input: Write a function that creates a web crawler in Python which starts from a given website URL and looks for other links within the page to continue scraping the content. The function should use the `requests` and `BeautifulSoup` libraries to handle HTTP requests and HTML parsing, respectively. Utilize a queue to manage the URLs to be visited and a set to keep track of visited URLs to avoid redundant processing. The crawling should stop when there are no more URLs in the queue to process.
  
  assert crawl('https://example.com')
  assert crawl('https://anotherexample.com') 
output: def crawl(start_url):
      import requests
      from bs4 import BeautifulSoup
      from urllib.parse import urljoin
      import re
  
      queue = [start_url]
      visited = set()
  
      while queue:
          url = queue.pop(0)
          if url in visited:
              continue
          visited.add(url)
  
          try:
              response = requests.get(url)
              if response.status_code != 200:
                  continue
  
              soup = BeautifulSoup(response.text, 'html.parser')
              for link in soup.find_all('a', href=True):
                  absolute_url = urljoin(url, link['href'])
                  if re.match(r'^https?://', absolute_url) and absolute_url not in visited:
                      queue.append(absolute_url)
          except requests.RequestException:
              continue