fixup! EZP-29289: Migrating ezxmltext with invalid name or id attributes

lib/FieldType/XmlText/Converter/RichText.php

@@ -169,6 +169,17 @@ protected function validateAttributeValues(DOMDocument $document, $contentFieldI
 
         $whitelist = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-';
         $replaceStr = '';
+        /*
+         * We want to pick elements which has id value
+         *  #1 not starting with a..z or '_'
+         *  #2 not a..z, '0..9', '_' or '-' after 1st character
+         * So, no xpath v2 to our disposal...
+         * 1st line : we check the 1st char(substring) in id, converts it to 'a' if it in whitelist(translate), then check if it string now starts with 'a'(starts-with), then we invert result(not)
+         *   : So we replace first char with 'a' if it is whitelisted, then we select the element if id value does not start with 'a'
+         * 2nd line:  now we check remaining(omit 1st char) part of string (substring), removes any character that *is* whitelisted(translate), then check if there are any non-whitelisted characters left(string-lenght)
+         * 3rd line: Due to the not() in 1st line, we pick all elements not matching that 1st line. That also includes elements not having a xml:id at all..
+         *   : So, we want to make sure we only pick elements which has a xml:id attribute.
+         */
         $nodes = $xpath->query("//*[
             (
                 not(starts-with(translate(substring(@xml:id, 1, 1), '$whitelist1st', '$replaceStr1st'), 'a'))