LibURL/Pattern: Implement helper for escaping a URL Pattern String

Author: https://github.com/shannonbooth
Commit: 6b85748f53
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3847
Reviewed-by: https://github.com/trflynn89

Libraries/LibURL/Pattern/String.cpp

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2025, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/StringBuilder.h>
+#include <LibURL/Pattern/String.h>
+
+namespace URL::Pattern {
+
+// https://urlpattern.spec.whatwg.org/#escape-a-pattern-string
+String escape_a_pattern_string(String const& input)
+{
+    // 1. Assert: input is an ASCII string.
+    VERIFY(all_of(input.code_points(), is_ascii));
+
+    // 2. Let result be the empty string.
+    StringBuilder result;
+
+    // 3. Let index be 0.
+    // 4. While index is less than input’s length:
+    for (auto c : input.bytes_as_string_view()) {
+        // 1. Let c be input[index].
+        // 2. Increment index by 1.
+
+        // 3. If c is one of:
+        //     * U+002B (+);
+        //     * U+002A (*);
+        //     * U+003F (?);
+        //     * U+003A (:);
+        //     * U+007B ({);
+        //     * U+007D (});
+        //     * U+0028 (();
+        //     * U+0029 ()); or
+        //     * U+005C (\),
+        //    then append U+005C (\) to the end of result.
+        if ("+*?:{}()\\"sv.contains(c))
+            result.append('\\');
+
+        // 4. Append c to the end of result.
+        result.append(c);
+    }
+
+    // 5. Return result.
+    return result.to_string_without_validation();
+}
+
+}