Fix regular expression for variable splitting. (project-chip#37493)

* Fix regular expression for variable splitting.

Previous code tried to escape every character, however python
linters complained about illegal escapes. The expression also
seemed a bit complex, so a character set seems clearer.

* Add brackets to force separator to also exist in the split

scripts/py_matter_yamltests/matter_yamltests/parser.py

@@ -1225,8 +1225,7 @@ def _config_variable_substitution(self, value):
             # But some other tests were relying on the fact that the expression was put 'as if' in
             # the generated code and was resolved before being sent over the wire. For such
             # expressions (e.g 'myVar + 1') we need to compute it before sending it over the wire.
-            delimiter_regex = "(\ |\(|\)|\+|\-|\*|\/|\%)"
-            tokens = re.split(delimiter_regex, value)
+            tokens = re.split("([- ()|+*/%])", value)
             if len(tokens) == 0:
                 return value