[ruby-core:124131] [Ruby Feature#8948] Frozen regex

11 Dec 2025

      Issue #8948 has been updated by matz (Yukihiro Matsumoto).

We found some gems inherited from Regexp and used instance variables on instances, so they would be broken if all Regexp objects become frozen. I think it's unacceptable for Ruby 4.0.
Frankly, I don't think freezing the Regexp objects is worth the effort/trouble.

Matz.

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Feature #8948: Frozen regex
https://bugs.ruby-lang.org/issues/8948#change-115583

* Author: sawa (Tsuyoshi Sawada)
* Status: Assigned
* Assignee: Eregon (Benoit Daloze)
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=begin
I see that frozen string was accepted for Ruby 2.1, and frozen array and hash are proposed in https://bugs.ruby-lang.org/issues/8909. I feel there is even more use case for a frozen regex, i.e., a regex literal that generates a regex only once. It is frequent to have a regex within a frequently repeated portion of code, and generating the same regex each time is a waste of resource. At the moment, we can have a code like:

    class Foo
      RE1 = /pattern1/
      RE2 = /pattern1/
      RE3 = /pattern1/
      def classify
        case self
        when RE1 then 1
        when RE2 then 2
        when RE3 then 3
        else 4
        end
      end
    end

but suppose we have a frozen `Regexp` literal `//f`. Then we can write like:

    class Foo
      def classify
        case self
        when /pattern1/f then 1
        when /pattern1/f then 2
        when /pattern1/f then 3
        else 4
        end
      end
    end

=end

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