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gh-135661: Fix parsing start and end tags in HTMLParser #135930

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Jul 3, 2025
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gh-135661: Fix parsing start and end tags in HTMLParser #135930

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gh-135661: Fix parsing start and end tags in HTMLParser
serhiy-storchaka Jun 13, 2025
182b16f
Fix Sphinx errors.
serhiy-storchaka Jun 25, 2025
436a8a9
Merge branch 'main' into htmlparser-tag
serhiy-storchaka Jul 2, 2025
ebf8ce3
Apply suggestions from code review
serhiy-storchaka Jul 2, 2025
d05303b
Merge remote-tracking branch 'refs/remotes/origin/htmlparser-tag' int…
serhiy-storchaka Jul 2, 2025
955db4e
Address review comments.
serhiy-storchaka Jul 2, 2025
66ec1a0
Merge branch 'main' into htmlparser-tag
serhiy-storchaka Jul 3, 2025
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Move to Security.
serhiy-storchaka Jul 3, 2025
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136 changes: 52 additions & 84 deletions Lib/html/parser.py
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Original file line number Diff line number Diff line change
Expand Up @@ -36,29 +36,33 @@
# explode, so don't do it.
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I don't know if you saw and heeded the warning or if you just got lucky, but it looks like you were able to change these regex!
Since you renamed locatestarttagend, the comment at line 34 should also be updated.

In addition, make sure that existing comments are still relevant. In particular I would appreciate this for comments linking to specific sections of the HTML5 standard.

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There are links below, they still work, although they now redirect to other address. I updated them.

On other hand, section numbers were changed. I updated them in places which I touched.

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Thanks!

# see http://www.w3.org/TR/html5/tokenization.html#tag-open-state
# and http://www.w3.org/TR/html5/tokenization.html#tag-name-state
tagfind_tolerant = re.compile(r'([a-zA-Z][^\t\n\r\f />\x00]*)(?:\s|/(?!>))*')
attrfind_tolerant = re.compile(
r'((?<=[\'"\s/])[^\s/>][^\s/=>]*)(\s*=+\s*'
r'(\'[^\']*\'|"[^"]*"|(?![\'"])[^>\s]*))?(?:\s|/(?!>))*')
locatestarttagend_tolerant = re.compile(r"""
<[a-zA-Z][^\t\n\r\f />\x00]* # tag name
(?:[\s/]* # optional whitespace before attribute name
(?:(?<=['"\s/])[^\s/>][^\s/=>]* # attribute name
(?:\s*=+\s* # value indicator
(?:'[^']*' # LITA-enclosed value
|"[^"]*" # LIT-enclosed value
|(?!['"])[^>\s]* # bare value
)
\s* # possibly followed by a space
)?(?:\s|/(?!>))*
)*
tagfind_tolerant = re.compile(r'([a-zA-Z][^\t\n\r\f />]*)(?:[\t\n\r\f ]|/(?!>))*')
attrfind_tolerant = re.compile(r"""
(
(?<=['"\t\n\r\f /])[^\t\n\r\f />][^\t\n\r\f /=>]* # attribute name
)
(= # value indicator
('[^']*' # LITA-enclosed value
|"[^"]*" # LIT-enclosed value
|(?!['"])[^>\t\n\r\f ]* # bare value
)
)?
\s* # trailing whitespace
(?:[\t\n\r\f ]|/(?!>))* # possibly followed by a space
""", re.VERBOSE)
locatetagend_tolerant = re.compile(r"""
[a-zA-Z][^\t\n\r\f />]* # tag name
[\t\n\r\f /]* # optional whitespace before attribute name
(?:(?<=['"\t\n\r\f /])[^\t\n\r\f />][^\t\n\r\f /=>]* # attribute name
(?:= # value indicator
(?:'[^']*' # LITA-enclosed value
|"[^"]*" # LIT-enclosed value
|(?!['"])[^>\t\n\r\f ]* # bare value
)
)?
[\t\n\r\f /]* # possibly followed by a space
)*
>?
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These changes make sense to me.

I also noticed that you removed the start from locatestarttagend_tolerant, presumably because you are now using it to find the end of end tags too (which can contain attributes, even if they are invalid).

This variable is not documented however I can see two options:

  • we consider it private and just rename it;
  • we create an alias to the old name for backward compatibility, in case someone was using it;

Note that before there was also a set of *_strict variable that got removed, so the _tolerant suffix is no longer needed and it was kept for backward compatibility. Since you are refactoring/renaming (some of) these variables, you might want to consider dropping the _tolerant suffix altogether (and possibly adding aliases to preserve backward compatibility), either in this or in a separate PR.

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Restored the removed variables. I will remove them in the main branch in the following PR.

""", re.VERBOSE)
endendtag = re.compile('>')
# the HTML 5 spec, section 8.1.2.2, doesn't allow spaces between
# </ and the tag name, so maybe this should be fixed
endtagfind = re.compile(r'</\s*([a-zA-Z][-.a-zA-Z0-9:_]*)\s*>')

# Character reference processing logic specific to attribute values
# See: https://html.spec.whatwg.org/multipage/parsing.html#named-character-reference-state
Expand Down Expand Up @@ -141,7 +145,8 @@ def get_starttag_text(self):

def set_cdata_mode(self, elem):
self.cdata_elem = elem.lower()
self.interesting = re.compile(r'</\s*%s\s*>' % self.cdata_elem, re.I)
self.interesting = re.compile(r'</%s(?=[\t\n\r\f />])' % self.cdata_elem,
re.IGNORECASE|re.ASCII)
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Any reason for adding re.ASCII here?

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Yes, it affects case-insensitive mode. Otherwise 'ſ' ~ 's' and 'ı' ~ 'i'. There may be more cases after adding support for title and textarea.

This is not actually a problem in the current code, but future changes could make this important.


def clear_cdata_mode(self):
self.interesting = interesting_normal
Expand All @@ -166,7 +171,7 @@ def goahead(self, end):
# & near the end and see if it's followed by a space or ;.
amppos = rawdata.rfind('&', max(i, n-34))
if (amppos >= 0 and
not re.compile(r'[\s;]').search(rawdata, amppos)):
not re.compile(r'[\t\n\r\f ;]').search(rawdata, amppos)):
break # wait till we get all the text
j = n
else:
Expand Down Expand Up @@ -381,76 +386,39 @@ def parse_starttag(self, i):
# or -1 if incomplete.
def check_for_whole_start_tag(self, i):
rawdata = self.rawdata
m = locatestarttagend_tolerant.match(rawdata, i)
if m:
j = m.end()
next = rawdata[j:j+1]
if next == ">":
return j + 1
if next == "/":
if rawdata.startswith("/>", j):
return j + 2
if rawdata.startswith("/", j):
# buffer boundary
return -1
# else bogus input
if j > i:
return j
else:
return i + 1
if next == "":
# end of input
return -1
if next in ("abcdefghijklmnopqrstuvwxyz=/"
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"):
# end of input in or before attribute value, or we have the
# '/' from a '/>' ending
return -1
if j > i:
return j
else:
return i + 1
raise AssertionError("we should not get here!")
match = locatetagend_tolerant.match(rawdata, i+1)
assert match
j = match.end()
if rawdata[j-1] != ">":
return -1
return j

# Internal -- parse endtag, return end or -1 if incomplete
def parse_endtag(self, i):
rawdata = self.rawdata
assert rawdata[i:i+2] == "</", "unexpected call to parse_endtag"
match = endendtag.search(rawdata, i+1) # >
if not match:
if rawdata.find('>', i+2) < 0:
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Suggested change
if rawdata.find('>', i+2) < 0:
if rawdata.rfind('>', i+2) < 0:

Probably inconsequential performance-wise, but using rfind seems more logical here (and possibly elsewhere).

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This check is not actually needed. It is simply an optimization for the case of truncated end tag, because it is faster than endtagopen.match() + locatetagend.match(). I do not know whether it really helps, but I left it as insurance against unpredicted performance degradation.

find may be faster than rfind in general, and in case of end tag, there is large chance to find ">" in first few characters.

return -1
gtpos = match.end()
match = endtagfind.match(rawdata, i) # </ + tag + >
if not match:
if self.cdata_elem is not None:
self.handle_data(rawdata[i:gtpos])
return gtpos
# find the name: w3.org/TR/html5/tokenization.html#tag-name-state
namematch = tagfind_tolerant.match(rawdata, i+2)
if not namematch:
# w3.org/TR/html5/tokenization.html#end-tag-open-state
if rawdata[i:i+3] == '</>':
return i+3
else:
return self.parse_bogus_comment(i)
tagname = namematch.group(1).lower()
# consume and ignore other stuff between the name and the >
# Note: this is not 100% correct, since we might have things like
# </tag attr=">">, but looking for > after the name should cover
# most of the cases and is much simpler
gtpos = rawdata.find('>', namematch.end())
self.handle_endtag(tagname)
return gtpos+1
if not endtagopen.match(rawdata, i): # </ + letter
# w3.org/TR/html5/tokenization.html#end-tag-open-state
if rawdata[i+2:i+3] == '>': # </> is ignored
return i+3
else:
return self.parse_bogus_comment(i)

elem = match.group(1).lower() # script or style
if self.cdata_elem is not None:
if elem != self.cdata_elem:
self.handle_data(rawdata[i:gtpos])
return gtpos
match = locatetagend_tolerant.match(rawdata, i+2)
assert match
j = match.end()
if rawdata[j-1] != ">":
return -1

self.handle_endtag(elem)
# find the name: w3.org/TR/html5/tokenization.html#tag-name-state
match = tagfind_tolerant.match(rawdata, i+2)
assert match
tag = match.group(1).lower()
self.handle_endtag(tag)
self.clear_cdata_mode()
return gtpos
return j

# Overridable -- finish processing of start+end tag: <tag.../>
def handle_startendtag(self, tag, attrs):
Expand Down
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