[ruby-core:94405] [Ruby master Bug#16108] gsub gives wrong results with regex backreferencing and triple backslash

Issue #16108 has been updated by alanwu (Alan Wu).


The source of your problem seem to be the behavior below:
```ruby
p ' \1 '.bytes # => [32, 92, 49, 32]
p ' \\ '.bytes # => [32, 92, 32]
p ' \ '.bytes  # => [32, 92, 32]
```
as you can see, two backslashes in a single quote string literal only gives one backslash in the resulting string.

This is future complicated by gsub interpreting the content of the second argument as a replacement directive. The means interpreting the backslashes for a second time. You want the final replacement to be "one backslash, followed by the first match group, then another backslash", or literally `\\1\` (`[92, 92, 49, 92]`). The replacement directive to express this is `\\\1\\` (`[92, 92, 92, 49, 92, 92]`), as we need to escape the first and last backslash by doubling them. We don't want to double the backslash right before "1", as we are not looking for a literal backslash there.

Now we need to construct a Ruby string literal we can put in the source code that would give us the replacement directive we want, which we could do by doubling all the backslashes:

```ruby
p '\\\\\\1\\\\'.bytes # => [92, 92, 92, 49, 92, 92]
```

We could get rid of one of the backslashes in the before "1", the single quote literal `'\1'` gives `[92, 49]`:
```ruby
p '\\\\\1\\\\'.bytes # => [92, 92, 92, 49, 92, 92]
```
We could also get rid of two backslashes after the 1 as `gsub` interprets the lone backslash at the end as a literal backslash.

This is too many backslashes for my taste, so I would prefer the block form. It takes the return value of block and substitute that for the mach verbatim. The special `$1` variable is set within the gsub block, which we can use to build the replacement we want:

```ruby
input.gsub(pattern) { ["\\", $1, "\\"].join }
```
---
Here is a test program for you:

```ruby
input = '\indexentry{\textbf{bold}|hyperpage}{2}'
pattern = /\\textbf\{([^\}]+)\}/

test = ->(replacement) {
  puts "result: #{input.gsub(pattern, replacement)}, replacement: #{replacement.bytes}.map(&:chr).join"
}
test.call('\\\1\\')
test.call('\\ \1\\')
test.call('\\\\\\1\\\\')
test.call('\\\\\\1\\')
test.call('\\\\\1\\')

$stdout.write "alternative: "
puts input.gsub(pattern) { ["\\", $1, "\\"].join }
```


----------------------------------------
Bug #16108: gsub gives wrong results with regex backreferencing and triple backslash
https://bugs.ruby-lang.org/issues/16108#change-80825

* Author: VivianUnger (Vivian Unger)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.3p62 (2019-04-16 revision 67580) [x64-mingw32]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
I have written a script to convert LaTeX indexing files (.idx) to Macrex backup format (.mbk), so that I can import LaTeX-embedded indexes into the Macrex indexing program. A problem arises when I try to convert bolded text. LaTeX indicates bolded text with the tag \textbf{} while Macrex wraps it in backslashes: \\.

In my test case, the input string is "\indexentry{\textbf{bold}|hyperpage}{2}", which I need to convert into "\indexentry{\bold\|hyperpage}{2}". For this I am using:

record.gsub(/\\textbf\{([^\}]+)\}/, '\\\1\\')

But instead of the expected output, I get:

\indexentry{\1\|hyperpage}{2}

...as if I only had \\ rather than \\\.

I have tried the same Regex in a search-and-replace in Notepad++ and it works as expected. It's only in Ruby that I get this unexpected result.

The kludgey workaround I have found is to leave a space before the \\:

record.gsub(/\\textbf\{([^\}]+)\}/, '\\ \1\\')

...giving the result:

\indexentry{\ bold\|hyperpage}{2}

But this won't do. Macrex complains and the extra space has to be edited out. Imagine if you have hundreds of lines with bold text in them!



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