preg_replace, working out the search and replace values

I've got a table with lots of content that assume the site and it's subdomains are on http://

New content is being created with https:// and we don't want to alter the old records but if they're called we want to change the http to an https. Not all links, just the ones that link back to the site.

so I've got
View attachment 227930
etc

I could just set up an array with each subdomain but I figured it'd be more elegant to use preg_replace. What would I use as the search and replace...

preg_replace, working out the search and replace values

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