From: "Mariusz Pękala" <skoot@...>
Date: 2007-06-22T19:55:02+09:00
Subject: Re: Do You Understand Regular Expressions?

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On 2007-06-21 23:12:32 +0900 (Thu, Jun), Rob Biedenharn wrote:
> On Jun 21, 2007, at 9:47 AM, Stephen Ball wrote:
> >It's because the pattern /.*/ matches everything, including the
> >absence of everything. Yes, with the proper regexs you can indeed have
> >tea and no tea at the same time. Certainly peculiar, but occasionally
> >useful.
>=20
> That still doesn't really explain why "hello".scan(/.*/) =3D> ["hello", =
=20
> ""]
>=20
> Why wouldn't it be ["hello", "", "", "", "", "", "", "", "", "", "", =20
> "", ... ] since I (or rather the OP) could continue to match zero =20
> characters (bytes) at the end of the string forever?  It does seem =20
> that it might be that a termination condition is checked a bit later =20
> than it should be in this case.

I would say the condition is checked at the right time, it's just the
condition is different: it allows checking a match for empty string
at the end of just-matched string, it does not allow checking empty
string after ampty string.

The interesting behaviour is:

irb(main):035:0> "hello".scan /.*?/
=3D> ["", "", "", "", "", ""]

The /.*?/ matches 'zero or more characters, preferring the shortest
match'. One could ask - where have the actual characters gone?
Note that it's not an infinite loop of empty strings.
After matching 'nothing', the start-position for next match is
increased, skipping one character, to prevent infinite loop of matching
nothing again.

*This* behavour may be considered weird, or buggy, and probably results
are not what was expected.

But look at:

irb(main):038:0> "hello".scan /h(.*)e/
=3D> [[""]]
irb(main):039:0> "hello".scan /h(.*)(.*)(.*)(.*)(.*)e/
=3D> [["", "", "", "", ""]]

Here 'nothing' matches many times, and definitely this *is* the expected
behaviour.



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