Issue #21176 has been reported by ima1zumi (Mari Imaizumi). ---------------------------------------- Bug #21176: Regression in case-insensitive matching for single-byte encodings https://bugs.ruby-lang.org/issues/21176 * Author: ima1zumi (Mari Imaizumi) * Status: Open * ruby -v: ruby 3.5.0dev (2025-02-28T09:32:36Z master db4ea95219) +PRISM [arm64-darwin24] * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- In Ruby 3.4.2, case-insensitive regex matching (`/i`) worked as expected for single-byte encodings like ISO-8859-x. However, in Ruby 3.5.0dev, characters such as `\u00F3 (ó)` and `\u00D3 (Ó)` are no longer considered equivalent under case-insensitive matching, causing the match to fail. The likely cause is #16145 , which appears to have broken handling of `0x80–0xFF` in single-byte encodings. ## Reproduction ```ruby enc = Encoding::ISO_8859_1 o_acute_lower = "\u00F3".encode(enc) # ó o_acute_upper = "\u00D3".encode(enc) # Ó puts /[x#{o_acute_lower}]/i =~ "abc#{o_acute_upper}" ``` - Ruby 3.4.2: outputs 3 (match successful) - Ruby 3.5.0dev: outputs nil (match fails) - ruby 3.5.0dev (2025-02-28T09:32:36Z master db4ea95219) +PRISM [arm64-darwin24] I will submit PR to fix this. -- https://bugs.ruby-lang.org/