create the most complicated regex parser ever in javascript

To create a complex regex parser in JavaScript, you need to follow the steps below:

1. Understand the problem that you are trying to solve and the requirements for the parser.
2. Break down the problem into smaller sub-problems or tasks that are easier to solve individually.
3. Define a grammar or set of rules that the parser needs to follow, and convert that grammar into a regular expression.
4. Create a parser function that takes the regular expression and applies it to a given input string, producing an output that matches the grammar.

Here is an example of a complex regex parser that can handle mathematical expressions with variable names, parenthesis, and nested functions:

index.tsx
function parseExpression(input) {
  const variableRegex = /\b([a-zA-Z][a-zA-Z0-9]*)\b/g; // matches variable names
  const functionRegex = /([a-zA-Z]+)\((.*?)\)/g; // matches function calls

  // replace variable names with a special character so they don't interfere with other parsing
  let output = input.replace(variableRegex, '_');

  // handle nested function calls using recursion
  while (functionRegex.test(output)) {
    output = output.replace(functionRegex, (match, func, args) => {
      return parseExpression(args);
    });
  }

  // handle parenthesis by recursively evaluating expressions inside them
  while (output.includes('(')) {
    output = output.replace(/\(([^()]*)\)/g, (match, inner) => {
      return parseExpression(inner);
    });
  }

  return output;
}
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In this example, we define two regular expressions to match variable names and function calls. We replace all variable names with a special character to prevent them from interfering with other parsing steps. We then use recursion to handle nested function calls and parenthesis by recursively evaluating the expressions inside them. The final output is a string that matches the grammar of mathematical expressions with variable names, parenthesis, and nested functions.