The page returned by /@handle is a serialized interface, and a first-match regex reads it in layout order. In the dev.to write-up, the first occurrence of the `"channelId"` key on all three pages checked sat inside a `gridChannelRenderer` nested in a `horizontalListRenderer`, which is the featured-channels shelf [5]. It is the right key, but it belongs to a different channel's record.
The offsets are worth working out. At 37% of a 1.7 MB document the first match is about 630 KB in; at 84% of 2.5 MB it is about 2.1 MB in [2]. The scan reads most of the page before it commits, then commits to whatever the shelf happens to list first.
Five other markers point at the page's own channel: the canonical link tag, the `og:url` meta tag, an `itemprop="identifier"` meta tag, the `"externalId"` key, and the `channel_id` parameter inside `"rssUrl"`. In this sample all five agreed with each other on every channel checked [11]. The value you want is in the document. The naive pattern still finds a channelId, just a different, equally well-formed UC ID, and it finds that one sooner.
Then the rate. Fifteen wrong out of sixteen is 93.75% [1], and that is a claim about that handle list, not yours. For it to transfer, your handles have to be channels whose pages render a featured shelf. The sample is stocked with exactly that kind of channel: a VEVO mirror at @BillieEilish, a second channel at @Mathologer, a translation at @veritasium, spinoffs at @mkbhd and @Fireship [4][14]. Resolve a list of small channels instead and more of them will have no shelf, your naive resolver will score better, and the failures will be intermittent and tied to page layout rather than to anything in your code. The wrong answer gives no sign of being wrong: no exception, the expected row count, plausible titles [8].
One piece of craft to copy. The author notes that `{22}` in those patterns is a validity check and not an over-match guard, because each pattern terminates at a closing quote and `"` is in neither `\w` nor `-`, so `UC[\w-]+` would capture the same characters [13]. A comment explaining what a quantifier is not doing is worth more than the quantifier.
If you already have a table keyed by handle, the check is the one that caught this: take the ID you stored, ask YouTube's public feed what that ID is called, and compare the answer to the handle you asked for [1]. The author found it via an unrelated log line, hours after publishing, when @veritasium came back as Veritasium en Francais [10].
