Add regular expressions lookbehind

Jason Orendorff jason.orendorff at
Wed Oct 2 05:15:29 PDT 2013


Here is how I interpret Waldemar's post:

On Mon, Sep 30, 2013 at 5:55 PM, 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 [...]

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.


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