grep lines that contain 1 character followed by another character

I’m working on my assignment and I’ve been stuck on this question, and I’ve tried looking for a solution online and my textbook.

The question is:

List all the lines in the f3.txt file that contain words with a character b not followed by a character e.

I’m aware you can do grep -i 'b' to find the lines that contain the letter b, but how can I make it so that it only shows the lines that contain b but not followed by the character e?

Answer

You can go for the following two solutions:

grep -ie 'b[^e]' input_file.txt

or

grep -ie 'b.' input_file.txt | grep -vi 'be'

The first one does use regex:

• 'b[^e]' means b followed by any symbol that is not e
• -i is to ignore case, with this option lines containing B or b that are not directly followed by e or E will be accepted

The second solution calls grep twice:

• the first time you look for patterns that contains b only to select those lines
• the resulting lines are filtered by the second grep using -v to reject lines containing be
• both grep are ignoring the case by using -i

• if b must absolutely be followed by another character then use b. (regex meaning b followed by any other char) otherwise if you want to also accept lines where b is not followed by any other character at all you can just use b in the first grep call instead of b..

  grep -ie 'b' input_file.txt | grep -vi 'be'
  

input:

BEBE
bebe
toto
abc
bobo

result:

abc
bobo