Manual:$wgEnableMagicLinks

Controls whether magic link functionality should be enabled.

Example
For example, to enable ISBN auto-linking, you can use:

If auto-linking was previously disabled and you've enabled it as described above, you'll most likely need to purge the cache of the affected pages. For purging all pages in a given namespace or all pages in all namespaces, you can use the purgeList.php maintenance script.