1
Write your PEG.js grammar
​x
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// Simple Arithmetics Grammar
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// ==========================
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//
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// Accepts expressions like "2 * (3 + 4)" and computes their value.
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​
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Expression
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  = head:Term tail:(_ ("+" / "-") _ Term)* {
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      return tail.reduce(function(result, element) {
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        if (element[1] === "+") { return result + element[3]; }
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        if (element[1] === "-") { return result - element[3]; }
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      }, head);
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    }
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​
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Term
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  = head:Factor tail:(_ ("*" / "/") _ Factor)* {
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      return tail.reduce(function(result, element) {
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        if (element[1] === "*") { return result * element[3]; }
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        if (element[1] === "/") { return result / element[3]; }
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      }, head);
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    }
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​
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Factor
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  = "(" _ expr:Expression _ ")" { return expr; }
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  / Integer
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​
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Integer "integer"
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  = _ [0-9]+ { return parseInt(text(), 10); }
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​
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_ "whitespace"
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  = [ \t\n\r]*
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Parser built successfully.0.81 kB, 32 ms, 25 kB/s

2
Test the generated parser with some input
Input parsed successfully.0.011 kB, 1 ms, 11 kB/s

Output

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Download the parser code