Bug with using - inside brackets [] with using Style/RedundantRegexpEscape

[ebeursken]

When creating a regex expression, Rubocop automatically says escaping a dash is not necessary.
When you do not escape a dash (-) inside brackets ([]) it means something else and causes unexpected behavior, since it is seen as a range value instead of an actual dash inside the regex.
Thus, this rubocop rule results in regex expressions that are not the same as with escaping those characters!

---

Expected behavior

No backslash is needed for a -
And thus within a regex expression, this - means an actual -

Actual behavior

Not escaping this - results in unexpected behavior when using this inside a [], so that the regex expression allows something that you did not expect.
When using dash inside those type of brackets without backslash, it can mean a range of characters (e.g. [a-d] means a, b, c or d)
See : https://ruby-doc.org/core-2.5.3/Regexp.html#class-Regexp-label-Character+Classes

Steps to reproduce the problem

Code without backslash, according to the rubocop style:

NAME = /([[[:alnum:]]+-_\.])+/

This seems to be a range instead of an actual dash, since it is used inside a []

Code with a backslash

NAME = /([[[:alnum:]]+\-_\.])+/

RuboCop version

0.88.0 (using Parser 2.7.1.4, rubocop-ast 0.2.0, running on ruby 2.7.1 x86_64-linux-musl)

[owst]

Hi @ebeursken - the issue of \- being reported as unnecessary was fixed in 0adf3cd and was released in v0.89.0 and later versions.

However, I wonder if there is a mistake in your regexp - you have [[[:alnum:]], which has one too many leading [s:

> /[[[:alnum:]]]/ =~ 'b'
=> nil

Edit: apologies, this example is incorrect - I had typoed on :alnum: and missed the trailing : in my pry session, but fixed when pasting here.

I don't know what you're trying to match, but maybe NAME = /([[:alnum:]+\-_\.])+/ is what you need?

[ebeursken]

Thank you for your quick response! @owst
The extra bracket at the start is closed just before before the last ), but thanks for sharing your thoughts!

[owst]

Hi @ebeursken apologies, I realised I typoed on my example above.

It seems Ruby allows redundant nesting of character classes, which I hadn't realised (maybe I can alter Style/RedundantRegexpCharacterClass to catch such patterns). For example:

[1] pry(main)> RUBY_VERSION
=> "2.7.1"
[2] pry(main)> /[a]/ =~ 'a'
=> 0
[3] pry(main)> /[[a]]/ =~ 'a'
=> 0
[4] pry(main)> /[[[a]]]/ =~ 'a'
=> 0

Therefore I believe your regexp:

 /([[[:alnum:]]+\-_\.])+/

is equivalent to

 /([[:alnum:]+\-_\.])+/

and then . does not need to be escaped within a character class, so we have:

 /([[:alnum:]+\-_.])+/