[Bug]: count-based --shard fallback selects zero tests on large suites (PCRE backtrack limit in --filter)

What happened?

On a large suite (~1700+ test classes), running pest --shard=1/4 without a tests/.pest/shards.json timings file makes shard 1 report No tests found and exit 1, even though hundreds of classes were assigned to it. Other shards run normally.

Root cause

In src/Plugins/Shard.php::handleArguments, when no timings file exists, tests are split into contiguous chunks:

// line 135
$testsToRun = (array_chunk($tests, max(1, (int) ceil(count($tests) / $total))))[$index - 1] ?? [];

and each chunk becomes a single |-joined regex:

// line 215
return addslashes(implode('|', $testsToRun));

Because --list-tests returns classes in discovery/alphabetical order, the first contiguous chunk is a cluster of classes sharing long common prefixes (e.g. Tests\Feature\Api\Admin\…). Joined into one unanchored alternation (~36 KB, ~436 alternatives), matching it triggers catastrophic backtracking: preg_match returns false with preg_last_error() === PREG_INTERNAL_ERROR. PHPUnit treats the errored filter as "matched nothing", so the shard runs zero tests and exits 1.

Why a committed shards.json hides it

The timings path distributes new (untimed) tests round-robin:

// line 128
$partitions[$i % $total][] = $test;

Interleaving scatters the clustered prefixes across shards, so no single filter is large/homogeneous enough to hit the limit. Any present timings file — even an empty {"timings":{}} — therefore avoids the bug; only the no-cache fallback is affected.

How to reproduce it

1. A suite with enough classes that ceil(total/shards) exceeds ~400 contiguous prefix-clustered classes.
2. Ensure tests/.pest/shards.json does not exist.
3. Run pest --shard=1/4.
4. Shard 1 → No tests found, exit 1.

Minimal confirmation of the mechanism:

$pat = '/'.addslashes(implode('|', $first436ContiguousClasses)).'/';
preg_match($pat, $someClass.'::x'); // false
echo preg_last_error();             // 1 (PREG_INTERNAL_ERROR)
// the same 436 classes interleaved → matches fine

Suggested fix

Make the no-cache fallback interleave like the new-tests path instead of using contiguous array_chunk — e.g. round-robin assignment ($i % $total). That breaks up the prefix clusters and keeps each shard's filter within PCRE limits. (Chunking/anchoring the emitted --filter, or writing it to a --filter file, would also help.)

Package Version

4.7.2 (fallback code identical since at least 4.3.2)

PHP Version

8.3

Operation System

Linux / macOS

Start in src/Plugins/Shard.php::handleArguments and inspect the no-cache fallback around the array_chunk assignment, comparing it with the round-robin timed-tests path. Reproduce with a large suite and pest --shard=1/4 without tests/.pest/shards.json. Done means shard 1 no longer reports No tests found when classes are assigned, while the existing shard behavior remains intact.