How to use OR with grep?

Question

I ran grep 'd(o|i)g' a.txt and a.txt contains dig but nothing was returned. Is this the right way to match dog or dig?

See also [Why does my regular expression work in X but not in Y?](https://unix.stackexchange.com/q/119905/170373), though the fact that alternation (`|`) only works in ERE is kinda there in the middle of all the other stuff. — ilkkachu, Feb 22 '22 at 14:29

score 7 · Accepted Answer · answered Feb 21 '22 at 17:09

7

Yes, d(o|i)g is the correct way to do it. You can also do d[oi]g since you are dealing with single characters.

You need to use the -E flag on your grep call to get extended regexes.

$ cat a.txt
bird
dog
cat
dug
$ grep 'd(o|i)g' a.txt
$ grep -E 'd(o|i)g' a.txt
dog

answered Feb 21 '22 at 17:09

Andy Lester

4

Standard way of doing it for this particular case: `grep -e dog -e dig a.txt` or `grep 'd[io]g' a.txt`, as you propose. – Kusalananda Feb 21 '22 at 19:03

score 5 · Answer 2 · answered Feb 21 '22 at 17:07

5

You need to escape the regex meta characters, like

> echo dig | grep "d\(o\|i\)g"
dig

, or you can switch to ERE ("extended RegEx"es):

echo dig | grep -E "d(o|i)g"
dig

answered Feb 21 '22 at 17:07

RudiC

6

`\|` with Basic Regular Expressions (BRE) is a GNU extension. POSIX BRE don't have an alternation operator. POSIX EREs (with `grep -E`) do. – Stéphane Chazelas Feb 21 '22 at 17:12

score 3 · Answer 3 · answered Feb 21 '22 at 23:26

3

You could also try grep 'd[io]g' but this only works for single characters.

answered Feb 21 '22 at 23:26

user10489

3 Answers3