Regex to match lines not containing a word

Question

I was trying to create a regex, to be used in a grep statement to match lines that do not contain a word. In the example below I have the words uhu, uhuu, uhuuu, uhuuu one in each line. I used an echo to do that in one command. Results are:

line1 = uh - end
line2 = uhu - end
line3 = uhuu - end
line4 = uhuuu - end
line5 = uhuuuu - end

Never mind the line number and the end word. So, I wanted to grep uhuu's with 2 and 4 repetitions only, that is line 3 and 5. I tried:

echo -e 'line1 = uh - end\nline2 = uhu - end\nline3 = uhuu - end\nline4 = uhuuu - end\nline5 = uhuuuu - end' | grep -E 'uh(u{2}|u{4})'
line3 = uhuu - end
line4 = uhuuu - end
line5 = uhuuuu - end

It brings line 3 and 5, but also line 4 because uhuuu has 2 u's, plus a 3rd one. So my question is, using Regex, is it possible to exclude that 4th line taking into account only the 'uhuuu' word?

I know I could accomplish this with a pipe and grep -v at the end of everything but I was wondering if this could be done using RegEx.

I checked this link but couldn find a way to make it work for my case. Thank you https://stackoverflow.com/questions/406230/regular-expression-to-match-line-that-doesnt-contain-a-word#=

@Devian That suffers from the same problem: `uhu{2}` matches with at least two trailing `u`s and so will match longer items. — Ouroborus, Dec 24 '16 at 21:08
how about using a [lookahead](http://www.regular-expressions.info/lookaround.html) something like `grep -P 'uh(?:u{2}|u{4})(?!u)'` ([see demo](https://www.regex101.com/r/eZNL0z/1)). — bobble bubble, Dec 24 '16 at 21:19
Thanks @bobblebubble, this works like a charm, even though I don't understand yet how lookahead (lookaround) works. — Adriano_Pinaffo, Dec 24 '16 at 23:01

score 1 · Answer 1 · edited Sep 24 '17 at 19:29

1

Use a word boundary meta character after grouping:

grep -E 'uh(u{2}|u{4})\b'

edited Sep 24 '17 at 19:29

Graham

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answered Dec 24 '16 at 22:02

revo

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That's right @revo, I had tried it before but had even forgotten. Thank you – Adriano_Pinaffo Dec 24 '16 at 23:03

Regex to match lines not containing a word

1 Answers1