Avoid unwanted behavior change in shlex.quote (see #9723).

I simplified the quote code to use a regex instead of a loop+test when I
moved pipes.quote to shlex in 5966eeb0457d; Ezio Melotti pointed out
that my regex contained redundant parts (now removed) and allowed
non-ASCII characters (now disallowed).

I think common UNIX shells don’t quote non-ASCII characters, but there’s
no harm in doing so.  We’ll see if users request a change.

Lib/shlex.py

@@ -276,7 +276,7 @@ def split(s, comments=False, posix=True):
     return list(lex)
 
 
-_find_unsafe = re.compile(r'[^\w\d@%_\-\+=:,\./]').search
+_find_unsafe = re.compile(r'[^\w@%\-\+=:,\./]', re.ASCII).search
 
 def quote(s):
     """Return a shell-escaped version of the string *s*."""

Lib/test/test_shlex.py

@@ -176,7 +176,8 @@ class ShlexTest(unittest.TestCase):
 
     def testQuote(self):
         safeunquoted = string.ascii_letters + string.digits + '@%_-+=:,./'
-        unsafe = '"`$\\!'
+        unicode_sample = '\xe9\xe0\xdf'  # e + acute accent, a + grave, sharp s
+        unsafe = '"`$\\!' + unicode_sample
 
         self.assertEqual(shlex.quote(''), "''")
         self.assertEqual(shlex.quote(safeunquoted), safeunquoted)