Issue #16145 has been updated by duerst (Martin Dürst). @zenspider I said that single-character character classes get reduced to just the actual character. So that would mean that your `"CAFÉ" =~ /[a]/i` gets reduced to `"CAFÉ" =~ /a/i`, and therefore works. That of course does not prove my guess, but it also doesn't disprove it. We'd need some other examples to test this further. ---------------------------------------- Bug #16145: regexp match error if mixing /i, character classes, and utf8 https://bugs.ruby-lang.org/issues/16145#change-105514 * Author: zenspider (Ryan Davis) * Status: Open * Priority: Normal * Backport: 2.5: UNKNOWN, 2.6: UNKNOWN ---------------------------------------- (reported on behalf of mage@mage.gold -- there appears to be an error in registration or login): See: ruby-talk @ X-Mail-Count: 440336 2.6.3 :049 > 'SHOP' =~ /[xo]/i => 2 2.6.3 :050 > 'CAFÉ' =~ /[é]/i => 3 2.6.3 :051 > 'CAFÉ' =~ /[xé]/i => nil 2.6.3 :052 > 'CAFÉ' =~ /[xÉ]/i => 3 Expected result: 2.6.3 :051 > 'CAFÉ' =~ /[xé]/i => 3 I tested it on random regex online pages. It does not match on https://regex101.com/ It matches on: https://regexr.com/ https://www.regextester.com/ https://www.freeformatter.com/regex-tester.html (Ignore case turned on). The reason I suppose it’s more like a bug than a feature is the fact that /[é]/i matches 'CAFÉ'. If the //i didn’t work for UTF-8 characters then the /[é]/i wouldn’t match it either. For example, [é] does not match 'CAFÉ' on https://regex101.com/ I could not find a page or a system that behaves the same way as Ruby does. For example, it matches in PostgreSQL 10 (under FreeBSD 12) too: # select 'CAFÉ'~ '[xé]'; ?column? ---------- f (1 row) # select 'CAFÉ' ~* '[xé]'; ?column? ---------- t (1 row) Tested it in IRB on macOS and FreeBSD. $ uname -a && ruby -v && locale Darwin xxx 18.7.0 Darwin Kernel Version 18.7.0: Thu Jun 20 18:42:21 PDT 2019; root:xnu-4903.270.47~4/RELEASE_X86_64 x86_64 ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-darwin18] LANG="en_US.UTF-8" LC_COLLATE="en_US.UTF-8" LC_CTYPE="en_US.UTF-8" LC_MESSAGES="en_US.UTF-8" LC_MONETARY="en_US.UTF-8" LC_NUMERIC="en_US.UTF-8" LC_TIME="en_US.UTF-8" LC_ALL="en_US.UTF-8" $ uname -a && ruby -v && locale FreeBSD xxx 12.0-RELEASE-p9 FreeBSD 12.0-RELEASE-p9 GENERIC amd64 ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-freebsd12.0] LANG=en_US.UTF-8 LC_CTYPE="en_US.UTF-8" LC_COLLATE="en_US.UTF-8" LC_TIME="en_US.UTF-8" LC_NUMERIC="en_US.UTF-8" LC_MONETARY="en_US.UTF-8" LC_MESSAGES="en_US.UTF-8" LC_ALL=en_US.UTF-8 I installed Ruby with RVM. -- https://bugs.ruby-lang.org/