Ruby Regex Replace Last Occurence of Grouping

Question

In Ruby regular expressions I would like to use gsub to replace a last occurrence of a grouping, if it occurs, otherwise, perform a replacement anyways at a default location. I am trying to replace the last occurrence of a number in the 40s (40...49). I have the following regular expression, which is correctly capturing the grouping I would like in '\3':

/(([1-3,5-9][0-9]|([4][0-9]))[a-z])*Foo/

Some sample strings I am using this regex on are:

12a23b34c45d56eFoo
12a45b34c46d89eFoo
45aFoo
Foo
12a23bFoo
12a23b445cFoo

Using https://regex101.com/, I see the last number in 40s is captured in '\3'. I would then like to somehow perform string.gsub(regex, '\3' => 'NEW') to replace this last occurrence or append before Foo if not present. My desired results would be:

12a23b34cNEWd56eFoo
12a45b34cNEWd89eFoo
NEWaFoo
NEWFoo
12a23bNEWFoo
12a23b4NEWcFoo

My intent is to only match on 2 digit numbers. So ideally I would want my regex to ignore 445. So I only want my regex to match the sequence of 2 digit numbers and then a letter. I'll update the question to have a sample of that. — fowlball1010
– fowlball1010, Commented May 14, 2015 at 13:23

Aleksei Matiushkin · Accepted Answer · 2015-05-14 13:23:04Z

1

If I correctly understood, you are interested in gsub with codeblock:

str.gsub(PATTERN) { |mtch|
  puts mtch                # the whole match
  puts $~[3]               # the third group
  mtch.gsub($~[3], 'NEW')  # the result
}

'abc'.gsub(/(b)(c)/) { |m| m.gsub($~[2], 'd') }
#⇒ "abd"

Probably you should handle the case when there are no 40-s occureneces at all, like:

gsub($~[1], "NEW$~[1]") if $~[3].nil?

To handle all the possible cases, one might declare the group for Foo:

#                           NOTE THE GROUP ⇓⇓⇓⇓⇓
▶ re = /(([1-3,5-9][0-9]|([4][0-9]))[a-z])*(Foo)/
#⇒ /(([1-3,5-9][0-9]|([4][0-9]))[a-z])*(Foo)/
▶ inp.gsub(re) do |mtch|
▷   $~[3].nil? ? mtch.gsub($~[4], "NEW#{$~[4]}") : mtch.gsub(/#{$~[3]}/, 'NEW')  
▷ end
#⇒ "12a23b34cNEWd56eFoo\n12a45b34cNEWd89eFoo\nNEWaFoo\nNEWFoo\n12a23bNEWFoo"

Hope it helps.

edited May 14, 2015 at 13:23

answered May 14, 2015 at 12:47

Aleksei Matiushkin

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3 Comments

fowlball1010 Over a year ago

The codeblock works well for the cases when there are 40s. However, the portion for when there are no 40s I can't get to work. Should that code be in the original codeblock as the result of a conditional of $~[3].nil? like you specified. I put it in there, but I get $~[1] nil as a parameter to gsub. The code fails for the strings of 'Foo' and '12a23bFoo'.

Aleksei Matiushkin Over a year ago

Since you have not specified what Foo is, I was unable to come with a complete answer. For plain Foo string, the following would do the trick: inp.gsub(re) { |mtch| mtch.gsub /#{$~[3]}Foo/, 'NEWFoo' }.

fowlball1010 Over a year ago

Foo is just a string. So I have a function that takes any of the aforementioned sample strings and returns the desired result with the regex matching. I tried the recommendation but I still don't get the result. I'm unsure how it would work with 'Foo' with /#{$~[3]}Foo/ since $~[3] is nil?

Ely · Accepted Answer · 2015-05-14 14:32:42Z

0

I suggest the following:

'12a23b34c45d56eFoo'.gsub(/(([1-3,5-9][0-9]|([4][0-9]))[a-z])*Foo/) {
  if Regexp.last_match[3].nil? then
    puts "Append before Foo"
  else 
    puts "Replace group 3"
  end 
}

You'd need to find a way to append or replace accordingly or maybe someone can edit with a concise code...

edited May 14, 2015 at 14:32

answered May 14, 2015 at 14:04

Ely

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