diff --git a/python/src/com/jetbrains/python/psi/impl/PyStringLiteralExpressionImpl.java b/python/src/com/jetbrains/python/psi/impl/PyStringLiteralExpressionImpl.java
index ea57e55c3142..26ab8638e783 100644
--- a/python/src/com/jetbrains/python/psi/impl/PyStringLiteralExpressionImpl.java
+++ b/python/src/com/jetbrains/python/psi/impl/PyStringLiteralExpressionImpl.java
@@ -351,7 +351,7 @@ public class PyStringLiteralExpressionImpl extends PyElementImpl implements PySt
         if (intersection != null && !intersection.isEmpty()) {
           final String value = fragment.getSecond();
           final String intersectedValue;
-          if (value.length() == 1 || value.length() == intersection.getLength()) {
+          if (value.codePointCount(0, value.length()) == 1 || value.length() == intersection.getLength()) {
             intersectedValue = value;
           }
           else {
@@ -367,7 +367,7 @@ public class PyStringLiteralExpressionImpl extends PyElementImpl implements PySt
 
     @Override
     public int getOffsetInHost(final int offsetInDecoded, @NotNull final TextRange rangeInsideHost) {
-      int offset = 0;
+      int offset = 0; // running offset in the decoded fragment
       int endOffset = -1;
       for (Pair<TextRange, String> fragment : myHost.getDecodedFragments()) {
         final TextRange encodedTextRange = fragment.getFirst();
@@ -379,13 +379,15 @@ public class PyStringLiteralExpressionImpl extends PyElementImpl implements PySt
           if (valueLength == 0) {
             return -1;
           }
-          else if (valueLength == 1) {
+          // A long unicode escape of form \U01234567 can be decoded into a surrogate pair
+          else if (value.codePointCount(0, valueLength) == 1) {
             if (offset == offsetInDecoded) {
               return intersection.getStartOffset();
             }
-            offset++;
+            offset += valueLength;
           }
           else {
+            // Literal fragment without escapes: it's safe to use intersection length instead of value length
             if (offset + intersectionLength >= offsetInDecoded) {
               final int delta = offsetInDecoded - offset;
               return intersection.getStartOffset() + delta;
diff --git a/python/testSrc/com/jetbrains/python/PyEditingTest.java b/python/testSrc/com/jetbrains/python/PyEditingTest.java
index 8c9ca82bed74..721f1f099e93 100644
--- a/python/testSrc/com/jetbrains/python/PyEditingTest.java
+++ b/python/testSrc/com/jetbrains/python/PyEditingTest.java
@@ -472,6 +472,16 @@ public class PyEditingTest extends PyTestCase {
                 ")");
   }
 
+  // PY-21840
+  public void testEditInjectedRegexpFragmentWithLongUnicodeEscape() {
+    myFixture.configureByText(PythonFileType.INSTANCE,
+                              "import re\n" +
+                              "re.compile(ur'\\U00010000<caret>')");
+    doTyping("t");
+    myFixture.checkResult("import re\n" +
+                          "re.compile(ur'\\U00010000t')");
+  }
+
   private String doTestTyping(final String text, final int offset, final char character) {
     final PsiFile file = WriteCommandAction.runWriteCommandAction(null, new Computable<PsiFile>() {
       @Override
diff --git a/python/testSrc/com/jetbrains/python/PyStringLiteralTest.java b/python/testSrc/com/jetbrains/python/PyStringLiteralTest.java
index 4efa8cf2a9f7..90b489535d43 100644
--- a/python/testSrc/com/jetbrains/python/PyStringLiteralTest.java
+++ b/python/testSrc/com/jetbrains/python/PyStringLiteralTest.java
@@ -115,6 +115,19 @@ public class PyStringLiteralTest extends PyTestCase {
     assertEquals(-1, escaper.getOffsetInHost(9, range));
   }
 
+  public void testEscaperOffsetInLongUnicodeEscape() {
+    final PyStringLiteralExpression expr = createLiteralFromText("u'XXX a\\U0001F600\\U0001F600b YYY'");
+    final LiteralTextEscaper<? extends PsiLanguageInjectionHost> escaper = expr.createLiteralTextEscaper();
+    final TextRange range = TextRange.create(6, 28);
+    assertEquals(6, escaper.getOffsetInHost(0, range));
+    assertEquals(7, escaper.getOffsetInHost(1, range));
+    // Each \\U0001F600 is represented as a surrogate pair, hence 2 characters-wide step in decoded text
+    assertEquals(17, escaper.getOffsetInHost(3, range)); 
+    assertEquals(27, escaper.getOffsetInHost(5, range));
+    assertEquals(28, escaper.getOffsetInHost(6, range));
+    assertEquals(-1, escaper.getOffsetInHost(7, range));
+  }
+
   public void testStringValue() {
     assertEquals("foo", createLiteralFromText("\"\"\"foo\"\"\"").getStringValue());
     assertEquals("foo", createLiteralFromText("u\"foo\"").getStringValue());