mathConstants

 private fun extractNumber(numString: String) = numString.toDoubleOrNull()

 private fun isValue(expression: String): Boolean {
    val validChars = "1234567890.-"

    for (i in expression.indices) {
       val char = expression[i]
       if (char !in validChars) return false
       if (expression.count { it == '.' } > 1) return false
       if (char == '-' && i != 0) return false
    }
    return true
 }

return when {
   isValue(expression) -> extractNumber(expression) ?: Double.MIN_VALUE
   expression == "PI" -> PI
   expression == "E" -> E
   else -> throw NumberFormatException()
}