Regex for 1 or 2 digits, optional non-alphanumeric, 2 known alphas
^[0-9]{1,2}[:.,-]?po$
Add any other allowable non-alphanumeric characters to the middle brackets to allow them to be parsed as well.
^\d{1,2}[\W_]?po$
\d defines a number and {1,2} means 1 or two of the expression before, \W defines a non word character.