Does Bourne Shell have a regex validator?

I am on a closed network (i.e. no connectivity to the internet). I have a bourne shell script that asks for the user to enter a regular expression for use with grep -P. Generally speaking, I like to do some form of input validation. Is there a way to test a string variable to see if it is a (valid) regex? (Copying things from the internet onto my system can be done, but it takes forever and is a PITA -- thus I am looking for way to do it natively.)