diff --git a/selfie-lib/src/commonTest/kotlin/com/diffplug/selfie/PerCharacterEscaperTest.kt b/selfie-lib/src/commonTest/kotlin/com/diffplug/selfie/PerCharacterEscaperTest.kt
new file mode 100644
index 00000000..373cbac9
--- /dev/null
+++ b/selfie-lib/src/commonTest/kotlin/com/diffplug/selfie/PerCharacterEscaperTest.kt
@@ -0,0 +1,91 @@
+/*
+ * Copyright (C) 2016-2023 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.selfie
+
+import com.diffplug.selfie.PerCharacterEscaper.Companion.selfEscape
+import com.diffplug.selfie.PerCharacterEscaper.Companion.specifiedEscape
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFails
+import kotlin.test.assertSame
+
+class PerCharacterEscaperTest {
+  @Test
+  fun performanceOptimizationSelf() {
+    val escaper = selfEscape("`123")
+    // if nothing gets changed, it should return the exact same value
+    val abc = "abc"
+    assertSame(abc, escaper.escape(abc))
+    assertSame(abc, escaper.unescape(abc))
+
+    // otherwise it should have the normal behavior
+    assertEquals("`1", escaper.escape("1"))
+    assertEquals("``", escaper.escape("`"))
+    assertEquals("abc`1`2`3``def", escaper.escape("abc123`def"))
+
+    // in both directions
+    assertEquals("1", escaper.unescape("`1"))
+    assertEquals("`", escaper.unescape("``"))
+    assertEquals("abc123`def", escaper.unescape("abc`1`2`3``def"))
+  }
+
+  @Test
+  fun performanceOptimizationSpecific() {
+    val escaper = specifiedEscape("`a1b2c3d")
+    // if nothing gets changed, it should return the exact same value
+    val abc = "abc"
+    assertSame(abc, escaper.escape(abc))
+    assertSame(abc, escaper.unescape(abc))
+
+    // otherwise it should have the normal behavior
+    assertEquals("`b", escaper.escape("1"))
+    assertEquals("`a", escaper.escape("`"))
+    assertEquals("abc`b`c`d`adef", escaper.escape("abc123`def"))
+
+    // in both directions
+    assertEquals("1", escaper.unescape("`b"))
+    assertEquals("`", escaper.unescape("`a"))
+    assertEquals("abc123`def", escaper.unescape("abc`1`2`3``def"))
+  }
+
+  @Test
+  fun cornerCasesSelf() {
+    val escaper = selfEscape("`123")
+    // cornercase - escape character without follow-on will throw an error
+    val exception = assertFails { escaper.unescape("`") }
+    assertEquals("Escape character '`' can't be the last character in a string.", exception.message)
+    // escape character followed by non-escape character is fine
+    assertEquals("a", escaper.unescape("`a"))
+  }
+
+  @Test
+  fun cornerCasesSpecific() {
+    val escaper = specifiedEscape("`a1b2c3d")
+    // cornercase - escape character without follow-on will throw an error
+    val exception = assertFails { escaper.unescape("`") }
+    assertEquals("Escape character '`' can't be the last character in a string.", exception.message)
+    // escape character followed by non-escape character is fine
+    assertEquals("e", escaper.unescape("`e"))
+  }
+
+  @Test
+  fun roundtrip() {
+    val escaper = selfEscape("`<>")
+    val roundtrip = { str: String? -> assertEquals(str, escaper.unescape(escaper.escape(str!!))) }
+    roundtrip("")
+    roundtrip("<local>~`/")
+  }
+}
diff --git a/selfie-lib/src/jsMain/kotlin/com/diffplug/selfie/PerCharacterEscaper.js.kt b/selfie-lib/src/jsMain/kotlin/com/diffplug/selfie/PerCharacterEscaper.js.kt
index e7aa74c2..bf1bce1b 100644
--- a/selfie-lib/src/jsMain/kotlin/com/diffplug/selfie/PerCharacterEscaper.js.kt
+++ b/selfie-lib/src/jsMain/kotlin/com/diffplug/selfie/PerCharacterEscaper.js.kt
@@ -15,12 +15,212 @@
  */
 package com.diffplug.selfie
 
-actual class PerCharacterEscaper {
-  actual fun escape(input: String): String = TODO()
-  actual fun unescape(input: String): String = TODO()
+/**
+ * If your escape policy is "'123", it means this:
+ * ```
+ * abc->abc
+ * 123->'1'2'3
+ * I won't->I won''t
+ * ```
+ */
+actual class PerCharacterEscaper
+/**
+ * The first character in the string will be uses as the escape character, and all characters will
+ * be escaped.
+ */
+private constructor(
+    private val escapeCodePoint: Int,
+    private val escapedCodePoints: IntArray,
+    private val escapedByCodePoints: IntArray
+) {
+  val MIN_SUPPLEMENTARY_CODE_POINT = 0x010000
+  val MAX_CODE_POINT = 0X10FFFF
+  val MIN_LOW_SURROGATE = '\uDC00'
+  val MIN_HIGH_SURROGATE = '\uD800'
+  private fun highSurrogate(codePoint: Int): Char {
+    return ((codePoint ushr 10) +
+            (MIN_HIGH_SURROGATE - (MIN_SUPPLEMENTARY_CODE_POINT ushr 10)).code)
+        .toChar()
+  }
+  private fun lowSurrogate(codePoint: Int): Char {
+    return ((codePoint and 0x3ff) + MIN_LOW_SURROGATE.code).toChar()
+  }
+  private fun toSurrogates(codePoint: Int, dst: CharArray, index: Int) {
+    // We write elements "backwards" to guarantee all-or-nothing
+    dst[index + 1] = lowSurrogate(codePoint)
+    dst[index] = highSurrogate(codePoint)
+  }
+  private fun toChars(codePoint: Int): CharArray {
+    return if (isBmpCodePoint(codePoint)) {
+      charArrayOf(codePoint.toChar())
+    } else if (isValidCodePoint(codePoint)) {
+      val result = CharArray(2)
+      toSurrogates(codePoint, result, 0)
+      result
+    } else {
+      throw IllegalArgumentException("Not a valid Unicode code point: $codePoint")
+    }
+  }
+  private fun isBmpCodePoint(codePoint: Int): Boolean {
+    return codePoint ushr 16 == 0
+  }
+  private fun isValidCodePoint(codePoint: Int): Boolean {
+    // Optimized form of:
+    //     codePoint >= MIN_CODE_POINT && codePoint <= MAX_CODE_POINT
+    val plane = codePoint ushr 16
+    return plane < MAX_CODE_POINT + 1 ushr 16
+  }
+  private fun charCount(codePoint: Int): Int {
+    return if (codePoint >= MIN_SUPPLEMENTARY_CODE_POINT) 2 else 1
+  }
+  private fun firstOffsetNeedingEscape(input: String): Int {
+    val length = input.length
+    var firstOffsetNeedingEscape = -1
+    var offset = 0
+    outer@ while (offset < length) {
+      val codepoint = codePointAt(input, offset)
+      for (escaped in escapedCodePoints) {
+        if (codepoint == escaped) {
+          firstOffsetNeedingEscape = offset
+          break@outer
+        }
+      }
+      offset += charCount(codepoint)
+    }
+    return firstOffsetNeedingEscape
+  }
+  actual fun escape(input: String): String {
+    val noEscapes = firstOffsetNeedingEscape(input)
+    return if (noEscapes == -1) {
+      input
+    } else {
+      val length = input.length
+      val needsEscapes = length - noEscapes
+      val builder = StringBuilder(noEscapes + 4 + needsEscapes * 5 / 4)
+      builder.append(input, 0, noEscapes)
+      var offset = noEscapes
+      while (offset < length) {
+        val codepoint = codePointAt(input, offset)
+        offset += charCount(codepoint)
+        val idx = indexOf(escapedCodePoints, codepoint)
+        if (idx == -1) {
+          builder.append(toChars(codepoint))
+        } else {
+          builder.append(toChars(escapeCodePoint))
+          builder.append(toChars(escapedByCodePoints[idx]))
+        }
+      }
+      builder.toString()
+    }
+  }
+  private fun firstOffsetNeedingUnescape(input: String): Int {
+    val length = input.length
+    var firstOffsetNeedingEscape = -1
+    var offset = 0
+    while (offset < length) {
+      val codepoint = codePointAt(input, offset)
+      if (codepoint == escapeCodePoint) {
+        firstOffsetNeedingEscape = offset
+        break
+      }
+      offset += charCount(codepoint)
+    }
+    return firstOffsetNeedingEscape
+  }
+  actual fun unescape(input: String): String {
+    val noEscapes = firstOffsetNeedingUnescape(input)
+    return if (noEscapes == -1) {
+      input
+    } else {
+      val length = input.length
+      val needsEscapes = length - noEscapes
+      val builder = StringBuilder(noEscapes + 4 + needsEscapes * 5 / 4)
+      builder.append(input, 0, noEscapes)
+      var offset = noEscapes
+      while (offset < length) {
+        var codepoint = codePointAt(input, offset)
+        offset += charCount(codepoint)
+        // if we need to escape something, escape it
+        if (codepoint == escapeCodePoint) {
+          if (offset < length) {
+            codepoint = codePointAt(input, offset)
+            val idx = indexOf(escapedByCodePoints, codepoint)
+            if (idx != -1) {
+              codepoint = escapedCodePoints[idx]
+            }
+            offset += charCount(codepoint)
+          } else {
+            throw IllegalArgumentException(
+                "Escape character '" +
+                    toChars(escapeCodePoint).concatToString(0, 0 + 1) +
+                    "' can't be the last character in a string.")
+          }
+        }
+        // we didn't escape it, append it raw
+        builder.append(toChars(codepoint))
+      }
+      builder.toString()
+    }
+  }
 
   actual companion object {
-    actual fun selfEscape(escapePolicy: String): PerCharacterEscaper = TODO()
-    actual fun specifiedEscape(escapePolicy: String): PerCharacterEscaper = TODO()
+    private fun indexOf(arr: IntArray, target: Int): Int {
+      for ((index, value) in arr.withIndex()) {
+        if (value == target) {
+          return index
+        }
+      }
+      return -1
+    }
+    private fun codePointAt(value: String, offset: Int): Int {
+      val codePoint = js("value.codePointAt(offset)")
+      return codePoint
+    }
+    private fun codePoints(value: String): IntArray {
+      val result = mutableListOf<Int>()
+      var offset = 0
+      while (offset < value.length) {
+        val codepoint = codePointAt(value, offset)
+        result.add(codepoint)
+        offset += 1
+      }
+
+      return result.toIntArray()
+    }
+
+    /**
+     * If your escape policy is "'123", it means this:
+     * ```
+     * abc->abc
+     * 123->'1'2'3
+     * I won't->I won''t
+     * ```
+     */
+    actual fun selfEscape(escapePolicy: String): PerCharacterEscaper {
+      val escapedCodePoints = codePoints(escapePolicy)
+      val escapeCodePoint = escapedCodePoints[0]
+      return PerCharacterEscaper(escapeCodePoint, escapedCodePoints, escapedCodePoints)
+    }
+
+    /**
+     * If your escape policy is "'a1b2c3d", it means this:
+     * ```
+     * abc->abc
+     * 123->'b'c'd
+     * I won't->I won'at
+     * ```
+     */
+    actual fun specifiedEscape(escapePolicy: String): PerCharacterEscaper {
+      val codePoints = codePoints(escapePolicy)
+      require(codePoints.size % 2 == 0)
+      val escapeCodePoint = codePoints[0]
+      val escapedCodePoints = IntArray(codePoints.size / 2)
+      val escapedByCodePoints = IntArray(codePoints.size / 2)
+      for (i in escapedCodePoints.indices) {
+        escapedCodePoints[i] = codePoints[2 * i]
+        escapedByCodePoints[i] = codePoints[2 * i + 1]
+      }
+      return PerCharacterEscaper(escapeCodePoint, escapedCodePoints, escapedByCodePoints)
+    }
   }
 }
diff --git a/selfie-lib/src/jvmTest/kotlin/com/diffplug/selfie/PerCharacterEscaperTest.kt b/selfie-lib/src/jvmTest/kotlin/com/diffplug/selfie/PerCharacterEscaperTest.kt
deleted file mode 100644
index ff2f39db..00000000
--- a/selfie-lib/src/jvmTest/kotlin/com/diffplug/selfie/PerCharacterEscaperTest.kt
+++ /dev/null
@@ -1,100 +0,0 @@
-/*
- * Copyright (C) 2016-2023 DiffPlug
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     https://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-package com.diffplug.selfie
-
-import com.diffplug.selfie.PerCharacterEscaper.Companion.selfEscape
-import com.diffplug.selfie.PerCharacterEscaper.Companion.specifiedEscape
-import java.util.function.Consumer
-import org.junit.jupiter.api.Assertions
-import org.junit.jupiter.api.Test
-
-class PerCharacterEscaperTest {
-  @Test
-  fun performanceOptimizationSelf() {
-    val escaper = selfEscape("`123")
-    // if nothing gets changed, it should return the exact same value
-    val abc = "abc"
-    Assertions.assertSame(abc, escaper.escape(abc))
-    Assertions.assertSame(abc, escaper.unescape(abc))
-
-    // otherwise it should have the normal behavior
-    Assertions.assertEquals("`1", escaper.escape("1"))
-    Assertions.assertEquals("``", escaper.escape("`"))
-    Assertions.assertEquals("abc`1`2`3``def", escaper.escape("abc123`def"))
-
-    // in both directions
-    Assertions.assertEquals("1", escaper.unescape("`1"))
-    Assertions.assertEquals("`", escaper.unescape("``"))
-    Assertions.assertEquals("abc123`def", escaper.unescape("abc`1`2`3``def"))
-  }
-
-  @Test
-  fun performanceOptimizationSpecific() {
-    val escaper = specifiedEscape("`a1b2c3d")
-    // if nothing gets changed, it should return the exact same value
-    val abc = "abc"
-    Assertions.assertSame(abc, escaper.escape(abc))
-    Assertions.assertSame(abc, escaper.unescape(abc))
-
-    // otherwise it should have the normal behavior
-    Assertions.assertEquals("`b", escaper.escape("1"))
-    Assertions.assertEquals("`a", escaper.escape("`"))
-    Assertions.assertEquals("abc`b`c`d`adef", escaper.escape("abc123`def"))
-
-    // in both directions
-    Assertions.assertEquals("1", escaper.unescape("`b"))
-    Assertions.assertEquals("`", escaper.unescape("`a"))
-    Assertions.assertEquals("abc123`def", escaper.unescape("abc`1`2`3``def"))
-  }
-
-  @Test
-  fun cornerCasesSelf() {
-    val escaper = selfEscape("`123")
-    // cornercase - escape character without follow-on will throw an error
-    val exception =
-        Assertions.assertThrows(java.lang.IllegalArgumentException::class.java) {
-          escaper.unescape("`")
-        }
-    Assertions.assertEquals(
-        "Escape character '`' can't be the last character in a string.", exception.message)
-    // escape character followed by non-escape character is fine
-    Assertions.assertEquals("a", escaper.unescape("`a"))
-  }
-
-  @Test
-  fun cornerCasesSpecific() {
-    val escaper = specifiedEscape("`a1b2c3d")
-    // cornercase - escape character without follow-on will throw an error
-    val exception =
-        Assertions.assertThrows(java.lang.IllegalArgumentException::class.java) {
-          escaper.unescape("`")
-        }
-    Assertions.assertEquals(
-        "Escape character '`' can't be the last character in a string.", exception.message)
-    // escape character followed by non-escape character is fine
-    Assertions.assertEquals("e", escaper.unescape("`e"))
-  }
-
-  @Test
-  fun roundtrip() {
-    val escaper = selfEscape("`<>")
-    val roundtrip = Consumer { str: String? ->
-      Assertions.assertEquals(str, escaper.unescape(escaper.escape(str!!)))
-    }
-    roundtrip.accept("")
-    roundtrip.accept("<local>~`/")
-  }
-}