Add regular expressions lookbehind

Sebastian Zartner sebastianzartner at gmail.com
Wed Oct 2 13:07:51 PDT 2013


I can try to help, though my time and technical knowledge about regexp
implementations are limited. So wouldn't someone of the people, who already
implemented a working regexp engine, be a better help? Did somebody of you
ever contact the e.g. the people behind PCRE?

Sebastian


On 2 October 2013 18:35, Brendan Eich <brendan at mozilla.com> wrote:

> I think this is right, we need help and are open to it from someone who
> can lift the weight. Waldemar may be able to help, I'm not sure how much
> (but he wrote the ES3 regexp spec, so at least as reviewer he is a key part
> of the solution).
>
> Sebastian, if you have time to help, that would be tremendous.
>
> /be
>
>  Jason Orendorff <mailto:jason.orendorff at gmail.**com<jason.orendorff at gmail.com>
>> >
>> October 2, 2013 5:15 AM
>>
>> Sebastian,
>>
>> Here is how I interpret Waldemar's post:
>>
>> On Mon, Sep 30, 2013 at 5:55 PM, 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 [...]
>>>
>>
>> This is the real problem. No one has taken on the work. It's a fair
>> amount of very technical work.
>>
>>  [...] 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?
>>>
>>
>> Sebastian, as you pointed out, these technical points have all been
>> addressed one way or another in practice. My guess is that TC39
>> *would* accept some commonly-implemented lookbehind behavior, if
>> someone put up the work. I could be wrong.
>>
>> -j
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>>
>> Waldemar Horwat <mailto:waldemar at google.com>
>> September 30, 2013 3:55 PM
>> 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
>>
>>
>>
>>
>>
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>>
>> Sebastian Zartner <mailto:sebastianzartner@**gmail.com<sebastianzartner at gmail.com>
>> >
>> September 28, 2013 1:54 PM
>>
>> 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>
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