Add `startswith` and `endswith` validators

`HasPrefix` and `HasSuffix` are both part of the `strings` package.
These seem like generally useful validations to include and cover some
subset of the use-cases of a general regex validator without having any
of the problems outlined by the validator docs.

baked_in.go

@@ -112,6 +112,8 @@ var (
 		"excludes":         excludes,
 		"excludesall":      excludesAll,
 		"excludesrune":     excludesRune,
+		"startswith":       startsWith,
+		"endswith":         endsWith,
 		"isbn":             isISBN,
 		"isbn10":           isISBN10,
 		"isbn13":           isISBN13,
@@ -650,6 +652,16 @@ func contains(fl FieldLevel) bool {
 	return strings.Contains(fl.Field().String(), fl.Param())
 }
 
+// StartsWith is the validation function for validating that the field's value starts with the text specified within the param.
+func startsWith(fl FieldLevel) bool {
+	return strings.HasPrefix(fl.Field().String(), fl.Param())
+}
+
+// EndsWith is the validation function for validating that the field's value ends with the text specified within the param.
+func endsWith(fl FieldLevel) bool {
+	return strings.HasSuffix(fl.Field().String(), fl.Param())
+}
+
 // FieldContains is the validation function for validating if the current field's value contains the field specified by the param's value.
 func fieldContains(fl FieldLevel) bool {
 	field := fl.Field()

doc.go

@@ -714,6 +714,18 @@ This validates that a string value does not contain the supplied rune value.
 
 	Usage: excludesrune=@
 
+Starts With
+
+This validates that a string value starts with the supplied string value
+
+	Usage: startswith=hello
+
+Ends With
+
+This validates that a string value ends with the supplied string value
+
+	Usage: endswith=goodbye
+
 International Standard Book Number
 
 This validates that a string value contains a valid isbn10 or isbn13 value.

validator_test.go

@@ -8555,3 +8555,59 @@ func TestDirValidation(t *testing.T) {
 		validate.Var(2, "dir")
 	}, "Bad field type int")
 }
+
+func TestStartsWithValidation(t *testing.T) {
+	tests := []struct {
+		Value       string `validate:"startswith=(/^ヮ^)/*:・ﾟ✧"`
+		Tag         string
+		ExpectedNil bool
+	}{
+		{Value: "(/^ヮ^)/*:・ﾟ✧ glitter", Tag: "startswith=(/^ヮ^)/*:・ﾟ✧", ExpectedNil: true},
+		{Value: "abcd",  Tag: "startswith=(/^ヮ^)/*:・ﾟ✧", ExpectedNil: false},
+	}
+
+	validate := New()
+
+	for i, s := range tests {
+		errs := validate.Var(s.Value, s.Tag)
+
+		if (s.ExpectedNil && errs != nil) || (!s.ExpectedNil && errs == nil) {
+			t.Fatalf("Index: %d failed Error: %s", i, errs)
+		}
+
+		errs = validate.Struct(s)
+
+		if (s.ExpectedNil && errs != nil) || (!s.ExpectedNil && errs == nil) {
+			t.Fatalf("Index: %d failed Error: %s", i, errs)
+		}
+	}
+}
+
+func TestEndsWithValidation(t *testing.T) {
+	tests := []struct {
+		Value       string `validate:"endswith=(/^ヮ^)/*:・ﾟ✧"`
+		Tag         string
+		ExpectedNil bool
+	}{
+		{Value: "glitter (/^ヮ^)/*:・ﾟ✧", Tag: "endswith=(/^ヮ^)/*:・ﾟ✧", ExpectedNil: true},
+		{Value: "(/^ヮ^)/*:・ﾟ✧ glitter", Tag: "endswith=(/^ヮ^)/*:・ﾟ✧", ExpectedNil: false},
+	}
+
+	validate := New()
+
+	for i, s := range tests {
+		errs := validate.Var(s.Value, s.Tag)
+
+		if (s.ExpectedNil && errs != nil) || (!s.ExpectedNil && errs == nil) {
+			t.Fatalf("Index: %d failed Error: %s", i, errs)
+		}
+
+		errs = validate.Struct(s)
+
+		if (s.ExpectedNil && errs != nil) || (!s.ExpectedNil && errs == nil) {
+			t.Fatalf("Index: %d failed Error: %s", i, errs)
+		}
+	}
+}
+
+