Regex that matches numbers, letters and dash but not the word net or NET

I'm not certain this is the best approach, but you could do something like this:

@"^(?(net$|NET$) |[0-9a-zA-Z-]+$)"

Basically it is checking if the string is "net" or "NET", and if so, it matches it against a space. That will always fail. Otherwise, it does your normal match.

Here's a more detailed explanation.

First of all, it starts with a basic if statement and a start ^ anchor: ^(?(if)then|else)

For the if statement, we check for "net" or | "NET". Each of these is in front of an end $ anchor so that we know it is not a substring. Here's the result: ^(?(net$|NET$)then|else)

Then if the statement is correct, I matched it against an space " ". Since we know the statement is already "net" or "NET", this part will always fail. That leaves us with this: ^(?(net$|NET$) |else)

Finally, we plug in your previous pattern into the else clause to get: ^(?(net$|NET$) |[0-9a-zA-Z-]+$)

After seeing the above answer, I remembered about negative lookaheads, which simplifys this by not needing a the dumb if statement. Using the lookahead, this answer would look like this:

@"^(?!net$|NET$)[0-9a-zA-Z-]+$"

Simply add a negative lookahead assertion at the beginning like so:

if (Regex.IsMatch(text, @"
    # Match word that is not NET
    ^              # Anchor to start of string.
    (?!net$)       # Assert that entire string is not NET.
    [0-9a-zA-Z-]+  # Match word.
    $              # Anchor to end of string.
    ", RegexOptions.IgnoreCase | RegexOptions.IgnorePatternWhitespace)) {
    // "Good" word that is not net, NET, Net,...
} else {
    // Not a "good" word.
}