func (p *Parser) backup() {
	p.pos -= p.width
}
backup steps back one rune. Can only be called once per call of next.
package jsonpath
import (
	"errors"
	"fmt"
	"regexp"
	"strconv"
	"strings"
	"unicode"
	"unicode/utf8"
)
const eof = -1
const (
	leftDelim  = "{"
	rightDelim = "}"
)
type Parser struct {
	Name  string
	Root  *ListNode
	input string
	cur   *ListNode
	pos   int
	start int
	width int
}
var (
	ErrSyntax        = errors.New("invalid syntax")
	dictKeyRex       = regexp.MustCompile(`^'([^']*)'$`)
	sliceOperatorRex = regexp.MustCompile(`^(-?[\d]*)(:-?[\d]*)?(:[\d]*)?$`)
)
func Parse(name, text string) (*Parser, error) {
	p := NewParser(name)
	err := p.Parse(text)
	if err != nil {
		p = nil
	}
	return p, err
}
func NewParser(name string) *Parser {
	return &Parser{
		Name: name,
	}
}
func parseAction(name, text string) (*Parser, error) {
	p, err := Parse(name, fmt.Sprintf("%s%s%s", leftDelim, text, rightDelim))
	if err != nil {
		return p, err
	}
	p.Root = p.Root.Nodes[0].(*ListNode)
	return p, nil
}
func (p *Parser) Parse(text string) error {
	p.input = text
	p.Root = newList()
	p.pos = 0
	return p.parseText(p.Root)
}
func (p *Parser) consumeText() string {
	value := p.input[p.start:p.pos]
	p.start = p.pos
	return value
}
func (p *Parser) next() rune {
	if int(p.pos) >= len(p.input) {
		p.width = 0
		return eof
	}
	r, w := utf8.DecodeRuneInString(p.input[p.pos:])
	p.width = w
	p.pos += p.width
	return r
}
func (p *Parser) peek() rune {
	r := p.next()
	p.backup()
	return r
}