Distinguish preg_match() engine failure from "no match" in all dispatchers

src/Dispatcher/CharCountBased.php

@@ -7,7 +7,6 @@
 
 use function assert;
 use function end;
-use function preg_match;
 
 /** @final */
 class CharCountBased extends RegexBasedAbstract
@@ -18,7 +17,7 @@ protected function dispatchVariableRoute(array $routeData, string $uri): ?Matche
         foreach ($routeData as $data) {
             assert(isset($data['suffix']));
 
-            if (preg_match($data['regex'], $uri . $data['suffix'], $matches) !== 1) {
+            if ($this->matchRoute($data['regex'], $uri . $data['suffix'], $matches) !== 1) {
                 continue;
             }
 

src/Dispatcher/GroupCountBased.php

@@ -6,7 +6,6 @@
 use FastRoute\Dispatcher\Result\Matched;
 
 use function count;
-use function preg_match;
 
 /** @final */
 class GroupCountBased extends RegexBasedAbstract
@@ -15,7 +14,7 @@ class GroupCountBased extends RegexBasedAbstract
     protected function dispatchVariableRoute(array $routeData, string $uri): ?Matched
     {
         foreach ($routeData as $data) {
-            if (preg_match($data['regex'], $uri, $matches) !== 1) {
+            if ($this->matchRoute($data['regex'], $uri, $matches) !== 1) {
                 continue;
             }
 

src/Dispatcher/GroupPosBased.php

@@ -6,7 +6,6 @@
 use FastRoute\Dispatcher\Result\Matched;
 
 use function assert;
-use function preg_match;
 
 /** @final */
 class GroupPosBased extends RegexBasedAbstract
@@ -15,7 +14,7 @@ class GroupPosBased extends RegexBasedAbstract
     protected function dispatchVariableRoute(array $routeData, string $uri): ?Matched
     {
         foreach ($routeData as $data) {
-            if (preg_match($data['regex'], $uri, $matches) !== 1) {
+            if ($this->matchRoute($data['regex'], $uri, $matches) !== 1) {
                 continue;
             }
 

src/Dispatcher/MarkBased.php

@@ -5,16 +5,14 @@
 
 use FastRoute\Dispatcher\Result\Matched;
 
-use function preg_match;
-
 /** @final */
 class MarkBased extends RegexBasedAbstract
 {
     /** @inheritDoc */
     protected function dispatchVariableRoute(array $routeData, string $uri): ?Matched
     {
         foreach ($routeData as $data) {
-            if (preg_match($data['regex'], $uri, $matches) !== 1) {
+            if ($this->matchRoute($data['regex'], $uri, $matches) !== 1) {
                 continue;
             }
 

src/Dispatcher/RegexBasedAbstract.php

@@ -8,6 +8,11 @@
 use FastRoute\Dispatcher\Result\Matched;
 use FastRoute\Dispatcher\Result\MethodNotAllowed;
 use FastRoute\Dispatcher\Result\NotMatched;
+use RuntimeException;
+
+use function preg_last_error_msg;
+use function preg_match;
+use function sprintf;
 
 /**
  * @internal
@@ -35,6 +40,26 @@ public function __construct(array $data)
     /** @param DynamicRouteChunks $routeData */
     abstract protected function dispatchVariableRoute(array $routeData, string $uri): ?Matched;
 
+    /**
+     * Matches a route regex, distinguishing a genuine "no match" (preg_match()
+     * returns 0) from a PCRE engine failure such as PREG_BACKTRACK_LIMIT_ERROR
+     * (returns false), which would otherwise fail unrelated routes sharing the
+     * same combined regex chunk.
+     *
+     * @param array<int|string, mixed>|null $matches
+     */
+    protected function matchRoute(string $regex, string $subject, ?array &$matches = null): int
+    {
+        $result = preg_match($regex, $subject, $matches);
+        if ($result === false) {
+            throw new RuntimeException(
+                sprintf('Regex matching failed for "%s": %s', $regex, preg_last_error_msg()),
+            );
+        }
+
+        return $result;
+    }
+
     public function dispatch(string $httpMethod, string $uri): Matched|NotMatched|MethodNotAllowed
     {
         if (isset($this->staticRouteMap[$httpMethod][$uri])) {

test/Dispatcher/DispatcherTestCase.php

@@ -13,8 +13,11 @@
 use FastRoute\Dispatcher\Result\NotMatched;
 use PHPUnit\Framework\Attributes as PHPUnit;
 use PHPUnit\Framework\TestCase;
+use RuntimeException;
 
 use function FastRoute\simpleDispatcher;
+use function ini_set;
+use function str_repeat;
 
 /** @phpstan-import-type ExtraParameters from DataGenerator */
 abstract class DispatcherTestCase extends TestCase
@@ -177,6 +180,36 @@ public function capturing(): void
         }, $this->generateDispatcherOptions());
     }
 
+    /**
+     * A PCRE engine failure must surface instead of being reported as
+     * NOT_FOUND for an unrelated route in the same combined regex chunk.
+     *
+     * @see https://github.com/nikic/FastRoute/issues/167
+     */
+    #[PHPUnit\Test]
+    public function regexEngineFailureIsNotSilentlyTreatedAsNotFound(): void
+    {
+        $previousLimit = ini_set('pcre.backtrack_limit', '1000');
+        self::assertNotFalse($previousLimit, 'Unable to lower pcre.backtrack_limit for this test');
+
+        try {
+            $this->expectException(RuntimeException::class);
+            $this->expectExceptionMessageMatches('/Backtrack limit exhausted/');
+
+            $dispatcher = simpleDispatcher(static function (ConfigureRoutes $r): void {
+                // Catastrophically backtracking pattern.
+                $r->addRoute('GET', '/{p:(?:a?a?)*}/complicated', 'complicated_pattern');
+                // Cheap route in the same chunk: it would match on its own, but the
+                // chunk's preg_match() call fails.
+                $r->addRoute('GET', '/{p:a+}', 'cheap_route');
+            }, $this->generateDispatcherOptions());
+
+            $dispatcher->dispatch('GET', '/' . str_repeat('a', 30));
+        } finally {
+            ini_set('pcre.backtrack_limit', $previousLimit);
+        }
+    }
+
     /** @return iterable<string, array{0: string, 1: string, 2: Closure(ConfigureRoutes):void, 3: string, 4?: array<string, string>}> */
     public static function provideFoundDispatchCases(): iterable
     {