<?php
$re = '~(?:<\w+|(?!^)\G)(?:\s+(?:src|href)=(?:"[^"]*"|\'[^\']*\'))*\s+(?!(?:href|src)=)\w+=\K(?|"([^\s"=]*)"|\'([^\s\'=]*)\')~u';
$str = "<mytag src=\"src_here\" b=\"yes\" href=\"href_here\"> becomes <mytag src=\"src_here\" b=yes href=\"href_here\">\n<mytag b='yes'> becomes <mytag b=yes>\nbut <script>var b=\"yes\"</script> stays intact\n<mytag b=\"no no\" c=\"no=no\"> stays intact\n<tag href=\"something\"> text <tag src=\"dddd\"> intact";
$subst = "$1";
echo $result;
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