metacharacters

Metacharacters are characters that have a specialized meaning in a given language or tool, rather than representing themselves literally. They are used to control pattern matching, text processing, command interpretation, and other operations. The exact set and behavior of metacharacters depend on the context and the engine being used, such as regular expressions or shell commands.

In regular expressions, metacharacters form the building blocks of patterns. Common ones include . which matches any character except newline, ^ and $ which anchor the start and end of a string, and * + ? which denote repetition, and [ ] for character classes. Parentheses group subpatterns, and | denotes alternation. Curly braces {m,n} specify precise repetitions. A backslash is used to escape metacharacters if their literal meaning is required, and to introduce escape sequences like \d or \w in flavors that support them. Inside character classes, many metacharacters lose their special meaning, with only a few exceptions such as ^, -, and ].

In command interpreters, metacharacters perform shell control and expansion. The asterisk * matches multiple filename characters, ? matches a single character, and [ ... ] creates a character class. Other metacharacters control input and output, such as > for redirection, | for piping, ; for command separation, and & to run a command in the background. Quoting and escaping are used to suppress or preserve special meaning.

Beyond regular expressions and shells, many programming languages and data formats treat punctuation as operators or delimiters, which can be described as metacharacter-like depending on usage. The concept emphasizes that some characters carry meaning beyond their literal glyph and may need escaping or quoting to be treated literally.