去除连续换行符，改为只有一个换行符

/**
 *    去除字符串中连续换行，只保留结尾处换行
 *    allStr 字符串
 *    return 去除连续换行后的字符串
 */
+ (NSString *)trimAllNewline:(NSString *)allStr {
    NSRegularExpression *regular = [[NSRegularExpression alloc] initWithPattern:@"\n{1,}" options:0 error:nil];
    NSString *newString = [regular stringByReplacingMatchesInString:allStr options:0 range:NSMakeRange(0, [allStr length]) withTemplate:@"\n"];
    return newString;
}