VideoMentions Search is built entirely using serverless technologies, so it should scale really well. There's no single server or database to act as a bottleneck. When you perform a search, your browser fires off a number of network requests to serverless function API endpoints that fetch the data from YouTube, then return a response. So if you do an "All time" search on a channel with 2 million videos, your laptop fan may kick on while your browser works hard firing off thousand and thousands of requests until it's fetched all 2 million videos, you run out of memory, or you manually hit the "Cancel" button - whichever comes first :)
Your regex idea is interesting, and I'll consider implementing some more complex rules like that if enough people request it. For now, my answer would be to either perform separate searches (like one for "yc" and another for "ycombinator"), or perform one search, then use ctrl/cmd+f to search within the matches displayed on the page.
Thanks for checking out my project, and for the great feedback!