New submission from Jirka Marsik :
When you have capture groups inside a negative lookaround assertion, the
strings captured by those capture groups can sometimes survive the failure of
the assertion and feature in the returned Match object.
Here it is illustrated with lookbehinds and lookaheads:
>>> re.search(r"(?>> re.search(r"(?!(a)c)ab", "ab").group(1)
'a'
Even though the search for the expression '(a)c' fails when trying to match
'c', the string 'a' is still reported as having been successfully matched by
capture group 1. The expected behavior would be for the capture group 1 to not
have a match.
Because of the following reasons, I believe this behavior is not intentional
and is the result of Python not cleaning up after the asserted subexpression
fails (e.g. by running the asserted subexpression in a new stack frame).
1) This behavior is not being systematically enforced.
We can observe this behavior only in certain cases. Modifying the expression
to use the branching operator `|` inside the asserted subexpression leads to
the expected behavior.
>>> re.search(r"(?>> re.search(r"(?!(a)c|(a)d)ab", "ab").group(1) is None
True
2) Other languages do not leak capture groups from negative lookarounds.
Node.js (ECMAScript):
> /(? /(?!(a)c)ab/.exec("ab")[1]
undefined
> /(? /(?!(a)c|(a)d)ab/.exec("ab")[1]
undefined
MRI (Ruby):
irb(main):001:0> /(?
irb(main):002:0> /(?!(a)c)ab/.match("ab")[1]
=> #
irb(main):003:0> /(?
irb(main):004:0> /(?!(a)c|(a)d)ab/.match("ab")[1]
=> #
JShell (Java):
jshell> Matcher m =
java.util.regex.Pattern.compile("(? m.find()
jshell> m.group(1)
$3 ==> null
jshell> Matcher m =
java.util.regex.Pattern.compile("(? m.find()
jshell> m.group(1)
$6 ==> null
jshell> Matcher m = java.util.regex.Pattern.compile("(?!(a)c)ab").matcher("ab")
m ==> java.util.regex.Matcher[pattern=(?!(a)c)ab region=0,2 lastmatch=]
jshell> m.find()
jshell> m.group(1)
$9 ==> null
jshell> Matcher m =
java.util.regex.Pattern.compile("(?!(a)c|(a)d)ab").matcher("ab")
m ==> java.util.regex.Matcher[pattern=(?!(a)c|(a)d)ab region=0,2 lastmatch=]
jshell> m.find()
jshell> m.group(1)
$12 ==> null
3) Not leaking capture groups from negative lookarounds is symmetric to how
capture groups are treated in failed matches.
When regular expression engines fail to match a regular expression, they do
not provide a partial match object that contains the state of capture groups at
the time when when the matcher failed. Instead, the state of the matcher is
discarded and some bottom value is returned (None, null or undefined).
Similarly, one would expect nested subexpressions to behave the same way, so
that capture groups from failed match attempts are discarded.
--
components: Regular Expressions
messages: 404479
nosy: ezio.melotti, jirkamarsik, mrabarnett
priority: normal
severity: normal
status: open
title: Negative lookaround assertions sometimes leak capture groups
type: behavior
versions: Python 3.9
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<https://bugs.python.org/issue45539>
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