What is meaning of [_|\_|\.]? in Javascript regexps?

Question

I have a js code：

/^([a-zA-Z0-9]+[_|\_|\.]?)*[a-zA-Z0-9]+@([a-zA-Z0-9]+[_|\_|\.]?)*[a-zA-Z0-9]+\.[a-zA-Z]{2,3}$/

But what's meaning of [_|\_|\.]?（js regexp）

It's nonsense, it's a character class which means `match _ or | or .` zero or one time. It could be shortened to `[|_.]?` but I doubt it is the intention of it's writer. — HamZa, Oct 03 '13 at 08:31
The regex is [written terribly](http://stackoverflow.com/questions/9687596/slow-regex-performance), and will perform poorly. For example, try it on `aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa@a.a`: http://www.regex101.com/r/bM1fK8 . Besides that, it doesn't support all valid domains (eg full Unicode), TLDs (eg .museum), or email names (eg `email+tag@example.com`). You can find a better pattern. — Kobi, Oct 03 '13 at 08:41

score 11 · Accepted Answer · edited May 23 '17 at 12:30

If we use a resource like Regexper, we can visualise this regular expression:

Example

In fact, we can even use resources like Regexper to visualise the entire expression.

Joe Simmons · Answer 2 · 2013-10-04T11:02:24.070

5

It matches a pipe character, an underscore, or a period.
It is unnecessarily convoluted, however. It could be simpler.

It could be shortened to this
[|_.]

edited Oct 04 '13 at 11:02

answered Oct 03 '13 at 08:32

Joe Simmons

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It's not my job to fix his code. He asked what that piece of regex did, I told him. – Joe Simmons Oct 03 '13 at 08:38
I don't want to come off as sounding mean, but I answered his question. Other people have already stated that the regex could be shortened, so why should I, too. – Joe Simmons Oct 03 '13 at 08:42
I updated my answer, by the way, despite it being answered 4 times elsewhere on this page. – Joe Simmons Oct 03 '13 at 08:48
As it's wrapped in square braces, the `.` doesn't need to be escaped. `[|_.]` works just as well. – James Donnelly Oct 03 '13 at 08:54
I disagree. I don't think a correct answer should ever be downvoted. Voted up? maybe not, but downvoted? no. – Joe Simmons Oct 03 '13 at 09:03
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How cute, someone removed their comments so I look like a moron. Why is it possible to remove comments so long after they're made? – Joe Simmons Nov 04 '13 at 00:01

score 5 · Answer 3 · answered Oct 03 '13 at 08:33

REGEX101 is a very good tool for understanding regular expression

Char class [_|\_|\.] 0 to 1 times [greedy] matches:

[_|\_|\. One of the following characters _|_|.
 [_|\_|\.] requires one of either "_", "|" or "."

See This Link of RegEx101 here Your Expression explanation

score 3 · Answer 4 · edited May 23 '17 at 12:03

[_|\_|\.] is probably meant to match an underscore (_) or a period (.), and should have been written as [_.].

I'm reasonably sure the author is using the pipe (|) to mean "or" (i.e., alternation), which isn't necessary inside a character class. As the other responders said, the pipe actually matches a literal pipe, but I don't believe that was the author's intent. It's a very common beginner's mistake.

The dot (.) is another special character that loses its special meaning when it appears in a character class. There's no need to escape it with a backslash as the author did, though it does no harm. And the underscore never has any special meaning; I won't even try to guess why the author listed it twice, once with a backslash and once without.

You didn't ask about it, but the ? doesn't belong there either. That's what makes the regex so horribly inefficient, as Kobi remarked. The idea was to match one or more alphanumerics, then optionally match a separator character (dot or underscore), which must be followed by some more alphanumerics, repeating as needed. Here's how I would write that:

[a-zA-Z0-9]+([_.][a-zA-Z0-9]+)*

If it runs out of alphanumerics and the next character is not _ or ., it skips that whole section and tries to match the next part. And if it can't do that, it can bail out immediately because no match is possible. But the way your regex is written, the separator is optional independently of the things it's supposed to separate, which makes it useless. The regex engine has to keep backing up, trying to match characters that it has already consumed in endless, pointless combinations before it can give up. And that, unfortunately, is another common mistake.

What is meaning of [_|\_|\.]? in Javascript regexps?

4 Answers4