F# Remove/Replace char in a specific string index

Question

I want to remove a char in a string. but not all the elements of that char in a string. example. i want "|red|red|red|red|" to turn into "red|red|red|red" So I want to create a function that checks if the first and last index of a string is a certain char and remove it if its the case.

so far i have come up with something like this:

let rec inputFormatter (s : string) : string =
    match s.[1] with
    |'|'|','|'.'|'-' -> // something that replaces the char with "" in the string s
                        (inputFormatter s)
    |_ -> match s.[(String.length s)] with
          |"|"|","|"."|"-" -> // same as above.
                              (inputFormatter s)
      |_ -> s 

Can anyone help me figure out what i could write in my function? Ofcourse you are also welcome to come up with an etirely different function if you find that more conveniet.

thanks in advance!

Answer 1

let replace elem (str:string) =    
    let len = String.length str
    if str.[0] = elem && str.[len-1] = elem then str.[1..len-2]
    else str

Usage:

replace '|' "|red|red|red|red|"

// val it : string = "red|red|red|red"

And here's a version working with string instead of char:

let replace elem (str:string) =    
    let lens = String.length str
    let lene = String.length elem    
    if lene <= lens && str.[0..lene-1] = elem && str.[lens-lene..lens-1] = elem then str.[lene..lens-lene-1]
    else str

UPDATE

As Mark suggested a better option is re-using Trim

let replace (elem:string) (str:string) = str.Trim(elem.ToCharArray())