As I understand it the problem with lookbehinds are not parentheses but
variable lengths. Therefore many regular expression flavors only allow
fixed-length lookbehinds[1], which could also be the restriction within
ECMAScript for now. This restriction could then be addressed at a later
point.

> Backtrack into capturing results?
Seems like at least PCRE does that. for parenthesed parts within
lookbehinds.

Sebastian

[1] http://www.regular-expressions.info/lookaround.html#limitbehind


On 1 October 2013 00:55, Waldemar Horwat <waldemar at google.com> wrote:

> No one has yet submitted a well-defined proposal for lookbehinds on the
> table.  Lookbehinds are difficult to translate into the language used by
> the spec and get quite fuzzy when the order of evaluation of parts of the
> regexp matters, which is what happens if capturing parentheses are
> involved.  Where do you start looking for the lookbehind?  Shortest first,
> longest first, or reverse string match?  Greedy or not?  Backtrack into
> capturing results?
>
>     Waldemar
>
>
>
> On 09/28/2013 01:54 PM, Sebastian Zartner wrote:
>
>> I wonder if the discussion about lookbehinds[1] and Marc Harter's
>> proposal for them[2] in the past led to anything.
>> I'd really like to see these implemented in ECMAScript specification and
>> it seems I am not the only one.[3][4][5] This even caused people to try to
>> mimic them.[6]
>> So I wanted to pick up the discussion again and ask, what info was
>> missing that they didn't get specified?
>>
>> Sebastian
>>
>> [1] https://mail.mozilla.org/**pipermail/es-discuss/2010-**
>> November/012164.html<https://mail.mozilla.org/pipermail/es-discuss/2010-November/012164.html>
>> [2] https://docs.google.com/**document/pub?id=**
>> 1EUHvr1SC72g6OPo5fJjelVESpd4nI**0D5NQpF3oUO5UM<https://docs.google.com/document/pub?id=1EUHvr1SC72g6OPo5fJjelVESpd4nI0D5NQpF3oUO5UM>
>> [3] http://stackoverflow.com/**questions/12273112/will-js-**
>> regex-ever-get-lookbehind<http://stackoverflow.com/questions/12273112/will-js-regex-ever-get-lookbehind>
>> [4] http://stackoverflow.com/**questions/13993793/error-**
>> using-both-lookahead-and-look-**behind-regex<http://stackoverflow.com/questions/13993793/error-using-both-lookahead-and-look-behind-regex>
>> [5] http://regexadvice.com/forums/**thread/85210.aspx<http://regexadvice.com/forums/thread/85210.aspx>
>> [6] http://blog.stevenlevithan.**com/archives/mimic-lookbehind-**
>> javascript<http://blog.stevenlevithan.com/archives/mimic-lookbehind-javascript>
>>
>>
>> ______________________________**_________________
>> es-discuss mailing list
>> es-discuss at mozilla.org
>> https://mail.mozilla.org/**listinfo/es-discuss<https://mail.mozilla.org/listinfo/es-discuss>
>>
>>
>
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As I understand it the problem with lookbehinds are not parentheses but
variable lengths. Therefore many regular expression flavors only allow
fixed-length lookbehinds[1], which could also be the restriction within
ECMAScript for now. This restriction could then be addressed at a later
point.

> Backtrack into capturing results?

Seems like at least PCRE does that. for parenthesed parts within
lookbehinds.

[1]: http://www.regular-expressions.info/lookaround.html#limitbehind