system: You are an expert security code reviewer. You will be shown a source code file along with a multiple-choice question listing several function or class names from that file. Your task is to identify which one contains the specified CWE vulnerability.

OUTPUT FORMAT: Respond with the single letter of the correct option, wrapped in \boxed{...}. Example: `\boxed{B}`. Output the boxed letter ONLY -- no explanation, no other text.

user: ```python
import os
import re
from urllib.parse import urlparse

from flask import (
    Flask,
    make_response,
    render_template,
    request,
)
import praw
import markdown
import requests
from weasyprint import HTML
import yaml


app = Flask(__name__)

file_dir = os.path.dirname(__file__)


class Config:
    def __init__(self):
        with open(os.path.join(file_dir, "secrets.yaml"), 'r') as stream:
            try:
                secrets = yaml.safe_load(stream)
            except yaml.YAMLError as exc:
                print(exc)
        self.hcaptcha_secret_key = secrets['hcaptcha']['secret_key']
        self.hcaptcha_verify_url = secrets['hcaptcha']['verify_url']

        self.reddit_client_id = secrets['reddit']['client_id']
        self.reddit_secret_key = secrets['reddit']['secret_key']


CONFIG = Config()

PATH_REGEX = re.compile(r'/r/\S+/comments/\S+/\S+/')


def clean_comment(comment: str) -> str:
    return comment.replace('\n', '  ')


def is_url_ok(url: str) -> bool:
    parsed_url = urlparse(url)

    if parsed_url.netloc != 'www.reddit.com' or not parsed_url.path or not PATH_REGEX.match(parsed_url.path):
        raise RuntimeError("URL must be the full URL of the thread. "
                           "For example: https://www.reddit.com/r/subreddit/comments/abc123/thread_title/")

    return True


def download(comment_url) -> None:
    reddit = praw.Reddit(user_agent='User-Agent: ThreadBackupMakerforReddit:v1.0 (by /u/pmheavy)',
                         client_id=CONFIG.reddit_client_id, client_secret=CONFIG.reddit_secret_key)
    submission = reddit.submission(url=comment_url)

    comments = [f"# {submission.title} #",
                f"#### Author: {submission.author} ####",
                f"#### URL: {submission.permalink} ####",
                "\n-----",
                f"{submission.url}",
                f"\n{submission.selftext}" if submission.selftext else "",
                "\n-----"]
    submission.comments.replace_more(limit=None)
    comment_queue = submission.comments[:]  # Seed with top-level
    while comment_queue:
        comment = comment_queue.pop(0)
        if comment.score >= 0:
            body = clean_comment(comment.body)
            if comment.parent_id.startswith('t1_'):
                parent_depth = comment.parent().depth
                comment.depth = comment.depth if comment.depth > parent_depth else parent_depth + comment.depth + 1
            comments.append(f"{'    '*comment.depth}* {body} [Author: {comment.author}] [Upvotes: {comment.score}]")
            comment_queue[0:0] = comment.replies

    markdown_string = "\n".join(comments)
    html = "".join(["<html><head></head><body>", markdown.markdown(markdown_string), "</body></html>"])
    return HTML(string=html).write_pdf()


@app.route('/')
def hello_world():
    return render_template('main.html')


@app.route('/', methods=['POST'])
def my_form_post():
    url = request.form['url']

    token = request.form['h-captcha-response']
    data = {'secret': CONFIG.hcaptcha_secret_key, 'response': token}
    hcaptcha_response = requests.post(url=CONFIG.hcaptcha_verify_url, data=data).json()
    if not hcaptcha_response['success']:
        return render_template('error.html', error_message="Captcha not passed.")

    try:
        if is_url_ok(url):
            response = make_response(download(url))
            response.headers["Cache-Control"] = "must-revalidate"
            response.headers["Pragma"] = "must-revalidate"
            response.headers["Content-type"] = "application/pdf"
            response.headers["Content-Disposition"] = f"attachment; filename={url}.pdf"
            return response
    except RuntimeError as e:
        return render_template('error.html', error_message=str(e))
    except praw.reddit.ClientException:
        return render_template('error.html', error_message="Unable to collect the thread.")
    except praw.reddit.RedditAPIException:
        return render_template('error.html', error_message="Unable to connect to Reddit.")


if __name__ == '__main__':
    app.run()

```

Which of the following functions or classes contains a `CWE-1333: Inefficient Regular Expression Complexity` vulnerability?

  A) hello_world
  B) download
  C) is_url_ok
  D) my_form_post
Answer: C