Add grammar regex test for invalid escaped brace

This new test is meant to answer the question "why doesn't `_arg`
include the backslash character while it is supported in the
`UNQUOTED_CHAR` parser rule?"

The answer to that question is that if we included the backslash
character, then this new test would fail because the
`SIMPLE_INTERPOLATION_PATTERN` regex would believe that `${foo:\}` is a
valid interpolation syntax, while it is not.

There is currently no other test that would fail if the backslash
character was added to `_arg`, motivating the need for such a test.

NB: this commit does *not* modify `_arg`, it just rewrites it in raw
string format to make it more readable.

omegaconf/grammar_parser.py

@@ -27,7 +27,7 @@
 _node_inter = f"\\${{\\s*{_node_path}\\s*}}"  # node interpolation ${foo.bar}
 _id = "[a-zA-Z_]\\w*"  # foo, foo_bar, abc123
 _resolver_name = f"({_id}(\\.{_id})*)?"  # foo, ns.bar3, ns_1.ns_2.b0z
-_arg = "[a-zA-Z_0-9/\\-\\+.$%*@?|]+"  # string representing a resolver argument
+_arg = r"[a-zA-Z_0-9/\-\+.$%*@?|]+"  # string representing a resolver argument
 _args = f"{_arg}(\\s*,\\s*{_arg})*"  # list of resolver arguments
 _resolver_inter = f"\\${{\\s*{_resolver_name}\\s*:\\s*{_args}?\\s*}}"  # ${foo:bar}
 _inter = f"({_node_inter}|{_resolver_inter})"  # any kind of interpolation

tests/test_grammar.py

@@ -660,6 +660,7 @@ def test_grammar_consistency(self, expression: str) -> None:
         ("${ns . f:var}", False),
         ("${$foo:bar}", False),
         ("${.foo:bar}", False),
+        (r"${foo:\}", False),
         # Valid according to the grammar but not matched by the regex.
         ("${foo.${bar}}", True),
         ("${foo:${bar}}", True),