I ran this with an arc linter on a private repository which doesn't have a regex to match severity:
[2023-04-18 10:33:24] EXCEPTION: (PhutilAggregateException) Some linters failed: - RuntimeException: strtolower(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/lint/engine/ArcanistLintEngine.php:264] arcanist(), tmphutil() #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<arcanist>/src/error/PhutilErrorHandler.php:261] #1 <#2> strtolower(NULL) called at [<arcanist>/src/lint/linter/ArcanistScriptAndRegexLinter.php:379] #2 <#2> ArcanistScriptAndRegexLinter::getMatchSeverity(array) called at [<arcanist>/src/lint/linter/ArcanistScriptAndRegexLinter.php:255] #3 <#2> ArcanistScriptAndRegexLinter::lintPath(string) called at [<arcanist>/src/lint/engine/ArcanistLintEngine.php:578] #4 <#2> ArcanistLintEngine::executeLinterOnPaths(ArcanistScriptAndRegexLinter, array) called at [<arcanist>/src/lint/engine/ArcanistLintEngine.php:536] #5 <#2> ArcanistLintEngine::executeLintersOnChunk(array, array) called at [<arcanist>/src/lint/engine/ArcanistLintEngine.php:474] #6 <#2> ArcanistLintEngine::executeLinters(array) called at [<arcanist>/src/lint/engine/ArcanistLintEngine.php:206] #7 ArcanistLintEngine::run() called at [<arcanist>/src/workflow/ArcanistLintWorkflow.php:219] #8 ArcanistLintWorkflow::run() called at [<arcanist>/scripts/arcanist.php:427]
(Reported by @goddenrich)