regexp: lookbehind error highlighting for python

python/src/com/jetbrains/python/psi/impl/PyStringLiteralExpressionImpl.java

@@ -463,6 +463,11 @@ public class PyStringLiteralExpressionImpl extends PyElementImpl implements PySt
     return false;
   }
 
+  @Override
+  public Lookbehind supportsLookbehind(@NotNull RegExpGroup lookbehindGroup) {
+    return Lookbehind.FIXED_LENGTH_ALTERNATION;
+  }
+
   @Override
   public Long getQuantifierValue(@NotNull RegExpNumber number) {
     try {

python/testData/regexp/lookbehind.py

@@ -0,0 +1,7 @@
+import re
+
+re.compile(r"(?<!a|b)");
+re.compile(r"(?<!<error descr="Alternation alternatives needs to have the same length inside lookbehind">a|bc</error>)");
+re.compile(r"(?<!a{3}})");
+re.compile(r"(?<!a<weak_warning descr="Fixed repetition range">{3,3}</weak_warning>})");
+re.compile(r"(?<!a<error descr="Unequal min and max in counted quantifier not allowed inside lookbehind">{3,4}</error>})");

python/testSrc/com/jetbrains/python/PyRegexpTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2000-2016 JetBrains s.r.o.
+ * Copyright 2000-2017 JetBrains s.r.o.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -39,6 +39,10 @@ import java.util.List;
  */
 public class PyRegexpTest extends PyTestCase {
 
+  public void testLookbehind() {
+    doTestHighlighting();
+  }
+
   public void testCountedQuantifier() {
     doTestHighlighting();
   }