Content deleted Content added
→Grammars: Concrete syntax |
→Definition: Moved one example to after the syntax section |
||
Line 91:
It is worth noticing that the primary dialect of concrete syntax parsing expression grammars does not have an explicit definition terminator or separator between definitions, although it is customary to begin a new definition on a new line; the <code>LEFTARROW</code> of the next definition is sufficient for finding the boundary, if one adds the constraint that a nonterminal in an <code>Expression</code> must not be followed by a <code>LEFTARROW</code>. However, some dialects may allow an explicit terminator, or outright require<ref name="ptKupries"/> it.
=== Example ===
This is a PEG that recognizes mathematical formulas that apply the basic five operations to non-negative integers.▼
<syntaxhighlight lang="peg">▼
Expr ← Sum▼
Sum ← Product (('+' / '-') Product)*▼
Product ← Power (('*' / '/') Power)*▼
Power ← Value ('^' Power)?▼
Value ← [0-9]+ / '(' Expr ')'▼
</syntaxhighlight>▼
In the above example, the terminal symbols are characters of text, represented by characters in single quotes, such as <code>'('</code> and <code>')'</code>. The range <code>[0-9]</code> is a shortcut for the ten characters from <code>'0'</code> to <code>'9'</code>. (This range syntax is the same as the syntax used by [[regular expression]]s.) The nonterminal symbols are the ones that expand to other rules: ''Value'', ''Power'', ''Product'', ''Sum'', and ''Expr''. Note that rules ''Sum'' and ''Product'' don't lead to desired left-associativity of these operations (they don't deal with associativity at all, and it has to be handled in post-processing step after parsing), and the ''Power'' rule (by referring to itself on the right) results in desired right-associativity of exponent. Also note that a rule like {{code|Sum ← Sum (('+' / '-') Product)?|peg}} (with intention to achieve left-associativity) would cause infinite recursion, so it cannot be used in practice even though it can be expressed in the grammar.▼
=== Semantics ===
Line 119 ⟶ 130:
The '''not-predicate''' expression !''e'' succeeds if ''e'' fails and fails if ''e'' succeeds, again consuming no input in either case.
===
▲This is a PEG that recognizes mathematical formulas that apply the basic five operations to non-negative integers.
▲<syntaxhighlight lang="peg">
▲Expr ← Sum
▲Sum ← Product (('+' / '-') Product)*
▲Product ← Power (('*' / '/') Power)*
▲Power ← Value ('^' Power)?
▲Value ← [0-9]+ / '(' Expr ')'
▲</syntaxhighlight>
▲In the above example, the terminal symbols are characters of text, represented by characters in single quotes, such as <code>'('</code> and <code>')'</code>. The range <code>[0-9]</code> is a shortcut for the ten characters from <code>'0'</code> to <code>'9'</code>. (This range syntax is the same as the syntax used by [[regular expression]]s.) The nonterminal symbols are the ones that expand to other rules: ''Value'', ''Power'', ''Product'', ''Sum'', and ''Expr''. Note that rules ''Sum'' and ''Product'' don't lead to desired left-associativity of these operations (they don't deal with associativity at all, and it has to be handled in post-processing step after parsing), and the ''Power'' rule (by referring to itself on the right) results in desired right-associativity of exponent. Also note that a rule like {{code|Sum ← Sum (('+' / '-') Product)?|peg}} (with intention to achieve left-associativity) would cause infinite recursion, so it cannot be used in practice even though it can be expressed in the grammar.
The following recursive rule matches standard C-style if/then/else statements in such a way that the optional "else" clause always binds to the innermost "if", because of the implicit prioritization of the '/' operator. (In a [[context-free grammar]], this construct yields the classic [[dangling else|dangling else ambiguity]].)
<syntaxhighlight lang="peg">
| |||