How to remove empty lines from a string in bash using parameter expansion?

Question

I have the following variable.

echo "|${VAR1}|"

which returns

|
ABC

XYZ|

How can I remove the empty lines, preserving line breaks and using parameter expansion? So that it would become

|ABC
XYZ|

p.s.: I know how to do it using pipe sed, but I would like to avoid the extra SED process:

VAR1=`echo "${VAR1}" | sed '/^\s*$/d'`

Answer 1

Remove the leading newlines, then replace any consecutive newlines with a single one.

#! /bin/bash

var='
ABC

XYZ'

expected='ABC
XYZ'

shopt -s extglob
var=${var##+($'\n')}
var=${var//+($'\n')/$'\n'}
[[ $var == $expected ]] && echo OK