Adding backreferenced value to its replacement

Question

I am trying to add a number from a backreference to another number, but I seem to get only concatenation:

textStr = "<testsuite errors=\"0\" tests=\"4\" time=\"4.867\" failures=\"0\" name=\"TestRateUs\">"

new_str = textStr.gsub(/(testsuite errors=\"0\" tests=\")(\d+)(\" time)/, '\1\2+4\3')

# => "<testsuite errors=\"0\" tests=\"4+4\" time=\"4.867\" failures=\"0\" name=\"TestRateUs\">"

I tried also using to_i on the backreferenced value, but I can't get the extracted value to add. Do I need to do something to the value to make it addable?

You cannot use a backreference like that since it is not evaluated yet. — Wiktor Stribiżew, Feb 02 '16 at 09:49
why was this marked down? The question was valid. If you can't do something then its ok to say you can't do it but why mark the question down? — amadain, Feb 02 '16 at 13:34
I have not done this. I think some won't like the fact that you use HTML as sample text for a regex manipulation. — Wiktor Stribiżew, Feb 02 '16 at 13:38
I liked your answer. I achieved it in 2 regexes - one to extract and the next to manipulate. Yours does it nicely in one go. Thx. Something to learn — amadain, Feb 02 '16 at 13:40

score 1 · Accepted Answer · answered Feb 02 '16 at 10:14

If you are manipulating XML, I'd suggest using some specific library for that. In this answer, I just want to show how to perform operations on the submatches.

You can sum up the values inside a block:

textStr="<testsuite errors=\"0\" tests=\"4\" time=\"4.867\" failures=\"0\" name=\"TestRateUs\">"
new_str = textStr.gsub(/(testsuite errors=\"0\" tests=\")(\d+)(\" time)/) do
    Regexp.last_match[1] + (Regexp.last_match[2].to_i + 4).to_s + Regexp.last_match[3]
end
puts new_str

See IDEONE demo

If we use {|m|...} we won't be able to access captured texts since m is equal to Regexp.last_match[0].to_s.

Adding backreferenced value to its replacement

1 Answers1