diff --git a/app/build.gradle.kts b/app/build.gradle.kts
index c7ed8ad2..11715ad7 100644
--- a/app/build.gradle.kts
+++ b/app/build.gradle.kts
@@ -146,4 +146,7 @@ dependencies {
     implementation(libs.bundles.room)
     ksp(libs.androidx.room.compiler)
     detektPlugins(libs.compose.detekt)
+    
+    // Unit testing dependencies
+    testImplementation("junit:junit:4.13.2")
 }
diff --git a/app/src/main/kotlin/org/fossify/keyboard/services/SimpleKeyboardIME.kt b/app/src/main/kotlin/org/fossify/keyboard/services/SimpleKeyboardIME.kt
index 45b1cbdb..7ac46c46 100644
--- a/app/src/main/kotlin/org/fossify/keyboard/services/SimpleKeyboardIME.kt
+++ b/app/src/main/kotlin/org/fossify/keyboard/services/SimpleKeyboardIME.kt
@@ -383,18 +383,50 @@ class SimpleKeyboardIME : InputMethodService(), OnKeyboardActionListener, Shared
                             } else 0
                             if (lastIndexEmpty >= 0) {
                                 val word = fullText.subSequence(lastIndexEmpty, fullText.length).trim().toString()
-                                val wordChars = word.toCharArray()
-                                val predictWord = StringBuilder()
-                                for (char in wordChars.size - 1 downTo 0) {
-                                    predictWord.append(wordChars[char])
-                                    val shouldChangeText = predictWord.reverse().toString()
-                                    if (cachedVNTelexData.containsKey(shouldChangeText)) {
-                                        inputConnection.setComposingRegion(fullText.length - shouldChangeText.length, fullText.length)
-                                        inputConnection.setComposingText(cachedVNTelexData[shouldChangeText], fullText.length)
+                                
+                                // Check for escape sequence FIRST (before single-char transformations)
+                                // Only applies when: 1) last two chars are same,
+                                // 2) no rule for doubled sequence, 3) rule exists for single char
+                                if (word.length >= 2) {
+                                    val lastTwo = word.takeLast(2)
+                                    if (lastTwo[0] == lastTwo[1]) {
+                                        val doubledSeq = lastTwo.lowercase()
+                                        val singleChar = lastTwo[0].toString().lowercase()
+                                        // If there's NO rule for the doubled sequence,
+                                        // but there IS a rule for single char, it's an escape
+                                        if (!cachedVNTelexData.containsKey(doubledSeq) && cachedVNTelexData.containsKey(singleChar)) {
+                                            // This is an escape sequence - delete last char to keep just one
+                                            inputConnection.deleteSurroundingText(1, 0)
+                                            return
+                                        }
+                                    }
+                                }
+                                
+                                // Then check for transformation rules (longest patterns first)
+                                for (i in word.indices) {
+                                    val partialWord = word.substring(i, word.length)
+                                    val partialWordLower = partialWord.lowercase()
+                                    if (cachedVNTelexData.containsKey(partialWordLower)) {
+                                        val replacement = cachedVNTelexData[partialWordLower]!!
+                                        // Preserve case: if first char is uppercase, capitalize replacement
+                                        val finalReplacement = if (
+                                            partialWord.firstOrNull()?.isUpperCase() == true &&
+                                            replacement.isNotEmpty()
+                                        ) {
+                                            replacement.replaceFirstChar { it.uppercase() }
+                                        } else {
+                                            replacement
+                                        }
+                                        inputConnection.setComposingRegion(
+                                            fullText.length - partialWordLower.length,
+                                            fullText.length
+                                        )
+                                        inputConnection.setComposingText(finalReplacement, fullText.length)
                                         inputConnection.setComposingRegion(fullText.length, fullText.length)
                                         return
                                     }
                                 }
+                                
                                 inputConnection.commitText(codeChar.toString(), 1)
                                 updateShiftKeyState()
                             }
diff --git a/app/src/main/res/xml/keys_letters_english_qwerty.xml b/app/src/main/res/xml/keys_letters_english_qwerty.xml
index e6fdf53d..2690dd16 100644
--- a/app/src/main/res/xml/keys_letters_english_qwerty.xml
+++ b/app/src/main/res/xml/keys_letters_english_qwerty.xml
@@ -43,7 +43,7 @@
             app:topSmallNumber="1" />
         <Key
             app:keyLabel="w"
-            app:popupCharacters="2"
+            app:popupCharacters="2ư"
             app:popupKeyboard="@xml/keyboard_popup_template"
             app:topSmallNumber="2" />
         <Key
@@ -78,7 +78,7 @@
             app:topSmallNumber="8" />
         <Key
             app:keyLabel="o"
-            app:popupCharacters="őöøóôòõ9ō"
+            app:popupCharacters="őöøóôòõ9ōơ"
             app:popupKeyboard="@xml/keyboard_popup_template"
             app:topSmallNumber="9" />
         <Key
@@ -93,7 +93,7 @@
             app:horizontalGap="5%"
             app:keyEdgeFlags="left"
             app:keyLabel="a"
-            app:popupCharacters="áàâãäåāæą"
+            app:popupCharacters="áàâãäåāæąă"
             app:popupKeyboard="@xml/keyboard_popup_template" />
         <Key
             app:keyLabel="s"
diff --git a/app/src/test/kotlin/org/fossify/keyboard/services/VietnameseTelexTest.kt b/app/src/test/kotlin/org/fossify/keyboard/services/VietnameseTelexTest.kt
new file mode 100644
index 00000000..362d23cc
--- /dev/null
+++ b/app/src/test/kotlin/org/fossify/keyboard/services/VietnameseTelexTest.kt
@@ -0,0 +1,284 @@
+package org.fossify.keyboard.services
+
+import org.junit.Assert.assertEquals
+import org.junit.Before
+import org.junit.Test
+
+class VietnameseTelexTest {
+
+    private lateinit var telexRules: HashMap<String, String>
+
+    @Before
+    fun setup() {
+        telexRules = hashMapOf(
+            "w" to "ư",
+            "a" to "ă",
+            "aw" to "ă",
+            "aa" to "â",
+            "dd" to "đ",
+            "ee" to "ê",
+            "oo" to "ô",
+            "ow" to "ơ",
+            "uw" to "ư",
+            "uow" to "ươ"
+        )
+    }
+
+    @Test
+    fun testShortestMatchFirst() {
+        val input = "aw"
+        val result = applyRulesShortestFirst(input)
+        assertEquals("aư", result)
+    }
+
+    @Test
+    fun testLongestMatchFirst() {
+        val input = "aw"
+        val result = applyRulesLongestFirst(input)
+        assertEquals("ă", result)
+    }
+
+    @Test
+    fun testDoubleA() {
+        val input = "aa"
+        val result = applyRulesLongestFirst(input)
+        assertEquals("â", result)
+    }
+
+    @Test
+    fun testTripleCharacter() {
+        val input = "uow"
+        val result = applyRulesLongestFirst(input)
+        assertEquals("ươ", result)
+    }
+
+    @Test
+    fun testNoTransformation() {
+        val input = "b"
+        val result = applyRulesLongestFirst(input)
+        assertEquals("b", result)
+    }
+
+    @Test
+    fun testDoubleWTransformation() {
+        val input = "ww"
+        val shortestFirstResult = applyRulesShortestFirst(input)
+        assertEquals("wư", shortestFirstResult)
+        
+        // With escape logic, "ww" should produce "w" (escape sequence)
+        val longestFirstResult = applyRulesLongestFirst(input)
+        assertEquals("w", longestFirstResult)
+    }
+
+    @Test
+    fun testDoubleDTransformation() {
+        val input = "dd"
+        val result = applyRulesLongestFirst(input)
+        assertEquals("đ", result)
+    }
+
+    @Test
+    fun testMixedCaseNotMatching() {
+        val input = "Ee"
+        val result = applyRulesCaseSensitive(input)
+        assertEquals("Ee", result)
+    }
+
+    @Test
+    fun testAllUppercaseNotMatching() {
+        val input = "EE"
+        val result = applyRulesCaseSensitive(input)
+        assertEquals("EE", result)
+    }
+
+    @Test
+    fun testPatternPrecedence() {
+        val input = "aw"
+        
+        val shortestFirstResult = applyRulesShortestFirst(input)
+        assertEquals("aư", shortestFirstResult)
+        
+        val longestFirstResult = applyRulesLongestFirst(input)
+        assertEquals("ă", longestFirstResult)
+    }
+
+    @Test
+    fun testSingleW() {
+        val input = "w"
+        val result = applyRulesLongestFirst(input)
+        assertEquals("ư", result)
+    }
+
+    @Test
+    fun testTripleW() {
+        val input = "www"
+        val result = applyRulesLongestFirst(input)
+        // "www" → lastTwo "ww" escape to "w", result is "ww"
+        assertEquals("ww", result)
+    }
+
+    @Test
+    fun testSpaceDoubleW() {
+        val input = "O ww"
+        val result = applyRulesLongestFirst(input)
+        // "ww" after space escapes to "w"
+        assertEquals("O w", result)
+    }
+
+    @Test
+    fun testUppercaseCasePreservation_Aw() {
+        val input = "Aw"
+        val result = applyRulesWithCasePreservation(input)
+        assertEquals("Ă", result)
+    }
+
+    @Test
+    fun testLowercaseCasePreservation_aw() {
+        val input = "aw"
+        val result = applyRulesWithCasePreservation(input)
+        assertEquals("ă", result)
+    }
+
+    @Test
+    fun testUppercaseCasePreservation_Ow() {
+        val input = "Ow"
+        val result = applyRulesWithCasePreservation(input)
+        assertEquals("Ơ", result)
+    }
+
+    @Test
+    fun testLowercaseCasePreservation_ow() {
+        val input = "ow"
+        val result = applyRulesWithCasePreservation(input)
+        assertEquals("ơ", result)
+    }
+
+    @Test
+    fun testEscapeSequence_ww() {
+        // Typing "ww" should produce "w" (escape transformation)
+        val result = applyRulesLongestFirst("ww")
+        assertEquals("Double 'w' should escape to literal 'w'", "w", result)
+    }
+
+    @Test
+    fun testEscapeAfterTransformation() {
+        // "ưw" is not a doubled character (last two are 'ư' and 'w'), so "w" transforms normally to "ư"
+        // This gives us "ư" (prefix) + "ư" (transformation of "w") = "ưư"
+        val result = applyRulesLongestFirst("ưw")
+        assertEquals("ưw should transform the 'w' to 'ư', giving 'ưư'", "ưư", result)
+    }
+
+    /**
+     * Helper function that applies transformation rules checking shortest patterns first.
+     * This demonstrates incorrect behavior when shorter patterns match before longer ones.
+     */
+    private fun applyRulesShortestFirst(word: String): String {
+        val wordChars = word.toCharArray()
+        val predictWord = StringBuilder()
+        
+        for (char in wordChars.size - 1 downTo 0) {
+            predictWord.append(wordChars[char])
+            val shouldChangeText = predictWord.reverse().toString()
+            val shouldChangeTextLower = shouldChangeText.lowercase()
+            
+            if (telexRules.containsKey(shouldChangeTextLower)) {
+                val prefix = word.substring(0, word.length - shouldChangeText.length)
+                return prefix + telexRules[shouldChangeTextLower]
+            }
+            
+            predictWord.reverse()
+        }
+        
+        return word
+    }
+
+    /**
+     * Helper function that applies transformation rules checking longest patterns first.
+     * This demonstrates correct behavior where longer patterns take precedence.
+     */
+    private fun applyRulesLongestFirst(word: String): String {
+        // Check for escape sequence FIRST (before single-char transformations)
+        // Only applies when: 1) last two chars are same,
+        // 2) no rule for doubled sequence, 3) rule exists for single char
+        if (word.length >= 2) {
+            val lastTwo = word.takeLast(2)
+            if (lastTwo[0] == lastTwo[1]) {
+                val doubledSeq = lastTwo.lowercase()
+                val singleChar = lastTwo[0].toString().lowercase()
+                // If there's NO rule for the doubled sequence, but there IS a rule for single char, it's an escape
+                if (!telexRules.containsKey(doubledSeq) && telexRules.containsKey(singleChar)) {
+                    // This is an escape sequence - return word with just one of the doubled char
+                    return word.substring(0, word.length - 1)
+                }
+            }
+        }
+        
+        // Then check for transformation rules (longest patterns first)
+        for (length in word.length downTo 1) {
+            val suffix = word.substring(word.length - length)
+            val suffixLower = suffix.lowercase()
+            if (telexRules.containsKey(suffixLower)) {
+                val prefix = word.substring(0, word.length - length)
+                return prefix + telexRules[suffixLower]!!
+            }
+        }
+        
+        return word
+    }
+
+    /**
+     * Helper function that applies transformation rules with case preservation.
+     * Matches case-insensitively but preserves the original case in output.
+     */
+    private fun applyRulesWithCasePreservation(word: String): String {
+        // Check for escape sequence FIRST (before single-char transformations)
+        if (word.length >= 2) {
+            val lastTwo = word.takeLast(2)
+            if (lastTwo[0] == lastTwo[1]) {
+                val doubledSeq = lastTwo.lowercase()
+                val singleChar = lastTwo[0].toString().lowercase()
+                // If there's NO rule for the doubled sequence, but there IS a rule for single char, it's an escape
+                if (!telexRules.containsKey(doubledSeq) && telexRules.containsKey(singleChar)) {
+                    // This is an escape sequence - return word with just one of the doubled char
+                    return word.substring(0, word.length - 1)
+                }
+            }
+        }
+        
+        // Then check for transformation rules (longest patterns first)
+        for (length in word.length downTo 1) {
+            val suffix = word.substring(word.length - length)
+            val suffixLower = suffix.lowercase()
+            if (telexRules.containsKey(suffixLower)) {
+                val prefix = word.substring(0, word.length - length)
+                val replacement = telexRules[suffixLower]!!
+                // Preserve case: if first char is uppercase, capitalize replacement
+                val finalReplacement = if (suffix.firstOrNull()?.isUpperCase() == true && replacement.isNotEmpty()) {
+                    replacement.replaceFirstChar { it.uppercase() }
+                } else {
+                    replacement
+                }
+                return prefix + finalReplacement
+            }
+        }
+        
+        return word
+    }
+
+    /**
+     * Helper function that applies rules with case-sensitive matching.
+     * Mixed-case input won't match lowercase rules.
+     */
+    private fun applyRulesCaseSensitive(word: String): String {
+        for (length in word.length downTo 1) {
+            val suffix = word.substring(word.length - length)
+            if (telexRules.containsKey(suffix)) {
+                val prefix = word.substring(0, word.length - length)
+                return prefix + telexRules[suffix]!!
+            }
+        }
+        
+        return word
+    }
+
+}