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Parsing expressions

Answer

let dig s = 
  match s with
  | "" -> []
  | _  -> if ('0'<=s.[0])&(s.[0]<='9') then [(s.[0],s.[1..])] else [];;

dig "12";;
rep dig "12";;

Alternativly, by mastering functional/lists' functions:

let dig2 = List.map sym ['0'..'9'] |> List.reduce (||);;
dig2 "12";;
let num2 = rep dig2;;
num2 "12";;

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