[BUGFIX] Improve selector validation performance (#1372)

Avoid [catastrophic backtracking](https://www.regular-expressions.info/catastrophic.html)
in selector validation regular expression by using possessive quantifier with
mutually exclusive alternations.

Also remove outdated description from DocBlock,
but add description for extended class summarizing differences.

Author: JakeQZ

CHANGELOG.md

@@ -18,6 +18,8 @@ Please also have a look at our
 
 ### Fixed
 
+- Improve performance of selector validation
+  (avoiding silent PCRE catastrophic failure) (#1372)
 - Use typesafe versions of PHP functions (#1368, #1370)
 
 ### Documentation

src/Property/KeyframeSelector.php

@@ -7,21 +7,41 @@
 class KeyframeSelector extends Selector
 {
     /**
-     * regexp for specificity calculations
+     * This differs from the parent class:
+     * - comma is not allowed unless escaped or quoted;
+     * - percentage value is allowed by itself.
      *
-     * @var string
+     * @var non-empty-string
      *
      * @internal since 8.5.2
      */
     public const SELECTOR_VALIDATION_RX = '/
-    ^(
-        (?:
-            [a-zA-Z0-9\\x{00A0}-\\x{FFFF}_^$|*="\'~\\[\\]()\\-\\s\\.:#+>]* # any sequence of valid unescaped characters
-            (?:\\\\.)?                                                     # a single escaped character
-            (?:([\'"]).*?(?<!\\\\)\\2)?                                    # a quoted text like [id="example"]
-        )*
-    )|
-    (\\d+%)                                                                # keyframe animation progress percentage (e.g. 50%)
-    $
-    /ux';
+        ^(
+            (?:
+                # any sequence of valid unescaped characters, except quotes
+                [a-zA-Z0-9\\x{00A0}-\\x{FFFF}_^$|*=~\\[\\]()\\-\\s\\.:#+>]++
+                |
+                # one or more escaped characters
+                (?:\\\\.)++
+                |
+                # quoted text, like in `[id="example"]`
+                (?:
+                    # opening quote
+                    ([\'"])
+                    (?:
+                        # sequence of characters except closing quote or backslash
+                        (?:(?!\\g{-1}|\\\\).)++
+                        |
+                        # one or more escaped characters
+                        (?:\\\\.)++
+                    )*+ # zero or more times
+                    # closing quote or end (unmatched quote is currently allowed)
+                    (?:\\g{-1}|$)
+                )
+            )*+ # zero or more times
+            |
+            # keyframe animation progress percentage (e.g. 50%), untrimmed
+            \\s*+(\\d++%)\\s*+
+        )$
+        /ux';
 }

src/Property/Selector.php

@@ -15,19 +15,34 @@
 class Selector implements Renderable
 {
     /**
-     * regexp for specificity calculations
-     *
-     * @var string
+     * @var non-empty-string
      *
      * @internal since 8.5.2
      */
     public const SELECTOR_VALIDATION_RX = '/
         ^(
             (?:
-                [a-zA-Z0-9\\x{00A0}-\\x{FFFF}_^$|*="\'~\\[\\]()\\-\\s\\.:#+>,]* # any sequence of valid unescaped characters
-                (?:\\\\.)?                                                      # a single escaped character
-                (?:([\'"]).*?(?<!\\\\)\\2)?                                     # a quoted text like [id="example"]
-            )*
+                # any sequence of valid unescaped characters, except quotes
+                [a-zA-Z0-9\\x{00A0}-\\x{FFFF}_^$|*=~\\[\\]()\\-\\s\\.:#+>,]++
+                |
+                # one or more escaped characters
+                (?:\\\\.)++
+                |
+                # quoted text, like in `[id="example"]`
+                (?:
+                    # opening quote
+                    ([\'"])
+                    (?:
+                        # sequence of characters except closing quote or backslash
+                        (?:(?!\\g{-1}|\\\\).)++
+                        |
+                        # one or more escaped characters
+                        (?:\\\\.)++
+                    )*+ # zero or more times
+                    # closing quote or end (unmatched quote is currently allowed)
+                    (?:\\g{-1}|$)
+                )
+            )*+ # zero or more times
         )$
         /ux';