From: Dave Burt <burtdav@...>
Date: 2004-05-15T13:13:51+09:00
Subject: Re: Regexp Error?

I would have thought, from that logic, that you could just as well expect an
infinite loop ("XXXXXX...") rather than just "XX" - why does /.*/ not keep
matching that same 0-char gap at the end?

"Ara.T.Howard" <ahoward@fattire.ngdc.noaa.gov> wrote in message
news:Pine.LNX.4.44.0405140634490.5586-100000@fattire.ngdc.noaa.gov...
>
> ^ and $ are special and they consume no chars and so are not really
'matched'
> in the same way...
>
> your regex says 'zero or more chars before the end of a string' so you get
>
>
>   ^ 1 2 3 - 4 5 6 $
>   --------------- 
>                  ^
>
> the first go then then scanning starts again - the problem is that it's
then
> looking for something potentially zero widthed followed by something zero
> widthed - which is always going to match (again).  i guess the difference
for
> the second match is that it does not advance the scanner ptr and can
therefore
> know it's done...  it does seem odd, but without that behaviour it would
be
> hard to match empty strings, line boundries, and other zero widthed
things...
> for instance if you did this
>
>   "123-456".gsub(/.*|$/, 'X')
>
> you would expect 'XX', where the second 'X' is inserted into a zero width
> position and '.*' does not include '$' and yet this is realy the same
exact
> behaviour - scanning is done again from the non-space before the end of
line,
> allowing you to finally match '$' which '.*' did not consume.
>
> regexs can be so tricky, i _try_ to use these rules with them
>
>   * always use both ^ and $ (this makes it a lot harder to write the
expression
>     too!)
>
>   * never use .* (or * at all really)
>
> the last is actually pretty important - we use a product here, ldm (local
data
> manager), that scans a huge memeory mapped queue full of data products
matched
> a list of actions against the product tags.  the list of actions use
regexps
> and all of ours had '.*' in them.  top showed the ldm process at around
30%
> cpu - reworking the patterns to not include '.*' dropped it off the rader.
>
> -a
> -- 
>
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>