I was using the ARVE video embedder plugin on WordPress.
Observing a page referencing either Vimeo or YouTube I saw the error:
Warning: preg_match(): Compilation failed: two named subpatterns have the same name at offset 75 in /wp-content/plugins/advanced-responsive-video-embedder/public/functions-shortcode-filters.php on line 255
Disabling plugins didn’t cure the issue
And reverting to the default TwentySeventeen theme didn’t resolve the issue
I configured another website with the plugin with the aim of comparing the two websites. There was no error!
ARVE has a handy Debug Info tab on its settings page. I used this to compare the two websites.
Here’s the part of interest:
ARVE Version: 8.9.10 ARVE-Pro Version: NOT INSTALLED WordPress Version: 4.9.8 PHP Version: 5.6.38
The working website was running version 7 of PHP.
I switched the version on the erroring site to 7.0. The error message disappeared.