Add regular expressions lookbehind

Brendan Eich brendan at mozilla.com
Wed Oct 2 09:35:41 PDT 2013


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>
> 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 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
> [2] 
> https://docs.google.com/document/pub?id=1EUHvr1SC72g6OPo5fJjelVESpd4nI0D5NQpF3oUO5UM
> [3] 
> 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
> [5] http://regexadvice.com/forums/thread/85210.aspx
> [6] http://blog.stevenlevithan.com/archives/mimic-lookbehind-javascript
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