εR ≡ Rε ≡ R

Regular expressions in UNIX have extended operations beyond the basic three (union, concatenation, and closure); however, they can all be represented by standard regular expressions. The '?' operation in UNIX regular expressions stands for 'zero or one occurrences of'; i.e., R? is the same as (R | ε).

In a finite automaton, a transition on ε means the automaton moves from one state to another without consuming a character; automatons with epsilon-transitions are one type of non-deterministic finite automaton. Epsilon-transitions are necessary to convert regular expressions into finite automata.