Assembly or cast unit drawing sheet number

Use match function to find "-" character

match(GetValue("NAME_BASE"),"*-*")

Use of mid fuction to return only characters after "-"

mid(GetValue("NAME_BASE"),(1+(find(GetValue("NAME_BASE"),"-")),2)

Combine all above in rule

if (match(GetValue("NAME_BASE"),"*-*"))
then mid(GetValue("NAME_BASE"),(1+(find(GetValue("NAME_BASE"),"-")),2)
else ""
endif