I just saw that there are implementations of ULID that provide uppercase only ULIDs, others (like the PHP implementation by @robinvdvleuten) provide lowercase ULIDs. The specification does not yet impose uppercase or lowercase but states that ULID is "case-insensitive". This is a great feature.

Nevertheless, I'd like to propose suggesting Uppercase ULID as "the right way". Mainly for two reasons:

• most of the libraries provide uppercase ULIDs. The existing libraries will probably become the "standard solution" for their language (given that they are well designed, tested and documented).
• Having different solutions in different languages could lead to problems in multi language environments. Consider a database that is accessed by more than one project with different languages across the projects. Consider Regex validation that will often be made up for the existing library and may lead to problems when other librarys or languages provide stuff.

I understand that this is debatable, as being flexible in your setup is a strength. But I also think, that having either uppercase or lowercase as the proposed (or imposed) way to implement ULIDs will help the Spec to spread because there is less potential for conflicts.

What do you think of this?