Regex to extract site-relative stylesheets from HTML page source

stylesheet regex

$regex = '/
  (?=                  # Lookahead:
    <link              # Open link tag
      (?:\s+[^>]*)?    # Any leading data (non-capturing)
      \s+rel=          # rel attribute preceded by one or more spaces
      ([\'"])          # Opening quote for attribute ($matches[1] for backreference)
      stylesheet       # Match attribute value
      \1               # Backreference to match quotes
      .*?              # Consume any subsequent data non-greedily
      \/?              # Optionally self-close
    >                  # Close link tag
  )                    # 
  <link                # Open link tag
    (?:\s+[^>]*)?      # Any leading data (non-capturing)
    \s+href=           # href attribute preceded by one or more spaces
    ([\'"])            # Opening quote for attribute ($matches[2] for backreference)
    (?!                # Negative lookahead:
      (?:https?:)?\/\/ # Not interested in non-local files (non-capturing)
    )                  # 
    (.+?)              # The href attribute value ($matches[3])
    \1                 # Backreference to match quotes
    .*?                # Consume any subsequent data non-greedily
    \/?                # Optionally self-close
  >                    # Close link tag
/mixs';