diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..694d4e9
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,29 @@
+BSD 3-Clause License
+
+Copyright (c) 2019, Fuyu
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright notice, this
+  list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+
+* Neither the name of the copyright holder nor the names of its
+  contributors may be used to endorse or promote products derived from
+  this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/input.go b/input.go
index 0acd88d..b6edb38 100644
--- a/input.go
+++ b/input.go
@@ -6,15 +6,21 @@ import (
 	"strconv"
 )
 
-func inputInt(kind reflect.Kind, value string) interface{} {
-	return int(inputSame(reflect.Int, value).(int64))
+// InputFunc is a function that converts the parameter of a validation rule to the desired type
+type InputFunc func(reflect.Kind, string) interface{}
+
+// InputInt always returns an int
+func InputInt(kind reflect.Kind, value string) interface{} {
+	return int(InputSame(reflect.Int, value).(int64))
 }
 
-func inputRegexp(kind reflect.Kind, value string) interface{} {
+// InputRegexp always returns a compiled regular expression
+func InputRegexp(kind reflect.Kind, value string) interface{} {
 	return regexp.MustCompile(value)
 }
 
-func inputSame(kind reflect.Kind, value string) interface{} {
+// InputSame returns the type matching the field
+func InputSame(kind reflect.Kind, value string) interface{} {
 	var val interface{}
 	var err error
 
diff --git a/rules.go b/rules.go
index 69dddd0..3656938 100644
--- a/rules.go
+++ b/rules.go
@@ -6,17 +6,25 @@ import (
 	"strings"
 )
 
-type listFunc func(reflect.Value, interface{}) bool
+// ValidationFunc is a function to validate a field
+type ValidationFunc func(reflect.Value, interface{}) bool
+
+// Kinds is a map with validation funcs for each reflect.Kind
+type Kinds map[reflect.Kind]ValidationFunc
+
 type listFuncInfo struct {
-	inputFunc func(reflect.Kind, string) interface{}
-	kinds     _kinds
+	inputFunc InputFunc
+	kinds     Kinds
 }
 
-type _kinds map[reflect.Kind]listFunc
+// AddRule adds a rule to the list of validation functions
+func AddRule(name string, inputFunc InputFunc, kinds Kinds) {
+	funcs[name] = listFuncInfo{inputFunc, kinds}
+}
 
 // nolint: dupl
 var funcs = map[string]listFuncInfo{
-	`required`: {nil, _kinds{
+	`required`: {nil, Kinds{
 		reflect.Ptr: func(rv reflect.Value, _ interface{}) bool {
 			return !rv.IsNil()
 		},
@@ -44,29 +52,29 @@ var funcs = map[string]listFuncInfo{
 	}},
 
 	// Strings
-	`prefix`: {inputSame, _kinds{
+	`prefix`: {InputSame, Kinds{
 		reflect.String: func(rv reflect.Value, val interface{}) bool {
 			return strings.HasPrefix(rv.String(), val.(string))
 		},
 	}},
-	`suffix`: {inputSame, _kinds{
+	`suffix`: {InputSame, Kinds{
 		reflect.String: func(rv reflect.Value, val interface{}) bool {
 			return strings.HasSuffix(rv.String(), val.(string))
 		},
 	}},
-	`contains`: {inputSame, _kinds{
+	`contains`: {InputSame, Kinds{
 		reflect.String: func(rv reflect.Value, val interface{}) bool {
 			return strings.Contains(rv.String(), val.(string))
 		},
 	}},
-	`regexp`: {inputRegexp, _kinds{
+	`regexp`: {InputRegexp, Kinds{
 		reflect.String: func(rv reflect.Value, val interface{}) bool {
 			return val.(*regexp.Regexp).MatchString(rv.String())
 		},
 	}},
 
 	// Comparisons
-	`eq`: {inputSame, _kinds{
+	`eq`: {InputSame, Kinds{
 		reflect.String: func(rv reflect.Value, val interface{}) bool {
 			return rv.String() == val.(string)
 		},
@@ -82,7 +90,29 @@ var funcs = map[string]listFuncInfo{
 	}},
 
 	// Integers
-	`gt`: {inputSame, _kinds{
+	`gte`: {InputSame, Kinds{
+		reflect.Int: func(rv reflect.Value, val interface{}) bool {
+			return rv.Int() >= val.(int64)
+		},
+		reflect.Uint: func(rv reflect.Value, val interface{}) bool {
+			return rv.Uint() >= val.(uint64)
+		},
+		reflect.Float64: func(rv reflect.Value, val interface{}) bool {
+			return rv.Float() >= val.(float64)
+		},
+	}},
+	`lte`: {InputSame, Kinds{
+		reflect.Int: func(rv reflect.Value, val interface{}) bool {
+			return rv.Int() <= val.(int64)
+		},
+		reflect.Uint: func(rv reflect.Value, val interface{}) bool {
+			return rv.Uint() <= val.(uint64)
+		},
+		reflect.Float64: func(rv reflect.Value, val interface{}) bool {
+			return rv.Float() <= val.(float64)
+		},
+	}},
+	`gt`: {InputSame, Kinds{
 		reflect.Int: func(rv reflect.Value, val interface{}) bool {
 			return rv.Int() > val.(int64)
 		},
@@ -93,7 +123,7 @@ var funcs = map[string]listFuncInfo{
 			return rv.Float() > val.(float64)
 		},
 	}},
-	`lt`: {inputSame, _kinds{
+	`lt`: {InputSame, Kinds{
 		reflect.Int: func(rv reflect.Value, val interface{}) bool {
 			return rv.Int() < val.(int64)
 		},
@@ -106,7 +136,7 @@ var funcs = map[string]listFuncInfo{
 	}},
 
 	// Slices, maps & strings
-	`len`: {inputInt, _kinds{
+	`len`: {InputInt, Kinds{
 		reflect.Slice: func(rv reflect.Value, val interface{}) bool {
 			return rv.Len() == val.(int)
 		},
@@ -117,7 +147,7 @@ var funcs = map[string]listFuncInfo{
 			return rv.Len() == val.(int)
 		},
 	}},
-	`min`: {inputInt, _kinds{
+	`min`: {InputInt, Kinds{
 		reflect.Slice: func(rv reflect.Value, val interface{}) bool {
 			return rv.Len() >= val.(int)
 		},
@@ -128,7 +158,7 @@ var funcs = map[string]listFuncInfo{
 			return rv.Len() >= val.(int)
 		},
 	}},
-	`max`: {inputInt, _kinds{
+	`max`: {InputInt, Kinds{
 		reflect.Slice: func(rv reflect.Value, val interface{}) bool {
 			return rv.Len() <= val.(int)
 		},
diff --git a/rules_test.go b/rules_test.go
index 46e2c6b..409c8fc 100644
--- a/rules_test.go
+++ b/rules_test.go
@@ -1,9 +1,29 @@
 package validate
 
 import (
+	"reflect"
 	"testing"
 )
 
+func TestAddRule(t *testing.T) {
+	type s struct {
+		A string `validate:"custom"`
+	}
+
+	AddRule(`custom`, nil, Kinds{
+		reflect.String: func(rv reflect.Value, _ interface{}) bool {
+			return rv.String() == `custom`
+		},
+	})
+
+	pass := s{`custom`}
+
+	fail := s{`somethingelse`}
+
+	check(t, pass, 0)
+	check(t, fail, 1)
+}
+
 func TestRuleRequired(t *testing.T) {
 	type s struct {
 		A *string     `validate:"required"`
@@ -18,9 +38,9 @@ func TestRuleRequired(t *testing.T) {
 	}
 
 	str := ``
-	var pass = s{&str, make([]int, 1), make([]int, 1), ` `, -1, 1, 0.01, ``, map[int]int{0: 1}}
+	pass := s{&str, make([]int, 1), make([]int, 1), ` `, -1, 1, 0.01, ``, map[int]int{0: 1}}
 
-	var fail = s{nil, nil, make([]int, 0), ``, 0, 0, 0.000, nil, nil}
+	fail := s{nil, nil, make([]int, 0), ``, 0, 0, 0.000, nil, nil}
 
 	check(t, pass, 0)
 	check(t, fail, 9)
@@ -32,9 +52,9 @@ func TestRulePrefixSuffix(t *testing.T) {
 		B string `validate:"suffix=@"`
 	}
 
-	var pass = s{`#a`, `a@`}
+	pass := s{`#a`, `a@`}
 
-	var fail = s{`a#`, `@a`}
+	fail := s{`a#`, `@a`}
 
 	check(t, pass, 0)
 	check(t, fail, 2)
@@ -45,12 +65,12 @@ func TestRuleContains(t *testing.T) {
 		A string `validate:"contains=%"`
 	}
 
-	var pass1 = s{`a%`}
-	var pass2 = s{`%a`}
-	var pass3 = s{`%`}
-	var pass4 = s{`a%a`}
+	pass1 := s{`a%`}
+	pass2 := s{`%a`}
+	pass3 := s{`%`}
+	pass4 := s{`a%a`}
 
-	var fail = s{`aa`}
+	fail := s{`aa`}
 
 	check(t, pass1, 0)
 	check(t, pass2, 0)
@@ -64,11 +84,11 @@ func TestRuleRegexp(t *testing.T) {
 		A string `validate:"regexp=^[0-9]$"`
 	}
 
-	var pass1 = s{`0`}
-	var pass2 = s{`7`}
+	pass1 := s{`0`}
+	pass2 := s{`7`}
 
-	var fail1 = s{`A`}
-	var fail2 = s{`11`}
+	fail1 := s{`A`}
+	fail2 := s{`11`}
 
 	check(t, pass1, 0)
 	check(t, pass2, 0)
@@ -83,16 +103,46 @@ func TestRuleEqGtLt(t *testing.T) {
 		C uint    `validate:"lt=1"`
 	}
 
-	var pass = s{3, 100001, 0}
+	pass := s{3, 100001, 0}
 
-	var fail1 = s{2, 1e5, 1}
-	var fail2 = s{4, 9999, 2}
+	fail1 := s{2, 1e5, 1}
+	fail2 := s{4, 9999, 2}
 
 	check(t, pass, 0)
 	check(t, fail1, 3)
 	check(t, fail2, 3)
 }
 
+func TestRuleGteLte(t *testing.T) {
+	type s struct {
+		U uint    `validate:"gte=0,lte=10"`
+		I int     `validate:"gte=-10,lte=0"`
+		F float64 `validate:"gte=0,lte=10"`
+	}
+
+	pass1 := s{0, -10, 0}
+	pass2 := s{10, 0, 10}
+
+	// Uint
+	fail1 := s{11, -10, 0}
+
+	// Int
+	fail2 := s{0, -11, 0}
+	fail3 := s{0, 1, 0}
+
+	// Float
+	fail4 := s{0, -10, -0.0001}
+	fail5 := s{0, -10, 10.0001}
+
+	check(t, pass1, 0)
+	check(t, pass2, 0)
+	check(t, fail1, 1)
+	check(t, fail2, 1)
+	check(t, fail3, 1)
+	check(t, fail4, 1)
+	check(t, fail5, 1)
+}
+
 func TestLenMinMax(t *testing.T) {
 	type s struct {
 		A string         `validate:"len=3"`
@@ -100,10 +150,10 @@ func TestLenMinMax(t *testing.T) {
 		C map[int]string `validate:"max=1"`
 	}
 
-	var pass = s{`abc`, []int{1, 2}, nil}
+	pass := s{`abc`, []int{1, 2}, nil}
 
-	var fail1 = s{`ab`, []int{1}, map[int]string{1: `a`, 2: `b`}}
-	var fail2 = s{`abcd`, nil, nil}
+	fail1 := s{`ab`, []int{1}, map[int]string{1: `a`, 2: `b`}}
+	fail2 := s{`abcd`, nil, nil}
 
 	check(t, pass, 0)
 	check(t, fail1, 3)
@@ -111,6 +161,7 @@ func TestLenMinMax(t *testing.T) {
 }
 
 func check(t *testing.T, c interface{}, errCount int) {
+	t.Helper()
 	errs := Validate(c)
 	if len(errs) != errCount {
 		t.Errorf(`Case %T(%v) should get %d errors, but got %v`, c, c, errCount, errs)
diff --git a/validate.go b/validate.go
index 2317edb..59216c8 100644
--- a/validate.go
+++ b/validate.go
@@ -7,6 +7,12 @@ import (
 	"sync"
 )
 
+type ValidateValuer interface {
+	ValidateValue() interface{}
+}
+
+var vvType = reflect.TypeOf((*ValidateValuer)(nil)).Elem()
+
 // Validate validates a variable
 func Validate(v interface{}) []ValidationError {
 	rv := reflect.ValueOf(v)
@@ -159,12 +165,24 @@ func getRules(rt reflect.Type) []rule {
 
 	for i := 0; i < rt.NumField(); i++ {
 		ft := rt.Field(i)
+
+		var valuer bool
+		if ft.Type.Implements(vvType) {
+			ft.Type = reflect.TypeOf(reflect.New(ft.Type).Interface().(ValidateValuer).ValidateValue())
+			valuer = true
+		}
+
 		kind := simplifyKind(ft.Type.Kind())
 
-		tags := strings.Split(ft.Tag.Get(`validate`), `,`)
-		rules = append(rules, getTagFuncs(i, ft, kind, tags)...)
+		var ptr bool
+		if kind == reflect.Ptr {
+			kind = ft.Type.Elem().Kind()
+			ptr = true
+		}
+		kind = simplifyKind(kind)
 
-		// TODO: Add validator interface
+		tags := strings.Split(ft.Tag.Get(`validate`), `,`)
+		rules = append(rules, getTagFuncs(i, ft, kind, tags, ptr, valuer)...)
 
 		switch kind {
 		case reflect.Slice, reflect.Struct, reflect.Interface:
@@ -185,7 +203,7 @@ func nest(ft reflect.StructField) func(reflect.Value) ([]ValidationError, bool)
 	}
 }
 
-func getTagFuncs(i int, ft reflect.StructField, kind reflect.Kind, tags []string) []rule {
+func getTagFuncs(i int, ft reflect.StructField, kind reflect.Kind, tags []string, ptr, valuer bool) []rule {
 	var rules []rule
 	for _, v := range tags {
 		if v == `` {
@@ -198,22 +216,42 @@ func getTagFuncs(i int, ft reflect.StructField, kind reflect.Kind, tags []string
 			value = parts[1]
 		}
 
-		if tag == `optional` {
-			rules = append(rules, rule{i, func(rv reflect.Value) ([]ValidationError, bool) {
-				check, _ := getTagFunc(`required`, ``, kind)
-				return nil, check(rv, nil)
-			}})
-			continue
+		kind := kind
+		ptr := ptr
+		if ptr && (tag == `optional` || strings.TrimPrefix(tag, `!`) == `required`) {
+			kind = reflect.Ptr
+			ptr = false
 		}
 
-		check, val := getTagFunc(tag, value, kind)
+		var f validateCheck
 
-		f := func(rv reflect.Value) ([]ValidationError, bool) {
-			if check(rv, val) {
-				return nil, true
+		if tag == `optional` {
+			f = func(rv reflect.Value) ([]ValidationError, bool) {
+				check, _ := getTagFunc(`required`, ``, kind)
+				return nil, check(rv, nil)
+			}
+		} else {
+			var not bool
+			if strings.HasPrefix(tag, `!`) {
+				not = true
 			}
 
-			return []ValidationError{{Field: []Field{{nil, &ft}}, Check: tag, Value: value}}, false
+			check, val := getTagFunc(strings.TrimPrefix(tag, `!`), value, kind)
+
+			f = func(rv reflect.Value) ([]ValidationError, bool) {
+				if check(rv, val) == !not {
+					return nil, true
+				}
+
+				return []ValidationError{{Field: []Field{{nil, &ft}}, Check: tag, Value: value}}, false
+			}
+		}
+
+		if ptr {
+			f = depointerFunc(f, ft, tag, value)
+		}
+		if valuer {
+			f = valuerFunc(f)
 		}
 
 		rules = append(rules, rule{i, f})
@@ -222,7 +260,26 @@ func getTagFuncs(i int, ft reflect.StructField, kind reflect.Kind, tags []string
 	return rules
 }
 
-func getTagFunc(tag, value string, kind reflect.Kind) (listFunc, interface{}) {
+type validateCheck = func(rv reflect.Value) ([]ValidationError, bool)
+
+func depointerFunc(f validateCheck, ft reflect.StructField, tag, value string) validateCheck {
+	return func(rv reflect.Value) ([]ValidationError, bool) {
+		if rv.IsNil() {
+			return []ValidationError{{Field: []Field{{nil, &ft}}, Check: tag, Value: value}}, false
+		}
+
+		return f(rv.Elem())
+	}
+}
+
+func valuerFunc(f validateCheck) validateCheck {
+	return func(rv reflect.Value) ([]ValidationError, bool) {
+		rv = reflect.ValueOf(rv.Interface().(ValidateValuer).ValidateValue())
+		return f(rv)
+	}
+}
+
+func getTagFunc(tag, value string, kind reflect.Kind) (ValidationFunc, interface{}) {
 	tagInfo, ok := funcs[tag]
 	if !ok {
 		panic(`Unknown validation ` + tag)
diff --git a/validate_test.go b/validate_test.go
index 035b0b9..96a83b4 100644
--- a/validate_test.go
+++ b/validate_test.go
@@ -10,10 +10,10 @@ func TestOptionalMultiple(t *testing.T) {
 		B int    `validate:"gt=3,lt=20"`
 	}
 
-	var pass1 = s{``, 4}
-	var pass2 = s{`a`, 19}
+	pass1 := s{``, 4}
+	pass2 := s{`a`, 19}
 
-	var fail = s{`b`, 3}
+	fail := s{`b`, 3}
 
 	check(t, pass1, 0)
 	check(t, pass2, 0)
@@ -34,11 +34,11 @@ func TestNesting(t *testing.T) {
 		B sb
 	}
 
-	var pass = s{[]sa{{`abc`}}, sb{12}}
+	pass := s{[]sa{{`abc`}}, sb{12}}
 
-	var fail1 = s{nil, sb{12}}
-	var fail2 = s{[]sa{{``}}, sb{12}}
-	var fail3 = s{[]sa{{``}}, sb{9}}
+	fail1 := s{nil, sb{12}}
+	fail2 := s{[]sa{{``}}, sb{12}}
+	fail3 := s{[]sa{{``}}, sb{9}}
 
 	check(t, pass, 0)
 	check(t, fail1, 1)
@@ -76,3 +76,78 @@ func TestValidationErrorField(t *testing.T) {
 		t.Fatal(`Expected errors to be A.B.C[0].D and A.B.C[2].D; got`, errs[0].Field, `and`, errs[1].Field)
 	}
 }
+
+func TestNot(t *testing.T) {
+	type s struct {
+		A string `validate:"!len=3"`
+		B int    `validate:"!eq=3"`
+	}
+
+	pass1 := s{`ab`, 2}
+	pass2 := s{`abcd`, 4}
+
+	fail := s{`abc`, 3}
+
+	check(t, pass1, 0)
+	check(t, pass2, 0)
+	check(t, fail, 2)
+
+	errs := Validate(fail)
+	if errs[0].Check != `!len` || errs[1].Check != `!eq` {
+		t.Errorf(`Checknames missing !, got "%s" and "%s"`, errs[0].Check, errs[1].Check)
+	}
+}
+
+func TestPtr(t *testing.T) {
+	type s struct {
+		A *int `validate:"eq=3"`
+		B *int `validate:"optional,eq=3"`
+	}
+
+	two := 2
+	three := 3
+
+	pass1 := s{&three, &three}
+	pass2 := s{&three, nil}
+
+	fail1 := s{&two, &two}
+	fail2 := s{nil, nil}
+
+	check(t, pass1, 0)
+	check(t, pass2, 0)
+	check(t, fail1, 2)
+	check(t, fail2, 1)
+}
+
+type val struct {
+	Int   int
+	Valid bool
+}
+
+func (v val) ValidateValue() interface{} {
+	if !v.Valid {
+		return (*int)(nil)
+	}
+
+	return &v.Int
+}
+
+func TestValidateValuer(t *testing.T) {
+	type s struct {
+		A val `validate:"required,lt=2"`
+		B val `validate:"optional,eq=3"`
+	}
+
+	pass1 := s{val{Valid: true}, val{}}
+	pass2 := s{val{Valid: true}, val{Int: 3, Valid: true}}
+
+	fail1 := s{}
+	fail2 := s{val{Int: 2, Valid: true}, val{}}
+	fail3 := s{val{Valid: true}, val{Int: 2, Valid: true}}
+
+	check(t, pass1, 0)
+	check(t, pass2, 0)
+	check(t, fail1, 1)
+	check(t, fail2, 1)
+	check(t, fail3, 1)
+}