substr

Substr is a string operation that extracts a contiguous portion of a string and returns it as a new string. It is implemented as a function or method in many programming languages and libraries, often under the name substr or a closely related variant. The operation is fundamental for text processing, parsing, and formatting tasks.

Typical usage and parameters vary by language. A common form is substr(string, start, length), where start identifies the first character to include and length specifies how many characters to take. The interpretation of start and length differs: many languages use 0-based indexing (the first character is at position 0), while others use 1-based indexing. Some languages allow length to be omitted to return the remainder of the string, and some support negative start values to count from the end.

Notable language variants include:

- C++: std::string substr(size_t pos = 0, size_t len = npos) with 0-based indexing; pos is the index of the first character, and len is the number of characters to include.

- PHP: substr(string, int start, int length) allows negative start to count from the end and can omit length to return to the string end.

- R: substr(string, start, stop) uses 1-based indexing and end-inclusive positions.

- SQL: SUBSTRING or SUBSTR functions in many dialects extract a substring with a start position and length; exact syntax varies by database system.

- JavaScript: substr is a legacy form that takes (start, length); modern code often uses substring or slice instead.

Examples vary by language, but the underlying idea remains the same: select a slice of characters from a string based on position and length. See also: substring, slice, right, left.