what regular expression mechanism to match "ski" but not "water ski"?

Question

What regular expression mechanism can be used to match "ski" but not "water ski"?

Or, match "ski" but not "ski sale"?

Could a short explanation be included?

Update: i mean, for example, it should be able to match "2009 ski competition" but not "2009 water ski competition".

Answer 1

For the updated part, you need to use negative look-behind. Something like

(?<!water\s*)(ski\b)

That should match the word ski, where water is not the previous word.

EDIT: I tweaked the regex above, and I meant negative look-behind, not back-referencing. For ski and ski competition but but not ski sale, you need negative look-ahead. E.g.:

(\bski)(?!\s*sale)

See http://www.regular-expressions.info/lookaround.html for some more explanation.

Answer 2

You could always do a multi pass search first remove all instances of water ski and ski sale from the string then do the IsMatch on that. In C# it would look like this.

string line = Console.ReadLine();
line = Regex.Replace(line, "water ski", "");
line = Regex.Replace("ski sale", "");
bool matchFound = line.IsMatch("ski");