Bad user input from some ustring pattern in some Lua module (or possibly the console). Possibly a user testing something.

Probably the relevant code should silence the error here - it already indicates to the user the match failed.

There's already a generous limit of 10_000 bytes for the pattern length. But it's not clear if it's possible to check the nesting depth without executing the regex. Else silencing the error might the better option.

$p = str_repeat('(', 200) . str_repeat(')', 200);
$p2 = str_repeat('(', 300) . str_repeat(')', 300);

preg_match("/^$p$/", "foo"); // No error
preg_match("/^$p2$/", "foo"); // Compilation failed: parentheses are too deeply nested at offset 

Change #1217455 had a related patch set uploaded (by Ammarpad; author: Ammarpad):

[mediawiki/extensions/Scribunto@master] UstringLibrary: Silence preg_match warnings

https://gerrit.wikimedia.org/r/1217455