In formal language theory and pattern matching, alternation is the union of two sets of strings or patterns. As a pattern, the alternation of a and b matches either a or b.
In formal language theory, alternation is commutative and associative. This is not in general true in pattern-matching languages.
In the SNOBOL language, regular expression syntax, and some other languages, alternation is a binary infix operator on patterns, notated "|".
- John E. Hopcroft and Jeffrey D. Ullman, Introduction to Automata Theory, Languages and Computation, Addison-Wesley Publishing, Reading Massachusetts, 1979. ISBN 0-201-02988-X.