How to include a right square bracket in a character class in R?

Question

This is addressed in this question and answered here (and this question is also cited), but none of them actually answer the question -- the answer provided to the first question sidesteps the issue. The more general regex answer doesn't appear to work in R:

gsub('[\\]`]', '', '`]')
# [1] "`]"

I'm trying to match a specific set of punctuation so [:punct:] is too general.

There's a certain order of presence. See https://stackoverflow.com/questions/32041265/how-to-escape-closed-bracket-in-regex-in-r/32041328#32041328 — hwnd, May 08 '17 at 05:02
@alistaire it does for this string, but no. That's just matching `]` -- the character class is `[]`. — MichaelChirico, May 08 '17 at 05:07
@MichaelChirico Are you sure about that? Then why does this _fail_: `gsub('[]', '', '``]')` ? — Tim Biegeleisen, May 08 '17 at 05:08
@MichaelChirico So maybe my answer _was_ working. Let me test it a bit to try and prove it. — Tim Biegeleisen, May 08 '17 at 05:09
@TimBiegeleisen when I was trying that myself, the string that it was failing on was: `-.,'\`<\"6\/]` — MichaelChirico, May 08 '17 at 05:10
This works: `gsub('[]a\`]', '', '\`]a')` which seems to indicate the whole thing must be the class. — alistaire, May 08 '17 at 05:10
[This reference](http://www.regular-expressions.info/charclass.html) says that regex implementations can be inconsistent about whether `[]]` works as a character class, it probably warrants careful testing. — Marius, May 08 '17 at 05:10

Tim Biegeleisen · Answer 1 · 2017-05-08T05:14:02.003

2

The right bracket inside the character class appear to be work for me unescaped, i.e. just use this:

gsub('[]`]', '', '`]')
[1] ""

It appears that the R interpreter "knows" that a right or left bracket inside a balanced character class (i.e. consisting of a proper left bracket and right bracket) is actually just a character.

Here is proof of this claim:

gsub('[]a-z`]', '', '`]abc')
[1] ""

If the first [] were a closed and empty character class, then the range a-z should not have worked, and the letters b and c should have remained untouched.

Also: If we try to substitute using an empty character class, we get an error, e.g.

gsub('[]', '', '`]')
Error in gsub("[]", "", "`]") : 
  invalid regular expression '[]', reason 'Missing ']''

edited May 08 '17 at 05:14

answered May 08 '17 at 04:59

Tim Biegeleisen

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no, the character class is then just `[]`, and then it matches `\`]` directly, so this won't work on the string `\` ]`, for example – MichaelChirico May 08 '17 at 05:00
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@hwnd Sorry, I didn't read the link and also you aren't Akrun, so I wasn't particularly scared either :-) If this is a duplicate you should close it then. – Tim Biegeleisen May 08 '17 at 05:14

score 0 · Answer 2 · answered May 08 '17 at 05:08

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Just escape the bracket and other special chars, e.g.,

gsub("\\]\\`", "", "]`")

answered May 08 '17 at 05:08

mkearney

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Yes, but this doesn't answer the character class problem. – Tim Biegeleisen May 08 '17 at 05:09

How to include a right square bracket in a character class in R?

2 Answers2