why is this regexp invalid: "\\(?1:\\)"

why is this regexp invalid: "\\(?1:\\)"

[Ilya Shlyakhter]

dear emacs users,

can you help me understand why the following simple
expression gives an "Invalid regexp" error:

(string-match "\\(?1:\\)" "")

while removing the explicit group number gives no error:

(string-match "\\(?:\\)" "")

thanks,

ilya

Re: why is this regexp invalid: "\\(?1:\\)"

[Stefan Monnier]

> can you help me understand why the following simple
> expression gives an "Invalid regexp" error:

> (string-match "\\(?1:\\)" "")

> while removing the explicit group number gives no error:

> (string-match "\\(?:\\)" "")

My guess is that you're using a version of Emacs that is too old to
support explicit group numbers (which was a new feature in Emacs-23.1,
IIRC).


        Stefan

Re: why is this regexp invalid: "\\(?1:\\)"

[Joel James Adamson]

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Ilya Shlyakhter <ilya_shl@alum.mit.edu> writes:

> (string-match "\\(?1:\\)" "")
> (string-match "\\(?:\\)" "")

I get 0 from both.

Emacs version: GNU Emacs 24.0.50.3 (x86_64-unknown-linux-gnu, GTK+
Version 2.20.1) of 2010-08-31 on chondestes.bio.unc.edu

Joel

-- 
Joel J. Adamson
FSF Member #8164
http://www.unc.edu/~adamsonj

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Re: why is this regexp invalid: "\\(?1:\\)"

[Pascal J. Bourguignon]

Joel James Adamson <adamsonj@email.unc.edu> writes:

> Ilya Shlyakhter <ilya_shl@alum.mit.edu> writes:
>
>> (string-match "\\(?1:\\)" "")
>> (string-match "\\(?:\\)" "")
>
> I get 0 from both.

Which is expected.  These regexps are not invalid, they're perfectly
valid, and match perfectly what they have to match.

Read (info "(elisp)Regexp Backslash").

Notice that the two regexps have a different meaning.

"\\(?1:\\)" matches exactly 0 characters, and assign them the group number 1.

"\\(?:\\)" matches exactly 0 characters, if it can (it's shy), and
assign them the next group number (which is 1 since it's the first
group).


In both case, when matching the empty string, the result should be 0,
since there are 0 character at the position 0 in the empty string.


-- 
__Pascal Bourguignon__                     http://www.informatimago.com/

Re: why is this regexp invalid: "\\(?1:\\)"

[David Kastrup]

pjb@informatimago.com (Pascal J. Bourguignon) writes:

> Joel James Adamson <adamsonj@email.unc.edu> writes:
>
>> Ilya Shlyakhter <ilya_shl@alum.mit.edu> writes:
>>
>>> (string-match "\\(?1:\\)" "")
>>> (string-match "\\(?:\\)" "")
>>
>> I get 0 from both.
>
> Which is expected.  These regexps are not invalid, they're perfectly
> valid, and match perfectly what they have to match.
>
> Read (info "(elisp)Regexp Backslash").
>
> Notice that the two regexps have a different meaning.
>
> "\\(?1:\\)" matches exactly 0 characters, and assign them the group number 1.
>
> "\\(?:\\)" matches exactly 0 characters, if it can (it's shy), and
> assign them the next group number (which is 1 since it's the first
> group).

I recommend you check again what a shy group is supposed to be.

> In both case, when matching the empty string, the result should be 0,
> since there are 0 character at the position 0 in the empty string.

That much is correct.

-- 
David Kastrup

Re: why is this regexp invalid: "\\(?1:\\)"

[Pascal J. Bourguignon]

David Kastrup <dak@gnu.org> writes:

> pjb@informatimago.com (Pascal J. Bourguignon) writes:
>
>> Joel James Adamson <adamsonj@email.unc.edu> writes:
>>
>>> Ilya Shlyakhter <ilya_shl@alum.mit.edu> writes:
>>>
>>>> (string-match "\\(?1:\\)" "")
>>>> (string-match "\\(?:\\)" "")
>>>
>>> I get 0 from both.
>>
>> Which is expected.  These regexps are not invalid, they're perfectly
>> valid, and match perfectly what they have to match.
>>
>> Read (info "(elisp)Regexp Backslash").
>>
>> Notice that the two regexps have a different meaning.
>>
>> "\\(?1:\\)" matches exactly 0 characters, and assign them the group number 1.
>>
>> "\\(?:\\)" matches exactly 0 characters, if it can (it's shy), and
>> assign them the next group number (which is 1 since it's the first
>> group).
>
> I recommend you check again what a shy group is supposed to be.

Right.  I confounded it with non-greedy.

"\\(?:\\)" matches exactly 0 characters, and do not assign them to any
group number.


>> In both case, when matching the empty string, the result should be 0,
>> since there are 0 character at the position 0 in the empty string.
>
> That much is correct.
>
> -- 
> David Kastrup

-- 
__Pascal Bourguignon__                     http://www.informatimago.com/