Your problem is a different one: the brackets.
()
are used for catching some data, but here you want to match a bracket. You must use
\(
and
\)
.
[]
stand for character ranges, if you want to match them, you must escape them also
\[\]
. I do not know if literal spaces are acceptable in a character range, better omit them.
I'd write
ValidationExpression="[0-9a-z\s\d-_\(\)\[\]]{1,50}"