sameletter

Sameletter is a term used in string processing and linguistics to describe a property of a string where two adjacent characters are identical. Formally, a word w = w1 w2 ... wn over an alphabet Σ contains a same-letter if there exists an index i with wi = w(i+1). Strings that do not contain any such adjacency are sometimes described as having no same-letter pairs or as no-adjacent-equals.

In computing, same-letter occurrences are central to several techniques. Regular expressions often use a backreference to detect them; for example, the pattern (.)\1 matches any instance of two consecutive identical characters. In data compression, particularly run-length encoding, runs of identical letters—i.e., consecutive same letters—are encoded as a single character followed by its run length. Detecting same-letter runs can also be important in text normalization, spell checking, and OCR error detection, where repeated letters may indicate a stylization or a processing error.

In linguistics and typography, same-letter adjacency is common in English and other languages, often arising from morpheme boundaries, affixation, or phonotactic constraints. Double letters such as in letter, committee, or balloon reflect historic pronunciation and spelling patterns, as well as simplifications in certain dialects.

Notes on terminology: sameletter is not a formal mathematical term with a universal definition, but a descriptive phrase used in programming, puzzle communities, and informal discussions to refer to the presence or absence of consecutive identical characters.

See also: run-length encoding, double letters, pattern matching, regular expressions.