How to manually expand a special variable (ex: ~ tilde) in bash

If the variable var is input by the user, eval should not be used to expand the tilde using

eval var=$var  # Do not use this!

The reason is: the user could by accident (or by purpose) type for example var="$(rm -rf $HOME/)" with possible disastrous consequences.

A better (and safer) way is to use Bash parameter expansion:

var="${var/#\~/$HOME}"