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Comment by comex | Hacker News Reader

I wish Awk had capture groups. It would fit in so well with typical Awk one-liners to be able to say:

    awk '/foo=([0-9]+)/ { print $1 }'

although I suppose the syntax would have to be different since $1 has a meaning already.

Yes, gawk has a function that returns capture groups, but it's a bit verbose for one-liners. Instead I switch to Perl:

    perl -nE 'if (/foo=([0-9]+)/) { say $1 }'

But I wish I could just use Awk.

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You can still do it with awk, but with different "ergonomics":

  awk -F= '$2 ~ /[0-9]+/ { print $2 }'

With imaginative choice of FS and RS you can push it very far.

Whether other people having to deal with such code will appreciate your imagination is another matter, though.

Edit: I missed the detail where you want to specifically match "foo" as lhs, and anywhere on the line. So the correct condition would be even lengthier : ^ ) You have a valid point. Captures would provide for shorter patterns.

I recently saw someone on StackOverflow asking for something similar: access, from the condition-action body, to the text which matched the regex.

So I added a feature to the TXR Lisp awk macro. There is now an Awk variable called res which holds the result of the condition. If the condition is a regex, then that has the matching part. The fact that the action is executing tells us that the result of the condition is true; but res gives us the specific true value, like the it in anaphoric if macros.

This made it into release 284.

Can use sed. One way is to use a "sentinal", for example, a non-printable character that is absent from the input.

   sed -n "s/SENTINAL//g;s/\(.*\)\(foo=\)\([0-9]*\)\(.*\)/SENTINAL\3/;/SENTINAL/!d;s/SENTINAL//;/./p"

shortened to

   x=$(echo x|tr x '\34');
   sed -n "s/$x//g;s/\(.*\)\(foo=\)\([0-9]*\)\(.*\)/$x\3/;/$x/!d;s/$x//;/./p"

Using flex is another option. Faster than AWK, Perl, sed and similarly ubiqitous.

   flex -8iCrf -o/dev/stdout << eof|cc -xc -O3 -std=c89 -pedantic -W -Wall -static /dev/stdin
     int fileno(FILE *);
     #define J BEGIN
     #define E ECHO
     int x;
   %option noyywrap noinput nounput
   %s x1 x2
   %%
   foo=[^\n] yyless(1);J x1;x++;
   <x1>[0-9]+ if(x==1)E;J x2;
   <x2>\n E;x=0;J 0;
   <x2>.
   .|\n
   %%
   int main(){ yylex();exit(0);}
   eof

I don't use `match` often, so I get the `match(str, regex)` order wrong. So yeah, it would be nice if gawk automatically provided the capture groups via some special variable.

Not really verbose for this particular example though:

    $ echo 'foo=42' | awk 'match($0, /foo=([0-9]+)/, m){print m[1]}'
    42
    $ echo 'foo=42' | perl -nE 'if (/foo=([0-9]+)/) { say $1 }'
    42
    # TIMTOWTDI 
    $ echo 'foo=42' | perl -nE 'say $1 if /foo=([0-9]+)/'
    42

GNU Awk version 5 has dynamic regexprs. [1]

[1] 3.6 using Dynamic Regexps : https://www.gnu.org/software/gawk/manual/html_node/Computed-...

I have to admit I never really learned awk, but whenever I want to do something that I think "awk would be god for this", I use perl. Are there things for which awk is a significantly better tool?

Ideal for embedded systems where resources are limited.

I wish grep had capture groups so I could do things like

    grep "\((\d+)\)" -print "$1" file.txt

ripgrep is a modern faster grep in Rust, some cons, many pros.

It'll do capture groups with search | replace

    # remove square brackets that surround digit characters
    $ echo '[52] apples [and] [31] mangoes' | rg '\[(\d+)]' -r '$1'
    52 apples [and] 31 mangoes

Pleasantly surprised to see an example from my ebook :)

Add `-o` option to get only the digits:

    $ echo '[52] apples [and] [31] mangoes' | rg -o '\[(\d+)]' -r '$1'
    52
    31

Congratulations (?) for making the top five google results for "ripgrep capture groups" [1].

I cite sources way more often than not, this time I got lazy after dithering over whether to go with the definitive ripgrep source page [2] or a decent looking third party(?) tutorial .. pressed for time I did neither.

[1] https://learnbyexample.github.io/learn_gnugrep_ripgrep/ripgr...

[2] https://github.com/BurntSushi/ripgrep

If PCRE is supported:

    grep -oP '\(\K\d+(?=\))'

The above will give all matches in a line though. You can remove `(?=\))` part if numbers are always enclosed in `()` or if you don't care about the `)`

Lookarounds are a bit messy. It's easier to just pipe into a second grep.

certainly gets rid of one type of edge case.

[1] gives some background about the issue and possible solution(s).

[1] : https://unix.stackexchange.com/questions/536657/how-to-refer...

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