123includes3

123includes3 is a term used in programming education to illustrate string inclusion and substring matching. It is not a formal standard but a mnemonic that developers use to discuss how to determine whether a given string contains a specified digit or sequence. In its simplest form, the problem concerns whether the character '3' appears anywhere in a string; more generally, it can refer to checking for the substring '123' or for sequences of digits 1, 2, and 3 in a given order, possibly with other characters in between.

Common interpretations of the term range from exact substring searches to more flexible pattern matching. Applications include validating user input, filtering or extracting data from text, and teaching basic concepts of search algorithms. The idea also appears in puzzle-oriented tutorials that ask learners to distinguish between containment of a single character, a specific substring, or a digit sequence with gaps.

Examples used in instructional material are typically presented in increasing complexity. A simple case might test whether '3' is in a string, as in a language feature like the includes operator. A slightly more advanced case checks for the literal substring '123'. A more challenging version uses a regular expression to determine whether the digits 1, 2, and 3 appear in that order, with possible intervening characters.

Origin and reception are informal, tied to teaching practices in beginner programming courses and online coding communities. See also substring search, includes operator, and pattern matching.