Source

File: wp-includes/rewrite.php

function url_to_postid( $url ) {
	global $wp_rewrite;

	/**
	 * Filters the URL to derive the post ID from.
	 *
	 * @since 2.2.0
	 *
	 * @param string $url The URL to derive the post ID from.
	 */
	$url = apply_filters( 'url_to_postid', $url );

	$url_host      = str_replace( 'www.', '', parse_url( $url, PHP_URL_HOST ) );
	$home_url_host = str_replace( 'www.', '', parse_url( home_url(), PHP_URL_HOST ) );

	// Bail early if the URL does not belong to this site.
	if ( $url_host && $url_host !== $home_url_host ) {
		return 0;
	}

	// First, check to see if there is a 'p=N' or 'page_id=N' to match against.
	if ( preg_match( '#[?&](p|page_id|attachment_id)=(\d+)#', $url, $values ) ) {
		$id = absint( $values[2] );
		if ( $id ) {
			return $id;
		}
	}

	// Get rid of the #anchor.
	$url_split = explode( '#', $url );
	$url       = $url_split[0];

	// Get rid of URL ?query=string.
	$url_split = explode( '?', $url );
	$url       = $url_split[0];

	// Set the correct URL scheme.
	$scheme = parse_url( home_url(), PHP_URL_SCHEME );
	$url    = set_url_scheme( $url, $scheme );

	// Add 'www.' if it is absent and should be there.
	if ( false !== strpos( home_url(), '://www.' ) && false === strpos( $url, '://www.' ) ) {
		$url = str_replace( '://', '://www.', $url );
	}

	// Strip 'www.' if it is present and shouldn't be.
	if ( false === strpos( home_url(), '://www.' ) ) {
		$url = str_replace( '://www.', '://', $url );
	}

	if ( trim( $url, '/' ) === home_url() && 'page' === get_option( 'show_on_front' ) ) {
		$page_on_front = get_option( 'page_on_front' );

		if ( $page_on_front && get_post( $page_on_front ) instanceof WP_Post ) {
			return (int) $page_on_front;
		}
	}

	// Check to see if we are using rewrite rules.
	$rewrite = $wp_rewrite->wp_rewrite_rules();

	// Not using rewrite rules, and 'p=N' and 'page_id=N' methods failed, so we're out of options.
	if ( empty( $rewrite ) ) {
		return 0;
	}

	// Strip 'index.php/' if we're not using path info permalinks.
	if ( ! $wp_rewrite->using_index_permalinks() ) {
		$url = str_replace( $wp_rewrite->index . '/', '', $url );
	}

	if ( false !== strpos( trailingslashit( $url ), home_url( '/' ) ) ) {
		// Chop off http://domain.com/[path].
		$url = str_replace( home_url(), '', $url );
	} else {
		// Chop off /path/to/blog.
		$home_path = parse_url( home_url( '/' ) );
		$home_path = isset( $home_path['path'] ) ? $home_path['path'] : '';
		$url       = preg_replace( sprintf( '#^%s#', preg_quote( $home_path ) ), '', trailingslashit( $url ) );
	}

	// Trim leading and lagging slashes.
	$url = trim( $url, '/' );

	$request              = $url;
	$post_type_query_vars = array();

	foreach ( get_post_types( array(), 'objects' ) as $post_type => $t ) {
		if ( ! empty( $t->query_var ) ) {
			$post_type_query_vars[ $t->query_var ] = $post_type;
		}
	}

	// Look for matches.
	$request_match = $request;
	foreach ( (array) $rewrite as $match => $query ) {

		// If the requesting file is the anchor of the match,
		// prepend it to the path info.
		if ( ! empty( $url ) && ( $url != $request ) && ( strpos( $match, $url ) === 0 ) ) {
			$request_match = $url . '/' . $request;
		}

		if ( preg_match( "#^$match#", $request_match, $matches ) ) {

			if ( $wp_rewrite->use_verbose_page_rules && preg_match( '/pagename=\$matches\[([0-9]+)\]/', $query, $varmatch ) ) {
				// This is a verbose page match, let's check to be sure about it.
				$page = get_page_by_path( $matches[ $varmatch[1] ] );
				if ( ! $page ) {
					continue;
				}

				$post_status_obj = get_post_status_object( $page->post_status );
				if ( ! $post_status_obj->public && ! $post_status_obj->protected
					&& ! $post_status_obj->private && $post_status_obj->exclude_from_search ) {
					continue;
				}
			}

			// Got a match.
			// Trim the query of everything up to the '?'.
			$query = preg_replace( '!^.+\?!', '', $query );

			// Substitute the substring matches into the query.
			$query = addslashes( WP_MatchesMapRegex::apply( $query, $matches ) );

			// Filter out non-public query vars.
			global $wp;
			parse_str( $query, $query_vars );
			$query = array();
			foreach ( (array) $query_vars as $key => $value ) {
				if ( in_array( (string) $key, $wp->public_query_vars, true ) ) {
					$query[ $key ] = $value;
					if ( isset( $post_type_query_vars[ $key ] ) ) {
						$query['post_type'] = $post_type_query_vars[ $key ];
						$query['name']      = $value;
					}
				}
			}

			// Resolve conflicts between posts with numeric slugs and date archive queries.
			$query = wp_resolve_numeric_slug_conflicts( $query );

			// Do the query.
			$query = new WP_Query( $query );
			if ( ! empty( $query->posts ) && $query->is_singular ) {
				return $query->post->ID;
			} else {
				return 0;
			}
		}
	}
	return 0;
}