R: how to find the first digit in a string

Question

    string = "ABC3JFD456"

Suppose I have the above string, and I wish to find what the first digit in the string is and store its value. In this case, I would want to store the value 3 (since it's the first-occuring digit in the string). grepl("\\d", string) only returns a logical value, but does not tell me anything about where or what the first digit is. Which regular expression should I use to find the value of the first digit?

Answer 1

Base R

regmatches(string, regexpr("\\d", string))
## [1] "3"

Or using stringi

library(stringi)
stri_extract_first(string, regex = "\\d")
## [1] "3"

Or using stringr

library(stringr)
str_extract(string, "\\d")
## [1] "3"

Answer 2

1) sub Try sub with the indicated regular expression which takes the shortest string until a digit, a digit and then everything following and replaces it with the digit:

sub(".*?(\\d).*", "\\1", string)

giving:

[1] "3"

This also works if string is a vector of strings.

2) strapplyc It would also be possible to use strapplyc from gsubfn in which case an even simpler regular expression could be used:

strapplyc(string, "\\d", simplify = TRUE)[1]

giving the same or use this which gives the same answer again but also works if string is a vector of strings:

sapply(strapplyc(string, "\\d"), "[[", 1)