You can do similar things, but it's not going to be pretty, as is often the case when working with Creo relations. Here's an example:
XX = "BLADE_1_2"
AB = "BLADE_A_B"
NUMBERS = "1234567890"
CHECKSTRING = XX
if extract(CHECKSTRING,1,5) == "BLADE" && search(NUMBERS,extract(CHECKSTRING,7,1)) != 0
CHECK = true
CHECK = false
This gives CHECK as "Yes", but if you change the CHECKSTRING to AB, it gives you a "No".
It uses the NUMBERS parameter as a storage of certain characters and then checks if a specific character is contained in that string. You can even do something like "search for the first occurrence of a '_' character, then look at the next character and see if it's a number". But it's cumbersome, and the lack of loops makes it really limited.