Count the characters matching alphabets

Try this expression, it should work for you assuming you want to only return the first instance (with count) of each character.  E.g., "Caterpillar" = "c1a2t1e1r2p1i1l2".

a!localVariables(
  local!alphabet: "abcdefghijklmnopqrstuvwxyz",
  local!input: "Umbrella",
  
  local!inputArray: a!forEach(
    items: 1+enumerate(len(local!input)),
    expression: lower(charat(local!input,fv!index))
  ),
  local!chars: union(local!inputArray,local!inputArray),
  
  joinarray(
    reject(
      fn!isnull,
      a!forEach(
        items: local!chars,
        expression: if(
          find(fv!item,local!alphabet,1)>0,
          concat(
            fv!item,
            count(wherecontains(fv!item,local!inputArray))
          ),
          null
        )
      )
    )
  )
)