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

You could do:

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

And if you don't want to inlcude the id= part you can using positive look-behind:

$ grep -Po '(?<=id=)[^;]*' file