upcase!

When I use upcase! for array in regexp,I got error.

ex.)

str = 'ID5'
str.ucase        # -> ID5
str.upcase!      # -> nil

When I use option -i(ignore case) for array in regexp,I got error.

ex.)

str = 'AaA'
/^#{str}$/ =~ 'AAA'             # -> nil
/^#{str}$/i =~ 'AAA'            # -> nil (This is strange)
/^#{str.upcase!}$/ =~ 'AAA'     # -> nil (This is also strange)
/^#{str.upcase}$/ =~ 'AAA'      # -> 0