diff --git a/kotlin-native/runtime/src/main/kotlin/kotlin/native/internal/test/TestRunner.kt b/kotlin-native/runtime/src/main/kotlin/kotlin/native/internal/test/TestRunner.kt
index 97ecbcd482e..b901f4c8fa9 100644
--- a/kotlin-native/runtime/src/main/kotlin/kotlin/native/internal/test/TestRunner.kt
+++ b/kotlin-native/runtime/src/main/kotlin/kotlin/native/internal/test/TestRunner.kt
@@ -174,7 +174,7 @@ internal class TestRunner(val suites: List<TestSuite>, args: Array<String>) {
     }
 
     private fun setLoggerFromArg(logger: String) {
-        when (logger.toUpperCase()) {
+        when (logger.uppercase()) {
             "GTEST" -> this.logger = GTestLogger()
             "TEAMCITY" -> this.logger = TeamCityLogger()
             "SIMPLE" -> this.logger = SimpleTestLogger()
diff --git a/kotlin-native/runtime/src/main/kotlin/kotlin/text/Char.kt b/kotlin-native/runtime/src/main/kotlin/kotlin/text/Char.kt
index 0b43eadc2a8..451707c03ca 100644
--- a/kotlin-native/runtime/src/main/kotlin/kotlin/text/Char.kt
+++ b/kotlin-native/runtime/src/main/kotlin/kotlin/text/Char.kt
@@ -269,7 +269,7 @@ internal actual fun checkRadix(radix: Int): Int {
 internal fun Char.Companion.toLowerCase(codePoint: Int): Int =
     if (codePoint < MIN_SUPPLEMENTARY_CODE_POINT) {
         @Suppress("DEPRECATION")
-        codePoint.toChar().toLowerCase().toInt()
+        codePoint.toChar().lowercaseChar().toInt()
     } else {
         codePoint // TODO: Implement this transformation for supplementary codepoints.
     }
@@ -278,7 +278,7 @@ internal fun Char.Companion.toLowerCase(codePoint: Int): Int =
 internal fun Char.Companion.toUpperCase(codePoint: Int): Int =
     if (codePoint < MIN_SUPPLEMENTARY_CODE_POINT) {
         @Suppress("DEPRECATION")
-        codePoint.toChar().toUpperCase().toInt()
+        codePoint.toChar().uppercaseChar().toInt()
     } else {
         codePoint // TODO: Implement this transformation for supplementary codepoints.
     }
diff --git a/kotlin-native/runtime/src/main/kotlin/kotlin/text/Strings.kt b/kotlin-native/runtime/src/main/kotlin/kotlin/text/Strings.kt
index b13d48c0519..aff73e9163a 100644
--- a/kotlin-native/runtime/src/main/kotlin/kotlin/text/Strings.kt
+++ b/kotlin-native/runtime/src/main/kotlin/kotlin/text/Strings.kt
@@ -256,6 +256,7 @@ private external fun toCharArray(string: String, start: Int, size: Int): CharArr
 @Deprecated("Use replaceFirstChar instead.", ReplaceWith("replaceFirstChar { if (it.isLowerCase()) it.titlecase() else it.toString() }"))
 @DeprecatedSinceKotlin(warningSince = "1.5")
 public actual fun String.capitalize(): String {
+    @Suppress("DEPRECATION")
     return if (isNotEmpty() && this[0].isLowerCase()) substring(0, 1).toUpperCase() + substring(1) else this
 }
 
@@ -268,6 +269,7 @@ public actual fun String.capitalize(): String {
 @Deprecated("Use replaceFirstChar instead.", ReplaceWith("replaceFirstChar { it.lowercase() }"))
 @DeprecatedSinceKotlin(warningSince = "1.5")
 public actual fun String.decapitalize(): String {
+    @Suppress("DEPRECATION")
     return if (isNotEmpty() && !this[0].isLowerCase()) substring(0, 1).toLowerCase() + substring(1) else this
 }
 
diff --git a/kotlin-native/runtime/src/main/kotlin/kotlin/text/regex/sets/CharSet.kt b/kotlin-native/runtime/src/main/kotlin/kotlin/text/regex/sets/CharSet.kt
index 76dce2f6e6e..d8df165dc99 100644
--- a/kotlin-native/runtime/src/main/kotlin/kotlin/text/regex/sets/CharSet.kt
+++ b/kotlin-native/runtime/src/main/kotlin/kotlin/text/regex/sets/CharSet.kt
@@ -28,13 +28,13 @@ package kotlin.text.regex
 open internal class CharSet(char: Char, val ignoreCase: Boolean = false) : LeafSet() {
 
     // We use only low case characters when working in case insensitive mode.
-    val char: Char = if (ignoreCase) char.toLowerCase() else char
+    val char: Char = if (ignoreCase) char.lowercaseChar() else char
 
     // Overrides =======================================================================================================
 
     override fun accepts(startIndex: Int, testString: CharSequence): Int {
         if (ignoreCase) {
-            return if (this.char == testString[startIndex].toLowerCase()) 1 else -1
+            return if (this.char == testString[startIndex].lowercaseChar()) 1 else -1
         } else {
             return if (this.char == testString[startIndex]) 1 else -1
         }
diff --git a/kotlin-native/runtime/src/main/kotlin/kotlin/text/regex/sets/RangeSet.kt b/kotlin-native/runtime/src/main/kotlin/kotlin/text/regex/sets/RangeSet.kt
index 2e8a23c659a..d5029bb853c 100644
--- a/kotlin-native/runtime/src/main/kotlin/kotlin/text/regex/sets/RangeSet.kt
+++ b/kotlin-native/runtime/src/main/kotlin/kotlin/text/regex/sets/RangeSet.kt
@@ -32,7 +32,7 @@ open internal class RangeSet(charClass: AbstractCharClass, val ignoreCase: Boole
     override fun accepts(startIndex: Int, testString: CharSequence): Int {
         if (ignoreCase) {
             val char = testString[startIndex]
-            return if (chars.contains(char.toUpperCase()) || chars.contains(char.toLowerCase())) 1 else -1
+            return if (chars.contains(char.uppercaseChar()) || chars.contains(char.lowercaseChar())) 1 else -1
         } else {
             return if (chars.contains(testString[startIndex])) 1 else -1
         }
diff --git a/kotlin-native/runtime/src/main/kotlin/kotlin/text/regex/sets/SupplementaryRangeSet.kt b/kotlin-native/runtime/src/main/kotlin/kotlin/text/regex/sets/SupplementaryRangeSet.kt
index d64e32a2060..81903dcde07 100644
--- a/kotlin-native/runtime/src/main/kotlin/kotlin/text/regex/sets/SupplementaryRangeSet.kt
+++ b/kotlin-native/runtime/src/main/kotlin/kotlin/text/regex/sets/SupplementaryRangeSet.kt
@@ -122,7 +122,7 @@ open internal class SupplementaryRangeSet(charClass: AbstractCharClass, val igno
 
     fun contains(char: Char): Boolean {
         if (ignoreCase) {
-            return chars.contains(char.toUpperCase()) || chars.contains(char.toLowerCase())
+            return chars.contains(char.uppercaseChar()) || chars.contains(char.lowercaseChar())
         } else {
             return chars.contains(char)
         }