BGP RegEx Confused bit

so i found this question in Cisco forum while learning BGP regex and i built a simple lab about it... in forum where i found it... op said that correct answer test gave was A, but it did'nt really work... i asked my teacher about it (courses mostly focus on CCNA stuff but sometimes we go into more complicated stuff) and we came up with this " ^.*_333_.+$ "

also im bit confused about underscore ( _ ) what does this symbol exactly do, few examples would be helpful because i tried reading few articles most say that it can be space,symbol,end of string,start of string, but mainly its used as space i guess.

thanks in advance