Regular expression in XPath - filter for attribute

Hi Community,

In a custom file type for an XLIFF I would like to filter for elements with certain attributes, to exclude them from translation.
However, the attributes contain different numbers, such as those marked with bold:

<trans-unit id="content.5262957">
<source xml:lang="fr-FR"><![CDATA[https://cdn.elucidat.com/5fd24b7fc9ccc/media/98c7c45c7a24bc161bb53cd5628001dc.PNG (For reference only)]]></source>
</trans-unit>
<trans-unit id="content.5262973">
<source xml:lang="fr-FR"><![CDATA[https://cdn.elucidat.com/5fd24b7fc9ccc/media/9d46c04fe792c2ff102b7c76bd779910.PNG (For reference only)]]></source>
</trans-unit>

I tried to define an XPath with regular expression to search for these:

//trans-unit[@id="(content\.)\d+"]

This expression within works if I try it in RegExr, but not Studio:

"(content\.)\d+"

I also tried this but did not work:

//trans-unit[contains(@id=,'content.')]/source

But if I add the below, I can filter out the given segments (but it'd take long to add all of them):

//trans-unit[@id="content.5204295"]

Can you please help, what am I doing wrong?

Thank you!

Greta