Detecting number repetition in R using regex - regex

Question

Shouldn't this code work for repeating number detection in R?
> grep(pattern = "\\d{2}", x = 1223)
[1] 1
> grep(pattern = "\\d{3}", x = 1223)
[1] 1
If we have 988 we should get true and if 123 we should get false.
Sounds like it isn't.
> grep(pattern = "\\d{2}", x = "1223")
[1] 1
> grep(pattern = "\\d{2}", x = "13")
[1] 1

Answer 1

You need to use backreferences:
> grep(pattern = "(\\d)\\1", x = "1224")
[1] 1
> grep(pattern = "(\\d)\\1{1,}", x = "1224")
[1] 1
> grep(pattern = "(\\d)\\1", x = "1234")
integer(0)
EDIT: Seems like you need to figure how it works: (\\d) creates a capture group for the \\d, which can be referred to using a backreference \\1. For example, if you have numbers like x2y and you want to find those where x is the same as y, then:
> grep(pattern = "(\\d)2\\1", x = "121")
[1] 1
> grep(pattern = "(\\d)2\\1", x = "124")
integer(0)
I'd strongly recommend that you read a basic tutorial on regular expressions.

Answer 2

I know the question explicitly says "using regex" in the title, but here is a non-regex method that could work, depending on what you want to do.
strings <- c("1223","1233","1234","113")
# detect consecutive repeat digits, or characters
(strings.rle <- lapply(strings, function(x)rle(unlist(strsplit(x,"")))))
[[1]]
Run Length Encoding
lengths: int [1:3] 1 2 1
values : chr [1:3] "1" "2" "3"
[[2]]
Run Length Encoding
lengths: int [1:3] 1 1 2
values : chr [1:3] "1" "2" "3"
[[3]]
Run Length Encoding
lengths: int [1:4] 1 1 1 1
values : chr [1:4] "1" "2" "3" "4"
[[4]]
Run Length Encoding
lengths: int [1:2] 2 1
values : chr [1:2] "1" "3"
Now you can work with strings.rle to do what you want
# which entries have consecutive repeat digits, or characters
strings[sapply(strings.rle, function(x) any(x$lengths > 1))]
[1] "1223" "1233" "113"
or
# which digits or characters are consecutively repeated?
lapply(strings.rle, function(x) x$values[which(x$lengths > 1)])
[[1]]
[1] "2"
[[2]]
[1] "3"
[[3]]
character(0)
[[4]]
[1] "1"