For set theory, we have notation to denote subsets, e.g.: $$S \subset X$$
When working with expressions (arithmetic, Boolean, etc.), is there a notation to denote expression $S$ is a valid sub-expression of $X$?
For instance, if we have $X := 5 \times (3+4)$, then we can say $5$, $3+4$, and $(3+4)$ are valid sub-expression of $X$, but $5 \times$ is not since it cannot be correctly evaluated, and neither is $2+3$ a valid sub-expression of $X$, since, well, obviously it isn't.
If there is no convenient notation, what is a clear way to word it (apparently "sub-expression" is not desirable to some)?