Regular expression validation

Hi,

is there any function in postgresql saying me if the given regular
expression is valid? I store regular expressions in a table and then
perform some searches using ~* operator. But when the stored regular
expression is invalid (eg '+') then the query with ~* fails (ERROR:
Invalid regular expression: ?, *, or + operand invalid.)  I'd like to have
a check constraint on that column storing regular expressions, so that no
one can insert invalid data and cause failing of other queries... Is there
any simple way?

thanks,                  kuba