diff --git a/src/Type/Regex/RegexGroupParser.php b/src/Type/Regex/RegexGroupParser.php
index e2ab45d31d..a9066f5b6b 100644
--- a/src/Type/Regex/RegexGroupParser.php
+++ b/src/Type/Regex/RegexGroupParser.php
@@ -448,18 +448,38 @@ private function getRootAlternation(TreeNode $group): ?TreeNode
 		if (
 			$group->getId() === '#capturing'
 			&& count($group->getChildren()) === 1
-			&& $group->getChild(0)->getId() === '#alternation'
 		) {
-			return $group->getChild(0);
+			$child = $group->getChild(0);
+			if ($child->getId() === '#alternation') {
+				return $child;
+			}
+
+			if (
+				$child->getId() === '#noncapturing'
+				&& count($child->getChildren()) === 1
+				&& $child->getChild(0)->getId() === '#alternation'
+			) {
+				return $child->getChild(0);
+			}
 		}
 
 		// 1st token within a named capturing group is a token holding the group-name
 		if (
 			$group->getId() === '#namedcapturing'
 			&& count($group->getChildren()) === 2
-			&& $group->getChild(1)->getId() === '#alternation'
 		) {
-			return $group->getChild(1);
+			$child = $group->getChild(1);
+			if ($child->getId() === '#alternation') {
+				return $child;
+			}
+
+			if (
+				$child->getId() === '#noncapturing'
+				&& count($child->getChildren()) === 1
+				&& $child->getChild(0)->getId() === '#alternation'
+			) {
+				return $child->getChild(0);
+			}
 		}
 
 		return null;
@@ -606,6 +626,10 @@ private function walkGroupAst(
 			$walkResult = $walkResult->numeric(TrinaryLogic::createNo());
 		}
 
+		if (in_array($ast->getId(), ['#lookahead', '#negativelookahead', '#lookbehind', '#negativelookbehind'], true)) {
+			return $walkResult;
+		}
+
 		foreach ($children as $child) {
 			$walkResult = $this->walkGroupAst(
 				$child,
@@ -620,6 +644,10 @@ private function walkGroupAst(
 
 	private function isMaybeEmptyNode(TreeNode $node, string $patternModifiers, bool &$isNonFalsy): bool
 	{
+		if (in_array($node->getId(), ['#lookahead', '#negativelookahead', '#lookbehind', '#negativelookbehind'], true)) {
+			return true;
+		}
+
 		if ($node->getId() === '#quantification') {
 			[$min] = $this->getQuantificationRange($node);
 
@@ -640,6 +668,25 @@ private function isMaybeEmptyNode(TreeNode $node, string $patternModifiers, bool
 			return $literal === '';
 		}
 
+		if ($node->getId() === '#alternation') {
+			$allNonFalsy = true;
+			foreach ($node->getChildren() as $child) {
+				$childNonFalsy = false;
+				if ($this->isMaybeEmptyNode($child, $patternModifiers, $childNonFalsy)) {
+					return true;
+				}
+				if ($childNonFalsy) {
+					continue;
+				}
+
+				$allNonFalsy = false;
+			}
+			if ($allNonFalsy) {
+				$isNonFalsy = true;
+			}
+			return false;
+		}
+
 		foreach ($node->getChildren() as $child) {
 			if (!$this->isMaybeEmptyNode($child, $patternModifiers, $isNonFalsy)) {
 				return false;
diff --git a/tests/PHPStan/Analyser/nsrt/preg_match_shapes.php b/tests/PHPStan/Analyser/nsrt/preg_match_shapes.php
index 8dac3aa80c..e710ea4572 100644
--- a/tests/PHPStan/Analyser/nsrt/preg_match_shapes.php
+++ b/tests/PHPStan/Analyser/nsrt/preg_match_shapes.php
@@ -1074,6 +1074,53 @@ function bug12792(string $string): void {
 	}
 }
 
+function bug12840(string $s): void {
+	// Empty alternation in non-capturing group
+	if (preg_match('~^((?:|\d+))x$~s', $s, $matches) === 1) {
+		assertType("array{non-falsy-string, ''|numeric-string}", $matches);
+	}
+
+	// Lookahead in alternation - lookahead is zero-width
+	if (preg_match('~^(a?(?:(?=x)|y))x$~s', $s, $matches) === 1) {
+		assertType("array{non-falsy-string, string}", $matches);
+	}
+
+	// Lookbehind in alternation - lookbehind is zero-width
+	if (preg_match('~^(a?(?:(?<=x)|y))x$~s', $s, $matches) === 1) {
+		assertType("array{non-falsy-string, string}", $matches);
+	}
+
+	// Negative lookahead - zero-width
+	if (preg_match('~^(a?(?:(?!z)|y))x$~s', $s, $matches) === 1) {
+		assertType("array{non-falsy-string, string}", $matches);
+	}
+
+	// Negative lookbehind - zero-width
+	if (preg_match('~^(a?(?:(?<!z)|y))x$~s', $s, $matches) === 1) {
+		assertType("array{non-falsy-string, string}", $matches);
+	}
+
+	// Lookahead alone in group should not produce non-empty-string (it's zero-width)
+	if (preg_match('~^((?=x))x$~s', $s, $matches) === 1) {
+		assertType("array{non-falsy-string, string}", $matches);
+	}
+
+	// Two non-capturing groups with empty alternation
+	if (preg_match('~^((?:|\d+)(?:|\w+))x$~s', $s, $matches) === 1) {
+		assertType("array{non-falsy-string, string}", $matches);
+	}
+
+	// Named capturing group with non-capturing wrapper alternation
+	if (preg_match('~^(?P<g>(?:|\d+))x$~s', $s, $matches) === 1) {
+		assertType("array{0: non-falsy-string, g: ''|numeric-string, 1: ''|numeric-string}", $matches);
+	}
+
+	// Lookahead in concatenation with literal - group still captures the literal
+	if (preg_match('~^((?=x)a)x$~s', $s, $matches) === 1) {
+		assertType("array{non-falsy-string, non-falsy-string}", $matches);
+	}
+}
+
 function testExtendedSyntaxEscapedHash(string $string): void {
 	// \# in extended mode should be treated as literal hash, not comment
 	if (preg_match('/^ ([\#.]) $/x', $string, $matches)) {