DNA GC percentage problem in F#

open System
open System.IO
open System.Collections.Generic
open System.Text
open System.Text.RegularExpressions

#load "Library1.fs"
open Library1

// Define your library scripting code here
let regex s = new Regex(s)
let (=~) s (re:Regex) = re.IsMatch(s)
let (<>~) s (re:Regex) = not (s =~ re)

let samplestring = "This is a string";;
let readLines filePath = System.IO.File.ReadLines("sample.txt");;

samplestring =~ regex "This"

let elementOf x e = Seq.exists ((=) e) x     //  elementOf "abc" 'c'

let filterString (s : string) (filter : string) =
  s 
  |> Seq.filter (fun x -> elementOf filter x)

let readDNAdata =
    "\dev\sample.txt"
    |> System.IO.File.ReadAllLines 
    |> String.concat ""

let parseDNAdata (line : string) =
  ((regex "(?<=\d)(?=[ACTG])").Split(line)).[0..1]

let GCpercent (dnaData : string []) =
  let id = dnaData.[0]
  let dnaseq = dnaData.[1]
  let allCount = dnaseq.Length
  let GCcount = (filterString dnaseq "GC") |> Seq.length
  (id, 100.0 * float(GCcount) / float(allCount), dnaseq)


let getDNAdata = 
  readDNAdata.Split '>'
  |> Seq.map parseDNAdata 
  |> Seq.map GCpercent


getDNAdata

ninjarobot,

Thanks. I did some changes to that function to eliminate the out of bounds and agree it is (almost certainly) there.

Thanks. This was my first F# program so I wasn't immediately sure how to do the match...with and really appreciate your example and will review/use/adapt it.