[ruby-core:105918] [Ruby master Feature#12745] String#(g)sub(!) should pass a MatchData to the block, not a String

Issue #12745 has been updated by jrochkind (jonathan rochkind).


I would love to see this resurrected. 

If you want the MatchData in a gsub block, at first it looks like you can just use `Regexp.last_match`. 

But I think this, along with variables like `$1`, may not be thread-safe? See for instance related at https://bugs.ruby-lang.org/issues/12689 and https://github.com/jruby/jruby/issues/3031#issuecomment-259057232 and https://github.com/jruby/jruby/issues/4868#issuecomment-347276140

If so, it makes having a clear fixed MatchData passed as an arg to block more than just a minor convenience, but necessary to do things like this in a thread-safe way at all? Or I may be misunderstanding. 

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Feature #12745: String#(g)sub(!) should pass a MatchData to the block, not a String
https://bugs.ruby-lang.org/issues/12745#change-94461

* Author: herwin (Herwin W)
* Status: Feedback
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
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A simplified (and stupid) example: replace some placeholders in a string with function calls

~~~ruby
def placeholder(val)
  raise 'Incorrect value' unless val == 'three'
  '3'
end

str = '1.2.[three].4'
str.gsub!(/\[(\w+)\]/) { |m| placeholder(m) }
~~~

This raises the 'incorrect value' because we don't pass the match 'three', but the full string '[three]'. It looks like we have 3 options to fix that:

1. Match `[three]` instead of `three` in the placeholder replacement method
2. Pass `m[1..-2]` instead of `m` to the method (or strip it in `placeholder`)
3. Use `$1` in the method call, ignore the value that's passed to the block

Options 1 and 2 look kind of code duplication to me (and they're possible in the simplified example, but might get tricky in real situations). I don't like option 3 because you completely ignore the value that's been passed to the block in favor of global variables, you can't use named captures, and writing code this way makes it incompatible with Rubinius.

I think it would be more logical to pass a `MatchData` (like what you'd get with `String#match`) instead of a `String` to the block. The `#to_s` returns the whole string, so in 90% of the use cases the code could remain unaltered, but the remaining 10% makes it a change that shouldn't be backported to 2.3.

Attached is a very naive patch to pass a matchdata to the block called by `String#sub`. The additional change in `rbinstall.rb` was required to run `make install`, which actually shows an incompatiblity (which I hadn't anticipated)

---Files--------------------------------
ruby_string_sub_matchdata.diff (952 Bytes)


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