Use grep to match a pattern in a line only once

Question

I have this:

 echo 12345 | grep -o '[[:digit:]]\{1,4\}'

Which gives this:

1234
5

I understand whats happening. How do I stop grep from trying to continue matching after 1 successful match?

How do I get only

1234

Answer 1

Do you want grep to stop matching or do you only care about the first match. You could use head if the later is true...

`grep stuff | head -n 1` 

Grep is a line based util so the -m 1 flag tells grep to stop after it matches the first line which when combined with head is pretty good in practice.

Answer 2

You need to do the grouping: \(...\) followed by the exact number of occurrence: \{<n>\} to do the job:

maci:~ san$ echo 12345 | grep -o '\([[:digit:]]\)\{4\}'
1234

Hope it helps. Cheers!!