Suggested RegExp Improvements

Brendan Eich brendan at
Fri Nov 12 15:04:08 PST 2010

On Nov 12, 2010, at 2:52 PM, Marc Harter wrote:

> After considering all the breadth this discussion could take maybe it
> would be wise to just focus on one issue at a time.  For me, the biggest
> missing feature is lookbehind.  Its common to most languages
> implementing the Perl-RegExp-syntax, it is very useful when looking for
> patterns that follow or don't follow a particular pattern.  I guess I'm
> confused why lookahead made it in but not lookbehind.

This was 1998, Netscape 4 work I did in '97 was based on Perl 4(!), but we proposed to ECMA TC39 TG1 (the JS group -- things were different then, including capitalization) something based on Perl 5. We didn't get everything, and we had to rationalize some obvious quirks.

I don't remember lookbehind (which emerged in Perl 5.005 in July '98) being left out on purpose. Waldemar may recall more, I'd handed him the JS keys inside to go do

If you are game to write a proposal or mini-spec (in the style of ES5 even), let me know. I'll chat with other TC39'ers next week about this.


> What do people
> think about including this feature?
> Marc
> On Fri, 2010-11-12 at 16:20 -0600, Marc Harter wrote:
>> I will start out with a disclaimer.  I have not read both ECMAScript
>> specifications for 3 and now 5, so I admit that I am not an expert in
>> the spec itself but as I user of JavaScript, I would like to get some
>> expert discussion over this topic as proposed enhancements to the
>> RegExp engine for Harmony.
>> I will start with a list of lacking features in JS as compared to Perl
>> provided by (
>>      * No \A or \Z anchors to match the start or end of the string.
>>        Use a caret or dollar instead. 
>>      * Lookbehind is not supported at all. Lookahead is fully
>>        supported. 
>>      * No atomic grouping or possessive quantifiers 
>>      * No Unicode support, except for matching single characters with
>>        \uFFFF 
>>      * No named capturing groups. Use numbered capturing groups
>>        instead. 
>>      * No mode modifiers to set matching options within the regular
>>        expression. 
>>      * No conditionals. 
>>      * No regular expression comments. Describe your regular
>>        expression with JavaScript // comments instead, outside the
>>        regular expression string. 
>> I don't know if all of these "need" to be in the language but there
>> have been some that I have personally wanted to use:
>>      * Lookbehind!  ECMAScript fully supports lookahead, why not
>>        lookbehind?  Seems like a big hole to me. 
>>      * Named capturing groups and comments (e.g.
>>  Mostly I argue for this because
>>        it makes RegExp matches more self-documenting.  Regular
>>        Expressions are already cryptic as it is.
>> I do like some of the new flags proposed in
>> ( but personally haven't used them but maybe
>> that is something also for discussion.
>> Marc Harter
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