Bin Dec Hex Oct Converter

Using Arduino Programming Questions
2
  1. How can I ensure that the user input in the serial monitor is a decimal value?

By rejecting characters other than '0' to '9' inclusive.

  1. If I wanted to convert from Hex to Bin, how would i do this and again, how would I ensure that the user input was Hex?

By rejecting characters other than '0' to '9' inclusive or 'a' to 'f' or 'A' to 'F'

Ultimately, everything is binary.