Table: RegExp flags `/g` and `/y`

README.md

Table: RegExp flags /g and /y

The following table was generated by the Node.js script:

	/	/g	/y	/yg
r.exec(s)	{i:0}	{i:1}	{i:1}	{i:1}
	.lI unch	.lI upd	.lI upd	.lI upd
r.test(s)	true	true	true	true
	.lI unch	.lI upd	.lI upd	.lI upd
s.match(r)	{i:0}	["#","#","#"]	{i:1}	["#","#"]
	.lI unch	.lI reset	.lI upd	.lI reset
s.matchAll(r)	TypeError	[{i:1}, {i:3}]	TypeError	[{i:1}]
		.lI unch		.lI unch
s.replace(r, 'x')	"x#-#"	"xx-x"	"#x-#"	"xx-#"
	.lI unch	.lI reset	.lI upd	.lI reset
s.replaceAll(r, 'x')	TypeError	"xx-x"	TypeError	"xx-#"
		.lI reset		.lI reset

Variables:

const r = /#/; r.lastIndex = 1;
const s = "##-#";

Abbreviations:

• {i:2}: a match object whose property .index has the value 2.
• .lI upd: .lastIndex is updated
• .lI reset: .lastIndex is reset to zero
• .lI unch: .lastIndex is unchanged

regexp-flags-gy.mjs

const INITIAL_LAST_INDEX = 1;
const FLAGS_LIST = ['', 'g', 'y', 'yg'];
const DEMO_STR = '##-#';
const DEMO_REGEXP_PATTERN = '#';
function pushRow(table, rowDescription, callback) {
  const row1 = [btick(rowDescription)];
  const row2 = [''];
  for (const flags of FLAGS_LIST) {
    const str = DEMO_STR;
    const regExp = new RegExp(DEMO_REGEXP_PATTERN, flags);
    regExp.lastIndex = INITIAL_LAST_INDEX;
    try {
      const result = callback(regExp, str);
      if (resultToString(result) === undefined) {
        throw new Error({rowDescription, regExp, str});
      }
      row1.push(resultToString(result));
      row2.push(lastIndexToString(regExp.lastIndex));
    } catch (err) {
      if (! (err instanceof TypeError)) {
        throw err;
      }
      row1.push(btick(err.constructor.name));
      row2.push('');
    }
  }
  table.push(row1);
  table.push(row2);
}

function lastIndexToString(lastIndex) {
  if (lastIndex === 0) {
    return '`.lI` reset';
  } else if (lastIndex === INITIAL_LAST_INDEX) {
    return '`.lI` unch';
  } else {
    return '`.lI` upd';
  }
}

function isMatchObject(value) {
  return value && typeof value === 'object' && 'index' in value;
}

function toMatchObjectString(matchObject) {
  return '{i:'+matchObject.index+'}';
}

function btick(str) {
  return '`' + str + '`';
}

function resultToString(result) {
  if (isMatchObject(result)) {
    return btick(toMatchObjectString(result));
  } else if (Array.isArray(result) && result.every(elem => isMatchObject(elem))) {
    return btick('[' + result.map(x => toMatchObjectString(x)).join(', ') + ']');
  } else if (['number', 'boolean', 'string'].includes(typeof result) || Array.isArray(result)) {
    return btick(JSON.stringify(result));
  } else {
    throw new Error(result);
  }
}

function extractColumn(table, colIndex) {
  return table.map(row => row[colIndex]);
}
function getWidthOfColumn(table, colIndex) {
  return Math.max(...extractColumn(table, colIndex).map(
    str => typeof str === 'string' ? str.length : 0));
}
const DASHES = Symbol('DASHES');
function printTable(table) {
  const columnWidths = [...table[0].keys()].map(
    (colIndex) => getWidthOfColumn(table, colIndex));
  for (const row of table) {
    let rowStr = row
      .map((cell, colIndex) => {
        if (cell === DASHES) {
          return '-'.repeat(columnWidths[colIndex]);
        } else {
          return cell.padEnd(columnWidths[colIndex])
        }
      })
      .join(' | ');
    rowStr = '| ' + rowStr + ' |';
    console.log(rowStr);
  }
}

function main() {
  const table = [];

  table.push(['', ...FLAGS_LIST.map(flags => btick('/'+flags))]);
  table.push(new Array(table[0].length).fill(DASHES));

  pushRow(table, 'r.exec(s)', (re, str) => re.exec(str));
  pushRow(table, 'r.test(s)', (re, str) => re.test(str));

  pushRow(table, 's.match(r)', (re, str) => str.match(re));
  pushRow(table, 's.matchAll(r)', (re, str) => [...str.matchAll(re)]);
  
  pushRow(table, "s.replace(r, 'x')", (re, str) => str.replace(re, 'x'));
  pushRow(table, "s.replaceAll(r, 'x')", (re, str) => str.replaceAll(re, 'x'));

  printTable(table);

  console.log();
  console.log('Variables:');
  console.log();
  console.log('```js');
  console.log(`const r = ${new RegExp(DEMO_REGEXP_PATTERN, '')}; r.lastIndex = 1;`);
  console.log(`const s = ${JSON.stringify(DEMO_STR)};`);
  console.log('```');
}

main();