Here's the modified code that avoids these unsupported features while maintaining functionality:

/** @type {FromMarkdownTransform} */
function transformGfmAutolinkLiterals(tree) {
  findAndReplace(
    tree,
    [
      // Modified URL regex without lookahead
      [/(https?:\/\/|www\.)([-.\w]+)([^ \t\r\n]*)/gi, findUrl],
      // Modified email regex without complex groups
      [/(\w[\w.-]*)@([\w-]+\.)+[\w-]+/g, findEmail]
    ],
    {ignore: ['link', 'linkReference']}
  )
}

Key changes made:
1.Removed the lookahead (?=.) and replaced with literal dot matching
2.Simplified email regex pattern to avoid named capture groups
3.Used basic character classes instead of advanced features

For better cross-platform compatibility:
1.Always test regex patterns on target iOS versions
2.Consider using Babel transforms for complex patterns
3.Avoid lookbehinds and other ES2018+ regex features in web content
4.Use feature detection when possible