Hacker News Reader

Comment by cedws | Hacker News Reader

I pretty much always consider regex expressions as the wrong solution. They're notoriously hard to get right.

There's a whole lot of faulty expressions out there for validating email addresses. I prefer to do less validation and let it fail. If the email address is wrong, whatever service you're using for sending emails will just reject it. If you really do need to validate email addresses, use something somebody else wrote that does it properly.

If you're working with some exotic format for which there isn't already an open source library, do what this guy says: parse it, don't try to validate it with regex: https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-va...

View on HN

Regex works very well for what it was originally designed: describing/validating regular languages. It can work ok if your language is simple and almost regular. They work very badly for validating non-regulars languages, even when extensions are added Perl-style to support that. And, unfortunately, most structured formats you might care to valdiate are in fact not regular languages at all.

Email addresses in particular are surprisingly complicated and far from being regular languages. I don't know how commonly real servers support the full feature set, but even if they just support non-ascii names they quickly become a pain.

Here, you have good advice: "I ... consider regex expressions as the wrong solution. They're notoriously hard to get right."

However, the conclusion of "use something somebody else wrote that does it properly", while valid, is asking a lot. As regex is hard to get right, don't assume the code you find on the web or book or via some other means works correctly.

My rule is if I didn't write it and can't wrap my head around the code to convince myself it is the right solution, I don't use it. And as I think others have written, there are some interactive online tests for regex expressions that can help.

I think the average developer has a better chance of finding a robust, battle tested library to do what they need than cooking up some regex of their own. Preferably, the library does not use regex at all and checks data more intelligently.

Sometimes valid email addresses will be rejected as invalid, and sometimes invalid email addresses are still successfully delivered. Validation guarantees nothing, and at most it should be a UI cue.

Reply on news.ycombinator.com