Editing Parsing expression grammar (section)

==== Practical significance ====
Direct recursion, be that left or right, is important in context-free grammars, because there recursion is the only way to describe repetition:
<syntaxhighlight lang="peg">
Sum   → Term | Sum '+' Term | Sum '-' Term
Args  → Arg | Arg ',' Args
</syntaxhighlight>
People trained in using context-free grammars often come to PEGs expecting to use the same idioms, but parsing expressions can do repetition without recursion:
<syntaxhighlight lang="peg">
Sum   ← Term ( '+' Term / '-' Term )*
Args  ← Arg ( ',' Arg )*
</syntaxhighlight>
A difference lies in the [[abstract syntax tree]]s generated: with recursion each <code>Sum</code> or <code>Args</code> can have at most two children, but with repetition there can be arbitrarily many. If later stages of processing require that such lists of children are recast as trees with bounded [[degree (graph theory)|degree]], for example microprocessor instructions for addition typically only allow two operands, then properties such as [[left-associative|left-associativity]] would be imposed after the PEG-directed parsing stage.

Therefore left-recursion is practically less likely to trouble a PEG packrat parser than, say, an LL(k) context-free parser, unless one insists on using context-free idioms. However, not all cases of recursion are about repetition.