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Re: Is this a /^*/ bug?
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From:
demerphq
Date:
March 25, 2013 16:33
Subject:
Re: Is this a /^*/ bug?
Message ID:
CANgJU+XBg-2E6jRyPtWqEQuAcxvV8-P8hhpCOr3ZNr5=TOKQdw@mail.gmail.com
On 25 March 2013 17:22, Tom Christiansen <tchrist@perl.com> wrote:
> demerphq <demerphq@gmail.com> wrote on Mon, 25 Mar 2013 17:01:59 BST:
>
> On 25 March 2013 16:55, Tom Christiansen <tchrist@perl.com> wrote:
>
>>> I realize this is nonsense, but I wonder if it is not a bug. Shouldn't the
>>> overall pattern still fail, not succeed?
>>>
>>> % perl -WE 'say "foo.bar" =~ /^.*.bar$/ || "FAIL"'
>>> 1
>>>
>>> % perl -WE 'say "foo.bar" =~ /^*.bar$/ || "FAIL"'
>>> ^* matches null string many times in regex; marked by <-- HERE in m/^* <-- HERE .bar$/ at -e line 1.
>>> 1
>>>
>>> % perl -WE 'say "foo.bar" =~ /(^*).bar$/ || "FAIL"'
>>> ^* matches null string many times in regex; marked by <-- HERE in m/(^* <-- HERE ).bar$/ at -e line 1.
>>> 1
>>>
>>> % perl -WE 'say "foo.bar" =~ /^.bar$/ || "FAIL"'
>>> FAIL
>>>
>>> Tested with v5.8.8, v5.14.0, v5.16.0, and v5.17.0-352-g3630f57.
>
>> There might be a case to change this from a warning to a fatal error.
>
>> But its a warning, and as such no, I dont think the pattern should fail:
>
>> $ perl -WE 'say "foo.bar" =~ /^*.bar$/ || "FAIL"' 2>&1 | splain
>> ^* matches null string many times in regex; marked by <-- HERE in m/^* <-- HERE
>> .bar$/ at -e line 1 (#1)
>> (W regexp) The pattern you've specified would be an infinite loop if the
>> regular expression engine didn't specifically check for that. The <-- HERE
>> shows in the regular expression about where the problem was discovered.
>> See perlre.
>
>> 1
>
>> Consider that ^* means "match at the start of the string 0 or more
>> times". So the pattern succeeds because it matches 0 times.
>
>> It warns because it could match at the same spot an infinite number of times.
>
>> All of this is as I expect.
>
> I read is as saying
>
> 1 Match the beginning of the string 0 or more times.
> 2 Match any one non-newline character.
> 3 Match the constant string "bar"
> 4 Match the end of the string, with optional newline slop
No arguments here.
> I think somehow the pointer tracking the position in the string to matched
> got bumped up inappropriately because of the constant-string-at-the-end
> optimization. It fails to account for the "foo" bar, which is why I
> believe it is in error.
Well I can see why there might be confusion here. But consider, do you
think that
perl -Mre=debug -WE 'say "foo.bar" =~ /.bar$/
should fail?
Because to me the two patterns are functionally identical. ^* or ^?
are to me no-ops, so you can just remove them from the pattern when
you analyze what it does.
> But you are saying that it is ok *not* to match the beginning of the
> string, since 0 or more includes 0, and here we did not match the
> beginning of the string at all.
>
> That's pretty darned ugly, is all I can say.
Maybe the helps: (# comments added by me):
$ perl -Mre=debug -WE 'say "foo.bar" =~ /^*.bar$/ || "FAIL"'
Compiling REx "^*.bar$"
^* matches null string many times in regex; marked by <-- HERE in m/^*
<-- HERE .bar$/ at -e line 1.
Final program:
1: CURLYX[0] {0,32767} (5)
3: BOL (4)
4: WHILEM[1/1] (0)
5: NOTHING (6)
6: REG_ANY (7)
7: EXACT <bar> (9)
9: EOL (10)
10: END (0)
floating "bar"$ at 1..1 (checking floating) minlen 4
# so this says that it must find a "bar" at the end of the string
Guessing start of match in sv for REx "^*.bar$" against "foo.bar"
Found floating substr "bar"$ at offset 4...
Starting position does not contradict /^/m...
Guessed: match at offset 3
# So it found "bar" at offset 4, and then decremented the start pos by
1 (because it needs 4 chars to match)
Matching REx "^*.bar$" against ".bar"
3 <foo> <.bar> | 1:CURLYX[0] {0,32767}(5)
3 <foo> <.bar> | 4: WHILEM[1/1](0)
whilem: matched 0 out of 0..32767
3 <foo> <.bar> | 3: BOL(4)
failed...
whilem: failed, trying continuation...
3 <foo> <.bar> | 5: NOTHING(6)
3 <foo> <.bar> | 6: REG_ANY(7)
4 <foo.> <bar> | 7: EXACT <bar>(9)
7 <foo.bar> <> | 9: EOL(10)
7 <foo.bar> <> | 10: END(0)
Match successful!
1
Freeing REx: "^*.bar$"
# And matched successfully at that position.
Yves
--
perl -Mre=debug -e "/just|another|perl|hacker/"
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