How to convert imperial units of length into metric?

Question

Here I am faced with an issue that I believe(or at least hope) was solved 1 million times already. What I got as the input is a string that represents a length of an object

Answer 1

Here is my solution. It uses eval() to evaluate the expression, but don't worry, the regex check at the end makes it completely safe.

function imperial2metric($number) {
    // Get rid of whitespace on both ends of the string.
    $number = trim($number);

    // This results in the number of feet getting multiplied by 12 when eval'd
    // which converts them to inches.
    $number = str_replace("'", '*12', $number);

    // We don't need the double quote.
    $number = str_replace('"', '', $number);

    // Convert other whitespace into a plus sign.
    $number = preg_replace('/\s+/', '+', $number);

    // Make sure they aren't making us eval() evil PHP code.
    if (preg_match('/[^0-9\/\.\+\*\-]/', $number)) {
        return false;
    } else {
        // Evaluate the expression we've built to get the number of inches.
        $inches = eval("return ($number);");

        // This is how you convert inches to meters according to Google calculator.
        $meters = $inches * 0.0254;

        // Returns it in meters. You may then convert to centimeters by
        // multiplying by 100, kilometers by dividing by 1000, etc.
        return $meters;
    }
}

So for example, the string

3' 2 1/2"

gets converted to the expression

3*12+2+1/2

which gets evaluated to

38.5

which finally gets converted to 0.9779 meters.