diff --git a/heracles/io.py b/heracles/io.py
index f1bb5ca..e47d458 100644
--- a/heracles/io.py
+++ b/heracles/io.py
@@ -21,7 +21,7 @@
 import logging
 import os
 import re
-from collections.abc import MutableMapping
+from collections.abc import MutableMapping, Sequence
 from pathlib import Path
 from types import MappingProxyType
 from typing import TYPE_CHECKING, Union
@@ -76,25 +76,36 @@ def _string_from_key(key: _DictKey) -> str:
     """
     Return string representation for a given key.
     """
-    if isinstance(key, tuple):
-        names = list(map(_string_from_key, key))
-        c = ";" if any("," in name for name in names) else ","
-        return c.join(names)
-    return re.sub(r"\W+", "_", str(key))
+    # recursive expansion for sequences
+    if isinstance(key, Sequence) and not isinstance(key, str):
+        return "-".join(map(_string_from_key, key))
+
+    # get string representation of key
+    s = str(key)
+
+    # escape literal "\"
+    s = s.replace("\\", "\\\\")
+
+    # escape literal "-"
+    s = s.replace("-", "\\-")
+
+    # substitute non-FITS characters by tilde
+    s = re.sub(r"[^ -~]+", "~", s, flags=re.ASCII)
+
+    return s
 
 
 def _key_from_string(s: str) -> _DictKey:
     """
     Return key for a given string representation.
     """
-    keys = s.split(";")
-    if len(keys) > 1:
-        return tuple(map(_key_from_string, keys))
-    keys = keys[0].split(",")
-    if len(keys) > 1:
-        return tuple(map(_key_from_string, keys))
-    key = keys[0]
-    return int(key) if key.isdigit() else key
+    parts = re.split(r"(?<!\\)-", s.replace("\\\\", "\0"))
+    if len(parts) > 1:
+        return tuple(map(_key_from_string, parts))
+    key = parts[0]
+    key = key.replace("\\-", "-")
+    key = key.replace("\0", "\\")
+    return int(key) if key.removeprefix("-").isdigit() else key
 
 
 def _get_next_extname(fits, prefix):
diff --git a/tests/test_io.py b/tests/test_io.py
index 9fe643d..212e74e 100644
--- a/tests/test_io.py
+++ b/tests/test_io.py
@@ -189,14 +189,19 @@ def test_string_from_key():
     assert _string_from_key(1) == "1"
     assert _string_from_key(("a",)) == "a"
     assert _string_from_key((1,)) == "1"
-    assert _string_from_key(("a", 1)) == "a,1"
-    assert _string_from_key(("a", "b", 1, 2)) == "a,b,1,2"
-    assert _string_from_key((("a", 1), "b")) == "a,1;b"
-    assert _string_from_key((("a", 1), ("b", 2))) == "a,1;b,2"
+    assert _string_from_key(("a", 1)) == "a-1"
+    assert _string_from_key(("a", "b", 1, 2)) == "a-b-1-2"
+
+    # flatten nested sequences
+    assert _string_from_key([("a", 1), "b"]) == "a-1-b"
+    assert _string_from_key([("a", 1), ("b", (2,))]) == "a-1-b-2"
 
     # test special chars
-    assert _string_from_key("a,b,c") == "a_b_c"
-    assert _string_from_key("!@#$%^&*()[]{};,.") == "_"
+    assert _string_from_key("a-b-c") == r"a\-b\-c"
+    assert _string_from_key(("a\\", 1)) == r"a\\-1"
+    assert _string_from_key(("a\\-", 1)) == r"a\\\--1"
+    assert _string_from_key(("a\\", -1)) == r"a\\-\-1"
+    assert _string_from_key("a€£") == "a~"
 
 
 def test_key_from_string():
@@ -204,10 +209,12 @@ def test_key_from_string():
 
     assert _key_from_string("a") == "a"
     assert _key_from_string("1") == 1
-    assert _key_from_string("a,1") == ("a", 1)
-    assert _key_from_string("a,b,1,2") == ("a", "b", 1, 2)
-    assert _key_from_string("a,1;b") == (("a", 1), "b")
-    assert _key_from_string("a,1;b,2") == (("a", 1), ("b", 2))
+    assert _key_from_string("a-1") == ("a", 1)
+    assert _key_from_string("a-b-1-2") == ("a", "b", 1, 2)
+    assert _key_from_string(r"a\-b\-c") == "a-b-c"
+    assert _key_from_string(r"a\\-1") == ("a\\", 1)
+    assert _key_from_string(r"a\\\-1") == "a\\-1"
+    assert _key_from_string(r"a\\-\-1") == ("a\\", -1)
 
 
 def test_write_read_maps(rng, tmp_path):
@@ -325,9 +332,11 @@ def test_write_read_cov(mock_cls, tmp_path):
     workdir = str(tmp_path)
 
     cov = {}
-    for k1, k2 in combinations_with_replacement(mock_cls, 2):
-        cl1, cl2 = mock_cls[k1], mock_cls[k2]
-        cov[k1, k2] = np.outer(cl1, cl2)
+    for (a1, b1, i1, j1), (a2, b2, i2, j2) in combinations_with_replacement(
+        mock_cls, 2
+    ):
+        cl1, cl2 = mock_cls[a1, b1, i1, j1], mock_cls[a2, b2, i2, j2]
+        cov[a1, b1, a2, b2, i1, j1, i2, j2] = np.outer(cl1, cl2)
 
     filename = "cov.fits"