Hi Marco,

I also do not know of a built-in way to do this, but here is my shot at an implementation in XQuery. It uses fast exponentiation [1] and throws a custom error if there is more than one "e" in the input.

Hope that helps (or is of interest), I had fun hacking it together,
  Leo

    declare function local:pow($n, $k) {
      if($k eq 0) then xs:decimal('1')
      else if($k eq 1) then $n
      else (
        let $d := $k idiv 2,
            $m := $k mod 2
        return if($m eq 0) then local:pow($n * $n, $d)
        else local:pow($n * $n, $d) * $n
      )
    };

    declare function local:parse-decimal($str as xs:string) as xs:decimal {
      let $parts := tokenize($str, '[eE]')
      return switch(count($parts))
        case 1 return xs:decimal($str)
        case 2 return (
          let $exp  := xs:long($parts[2]),
              $base := xs:decimal($parts[1]),
              $b10  := if($exp gt 0) then xs:decimal('10') else xs:decimal('0.1')
          return $base * local:pow($b10, $exp)
        )
        default return error(xs:QName('local:NODECIML'), 'Wrong format: ' || $str)
    };

    local:parse-decimal('123.45e-300')

[1] https://en.wikipedia.org/wiki/Exponentiation_by_squaring

Am Mittwoch, 01. November 2017 13:21 CET, Marco Lettere <m.lettere@gmail.com> schrieb:
> Thanks Kristian,
> unfortunately this introduces rounding errors which I'm trying to avoid
> because managing units, measures and monetary amounts.
> M.
>
> Il 01 nov 2017 12:57 PM, "Kristian Kankainen" <kristian@keeleleek.ee> ha
> scritto:
>
> You need to use xs:float or xs:double instead of xs:decimal to be able to
> use the 'e' or 'E' as the exponent separator.
>
> Br,
> Kristian K1. nov 2017 13:18 kirjutas kuupäeval Marco Lettere <
> m.lettere@gmail.com>:
> >
> > Hi all,
> >
> > I thought of asking this in parallel of hacking my own parsing procedure
> > ...
> >
> > Is there a native way to parse scientific notation string into
> > xs:decimal since a direct casting (xs:decimal("1e1")) is not allowed?
> >
> > Thanks,
> >
> > Marco.
> >