perl regular expression substitution with groups

Question

I have the following json input

... "somefield":"somevalue", "time":"timevalue", "anotherfield":"value" ...

inside my ksh script I wish to replace timevalue with my value. So I created this regular expression using groups with works just fine

data=`cat somefile.json`
echo $data | perl -pe "s|(.*time\"\s*\:\s*\").*?(\".*)|\1%TIME%\2|g" | another-script.sh

... "somefield":"somevalue", "time":"%TIME%", "anotherfield":"value" ...

However ... I cannot use number as substitution because perl uses numbers to define groups .. so this one obviously doen't work

perl -pe "s|(.*time\"\s*\:\s*\").*?(\".*)|\120:00:00\2|g"

I can overcome this by doing two step substitution

perl -pe "s|(.*time\"\s*\:\s*\").*?(\".*)|\1%TIME%\2|g" | perl -pe "s|%TIME%|20:00:00|"

... "somefield":"somevalue", "time":"20:00:00", "anotherfield":"value" ...

but I am sure there is a better and more elegant way doing it

Answer 1

Whilst you could do this with regexes, it would be so much easier with the right tool

jq '.time="20:00:00"' somefile.json 

If you particularly wish to use Perl, the core Perl distribution has included a JSON parser since 2011, so you could do something like:

perl -MJSON::PP=decode_json,encode_json -0 -E '$j = decode_json(<>); $j->{time} = "20:00:00"; say encode_json($j)' somefile.json

Answer 2

Perl doesn't use \\1 for substitution. If you had enabled warnings (eg with perl -w ), perl would have told you it's $1 . Which can be disambiguated from surrounding digits by adding { } :

perl -pe 's|(.*time"\s*:\s*").*?(".*)|${1}20:00:00$2|g'

(I also removed all the redundant backslashes from the regex.)

On another note, what's the point of matching .* if you're just going to replace it by itself? Couldn't it just be

perl -pe 's|(time"\s*:\s*").*?(")|${1}20:00:00$2|g'

?

I'm not a big fan of .* or .*? . If you're trying to match the inside of a quoted string, it would be better to be specific:

perl -pe 's|(time"\s*:\s*")[^"]*(")|${1}20:00:00$2|g'

We're not trying to validate the input string, so now there's really no reason to match that final " (and replace it by itself) either:

perl -pe 's|(time"\s*:\s*")[^"]*|${1}20:00:00|g'

If your perl is not ancient (5.10+), you can use \\K to "keep" leading parts of the string, ie not include it in the match:

perl -pe 's|time"\s*:\s*"\K[^"]*|20:00:00|g'

Now only the [^"]* part will be substituted, saving us from having to do any capturing.