如果找到，Regex得到结果，如果没有，则得到另一个结果。

Question

我正试图从m3u8文件中获得最大分辨率。

下面是示例m3u8内容：

#EXTM3U
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=149000,RESOLUTION=320x180,CODECS="avc1.77.30, mp4a.40.2",CLOSED-CAPTIONS=NONE
https://URL/index_0_av.m3u8?null=0&id=AgBuNs5OuBzaVwrmOF+jXvWJ+HkjL%2fShsITuRsvq00%2f+IYwrOKyjWNrak9RN+Vq0bDNLLJZ47ZVL1w%3d%3d&hdntl=exp=1597650826~acl=%2f*~data=hdntl~hmac=9a46c5ffe594c611b03e0dfbd8e5981aab024fae8492abc40f0fcd7c854af59a
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=298000,RESOLUTION=320x180,CODECS="avc1.77.30, mp4a.40.2",CLOSED-CAPTIONS=NONE
https://URL/index_1_av.m3u8?null=0&id=AgBuNs5OuBzaVwrmOF+jXvWJ+HkjL%2fShsITuRsvq00%2f+IYwrOKyjWNrak9RN+Vq0bDNLLJZ47ZVL1w%3d%3d&hdntl=exp=1597650826~acl=%2f*~data=hdntl~hmac=9a46c5ffe594c611b03e0dfbd8e5981aab024fae8492abc40f0fcd7c854af59a
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=499000,RESOLUTION=640x360,CODECS="avc1.64001f, mp4a.40.2",CLOSED-CAPTIONS=NONE
https://URL/index_2_av.m3u8?null=0&id=AgBuNs5OuBzaVwrmOF+jXvWJ+HkjL%2fShsITuRsvq00%2f+IYwrOKyjWNrak9RN+Vq0bDNLLJZ47ZVL1w%3d%3d&hdntl=exp=1597650826~acl=%2f*~data=hdntl~hmac=9a46c5ffe594c611b03e0dfbd8e5981aab024fae8492abc40f0fcd7c854af59a
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=800000,RESOLUTION=640x360,CODECS="avc1.64001f, mp4a.40.2",CLOSED-CAPTIONS=NONE
https://URL/index_3_av.m3u8?null=0&id=AgBuNs5OuBzaVwrmOF+jXvWJ+HkjL%2fShsITuRsvq00%2f+IYwrOKyjWNrak9RN+Vq0bDNLLJZ47ZVL1w%3d%3d&hdntl=exp=1597650826~acl=%2f*~data=hdntl~hmac=9a46c5ffe594c611b03e0dfbd8e5981aab024fae8492abc40f0fcd7c854af59a
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=1062000,RESOLUTION=960x540,CODECS="avc1.77.30, mp4a.40.2",CLOSED-CAPTIONS=NONE
https://URL/index_4_av.m3u8?null=0&id=AgBuNs5OuBzaVwrmOF+jXvWJ+HkjL%2fShsITuRsvq00%2f+IYwrOKyjWNrak9RN+Vq0bDNLLJZ47ZVL1w%3d%3d&hdntl=exp=1597650826~acl=%2f*~data=hdntl~hmac=9a46c5ffe594c611b03e0dfbd8e5981aab024fae8492abc40f0fcd7c854af59a
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=1361000,RESOLUTION=960x540,CODECS="avc1.77.30, mp4a.40.2",CLOSED-CAPTIONS=NONE
https://URL/index_5_av.m3u8?null=0&id=AgBuNs5OuBzaVwrmOF+jXvWJ+HkjL%2fShsITuRsvq00%2f+IYwrOKyjWNrak9RN+Vq0bDNLLJZ47ZVL1w%3d%3d&hdntl=exp=1597650826~acl=%2f*~data=hdntl~hmac=9a46c5ffe594c611b03e0dfbd8e5981aab024fae8492abc40f0fcd7c854af59a
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=1503000,RESOLUTION=1280x720,CODECS="avc1.64001f, mp4a.40.2",CLOSED-CAPTIONS=NONE
https://URL/index_6_av.m3u8?null=0&id=AgBuNs5OuBzaVwrmOF+jXvWJ+HkjL%2fShsITuRsvq00%2f+IYwrOKyjWNrak9RN+Vq0bDNLLJZ47ZVL1w%3d%3d&hdntl=exp=1597650826~acl=%2f*~data=hdntl~hmac=9a46c5ffe594c611b03e0dfbd8e5981aab024fae8492abc40f0fcd7c854af59a

我使用这个正则表达式来获得最大分辨率，这是最后一个网址"av.m3u8?““。

(https.*\w*index_6\w*.*)"

我的问题是，如果找不到“720 p的index_6”，我如何才能得到"index_5“，这样结果就不会是空的？

Answer 1

您可以使用以下模式：

^https.*index_\d+.*\Z

演示。

发生了什么变化：

• 将6替换为\d+，这将匹配一个或多个数字。
• 在末尾添加\Z (即弦端锚固)，仅与最后一次匹配。
• 捕获组是多余的，所以它被删除了。
• 这两个\w*看起来也是多余的(因为它们要么跟在后面，要么前面有.*)。如果需要，您可以添加更多的限制。

注:上述解决方案只有在索引按升序排列时才能工作。问题中提到，它应该与最后一个URL相匹配，所以我假设它们是有序的。如果不是这样的话，你可以写这样的东西：

var matches = Regex.Matches(input, @"^https.*index_(\d{1,9}).*$", RegexOptions.Multiline);
if (matches.Count > 0)
{
    var targetMatch = matches
                      .OfType<Match>()
                      .OrderByDescending(m => int.Parse(m.Groups[1].Value))
                      .First();
    string targetUrl = targetMatch.Value;
}