A few things to help understand:
Parentheses enclose a sub-pattern, which is evaluated first (just like parentheses in PHP) and can have repetition modifiers append after the closing paren (not used in this case).
"|" is the "or" operator, so it matches if either side of the "|" matches.
I used "#" as the pattern delimiter, but it can be any of most special characters -- but you then have to escape that character within the pattern with a back-slash if you want that literal character to match. The "i" following the closing "#" makes the whole pattern case-Insensitive, so "N/A" or "n/a" would both match.
The "" anchors the pattern to the start of the string, and the "$" to the end of the string.
 is used to define a "character class": it matches on any of the characters or ranges of characters it contains. I used a "+" repetition modifier after it, meaning to match on one or more instances of those characters. ("*" would be used if you wanted to match on zero or more instances.)