如何制定特定的正则表达式

Question

I'm trying find a regex to grab gif images posted in a chatroom. Gifs are posted using a colon followed by text and/or numbers to describe the image. The chatroom is set up like this

user1:hello i'm user1 :hi
user2::heythere1 hi user1

The gifs in this example are :hi and :heythere1 .

The regex I have so far is grep -oE ':[a-zA-Z0-9]+' But this also returns :hello since every username is also followed by a colon. :hello in this example is not a gif. It is just someone saying hello.

Is there a way to alter this regex so that it only returns :hi and :heythere1 ?

Answer 1

Assuming all lines in your text file begin with a username and a colon, you could do this (I have used the same regex as yours):

cut -f2- -d: file | grep -oE ':[a-zA-Z0-9]+'

Input:

user1:hello i'm user1 :hi :h2
user2::heythere1 hi user1

Output:

:hi
:h2
:heythere1