diff --git a/metrics-server/vendor/k8s.io/apimachinery/pkg/labels/selector.go b/metrics-server/vendor/k8s.io/apimachinery/pkg/labels/selector.go
new file mode 100644
index 0000000..b301b42
--- /dev/null
+++ b/metrics-server/vendor/k8s.io/apimachinery/pkg/labels/selector.go
@@ -0,0 +1,879 @@
+/*
+Copyright 2014 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package labels
+
+import (
+	"bytes"
+	"fmt"
+	"sort"
+	"strconv"
+	"strings"
+
+	"github.com/golang/glog"
+	"k8s.io/apimachinery/pkg/selection"
+	"k8s.io/apimachinery/pkg/util/sets"
+	"k8s.io/apimachinery/pkg/util/validation"
+)
+
+// Requirements is AND of all requirements.
+type Requirements []Requirement
+
+// Selector represents a label selector.
+type Selector interface {
+	// Matches returns true if this selector matches the given set of labels.
+	Matches(Labels) bool
+
+	// Empty returns true if this selector does not restrict the selection space.
+	Empty() bool
+
+	// String returns a human readable string that represents this selector.
+	String() string
+
+	// Add adds requirements to the Selector
+	Add(r ...Requirement) Selector
+
+	// Requirements converts this interface into Requirements to expose
+	// more detailed selection information.
+	// If there are querying parameters, it will return converted requirements and selectable=true.
+	// If this selector doesn't want to select anything, it will return selectable=false.
+	Requirements() (requirements Requirements, selectable bool)
+
+	// Make a deep copy of the selector.
+	DeepCopySelector() Selector
+}
+
+// Everything returns a selector that matches all labels.
+func Everything() Selector {
+	return internalSelector{}
+}
+
+type nothingSelector struct{}
+
+func (n nothingSelector) Matches(_ Labels) bool              { return false }
+func (n nothingSelector) Empty() bool                        { return false }
+func (n nothingSelector) String() string                     { return "" }
+func (n nothingSelector) Add(_ ...Requirement) Selector      { return n }
+func (n nothingSelector) Requirements() (Requirements, bool) { return nil, false }
+func (n nothingSelector) DeepCopySelector() Selector         { return n }
+
+// Nothing returns a selector that matches no labels
+func Nothing() Selector {
+	return nothingSelector{}
+}
+
+// NewSelector returns a nil selector
+func NewSelector() Selector {
+	return internalSelector(nil)
+}
+
+type internalSelector []Requirement
+
+func (s internalSelector) DeepCopy() internalSelector {
+	if s == nil {
+		return nil
+	}
+	result := make([]Requirement, len(s))
+	for i := range s {
+		s[i].DeepCopyInto(&result[i])
+	}
+	return result
+}
+
+func (s internalSelector) DeepCopySelector() Selector {
+	return s.DeepCopy()
+}
+
+// ByKey sorts requirements by key to obtain deterministic parser
+type ByKey []Requirement
+
+func (a ByKey) Len() int { return len(a) }
+
+func (a ByKey) Swap(i, j int) { a[i], a[j] = a[j], a[i] }
+
+func (a ByKey) Less(i, j int) bool { return a[i].key < a[j].key }
+
+// Requirement contains values, a key, and an operator that relates the key and values.
+// The zero value of Requirement is invalid.
+// Requirement implements both set based match and exact match
+// Requirement should be initialized via NewRequirement constructor for creating a valid Requirement.
+// +k8s:deepcopy-gen=true
+type Requirement struct {
+	key      string
+	operator selection.Operator
+	// In huge majority of cases we have at most one value here.
+	// It is generally faster to operate on a single-element slice
+	// than on a single-element map, so we have a slice here.
+	strValues []string
+}
+
+// NewRequirement is the constructor for a Requirement.
+// If any of these rules is violated, an error is returned:
+// (1) The operator can only be In, NotIn, Equals, DoubleEquals, NotEquals, Exists, or DoesNotExist.
+// (2) If the operator is In or NotIn, the values set must be non-empty.
+// (3) If the operator is Equals, DoubleEquals, or NotEquals, the values set must contain one value.
+// (4) If the operator is Exists or DoesNotExist, the value set must be empty.
+// (5) If the operator is Gt or Lt, the values set must contain only one value, which will be interpreted as an integer.
+// (6) The key is invalid due to its length, or sequence
+//     of characters. See validateLabelKey for more details.
+//
+// The empty string is a valid value in the input values set.
+func NewRequirement(key string, op selection.Operator, vals []string) (*Requirement, error) {
+	if err := validateLabelKey(key); err != nil {
+		return nil, err
+	}
+	switch op {
+	case selection.In, selection.NotIn:
+		if len(vals) == 0 {
+			return nil, fmt.Errorf("for 'in', 'notin' operators, values set can't be empty")
+		}
+	case selection.Equals, selection.DoubleEquals, selection.NotEquals:
+		if len(vals) != 1 {
+			return nil, fmt.Errorf("exact-match compatibility requires one single value")
+		}
+	case selection.Exists, selection.DoesNotExist:
+		if len(vals) != 0 {
+			return nil, fmt.Errorf("values set must be empty for exists and does not exist")
+		}
+	case selection.GreaterThan, selection.LessThan:
+		if len(vals) != 1 {
+			return nil, fmt.Errorf("for 'Gt', 'Lt' operators, exactly one value is required")
+		}
+		for i := range vals {
+			if _, err := strconv.ParseInt(vals[i], 10, 64); err != nil {
+				return nil, fmt.Errorf("for 'Gt', 'Lt' operators, the value must be an integer")
+			}
+		}
+	default:
+		return nil, fmt.Errorf("operator '%v' is not recognized", op)
+	}
+
+	for i := range vals {
+		if err := validateLabelValue(vals[i]); err != nil {
+			return nil, err
+		}
+	}
+	sort.Strings(vals)
+	return &Requirement{key: key, operator: op, strValues: vals}, nil
+}
+
+func (r *Requirement) hasValue(value string) bool {
+	for i := range r.strValues {
+		if r.strValues[i] == value {
+			return true
+		}
+	}
+	return false
+}
+
+// Matches returns true if the Requirement matches the input Labels.
+// There is a match in the following cases:
+// (1) The operator is Exists and Labels has the Requirement's key.
+// (2) The operator is In, Labels has the Requirement's key and Labels'
+//     value for that key is in Requirement's value set.
+// (3) The operator is NotIn, Labels has the Requirement's key and
+//     Labels' value for that key is not in Requirement's value set.
+// (4) The operator is DoesNotExist or NotIn and Labels does not have the
+//     Requirement's key.
+// (5) The operator is GreaterThanOperator or LessThanOperator, and Labels has
+//     the Requirement's key and the corresponding value satisfies mathematical inequality.
+func (r *Requirement) Matches(ls Labels) bool {
+	switch r.operator {
+	case selection.In, selection.Equals, selection.DoubleEquals:
+		if !ls.Has(r.key) {
+			return false
+		}
+		return r.hasValue(ls.Get(r.key))
+	case selection.NotIn, selection.NotEquals:
+		if !ls.Has(r.key) {
+			return true
+		}
+		return !r.hasValue(ls.Get(r.key))
+	case selection.Exists:
+		return ls.Has(r.key)
+	case selection.DoesNotExist:
+		return !ls.Has(r.key)
+	case selection.GreaterThan, selection.LessThan:
+		if !ls.Has(r.key) {
+			return false
+		}
+		lsValue, err := strconv.ParseInt(ls.Get(r.key), 10, 64)
+		if err != nil {
+			glog.V(10).Infof("ParseInt failed for value %+v in label %+v, %+v", ls.Get(r.key), ls, err)
+			return false
+		}
+
+		// There should be only one strValue in r.strValues, and can be converted to a integer.
+		if len(r.strValues) != 1 {
+			glog.V(10).Infof("Invalid values count %+v of requirement %#v, for 'Gt', 'Lt' operators, exactly one value is required", len(r.strValues), r)
+			return false
+		}
+
+		var rValue int64
+		for i := range r.strValues {
+			rValue, err = strconv.ParseInt(r.strValues[i], 10, 64)
+			if err != nil {
+				glog.V(10).Infof("ParseInt failed for value %+v in requirement %#v, for 'Gt', 'Lt' operators, the value must be an integer", r.strValues[i], r)
+				return false
+			}
+		}
+		return (r.operator == selection.GreaterThan && lsValue > rValue) || (r.operator == selection.LessThan && lsValue < rValue)
+	default:
+		return false
+	}
+}
+
+// Key returns requirement key
+func (r *Requirement) Key() string {
+	return r.key
+}
+
+// Operator returns requirement operator
+func (r *Requirement) Operator() selection.Operator {
+	return r.operator
+}
+
+// Values returns requirement values
+func (r *Requirement) Values() sets.String {
+	ret := sets.String{}
+	for i := range r.strValues {
+		ret.Insert(r.strValues[i])
+	}
+	return ret
+}
+
+// Empty returns true if the internalSelector doesn't restrict selection space
+func (lsel internalSelector) Empty() bool {
+	if lsel == nil {
+		return true
+	}
+	return len(lsel) == 0
+}
+
+// String returns a human-readable string that represents this
+// Requirement. If called on an invalid Requirement, an error is
+// returned. See NewRequirement for creating a valid Requirement.
+func (r *Requirement) String() string {
+	var buffer bytes.Buffer
+	if r.operator == selection.DoesNotExist {
+		buffer.WriteString("!")
+	}
+	buffer.WriteString(r.key)
+
+	switch r.operator {
+	case selection.Equals:
+		buffer.WriteString("=")
+	case selection.DoubleEquals:
+		buffer.WriteString("==")
+	case selection.NotEquals:
+		buffer.WriteString("!=")
+	case selection.In:
+		buffer.WriteString(" in ")
+	case selection.NotIn:
+		buffer.WriteString(" notin ")
+	case selection.GreaterThan:
+		buffer.WriteString(">")
+	case selection.LessThan:
+		buffer.WriteString("<")
+	case selection.Exists, selection.DoesNotExist:
+		return buffer.String()
+	}
+
+	switch r.operator {
+	case selection.In, selection.NotIn:
+		buffer.WriteString("(")
+	}
+	if len(r.strValues) == 1 {
+		buffer.WriteString(r.strValues[0])
+	} else { // only > 1 since == 0 prohibited by NewRequirement
+		buffer.WriteString(strings.Join(r.strValues, ","))
+	}
+
+	switch r.operator {
+	case selection.In, selection.NotIn:
+		buffer.WriteString(")")
+	}
+	return buffer.String()
+}
+
+// Add adds requirements to the selector. It copies the current selector returning a new one
+func (lsel internalSelector) Add(reqs ...Requirement) Selector {
+	var sel internalSelector
+	for ix := range lsel {
+		sel = append(sel, lsel[ix])
+	}
+	for _, r := range reqs {
+		sel = append(sel, r)
+	}
+	sort.Sort(ByKey(sel))
+	return sel
+}
+
+// Matches for a internalSelector returns true if all
+// its Requirements match the input Labels. If any
+// Requirement does not match, false is returned.
+func (lsel internalSelector) Matches(l Labels) bool {
+	for ix := range lsel {
+		if matches := lsel[ix].Matches(l); !matches {
+			return false
+		}
+	}
+	return true
+}
+
+func (lsel internalSelector) Requirements() (Requirements, bool) { return Requirements(lsel), true }
+
+// String returns a comma-separated string of all
+// the internalSelector Requirements' human-readable strings.
+func (lsel internalSelector) String() string {
+	var reqs []string
+	for ix := range lsel {
+		reqs = append(reqs, lsel[ix].String())
+	}
+	return strings.Join(reqs, ",")
+}
+
+// Token represents constant definition for lexer token
+type Token int
+
+const (
+	// ErrorToken represents scan error
+	ErrorToken Token = iota
+	// EndOfStringToken represents end of string
+	EndOfStringToken
+	// ClosedParToken represents close parenthesis
+	ClosedParToken
+	// CommaToken represents the comma
+	CommaToken
+	// DoesNotExistToken represents logic not
+	DoesNotExistToken
+	// DoubleEqualsToken represents double equals
+	DoubleEqualsToken
+	// EqualsToken represents equal
+	EqualsToken
+	// GreaterThanToken represents greater than
+	GreaterThanToken
+	// IdentifierToken represents identifier, e.g. keys and values
+	IdentifierToken
+	// InToken represents in
+	InToken
+	// LessThanToken represents less than
+	LessThanToken
+	// NotEqualsToken represents not equal
+	NotEqualsToken
+	// NotInToken represents not in
+	NotInToken
+	// OpenParToken represents open parenthesis
+	OpenParToken
+)
+
+// string2token contains the mapping between lexer Token and token literal
+// (except IdentifierToken, EndOfStringToken and ErrorToken since it makes no sense)
+var string2token = map[string]Token{
+	")":     ClosedParToken,
+	",":     CommaToken,
+	"!":     DoesNotExistToken,
+	"==":    DoubleEqualsToken,
+	"=":     EqualsToken,
+	">":     GreaterThanToken,
+	"in":    InToken,
+	"<":     LessThanToken,
+	"!=":    NotEqualsToken,
+	"notin": NotInToken,
+	"(":     OpenParToken,
+}
+
+// ScannedItem contains the Token and the literal produced by the lexer.
+type ScannedItem struct {
+	tok     Token
+	literal string
+}
+
+// isWhitespace returns true if the rune is a space, tab, or newline.
+func isWhitespace(ch byte) bool {
+	return ch == ' ' || ch == '\t' || ch == '\r' || ch == '\n'
+}
+
+// isSpecialSymbol detect if the character ch can be an operator
+func isSpecialSymbol(ch byte) bool {
+	switch ch {
+	case '=', '!', '(', ')', ',', '>', '<':
+		return true
+	}
+	return false
+}
+
+// Lexer represents the Lexer struct for label selector.
+// It contains necessary informationt to tokenize the input string
+type Lexer struct {
+	// s stores the string to be tokenized
+	s string
+	// pos is the position currently tokenized
+	pos int
+}
+
+// read return the character currently lexed
+// increment the position and check the buffer overflow
+func (l *Lexer) read() (b byte) {
+	b = 0
+	if l.pos < len(l.s) {
+		b = l.s[l.pos]
+		l.pos++
+	}
+	return b
+}
+
+// unread 'undoes' the last read character
+func (l *Lexer) unread() {
+	l.pos--
+}
+
+// scanIDOrKeyword scans string to recognize literal token (for example 'in') or an identifier.
+func (l *Lexer) scanIDOrKeyword() (tok Token, lit string) {
+	var buffer []byte
+IdentifierLoop:
+	for {
+		switch ch := l.read(); {
+		case ch == 0:
+			break IdentifierLoop
+		case isSpecialSymbol(ch) || isWhitespace(ch):
+			l.unread()
+			break IdentifierLoop
+		default:
+			buffer = append(buffer, ch)
+		}
+	}
+	s := string(buffer)
+	if val, ok := string2token[s]; ok { // is a literal token?
+		return val, s
+	}
+	return IdentifierToken, s // otherwise is an identifier
+}
+
+// scanSpecialSymbol scans string starting with special symbol.
+// special symbol identify non literal operators. "!=", "==", "="
+func (l *Lexer) scanSpecialSymbol() (Token, string) {
+	lastScannedItem := ScannedItem{}
+	var buffer []byte
+SpecialSymbolLoop:
+	for {
+		switch ch := l.read(); {
+		case ch == 0:
+			break SpecialSymbolLoop
+		case isSpecialSymbol(ch):
+			buffer = append(buffer, ch)
+			if token, ok := string2token[string(buffer)]; ok {
+				lastScannedItem = ScannedItem{tok: token, literal: string(buffer)}
+			} else if lastScannedItem.tok != 0 {
+				l.unread()
+				break SpecialSymbolLoop
+			}
+		default:
+			l.unread()
+			break SpecialSymbolLoop
+		}
+	}
+	if lastScannedItem.tok == 0 {
+		return ErrorToken, fmt.Sprintf("error expected: keyword found '%s'", buffer)
+	}
+	return lastScannedItem.tok, lastScannedItem.literal
+}
+
+// skipWhiteSpaces consumes all blank characters
+// returning the first non blank character
+func (l *Lexer) skipWhiteSpaces(ch byte) byte {
+	for {
+		if !isWhitespace(ch) {
+			return ch
+		}
+		ch = l.read()
+	}
+}
+
+// Lex returns a pair of Token and the literal
+// literal is meaningfull only for IdentifierToken token
+func (l *Lexer) Lex() (tok Token, lit string) {
+	switch ch := l.skipWhiteSpaces(l.read()); {
+	case ch == 0:
+		return EndOfStringToken, ""
+	case isSpecialSymbol(ch):
+		l.unread()
+		return l.scanSpecialSymbol()
+	default:
+		l.unread()
+		return l.scanIDOrKeyword()
+	}
+}
+
+// Parser data structure contains the label selector parser data structure
+type Parser struct {
+	l            *Lexer
+	scannedItems []ScannedItem
+	position     int
+}
+
+// ParserContext represents context during parsing:
+// some literal for example 'in' and 'notin' can be
+// recognized as operator for example 'x in (a)' but
+// it can be recognized as value for example 'value in (in)'
+type ParserContext int
+
+const (
+	// KeyAndOperator represents key and operator
+	KeyAndOperator ParserContext = iota
+	// Values represents values
+	Values
+)
+
+// lookahead func returns the current token and string. No increment of current position
+func (p *Parser) lookahead(context ParserContext) (Token, string) {
+	tok, lit := p.scannedItems[p.position].tok, p.scannedItems[p.position].literal
+	if context == Values {
+		switch tok {
+		case InToken, NotInToken:
+			tok = IdentifierToken
+		}
+	}
+	return tok, lit
+}
+
+// consume returns current token and string. Increments the position
+func (p *Parser) consume(context ParserContext) (Token, string) {
+	p.position++
+	tok, lit := p.scannedItems[p.position-1].tok, p.scannedItems[p.position-1].literal
+	if context == Values {
+		switch tok {
+		case InToken, NotInToken:
+			tok = IdentifierToken
+		}
+	}
+	return tok, lit
+}
+
+// scan runs through the input string and stores the ScannedItem in an array
+// Parser can now lookahead and consume the tokens
+func (p *Parser) scan() {
+	for {
+		token, literal := p.l.Lex()
+		p.scannedItems = append(p.scannedItems, ScannedItem{token, literal})
+		if token == EndOfStringToken {
+			break
+		}
+	}
+}
+
+// parse runs the left recursive descending algorithm
+// on input string. It returns a list of Requirement objects.
+func (p *Parser) parse() (internalSelector, error) {
+	p.scan() // init scannedItems
+
+	var requirements internalSelector
+	for {
+		tok, lit := p.lookahead(Values)
+		switch tok {
+		case IdentifierToken, DoesNotExistToken:
+			r, err := p.parseRequirement()
+			if err != nil {
+				return nil, fmt.Errorf("unable to parse requirement: %v", err)
+			}
+			requirements = append(requirements, *r)
+			t, l := p.consume(Values)
+			switch t {
+			case EndOfStringToken:
+				return requirements, nil
+			case CommaToken:
+				t2, l2 := p.lookahead(Values)
+				if t2 != IdentifierToken && t2 != DoesNotExistToken {
+					return nil, fmt.Errorf("found '%s', expected: identifier after ','", l2)
+				}
+			default:
+				return nil, fmt.Errorf("found '%s', expected: ',' or 'end of string'", l)
+			}
+		case EndOfStringToken:
+			return requirements, nil
+		default:
+			return nil, fmt.Errorf("found '%s', expected: !, identifier, or 'end of string'", lit)
+		}
+	}
+}
+
+func (p *Parser) parseRequirement() (*Requirement, error) {
+	key, operator, err := p.parseKeyAndInferOperator()
+	if err != nil {
+		return nil, err
+	}
+	if operator == selection.Exists || operator == selection.DoesNotExist { // operator found lookahead set checked
+		return NewRequirement(key, operator, []string{})
+	}
+	operator, err = p.parseOperator()
+	if err != nil {
+		return nil, err
+	}
+	var values sets.String
+	switch operator {
+	case selection.In, selection.NotIn:
+		values, err = p.parseValues()
+	case selection.Equals, selection.DoubleEquals, selection.NotEquals, selection.GreaterThan, selection.LessThan:
+		values, err = p.parseExactValue()
+	}
+	if err != nil {
+		return nil, err
+	}
+	return NewRequirement(key, operator, values.List())
+
+}
+
+// parseKeyAndInferOperator parse literals.
+// in case of no operator '!, in, notin, ==, =, !=' are found
+// the 'exists' operator is inferred
+func (p *Parser) parseKeyAndInferOperator() (string, selection.Operator, error) {
+	var operator selection.Operator
+	tok, literal := p.consume(Values)
+	if tok == DoesNotExistToken {
+		operator = selection.DoesNotExist
+		tok, literal = p.consume(Values)
+	}
+	if tok != IdentifierToken {
+		err := fmt.Errorf("found '%s', expected: identifier", literal)
+		return "", "", err
+	}
+	if err := validateLabelKey(literal); err != nil {
+		return "", "", err
+	}
+	if t, _ := p.lookahead(Values); t == EndOfStringToken || t == CommaToken {
+		if operator != selection.DoesNotExist {
+			operator = selection.Exists
+		}
+	}
+	return literal, operator, nil
+}
+
+// parseOperator return operator and eventually matchType
+// matchType can be exact
+func (p *Parser) parseOperator() (op selection.Operator, err error) {
+	tok, lit := p.consume(KeyAndOperator)
+	switch tok {
+	// DoesNotExistToken shouldn't be here because it's a unary operator, not a binary operator
+	case InToken:
+		op = selection.In
+	case EqualsToken:
+		op = selection.Equals
+	case DoubleEqualsToken:
+		op = selection.DoubleEquals
+	case GreaterThanToken:
+		op = selection.GreaterThan
+	case LessThanToken:
+		op = selection.LessThan
+	case NotInToken:
+		op = selection.NotIn
+	case NotEqualsToken:
+		op = selection.NotEquals
+	default:
+		return "", fmt.Errorf("found '%s', expected: '=', '!=', '==', 'in', notin'", lit)
+	}
+	return op, nil
+}
+
+// parseValues parses the values for set based matching (x,y,z)
+func (p *Parser) parseValues() (sets.String, error) {
+	tok, lit := p.consume(Values)
+	if tok != OpenParToken {
+		return nil, fmt.Errorf("found '%s' expected: '('", lit)
+	}
+	tok, lit = p.lookahead(Values)
+	switch tok {
+	case IdentifierToken, CommaToken:
+		s, err := p.parseIdentifiersList() // handles general cases
+		if err != nil {
+			return s, err
+		}
+		if tok, _ = p.consume(Values); tok != ClosedParToken {
+			return nil, fmt.Errorf("found '%s', expected: ')'", lit)
+		}
+		return s, nil
+	case ClosedParToken: // handles "()"
+		p.consume(Values)
+		return sets.NewString(""), nil
+	default:
+		return nil, fmt.Errorf("found '%s', expected: ',', ')' or identifier", lit)
+	}
+}
+
+// parseIdentifiersList parses a (possibly empty) list of
+// of comma separated (possibly empty) identifiers
+func (p *Parser) parseIdentifiersList() (sets.String, error) {
+	s := sets.NewString()
+	for {
+		tok, lit := p.consume(Values)
+		switch tok {
+		case IdentifierToken:
+			s.Insert(lit)
+			tok2, lit2 := p.lookahead(Values)
+			switch tok2 {
+			case CommaToken:
+				continue
+			case ClosedParToken:
+				return s, nil
+			default:
+				return nil, fmt.Errorf("found '%s', expected: ',' or ')'", lit2)
+			}
+		case CommaToken: // handled here since we can have "(,"
+			if s.Len() == 0 {
+				s.Insert("") // to handle (,
+			}
+			tok2, _ := p.lookahead(Values)
+			if tok2 == ClosedParToken {
+				s.Insert("") // to handle ,)  Double "" removed by StringSet
+				return s, nil
+			}
+			if tok2 == CommaToken {
+				p.consume(Values)
+				s.Insert("") // to handle ,, Double "" removed by StringSet
+			}
+		default: // it can be operator
+			return s, fmt.Errorf("found '%s', expected: ',', or identifier", lit)
+		}
+	}
+}
+
+// parseExactValue parses the only value for exact match style
+func (p *Parser) parseExactValue() (sets.String, error) {
+	s := sets.NewString()
+	tok, lit := p.lookahead(Values)
+	if tok == EndOfStringToken || tok == CommaToken {
+		s.Insert("")
+		return s, nil
+	}
+	tok, lit = p.consume(Values)
+	if tok == IdentifierToken {
+		s.Insert(lit)
+		return s, nil
+	}
+	return nil, fmt.Errorf("found '%s', expected: identifier", lit)
+}
+
+// Parse takes a string representing a selector and returns a selector
+// object, or an error. This parsing function differs from ParseSelector
+// as they parse different selectors with different syntaxes.
+// The input will cause an error if it does not follow this form:
+//
+//  <selector-syntax>         ::= <requirement> | <requirement> "," <selector-syntax>
+//  <requirement>             ::= [!] KEY [ <set-based-restriction> | <exact-match-restriction> ]
+//  <set-based-restriction>   ::= "" | <inclusion-exclusion> <value-set>
+//  <inclusion-exclusion>     ::= <inclusion> | <exclusion>
+//  <exclusion>               ::= "notin"
+//  <inclusion>               ::= "in"
+//  <value-set>               ::= "(" <values> ")"
+//  <values>                  ::= VALUE | VALUE "," <values>
+//  <exact-match-restriction> ::= ["="|"=="|"!="] VALUE
+//
+// KEY is a sequence of one or more characters following [ DNS_SUBDOMAIN "/" ] DNS_LABEL. Max length is 63 characters.
+// VALUE is a sequence of zero or more characters "([A-Za-z0-9_-\.])". Max length is 63 characters.
+// Delimiter is white space: (' ', '\t')
+// Example of valid syntax:
+//  "x in (foo,,baz),y,z notin ()"
+//
+// Note:
+//  (1) Inclusion - " in " - denotes that the KEY exists and is equal to any of the
+//      VALUEs in its requirement
+//  (2) Exclusion - " notin " - denotes that the KEY is not equal to any
+//      of the VALUEs in its requirement or does not exist
+//  (3) The empty string is a valid VALUE
+//  (4) A requirement with just a KEY - as in "y" above - denotes that
+//      the KEY exists and can be any VALUE.
+//  (5) A requirement with just !KEY requires that the KEY not exist.
+//
+func Parse(selector string) (Selector, error) {
+	parsedSelector, err := parse(selector)
+	if err == nil {
+		return parsedSelector, nil
+	}
+	return nil, err
+}
+
+// parse parses the string representation of the selector and returns the internalSelector struct.
+// The callers of this method can then decide how to return the internalSelector struct to their
+// callers. This function has two callers now, one returns a Selector interface and the other
+// returns a list of requirements.
+func parse(selector string) (internalSelector, error) {
+	p := &Parser{l: &Lexer{s: selector, pos: 0}}
+	items, err := p.parse()
+	if err != nil {
+		return nil, err
+	}
+	sort.Sort(ByKey(items)) // sort to grant determistic parsing
+	return internalSelector(items), err
+}
+
+func validateLabelKey(k string) error {
+	if errs := validation.IsQualifiedName(k); len(errs) != 0 {
+		return fmt.Errorf("invalid label key %q: %s", k, strings.Join(errs, "; "))
+	}
+	return nil
+}
+
+func validateLabelValue(v string) error {
+	if errs := validation.IsValidLabelValue(v); len(errs) != 0 {
+		return fmt.Errorf("invalid label value: %q: %s", v, strings.Join(errs, "; "))
+	}
+	return nil
+}
+
+// SelectorFromSet returns a Selector which will match exactly the given Set. A
+// nil and empty Sets are considered equivalent to Everything().
+func SelectorFromSet(ls Set) Selector {
+	if ls == nil || len(ls) == 0 {
+		return internalSelector{}
+	}
+	var requirements internalSelector
+	for label, value := range ls {
+		r, err := NewRequirement(label, selection.Equals, []string{value})
+		if err == nil {
+			requirements = append(requirements, *r)
+		} else {
+			//TODO: double check errors when input comes from serialization?
+			return internalSelector{}
+		}
+	}
+	// sort to have deterministic string representation
+	sort.Sort(ByKey(requirements))
+	return requirements
+}
+
+// SelectorFromValidatedSet returns a Selector which will match exactly the given Set.
+// A nil and empty Sets are considered equivalent to Everything().
+// It assumes that Set is already validated and doesn't do any validation.
+func SelectorFromValidatedSet(ls Set) Selector {
+	if ls == nil || len(ls) == 0 {
+		return internalSelector{}
+	}
+	var requirements internalSelector
+	for label, value := range ls {
+		requirements = append(requirements, Requirement{key: label, operator: selection.Equals, strValues: []string{value}})
+	}
+	// sort to have deterministic string representation
+	sort.Sort(ByKey(requirements))
+	return requirements
+}
+
+// ParseToRequirements takes a string representing a selector and returns a list of
+// requirements. This function is suitable for those callers that perform additional
+// processing on selector requirements.
+// See the documentation for Parse() function for more details.
+// TODO: Consider exporting the internalSelector type instead.
+func ParseToRequirements(selector string) ([]Requirement, error) {
+	return parse(selector)
+}