Add a function: `wp_spaces_regexp()`. It is not used yet.

This function is required by a bunch of patches by miqrogroove. Needs filter docs.

Props miqrogroove.
See #27588.

Built from https://develop.svn.wordpress.org/trunk@28708


git-svn-id: http://core.svn.wordpress.org/trunk@28524 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/formatting.php

@@ -3844,3 +3844,24 @@ function get_url_in_content( $content ) {
 
 	return false;
 }
+
+/**
+ * Returns the regexp for common whitespace characters.
+ *
+ * By default, spaces include new lines, tabs, nbsp entities, and the UTF-8 nbsp.
+ * This is designed to replace the PCRE \s sequence.  In ticket #22692, that
+ * sequence was found to be unreliable due to random inclusion of the A0 byte.
+ *
+ * @since 4.0.0
+ *
+ * @return string The spaces regexp.
+ */
+function wp_spaces_regexp() {
+	static $spaces;
+
+	if ( empty( $spaces ) ) {
+		$spaces = apply_filters( 'wp_spaces_regexp', '[\r\n\t ]|\xC2\xA0| ' );
+	}
+
+	return $spaces;
+}