From: dblack@...
Date: 2007-06-22T01:33:40+09:00
Subject: Re: Do You Understand Regular Expressions?

Hi --

On Fri, 22 Jun 2007, Stephen Ball wrote:

> On 6/21/07, dblack@wobblini.net <dblack@wobblini.net> wrote:
> [snip]
>> > So: since * matches "zero or more" characters when it starts the
>> > search for .* it matches the absence (the 'zero') and then matches the
>> > string (the 'or more').
>> 
>> It's the other way around, though; it matches "hello" *first*, and
>> then "".  So the zero-matching (which I admit I'm among those who find
>> unexpected) is happening at the end.
>> 
>
> Ah, but notice:
>
> "hello".scan(/.*$/)
> => ["hello", ""]
>
> "hello".scan(/^.*/)
> => ["hello"]
>
> Strange indeed, but it seems that's how it's working. Although I
> suspect I'm not fully grasping the subtleties introduced by the *
> character.
>
> Hmm, the more I think on it I think I have an answer:
>
> The /^.*/ pattern specifies that the string must start with anything
> (e.g. it must have at least one character) and then zero or more
> characters following.
>
> The /.*$/ pattern has no restriction since the anchor is on the side
> with the * character. So it's parsed as "zero or more of anything
> before the end of the string".
>
> So, if that's correct, you are right that the absence is matched last.
> Verified by the fact that the absence follows the string in the
> pattern match.

Yes, that was what I was mostly going by :-)


David

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