Comment by peterwwillis

By writing regexs by hand, you can accidentally introduce an obviously backtracking pattern such as * .=. *. By programmatically composing them, a program can analyze each regex group to find simple problems, and then combine them in ways that will avoid backtracking.

This isn't even why you should compose them programmatically, though. Perl allows you to compose a regex with in-line comments (https://perldoc.perl.org/perlfaq6.html#How-can-I-hope-to-use...), but it's still a hand-crafted regex, which is error-prone, much like composing code by hand. If you can get a machine to generate it for you, you avoid unintentional human-introduced bugs, as well as make it easier to read and reason about.

If you have a ton of regex's, or they are super important to your business, you should consider not editing them by hand. There's only so much test cases can do to prevent bugs.