Support for negative numbers, PI and E can be used directly.

[foamzzz]

The original function in expression_calc.cpp is as follows:

void eXpressionCalc::setExpression(Expression_t expression) {
    Expr = std::move(expression);
}

The expression obtained from the original function has been processed to support calculations involving negative numbers, such as -10-(2*(-PI))+E, Result: -0.998533. Additionally, PI and E no longer require manually added parentheses, making the process more intelligent.

void eXpressionCalc::setExpression(Expression_t expression) {
    std::string modified_expr = expression;

    // 1. If the first character is '-', prepend '0' for negative number
    if (!modified_expr.empty() && modified_expr[0] == '-') {
        modified_expr = "0" + modified_expr;  // Change "-2" to "0-2"
    }

    // 2. Replace all "(-" with "(0-" to handle negative numbers in parentheses
    size_t pos = 0;
    while ((pos = modified_expr.find("(-", pos)) != std::string::npos) {
        modified_expr.replace(pos, 2, "(0-");
        pos += 3;  // Skip past "(0-"
    }

    // 3. Append "()" to "PI" if not followed by '('
    pos = 0;
    while ((pos = modified_expr.find("PI", pos)) != std::string::npos) {
        if (pos + 2 >= modified_expr.size() || modified_expr[pos + 2] != '(') {
            modified_expr.insert(pos + 2, "()");
            pos += 4;  // Skip over "PI()"
        } else {
            pos += 3;  // Skip over "PI("
        }
    }

    // 4. Append "()" to "E" if not followed by '('
    pos = 0;
    while ((pos = modified_expr.find("E", pos)) != std::string::npos) {
        if (pos + 1 >= modified_expr.size() || modified_expr[pos + 1] != '(') {
            modified_expr.insert(pos + 1, "()");
            pos += 3;  // Skip over "E()"
        } else {
            pos += 2;  // Skip over "E("
        }
    }

    // 5. Assign the processed expression to internal member
    Expr = std::move(modified_expr);
}

The test.sh and readme.md have been updated with examples and a few minor modifications.