Extract a regular expression match

tovare_12677 0
Viewed 113430

I'm trying to extract a number from a string.

And do something like [0-9]+ on the string "aaa12xxx" and get "12".

I thought it would be something like:

> grep("[0-9]+", "aaa12xxx", value=TRUE)
[1] "aaa12xxx"

And then I figured... 

> sub("[0-9]+", "\\1", "aaa12xxx")
[1] "aaaxxx"

But I got some form of response doing:

> sub("[0-9]+", "ARGH!", "aaa12xxx")
[1] "aaaARGH!xxx"

There's a small detail I'm missing.

r regex
12 Answers

Use the new stringr package which wraps all the existing regular expression operates in a consistent syntax and adds a few that are missing:

library(stringr)
str_locate("aaa12xxx", "[0-9]+")
#      start end
# [1,]     4   5
str_extract("aaa12xxx", "[0-9]+")
# [1] "12"
hadley_166320

For your specific case you could remove all not numbers:

gsub("[^0-9]", "", "aaa12xxxx")
# [1] "12"

It won't work in more complex cases

gsub("[^0-9]", "", "aaa12xxxx34")
# [1] "1234"
user_1687470

You can use PERL regexs' lazy matching:

> sub(".*?([0-9]+).*", "\\1", "aaa12xx99",perl=TRUE)
[1] "12"

Trying to substitute out non-digits will lead to an error in this case.

user_2125930

One way would be this: 

test <- regexpr("[0-9]+","aaa12456xxx")

Now, notice regexpr gives you the starting and ending indices of the string:

    > test
[1] 4
attr(,"match.length")
[1] 5

So you can use that info with substr function

substr("aaa12456xxx",test,test+attr(test,"match.length")-1)

I'm sure there is a more elegant way to do this, but this was the fastest way I could find. Alternatively, you can use sub/gsub to strip out what you don't want to leave what you do want.

user_2254680

A solution for this question

library(stringr)
str_extract_all("aaa12xxx", regex("[[:digit:]]{1,}"))
# [[1]]
# [1] "12"

[[:digit:]]: digit [0-9]

{1,}: Matches at least 1 times

user_123363050

Using the package unglue we would do the following: 

# install.packages("unglue")
library(unglue)
unglue_vec(c("aaa12xxx", "aaaARGH!xxx"), "{prefix}{number=\\d+}{suffix}", var = "number")
#> [1] "12" NA

Created on 2019-11-06 by the reprex package (v0.3.0)

Use the convert argument to convert to a number automatically : 

unglue_vec(
  c("aaa12xxx", "aaaARGH!xxx"), 
  "{prefix}{number=\\d+}{suffix}", 
  var = "number", 
  convert = TRUE)
#> [1] 12 NA
user_22704750
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