R string containing only one type of character

Question

I would like to check if a string contain only one type of character

For example

INPUT: 

str = "AAAAB"
char = "A"

OUTPUT:

str contains only char = FALSE

With grepl(char,str) the result is TRUE but I want it to be FALSE.

Many thanks

Answer 1

If you want to check for a specific character (in char):

str <- "AAAAB"
char = "A"
all(unlist(strsplit(str, "")) == char)
#[1] FALSE

str <- "AAAA"
char = "A"
all(unlist(strsplit(str, "")) == char)
#[1] TRUE

Or, if you want to check if the string contains only one unique character (any one):

str <- "AAAAB"
length(unique(unlist(strsplit(str, "")))) == 1
#[1] FALSE

str = "AAAA"
length(unique(unlist(strsplit(str, "")))) == 1
#[1] TRUE

Answer 2

You need to use not operator for regex, in your case this would be:

> !grepl("[^A]", "AAA")
[1] TRUE
> !grepl("[^A]", "AAAB")
[1] FALSE

With variables:

grepl(paste0("[^", char, "]"), srt)