Metazeichen

Metazeichen, or metacharacters, are symbols with special meaning inside a formal notation, such as regular expressions, shell patterns, or command languages. They do not stand for literal characters themselves, but instead influence how input is interpreted, matched, or parsed. The exact set and behavior of metazeichen vary by system, and most implementations provide a mechanism to escape them to obtain their literal value.

In regular expressions, metazeichen control pattern matching and grouping. Examples include . which matches any single character (except newline in many flavors); ^ and $ for anchors; * and + for repetition; ? for optional elements; [ ] for character classes; ( ) for grouping; | for alternation; {m,n} for quantified repetitions; and the backslash \ to escape a metazeichen. Different regex engines may introduce or restrict certain metazeichen or modify their behavior with flags.

In shell globbing, metazeichen form filename patterns. The asterisk * matches any string, the question mark ? matches a single character, and square brackets [ ] denote a character class or range. Glob patterns are designed for pathname expansion rather than arbitrary text processing, and they differ from regular expressions in syntax and capabilities.

In SQL and other query languages, metazeichen often appear as wildcards. In SQL's LIKE operator, % stands for any sequence of characters and _ for a single character. Many systems also offer an ESCAPE clause to treat these as literals. Other languages may define additional meta-symbols for pattern matching or parsing.

Because metazeichen are not universal, their exact meaning depends on the context and language. Users should consult the respective documentation and apply proper escaping to avoid unintended matches. See also: regular expressions, shell glob patterns, SQL LIKE.