fix empty token parse in capturing group

Author: mvorisek

src/Type/Regex/RegexGroupParser.php

@@ -80,6 +80,7 @@ public function parseGroups(string $regex): ?array
 		}
 
 		$this->updateAlternationAstRemoveVerticalBarsAndAddEmptyToken($ast);
+		$this->updateCapturingAstAddEmptyToken($ast);
 
 		$captureOnlyNamed = false;
 		if ($this->phpVersion->supportsPregCaptureOnlyNamedGroups()) {
@@ -131,6 +132,21 @@ private function updateAlternationAstRemoveVerticalBarsAndAddEmptyToken(TreeNode
 		$ast->setChildren(array_values($children));
 	}
 
+	private function updateCapturingAstAddEmptyToken(TreeNode $ast): void
+	{
+		foreach ($ast->getChildren() as $child) {
+			$this->updateCapturingAstAddEmptyToken($child);
+		}
+
+		if ($ast->getId() !== '#capturing' || $ast->getChildren() !== []) {
+			return;
+		}
+
+		$emptyAlternationAst = new TreeNode('#alternation', null, [], $ast);
+		$emptyAlternationAst->setChildren([$this->createEmptyTokenTreeNode($emptyAlternationAst)]);
+		$ast->setChildren([$emptyAlternationAst]);
+	}
+
 	private function walkRegexAst(
 		TreeNode $ast,
 		?RegexAlternation $alternation,

tests/PHPStan/Analyser/nsrt/preg_match_shapes.php

@@ -907,7 +907,7 @@ function bugEmptySubexpression (string $string): void {
 	}
 
 	if (preg_match('/()/', $string, $matches)) {
-		assertType("array{string, string}", $matches); // could be array{'', ''}
+		assertType("array{string, ''}", $matches); // could be array{'', ''}
 	}
 
 	if (preg_match('/|/', $string, $matches)) {
@@ -939,7 +939,7 @@ function bugEmptySubexpression (string $string): void {
 	}
 
 	if (preg_match('~((a)|()|(b))~', $string, $matches)) {
-		assertType("array{0: string, 1: string, 2?: ''|'a', 3?: string, 4?: 'b'}", $matches);
+		assertType("array{0: string, 1: ''|'a'|'b', 2?: ''|'a', 3?: '', 4?: 'b'}", $matches);
 	}
 }