取出字符串中的指定内容

- (NSString *)CopyPartTextToString : (NSString *)SourceData
									 FirstPlaceIndex : (NSInteger) FirstPlaceIndex
										LastPlaceIndex : (NSInteger) LastPlaceIndex {
	if (FirstPlaceIndex < 1) FirstPlaceIndex = 1;
	if (LastPlaceIndex > [SourceData length]) LastPlaceIndex = [SourceData length];
	if (FirstPlaceIndex > LastPlaceIndex) return nil;
	
	NSRange range;
	range.location = FirstPlaceIndex - 1;
	range.length = LastPlaceIndex - FirstPlaceIndex + 1;
	
	return [SourceData substringWithRange:range];
}