Revert using regex Pattern in String.replace

Use String.indexOf(..., ignoreCase) instead in all branches to preserve
compatibility with behavior before 1.4.20 that used String.split which
essentially relied on that String.indexOf

#KT-43745 Fixed

libraries/stdlib/jvm/src/kotlin/text/StringsJVM.kt

@@ -77,17 +77,7 @@ public actual fun String.replace(oldChar: Char, newChar: Char, ignoreCase: Boole
  */
 @Suppress("ACTUAL_FUNCTION_WITH_DEFAULT_ARGUMENTS")
 public actual fun String.replace(oldValue: String, newValue: String, ignoreCase: Boolean = false): String {
-    if (ignoreCase) {
+    run {
-        val matcher = Pattern.compile(oldValue, Pattern.LITERAL or Pattern.CASE_INSENSITIVE).matcher(this)
-        if (!matcher.find()) return this
-        val stringBuilder = StringBuilder()
-        var i = 0
-        do {
-            stringBuilder.append(this, i, matcher.start()).append(newValue)
-            i = matcher.end()
-        } while (matcher.find())
-        return stringBuilder.append(this, i, length).toString()
-    } else {
         var occurrenceIndex: Int = indexOf(oldValue, 0, ignoreCase)
         // FAST PATH: no match
         if (occurrenceIndex < 0) return this

libraries/stdlib/test/text/StringTest.kt

@@ -896,6 +896,22 @@ class StringTest {
 
         assertEquals("-a-b-b-A-b-", input.replace("", "-"))
         assertEquals("-a-b-b-A-b-", input.replace("", "-", ignoreCase = true))
+
+        fun testIgnoreCase(chars: String) {
+            for ((i, c) in chars.withIndex()) {
+                val message = "Char: $c (${c.toInt()})"
+                val expectOneReplaced = chars.replaceRange(i..i, "_")
+                val expectAllReplaced = "_".repeat(chars.length)
+                assertEquals(expectOneReplaced, chars.replace(c, '_'), message)
+                assertEquals(expectAllReplaced, chars.replace(c, '_', ignoreCase = true), "$message, ignoreCase")
+                assertEquals(expectOneReplaced, chars.replace(c.toString(), "_"), "$message, as string")
+                assertEquals(expectAllReplaced, chars.replace(c.toString(), "_", ignoreCase = true), "$message, as string, ignoreCase")
+            }
+        }
+
+        testIgnoreCase("üÜ")
+        testIgnoreCase("öÖ")
+        testIgnoreCase("äÄ")
     }
 
     @Test fun replaceFirst() {