Question

The user will input a string similar to 151+328. I need a function to detect the user inputted an operation between two numbers and return them. How to do it? I know how to do the first part with regexes (i.e. [0-9]+\+[0-9], but I don’t have a clue about the second part.

tldr:

function('10+296', '+', 1)='10'    
function('10+296', '+', 2)='296'    
function('perpderpherp', 'derp', 2)='herp'

Answer 1

This function uses a regex to make sure the string is in the following format: [0-9]+[0-9] and then explode to split and return a part:

function splitAndReturn($str, $delimiter, $part) {
    $pattern = '/^\d+\+\d+$/';
    if (preg_match($pattern, $str)) {
        $parts = explode($delimiter, $str);
        return $parts[$part-1];
    }
    return false;
}

echo splitAndReturn('10+296', '+', 1); //=10
echo splitAndReturn('10+296', '+', 2); //=296
echo splitAndReturn('potatocarrotbanana', 'carrot', 1); // returns false because it doesn't comply with the "detect only [0-9]+[0-9]" rule you stated.

---

This function uses explode to split the string at the delimiter and return any of the parts:

function splitAndReturn($str, $delimiter, $part) {
    $parts = explode($delimiter, $str);
    return $parts[$part-1];
}

echo splitAndReturn('10+296', '+', 1); //=10
echo splitAndReturn('10+296', '+', 2); //=296
echo splitAndReturn('potatocarrotbanana', 'carrot', 1); //='potato'

If the delimiter doesn’t exist, it’ll just return the original string.