var $url_re = "
    ~
    (
      (?:
        https?://                                   # http or https protocol
        |                                           # or
        (?://)?www\d{0,3}[.]                        # www., www1., www2. www999. optionally starting with //
      )
      (?:
        [^\s\[\]\(\)\{\}<>`'\",«»“”‘’\?&/!;:]+      # domain
      )
      (?:                                           # One or more ungreedy:
        [^\s\[\]\(\)\{\}<>`'\"«»“”‘’]+              # Run of non-space, non-brackets, non-punctuation
        |                                           # or
        \([^\s\(\)]*\)                              # balanced parents ()
        |                                           # or
        \[[^\s\[\]]*\]                              # balanced parents []
        |                                           # or
        \{[^\s\{\}]*\}                              # balanced parents {}
      )+
      (?:                                           # One or more greedy:
        [^\s\[\]\(\)\{\}<>`'\",«»“”‘’\.\?&!;:]+     # Run of non-space, non-brackets, non-punctuation
        |                                           # or
        \([^\s\(\)]*\)                              # balanced parents ()
        |                                           # or
        \[[^\s\[\]]*\]                              # balanced parents []
        |                                           # or
        \{[^\s\{\}]*\}                              # balanced parents {}
      )+
    )
    ~xiu
";