How to use regex to match upto third forward slash in R using gsub?

Question

So this question is relating to specifically how R handles regex - I would like to find some regex in conjunction with gsub to extract out the text all but before the 3rd forward slash.

Here are some string examples:

/google.com/images/video 
/msn.com/bing/chat
/bbc.com/video

I would like to obtain the following strings only:

/google.com/images
/msn.com/bing
/bbc.com/video

So it is not keeping the information after the 3rd forward slash.

I cannot seem to get any regex working along with using gsub to solve this!

The closest I have got is:

gsub(pattern = "/[A-Za-z0-9_.-]/[A-Za-z0-9_.-]*$", replacement = "", x = the_data_above )

I think R has some issues regarding forward slashes and escaping them.

Take a look at this https://stackoverflow.com/questions/56002107/regexp-select-everything-before-the-3rd-slash — moys, Feb 26 '20 at 14:12
If you are not concerned with the # of slashes and just want everything before the last slash you could use `dirname()` — Andrew, Feb 26 '20 at 14:21
@Andrew Not what i need - I need it to stop after a certain slash — Beans On Toast, Feb 26 '20 at 14:22
Is this what you are looking for? `gsub("(/.+?/.+?)/.*", "\\1", text)` — Andrew, Feb 26 '20 at 14:24
`gsub` does not extract, it *replaces*. Use `stringr::str_extract` to make your life easier when extracting regex matches. — Wiktor Stribiżew, Feb 26 '20 at 14:31

score 2 · Accepted Answer · answered Feb 26 '20 at 14:23

2

From the start of the string match two instances of slash and following non-slash characters followed by anything and replace with the two instances.

paths <- c("/google.com/images/video", "/msn.com/bing/chat", "/bbc.com/video")
sub("^((/[^/]*){2}).*", "\\1", paths)
## [1] "/google.com/images" "/msn.com/bing"      "/bbc.com/video"

answered Feb 26 '20 at 14:23

G. Grothendieck

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can i retain the slash after the word bing so it returns /msn.com/bing/ in the output? – Beans On Toast Feb 26 '20 at 14:30
`sub("^((/[^/]*){2}/?).*", "\\1", paths)` – G. Grothendieck Feb 26 '20 at 14:31

score 2 · Answer 2 · answered Feb 26 '20 at 14:26

2

You can take advantage of lazy (vs greedy) matching by adding the ? after the quantifier (+ in this case) within your capture group:

gsub("(/.+?/.+?)/.*", "\\1", text)
[1] "/google.com/images" "/msn.com/bing"      "/bbc.com/video"

Data:

text <- c("/google.com/images/video",
"/msn.com/bing/chat",
"/bbc.com/video")

answered Feb 26 '20 at 14:26

Andrew

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can i retain the slash after the word bing? is that possible? – Beans On Toast Feb 26 '20 at 14:30
Sure thing, just change the pattern to `"(/.+?/.+?/?).*"` – Andrew Feb 26 '20 at 14:37

score 0 · Answer 3 · answered Feb 26 '20 at 14:10

0

Try this out:

^\/[A-Za-z0-9_.-]+\/[A-Za-z0-9_.-]+

As seen here: https://regex101.com/r/9ZYppe/1

Your problem arises from the fact that [A-Za-z0-9_.-] matches only one such character. You need to use the + operator to specify that there are multiple of them. Also, the $ at the end is pretty unnecessary because using ^ to assert the start of the sentence solves a great many problems.

answered Feb 26 '20 at 14:10

Robo Mop

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I can't seem to extract the strings using this code in R - it seemingly only extracts the information after the 3rd slash – Beans On Toast Feb 26 '20 at 14:20
@BeansOnToast It might be because the `.` hasn't been escaped. Try this: `^\/[A-Za-z0-9_\.-]+\/[A-Za-z0-9_\.-]+` – Robo Mop Feb 26 '20 at 14:23
Nope - when using this code `gsub(pattern = "^\\/[A-Za-z0-9_\\.-]+\\/[A-Za-z0-9_\\.-]+",x = t, replacement = "")` it should replace those things after the third slash with a blank character but it is not - is there an alternative way to do this instead? – Beans On Toast Feb 26 '20 at 14:25

How to use regex to match upto third forward slash in R using gsub?

3 Answers3