sebastianbergmann/comparator · error · ComparisonFailure

Failed asserting that

Error message

Failed asserting that %s matches expected %s.

What it means

Thrown by ScalarComparator::assertEquals when two string values differ. Before throwing, the comparator strips the overlong common prefix and suffix of the two strings so the ComparisonFailure diff shows only the diverging portion; the %s placeholders carry the exported actual and expected strings.

Solutions

1. Read the ComparisonFailure diff; it already trims the overlong common prefix/suffix to show just the diverging region of the two strings.
2. Inspect both strings for invisible differences: trailing whitespace, newline style, encoding, or multi-byte characters.
3. Use mb_strtolower or normalize both strings if case or encoding differences are expected.
4. Compare a substring or a computed value rather than full strings when the assertion targets specific content.
5. Correct the code path producing the actual string so it matches the expected value.

When it happens

Trigger: Thrown at src/ScalarComparator.php:84 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of sebastianbergmann/comparator@00837a9d22 (2026-09-15). Data as JSON: /api/errors/ab0068191857e7b7. Report an issue: GitHub.

Appendix: source

Thrown at src/ScalarComparator.php:84

        }

        if ($expectedToCompare !== $actualToCompare && is_string($expected) && is_string($actual)) {
            [$cutExpected, $cutActual] = self::removeOverlongCommonPrefix($expected, $actual);
            [$cutExpected, $cutActual] = self::removeOverlongCommonSuffix($cutExpected, $cutActual);

            throw new ComparisonFailure(
                $expected,
                $actual,
                $exporter->export($cutExpected),
                $exporter->export($cutActual),
                'Failed asserting that two strings are equal.',
                $this->contextLines(),
            );
        }

        /** @phpstan-ignore notEqual.notAllowed */
        if ($expectedToCompare != $actualToCompare) {
            throw new ComparisonFailure(
                $expected,
                $actual,
                // no diff is required
                '',
                '',
                sprintf(
                    'Failed asserting that %s matches expected %s.',
                    $exporter->export($actual),
                    $exporter->export($expected),
                ),
                $this->contextLines(),
            );
        }
    }

    /**
     * @return array{string, string}
     */

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