apg-js-examples/src/apg-exp/recursive.js

Examples of using the suite of apg-js applications and libraries.

/*  *************************************************************************************
 *   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](http://perl6.org/archive/doc/design/apo/A05.html).
// 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}`);
  }
})();