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I need a single-pass regex for unix grep that contains, say alpha, but does not contain beta.

grep 'alpha' <> | grep -v 'beta'
ndemou
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Wilderness
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    Please post a sample input and expected output. How do you expect the Not 'y' not to match all lines except 'x' ?. Which is another way of saying you may want a grep 1 pass, but you probably need a grep 2 pass OR awk or perl script for a onepass. Incidentally, that is not my down vote. Maybe someone will explain why this is a bad question?! Good luck. – shellter May 19 '11 at 18:42
  • I think this is definitely a reasonable question to ask (so +1 from me) especially as I have seen it asked before, and have even asked it myself. – nohat May 19 '11 at 20:17
  • @shellter: I knew various ways using awk, sed and perl to do it. Even the grep command can do it with a pipe (added a sample line in the question). I just wanted to see if it could be done in one pass. It looks like it can be done (Mr47's answer below) and I got to learn look-ahead and look-behind in perl. It's fun learning new tricks in any language. I don't understand why you think this is a bad question. And I up-voted your answer too. :) – Wilderness May 19 '11 at 21:00
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    Please re-read my comment. 'That is ***not*** my downvote'.. In fact after seeing that you had 2 downvotes, I did give you a vote. I agree with you about learning new techniques. Gotta go. good luck! – shellter May 19 '11 at 21:25
  • I know you didn't down-vote. It would have been ok even if you did. Was just trying to learn something new. – Wilderness May 19 '11 at 22:06
  • Arg! Ok... Given your original post, there was no way to assume (except that you wanted one regexp). that you knew about awk/perl AND my real complaint was the lack of sample input and output. ;-) Best wishes! and keep on learning new techniques! – shellter May 19 '11 at 22:11
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    Agreed. Will be more elaborate next time. Thanks for your time ! – Wilderness May 19 '11 at 23:46

7 Answers7

53

The other answers here show some ways you can contort different varieties of regex to do this, although I think it does turn out that the answer is, in general, “don’t do that”. Such regular expressions are much harder to read and probably slower to execute than just combining two regular expressions using the boolean logic of whatever language you are using. If you’re using the grep command at a unix shell prompt, just pipe the results of one to the other:

grep "alpha" | grep -v "beta"

I use this kind of construct all the time to winnow down excessive results from grep. If you have an idea of which result set will be smaller, put that one first in the pipeline to get the best performance, as the second command only has to process the output from the first, and not the entire input.

nohat
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    Yes, but the reason you'd cram all this into a single grep command is usually for use in the tail -f command or something else that uses a data stream that can only be piped into a single command. – Ernie Jun 01 '15 at 18:27
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    This solution only works if you're not interested in the context, i.e. it doesn't work well with the `-A`, `-B` and `-C` options that `grep` has. – HelloGoodbye Sep 08 '15 at 14:20
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    This also doesn't work in the important (to me at least) case where the filename might contain the string `beta`. – Tom Feb 15 '18 at 11:55
  • @Tom, try `grep -l "alpha" | grep -v "beta"`. The first `grep` returns the file names. – Elliott May 23 '22 at 00:21
  • @Elliott - not sure how that helps? The issue is with a file called `beta.py` which contain the string `alpha`. These should be returned in the results but aren't. It could be worked around with `grep "alpha" | grep -v "[*:]+:.*beta" or similar, I guess. – Tom May 23 '22 at 09:40
  • This does not preserve the nice colors of the first grep for me. – Hunaphu Dec 09 '22 at 19:34
35

Well as we're all posting answers, here it is in awk ;-)

awk '/x/ && !/y/' infile

I hope this helps.

shellter
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26

^((?!beta).)*alpha((?!beta).)*$ would do the trick I think.

Mr47
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  • I'm pretty sure that POSIX `grep` doesn't support syntax like that! – Gabe May 19 '11 at 18:43
  • I didn't test it, but I'm pretty sure my version of grep supports syntax like this. Could be wrong though. – Mr47 May 19 '11 at 18:45
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    Could you please explain how the '('s and '?' work here? I am confused why you have 2 '(' in the beginning. – Wilderness May 19 '11 at 18:47
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    This is a PCRE (Perl-Compatible Regular Expression), so you'll need the -P option for GNU Grep. The (?!...) things are zero-width negative lookahead assertions. I suggest `perldoc perlre` for an explanation of lookahead assertions. – nohat May 19 '11 at 20:27
  • Is Perl itself suited for inline things like this, or is there a reason to use a Perl mode in grep over native Perl? Almost everything I find is written *in* Perl, as in a script or interpreter and not a standalone expression - at that point I could traverse a string upside down and backwards with loops and functions and everything. – John P May 19 '19 at 05:21
  • verified working with `grep -P` Also,surprisingly, the `^$` is required. – marinara Dec 23 '20 at 09:59
  • It also works for me, with grep -P option – amareno Aug 10 '23 at 09:36
4

I'm pretty sure this isn't possible with true regular expressions. The [^y]*x[^y]* example would match yxy, since the * allows zero or more non-y matches.

EDIT:

Actually, this seems to work: ^[^y]*x[^y]*$. It basically means "match any line that starts with zero or more non-y characters, then has an x, then ends with zero or more non-y characters".

Shea Levy
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0

Try using the excludes operator: [^y]*x[^y]*

sblundy
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    `[^y]*` matches the string `y` because there are zero non-y characters in that string. – CanSpice May 19 '11 at 18:42
  • Yeah, so? My example is [^y]*x[^y]*. – sblundy May 19 '11 at 18:54
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    Note that I answer the question at the same level of abstraction as the question itself. – sblundy May 19 '11 at 18:57
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    The questioner wants to match strings that contain `alpha` but not `beta`. The string `alphabeta` does not meet the questioner's criterion (it contains the string `beta`) yet your regular expression will return true because, before the substring `alpha`, there are zero or more occurrences of the string `beta`. – CanSpice May 19 '11 at 19:01
  • That depends upon boundary conditions, which the question didn't ask about. – sblundy May 19 '11 at 19:04
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Q: How to match x but not y in grep without pipe if y is a directory

A: grep x --exclude-dir='y'

Hunaphu
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-3

Simplest solution:

grep "alpha" * | grep -v "beta"

Please take care of gaps and double quotes.

Greg
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Deepshikha Satish
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