Add regular expressions lookbehind

Sebastian Zartner sebastianzartner at
Wed Oct 2 05:04:20 PDT 2013

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

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



On 1 October 2013 00:55, Waldemar Horwat <waldemar at> 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]**pipermail/es-discuss/2010-**
>> November/012164.html<>
>> [2]**document/pub?id=**
>> 1EUHvr1SC72g6OPo5fJjelVESpd4nI**0D5NQpF3oUO5UM<>
>> [3]**questions/12273112/will-js-**
>> regex-ever-get-lookbehind<>
>> [4]**questions/13993793/error-**
>> using-both-lookahead-and-look-**behind-regex<>
>> [5]**thread/85210.aspx<>
>> [6] http://blog.stevenlevithan.**com/archives/mimic-lookbehind-**
>> javascript<>
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