Regex: Does /w means [a-zA-Z] or [a-zA-Z0-9_] as most tutorials mention \w -Matches the word characters?

Question

I have just started with Regular Expressions and was to solving this question in which the task is to check whether that username is valid. A valid username will have the following properties:

1. The username can contain alphanumeric characters and/or underscores(_).

2. The username must start with an alphabetic character.

3. 8<=(Username Length)<=30.

I am using this as my reference that says

\w Matches the word characters.

and I came up with a solution like this String pattern = "^\\w(\\d|\\w|_){7,29}$"; which is not the correct solution. And after searching for a while I found the correct solution is

String pattern = "^[a-zA-Z][a-zA-Z0-9_]{7,29}$"; which is pretty clear to understand.

What I want to confirm is (\\w|\\d|_) equivalent to [a-zA-Z0-9_] or not?

I think they are because String pattern = "^[a-zA-z](\\w|\\d|_){7,29}$"; is accecpted for all test cases.

Also, this stackoverflow post has two different equivalent expressions for \\w as answers with one upvote each, want to know which one is correct [A-Za-z\s] or [A-Za-z0-9_] ?

Answer 1

Yes, according to the Java summary of regular expression constructs found here: https://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html,

\d  A digit: [0-9]
\w  A word character: [a-zA-Z_0-9]

So (\w|\d|_) is equivalent to ([a-zA-Z_0-9]|[0-9]|_), where the extra underscore as well as \d is redundant since it's included as part of \w.

(\w|\d|_) is equivalent to (\w)

Answer 2

Okay so after thinking over this for a while and trying some different solution to the question

\w is, in fact, equivalent to [A-Za-z0-9_] which is also given in the official documentation. https://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html

not [a-zA-Z\s] as stated in this answer.

and as for the question String pattern = ^[a-zA-Z]\\w{7,29}; is accepted for all the test cases and seems to me the shortest answer possible.

And therfore although (\\w|\\d|_) is equivalent to [a-zA-Z0-9_] but only using \\w is sufficient.

P.S. Always stick to official documentation when in doubt during the learning phase and not anybody's answer or tutorial anywhere. Hope this helps someone with the same doubt.

Edit: Thank you @4castle @trey for your suggestions.

Answer 3

In regex \w is equivalent to [a-zA-Z0-9_] so it will match letters a, B, 3, _ etc. To match words you would have to use \w+. The plus meaning one or more times. https://regex101.com is a great website for testing regex and finding out what they do.

Answer 4

\w stands for “word character”. Exactly which characters it matches differs between regex engines.

1. In all engines, it will include [A-Za-z].
2. In most, the underscore and digits are also included.
3. In some engines, word characters from other languages may also match.

The best way to find out is to do a couple of tests with the regex engine you are using. write a test string and search by regex \w to see what it matches.