Regex on substrings

Peter van der Zee ecma at
Thu Jun 2 10:32:52 PDT 2011

On Thu, Jun 2, 2011 at 7:17 PM, Mike Samuel <mikesamuel at> wrote:
> 2011/6/2 Peter van der Zee <ecma at>:
>> A problem I faced recently is the inability to apply regular
>> expressions to a substring of a string without explicitly taking the
>> substring first. So I'm wondering how much trouble it would be to
>> extend the RegExp api to this...
>> RegExp.prototype.test = function(string[, start=0[, stop=string.length]]){ };
>> RegExp.prototype.exec = function(string[, start=0[, stop=string.length]]){ };
> What do the ^ and $ assertions in non-multiline mode mean when start
> and stop are not [0, string.length)?
> Do they match at the beginning and end of the substring?

They mean exactly the same as they would do for
regex.test(string.substring(start, stop))

>> Optionally, it might be handy to have .test return a number,
>> indicating the length of the (first) match. If zero, there was no
>> match. This would however break with scripts that explicitly check for
>> === false.
> Using zero to indicate that there is no match will conflate zero
> length matches with no match.
> Consider
>    /\b/.test("a-b") === true

That's a good point. It suppose could return false, but that wouldn't
be very backcompat for these cases. It could return -1 but that would
definately be backcompat breaking.

Well, it would have been nice. But if the tax on returning match
length for .test is too big I'm not feeling that strong about it

- peter

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