Evaluate expression in parentheses, in String

Question

In Java, if I have a string like "(3+5)x + x^(6/2)", how could I replace all expressions in parentheses with their evaluations, to get the string "8x + x^3"?

Answer 1

It depends on what you are trying to do. For more complex cases, you could/should use a parser generator like ANTLR. If the expressions are not more complex then your examples (simple arithmetics), you could just try to analyze the expressions with JavaScript/Nashorn.

Using the solution of Use the backreference in a regex to replace a text dynamically you could do:

public static void main(String[] args) throws ScriptException {

    ScriptEngine engine = new ScriptEngineManager().getEngineByName("nashorn");
    String x = "(3+5)x + x^(6/2)";

    // can also be extended to expressions like 3+5*5 with 
    // Pattern.compile("\\(\\d+([+/*-]\\d+)+\\)")
    Pattern simpleArithmeticExpr = Pattern.compile("\\(\\d+[+/]\\d+\\)"); 
    Matcher matcher = simpleArithmeticExpr.matcher(x);
    StringBuffer sb = new StringBuffer();
    while (matcher.find()) {
        String expr = matcher.group();
        String evaluatedExpr = String.valueOf(engine.eval(expr));
        matcher.appendReplacement(sb, evaluatedExpr );
    }
    matcher.appendTail(sb);
    System.out.println(sb); // 8x + x^3

}

If the Javascript solution is to slow/heavyweight, you could also parse it yourself.