Merge pull request #208 from julianladisch/uuids

Add guides/uuids.md

guides/index.md

@@ -133,6 +133,7 @@ These are listed in the [Start](/start/) section.
 - For Contextual Query Language (CQL) examples, see the [Glossary](/reference/glossary/#cql), the FOLIO [CQL to PostgreSQL JSON converter](https://github.com/folio-org/cql2pgjson-java), the [API docs](/reference/api/), and the debug output for tests in each backend module.
 - Use [raml-cop](/guides/raml-cop/) to assess RAMLs, schema, and examples. A guide to its use with some explanations of its messages.
 - [Conduct cross-module joins via their APIs](cross-module-joins).
+- [How to check for a valid UUID](uuids).
 
 ### Tutorials
 

guides/uuids.md

@@ -0,0 +1,18 @@
+---
+layout: page
+title: How to check for a valid UUID
+permalink: /guides/uuids/
+menuInclude: no
+menuTopTitle: Guides
+---
+
+The regexp
+`^[a-fA-F0-9]{8}-[a-fA-F0-9]{4}-[1-5][a-fA-F0-9]{3}-[89abAB][a-fA-F0-9]{3}-[a-fA-F0-9]{12}$`
+matches any valid UUID. This accepts v1, v2, v3, v4 and v5 UUIDs as FOLIO allows all of them,
+but it excludes the Nil UUID `00000000-0000-0000-0000-000000000000` that usually serves as a
+NullObject to be used for the Null Object Pattern.
+
+A UUID has the form xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx where the version M must be [1-5]
+and the variant N must be [89abAB]; see
+[UUID at Wikipedia](https://en.wikipedia.org/wiki/Universally_unique_identifier).
+

reference/glossary.md

@@ -259,6 +259,12 @@ is "searchRetrieve Version 1.0, OASIS Standard".
 
 See [Stripes](#stripes) and the various [Stripes entities](#stripes-entities) explained above.
 
+#### UUID
+
+Universally unique identifier, a 128-bit number, see
+[UUID at Wikipedia](https://en.wikipedia.org/wiki/Universally_unique_identifier)
+and [How to check for a valid UUID](/guides/uuids/).
+
 #### Vert.x
 
 [Vert.x](http://vertx.io) is a toolkit for building scalable, reactive