/* *************************************************************************************
* copyright: Copyright (c) 2021 Lowell D. Thomas, all rights reserved
* license: BSD-2-Clause (https://opensource.org/licenses/BSD-2-Clause)
* ********************************************************************************* */regexes often have trouble handling recursion.
Matching pairs of parentheses is a classic recursion problem and is handled easily with apg parsers
and by extension apg-exp.
Here is a simple demonstration of matching paired parentheses and capturing all of the recursive matches.
The parenthesis pairs are found and all values of the recursive rule R are retained.
(function recursive() {
try {
const apgJs = require('apg-js');
const { apgExp } = apgJs;
let grammar = '';
let txt;
grammar += 'R = (open text R text close) / (open text close)\n';
grammar += 'open = %d40 ; open paren "("\n';
grammar += 'close = %d41 ; close paren ")"\n';
grammar += 'text = *(%d32-39/%d42-126); any characters but "()"\n';eslint-disable-next-line new-cap
const exp = new apgExp(grammar, '');
console.log();
console.log('Demonstrate true recursion - matching paired parentheses and more.');
console.log();
const str = 'find the matched parentheses (down 1(down 2(down 3(the middle)up 3)up 2)up 1)';
console.log('grammar:');
console.log(exp.source);
console.log();
console.log(` input: ${str}`);
const result = exp.exec(str);
if (result) {
txt = result.toText();
console.log(txt);
txt = exp.toText();
console.log(txt);
} else {
console.log(' result: null');
}
} catch (e) {
console.log(`EXCEPTION: ${e.message}`);
}
})();