String: extract wanted character instead of removing unwanted - r

Question

I was wandering if in R their is a function like KeepChar("abcde....xyz", some_text) that you feed with all the desired character that you want to keep, and returns the strings with only the desired character left in it. Here the function would only keep the letters of the alphabet in lower case. I would like something that looks like this:
some_text <- "Hel-_l0o W#oRr^ld"
some_text <- KeepChar("abcdefghijklmnopqrstuvwxyz ", some_text)
some_text
> "hello world"
I feel that the removing method that I am currently using gsub("#\\w+", "", some_text), tm_map(some_text, stripWhitespace) or str_replace_all(some_text,"[^[:graph:]]", " ") takes a lot of time and coding line with a constant risk of forgetting to remove a specific character, especially when you already know exactly what you want to keep.
Why I ask this question is because I am coding a plateform to process sentiment analysis on texts from various sources like twitter and I want to make sure not to forget to remove any unwanted character.

Answer 1

To handle a pattern without using regex I will try this:
string <- "Hel-_l0o W#oRr^ld"
pattern <- "abcdefghijklmnopqrstuvwxyz"
KeepChar = function(pattern, string){
splitted_string <- unlist(strsplit(string, ""))
splitted_pattern <- unlist(strsplit(pattern, ""))
ids_string <- splitted_string %in% splitted_pattern
return(paste(splitted_string[ids_string], sep = "", collapse = ""))
}
some_text <- KeepChar(pattern = pattern, string = string)

Answer 2

You can try this:
some_text <- "Hel-_l0o W#oRr^ld"
gsub("[^[:alpha:] ]", "", some_text)#will return all characters
gsub("[^[:lower:] ]", "", some_text)#will return only lower characters alongwith space
gsub("[^[:upper:] ]", "", some_text)#will return higher case characters alongwith space
You can also look at the page https://stat.ethz.ch/R-manual/R-devel/library/base/html/regex.html to see the matches available in R