How to if on a 2 string types

I need an if condition on a 2 types string:

1) "/A/Ababa"
2) "/A"

Something like this:

    if(myString is of the first type){
      must return Ababa
     }

then

    if(myString is of the second type){
      must return null
     }

How can i do correctly? Maybe Regex? Substring? Any other ideas?