I once asked a software developer at work how many times we called fork() in our code. I’ll admit, it was a very large project, but I expected the answer to be — at most — two digits. The developer came back and read off some number from a piece of paper that was in the millions. I told them there was no way we had millions of calls to fork() and, of course, we didn’t. The problem was the developer wasn’t clear on the difference between a regular expression and a glob.
Tools like grep use regular expressions to create search patterns. I might write
[Hh]ack ?a ?[Dd]ay as a regular expression to match things like “HackaDay” and “Hack a day” and, even, “Hackaday” using a tool like grep, awk, or many programming languages.