Question

In my app I need to be able to find all number substrings, then scan each one, find the first one that matches a range (such as between 5 and 15) and replace THAT instance with another string “X”.

My test string s = "1 foo 100 bar 10 gee 1"

My initial pattern is any string of 1 or more digits, eg, re = Regexp.new(/\d+/)

matches = s.scan(re) gives ["1", "100", "10", "1"]

If I want to replace the Nth match, and only the Nth match, with “X” how do I?

For example if I want to replace the third match “10” (matches[2]) I can’t just say
s[matches[2]] = "X" because that does two replacements

"1 foo X0 bar X gee 1"

Any help would be appreciated!

Answer 1

String#gsub has a form that takes a block. It yields to the block for each match, and replaces the match with the result of the block. So:

first = true
"1 foo 100 bar 10 gee 1 12".gsub(/\d+/) do |digits|
  number = digits.to_i
  if number >= 5 && number <= 15 && first
    # do the replacement
    first = false
    'X'
  else
    # don't replace; i.e. replace with itself
    digits
  end
end
# => "1 foo 100 bar X gee 1 12"