diff --git a/scp.py b/scp.py
index dc929de..6cc094a 100644
--- a/scp.py
+++ b/scp.py
@@ -14,21 +14,25 @@
 import types
 
 
-# this is quote from the shlex module, added in py3.3
-_find_unsafe = re.compile(br'[^\w@%+=:,./~-]').search
+safe_shell_chars = set("ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+                       "abcdefghijklmnopqrstuvwxyz"
+                       "0123456789"
+                       "-+=/:.,%_")
 
 
 def _sh_quote(s):
-    """Return a shell-escaped version of the string `s`."""
-    if not s:
-        return b""
-    if _find_unsafe(s) is None:
+    r"""Given bl"a, returns "bl\\"a".
+    """
+    if isinstance(s, bytes):
+        s = s.decode('utf-8')
+    if not s or any(c not in safe_shell_chars for c in s):
+        return '"%s"' % (s.replace('\\', '\\\\')
+                          .replace('"', '\\"')
+                          .replace('`', '\\`')
+                          .replace('$', '\\$'))
+    else:
         return s
 
-    # use single quotes, and put single quotes into double quotes
-    # the string $'b is then quoted as '$'"'"'b'
-    return b"'" + s.replace(b"'", b"'\"'\"'") + b"'"
-
 
 # Unicode conversion functions; assume UTF-8