Filter Exact Match with Grep

Question

I have been using grep -w extensively but recently I noticed that it is not solving my problem.

Let say I have a file with the following contents:

$  cat Log.txt
aaa xxx zzz 
bbb xxx aa
cccaaa yy aa
scn-aaa

I want to filter all rows which have the word "aaa" exactly match. That means the words "cccaaa" and "scn-aaa" should not come out.

I tried with grep -w but no luck

$ grep -w "aaa" Log.txt
aaa xxx zzz
scn-aaa

$ grep -w "\<aaa\>" Log.txt
aaa xxx zzz
scn-aaa

I also tried -Fx but did not help.

Please let me know how I can achieve this with grep command.

Note: Each line might have multiple columns which is not fixed.

Answer 1

I used many of the suggestions but what worked BEST for me is the following command:

grep -E '(^|\s)'<PATTERN>'($|\s)' <FILENAME>

Below is an example:

$ cat Log.txt
aaa xxx zzz
bbb xxx aa
cccaaa yy aa
scn-aaa

$ i=aaa

$ grep -E '(^|\s)'${i}'($|\s)' Log.txt
aaa xxx zzz

Thanks everyone for your suggestions :-)

Answer 2

You can try:

grep -P '(?<!\w-)(aaa)(?![\w-])'

it matches aaa that are not followed or preceded by one of a-zA-Z0-9 or a -.

• ?<! is a negative lookbehind - makes sure that aaa is not preceded by \w-

• ?! is a negative lookahead - makes sure that aaa is not followed bye \w-

Answer 3

Can also be done with awk

awk -F"[^[:alnum:]_-]" '{f=0;for (i=1;i<=NF;i++) if ($i=="aaa") f=1}f' file
aaa xxx zzz
cccaaa yy aaa

Here we set that field separator is not any alpha, numeric nor _ and -.
Then test every field, one by one. If one hit is found, print the line.

---

For some reason, even if we set correct separators, word boundary fails in awk, so do not use this:

awk -F"[^[:alnum:]_-]" '/\<aaa\>/' file
aaa xxx zzz
cccaaa yy aaa
scn-aaa

Answer 4

grep -w counts '-' as beginning a word boundary, which is why it is catching scn-aaa. In short, you want to do what -w does, but with a different definition of what constitutes a valid word character.

For grep, a word character is [_[:alnum:]], i.e. any letter or number or the underscore character. So we can roll our own grep -w like match by:

grep -E '(^|[^[:alnum:]_-])aaa($|[^[:alnum:]_-])'

That is, match aaa when preceded by and followed by the beginning or end of the string, or a non-word character, where we count '-' also as being a word character.