.mu.Lock()
        re.machine = append(re.machine, z)
        re.mu.Unlock()
}

// 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 {
                panic(`regexp: Compile(` + quote(str) + `): ` + error.Error())
        }
        return regexp
}

// MustCompilePOSIX is like CompilePOSIX but panics if the expression cannot be parsed.
// It simplifies safe initialization of global variables holding compiled regular
// expressions.
func MustCompilePOSIX(str string) *Regexp {
        regexp, error := CompilePOSIX(str)
        if error != nil {
                panic(`regexp: CompilePOSIX(` + quote(str) + `): ` + error.Error())
        }
        return regexp
}

func quote(s string) string {
        if strconv.CanBackquote(s) {
                return "`" + s + "`"
        }
        return strconv.Quote(s)
}

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

// SubexpNames returns the names of the parenthesized subexpressions
// in this Regexp. The name for the first sub-expression is names[1],
// so that if m is a match slice, the name for m[i] is SubexpNames()[i].
// Since the Regexp as a whole cannot be named, names[0] is always
// the empty string. The slice should not be modified.
func (re *Regexp) SubexpNames() []string {
        return re.subexpNames
}

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) syntax.EmptyOp
}

// inputString scans a string.
type inputString struct {
        str string
}

func (i *inputString) step(pos int) (rune, int) {
        if pos < len(i.str) {
                c := i.str[pos]
                if c < utf8.RuneSelf {
                        return rune(c), 1
                }
                return utf8.DecodeRuneInString(i.str[pos:])
        }
        return endOfText, 0
}

func (i *inputString) canCheckPrefix() bool {
        return true
}

func (i *inputString) hasPrefix(re *Regexp) bool {
        return strings.HasPrefix(i.str, re.prefix)
}

func (i *inputString) index(re *Regexp, pos int) int {
        return strings.Index(i.str[pos:], re.prefix)
}

func (i *inputString) context(pos int) syntax.EmptyOp {
        r1, r2 := endOfText, endOfText
        // 0 < pos && pos <= len(i.str)
        if uint(pos-1) < uint(len(i.str)) {
                r1 = rune(i.str[pos-1])
                if r1 >= utf8.RuneSelf {
                        r1, _ = utf8.DecodeLastRuneInString(i.str[:pos])
                }
        }
        // 0 <= pos && pos < len(i.str)
        if uint(pos) < uint(len(i.str)) {
                r2 = rune(i.str[pos])
                if r2 >= utf8.RuneSelf {
                        r2, _ = utf8.DecodeRuneInString(i.str[pos:])
                }
        }
        return syntax.EmptyOpContext(r1, r2)
}

// inputBytes scans a byte slice.
type inputBytes struct {
        str []byte
}

func (i *inputBytes) step(pos int) (rune, int) {
        if pos < len(i.str) {
                c := i.str[pos]
                if c < utf8.RuneSelf {
                        return rune(c), 1
                }
                return utf8.DecodeRune(i.str[pos:])
        }
        return endOfText, 0
}

func (i *inputBytes) canCheckPrefix() bool {
        return true
}

func (i *inputBytes) hasPrefix(re *Regexp) bool {
        return bytes.HasPrefix(i.str, re.prefixBytes)
}

func (i *inputBytes) index(re *Regexp, pos int) int {
        return bytes.Index(i.str[pos:], re.prefixBytes)
}

func (i *inputBytes) context(pos int) syntax.EmptyOp {
        r1, r2 := endOfText, endOfText
        // 0 < pos && pos <= len(i.str)
        if uint(pos-1) < uint(len(i.str)) {
                r1 = rune(i.str[pos-1])
                if r1 >= utf8.RuneSelf {
                        r1, _ = utf8.DecodeLastRune(i.str[:pos])
                }
        }
        // 0 <= pos && pos < len(i.str)
        if uint(pos) < uint(len(i.str)) {
                r2 = rune(i.str[pos])
                if r2 >= utf8.RuneSelf {
                        r2, _ = utf8.DecodeRune(i.str[pos:])
                }
        }
        return syntax.EmptyOpContext(r1, r2)
}

// inputReader scans a RuneReader.
type inputReader struct {
        r     io.RuneReader
        atEOT bool
        pos   int
}

func (i *inputReader) step(pos int) (rune, int) {
        if !i.atEOT && pos != i.pos {
                return endOfText, 0

        }
        r, w, err := i.r.ReadRune()
        if err != nil {
                i.atEOT = true
                return endOfText, 0
        }
        i.pos += w
        return r, w
}

func (i *inputReader) canCheckPrefix() bool {
        return false
}

func (i *inputReader) hasPrefix(re *Regexp) bool {
        return false
}

func (i *inputReader) index(re *Regexp, pos int) int {
        return -1
}

func (i *inputReader) context(pos int) syntax.EmptyOp {
        return 0
}

// LiteralPrefix returns a literal string that must begin any match
// of the regular expression re. It returns the boolean true if the
// literal string comprises the entire regular expression.
func (re *Regexp) LiteralPrefix() (prefix string, complete bool) {
        return re.prefix, re.prefixComplete
}

// MatchReader reports whether the Regexp matches the text read by the
// RuneReader.
func (re *Regexp) MatchReader(r io.RuneReader) bool {
        return re.doMatch(r, nil, "")
}

// MatchString reports whether the Regexp matches the string s.
func (re *Regexp) MatchString(s string) bool {
        return re.doMatch(nil, nil, s)
}

// Match reports whether the Regexp matches the byte slice b.
func (re *Regexp) Match(b []byte) bool {
        return re.doMatch(nil, b, "")
}

// MatchReader checks whether a textual regular expression matches the text
// read by the RuneReader. More complicated queries need to use Compile and
// the full Regexp interface.
func MatchReader(pattern string, r io.RuneReader) (matched bool, err error) {
        re, err := Compile(pattern)
        if err != nil {
                return false, err
        }
        return re.MatchReader(r), nil
}

// 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, err 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, err error) {
        re, err := Compile(pattern)
        if err != nil {
                return false, err
        }
        return re.Match(b), nil
}

// ReplaceAllString returns a copy of src, replacing matches of the Regexp
// with the replacement string repl. Inside repl, $ signs are interpreted as
// in Expand, so for instance $1 represents the text of the first submatch.
func (re *Regexp) ReplaceAllString(src, repl string) string {
        n := 2
        if strings.Contains(repl, "$") {
                n = 2 * (re.numSubexp + 1)
        }
        b := re.replaceAll(nil, src, n, func(dst []byte, match []int) []byte {
                return re.expand(dst, repl, nil, src, match)
        })
        return string(b)
}

// ReplaceAllLiteralString returns a copy of src, replacing matches of the Regexp
// with the replacement string repl. The replacement repl is substituted directly,
// without using Expand.
func (re *Regexp) ReplaceAllLiteralString(src, repl string) string {
        return string(re.replaceAll(nil, src, 2, func(dst []byte, match []int) []byte {
                return append(dst, repl...)
        }))
}

// ReplaceAllStringFunc returns a copy of src in which all matches of the
// Regexp have been replaced by the return value of function repl applied
// to the matched substring. The replacement returned by repl is substituted
// directly, without using Expand.
func (re *Regexp) ReplaceAllStringFunc(src string, repl func(string) string) string {
        b := re.replaceAll(nil, src, 2, func(dst []byte, match []int) []byte {
                return append(dst, repl(src[match[0]:match[1]])...)
        })
        return string(b)
}

func (re *Regexp) replaceAll(bsrc []byte, src string, nmatch int, repl func(dst []byte, m []int) []byte) []byte {
        lastMatchEnd := 0 // end position of the most recent match
        searchPos := 0    // position where we next look for a match
        var buf []byte
        var endPos int
        if bsrc != nil {
                endPos = len(bsrc)
        } else {
                endPos = len(src)
        }
        if nmatch > re.prog.NumCap {
                nmatch = re.prog.NumCap
        }

        var dstCap [2]int
        for searchPos <= endPos {
                a := re.doExecute(nil, bsrc, src, searchPos, nmatch, dstCap[:0])
                if len(a) == 0 {
                        break // no more matches
                }

                // Copy the unmatched characters before this match.
                if bsrc != nil {
                        buf = append(buf, bsrc[lastMatchEnd:a[0]]...)
                } else {
                        buf = append(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 {
                        buf = repl(buf, a)
                }
                lastMatchEnd = a[1]

                // Advance past this match; always advance at least one character.
                var width int
                if bsrc != nil {
                        _, width = utf8.DecodeRune(bsrc[searchPos:])
                } else {
                        _, 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.
        if bsrc != nil {
                buf = append(buf, bsrc[lastMatchEnd:]...)
        } else {
                buf = append(buf, src[lastMatchEnd:]...)
        }

        return buf
}

// ReplaceAll returns a copy of src, replacing matches of the Regexp
// with the replacement text repl. Inside repl, $ signs are interpreted as
// in Expand, so for instance $1 represents the text of the first submatch.
func (re *Regexp) ReplaceAll(src, repl []byte) []byte {
        n := 2
        if bytes.IndexByte(repl, '$') >= 0 {
                n = 2 * (re.numSubexp + 1)
        }
        srepl := ""
        b := re.replaceAll(src, "", n, func(dst []byte, match []int) []byte {
                if len(srepl) != len(repl) {
                        srepl = string(repl)
                }
                return re.expand(dst, srepl, src, "", match)
        })
        return b
}

// ReplaceAllLiteral returns a copy of src, replacing matches of the Regexp
// with the replacement bytes repl. The replacement repl is substituted directly,
// without using Expand.
func (re *Regexp) ReplaceAllLiteral(src, repl []byte) []byte {
        return re.replaceAll(src, "", 2, func(dst []byte, match []int) []byte {
                return append(dst, repl...)
        })
}

// ReplaceAllFunc returns a copy of src in which all matches of the
// Regexp have been replaced by the return value of function repl applied
// to the matched byte slice. The replacement returned by repl is substituted
// directly, without using Expand.
func (re *Regexp) ReplaceAllFunc(src []byte, repl func([]byte) []byte) []byte {
        return re.replaceAll(src, "", 2, func(dst []byte, match []int) []byte {
                return append(dst, repl(src[match[0]:match[1]])...)
        })
}

// Bitmap used by func special to check whether a character needs to be escaped.
var specialBytes [16]byte

// special reports whether byte b needs to be escaped by QuoteMeta.
func special(b byte) bool {
        return b < utf8.RuneSelf && specialBytes[b%16]&(1<<(b/16)) != 0
}

func init() {
        for _, b := range []byte(`\.+*?()|[]{}^$`) {
                specialBytes[b%16] |= 1 << (b / 16)
        }
}

// 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 {
        // A byte loop is correct because all metacharacters are ASCII.
        var i int
        for i = 0; i < len(s); i++ {
                if special(s[i]) {
                        break
                }
        }
        // No meta characters found, so return original string.
        if i >= len(s) {
                return s
        }

        b := make([]byte, 2*len(s)-i)
        copy(b, s[:i])
        j := i
        for ; i < len(s); i++ {
                if special(s[i]) {
                        b[j] = '\\'
                        j++
                }
                b[j] = s[i]
                j++
        }
        return string(b[:j])
}

// The number of capture values in the program may correspond
// to fewer capturing expressions than are in the regexp.
// For example, "(a){0}" turns into an empty program, so the
// maximum capture in the program is 0 but we need to return
// an expression for \1.  Pad appends -1s to the slice a as needed.
func (re *Regexp) pad(a []int) []int {
        if a == nil {
                // No match.
                return nil
        }
        n := (1 + re.numSubexp) * 2
        for len(a) < n {
                a = append(a, -1)
        }
        return a
}

// 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)) {
        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(nil, b, s, pos, re.prog.NumCap, nil)
                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
                        // TODO: use step()
                        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(re.pad(matches))
                        i++
                }
        }
}

// Find returns a slice holding the text of the leftmost match in b of the regular expression.
// A return value of nil indicates no match.
func (re *Regexp) Find(b []byte) []byte {
        var dstCap [2]int
        a := re.doExecute(nil, b, "", 0, 2, dstCap[:0])
        if a == nil {
                return nil
        }
        return b[a[0]:a[1]]
}

// FindIndex returns a two-element slice of integers defining the location of
// the leftmost match in b of the regular expression. The match itself is at
// b[loc[0]:loc[1]].
// A return value of nil indicates no match.
func (re *Regexp) FindIndex(b []byte) (loc []int) {
        a := re.doExecute(nil, b, "", 0, 2, nil)
        if a == nil {
                return nil
        }
        return a[0:2]
}

// FindString returns a string holding the text of the leftmost match in s of the regular
// expression. If there is no match, the return value is an empty string,
// but it will also be empty if the regular expression successfully matches
// an empty string. Use FindStringIndex or FindStringSubmatch if it is
// necessary to distinguish these cases.
func (re *Regexp) FindString(s string) string {
        var dstCap [2]int
        a := re.doExecute(nil, nil, s, 0, 2, dstCap[:0])
        if a == nil {
                return ""
        }
        return s[a[0]:a[1]]
}

// FindStringIndex returns a two-element slice of integers defining the
// location of the leftmost match in s of the regular expression. The match
// itself is at s[loc[0]:loc[1]].
// A return value of nil indicates no match.
func (re *Regexp) FindStringIndex(s string) (loc []int) {
        a := re.doExecute(nil, nil, s, 0, 2, nil)
        if a == nil {
                return nil
        }
        return a[0:2]
}

// FindReaderIndex returns a two-element slice of integers defining the
// location of the leftmost match of the regular expression in text read from
// the RuneReader. The match text was found in the input stream at
// byte offset loc[0] through loc[1]-1.
// A return value of nil indicates no match.
func (re *Regexp) FindReaderIndex(r io.RuneReader) (loc []int) {
        a := re.doExecute(r, nil, "", 0, 2, nil)
        if a == nil {
                return nil
        }
        return a[0:2]
}

// FindSubmatch returns a slice of slices holding the text of the leftmost
// match of the regular expression in b and the matches, if any, of its
// subexpressions, as defined by the 'Submatch' descriptions in the package
// comment.
// A return value of nil indicates no match.
func (re *Regexp) FindSubmatch(b []byte) [][]byte {
        var dstCap [4]int
        a := re.doExecute(nil, b, "", 0, re.prog.NumCap, dstCap[:0])
        if a == nil {
                return nil
        }
        ret := make([][]byte, 1+re.numSubexp)
        for i := range ret {
                if 2*i < len(a) && a[2*i] >= 0 {
                        ret[i] = b[a[2*i]:a[2*i+1]]
                }
        }
        return ret
}

// Expand appends template to dst and returns the result; during the
// append, Expand replaces variables in the template with corresponding
// matches drawn from src. The match slice should have been returned by
// FindSubmatchIndex.
//
// In the template, a variable is denoted by a substring of the form
// $name or ${name}, where name is a non-empty sequence of letters,
// digits, and underscores. A purely numeric name like $1 refers to
// the submatch with the corresponding index; other names refer to
// capturing parentheses named with the (?P<name>...) syntax. A
// reference to an out of range or unmatched index or a name that is not
// present in the regular expression is replaced with an empty slice.
//
// In the $name form, name is taken to be as long as possible: $1x is
// equivalent to ${1x}, not ${1}x, and, $10 is equivalent to ${10}, not ${1}0.
//
// To insert a literal $ in the output, use $$ in the template.
func (re *Regexp) Expand(dst []byte, template []byte, src []byte, match []int) []byte {
        return re.expand(dst, string(template), src, "", match)
}

// ExpandString is like Expand but the template and source are strings.
// It appends to and returns a byte slice in order to give the calling
// code control over allocation.
func (re *Regexp) ExpandString(dst []byte, template string, src string, match []int) []byte {
        return re.expand(dst, template, nil, src, match)
}

func (re *Regexp) expand(dst []byte, template string, bsrc []byte, src string, match []int) []byte {
        for len(template) > 0 {
                i := strings.Index(template, "$")
                if i < 0 {
                        break
                }
                dst = append(dst, template[:i]...)
                template = template[i:]
                if len(template) > 1 && template[1] == '$' {
                        // Treat $$ as $.
                        dst = append(dst, '$')
                        template = template[2:]
                        continue
                }
                name, num, rest, ok := extract(template)
                if !ok {
                        // Malformed; treat $ as raw text.
                        dst = append(dst, '$')
                        template = template[1:]
                        continue
                }
                template = rest
                if num >= 0 {
                        if 2*num+1 < len(match) && match[2*num] >= 0 {
                                if bsrc != nil {
                                        dst = append(dst, bsrc[match[2*num]:match[2*num+1]]...)
                                } else {
                                        dst = append(dst, src[match[2*num]:match[2*num+1]]...)
                                }
                        }
                } else {
                        for i, namei := range re.subexpNames {
                                if name == namei && 2*i+1 < len(match) && match[2*i] >= 0 {
                                        if bsrc != nil {
                                                dst = append(dst, bsrc[match[2*i]:match[2*i+1]]...)
                                        } else {
                                                dst = append(dst, src[match[2*i]:match[2*i+1]]...)
                                        }
                                        break
                                }
                        }
                }
        }
        dst = append(dst, template...)
        return dst
}

// extract returns the name from a leading "$name" or "${name}" in str.
// If it is a number, extract returns num set to that number; otherwise num = -1.
func extract(str string) (name string, num int, rest string, ok bool) {
        if len(str) < 2 || str[0] != '$' {
                return
        }
        brace := false
        if str[1] == '{' {
                brace = true
                str = str[2:]
        } else {
                str = str[1:]
        }
        i := 0
        for i < len(str) {
                rune, size := utf8.DecodeRuneInString(str[i:])
                if !unicode.IsLetter(rune) && !unicode.IsDigit(rune) && rune != '_' {
                        break
                }
                i += size
        }
        if i == 0 {
                // empty name is not okay
                return
        }
        name = str[:i]
        if brace {
                if i >= len(str) || str[i] != '}' {
                        // missing closing brace
                        return
                }
                i++
        }

        // Parse number.
        num = 0
        for i := 0; i < len(name); i++ {
                if name[i] < '0' || '9' < name[i] || num >= 1e8 {
                        num = -1
                        break
                }
                num = num*10 + int(name[i]) - '0'
        }
        // Disallow leading zeros.
        if name[0] == '0' && len(name) > 1 {
                num = -1
        }

        rest = str[i:]
        ok = true
        return
}

// FindSubmatchIndex returns a slice holding the index pairs identifying the
// leftmost match of the regular expression in b and the matches, if any, of
// its subexpressions, as defined by the 'Submatch' and 'Index' descriptions
// in the package comment.
// A return value of nil indicates no match.
func (re *Regexp) FindSubmatchIndex(b []byte) []int {
        return re.pad(re.doExecute(nil, b, "", 0, re.prog.NumCap, nil))
}

// FindStringSubmatch returns a slice of strings holding the text of the
// leftmost match of the regular expression in s and the matches, if any, of
// its subexpressions, as defined by the 'Submatch' description in the
// package comment.
// A return value of nil indicates no match.
func (re *Regexp) FindStringSubmatch(s string) []string {
        var dstCap [4]int
        a := re.doExecute(nil, nil, s, 0, re.prog.NumCap, dstCap[:0])
        if a == nil {
                return nil
        }
        ret := make([]string, 1+re.numSubexp)
        for i := range ret {
                if 2*i < len(a) && a[2*i] >= 0 {
                        ret[i] = s[a[2*i]:a[2*i+1]]
                }
        }
        return ret
}

// FindStringSubmatchIndex returns a slice holding the index pairs
// identifying the leftmost match of the regular expression in s and the
// matches, if any, of its subexpressions, as defined by the 'Submatch' and
// 'Index' descriptions in the package comment.
// A return value of nil indicates no match.
func (re *Regexp) FindStringSubmatchIndex(s string) []int {
        return re.pad(re.doExecute(nil, nil, s, 0, re.prog.NumCap, nil))
}

// FindReaderSubmatchIndex returns a slice holding the index pairs
// identifying the leftmost match of the regular expression of text read by
// the RuneReader, and the matches, if any, of its subexpressions, as defined
// by the 'Submatch' and 'Index' descriptions in the package comment. A
// return value of nil indicates no match.
func (re *Regexp) FindReaderSubmatchIndex(r io.RuneReader) []int {
        return re.pad(re.doExecute(r, nil, "", 0, re.prog.NumCap, nil))
}

const startSize = 10 // The size at which to start a slice in the 'All' routines.

// FindAll is the 'All' version of Find; it returns a slice of all successive
// matches of the expression, as defined by the 'All' description in the
// package comment.
// A return value of nil indicates no match.
func (re *Regexp) FindAll(b []byte, n int) [][]byte {
        if n < 0 {
                n = len(b) + 1
        }
        result := make([][]byte, 0, startSize)
        re.allMatches("", b, n, func(match []int) {
                result = append(result, b[match[0]:match[1]])
        })
        if len(result) == 0 {
                return nil
        }
        return result
}

// FindAllIndex is the 'All' version of FindIndex; it returns a slice of all
// successive matches of the expression, as defined by the 'All' description
// in the package comment.
// A return value of nil indicates no match.
func (re *Regexp) FindAllIndex(b []byte, n int) [][]int {
        if n < 0 {
                n = len(b) + 1
        }
        result := make([][]int, 0, startSize)
        re.allMatches("", b, n, func(match []int) {
                result = append(result, match[0:2])
        })
        if len(result) == 0 {
                return nil
        }
        return result
}

// FindAllString is the 'All' version of FindString; it returns a slice of all
// successive matches of the expression, as defined by the 'All' description
// in the package comment.
// A return value of nil indicates no match.
func (re *Regexp) FindAllString(s string, n int) []string {
        if n < 0 {
                n = len(s) + 1
        }
        result := make([]string, 0, startSize)
        re.allMatches(s, nil, n, func(match []int) {
                result = append(result, s[match[0]:match[1]])
        })
        if len(result) == 0 {
                return nil
        }
        return result
}

// FindAllStringIndex is the 'All' version of FindStringIndex; it returns a
// slice of all successive matches of the expression, as defined by the 'All'
// description in the package comment.
// A return value of nil indicates no match.
func (re *Regexp) FindAllStringIndex(s string, n int) [][]int {
        if n < 0 {
                n = len(s) + 1
        }
        result := make([][]int, 0, startSize)
        re.allMatches(s, nil, n, func(match []int) {
                result = append(result, match[0:2])
        })
        if len(result) == 0 {
                return nil
        }
        return result
}

// FindAllSubmatch is the 'All' version of FindSubmatch; it returns a slice
// of all successive matches of the expression, as defined by the 'All'
// description in the package comment.
// A return value of nil indicates no match.
func (re *Regexp) FindAllSubmatch(b []byte, n int) [][][]byte {
        if n < 0 {
                n = len(b) + 1
        }
        result := make([][][]byte, 0, startSize)
        re.allMatches("", b, n, func(match []int) {
                slice := make([][]byte, len(match)/2)
                for j := range slice {
                        if match[2*j] >= 0 {
                                slice[j] = b[match[2*j]:match[2*j+1]]
                        }
                }
                result = append(result, slice)
        })
        if len(result) == 0 {
                return nil
        }
        return result
}

// FindAllSubmatchIndex is the 'All' version of FindSubmatchIndex; it returns
// a slice of all successive matches of the expression, as defined by the
// 'All' description in the package comment.
// A return value of nil indicates no match.
func (re *Regexp) FindAllSubmatchIndex(b []byte, n int) [][]int {
        if n < 0 {
                n = len(b) + 1
        }
        result := make([][]int, 0, startSize)
        re.allMatches("", b, n, func(match []int) {
                result = append(result, match)
        })
        if len(result) == 0 {
                return nil
        }
        return result
}

// FindAllStringSubmatch is the 'All' version of FindStringSubmatch; it
// returns a slice of all successive matches of the expression, as defined by
// the 'All' description in the package comment.
// A return value of nil indicates no match.
func (re *Regexp) FindAllStringSubmatch(s string, n int) [][]string {
        if n < 0 {
                n = len(s) + 1
        }
        result := make([][]string, 0, startSize)
        re.allMatches(s, nil, n, func(match []int) {
                slice := make([]string, len(match)/2)
                for j := range slice {
                        if match[2*j] >= 0 {
                                slice[j] = s[match[2*j]:match[2*j+1]]
                        }
                }
                result = append(result, slice)
        })
        if len(result) == 0 {
                return nil
        }
        return result
}

// FindAllStringSubmatchIndex is the 'All' version of
// FindStringSubmatchIndex; it returns a slice of all successive matches of
// the expression, as defined by the 'All' description in the package
// comment.
// A return value of nil indicates no match.
func (re *Regexp) FindAllStringSubmatchIndex(s string, n int) [][]int {
        if n < 0 {
                n = len(s) + 1
        }
        result := make([][]int, 0, startSize)
        re.allMatches(s, nil, n, func(match []int) {
                result = append(result, match)
        })
        if len(result) == 0 {
                return nil
        }
        return result
}

// Split slices s into substrings separated by the expression and returns a slice of
// the substrings between those expression matches.
//
// The slice returned by this method consists of all the substrings of s
// not contained in the slice returned by FindAllString. When called on an expression
// that contains no metacharacters, it is equivalent to strings.SplitN.
//
// Example:
//   s := regexp.MustCompile("a*").Split("abaabaccadaaae", 5)
//   // s: ["", "b", "b", "c", "cadaaae"]
//
// The count determines the number of substrings to return:
//   n > 0: at most n substrings; the last substring will be the unsplit remainder.
//   n == 0: the result is nil (zero substrings)
//   n < 0: all substrings
func (re *Regexp) Split(s string, n int) []string {

        if n == 0 {
                return nil
        }

        if len(re.expr) > 0 && len(s) == 0 {
                return []string{""}
        }

        matches := re.FindAllStringIndex(s, n)
        strings := make([]string, 0, len(matches))

        beg := 0
        end := 0
        for _, match := range matches {
                if n > 0 && len(strings) >= n-1 {
                        break
                }

                end = match[0]
                if match[1] != 0 {
                        strings = append(strings, s[beg:end])
                }
                beg = match[1]
        }

        if end != len(s) {
                strings = append(strings, s[beg:])
        }

        return strings
}