Conditional string search

Hi, I need to search/grep userid and its value based on a condition that the user id is under the heading "*central_test*" in a big file of about 70808MB. The * represents some strings before and after the string "centrla_test". Could anyone please guide. OS is hp-ux.

Please provide an example of input file format as well as output expected : people will then be able to provide a more accurate answer.

Hi ctsgnb, the only constraint is that itz a config file of an appln which i'm not supposed to reveal. I'm really sorry; i'm constrained.
I tried with $grep "<user_id>" <file_name> . but this didn't work.