underscore

The underscore is a character used in typography and computing, represented by the symbol _. In ASCII it corresponds to code 95, and in Unicode it is U+005F. It is commonly used as a word separator or placeholder in places where spaces are not practical, and it appears in various naming conventions and text-editing practices.

In programming and data processing, the underscore serves multiple roles. It is widely used to form multiword identifiers in snake_case (for example, my_variable_name) to improve readability. Many languages permit underscores in identifiers, though some have conventions or restrictions on leading or trailing underscores. For example, in Python a single leading underscore signals a non-public attribute, double leading underscores trigger name mangling, and a trailing underscore can avoid keyword conflicts. In other languages, underscores often act as convenient separators rather than carrying semantic meaning.

In SQL, the underscore functions as a single-character wildcard within LIKE patterns; for example, name LIKE 'A_e' would match names with a three-character form starting with A and ending with e. This usage is distinct from the percent sign (%), which matches any sequence of characters.

Apart from programming, the underscore also appears in tools and libraries. Underscore.js is a JavaScript utility library that provides functional helpers such as map, reduce, and filter, and inspired similar libraries like Lodash. In typography and historical manuscript practices, underscores were sometimes used to indicate underlining or to mask letters in redacted text. In file systems and URLs, underscores are widely used as safe, readable characters that can replace spaces.

Overall, the underscore is a versatile symbol with broad applications in naming, pattern matching, and code organization.