Thanks to an other post on the list, I've been able to solve my problem.
First, the solution: using the splitregexp filter operator!
example: (transform something like "SVR203" into "SVR")
<$set name="codeFamily" filter="[[$ref$]splitregexp[\d+]join[]]">
Now, the cause of my trouble: Looking for a regexp, I seartch into the string operators. Only had a glance at other operator afterwards. Missed that splitregexp operator in the middle of the first batch of operator (which have no general purpose given like strings operators for instance). Only saw the regexp operator, which was not what I needed.
Suggestion: the regexp operator notice could hint at the splitregexp operator!