You need to use the isMatch function if you want to use a regular expression.
https://docs.mendix.com/refguide/string-function-calls/#ismatch
isMatch is anchored, so you need to take this into account. I've not tested it, but try something like this...
isMatch($String, '.*[A-Za-z].*')
Good luck!
Hi Jasper,
If I'm not wrong, you are checking with regular expression which can perform with isMatch() function only.
contains() can be used with particular format or letters like 'bggg'.
so, try same with isMatch() function.
Hope it helps.