Wildcard Replacement

A string $S$ whose characters are one of the following 5: $($, $)$, $+$, $-$ and $?$, is said to be a *valid string* if one of the following conditions holds: - $S = \ ?$ - $S = (x+y)$, where $x$ and $y$ are valid strings. - $S = (x-y)$, where $x$ and $y$ are valid strings. For example, strings $?$, $(?-?)$, $(?+(?-?))$ are valid strings, while strings $(?-+)$, $??$, $(?)$, $?-?$, $(?+?))$ are no