# Gsub producing different results running the same code
**URL:** https://meta.discourse.org/t/gsub-producing-different-results-running-the-same-code/203837
**Category:** Support
**Created:** [September 17, 2021, 5:17pm UTC](https://meta.discourse.org/t/gsub-producing-different-results-running-the-same-code/203837 "2021-09-17T17:17:52Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@pfaffman](https://meta.discourse.org/u/pfaffman)
#### Post date: [September 17, 2021, 5:17pm UTC](https://meta.discourse.org/t/gsub-producing-different-results-running-the-same-code/203837/1 "2021-09-17T17:17:52Z")
Maybe this is a stackoverflow question and I don’t understand what `gsub` does ~~, but this looks like bizarre ruby behavior that I wonder if it could somehow be the ruby image?~~ I get the same results in `irb` on my local machine.
I thought this might be some behavior of `dup` that I wasn’t understanding, but I can replicate the behavior if I define the string twice. The first time the `gsub` fails to insert the URL, but subsequent runs of the same data include it as I expect.
```plaintext
[1] pry(main)> save=%(On Android, click the three-dot icon in the upper right corner and select Relations from the popup menu. This function wasn't working for me u
ntil yesterday, so perhaps on Android, it's still in A/B testing. [upl-image-preview url=https://somehost.s3.eu-central-1.amazonaws.com/2021-08-29/1630236738-85280-screen-shot-2021-08
-29-at-83023-pm.png]
[1] pry(main)*
[1] pry(main)* )
=> "On Android, click the three-dot icon in the upper right corner and select Relations from the popup menu. This function wasn't working for me until yesterday, so
perhaps on Android, it's still in A/B testing. [upl-image-preview url=https://somehost.s3.eu-central-1.amazonaws.com/2021-08-29/1630236738-85280-screen-shot-2021-08-29-at-83023-pm.
png]
\n\n"
[2] pry(main)> s=save.dup
=> "On Android, click the three-dot icon in the upper right corner and select Relations from the popup menu. This function wasn't working for me until yesterday, so
perhaps on Android, it's still in A/B testing. [upl-image-preview url=https://somehost.s3.eu-central-1.amazonaws.com/2021-08-29/1630236738-85280-screen-shot-2021-08-29-at-83023-pm.
png]
\n\n"
[3] pry(main)> s.gsub!(/.+?<\/UPL-IMAGE-PREVIEW>/i,"\nIMAGEISHERE\n#{$1}\n")
=> "On Android, click the three-dot icon in the upper right corner and select Relations from the popup menu. This function wasn't working for me until yesterday, so
perhaps on Android, it's still in A/B testing. \nIMAGEISHERE\n\n
\n\n"
[4] pry(main)> s=save.dup
=> "On Android, click the three-dot icon in the upper right corner and select Relations from the popup menu. This function wasn't working for me until yesterday, so perhaps on Android, it's still in A/B testing. [upl-image-preview url=https://somehost.s3.eu-central-1.amazonaws.com/2021-08-29/1630236738-85280-screen-shot-2021-08-29-at-83023-pm.png]
\n\n"
[5] pry(main)> s.gsub!(/.+?<\/UPL-IMAGE-PREVIEW>/i,"\nIMAGEISHERE\n#{$1}\n")
=> "On Android, click the three-dot icon in the upper right corner and select Relations from the popup menu. This function wasn't working for me until yesterday, so perhaps on Android, it's still in A/B testing. \nIMAGEISHERE\nhttps://somehost.s3.eu-central-1.amazonaws.com/2021-08-29/1630236738-85280-screen-shot-2021-08-29-at-83023-pm.png\n
\n\n"
```
Subsequent runs of
```plaintext
s=save.dup
s.gsub!(/.+?<\/UPL-IMAGE-PREVIEW>/i,"\nIMAGEISHERE\n#{$1}\n")
```
produce the URL replacement as I expect.
work as expected.
I was having a similar problem the other day that I described at [this post](https://meta.discourse.org/t/bizarre-off-by-one-post-update-error/203669), but in a previous edit. That code was
```plaintext
def fix_slack_posts
SiteSetting.min_post_length = 2
reg=/(\*\*)(This topic was automatically generated from Slack. You can find the original thread \[here\].+?\))(\*\*\.)?\s*?([a-zA-Z, ()]* : )(.*)/m
preg = /([a-zA-Z, ()]+? : )(.*)/m
topic_posts = Post.where("raw like '**This topic was automatically%'")
topic_posts.each do |tpost|
begin
tpost.raw.gsub!(reg,"#{$5}\n\n#{$2}.")
tpost.save!
tpost.rebake!
rescue
puts "Can't update topic post #{tpost.raw}"
end
posts = Post.where(topic_id: tpost.topic_id).where("post_number > 1")
posts.each do |post|
if post.raw.gsub(preg,"#{$2}").length>=10
begin
post.raw.gsub!(preg,"#{$2}")
post.save!
post.rebake!
rescue
puts "#{post.id}--cannot save #{post.raw}. "
end
end
end
end
SiteSetting.min_post_length = 10
end
```
And what was happening is that the `tpost.raw` on the second iteration of the loop was getting the value of the last `post.raw` on the previous iteration.
I thought I was going crazy, but I have seen this
---
### Author:  [@pfaffman](https://meta.discourse.org/u/pfaffman)
#### Post date: [September 17, 2021, 5:29pm UTC](https://meta.discourse.org/t/gsub-producing-different-results-running-the-same-code/203837/2 "2021-09-17T17:29:18Z")
Here’s a simpler oddity. This version has gsub not working on the first run, but working on the second:
```ruby
[1] pry(main)> s=%(On Android, click the three-dot icon in the upper right corner and select Relations from the popup menu. This function wasn't working for me until yesterday, so perhaps on Android, it's still in A/B testing. [upl-image-preview url=https://somehost.s3.eu-central-1.amazonaws.com/2021-08-29/1630236738-85280-screen-shot-2021-08-29-at-83023-pm.png]
[1] pry(main)*
[1] pry(main)* )
=> "On Android, click the three-dot icon in the upper right corner and select Relations from the popup menu. This function wasn't working for me until yesterday, so perhaps on Android, it's still in A/B testing. [upl-image-preview url=https://somehost.s3.eu-central-1.amazonaws.com/2021-08-29/1630236738-85280-screen-shot-2021-08-29-at-83023-pm.png]
\n\n"
[2] pry(main)> s.gsub(/.+?<\/UPL-IMAGE-PREVIEW>/mi,"\nIMAGEISHERE\n#{$1}\n")
=> "On Android, click the three-dot icon in the upper right corner and select Relations from the popup menu. This function wasn't working for me until yesterday, so perhaps on Android, it's still in A/B testing. \nIMAGEISHERE\n\n
\n\n"
[3] pry(main)> s.gsub(/.+?<\/UPL-IMAGE-PREVIEW>/mi,"\nIMAGEISHERE\n#{$1}\n")
=> "On Android, click the three-dot icon in the upper right corner and select Relations from the popup menu. This function wasn't working for me until yesterday, so perhaps on Android, it's still in A/B testing. \nIMAGEISHERE\nhttps://somehost.s3.eu-central-1.amazonaws.com/2021-08-29/1630236738-85280-screen-shot-2021-08-29-at-83023-pm.png\n
\n\n"
```
---
### Author:  [@david](https://meta.discourse.org/u/david)
#### Post date: [September 17, 2021, 8:43pm UTC](https://meta.discourse.org/t/gsub-producing-different-results-running-the-same-code/203837/3 "2021-09-17T20:43:49Z")
Variables like `$1` are populated **after** a regex match is performed. Simplifying your example a little:
```ruby
s.gsub('original(value)', "replacement#{$1}")
```
The `"replacement#{$1}"` expression is evaluated **before** the gsub function is called. So `$1` will be leftover from some previous regex task. (That’s why your second attempt works - it takes `$1` from the first attempt)
There are a couple of options to solve this problem. [gsub](https://apidock.com/ruby/String/gsub) has lots of different functionalities.
My preference would be to pass a block to gsub. The block is evaluated after the regex has been matched, so `$1` will work as you expect:
```ruby
s.gsub('original(value)') { |match| "replacement#{$1}" }
```
Or you can use the ‘backreference’ feature of gsub. I’m not a fan of this syntax, but it does work. Rather than using `"replacement#{$1}"`, you use `'replacement\1'` (or `\2`, or `\3`, etc.)
```ruby
s.gsub('original(value)', 'replacement\1')
```
---
### Author:  [@pfaffman](https://meta.discourse.org/u/pfaffman)
#### Post date: [September 17, 2021, 8:47pm UTC](https://meta.discourse.org/t/gsub-producing-different-results-running-the-same-code/203837/4 "2021-09-17T20:47:49Z")
> [@david](#):
>
> So `$1` will be leftover from some previous regex task.
OMG. Thank you. I thought I was going crazy.
> [@david](#):
>
> My preference would be to pass a block to gsub. The block is evaluated after the regex has been matched, so `$1` will work as you expect
Brilliant.
> [@david](#):
>
> Or you can use the ‘backreference’ feature of gsub.
I’d always wondered about those backslashes. Five years later, I get it.
This totally was a stack exchange question. 🤷♂️
A million thanks.
---
### Author:  [@system](https://meta.discourse.org/u/system)
#### Post date: [October 17, 2021, 8:47pm UTC](https://meta.discourse.org/t/gsub-producing-different-results-running-the-same-code/203837/5 "2021-10-17T20:47:55Z")
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