ripgrep can do it! :)

    rg 'foo=([0-9]+)' --replace '$1'

or more succinctly:

    rg 'foo=([0-9]+)' -r '$1'

Example:

    $ echo 'quux=123 foo=123 bar=123' | rg 'foo=([0-9]+)' -r '$1' 
    quux=123 123 bar=123

Named groups work too:

    $ echo 'quux=123 foo=123 bar=123' | rg 'foo=(?P<digits>[0-9]+)' -or '$digits'
    123

You can also replace the whole match by combining --replace with --only-matching:

    $ echo 'quux=123 foo=123 bar=123' | rg 'foo=([0-9]+)' -or '$1'
    123

Of course, I understand capturing groups are useful to have in awk when you're using awk. ripgrep can only handle very simplistic cases. But they tend to be quite common.