Editing Packrat parser (section)

=== Rules ===
A derivation rule is composed by a nonterminal symbol and an expression <math>S \rightarrow \alpha</math>.

A special expression <math>\alpha_s</math> is the starting point of the grammar.<ref name=":1" /> In case no <math>\alpha_s</math> is specified, the first expression of the first rule is used.

An input string is considered accepted by the parser if the <math> \alpha_s </math> is recognized. As a side-effect, a string <math> x </math> can be recognized by the parser even if it was not fully consumed.<ref name=":1" /> 

An extreme case of this rule is that the grammar <math> S \rightarrow x* </math> matches any string.

This can be avoided by rewriting the grammar as <math> S \rightarrow x*!. </math>