[ to replace "["
] to replace "]"

Remember not to switch from html editor when using it

<?php make_clickable( $ret ) ?>

no need to write complete

<a href ...

etc, hyperlink plain text will appears as clickable link.

Regularly put it in wp-content/themes/some_theme/function.php as

...
add_filter( 'the_content', 'make_clickable' );
...