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// 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 syntax

import (
        "sort"
        "strings"
        "unicode"
        "unicode/utf8"
)

// An Error describes a failure to parse a regular expression
// and gives the offending expression.
type Error struct {
        Code ErrorCode
        Expr string
}

func (e *Error) Error() string {
        return "error parsing regexp: " + e.Code.String() + ": `" + e.Expr + "`"
}

// An ErrorCode describes a failure to parse a regular expression.
type ErrorCode string

const (
        // Unexpected error
        ErrInternalError ErrorCode = "regexp/syntax: internal error"

        // Parse errors
        ErrInvalidCharClass      ErrorCode = "invalid character class"
        ErrInvalidCharRange      ErrorCode = "invalid character class range"
        ErrInvalidEscape         ErrorCode = "invalid escape sequence"
        ErrInvalidNamedCapture   ErrorCode = "invalid named capture"
        ErrInvalidPerlOp         ErrorCode = "invalid or unsupported Perl syntax"
        ErrInvalidRepeatOp       ErrorCode = "invalid nested repetition operator"
        ErrInvalidRepeatSize     ErrorCode = "invalid repeat count"
        ErrInvalidUTF8           ErrorCode = "invalid UTF-8"
        ErrMissingBracket        ErrorCode = "missing closing ]"
        ErrMissingParen          ErrorCode = "missing closing )"
        ErrMissingRepeatArgument ErrorCode = "missing argument to repetition operator"
        ErrTrailingBackslash     ErrorCode = "trailing backslash at end of expression"
)

func (e ErrorCode) String() string {
        return string(e)
}

// Flags control the behavior of the parser and record information about regexp context.
type Flags uint16

const (
        FoldCase      Flags = 1 << iota // case-insensitive match
        Literal                         // treat pattern as literal string
        ClassNL                         // allow character classes like [^a-z] and [[:space:]] to match newline
        DotNL                           // allow . to match newline
        OneLine                         // treat ^ and $ as only matching at beginning and end of text
        NonGreedy                       // make repetition operators default to non-greedy
        PerlX                           // allow Perl extensions
        UnicodeGroups                   // allow \p{Han}, \P{Han} for Unicode group and negation
        WasDollar                       // regexp OpEndText was $, not \z
        Simple                          // regexp contains no counted repetition

        MatchNL = ClassNL | DotNL

        Perl        = ClassNL | OneLine | PerlX | UnicodeGroups // as close to Perl as possible
        POSIX Flags = 0                                         // POSIX syntax
)

// Pseudo-ops for parsing stack.
const (
        opLeftParen = opPseudo + iota
        opVerticalBar
)

type parser struct {
        flags       Flags     // parse mode flags
        stack       []*Regexp // stack of parsed expressions
        free        *Regexp
        numCap      int // number of capturing groups seen
        wholeRegexp string
        tmpClass    []rune // temporary char class work space
}

func (p *parser) newRegexp(op Op) *Regexp {
        re := p.free
        if re != nil {
                p.free = re.Sub0[0]
                *re = Regexp{}
        } else {
                re = new(Regexp)
        }
        re.Op = op
        return re
}

func (p *parser) reuse(re *Regexp) {
        re.Sub0[0] = p.free
        p.free = re
}

// Parse stack manipulation.

// push pushes the regexp re onto the parse stack and returns the regexp.
func (p *parser) push(re *Regexp) *Regexp {
        if re.Op == OpCharClass && len(re.Rune) == 2 && re.Rune[0] == re.Rune[1] {
                // Single rune.
                if p.maybeConcat(re.Rune[0], p.flags&^FoldCase) {
                        return nil
                }
                re.Op = OpLiteral
                re.Rune = re.Rune[:1]
                re.Flags = p.flags &^ FoldCase
        } else if re.Op == OpCharClass && len(re.Rune) == 4 &&
                re.Rune[0] == re.Rune[1] && re.Rune[2] == re.Rune[3] &&
                unicode.SimpleFold(re.Rune[0]) == re.Rune[2] &&
                unicode.SimpleFold(re.Rune[2]) == re.Rune[0] ||
                re.Op == OpCharClass && len(re.Rune) == 2 &&
                        re.Rune[0]+1 == re.Rune[1] &&
                        unicode.SimpleFold(re.Rune[0]) == re.Rune[1] &&
                        unicode.SimpleFold(re.Rune[1]) == re.Rune[0] {
                // Case-insensitive rune like [Aa] or [Δδ].
                if p.maybeConcat(re.Rune[0], p.flags|FoldCase) {
                        return nil
                }

                // Rewrite as (case-insensitive) literal.
                re.Op = OpLiteral
                re.Rune = re.Rune[:1]
                re.Flags = p.flags | FoldCase
        } else {
                // Incremental concatenation.
                p.maybeConcat(-1, 0)
        }

        p.stack = append(p.stack, re)
        return re
}

// maybeConcat implements incremental concatenation
// of literal runes into string nodes.  The parser calls this
// before each push, so only the top fragment of the stack
// might need processing.  Since this is called before a push,
// the topmost literal is no longer subject to operators like *
// (Otherwise ab* would turn into (ab)*.)
// If r >= 0 and there's a node left over, maybeConcat uses it
// to push r with the given flags.
// maybeConcat reports whether r was pushed.
func (p *parser) maybeConcat(r rune, flags Flags) bool {
        n := len(p.stack)
        if n < 2 {
                return false
        }

        re1 := p.stack[n-1]
        re2 := p.stack[n-2]
        if re1.Op != OpLiteral || re2.Op != OpLiteral || re1.Flags&FoldCase != re2.Flags&FoldCase {
                return false
        }

        // Push re1 into re2.
        re2.Rune = append(re2.Rune, re1.Rune...)

        // Reuse re1 if possible.
        if r >= 0 {
                re1.Rune = re1.Rune0[:1]
                re1.Rune[0] = r
                re1.Flags = flags
                return true
        }

        p.stack = p.stack[:n-1]
        p.reuse(re1)
        return false // did not push r
}

// newLiteral returns a new OpLiteral Regexp with the given flags
func (p *parser) newLiteral(r rune, flags Flags) *Regexp {
        re := p.newRegexp(OpLiteral)
        re.Flags = flags
        if flags&FoldCase != 0 {
                r = minFoldRune(r)
        }
        re.Rune0[0] = r
        re.Rune = re.Rune0[:1]
        return re
}

// minFoldRune returns the minimum rune fold-equivalent to r.
func minFoldRune(r rune) rune {
        if r < MinFold || r > MaxFold {
                return r
        }
        min := r
        r0 := r
        for r = unicode.SimpleFold(r); r != r0; r = unicode.SimpleFold(r) {
                if min > r {
                        min = r
                }
        }
        return min
}

// literal pushes a literal regexp for the rune r on the stack
// and returns that regexp.
func (p *parser) literal(r rune) {
        p.push(p.newLiteral(r, p.flags))
}

// op pushes a regexp with the given op onto the stack
// and returns that regexp.
func (p *parser) op(op Op) *Regexp {
        re := p.newRegexp(op)
        re.Flags = p.flags
        return p.push(re)
}

// repeat replaces the top stack element with itself repeated according to op, min, max.
// before is the regexp suffix starting at the repetition operator.
// after is the regexp suffix following after the repetition operator.
// repeat returns an updated 'after' and an error, if any.
func (p *parser) repeat(op Op, min, max int, before, after, lastRepeat string) (string, error) {
        flags := p.flags
        if p.flags&PerlX != 0 {
                if len(after) > 0 && after[0] == '?' {
                        after = after[1:]
                        flags ^= NonGreedy
                }
                if lastRepeat != "" {
                        // In Perl it is not allowed to stack repetition operators:
                        // a** is a syntax error, not a doubled star, and a++ means
                        // something else entirely, which we don't support!
                        return "", &Error{ErrInvalidRepeatOp, lastRepeat[:len(lastRepeat)-len(after)]}
                }
        }
        n := len(p.stack)
        if n == 0 {
                return "", &Error{ErrMissingRepeatArgument, before[:len(before)-len(after)]}
        }
        sub := p.stack[n-1]
        if sub.Op >= opPseudo {
                return "", &Error{ErrMissingRepeatArgument, before[:len(before)-len(after)]}
        }
        re := p.newRegexp(op)
        re.Min = min
        re.Max = max
        re.Flags = flags
        re.Sub = re.Sub0[:1]
        re.Sub[0] = sub
        p.stack[n-1] = re
        return after, nil
}

// concat replaces the top of the stack (above the topmost '|' or '(') with its concatenation.
func (p *parser) concat() *Regexp {
        p.maybeConcat(-1, 0)

        // Scan down to find pseudo-operator | or (.
        i := len(p.stack)
        for i > 0 && p.stack[i-1].Op < opPseudo {
                i--
        }
        subs := p.stack[i:]
        p.stack = p.stack[:i]

        // Empty concatenation is special case.
        if len(subs) == 0 {
                return p.push(p.newRegexp(OpEmptyMatch))
        }

        return p.push(p.collapse(subs, OpConcat))
}

// alternate replaces the top of the stack (above the topmost '(') with its alternation.
func (p *parser) alternate() *Regexp {
        // Scan down to find pseudo-operator (.
        // There are no | above (.
        i := len(p.stack)
        for i > 0 && p.stack[i-1].Op < opPseudo {
                i--
        }
        subs := p.stack[i:]
        p.stack = p.stack[:i]

        // Make sure top class is clean.
        // All the others already are (see swapVerticalBar).
        if len(subs) > 0 {
                cleanAlt(subs[len(subs)-1])
        }

        // Empty alternate is special case
        // (shouldn't happen but easy to handle).
        if len(subs) == 0 {
                return p.push(p.newRegexp(OpNoMatch))
        }

        return p.push(p.collapse(subs, OpAlternate))
}

// cleanAlt cleans re for eventual inclusion in an alternation.
func cleanAlt(re *Regexp) {
        switch re.Op {
        case OpCharClass:
                re.Rune = cleanClass(&re.Rune)
                if len(re.Rune) == 2 && re.Rune[0] == 0 && re.Rune[1] == unicode.MaxRune {
                        re.Rune = nil
                        re.Op = OpAnyChar
                        return
                }
                if len(re.Rune) == 4 && re.Rune[0] == 0 && re.Rune[1] == '\n'-1 && re.Rune[2] == '\n'+1 && re.Rune[3] == unicode.MaxRune {
                        re.Rune = nil
                        re.Op = OpAnyCharNotNL
                        return
                }
                if cap(re.Rune)-len(re.Rune) > 100 {
                        // re.Rune will not grow any more.
                        // Make a copy or inline to reclaim storage.
                        re.Rune = append(re.Rune0[:0], re.Rune...)
                }
        }
}

// collapse returns the result of applying op to sub.
// If sub contains op nodes, they all get hoisted up
// so that there is never a concat of a concat or an
// alternate of an alternate.
func (p *parser) collapse(subs []*Regexp, op Op) *Regexp {
        if len(subs) == 1 {
                return subs[0]
        }
        re := p.newRegexp(op)
        re.Sub = re.Sub0[:0]
        for _, sub := range subs {
                if sub.Op == op {
                        re.Sub = append(re.Sub, sub.Sub...)
                        p.reuse(sub)
                } else {
                        re.Sub = append(re.Sub, sub)
                }
        }
        if op == OpAlternate {
                re.Sub = p.factor(re.Sub, re.Flags)
                if len(re.Sub) == 1 {
                        old := re
                        re = re.Sub[0]
                        p.reuse(old)
                }
        }
        return re
}

// factor factors common prefixes from the alternation list sub.
// It returns a replacement list that reuses the same storage and
// frees (passes to p.reuse) any removed *Regexps.
//
// For example,
//     ABC|ABD|AEF|BCX|BCY
// simplifies by literal prefix extraction to
//     A(B(C|D)|EF)|BC(X|Y)
// which simplifies by character class introduction to
//     A(B[CD]|EF)|BC[XY]
//
func (p *parser) factor(sub []*Regexp, flags Flags) []*Regexp {
        if len(sub) < 2 {
                return sub
        }

        // Round 1: Factor out common literal prefixes.
        var str []rune
        var strflags Flags
        start := 0
        out := sub[:0]
        for i := 0; i <= len(sub); i++ {
                // Invariant: the Regexps that were in sub[0:start] have been
                // used or marked for reuse, and the slice space has been reused
                // for out (len(out) <= start).
                //
                // Invariant: sub[start:i] consists of regexps that all begin
                // with str as modified by strflags.
                var istr []rune
                var iflags Flags
                if i < len(sub) {
                        istr, iflags = p.leadingString(sub[i])
                        if iflags == strflags {
                                same := 0
                                for same < len(str) && same < len(istr) && str[same] == istr[same] {
                                        same++
                                }
                                if same > 0 {
                                        // Matches at least one rune in current range.
                                        // Keep going around.
                                        str = str[:same]
                                        continue
                                }
                        }
                }

                // Found end of a run with common leading literal string:
                // sub[start:i] all begin with str[0:len(str)], but sub[i]
                // does not even begin with str[0].
                //
                // Factor out common string and append factored expression to out.
                if i == start {
                        // Nothing to do - run of length 0.
                } else if i == start+1 {
                        // Just one: don't bother factoring.
                        out = append(out, sub[start])
                } else {
                        // Construct factored form: prefix(suffix1|suffix2|...)
                        prefix := p.newRegexp(OpLiteral)
                        prefix.Flags = strflags
                        prefix.Rune = append(prefix.Rune[:0], str...)

                        for j := start; j < i; j++ {
                                sub[j] = p.removeLeadingString(sub[j], len(str))
                        }
                        suffix := p.collapse(sub[start:i], OpAlternate) // recurse

                        re := p.newRegexp(OpConcat)
                        re.Sub = append(re.Sub[:0], prefix, suffix)
                        out = append(out, re)
                }

                // Prepare for next iteration.
                start = i
                str = istr
                strflags = iflags
        }
        sub = out

        // Round 2: Factor out common complex prefixes,
        // just the first piece of each concatenation,
        // whatever it is.  This is good enough a lot of the time.
        start = 0
        out = sub[:0]
        var first *Regexp
        for i := 0; i <= len(sub); i++ {
                // Invariant: the Regexps that were in sub[0:start] have been
                // used or marked for reuse, and the slice space has been reused
                // for out (len(out) <= start).
                //
                // Invariant: sub[start:i] consists of regexps that all begin with ifirst.
                var ifirst *Regexp
                if i < len(sub) {
                        ifirst = p.leadingRegexp(sub[i])
                        if first != nil && first.Equal(ifirst) {
                                continue
                        }
                }

                // Found end of a run with common leading regexp:
                // sub[start:i] all begin with first but sub[i] does not.
                //
                // Factor out common regexp and append factored expression to out.
                if i == start {
                        // Nothing to do - run of length 0.
                } else if i == start+1 {
                        // Just one: don't bother factoring.
                        out = append(out, sub[start])
                } else {
                        // Construct factored form: prefix(suffix1|suffix2|...)
                        prefix := first
                        for j := start; j < i; j++ {
                                reuse := j != start // prefix came from sub[start] 
                                sub[j] = p.removeLeadingRegexp(sub[j], reuse)
                        }
                        suffix := p.collapse(sub[start:i], OpAlternate) // recurse

                        re := p.newRegexp(OpConcat)
                        re.Sub = append(re.Sub[:0], prefix, suffix)
                        out = append(out, re)
                }

                // Prepare for next iteration.
                start = i
                first = ifirst
        }
        sub = out

        // Round 3: Collapse runs of single literals into character classes.
        start = 0
        out = sub[:0]
        for i := 0; i <= len(sub); i++ {
                // Invariant: the Regexps that were in sub[0:start] have been
                // used or marked for reuse, and the slice space has been reused
                // for out (len(out) <= start).
                //
                // Invariant: sub[start:i] consists of regexps that are either
                // literal runes or character classes.
                if i < len(sub) && isCharClass(sub[i]) {
                        continue
                }

                // sub[i] is not a char or char class;
                // emit char class for sub[start:i]...
                if i == start {
                        // Nothing to do - run of length 0.
                } else if i == start+1 {
                        out = append(out, sub[start])
                } else {
                        // Make new char class.
                        // Start with most complex regexp in sub[start].
                        max := start
                        for j := start + 1; j < i; j++ {
                                if sub[max].Op < sub[j].Op || sub[max].Op == sub[j].Op && len(sub[max].Rune) < len(sub[j].Rune) {
                                        max = j
                                }
                        }
                        sub[start], sub[max] = sub[max], sub[start]

                        for j := start + 1; j < i; j++ {
                                mergeCharClass(sub[start], sub[j])
                                p.reuse(sub[j])
                        }
                        cleanAlt(sub[start])
                        out = append(out, sub[start])
                }

                // ... and then emit sub[i].
                if i < len(sub) {
                        out = append(out, sub[i])
                }
                start = i + 1
        }
        sub = out

        // Round 4: Collapse runs of empty matches into a single empty match.
        start = 0
        out = sub[:0]
        for i := range sub {
                if i+1 < len(sub) && sub[i].Op == OpEmptyMatch && sub[i+1].Op == OpEmptyMatch {
                        continue
                }
                out = append(out, sub[i])
        }
        sub = out

        return sub
}

// leadingString returns the leading literal string that re begins with.
// The string refers to storage in re or its children.
func (p *parser) leadingString(re *Regexp) ([]rune, Flags) {
        if re.Op == OpConcat && len(re.Sub) > 0 {
                re = re.Sub[0]
        }
        if re.Op != OpLiteral {
                return nil, 0
        }
        return re.Rune, re.Flags & FoldCase
}

// removeLeadingString removes the first n leading runes
// from the beginning of re.  It returns the replacement for re.
func (p *parser) removeLeadingString(re *Regexp, n int) *Regexp {
        if re.Op == OpConcat && len(re.Sub) > 0 {
                // Removing a leading string in a concatenation
                // might simplify the concatenation.
                sub := re.Sub[0]
                sub = p.removeLeadingString(sub, n)
                re.Sub[0] = sub
                if sub.Op == OpEmptyMatch {
                        p.reuse(sub)
                        switch len(re.Sub) {
                        case 0, 1:
                                // Impossible but handle.
                                re.Op = OpEmptyMatch
                                re.Sub = nil
                        case 2:
                                old := re
                                re = re.Sub[1]
                                p.reuse(old)
                        default:
                                copy(re.Sub, re.Sub[1:])
                                re.Sub = re.Sub[:len(re.Sub)-1]
                        }
                }
                return re
        }

        if re.Op == OpLiteral {
                re.Rune = re.Rune[:copy(re.Rune, re.Rune[n:])]
                if len(re.Rune) == 0 {
                        re.Op = OpEmptyMatch
                }
        }
        return re
}

// leadingRegexp returns the leading regexp that re begins with.
// The regexp refers to storage in re or its children.
func (p *parser) leadingRegexp(re *Regexp) *Regexp {
        if re.Op == OpEmptyMatch {
                return nil
        }
        if re.Op == OpConcat && len(re.Sub) > 0 {
                sub := re.Sub[0]
                if sub.Op == OpEmptyMatch {
                        return nil
                }
                return sub
        }
        return re
}

// removeLeadingRegexp removes the leading regexp in re.
// It returns the replacement for re.
// If reuse is true, it passes the removed regexp (if no longer needed) to p.reuse.
func (p *parser) removeLeadingRegexp(re *Regexp, reuse bool) *Regexp {
        if re.Op == OpConcat && len(re.Sub) > 0 {
                if reuse {
                        p.reuse(re.Sub[0])
                }
                re.Sub = re.Sub[:copy(re.Sub, re.Sub[1:])]
                switch len(re.Sub) {
                case 0:
                        re.Op = OpEmptyMatch
                        re.Sub = nil
                case 1:
                        old := re
                        re = re.Sub[0]
                        p.reuse(old)
                }
                return re
        }
        if reuse {
                p.reuse(re)
        }
        return p.newRegexp(OpEmptyMatch)
}

func literalRegexp(s string, flags Flags) *Regexp {
        re := &Regexp{Op: OpLiteral}
        re.Flags = flags
        re.Rune = re.Rune0[:0] // use local storage for small strings
        for _, c := range s {
                if len(re.Rune) >= cap(re.Rune) {
                        // string is too long to fit in Rune0.  let Go handle it
                        re.Rune = []rune(s)
                        break
                }
                re.Rune = append(re.Rune, c)
        }
        return re
}

// Parsing.

func Parse(s string, flags Flags) (*Regexp, error) {
        if flags&Literal != 0 {
                // Trivial parser for literal string.
                if err := checkUTF8(s); err != nil {
                        return nil, err
                }
                return literalRegexp(s, flags), nil
        }

        // Otherwise, must do real work.
        var (
                p          parser
                err        error
                c          rune
                op         Op
                lastRepeat string
                min, max   int
        )
        p.flags = flags
        p.wholeRegexp = s
        t := s
        for t != "" {
                repeat := ""
        BigSwitch:
                switch t[0] {
                default:
                        if c, t, err = nextRune(t); err != nil {
                                return nil, err
                        }
                        p.literal(c)

                case '(':
                        if p.flags&PerlX != 0 && len(t) >= 2 && t[1] == '?' {
                                // Flag changes and non-capturing groups.
                                if t, err = p.parsePerlFlags(t); err != nil {
                                        return nil, err
                                }
                                break
                        }
                        p.numCap++
                        p.op(opLeftParen).Cap = p.numCap
                        t = t[1:]
                case '|':
                        if err = p.parseVerticalBar(); err != nil {
                                return nil, err
                        }
                        t = t[1:]
                case ')':
                        if err = p.parseRightParen(); err != nil {
                                return nil, err
                        }
                        t = t[1:]
                case '^':
                        if p.flags&OneLine != 0 {
                                p.op(OpBeginText)
                        } else {
                                p.op(OpBeginLine)
                        }
                        t = t[1:]
                case '$':
                        if p.flags&OneLine != 0 {
                                p.op(OpEndText).Flags |= WasDollar
                        } else {
                                p.op(OpEndLine)
                        }
                        t = t[1:]
                case '.':
                        if p.flags&DotNL != 0 {
                                p.op(OpAnyChar)
                        } else {
                                p.op(OpAnyCharNotNL)
                        }
                        t = t[1:]
                case '[':
                        if t, err = p.parseClass(t); err != nil {
                                return nil, err
                        }
                case '*', '+', '?':
                        before := t
                        switch t[0] {
                        case '*':
                                op = OpStar
                        case '+':
                                op = OpPlus
                        case '?':
                                op = OpQuest
                        }
                        after := t[1:]
                        if after, err = p.repeat(op, min, max, before, after, lastRepeat); err != nil {
                                return nil, err
                        }
                        repeat = before
                        t = after
                case '{':
                        op = OpRepeat
                        before := t
                        min, max, after, ok := p.parseRepeat(t)
                        if !ok {
                                // If the repeat cannot be parsed, { is a literal.
                                p.literal('{')
                                t = t[1:]
                                break
                        }
                        if min < 0 || min > 1000 || max > 1000 || max >= 0 && min > max {
                                // Numbers were too big, or max is present and min > max.
                                return nil, &Error{ErrInvalidRepeatSize, before[:len(before)-len(after)]}
                        }
                        if after, err = p.repeat(op, min, max, before, after, lastRepeat); err != nil {
                                return nil, err
                        }
                        repeat = before
                        t = after
                case '\\':
                        if p.flags&PerlX != 0 && len(t) >= 2 {
                                switch t[1] {
                                case 'A':
                                        p.op(OpBeginText)
                                        t = t[2:]
                                        break BigSwitch
                                case 'b':
                                        p.op(OpWordBoundary)
                                        t = t[2:]
                                        break BigSwitch
                                case 'B':
                                        p.op(OpNoWordBoundary)
                                        t = t[2:]
                                        break BigSwitch
                                case 'C':
                                        // any byte; not supported
                                        return nil, &Error{ErrInvalidEscape, t[:2]}
                                case 'Q':
                                        // \Q ... \E: the ... is always literals
                                        var lit string
                                        if i := strings.Index(t, `\E`); i < 0 {
                                                lit = t[2:]
                                                t = ""
                                        } else {
                                                lit = t[2:i]
                                                t = t[i+2:]
                                        }
                                        p.push(literalRegexp(lit, p.flags))
                                        break BigSwitch
                                case 'z':
                                        p.op(OpEndText)
                                        t = t[2:]
                                        break BigSwitch
                                }
                        }

                        re := p.newRegexp(OpCharClass)
                        re.Flags = p.flags

                        // Look for Unicode character group like \p{Han}
                        if len(t) >= 2 && (t[1] == 'p' || t[1] == 'P') {
                                r, rest, err := p.parseUnicodeClass(t, re.Rune0[:0])
                                if err != nil {
                                        return nil, err
                                }
                                if r != nil {
                                        re.Rune = r
                                        t = rest
                                        p.push(re)
                                        break BigSwitch
                                }
                        }

                        // Perl character class escape.
                        if r, rest := p.parsePerlClassEscape(t, re.Rune0[:0]); r != nil {
                                re.Rune = r
                                t = rest
                                p.push(re)
                                break BigSwitch
                        }
                        p.reuse(re)

                        // Ordinary single-character escape.
                        if c, t, err = p.parseEscape(t); err != nil {
                                return nil, err
                        }
                        p.literal(c)
                }
                lastRepeat = repeat
        }

        p.concat()
        if p.swapVerticalBar() {
                // pop vertical bar
                p.stack = p.stack[:len(p.stack)-1]
        }
        p.alternate()

        n := len(p.stack)
        if n != 1 {
                return nil, &Error{ErrMissingParen, s}
        }
        return p.stack[0], nil
}

// parseRepeat parses {min} (max=min) or {min,} (max=-1) or {min,max}.
// If s is not of that form, it returns ok == false.
// If s has the right form but the values are too big, it returns min == -1, ok == true.
func (p *parser) parseRepeat(s string) (min, max int, rest string, ok bool) {
        if s == "" || s[0] != '{' {
                return
        }
        s = s[1:]
        var ok1 bool
        if min, s, ok1 = p.parseInt(s); !ok1 {
                return
        }
        if s == "" {
                return
        }
        if s[0] != ',' {
                max = min
        } else {
                s = s[1:]
                if s == "" {
                        return
                }
                if s[0] == '}' {
                        max = -1
                } else if max, s, ok1 = p.parseInt(s); !ok1 {
                        return
                } else if max < 0 {
                        // parseInt found too big a number
                        min = -1
                }
        }
        if s == "" || s[0] != '}' {
                return
        }
        rest = s[1:]
        ok = true
        return
}

// parsePerlFlags parses a Perl flag setting or non-capturing group or both,
// like (?i) or (?: or (?i:.  It removes the prefix from s and updates the parse state.
// The caller must have ensured that s begins with "(?".
func (p *parser) parsePerlFlags(s string) (rest string, err error) {
        t := s

        // Check for named captures, first introduced in Python's regexp library.
        // As usual, there are three slightly different syntaxes:
        //
        //   (?P<name>expr)   the original, introduced by Python
        //   (?<name>expr)    the .NET alteration, adopted by Perl 5.10
        //   (?'name'expr)    another .NET alteration, adopted by Perl 5.10
        //
        // Perl 5.10 gave in and implemented the Python version too,
        // but they claim that the last two are the preferred forms.
        // PCRE and languages based on it (specifically, PHP and Ruby)
        // support all three as well.  EcmaScript 4 uses only the Python form.
        //
        // In both the open source world (via Code Search) and the
        // Google source tree, (?P<expr>name) is the dominant form,
        // so that's the one we implement.  One is enough.
        if len(t) > 4 && t[2] == 'P' && t[3] == '<' {
                // Pull out name.
                end := strings.IndexRune(t, '>')
                if end < 0 {
                        if err = checkUTF8(t); err != nil {
                                return "", err
                        }
                        return "", &Error{ErrInvalidNamedCapture, s}
                }

                capture := t[:end+1] // "(?P<name>"
                name := t[4:end]     // "name"
                if err = checkUTF8(name); err != nil {
                        return "", err
                }
                if !isValidCaptureName(name) {
                        return "", &Error{ErrInvalidNamedCapture, capture}
                }

                // Like ordinary capture, but named.
                p.numCap++
                re := p.op(opLeftParen)
                re.Cap = p.numCap
                re.Name = name
                return t[end+1:], nil
        }

        // Non-capturing group.  Might also twiddle Perl flags.
        var c rune
        t = t[2:] // skip (?
        flags := p.flags
        sign := +1
        sawFlag := false
Loop:
        for t != "" {
                if c, t, err = nextRune(t); err != nil {
                        return "", err
                }
                switch c {
                default:
                        break Loop

                // Flags.
                case 'i':
                        flags |= FoldCase
                        sawFlag = true
                case 'm':
                        flags &^= OneLine
                        sawFlag = true
                case 's':
                        flags |= DotNL
                        sawFlag = true
                case 'U':
                        flags |= NonGreedy
                        sawFlag = true

                // Switch to negation.
                case '-':
                        if sign < 0 {
                                break Loop
                        }
                        sign = -1
                        // Invert flags so that | above turn into &^ and vice versa.
                        // We'll invert flags again before using it below.
                        flags = ^flags
                        sawFlag = false

                // End of flags, starting group or not.
                case ':', ')':
                        if sign < 0 {
                                if !sawFlag {
                                        break Loop
                                }
                                flags = ^flags
                        }
                        if c == ':' {
                                // Open new group
                                p.op(opLeftParen)
                        }
                        p.flags = flags
                        return t, nil
                }
        }

        return "", &Error{ErrInvalidPerlOp, s[:len(s)-len(t)]}
}

// isValidCaptureName reports whether name
// is a valid capture name: [A-Za-z0-9_]+.
// PCRE limits names to 32 bytes.
// Python rejects names starting with digits.
// We don't enforce either of those.
func isValidCaptureName(name string) bool {
        if name == "" {
                return false
        }
        for _, c := range name {
                if c != '_' && !isalnum(c) {
                        return false
                }
        }
        return true
}

// parseInt parses a decimal integer.
func (p *parser) parseInt(s string) (n int, rest string, ok bool) {
        if s == "" || s[0] < '0' || '9' < s[0] {
                return
        }
        // Disallow leading zeros.
        if len(s) >= 2 && s[0] == '0' && '0' <= s[1] && s[1] <= '9' {
                return
        }
        t := s
        for s != "" && '0' <= s[0] && s[0] <= '9' {
                s = s[1:]
        }
        rest = s
        ok = true
        // Have digits, compute value.
        t = t[:len(t)-len(s)]
        for i := 0; i < len(t); i++ {
                // Avoid overflow.
                if n >= 1e8 {
                        n = -1
                        break
                }
                n = n*10 + int(t[i]) - '0'
        }
        return
}

// can this be represented as a character class?
// single-rune literal string, char class, ., and .|\n.
func isCharClass(re *Regexp) bool {
        return re.Op == OpLiteral && len(re.Rune) == 1 ||
                re.Op == OpCharClass ||
                re.Op == OpAnyCharNotNL ||
                re.Op == OpAnyChar
}

// does re match r?
func matchRune(re *Regexp, r rune) bool {
        switch re.Op {
        case OpLiteral:
                return len(re.Rune) == 1 && re.Rune[0] == r
        case OpCharClass:
                for i := 0; i < len(re.Rune); i += 2 {
                        if re.Rune[i] <= r && r <= re.Rune[i+1] {
                                return true
                        }
                }
                return false
        case OpAnyCharNotNL:
                return r != '\n'
        case OpAnyChar:
                return true
        }
        return false
}

// parseVerticalBar handles a | in the input.
func (p *parser) parseVerticalBar() error {
        p.concat()

        // The concatenation we just parsed is on top of the stack.
        // If it sits above an opVerticalBar, swap it below
        // (things below an opVerticalBar become an alternation).
        // Otherwise, push a new vertical bar.
        if !p.swapVerticalBar() {
                p.op(opVerticalBar)
        }

        return nil
}

// mergeCharClass makes dst = dst|src.
// The caller must ensure that dst.Op >= src.Op,
// to reduce the amount of copying.
func mergeCharClass(dst, src *Regexp) {
        switch dst.Op {
        case OpAnyChar:
                // src doesn't add anything.
        case OpAnyCharNotNL:
                // src might add \n
                if matchRune(src, '\n') {
                        dst.Op = OpAnyChar
                }
        case OpCharClass:
                // src is simpler, so either literal or char class
                if src.Op == OpLiteral {
                        dst.Rune = appendLiteral(dst.Rune, src.Rune[0], src.Flags)
                } else {
                        dst.Rune = appendClass(dst.Rune, src.Rune)
                }
        case OpLiteral:
                // both literal
                if src.Rune[0] == dst.Rune[0] && src.Flags == dst.Flags {
                        break
                }
                dst.Op = OpCharClass
                dst.Rune = appendLiteral(dst.Rune[:0], dst.Rune[0], dst.Flags)
                dst.Rune = appendLiteral(dst.Rune, src.Rune[0], src.Flags)
        }
}

// If the top of the stack is an element followed by an opVerticalBar
// swapVerticalBar swaps the two and returns true.
// Otherwise it returns false.
func (p *parser) swapVerticalBar() bool {
        // If above and below vertical bar are literal or char class,
        // can merge into a single char class.
        n := len(p.stack)
        if n >= 3 && p.stack[n-2].Op == opVerticalBar && isCharClass(p.stack[n-1]) && isCharClass(p.stack[n-3]) {
                re1 := p.stack[n-1]
                re3 := p.stack[n-3]
                // Make re3 the more complex of the two.
                if re1.Op > re3.Op {
                        re1, re3 = re3, re1
                        p.stack[n-3] = re3
                }
                mergeCharClass(re3, re1)
                p.reuse(re1)
                p.stack = p.stack[:n-1]
                return true
        }

        if n >= 2 {
                re1 := p.stack[n-1]
                re2 := p.stack[n-2]
                if re2.Op == opVerticalBar {
                        if n >= 3 {
                                // Now out of reach.
                                // Clean opportunistically.
                                cleanAlt(p.stack[n-3])
                        }
                        p.stack[n-2] = re1
                        p.stack[n-1] = re2
                        return true
                }
        }
        return false
}

// parseRightParen handles a ) in the input.
func (p *parser) parseRightParen() error {
        p.concat()
        if p.swapVerticalBar() {
                // pop vertical bar
                p.stack = p.stack[:len(p.stack)-1]
        }
        p.alternate()

        n := len(p.stack)
        if n < 2 {
                return &Error{ErrInternalError, ""}
        }
        re1 := p.stack[n-1]
        re2 := p.stack[n-2]
        p.stack = p.stack[:n-2]
        if re2.Op != opLeftParen {
                return &Error{ErrMissingParen, p.wholeRegexp}
        }
        // Restore flags at time of paren.
        p.flags = re2.Flags
        if re2.Cap == 0 {
                // Just for grouping.
                p.push(re1)
        } else {
                re2.Op = OpCapture
                re2.Sub = re2.Sub0[:1]
                re2.Sub[0] = re1
                p.push(re2)
        }
        return nil
}

// parseEscape parses an escape sequence at the beginning of s
// and returns the rune.
func (p *parser) parseEscape(s string) (r rune, rest string, err error) {
        t := s[1:]
        if t == "" {
                return 0, "", &Error{ErrTrailingBackslash, ""}
        }
        c, t, err := nextRune(t)
        if err != nil {
                return 0, "", err
        }

Switch:
        switch c {
        default:
                if c < utf8.RuneSelf && !isalnum(c) {
                        // Escaped non-word characters are always themselves.
                        // PCRE is not quite so rigorous: it accepts things like
                        // \q, but we don't.  We once rejected \_, but too many
                        // programs and people insist on using it, so allow \_.
                        return c, t, nil
                }

        // Octal escapes.
        case '1', '2', '3', '4', '5', '6', '7':
                // Single non-zero digit is a backreference; not supported
                if t == "" || t[0] < '0' || t[0] > '7' {
                        break
                }
                fallthrough
        case '0':
                // Consume up to three octal digits; already have one.
                r = c - '0'
                for i := 1; i < 3; i++ {
                        if t == "" || t[0] < '0' || t[0] > '7' {
                                break
                        }
                        r = r*8 + rune(t[0]) - '0'
                        t = t[1:]
                }
                return r, t, nil

        // Hexadecimal escapes.
        case 'x':
                if t == "" {
                        break
                }
                if c, t, err = nextRune(t); err != nil {
                        return 0, "", err
                }
                if c == '{' {
                        // Any number of digits in braces.
                        // Perl accepts any text at all; it ignores all text
                        // after the first non-hex digit.  We require only hex digits,
                        // and at least one.
                        nhex := 0
                        r = 0
                        for {
                                if t == "" {
                                        break Switch
                                }
                                if c, t, err = nextRune(t); err != nil {
                                        return 0, "", err
                                }
                                if c == '}' {
                                        break
                                }
                                v := unhex(c)
                                if v < 0 {
                                        break Switch
                                }
                                r = r*16 + v
                                if r > unicode.MaxRune {
                                        break Switch
                                }
                                nhex++
                        }
                        if nhex == 0 {
                                break Switch
                        }
                        return r, t, nil
                }

                // Easy case: two hex digits.
                x := unhex(c)
                if c, t, err = nextRune(t); err != nil {
                        return 0, "", err
                }
                y := unhex(c)
                if x < 0 || y < 0 {
                        break
                }
                return x*16 + y, t, nil

        // C escapes.  There is no case 'b', to avoid misparsing
        // the Perl word-boundary \b as the C backspace \b
        // when in POSIX mode.  In Perl, /\b/ means word-boundary
        // but /[\b]/ means backspace.  We don't support that.
        // If you want a backspace, embed a literal backspace
        // character or use \x08.
        case 'a':
                return '\a', t, err
        case 'f':
                return '\f', t, err
        case 'n':
                return '\n', t, err
        case 'r':
                return '\r', t, err
        case 't':
                return '\t', t, err
        case 'v':
                return '\v', t, err
        }
        return 0, "", &Error{ErrInvalidEscape, s[:len(s)-len(t)]}
}

// parseClassChar parses a character class character at the beginning of s
// and returns it.
func (p *parser) parseClassChar(s, wholeClass string) (r rune, rest string, err error) {
        if s == "" {
                return 0, "", &Error{Code: ErrMissingBracket, Expr: wholeClass}
        }

        // Allow regular escape sequences even though
        // many need not be escaped in this context.
        if s[0] == '\\' {
                return p.parseEscape(s)
        }

        return nextRune(s)
}

type charGroup struct {
        sign  int
        class []rune
}

// parsePerlClassEscape parses a leading Perl character class escape like \d
// from the beginning of s.  If one is present, it appends the characters to r
// and returns the new slice r and the remainder of the string.
func (p *parser) parsePerlClassEscape(s string, r []rune) (out []rune, rest string) {
        if p.flags&PerlX == 0 || len(s) < 2 || s[0] != '\\' {
                return
        }
        g := perlGroup[s[0:2]]
        if g.sign == 0 {
                return
        }
        return p.appendGroup(r, g), s[2:]
}

// parseNamedClass parses a leading POSIX named character class like [:alnum:]
// from the beginning of s.  If one is present, it appends the characters to r
// and returns the new slice r and the remainder of the string.
func (p *parser) parseNamedClass(s string, r []rune) (out []rune, rest string, err error) {
        if len(s) < 2 || s[0] != '[' || s[1] != ':' {
                return
        }

        i := strings.Index(s[2:], ":]")
        if i < 0 {
                return
        }
        i += 2
        name, s := s[0:i+2], s[i+2:]
        g := posixGroup[name]
        if g.sign == 0 {
                return nil, "", &Error{ErrInvalidCharRange, name}
        }
        return p.appendGroup(r, g), s, nil
}

func (p *parser) appendGroup(r []rune, g charGroup) []rune {
        if p.flags&FoldCase == 0 {
                if g.sign < 0 {
                        r = appendNegatedClass(r, g.class)
                } else {
                        r = appendClass(r, g.class)
                }
        } else {
                tmp := p.tmpClass[:0]
                tmp = appendFoldedClass(tmp, g.class)
                p.tmpClass = tmp
                tmp = cleanClass(&p.tmpClass)
                if g.sign < 0 {
                        r = appendNegatedClass(r, tmp)
                } else {
                        r = appendClass(r, tmp)
                }
        }
        return r
}

var anyTable = &unicode.RangeTable{
        R16: []unicode.Range16{{Lo: 0, Hi: 1<<16 - 1, Stride: 1}},
        R32: []unicode.Range32{{Lo: 1 << 16, Hi: unicode.MaxRune, Stride: 1}},
}

// unicodeTable returns the unicode.RangeTable identified by name
// and the table of additional fold-equivalent code points.
func unicodeTable(name string) (*unicode.RangeTable, *unicode.RangeTable) {
        // Special case: "Any" means any.
        if name == "Any" {
                return anyTable, anyTable
        }
        if t := unicode.Categories[name]; t != nil {
                return t, unicode.FoldCategory[name]
        }
        if t := unicode.Scripts[name]; t != nil {
                return t, unicode.FoldScript[name]
        }
        return nil, nil
}

// parseUnicodeClass parses a leading Unicode character class like \p{Han}
// from the beginning of s.  If one is present, it appends the characters to r
// and returns the new slice r and the remainder of the string.
func (p *parser) parseUnicodeClass(s string, r []rune) (out []rune, rest string, err error) {
        if p.flags&UnicodeGroups == 0 || len(s) < 2 || s[0] != '\\' || s[1] != 'p' && s[1] != 'P' {
                return
        }

        // Committed to parse or return error.
        sign := +1
        if s[1] == 'P' {
                sign = -1
        }
        t := s[2:]
        c, t, err := nextRune(t)
        if err != nil {
                return
        }
        var seq, name string
        if c != '{' {
                // Single-letter name.
                seq = s[:len(s)-len(t)]
                name = seq[2:]
        } else {
                // Name is in braces.
                end := strings.IndexRune(s, '}')
                if end < 0 {
                        if err = checkUTF8(s); err != nil {
                                return
                        }
                        return nil, "", &Error{ErrInvalidCharRange, s}
                }
                seq, t = s[:end+1], s[end+1:]
                name = s[3:end]
                if err = checkUTF8(name); err != nil {
                        return
                }
        }

        // Group can have leading negation too.  \p{^Han} == \P{Han}, \P{^Han} == \p{Han}.
        if name != "" && name[0] == '^' {
                sign = -sign
                name = name[1:]
        }

        tab, fold := unicodeTable(name)
        if tab == nil {
                return nil, "", &Error{ErrInvalidCharRange, seq}
        }

        if p.flags&FoldCase == 0 || fold == nil {
                if sign > 0 {
                        r = appendTable(r, tab)
                } else {
                        r = appendNegatedTable(r, tab)
                }
        } else {
                // Merge and clean tab and fold in a temporary buffer.
                // This is necessary for the negative case and just tidy
                // for the positive case.
                tmp := p.tmpClass[:0]
                tmp = appendTable(tmp, tab)
                tmp = appendTable(tmp, fold)
                p.tmpClass = tmp
                tmp = cleanClass(&p.tmpClass)
                if sign > 0 {
                        r = appendClass(r, tmp)
                } else {
                        r = appendNegatedClass(r, tmp)
                }
        }
        return r, t, nil
}

// parseClass parses a character class at the beginning of s
// and pushes it onto the parse stack.
func (p *parser) parseClass(s string) (rest string, err error) {
        t := s[1:] // chop [
        re := p.newRegexp(OpCharClass)
        re.Flags = p.flags
        re.Rune = re.Rune0[:0]

        sign := +1
        if t != "" && t[0] == '^' {
                sign = -1
                t = t[1:]

                // If character class does not match \n, add it here,
                // so that negation later will do the right thing.
                if p.flags&ClassNL == 0 {
                        re.Rune = append(re.Rune, '\n', '\n')
                }
        }

        class := re.Rune
        first := true // ] and - are okay as first char in class
        for t == "" || t[0] != ']' || first {
                // POSIX: - is only okay unescaped as first or last in class.
                // Perl: - is okay anywhere.
                if t != "" && t[0] == '-' && p.flags&PerlX == 0 && !first && (len(t) == 1 || t[1] != ']') {
                        _, size := utf8.DecodeRuneInString(t[1:])
                        return "", &Error{Code: ErrInvalidCharRange, Expr: t[:1+size]}
                }
                first = false

                // Look for POSIX [:alnum:] etc.
                if len(t) > 2 && t[0] == '[' && t[1] == ':' {
                        nclass, nt, err := p.parseNamedClass(t, class)
                        if err != nil {
                                return "", err
                        }
                        if nclass != nil {
                                class, t = nclass, nt
                                continue
                        }
                }

                // Look for Unicode character group like \p{Han}.
                nclass, nt, err := p.parseUnicodeClass(t, class)
                if err != nil {
                        return "", err
                }
                if nclass != nil {
                        class, t = nclass, nt
                        continue
                }

                // Look for Perl character class symbols (extension).
                if nclass, nt := p.parsePerlClassEscape(t, class); nclass != nil {
                        class, t = nclass, nt
                        continue
                }

                // Single character or simple range.
                rng := t
                var lo, hi rune
                if lo, t, err = p.parseClassChar(t, s); err != nil {
                        return "", err
                }
                hi = lo
                // [a-] means (a|-) so check for final ].
                if len(t) >= 2 && t[0] == '-' && t[1] != ']' {
                        t = t[1:]
                        if hi, t, err = p.parseClassChar(t, s); err != nil {
                                return "", err
                        }
                        if hi < lo {
                                rng = rng[:len(rng)-len(t)]
                                return "", &Error{Code: ErrInvalidCharRange, Expr: rng}
                        }
                }
                if p.flags&FoldCase == 0 {
                        class = AppendRange(class, lo, hi)
                } else {
                        class = appendFoldedRange(class, lo, hi)
                }
        }
        t = t[1:] // chop ]

        // Use &re.Rune instead of &class to avoid allocation.
        re.Rune = class
        class = cleanClass(&re.Rune)
        if sign < 0 {
                class = negateClass(class)
        }
        re.Rune = class
        p.push(re)
        return t, nil
}

// cleanClass sorts the ranges (pairs of elements of r),
// merges them, and eliminates duplicates.
func cleanClass(rp *[]rune) []rune {

        // Sort by lo increasing, hi decreasing to break ties.
        sort.Sort(ranges{rp})

        r := *rp
        if len(r) < 2 {
                return r
        }

        // Merge abutting, overlapping.
        w := 2 // write index
        for i := 2; i < len(r); i += 2 {
                lo, hi := r[i], r[i+1]
                if lo <= r[w-1]+1 {
                        // merge with previous range
                        if hi > r[w-1] {
                                r[w-1] = hi
                        }
                        continue
                }
                // new disjoint range
                r[w] = lo
                r[w+1] = hi
                w += 2
        }

        return r[:w]
}

// appendLiteral returns the result of appending the literal x to the class r.
func appendLiteral(r []rune, x rune, flags Flags) []rune {
        if flags&FoldCase != 0 {
                return appendFoldedRange(r, x, x)
        }
        return AppendRange(r, x, x)
}

// appendRange returns the result of appending the range lo-hi to the class r.
func AppendRange(r []rune, lo, hi rune) []rune {
        // Expand last range or next to last range if it overlaps or abuts.
        // Checking two ranges helps when appending case-folded
        // alphabets, so that one range can be expanding A-Z and the
        // other expanding a-z.
        n := len(r)
        for i := 2; i <= 4; i += 2 { // twice, using i=2, i=4
                if n >= i {
                        rlo, rhi := r[n-i], r[n-i+1]
                        if lo <= rhi+1 && rlo <= hi+1 {
                                if lo < rlo {
                                        r[n-i] = lo
                                }
                                if hi > rhi {
                                        r[n-i+1] = hi
                                }
                                return r
                        }
                }
        }

        return append(r, lo, hi)
}

const (
        // minimum and maximum runes involved in folding.
        // checked during test.
        MinFold = 0x0041
        MaxFold = 0x1044f
)

// appendFoldedRange returns the result of appending the range lo-hi
// and its case folding-equivalent runes to the class r.
func appendFoldedRange(r []rune, lo, hi rune) []rune {
        // Optimizations.
        if lo <= MinFold && hi >= MaxFold {
                // Range is full: folding can't add more.
                return AppendRange(r, lo, hi)
        }
        if hi < MinFold || lo > MaxFold {
                // Range is outside folding possibilities.
                return AppendRange(r, lo, hi)
        }
        if lo < MinFold {
                // [lo, MinFold-1] needs no folding.
                r = AppendRange(r, lo, MinFold-1)
                lo = MinFold
        }
        if hi > MaxFold {
                // [MaxFold+1, hi] needs no folding.
                r = AppendRange(r, MaxFold+1, hi)
                hi = MaxFold
        }

        // Brute force.  Depend on AppendRange to coalesce ranges on the fly.
        for c := lo; c <= hi; c++ {
                r = AppendRange(r, c, c)
                f := unicode.SimpleFold(c)
                for f != c {
                        r = AppendRange(r, f, f)
                        f = unicode.SimpleFold(f)
                }
        }
        return r
}

// appendClass returns the result of appending the class x to the class r.
// It assume x is clean.
func appendClass(r []rune, x []rune) []rune {
        for i := 0; i < len(x); i += 2 {
                r = AppendRange(r, x[i], x[i+1])
        }
        return r
}

// appendFolded returns the result of appending the case folding of the class x to the class r.
func appendFoldedClass(r []rune, x []rune) []rune {
        for i := 0; i < len(x); i += 2 {
                r = appendFoldedRange(r, x[i], x[i+1])
        }
        return r
}

// appendNegatedClass returns the result of appending the negation of the class x to the class r.
// It assumes x is clean.
func appendNegatedClass(r []rune, x []rune) []rune {
        nextLo := '\u0000'
        for i := 0; i < len(x); i += 2 {
                lo, hi := x[i], x[i+1]
                if nextLo <= lo-1 {
                        r = AppendRange(r, nextLo, lo-1)
                }
                nextLo = hi + 1
        }
        if nextLo <= unicode.MaxRune {
                r = AppendRange(r, nextLo, unicode.MaxRune)
        }
        return r
}

// appendTable returns the result of appending x to the class r.
func appendTable(r []rune, x *unicode.RangeTable) []rune {
        for _, xr := range x.R16 {
                lo, hi, stride := rune(xr.Lo), rune(xr.Hi), rune(xr.Stride)
                if stride == 1 {
                        r = AppendRange(r, lo, hi)
                        continue
                }
                for c := lo; c <= hi; c += stride {
                        r = AppendRange(r, c, c)
                }
        }
        for _, xr := range x.R32 {
                lo, hi, stride := rune(xr.Lo), rune(xr.Hi), rune(xr.Stride)
                if stride == 1 {
                        r = AppendRange(r, lo, hi)
                        continue
                }
                for c := lo; c <= hi; c += stride {
                        r = AppendRange(r, c, c)
                }
        }
        return r
}

// appendNegatedTable returns the result of appending the negation of x to the class r.
func appendNegatedTable(r []rune, x *unicode.RangeTable) []rune {
        nextLo := '\u0000' // lo end of next class to add
        for _, xr := range x.R16 {
                lo, hi, stride := rune(xr.Lo), rune(xr.Hi), rune(xr.Stride)
                if stride == 1 {
                        if nextLo <= lo-1 {
                                r = AppendRange(r, nextLo, lo-1)
                        }
                        nextLo = hi + 1
                        continue
                }
                for c := lo; c <= hi; c += stride {
                        if nextLo <= c-1 {
                                r = AppendRange(r, nextLo, c-1)
                        }
                        nextLo = c + 1
                }
        }
        for _, xr := range x.R32 {
                lo, hi, stride := rune(xr.Lo), rune(xr.Hi), rune(xr.Stride)
                if stride == 1 {
                        if nextLo <= lo-1 {
                                r = AppendRange(r, nextLo, lo-1)
                        }
                        nextLo = hi + 1
                        continue
                }
                for c := lo; c <= hi; c += stride {
                        if nextLo <= c-1 {
                                r = AppendRange(r, nextLo, c-1)
                        }
                        nextLo = c + 1
                }
        }
        if nextLo <= unicode.MaxRune {
                r = AppendRange(r, nextLo, unicode.MaxRune)
        }
        return r
}

// negateClass overwrites r and returns r's negation.
// It assumes the class r is already clean.
func negateClass(r []rune) []rune {
        nextLo := '\u0000' // lo end of next class to add
        w := 0             // write index
        for i := 0; i < len(r); i += 2 {
                lo, hi := r[i], r[i+1]
                if nextLo <= lo-1 {
                        r[w] = nextLo
                        r[w+1] = lo - 1
                        w += 2
                }
                nextLo = hi + 1
        }
        r = r[:w]
        if nextLo <= unicode.MaxRune {
                // It's possible for the negation to have one more
                // range - this one - than the original class, so use append.
                r = append(r, nextLo, unicode.MaxRune)
        }
        return r
}

// ranges implements sort.Interface on a []rune.
// The choice of receiver type definition is strange
// but avoids an allocation since we already have
// a *[]rune.
type ranges struct {
        p *[]rune
}

func (ra ranges) Less(i, j int) bool {
        p := *ra.p
        i *= 2
        j *= 2
        return p[i] < p[j] || p[i] == p[j] && p[i+1] > p[j+1]
}

func (ra ranges) Len() int {
        return len(*ra.p) / 2
}

func (ra ranges) Swap(i, j int) {
        p := *ra.p
        i *= 2
        j *= 2
        p[i], p[i+1], p[j], p[j+1] = p[j], p[j+1], p[i], p[i+1]
}

func checkUTF8(s string) error {
        for s != "" {
                rune, size := utf8.DecodeRuneInString(s)
                if rune == utf8.RuneError && size == 1 {
                        return &Error{Code: ErrInvalidUTF8, Expr: s}
                }
                s = s[size:]
        }
        return nil
}

func nextRune(s string) (c rune, t string, err error) {
        c, size := utf8.DecodeRuneInString(s)
        if c == utf8.RuneError && size == 1 {
                return 0, "", &Error{Code: ErrInvalidUTF8, Expr: s}
        }
        return c, s[size:], nil
}

func isalnum(c rune) bool {
        return '0' <= c && c <= '9' || 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z'
}

func unhex(c rune) rune {
        if '0' <= c && c <= '9' {
                return c - '0'
        }
        if 'a' <= c && c <= 'f' {
                return c - 'a' + 10
        }
        if 'A' <= c && c <= 'F' {
                return c - 'A' + 10
        }
        return -1
}