OS X 10.7+ and iOS 3.2+
Use the native regexp solution provided by hfossli.
Otherwise
Either use your favorite regexp library or use the following Cocoa-native solution:
NSString *theString = @" Hello this is a long string! ";
NSCharacterSet *whitespaces = [NSCharacterSet whitespaceCharacterSet];
NSPredicate *noEmptyStrings = [NSPredicate predicateWithFormat:@"SELF != ''"];
NSArray *parts = [theString componentsSeparatedByCharactersInSet:whitespaces];
NSArray *filteredArray = [parts filteredArrayUsingPredicate:noEmptyStrings];
theString = [filteredArray componentsJoinedByString:@" "];