If you're just learning, or have complete trust over what goes into the regex, skip the rest of this.

However, someone should mention that taking regex input from users is fraught with security issues. If you don't have complete trust over what's read into the regex, you should consider something like re::engine::RE2, which limits some constructs and mitigates denial-of-service.

I have only touched the tip of the iceberg; you should search for "perl regex security" and similar to be more bullet-proof-resistant.