A test input is sample data supplied to a rule or program so you can observe how it behaves. For a regular expression, it is the text checked against the pattern; the result can show whether the pattern matches and which parts of the text are selected.
In cybersecurity, test inputs help verify validation rules, filters, and log-parsing logic before they are relied on. Include ordinary examples as well as edge cases, such as empty, unusually long, or malformed strings. Testing can reveal mistakes in a pattern, but it does not prove that a rule is secure: review the logic and consider how attackers might craft input to bypass checks or trigger excessive processing.