Nonwildcard

Nonwildcard is a term used in computing to describe the literal characters in a search or matching pattern that are not treated as wildcards. Wildcards are special symbols that can stand for one or more unspecified characters, while nonwildcards must match exactly. The distinction helps users understand which parts of a pattern are fixed and which parts can vary.

In glob patterns used for filesystem matching, nonwildcards are the characters that must appear as written. For example, in the pattern data_2023*.txt, the segments data_2023 and .txt are nonwildcards, while the asterisk (*) is a wildcard that can match any sequence of characters. To search for a literal asterisk, it must be escaped (for example data_2023\*.txt), in which case the asterisk is treated as a nonwildcard.

In SQL, pattern matching with LIKE uses wildcards such as underscores and percent signs. Nonwildcards are the other characters in the pattern. For instance, in the pattern 'A__B%.csv', A and B and the period are nonwildcards, while the underscores and the percent sign are wildcards. To search for a literal underscore or percent sign, an escape mechanism is used (ESCAPE clause or appropriate escape character).

In regular expressions, most characters are literals unless they have special meaning. Nonwildcards correspond to literal characters in the pattern, whereas metacharacters such as . * + ? ( ) [ { | ^ $ have wildcard-like effects and can be made literal by escaping.

Understanding nonwildcards aids in precise matching, proper escaping, and potential performance gains by leveraging exact character matches when possible.