Expressions with an underscore in a variable name fail to evaluate

To reproduce:

1. Create an expression named infixExpression_0 and set its symbolic value to 0
2. Create a second expression named infixExpression_1 and set its symbolic value to {infixExpression_0}. This will show "Evaluation failed" for its computed value.

As a workaround, rename infixExpression_0 to infixExpression0 and update infixExpression_1's symbolic value. The computed value will now be correct.

@jwilson1881 @rohit.krishna