diff --git a/parso/python/diff.py b/parso/python/diff.py
index 25687fd..41f1794 100644
--- a/parso/python/diff.py
+++ b/parso/python/diff.py
@@ -388,7 +388,7 @@ class DiffParser(object):
                     # endmarker or another dedented code block.
                     typ, string, start_pos, prefix = next(tokens)
                     if '\n' in prefix or '\r' in prefix:
-                        prefix = re.sub(r'(<=\n|\r)[^\n\r]+$', '', prefix)
+                        prefix = re.sub(r'(?<=\n|\r)[^\n\r]+$', '', prefix)
                     else:
                         prefix = ''
                     yield PythonToken(
diff --git a/test/test_diff_parser.py b/test/test_diff_parser.py
index 4e80bd2..5946f8d 100644
--- a/test/test_diff_parser.py
+++ b/test/test_diff_parser.py
@@ -956,6 +956,9 @@ def test_wrong_backslash(differ):
 
 
 def test_random_unicode_characters(differ):
+    """
+    Those issues were all found with the fuzzer.
+    """
     differ.initialize('')
     differ.parse('\x1dĔBϞɛˁşʑ˳˻ȣſéÎ\x90̕ȟòwʘ\x1dĔBϞɛˁşʑ˳˻ȣſéÎ', parsers=1, expect_error_leaves=True)
     differ.parse('\r\r', parsers=1)
@@ -965,4 +968,5 @@ def test_random_unicode_characters(differ):
     differ.parse(s, parsers=1, expect_error_leaves=True)
     differ.parse('')
     differ.parse(s + '\n', parsers=1, expect_error_leaves=True)
+    differ.parse('   result = (\r\f\x17\t\x11res)', parsers=2, expect_error_leaves=True)
     differ.parse('')