input: Write a function to create a web crawler in Python that starts from a given website URL, looks for other links in the page, and scrapes the page content. The crawler should use a breadth-first search approach to visit all linked pages from the starting URL, avoiding revisiting any page. If the crawler encounters a page that cannot be reached, it should skip that page and continue with the next one in the queue.
  
  assert crawl('http://example.com/') is not None
  assert crawl('http://invalid.url') is None
  assert crawl('http://nonexistent.url') is None
  assert crawl('') is None
output: def crawl(url):
      from urllib.request import urlopen
      from urllib.error import URLError, HTTPError
      from bs4 import BeautifulSoup
      from collections import deque
      import re
  
      if not url:
          return None
  
      visited = set()
      queue = deque([url])
  
      while queue:
          current_url = queue.popleft()
          if current_url in visited:
              continue
          visited.add(current_url)
  
          try:
              response = urlopen(current_url)
              soup = BeautifulSoup(response.read(), 'html.parser')
              # Scrape content here if needed
              for link in soup.find_all('a', href=True):
                  new_url = link['href']
                  if new_url.startswith('/'):
                      new_url = current_url + new_url
                  if new_url not in visited:
                      queue.append(new_url)
          except (HTTPError, URLError):
              continue
  
      return visited