Exactly the same issue with https://www.stmaryoftheangels.org.uk just now. “Warning: preg_match(): Compilation failed: unrecognized character after (?< at offset 4”. If you log out of the admin panel you can’t get back in.
Thankfully backed up the Files and DB beforehand so could roll back to prior version and this fixed the issue. I’m not going to upgrade to 4.9 until the issue is fixed, either Easily or with www.ads-software.com.