Edit: class-wp-url-pattern-prefixer.php

<?php
/**
 * Class 'WP_URL_Pattern_Prefixer'.
 *
 * @package WordPress
 * @subpackage Speculative Loading
 * @since 6.8.0
 */

/**
 * Class for prefixing URL patterns.
 *
 * This class is intended primarily for use as part of the speculative loading feature.
 *
 * @since 6.8.0
 * @access private
 */
class WP_URL_Pattern_Prefixer {

/**
	 * Map of `$context_string => $base_path` pairs.
	 *
	 * @since 6.8.0
	 * @var array<string, string>
	 */
	private $contexts;

/**
	 * Constructor.
	 *
	 * @since 6.8.0
	 *
	 * @param array<string, string> $contexts Optional. Map of `$context_string => $base_path` pairs. Default is the
	 *                                        contexts returned by the
	 *                                        {@see WP_URL_Pattern_Prefixer::get_default_contexts()} method.
	 */
	public function __construct( array $contexts = array() ) {
		if ( count( $contexts ) > 0 ) {
			$this->contexts = array_map(
				static function ( string $str ): string {
					return self::escape_pattern_string( trailingslashit( $str ) );
				},
				$contexts
			);
		} else {
			$this->contexts = self::get_default_contexts();
		}
	}

/**
	 * Prefixes the given URL path pattern with the base path for the given context.
	 *
	 * This ensures that these path patterns work correctly on WordPress subdirectory sites, for example in a multisite
	 * network, or when WordPress itself is installed in a subdirectory of the hostname.
	 *
	 * The given URL path pattern is only prefixed if it does not already include the expected prefix.
	 *
	 * @since 6.8.0
	 *
	 * @param string $path_pattern URL pattern starting with the path segment.
	 * @param string $context      Optional. Context to use for prefixing the path pattern. Default 'home'.
	 * @return string URL pattern, prefixed as necessary.
	 */
	public function prefix_path_pattern( string $path_pattern, string $context = 'home' ): string {
		// If context path does not exist, the context is invalid.
		if ( ! isset( $this->contexts[ $context ] ) ) {
			_doing_it_wrong(
				__FUNCTION__,
				esc_html(
					sprintf(
						/* translators: %s: context string */
						__( 'Invalid URL pattern context %s.' ),
						$context
					)
				),
				'6.8.0'
			);
			return $path_pattern;
		}

/*
		 * In the event that the context path contains a :, ? or # (which can cause the URL pattern parser to switch to
		 * another state, though only the latter two should be percent encoded anyway), it additionally needs to be
		 * enclosed in grouping braces. The final forward slash (trailingslashit ensures there is one) affects the
		 * meaning of the * wildcard, so is left outside the braces.
		 */
		$context_path         = $this->contexts[ $context ];
		$escaped_context_path = $context_path;
		if ( strcspn( $context_path, ':?#' ) !== strlen( $context_path ) ) {
			$escaped_context_path = '{' . substr( $context_path, 0, -1 ) . '}/';
		}

/*
		 * If the path already starts with the context path (including '/'), remove it first
		 * since it is about to be added back.
		 */
		if ( str_starts_with( $path_pattern, $context_path ) ) {
			$path_pattern = substr( $path_pattern, strlen( $context_path ) );
		}

return $escaped_context_path . ltrim( $path_pattern, '/' );
	}

/**
	 * Returns the default contexts used by the class.
	 *
	 * @since 6.8.0
	 *
	 * @return array<string, string> Map of `$context_string => $base_path` pairs.
	 */
	public static function get_default_contexts(): array {
		return array(
			'home'       => self::escape_pattern_string( trailingslashit( (string) wp_parse_url( home_url( '/' ), PHP_URL_PATH ) ) ),
			'site'       => self::escape_pattern_string( trailingslashit( (string) wp_parse_url( site_url( '/' ), PHP_URL_PATH ) ) ),
			'uploads'    => self::escape_pattern_string( trailingslashit( (string) wp_parse_url( wp_upload_dir( null, false )['baseurl'], PHP_URL_PATH ) ) ),
			'content'    => self::escape_pattern_string( trailingslashit( (string) wp_parse_url( content_url(), PHP_URL_PATH ) ) ),
			'plugins'    => self::escape_pattern_string( trailingslashit( (string) wp_parse_url( plugins_url(), PHP_URL_PATH ) ) ),
			'template'   => self::escape_pattern_string( trailingslashit( (string) wp_parse_url( get_stylesheet_directory_uri(), PHP_URL_PATH ) ) ),
			'stylesheet' => self::escape_pattern_string( trailingslashit( (string) wp_parse_url( get_template_directory_uri(), PHP_URL_PATH ) ) ),
		);
	}

/**
	 * Escapes a string for use in a URL pattern component.
	 *
	 * @since 6.8.0
	 * @see https://urlpattern.spec.whatwg.org/#escape-a-pattern-string
	 *
	 * @param string $str String to be escaped.
	 * @return string String with backslashes added where required.
	 */
	private static function escape_pattern_string( string $str ): string {
		return addcslashes( $str, '+*?:{}()\\' );
	}
}

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