diff -r 56cc9de5c024 Lib/idlelib/AutoComplete.py
--- a/Lib/idlelib/AutoComplete.py	Thu May 31 22:41:51 2012 +0200
+++ b/Lib/idlelib/AutoComplete.py	Sat Jun 02 12:48:40 2012 -0700
@@ -12,6 +12,13 @@
 # This string includes all chars that may be in a file name (without a path
 # separator)
 FILENAME_CHARS = string.ascii_letters + string.digits + os.curdir + "._~#$:-"
+# This string includes all chars not allowed in a filename
+if os.name == 'posix':
+    INVALID_CHARS = '\0/'
+elif os.name == 'mac':
+    INVALID_CHARS = '\0:'
+else:
+    INVALID_CHARS = '\0\\/:*?"<>|'
 # This string includes all chars that may be in an identifier
 ID_CHARS = string.ascii_letters + string.digits + "_"
 
@@ -126,11 +133,11 @@
         if hp.is_in_string() and (not mode or mode==COMPLETE_FILES):
             self._remove_autocomplete_window()
             mode = COMPLETE_FILES
-            while i and curline[i-1] in FILENAME_CHARS:
+            while i and curline[i-1] not in INVALID_CHARS:
                 i -= 1
             comp_start = curline[i:j]
             j = i
-            while i and curline[i-1] in FILENAME_CHARS + SEPS:
+            while i and curline[i-1] not in INVALID_CHARS.relpace(SEPS, ''):
                 i -= 1
             comp_what = curline[i:j]
         elif hp.is_in_code() and (not mode or mode==COMPLETE_ATTRIBUTES):