README.md

Automatically generated result table

const s='##-#';

const r=/#/g; r.lastIndex=1;
r.exec(s)             .index=1       .lastIndex updated  
r.test(s)             true           .lastIndex updated  
s.replace(r, 'x')     "xx-x"         .lastIndex reset    
s.replaceAll(r, 'x')  "xx-x"         .lastIndex reset    
s.match(r)            ["#","#","#"]  .lastIndex reset    
s.matchAll(r)         [["#"],["#"]]  .lastIndex unchanged

const r=/#/y; r.lastIndex=1;
r.exec(s)             .index=1   .lastIndex updated  
r.test(s)             true       .lastIndex updated  
s.replace(r, 'x')     "#x-#"     .lastIndex updated  
s.replaceAll(r, 'x')  TypeError
s.match(r)            .index=1   .lastIndex updated  
s.matchAll(r)         TypeError

const r=/#/yg; r.lastIndex=1;
r.exec(s)             .index=1   .lastIndex updated  
r.test(s)             true       .lastIndex updated  
s.replace(r, 'x')     "xx-#"     .lastIndex reset    
s.replaceAll(r, 'x')  "xx-#"     .lastIndex reset    
s.match(r)            ["#","#"]  .lastIndex reset    
s.matchAll(r)         [["#"]]    .lastIndex unchanged

(Older versions of .matchAll() don’t throw a TypeError if /g is not set.)

regexp-lastindex.cjs

require('string.prototype.replaceall').shim();

function printUsage(flags, name, callback) {
  let output = [];
  output.push(name)
  const re = new RegExp('#', flags);
  re.lastIndex = 1;
  try {
    const result = callback(re, '##-#', 'x');
    if (result && typeof result === 'object' && 'index' in result) {
      output.push('.index='+result.index);
    } else if (['number', 'boolean', 'string'].includes(typeof result) || Array.isArray(result)) {
      output.push(JSON.stringify(result));
    } else {
      output.push('');
    }
    if (re.lastIndex === 0) {
      output.push('.lastIndex reset');
    } else if (re.lastIndex === 1) {
      output.push('.lastIndex unchanged');
    } else {
      output.push('.lastIndex updated');
    }
  } catch (err) {
    output.push(err.constructor.name);
    output.push('');
  }
  return output;
}

function extractColumn(table, colIndex) {
  return table.map(row => row[colIndex]);
}
function printTable(table) {
  const columnWidths = [...table[0].keys()].map(
    (i) => Math.max(...extractColumn(table, i).map(str => str.length)));
  for (const row of table) {
    const rowStr = row
      .map((cell, colIndex) => cell.padEnd(columnWidths[colIndex]))
      .join('  ');
    console.log(rowStr);
  }
}

console.log("const s='##-#';");
console.log();

for (const flags of ['g', 'y', 'yg']) {
  console.log(`const r=/#/${flags}; r.lastIndex=1;`);
  printTable([
    printUsage(flags, 'r.exec(s)',
      (re, str) => re.exec(str)),
    printUsage(flags, 'r.test(s)',
      (re, str) => re.test(str)),
    
    printUsage(flags, "s.replace(r, 'x')",
      (re, str, replacement) => str.replace(re, replacement)),
    printUsage(flags, "s.replaceAll(r, 'x')",
      (re, str, replacement) => str.replaceAll(re, replacement)),
    
    printUsage(flags, 's.match(r)',
      (re, str) => str.match(re)),
    printUsage(flags, 's.matchAll(r)',
      (re, str) => [...str.matchAll(re)]),
  ]);
  console.log();
}