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So I've been trying to use grep to extract some stuff from a Mercurial log and trying to avoid using awk just to see if I can get by without it here, and failing because neither grep nor egrep support full modern regex.

So this happens with grep:

$   hg log <filename> | grep changeset
changeset:   3651:d23495ab1168
changeset:   2974:6aa71cb2c575
changeset:   2756:9dd7fb635678
changeset:   2532:d3ced9af4d6c
changeset:   2459:9d5f5553b851
changeset:   1835:4558836beed1
changeset:   1628:517d0239e830
changeset:   1486:114bce51254d
changeset:   1378:2b968e7fbd19
changeset:   1374:4e7772e48d00

This is what I want in terms of output results. But to get better regex support, I tried doing this with ack, aaaand....nothing. What am I missing? Checked a bunch of examples and can see no difference in what I'm doing here.

$  hg log <filename> | ack changeset
$

There is no output.

Maybe I am misunderstanding this tool? egrep did not have enough support either. I just want to use \s :(

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  • 1
    What version of ack do you have? You example works fine for me with version 2.14. – Stephen Kitt Apr 16 '15 at 22:45
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    Huh, 1.39. Weird, I just sudo apt-get'd it....why am I out of date... – temporary_user_name Apr 16 '15 at 22:50
  • What distribution are you using? Even Debian oldstable has 1.92... – Stephen Kitt Apr 16 '15 at 22:54
  • Ubuntu 12.04. Trying to install from this and it is not going well, can't find an actual download link. – temporary_user_name Apr 16 '15 at 22:57
  • Try downloading it from Ubuntu instead... – Stephen Kitt Apr 16 '15 at 23:00
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Saw this on the ack documentation page, maybe it would help?

 --[no]filter                  Force ack to treat standard input as a pipe
                            (--filter) or tty (--nofilter)
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  • Unfortunately that (bizarrely) just caused ack to inform me of its version info and basic usage. – temporary_user_name Apr 16 '15 at 22:40
  • sorry that didn't work. – FrankRalphBob Apr 16 '15 at 22:49
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try entering 

awk '/changeset/ {print}'
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  • That only partially works and raises a host of new questions. As you wrote it, it produces the same output as above. But when I adjust the pattern to be /\s\d+(?=:)/ (digits followed by a colon and preceded by whitespace) which should match all the same lines, then it gives zero output. Moreover, why can I just write {print} rather than {print(something);} ? – temporary_user_name Apr 16 '15 at 22:46
  • @temporary_user_name That's because awk only recognizes Extended Regular Expressions (ERE), while \s, \d and (?= are Perl Compatible Regular Expressions (PCRE). – Jens Nov 21 '20 at 8:51
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Super late to the party, but with ack v3.3.1 piping works.

$ git log | ack 'bar'

$ ack 'regex' log | ack 'bar'

I ran into this because i needed to run a complex regex that i could not do in one line and while under a time crunch.

Hope this helps someone

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If you, by "modern regular expressions", mean Perl-compatible expressions (PCRE), you could always opt for using Perl:

hg log something | perl -ne '/pattern/ && print'

where pattern is your PCRE regular expression.

GNU grep supports PCRE if given the -P command line option (I don't know how complete the support is though) and GNU grep also supports \s out of the box as a shorthand for the standard pattern [[:blank:]] (matches a tab or a space). There is also a pcre2grep utility, part of the PCRE2 library package.

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ack is a default package installed on Debian based distributions. If you see a message like this when you run ack -h then you need to use ack-grep or follow the step at http://beyondgrep.com/install/ on Renaming ack-grep on Debian-derived distros

Try hg log <filename> | ack-grep changeset to see if ack-grep is installed properly.

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