CAN-DO is a handy function, but as Tom says can be risky if you don't fully understand it. After 5 years I thought I understood it, but haven't come across the # before!
Anyway, FYI there are a few other special characters (effective in first parameter only):
* - Wildcard for any character
. - Wildcard for single character
! - Rejects any value matching the following string
Out of curiosity I tried to figure out the # symbol; it seems to be ignored in the first parameter, but only if the first character of the ID..
Does anyone else know the full details for this function? The documentation is not entirely helpful, only mentioning '*' and '!'. And it looks like the very last statement on the doc page was meant for the CAN-FIND page..