Bug #15816

String#casecmp compares uppercase characters instead of lowercase

Added by jonathanhefner (Jonathan Hefner) about 2 months ago. Updated about 1 month ago.

Target version:


The current implementation of String#casecmp converts characters to uppercase before comparing them. However, all references I've found for strcasecmp (the C function on which String#casecmp is based) indicate characters should be converted to lowercase before being compared.

For example, this man page says:

The POSIX.1-2008 standard says ... shall behave as if the strings had been converted to lowercase and then a byte comparison performed.

The difference in behavior is apparent when comparing / sorting strings containing [, \, ], ^, _, or ` (the characters that occur between Z and a). Converting to lowercase sorts these punctuation characters before A-z along with most of the other punctuation in ASCII, but converting to uppercase sorts these characters after A-z instead.


casecmp-lowercase.patch (1.3 KB) casecmp-lowercase.patch jeremyevans0 (Jeremy Evans), 05/09/2019 03:37 AM


Updated by jeremyevans0 (Jeremy Evans) about 1 month ago

The documentation of String#casecmp does not specify how it is is implemented, so it seems fair to consider switching. However, this change is likely to cause backwards compatibility issues. While it seems unlikely there are many applications relying on the current behavior, I would guess there are at least a few.

Considering that String#casecmp? uses lowercase and not uppercase, I think making such a change is reasonable, but we may want to delay making this change until Ruby 3.

Attached is a patch if we want to make this change.

Updated by mame (Yusuke Endoh) about 1 month ago

Until ruby 1.8.7, it seemed to use downcase. It was changed at r14227 to support encoding. I think the behavior change was not intended, so this is merely a bug?

# ./bin/ruby-1.8.7-p374 -e 'p "a".casecmp("[")'

# ./bin/ruby-1.9.0-0 -e 'p "a".casecmp("[")'

Also available in: Atom PDF