Checking if looked up value is a number

I'm new to irules. I've chosen to use sub tables to look up info. There are some samples on editing and viewing sub tables so it was why I chose this way for now.

I don't have my code in front of me.

Basically I lookup from subtable and if equals "" I add a default value of 10.

If someone changes this entry to not numerical. How do I check if it is numerical? Expr function?