func TokeniseExpression(string expr) {
    numcache = ""
    exprTokens = []

    for char in expr.Replace(" ", "") {
        if precedences.ContainsKey(char) {
            // is an operator
            if len(numcache) > 0 {
                // numcache has a number in it
                exprTokens += ('number', numcache)
                // add token type 'number'
            }
            exprTokens += ('operator', char)
            // char is an operator type, add token
        } else if char is a digit {
            //1, 2, 3, etc.
            numcache += char
            // add to numcache
        }
    }

    if len(numcache) > 0 then exprTokens += ('number', numcache)
    // this is for the number at the end of the expression, which
    //     hasn't been 'popped' off it yet

    return exprTokens
}