A pattern describing which strings are meant. Powerful, terse, and the commonest place a subtle error hides.
Clipped from regular expression, a term formalised by the mathematician Stephen Cole Kleene in 1951 and brought into computing use in the 1960s; regular traces to Latin regula (a rule) and expression to Latin exprimere (to press out), but regex as a unit is a modern coinage of the clipping itself.
A pattern describing which strings are meant. Powerful and terse by design, and the commonest place a small mistake in the pattern hides in plain sight.