csfieldguide/chapters/content/en/glossary/regular-expression.md
# Regular expression
A formula used to describe a pattern in a text that is to be matched or searched for. These are typically used for finding elements of a program (such as variable names) and checking input in forms (such as checking that an email address has the right format).
The logic for regular expressions and finite state automata (FSA) are interchangeable – every regular expression has an equivalent FSA, and vice versa.