BBCode ("Bulletin Board Code") is a lightweight markup language used to format messages in many Internet forum software. It was first introduced in 1998. The available "tags" of BBCode are usually indicated by square brackets (<code>[</code> and <code>]</code>) surrounding a keyword, and are parsed before being translated into HTML.
BBCode is typically implemented by applying a series of regular expression string-replace operations upon the input. Because regular expressions are limited in analyzing the structure of text input, this has the artifact that any non-hierarchical BBCode input will be transformed into invalid non-hierarchical HTML without error.
Applying traditional parsing techniques is made difficult by ambiguities in the markup, such as in , where the input can either be interpreted as "text" quoted from someone called , or the bolded text "text" surrounded by and , i.e. <code>[quote=text[/quote]</code>.