grep lines that contain 1 character followed by another character

Question

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 1

This will find a "b" that is not followed by "e":

$ echo "one be
two
bring
brought" | egrep 'b[^e]'

Or if perl is available but egrep is not:

$ echo "one be
two
bring
brought" | perl -ne 'print if /b[^e]/;'

And if you want to find lines with "b" not followed by "e" but no words that contain "be" (using the \\w perl metacharacter to catch another character after the b), and avoiding any words that end with b:

$ echo "lab
bribe
two
bring
brought" | perl -ne 'print if /b\w/ && ! /be/'

So the final call would:

$ perl -ne 'print if /b\w/ && ! /be/' f3.txt

Answer 2

Exluding "edge" words that may exist and break the exercise, like lab , bribe and bob :

$ a="one                      
two
lab
bake
bob
aberon
bee
bell
bribe
bright
eee" 

$ echo "$a" |grep -v 'be' |grep 'b.'
bake
bob
bright

Answer 3

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