local np = 'abc_123'
local a = np:match('^[a-zA-Z0-9_][a-zA-Z0-9_][a-zA-Z0-9_]+$' )
print (a)
local np2 = 'ab'
local b = np2:match('^[a-zA-Z0-9_][a-zA-Z0-9_][a-zA-Z0-9_]+$' )
print (b)