MODX Redactor replace inline CSS and add SrcSet: http://jens-wittmann.de/codeschnipsel/redactor-editor.html

redactorReplace

<?php
$o = $input;

// replace inline css
$o = preg_replace('/(<[^>]+) style=".*"/iU', '$1', $o);

// remove paragraph around img and iframe
$o = preg_replace('/<p>\s*(<img.*>)\s*<\/p>/iU', '$1', $o);
$o = preg_replace('/<p>\s*(<iframe.*>\s*<\/iframe>)\s*<\/p>/iU', '$1', $o);

// add srcset
$o = preg_replace('/<img src="(.*)".*>/iU', '<img src="[[pthumb? &input=`$1` &options=`w=1000`]]" srcset="[[pthumb? &input=`$1` &options=`w=500`]] 500w, [[pthumb? &input=`$1` &options=`w=1500`]] 1500w" $2>', $o);

return $o;