input: Write a function named crawl that takes in a URL (string) as input and performs the following tasks:
  1. Starts from the given URL and looks for other links on the page.
  2. Scans and scrapes the content of the page.
  3. Continues to do so recursively for all the new links found.
  4. Returns None.
  
  assert crawl('https://example.com') == None
output: def crawl(url):
      import requests
      from bs4 import BeautifulSoup
      import re
  
      def get_links(url):
          response = requests.get(url)
          soup = BeautifulSoup(response.text, 'html.parser')
          links = set()
          for a_tag in soup.find_all('a', href=True):
              href = a_tag['href']
              if href.startswith('http'):
                  links.add(href)
              elif href.startswith('/'):
                  links.add(url + href)
          return links
  
      def scrape_content(url):
          response = requests.get(url)
          soup = BeautifulSoup(response.text, 'html.parser')
          return soup.get_text()
  
      visited = set()
      to_visit = [url]
  
      while to_visit:
          current_url = to_visit.pop()
          if current_url not in visited:
              visited.add(current_url)
              links = get_links(current_url)
              for link in links:
                  if link not in visited:
                      to_visit.append(link)
              scrape_content(current_url)
  
      return None