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..685d0d5 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 +90,7 @@ var funcs = map[string]listFuncInfo{
 	}},
 
 	// Integers
-	`gt`: {inputSame, _kinds{
+	`gt`: {InputSame, Kinds{
 		reflect.Int: func(rv reflect.Value, val interface{}) bool {
 			return rv.Int() > val.(int64)
 		},
@@ -93,7 +101,7 @@ 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 +114,7 @@ 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 +125,7 @@ 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 +136,7 @@ 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..32d632d 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`
+		},
+	})
+
+	var pass = s{`custom`}
+
+	var fail = s{`somethingelse`}
+
+	check(t, pass, 0)
+	check(t, fail, 1)
+}
+
 func TestRuleRequired(t *testing.T) {
 	type s struct {
 		A *string     `validate:"required"`
diff --git a/validate.go b/validate.go
index 2317edb..f1cc1aa 100644
--- a/validate.go
+++ b/validate.go
@@ -222,7 +222,7 @@ func getTagFuncs(i int, ft reflect.StructField, kind reflect.Kind, tags []string
 	return rules
 }
 
-func getTagFunc(tag, value string, kind reflect.Kind) (listFunc, interface{}) {
+func getTagFunc(tag, value string, kind reflect.Kind) (ValidationFunc, interface{}) {
 	tagInfo, ok := funcs[tag]
 	if !ok {
 		panic(`Unknown validation ` + tag)