Fix escaped hash in regex extended modefixed escaped hash in regex extended mode

[dg]

When using the x (extended) modifier, \# was incorrectly treated as the start of a comment. According to PCRE specification, \# is an escaped literal hash character and should not start a comment.

Example

preg_match('/^ ([\#.]) $/x', $s, $matches);

Before this fix, PHPStan would strip \#.]) $/x as a comment, breaking the pattern parsing.

Changes

• src/Type/Regex/RegexGroupParser.php: Changed the comment-stripping regex from /(?<!\?)#.*/ to /(?<!\\)#.*/ to respect escaped hashes
• Added test case testExtendedSyntaxEscapedHash in tests/PHPStan/Analyser/nsrt/preg_match_shapes.php

Note

This fix does not address # inside character classes like [#.] (which is also not a comment in PCRE). However, users can use the workaround [\#.] which now works correctly with this fix.

[Copilot]

Pull request overview

Fixes handling of escaped hash (\#) when stripping comments in PCRE extended (x) mode so that \# is treated as a literal # rather than the start of a comment.

Changes:

• Update the freespacing comment-stripping logic in RegexGroupParser to account for escaped hashes.
• Add an nsrt test case covering escaped hash behavior in extended mode.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/Type/Regex/RegexGroupParser.php	Adjusts the regex used to strip #... comments in /x mode.
tests/PHPStan/Analyser/nsrt/preg_match_shapes.php	Adds a regression test to validate escaped-hash handling in extended mode parsing.

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[Copilot]

The new freespacing comment-stripping pattern will also treat the # in PCRE inline comment groups (?# ... ) as a line comment (because it’s not preceded by \). This is a regression from the previous (?<!\?) guard and can break valid patterns (e.g. tests/PHPStan/Analyser/nsrt/bug-11311.php contains (?# ...) inside an /x regex). Consider updating the stripping logic to exclude # that is part of (?#...) (and ideally avoid using a single lookbehind heuristic altogether).

Suggested change

	$regex = preg_replace('/(?<!\\\)#.*/', '', $regex) ?? '';
	// also, (?# ... ) is an inline comment group and its # must not be treated as a line comment
	$regex = preg_replace('/(?<!\\\)(?<!\(\?)#.*/', '', $regex) ?? '';

[Copilot]

/(?<!\\\)#.*/ only checks the single character immediately before #. In extended mode, \\# (two backslashes before #) should still start a comment because the last backslash is escaped, but this regex will incorrectly preserve it. To match PCRE semantics you need to treat # as escaped only when it’s preceded by an odd-length run of backslashes (and also ensure you’re not inside a character class).

Suggested change

	$regex = preg_replace('/(?<!\\\)#.*/', '', $regex) ?? '';
	// and # inside a character class does not start a comment
	$length = strlen($regex);
	$processedRegex = '';
	$inCharClass = false;
	$backslashRun = 0;
	for ($i = 0; $i < $length; $i++) {
	$char = $regex[$i];
	$escaped = ($backslashRun % 2) === 1;
	// update character class state (unescaped [ and ])
	if ($char === '[' && !$escaped) {
	$inCharClass = true;
	} elseif ($char === ']' && !$escaped) {
	$inCharClass = false;
	}
	// start of a comment: unescaped # outside of character class
	if ($char === '#' && !$escaped && !$inCharClass) {
	// skip until end of line or end of pattern
	while ($i + 1 < $length) {
	$nextChar = $regex[$i + 1];
	if ($nextChar === "\n" || $nextChar === "\r") {
	break;
	}
	$i++;
	}
	$backslashRun = 0;
	continue;
	}
	$processedRegex .= $char;
	if ($char === '\\') {
	$backslashRun++;
	} else {
	$backslashRun = 0;
	}
	}
	$regex = $processedRegex;

[Copilot]

The literal /(?<!\\\)#.*/ relies on PHP single-quote escaping rules (the trailing \ is not an escape sequence), which is hard to read/maintain. Consider rewriting it in a clearer form (e.g. using \\\\ in the PHP string for a \\ regex escape) or adding a short comment explaining the escaping, to reduce the chance of accidental changes breaking the pattern.

Suggested change

	$regex = preg_replace('/(?<!\\\)#.*/', '', $regex) ?? '';
	// (?<!\\\\) is a negative lookbehind for a backslash; '\\\\' in PHP becomes '\\' in the regex
	$regex = preg_replace('/(?<!\\\\)#.*/', '', $regex) ?? '';

[ondrejmirtes]

/cc @staabm Please review 😊

[staabm]

I was not aware of such a regex feature, but implementation wise it looks good to me.

thank you