diff --git a/pycrate_asn1rt/asnobj_str.py b/pycrate_asn1rt/asnobj_str.py
index 064f13e..539a5f6 100644
--- a/pycrate_asn1rt/asnobj_str.py
+++ b/pycrate_asn1rt/asnobj_str.py
@@ -286,7 +286,7 @@ Specific constraints attributes:
                     else:
                         s = uint_to_bytes(self._val[0], self._val[1]).decode('ascii')
                     if is_printable(s):
-                        return ret + ' -- %s --' % s
+                        return ret + ' -- %r --' % s
                     else:
                         return ret
                 except Exception:
@@ -1325,7 +1325,7 @@ Specific constraints attributes:
                 try:
                     s = self._val.decode('ascii')
                     if is_printable(s):
-                        return ret + ' -- %s --' % s
+                        return ret + ' -- %r --' % s
                     else:
                         return ret
                 except Exception:
diff --git a/pycrate_asn1rt/utils.py b/pycrate_asn1rt/utils.py
index b6c2509..40965be 100644
--- a/pycrate_asn1rt/utils.py
+++ b/pycrate_asn1rt/utils.py
@@ -361,11 +361,11 @@ def extract_charstr(text=''):
                            re.subn('\s{0,}\n\s{0,}', '', text[1:cur])[0]
 
 
-printable_str = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'\
-                '!"#$%&\'()*+,-./:;<=>?@[\\]^_`{|}~ \t\n\r\x0b\x0c'
+_printable_str = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'\
+                 '!"#$%&\'()*+,-./:;<=>?@[\\]^_`{|}~ \t\n\r\x0b\x0c'
 
 def is_printable(w):
-    return all(c in printable_str for c in w)
+    return all(c in _printable_str for c in w)
 
 #------------------------------------------------------------------------------#
 # integer factorization and rounding routine