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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 (
    	"os"
    	"sort"
    	"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) String() 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
    	numCap      int       // number of capturing groups seen
    	wholeRegexp string
    }
    
    // Parse stack manipulation.
    
    // push pushes the regexp re onto the parse stack and returns the regexp.
    func (p *parser) push(re *Regexp) *Regexp {
    	// TODO: automatic concatenation
    	// TODO: turn character class into literal
    	// TODO: compute simple
    
    	p.stack = append(p.stack, re)
    	return re
    }
    
    // newLiteral returns a new OpLiteral Regexp with the given flags
    func newLiteral(r int, flags Flags) *Regexp {
    	re := &Regexp{
    		Op:    OpLiteral,
    		Flags: flags,
    	}
    	re.Rune0[0] = r
    	re.Rune = re.Rune0[:1]
    	return re
    }
    
    // literal pushes a literal regexp for the rune r on the stack
    // and returns that regexp.
    func (p *parser) literal(r int) *Regexp {
    	return p.push(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 {
    	return p.push(&Regexp{Op: op, Flags: p.flags})
    }
    
    // repeat replaces the top stack element with itself repeated
    // according to op.
    func (p *parser) repeat(op Op, opstr string) os.Error {
    	n := len(p.stack)
    	if n == 0 {
    		return &Error{ErrMissingRepeatArgument, opstr}
    	}
    	sub := p.stack[n-1]
    	re := &Regexp{
    		Op: op,
    	}
    	re.Sub = re.Sub0[:1]
    	re.Sub[0] = sub
    	p.stack[n-1] = re
    	return nil
    }
    
    // concat replaces the top of the stack (above the topmost '|' or '(') with its concatenation.
    func (p *parser) concat() *Regexp {
    	// TODO: Flatten concats.
    
    	// Scan down to find pseudo-operator | or (.
    	i := len(p.stack)
    	for i > 0 && p.stack[i-1].Op < opPseudo {
    		i--
    	}
    	sub := p.stack[i:]
    	p.stack = p.stack[:i]
    
    	var re *Regexp
    	switch len(sub) {
    	case 0:
    		re = &Regexp{Op: OpEmptyMatch}
    	case 1:
    		re = sub[0]
    	default:
    		re = &Regexp{Op: OpConcat}
    		re.Sub = append(re.Sub0[:0], sub...)
    	}
    	return p.push(re)
    }
    
    // alternate replaces the top of the stack (above the topmost '(') with its alternation.
    func (p *parser) alternate() *Regexp {
    	// TODO: Flatten alternates.
    
    	// Scan down to find pseudo-operator (.
    	// There are no | above (.
    	i := len(p.stack)
    	for i > 0 && p.stack[i-1].Op < opPseudo {
    		i--
    	}
    	sub := p.stack[i:]
    	p.stack = p.stack[:i]
    
    	var re *Regexp
    	switch len(sub) {
    	case 0:
    		re = &Regexp{Op: OpNoMatch}
    	case 1:
    		re = sub[0]
    	default:
    		re = &Regexp{Op: OpAlternate}
    		re.Sub = append(re.Sub0[:0], sub...)
    	}
    	return p.push(re)
    }
    
    // Parsing.
    
    func Parse(s string, flags Flags) (*Regexp, os.Error) {
    	if flags&Literal != 0 {
    		// Trivial parser for literal string.
    		if err := checkUTF8(s); err != nil {
    			return nil, err
    		}
    		re := &Regexp{
    			Op:    OpLiteral,
    			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 = []int(s)
    				break
    			}
    			re.Rune = append(re.Rune, c)
    		}
    		return re, nil
    	}
    
    	// Otherwise, must do real work.
    	var (
    		p   parser
    		err os.Error
    		c   int
    		op  Op
    	)
    	p.flags = flags
    	p.wholeRegexp = s
    	t := s
    	for t != "" {
    		switch t[0] {
    		default:
    			if c, t, err = nextRune(t); err != nil {
    				return nil, err
    			}
    			p.literal(c)
    
    		case '(':
    			// TODO: Actual Perl flag parsing.
    			if len(t) >= 3 && t[1] == '?' && t[2] == ':' {
    				// non-capturing paren
    				p.op(opLeftParen)
    				t = t[3:]
    				break
    			}
    			p.numCap++
    			p.op(opLeftParen).Cap = p.numCap
    			t = t[1:]
    		case '|':
    			p.concat()
    			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 '*', '+', '?':
    			switch t[0] {
    			case '*':
    				op = OpStar
    			case '+':
    				op = OpPlus
    			case '?':
    				op = OpQuest
    			}
    			// TODO: greedy
    			if err = p.repeat(op, t[0:1]); err != nil {
    				return nil, err
    			}
    			t = t[1:]
    		case '{':
    			return nil, os.NewError("repeat not implemented")
    		case '\\':
    			return nil, os.NewError("escape not implemented")
    		}
    	}
    
    	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
    }
    
    // parseVerticalBar handles a | in the input.
    func (p *parser) parseVerticalBar() os.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
    }
    
    // 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 n := len(p.stack); n >= 2 {
    		re1 := p.stack[n-1]
    		re2 := p.stack[n-2]
    		if re2.Op == opVerticalBar {
    			p.stack[n-2] = re1
    			p.stack[n-1] = re2
    			return true
    		}
    	}
    	return false
    }
    
    // parseRightParen handles a ) in the input.
    func (p *parser) parseRightParen() os.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}
    	}
    	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
    }
    
    // parseClassChar parses a character class character at the beginning of s
    // and returns it.
    func (p *parser) parseClassChar(s, wholeClass string) (r int, rest string, err os.Error) {
    	if s == "" {
    		return 0, "", &Error{Code: ErrMissingBracket, Expr: wholeClass}
    	}
    
    	// TODO: Escapes
    
    	return nextRune(s)
    }
    
    // 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 os.Error) {
    	t := s[1:] // chop [
    	re := &Regexp{Op: OpCharClass, 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
    
    		// TODO: Look for [:alnum:]
    		// TODO: Look for Unicode group.
    		// TODO: Look for Perl group.
    
    		// Single character or simple range.
    		rng := t
    		var lo, hi int
    		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}
    			}
    		}
    
    		// Expand last range if overlaps or abuts.
    		if n := len(class); n > 0 {
    			clo, chi := class[n-2], class[n-1]
    			if lo <= chi+1 && clo <= hi+1 {
    				if lo < clo {
    					class[n-2] = lo
    				}
    				if hi > chi {
    					class[n-1] = hi
    				}
    				continue
    			}
    		}
    
    		class = append(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 *[]int) []int {
    	// Sort by lo increasing, hi decreasing to break ties.
    	sort.Sort(ranges{rp})
    
    	r := *rp
    	// 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]
    }
    
    // negateClass overwrites r and returns r's negation.
    // It assumes the class r is already clean.
    func negateClass(r []int) []int {
    	nextLo := 0 // 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
    	}
    	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[:w], 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 *[]int.
    type ranges struct {
    	p *[]int
    }
    
    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) os.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 int, t string, err os.Error) {
    	c, size := utf8.DecodeRuneInString(s)
    	if c == utf8.RuneError && size == 1 {
    		return 0, "", &Error{Code: ErrInvalidUTF8, Expr: s}
    	}
    	return c, s[size:], nil
    }