fix: count string by RuneCountInString in StringLenBetween()

.changes/unreleased/BUG FIXES-20250903-155819.yaml

@@ -0,0 +1,5 @@
+kind: BUG FIXES
+body: ' tfsdk: Fixed `StringLenBetween` function to correctly count multibyte characters'
+time: 2025-09-03T15:58:19.587736995Z
+custom:
+    Issue: "1516"

helper/validation/strings.go

@@ -8,9 +8,11 @@ import (
 	"fmt"
 	"regexp"
 	"strings"
+	"unicode/utf8"
 
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/structure"
+	"golang.org/x/text/unicode/norm"
 )
 
 // StringIsNotEmpty is a ValidateFunc that ensures a string is not empty
@@ -78,8 +80,9 @@ func StringLenBetween(minVal, maxVal int) schema.SchemaValidateFunc {
 			errors = append(errors, fmt.Errorf("expected type of %s to be string", k))
 			return warnings, errors
 		}
-
-		if len(v) < minVal || len(v) > maxVal {
+		v = norm.NFC.String(v)
+		l := utf8.RuneCountInString(v)
+		if l < minVal || l > maxVal {
 			errors = append(errors, fmt.Errorf("expected length of %s to be in the range (%d - %d), got %s", k, minVal, maxVal, v))
 		}
 

helper/validation/strings_test.go

@@ -259,6 +259,27 @@ func TestValidationStringLenBetween(t *testing.T) {
 			f:           StringLenBetween(1, 5),
 			expectedErr: regexp.MustCompile(`expected type of [\w]+ to be string`),
 		},
+		{
+			val: "日", // 3 bytes char, length must be 1
+			f:   StringLenBetween(1, 1),
+		},
+		{
+			val: "😊", // 4 bytes char, length must be 1
+			f:   StringLenBetween(1, 1),
+		},
+		{
+			val: "😊😭", // 4 bytes chars, length must be 2
+			f:   StringLenBetween(1, 2),
+		},
+		{
+			val: "が", // 2 code points
+			f:   StringLenBetween(1, 1),
+		},
+		{
+			val:         "👨‍👩‍👧‍👦", // multi code points but can't be normalized
+			f:           StringLenBetween(1, 1),
+			expectedErr: regexp.MustCompile(`expected length of [\w]+ to be in the range \(1 \- 1\), got 👨‍👩‍👧‍👦`),
+		},
 	})
 }