function getUnitsInExpression(expression) {
  var uniqueUnits = []
  var uniqueLowerCaseUnits = []
  var unitRegEx = /[\.0-9]([%a-z]+)/gi
  var matches = unitRegEx.exec(expression)

  while (matches) {
    if (!matches || !matches[1]) {
      continue
    }

    if (uniqueLowerCaseUnits.indexOf(matches[1].toLowerCase()) === -1) {
      uniqueUnits.push(matches[1])
      uniqueLowerCaseUnits.push(matches[1].toLowerCase())
    }

    matches = unitRegEx.exec(expression)
  }

  return uniqueUnits
}
Checks what units are used in an expression

@param {String} expression
@returns {Array}