Groups in conditional regex subpatterns are optional and conditional subpatterns might match an empty string

Amp-Thread-ID: https://ampcode.com/threads/T-01a177b5-960f-71d2-a3e2-fa7649f39d2c

Author: ondrejmirtes

src/Type/Regex/RegexGroupParser.php

@@ -53,6 +53,8 @@ final class RegexGroupParser
 
 	private const ASSERTIONS = ['#lookahead', '#lookbehind', '#negativelookahead', '#negativelookbehind'];
 
+	private const CONDITIONS = ['#namedcondition', '#absolutecondition', '#relativecondition', '#assertioncondition'];
+
 	private static ?RegexParser $parser = null;
 
 	/** @var array<string, ?RegexNode> */
@@ -291,8 +293,12 @@ private function walkRegexAst(
 				true,
 			);
 			$parentGroup = $group;
-		} elseif (in_array($ast->getId(), self::NEGATIVE_ASSERTIONS, true)) {
-			// capturing groups in a negative assertion are never set when the whole pattern matches
+		} elseif (
+			in_array($ast->getId(), self::NEGATIVE_ASSERTIONS, true)
+			|| in_array($ast->getId(), self::CONDITIONS, true)
+		) {
+			// capturing groups in a negative assertion are never set when the whole pattern matches,
+			// capturing groups in a conditional subpattern are set only when its condition says so
 			$group = new RegexNonCapturingGroup(
 				$alternation,
 				true,
@@ -515,6 +521,11 @@ private function walkGroupAst(
 			return $walkResult;
 		}
 
+		if (in_array($ast->getId(), self::CONDITIONS, true)) {
+			// which branch of a conditional subpattern matches is not known, it might match an empty string
+			return $walkResult->onlyLiterals(null)->numeric(TrinaryLogic::createNo());
+		}
+
 		$children = $ast->getChildren();
 		$quantifiedLiterals = null;
 
@@ -785,7 +796,7 @@ private function repeatLiterals(array $literals, int $min, int $max): ?array
 
 	private function isMaybeEmptyNode(RegexNode $node, string $patternModifiers, bool &$isNonFalsy): bool
 	{
-		if (in_array($node->getId(), self::ASSERTIONS, true)) {
+		if (in_array($node->getId(), self::ASSERTIONS, true) || in_array($node->getId(), self::CONDITIONS, true)) {
 			return true;
 		}
 

tests/PHPStan/Analyser/nsrt/preg_match_shapes.php

@@ -1175,3 +1175,20 @@ function testBranchResetNotInOnlyTopLevelAlternation(string $s): void {
 		assertType('array<string>', $matches);
 	}
 }
+
+function testConditionalSubpatterns(string $s): void {
+	// from PCRE2 testdata/testinput1
+	if (preg_match('/^(\()?blah(?(1)(\)))$/', $s, $matches)) {
+		assertType("list{0: non-falsy-string, 1?: ''|'(', 2?: ')'}", $matches);
+	}
+
+	// from PCRE2 testdata/testinput2
+	if (preg_match('/((?(-1)a))/', $s, $matches)) {
+		assertType('array{string, string}', $matches);
+	}
+
+	// from PCRE2 testdata/testinput1
+	if (preg_match('/(?(DEFINE)(?<foo>bar))(?<![-a-z0-9])word/', $s, $matches)) {
+		assertType("array{0: non-falsy-string, foo: 'bar', 1: 'bar'}|array{non-falsy-string}", $matches);
+	}
+}