PEGs are in turn able to recognize recursive syntax which these constructs cannot.
Features and operators of these programming elements are employed in parsing expression grammars. PEGs are in turn able to recognize recursive syntax which these constructs cannot. These program elements share a name with formal constructs that describe languages that are equivalent to finite (*) automata according to Kleene's theorem and that are closed under the union, concatenation, and star operations. Languages that are not context-free and are not part of the namesake class of these constructs can be recognized when features such as backreferences and lookahead are added. For 10 points, identify these programming constructs that match strings against patterns, implemented in tools like "awk" and "grep", and as a primary feature of Perl.
Features and operators of these programming elements are employed in parsing expression grammars.
{regular expression}s [accept {regex}s or {regexp}s] (4) [ES]