Maximum Valid Matrix Expression

Given a rectangular 2D matrix whose cells are strings containing either a single digit ("0" through "9"), "+", or "-", return the maximum value of any valid expression that can be read contiguously from left to right within one row or from top to bottom within one column.

A valid expression:

• starts with a digit;
• alternates strictly between a digit and an operator (+ or -), so it never contains consecutive digits or consecutive operators; and
• may start and end at any cells along the chosen row or column. A single digit is therefore a valid expression.

Evaluate expressions from left to right using ordinary addition and subtraction. You may not change direction or skip cells. Return the maximum integer value.

Examples:

• In [["9", "+", "3", "6"], ["+", "7", "-", "2"]], "9+36" is invalid because "3" and "6" are consecutive digits, while "9+3" is valid.
• For [["9", "+", "3", "6", "-"], ["+", "7", "-", "2", "-"], ["9", "+", "1", "-", "3"]], the maximum is 18, obtained from the top-to-bottom expression 9+9 in column 0.

Constraints: the matrix is non-empty and rectangular, has at most 100 rows and 100 columns, and every cell is one of the three permitted forms.