Re: gsub bug? 2 results from anchored gsub

["Soni L." <fakedme@...>]

On 27/07/15 12:05 AM, Soni L. wrote:
> On 26/07/15 11:40 PM, Daurnimator wrote:
> > I want to ensure that a string always ends in a single "/".
> > If it has more than one, the extras should be removed
> > If it has none, a "/" should be appended.
> >
> > "/*$" should match all the '/' at the end of the string, and replace
> > them with a single "/".
> > I got an unexpected result:
> >
> >      > ("d//"):gsub("/*$", "/")
> >      d// 2
> >
> > This result suggests that there is an empty string being matched
> > between the last "/" and the end of the string.
> > It's matching the // and replacing that with "/"; but then it gets
> > confused and matches the empty string at the end, and ends up
> > inserting an extra /
> > Using 'print' as the match confirms:
> >
> >      > ("d//"):gsub("/*$", print)
> >      //
> >
> >      d// 2
> >
> > Is this a bug in string.gsub?
> > It seems odd to me that you could get 2 replacements for an anchored 
> > match.
> > Though as far as I can see, a strict reading of the manual doesn't 
> > disallow it.
> >
> >
> > Daurn.
> $ doesn't consume the end of the string?
>
> You'll probably find this issue in most pattern matchers?
I've been writing a pattern matcher lately, so let's look at what it'd 
do (a bit simplified to be easier to read):

Pattern: /*$ ->
Root[GreedyZeroOrMore["/"], EndOfString]

Matcher:

Cursor position: 0
d//
^
Matched /*, cursor position: 0
d//
^
Doesn't match $. Put char on buffer, increment cursor position and repeat.

Cursor position: 1
d//
 ^
Matched /*, cursor position: 3
d//
   ^
Matched $, cursor position: 3
End of pattern, put replacement on buffer (in this case "/"). Repeat.

Cursor position: 3
d//
   ^
Matched /*, cursor position: 3
d//
   ^
Matched $, cursor position: 3
End of pattern, put replacement on buffer (in this case "/"). Start 
cursor position == end cursor position, so advance cursor.

Cursor position: 4
d//
    ^
End of string, return buffer.

So you end up with 2 matches and "d//".

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