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It's not a good idead to parse HTML text using regular expressions

See original GitHub issue

In w3lib.html regular expressions are used to parse HTML texts:

_ent_re = re.compile(r'&((?P<named>[a-z\d]+)|#(?P<dec>\d+)|#x(?P<hex>[a-f\d]+))(?P<semicolon>;?)', re.IGNORECASE)
_tag_re = re.compile(r'<[a-zA-Z\/!].*?>', re.DOTALL)
_baseurl_re = re.compile(six.u(r'<base\s[^>]*href\s*=\s*[\"\']\s*([^\"\'\s]+)\s*[\"\']'), re.I)
_meta_refresh_re = re.compile(six.u(r'<meta\s[^>]*http-equiv[^>]*refresh[^>]*content\s*=\s*(?P<quote>["\'])(?P<int>(\d*\.)?\d+)\s*;\s*url=\s*(?P<url>.*?)(?P=quote)'), re.DOTALL | re.IGNORECASE)
_cdata_re = re.compile(r'((?P<cdata_s><!\[CDATA\[)(?P<cdata_d>.*?)(?P<cdata_e>\]\]>))', re.DOTALL)

However this is definitely incorrect when it involves commented contents, e.g.

>>> from w3lib import html
>>> html.get_base_url("""<!-- <base href="http://example.com/" /> -->""")
'http://example.com/'

Introducing “heavier” utilities like lxml would solve this issue easily, but that might be an awful idea as w3lib aims to be lightweight & fast.
Or maybe we could implement some quick parser merely for eliminating the commented parts.

Any ideas?

Issue Analytics

State:
Created 7 years ago
Comments:5 (1 by maintainers)

Top GitHub Comments

1reaction

openandclosecommented, Oct 27, 2019

Hello, just for your reference.

I recently tested w3lib’s prescan against 500 most popular websites. I found three bugs (or different behaviors from html5 spec).

books.google.com: <meta http-equiv="content-type"content="text/html; charset=UTF-8"> (no space between attributes)

mega.nz: <meta http-equiv="Content-Type" content="text/html, charset=UTF-8" /> (comma, not semicolon)

stuff.co.nz: doc.write('<body onload=[...] <meta charset="utf-8"/> (matching ‘<body’)

validator’s, jsdom’s and html5lib-python’s prescan parsers get encoding successfully.

…I don’t know it is a good idea to fix these and make prescan regex even more complex.

1reaction

kmikecommented, Aug 12, 2016

@starrify I believe the goal was indeed speed; also, these regexes may take e.g. only first 4096 bytes of the page, without the rest. Ideas about a proper solution are welcome! It should

a) be almost as fast as these regexes; b) work on arbitrarily truncated HTML files.