Editing Parsing expression grammar (section)

=== Ambiguity detection and influence of rule order on language that is matched ===
LL(k) and LR(k) parser generators will fail to complete when the input grammar is ambiguous. This is a feature in the common case that the grammar is intended to be unambiguous but is defective. A PEG parser generator will resolve unintended ambiguities earliest-match-first, which may be arbitrary and lead to surprising parses.

The ordering of productions in a PEG grammar affects not only the resolution of ambiguity, but also the ''language matched''. For example, consider the first PEG example in Ford's paper<ref name="For04" />
(example rewritten in pegjs.org/online notation, and labelled {{tmath|G_1}} and {{tmath|G_2}}):

* {{tmath|G_1}}:&nbsp;&nbsp;{{code|code= A = "a" "b" / "a"}}
* {{tmath|G_2}}:&nbsp;&nbsp;{{code|code= A = "a" / "a" "b"}}

Ford notes that ''The second alternative in the latter PEG rule will never succeed because the first choice is always taken if the input string ... begins with 'a'.''.<ref name="For04" /> 
Specifically, {{tmath|L(G_1)}} (i.e., the language matched by {{tmath|G_1}}) includes the input "ab", but {{tmath|L(G_2)}} does not. 
Thus, adding a new option to a PEG grammar can ''remove'' strings from the language matched, e.g. {{tmath|G_2}} is the addition of a rule to the single-production grammar&nbsp;{{code|code= A = "a" "b"}}, which contains a string not matched by {{tmath|G_2}}.
Furthermore, constructing a grammar to match {{tmath|L(G_1) \cup L(G_2)}} from PEG grammars {{tmath|G_1}} and {{tmath|G_2}} is not always a trivial task.
This is in stark contrast to CFG's, in which the addition of a new production cannot remove strings (though, it can introduce problems in the form of ambiguity),
and a (potentially ambiguous) grammar for {{tmath|L(G_1) \cup L(G_2)}} can be constructed
<syntaxhighlight lang="apl">
S → start(G1) | start(G2)
</syntaxhighlight>