Extract websites links from a text in R

r

Solution

regmatches This approach uses `regexpr`/`grepgexpr` and `regmatches`. I expanded the test data to include more examples.

text1 <- c("s@1212a www.abcd.com www.cats.com", 
           "www.boo.com", 
           "asdf",
           "blargwww.test.comasdf")

# Regular expressions take some practice.
# check out ?regex or the wikipedia page on regular expressions
# for more info on creating them yourself.
pattern <- "www\\..*?\\.com"
# Get information about where the pattern matches text1
m <- gregexpr(pattern, text1)
# Extract the matches from text1
regmatches(text1, m)

Which gives

> regmatches(text1, m) ##
[[1]]
[1] "www.abcd.com" "www.cats.com"

[[2]]
[1] "www.boo.com"

[[3]]
character(0)

[[4]]
[1] "www.test.com"

Notice it returns a list. If we want a vector you can just use `unlist` on the result. This is because we used `gregexpr` which implies there could be multiple matches in our string. If we know there is at most one match we could use `regexpr` instead

> m <- regexpr(pattern, text1)
> regmatches(text1, m)
[1] "www.abcd.com" "www.boo.com"  "www.test.com"

Notice, however, that this returns all results as a vector and only returns a single result from each string (note that www.cats.com isn't in the results). On the whole, though, I think either of these two methods is preferable to the `gsub` method because that way will return the entire input if there is no result found. For example take a look:

> gsub(text1, pattern=".*(www\\..*?\\.com).*", replace="\\1")
[1] "www.abcd.com" "www.boo.com"  "asdf"         "www.test.com"

And that's even after modifying the pattern to be a little more robust. We still get 'asdf' in the results even though it clearly doesn't match the pattern.

Shameless silly self promotion: `regmatches` was introduced with R 2.14 so if you're stuck with an earlier version of R you might be out of luck. Unless you're able to install the future2.14 package from my github repo which provides some support for functions introduced in 2.14 to earlier versions of R.

strapplyc. An alternative which gives the same result as ## above is:

library(gsubfn)
strapplyc(test1, pattern)

The regular expression Here is some explanation on how to decipher the regular expression:

pattern <- "www\\..*?\\.com"

Explanation:

`www` matches the www portion

`\\.` We need to escape an actual 'dot' character using `\\` because a plain `.` represents "any character" in regular expressions.

`.*?` The `.` represents any character, the `*` tells to match 0 or more times, and the `?` following the `*` tells it to not be greedy. Otherwise "asdf www.cats.com www.dogs.com asdf" would match all of "www.cats.com www.dogs.com" as a single match instead of recognizing that there are two matches in there.

`\\.` Once again we need to escape an actual dot character

`com` This part matches the ending 'com' that we want to match

Putting it all together it says: start with www. then match any characters until you reach the first ".com"

Problem

Original source