From: "matz (Yukihiro Matsumoto) via ruby-core" <ruby-core@...>
Date: 2024-12-18T06:11:41+00:00
Subject: [ruby-core:120289] [Ruby master Feature#20885] String#gsub?

Issue #20885 has been updated by matz (Yukihiro Matsumoto).


I am not positive for adding `gsub?`. It's not a predicate, after all.

Matz.


----------------------------------------
Feature #20885: String#gsub?
https://bugs.ruby-lang.org/issues/20885#change-111054

* Author: Dan0042 (Daniel DeLorme)
* Status: Open
----------------------------------------
I would like a variation of sub/gsub that returns a new string if there is a match, or nil otherwise.

This can be currently done with `str.dup.gsub!(...)` at the mere cost of 2 unnecessary extra allocations due to String#dup

TBH I'm not sure we need yet another gsub method, but what gets me here is that `gsub?` is the primitive operation from which `gsub` and `gsub!` are made. Internally, these two methods work like this:

```ruby
def gsub!(...)
  modified = gsub?(...) and replace(modified)
end

def gsub(...)
  gsub?(...) or dup
end
```

We can efficiently derive these two methods from `gsub?`, but we cannot efficiently implement `gsub?` in ruby, so it feels regrettable to me that `gsub?` is not available as a built-in.

Use cases include:

```ruby
# ensure gsub has modified the string
newstr = str.gsub?(rx,repl) or raise "str did not contain expected value"

# take an action if the string is modified
if str = obj.title.gsub?(rx,repl)
  obj.title = str
  obj.save
end

# avoid allocating a new string if there is no change
str = str.gsub?(rx,repl) || str
```



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