diff --git a/pycodestyle.py b/pycodestyle.py
index 9df2ae4c..dbd2afa7 100755
--- a/pycodestyle.py
+++ b/pycodestyle.py
@@ -1160,6 +1160,22 @@ def imports_on_separate_lines(logical_line):
             yield found, "E401 multiple imports on one line"
 
 
+_STRING_PREFIXES = frozenset(('u', 'U', 'b', 'B', 'r', 'R'))
+
+
+def _is_string_literal(line):
+    if line:
+        first_char = line[0]
+        if first_char in _STRING_PREFIXES:
+            first_char = line[1]
+        return first_char == '"' or first_char == "'"
+    return False
+
+
+_ALLOWED_KEYWORDS_IN_IMPORTS = (
+    'try', 'except', 'else', 'finally', 'with', 'if', 'elif')
+
+
 @register_check
 def module_imports_on_top_of_file(
         logical_line, indent_level, checker_state, noqa):
@@ -1178,15 +1194,6 @@ def module_imports_on_top_of_file(
 
     Okay: if x:\n    import os
     """  # noqa
-    def is_string_literal(line):
-        if line[0] in 'uUbB':
-            line = line[1:]
-        if line and line[0] in 'rR':
-            line = line[1:]
-        return line and (line[0] == '"' or line[0] == "'")
-
-    allowed_keywords = (
-        'try', 'except', 'else', 'finally', 'with', 'if', 'elif')
 
     if indent_level:  # Allow imports in conditional statement/function
         return
@@ -1194,25 +1201,25 @@ def is_string_literal(line):
         return
     if noqa:
         return
-    line = logical_line
-    if line.startswith('import ') or line.startswith('from '):
+    if logical_line.startswith(('import ', 'from ')):
         if checker_state.get('seen_non_imports', False):
             yield 0, "E402 module level import not at top of file"
-    elif re.match(DUNDER_REGEX, line):
-        return
-    elif any(line.startswith(kw) for kw in allowed_keywords):
-        # Allow certain keywords intermixed with imports in order to
-        # support conditional or filtered importing
-        return
-    elif is_string_literal(line):
-        # The first literal is a docstring, allow it. Otherwise, report
-        # error.
-        if checker_state.get('seen_docstring', False):
-            checker_state['seen_non_imports'] = True
+    elif not checker_state.get('seen_non_imports', False):
+        if DUNDER_REGEX.match(logical_line):
+            return
+        elif logical_line.startswith(_ALLOWED_KEYWORDS_IN_IMPORTS):
+            # Allow certain keywords intermixed with imports in order to
+            # support conditional or filtered importing
+            return
+        elif _is_string_literal(logical_line):
+            # The first literal is a docstring, allow it. Otherwise,
+            # report error.
+            if checker_state.get('seen_docstring', False):
+                checker_state['seen_non_imports'] = True
+            else:
+                checker_state['seen_docstring'] = True
         else:
-            checker_state['seen_docstring'] = True
-    else:
-        checker_state['seen_non_imports'] = True
+            checker_state['seen_non_imports'] = True
 
 
 @register_check