assertRegExp became assertMatchesRegularExpression

8.x deprecates a set of assertions with names that were shortened years ago, and 9 removes them — so the warnings are a deadline rather than noise.

// deprecated
$this->assertRegExp('/^ORD-d+$/', $ref);
$this->assertNotRegExp('/error/', $body);
$this->assertFileNotExists($path);
$this->assertContains('needle', $haystack);   // string form

// what to write
$this->assertMatchesRegularExpression('/^ORD-d+$/', $ref);
$this->assertDoesNotMatchRegularExpression('/error/', $body);
$this->assertFileDoesNotExist($path);
$this->assertStringContainsString('needle', $haystack);

The assertContains change is the one that matters, because it is not a rename — the old function accepted both arrays and strings, and the split into assertContains for arrays and assertStringContainsString for strings changes behaviour. A blind search and replace gets it wrong in one direction or the other. The rest are mechanical and Rector will do them.