Extracting the 1st non-zero digit from a numeric in R

This works with positive inputs:

firstdigit <- function(x){ floor(x / 10 ^ floor(log10(x))) }
firstdigit(c(123.01, 0.56, 0.078, 0.0092))
# 1 5 7 9

You can use sub() from base R:

x <- c("123.01", "0.56", "0.078", "0.0092", "-1.2", "-0.3", "-0.04", "100")

sub("[^1-9]*(.).*", "\\1", x)

or with gsub()

y <- gsub("[^1-9]", "", x)
substr(y, 1,1)

This last you can modify to find the second digit:

y <- gsub("[^1-9]", "", x)
substr(y, 2,2)

From the comments:
For this case the problem needs to be worded differently.
"Extract the second digit after leading zeros" or "extract the second significant figure".

y <- sub("^[^1-9]*", "", x)
y <- gsub("\\.", "", y)
substr(y, 2,2)

You can use regex to extract. Using the stringr package:

set.seed(1)
x <- c(123.01, 0.56, 0.078, 0.0092)
x
#> [1] 123.0100   0.5600   0.0780   0.0092

stringr::str_extract(as.character(x), "([1-9]{1})")
#> [1] "1" "5" "7" "9"

You can convert back to numeric as necessary.

We can use gsub to remove leading 0's, trimws to remove dot and 0 or more zeroes and then extract first character using substr.

x <- c(123.01, 0.56, 0.078, 0.0092)
substr(trimws(gsub('^0', '', x), "left", "\\.0*"), 1, 1)
#[1] "1" "5" "7" "9"