Characters in keys

Keys containing only lowercase latin letters (a to z) and the underscore (_). First and last characters must be letters. Most simple keys should fall into this category.

Keys that have one or more colon (:) characters inside in addition to the letters (a to z) and underscores (_). The colon is often used as a hierarchy separator character.

Lowercase letters from the latin script are usually preferred for keys, but sometimes uppercase letters are used (for instance for known abbreviations) or letters from other scripts (for instance for keys only relevant to an area where that script is used predominantly).

Keys that contain whitespace characters such as space, tab, new line, carriage return, or whitespace characters from other alphabets. Whitespace in keys can be confusing, especially at the beginning or end of the key, because they are invisible. Generally the whitespace should be removed or the underscore (_) used instead.

Keys that contain possibly problematic characters: =+/&<>;'"?%#@\, or control characters. These characters can be problematic, because they are used to quote strings in different programming languages or have special meanings in XML, HTML, URLs, and other places. The equal sign is used often as separator between tag keys and values. Keys that appear in this list are not necessarily wrong though. But in many cases they are just results of some error and should be fixed.

Keys that don't fit in any of the other categories.