How to get Youtube channel details using Youtube data API if channel has custom url

Simplest solution, using API only, is to just use Search:list method of YouTube Data API. From what I can tell (mind you, this is from my own research, official docs say nothing on this subject!), if you search using the custom URL component, with "channel" result type filter and "relevance" (default) sorting, first result should be what you're looking for.

So the following query gets 16 results, with the first one being the one you're looking for. Same goes for all other custom channel URLs I tested, so I think this is the most reliable way of doing this.

GET https://www.googleapis.com/youtube/v3/search?part=id%2Csnippet&q=annacavalli&type=channel&key={YOUR_API_KEY}

The other idea is just scraping YouTube page at the custom URL, where you can find ChannelID in one of the meta tags in HTML code. But that's ineffective, unreliable and AFAIK in violation of YouTube terms of use.

Edit: Well, it returns no results for smaller channels, so it's not reliable at all.