How to grep out substring which can change?

Question

Basically I have a very large text file and each line contains

tag=yyyyy;id=xxxxx;db_ref=zzzzz; 

What I want is to grep out the id, but the id

Answer 1

Via grep:

grep -o 'id=[^;]*'

Via awk:

awk -F';' '{ print $2}' testlog
id=xxxxx

edit: see sudo_O's answer for the look-behind. it's more to the point of your question, IMO.