Here is a way to return your desired output:
library(stringr)
min(which(!is.na(suppressWarnings(as.numeric(str_split(string, " ", simplify = TRUE))))))
This is how it works:
str_split(string, " ", simplify = TRUE) # converts your string to a vector/matrix, splitting at space
as.numeric(...) # tries to convert each element to a number, returning NA when it fails
suppressWarnings(...) # suppresses the warnings generated by as.numeric
!is.na(...) # returns true for the values that are not NA (i.e. the numbers)
which(...) # returns the position for each TRUE values
min(...) # returns the first position
The output:
min(which(!is.na(suppressWarnings(as.numeric(str_split(string1, " ", simplify = TRUE))))))
[1] 9
min(which(!is.na(suppressWarnings(as.numeric(str_split(string2, " ", simplify = TRUE))))))
[1] 1
min(which(!is.na(suppressWarnings(as.numeric(str_split(string3, " ", simplify = TRUE))))))
[1] 5