I didn't use the same object as you did but another that I understand a bit more and that looks more like Java's one ;)
NSString* string = @"CITY SANDWITCH 67200 STRASBOURG";
NSString* search = @"SAND"; // work also with 672 or STRAS
NSString* regexString = [NSString stringWithFormat: @".*\\b(%@[\\w]*)[\\s*\\w*]*", search];
NSError *error = NULL;
NSRegularExpression *regex = [NSRegularExpression
regularExpressionWithPattern:regexString
options:0
error:&error];
NSString *modifiedString = [regex stringByReplacingMatchesInString:string options:0 range:NSMakeRange(0, [string length]) withTemplate:@"$1"];
NSLog(@"-->%@", modifiedString);