TYPO3 Core

GeneralUtility::array2xml is very restrictive when it comes to tagnames. Currently only alphanumeric characters, '-' and '_' are allowed in tagnames which makes working with FlexForms or namespaced XML impossible (because '.' and ':' are discarded).

The following line is responsible for the restriction:

// The tag name is cleaned up so only alphanumeric chars (plus - and _) are in there and not longer than 100 chars either.
$tagName = substr(preg_replace('/[^[:alnum:]_-]/', '', $tagName), 0, 100);

There are multiple ways this could be handled in the future:

1. Lift restriction to match the W3 XML spec: https://www.w3.org/TR/2008/REC-xml-20081126/#NT-Name ( potentially breaking )
2. Add an option to explicitly allow certain characters. The resulting call could look like this:

GeneralUtility::array2xml($array, $NSprefix, $level, $docTag, $spaceInd, [ 'allowInTagName' => ['.', ':'] ]);