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Re: Do nothing? or Do something important in a very obscure way?

by blakem (Monsignor)
on Aug 23, 2002 at 10:49 UTC ( [id://192299]=note: print w/replies, xml ) Need Help??


in reply to Do nothing? or Do something important in a very obscure way?

/x?/ means match x zero or one times, but prefer 1...
i.e. Match it if you can

/x??/ means match x zero or one times, but prefer 0...
i.e. Match it if you have to

For instance: 

% perl -le '"wxy" =~ /(wx?)/;   print $1'
1:wx
% perl -le '"wxy" =~ /(wx??)/;  print $1'
1:w

% perl -le '"wxy" =~ /(wx?y)/;  print $1'
1:wxy
% perl -le '"wxy" =~ /(wx??y)/; print $1'
1:wxy

[download]
The notation is a little confusing because the two question marks in a row mean different things. /x?/ is really just syntatic sugar for /x{0,1}/. The second question mark turns on min matching. When you look at it this way, the difference between /x{0,1}/ and /x{0,1}?/ should be more clear.

-Blake

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Re: Re: Do nothing? or Do something important in a very obscure way?
by BrowserUk (Patriarch) on Aug 23, 2002 at 10:58 UTC

    Makes sense in terms of Perl5 regex's, but in the original context of Exegesis5 where the 'x??' and 'x*?' are followed by the Perl6 'don't backtrack' ':' operator, I am not so sure?

    What's this about a "crooked mitre"? I'm good at woodwork!
[reply]
      Ah, I see now.... /x??:/ will behave exactly like /x{0}/
      In english /x??:/ would read something like this:

      Match 'x' zero or one times, prefer 0 over 1, lock in the first value that allows this atom to match (i.e. don't backtrack).

      Since the 0 will always cause the atom to match, the engine will never be allowed to try the 1.

      I know thats not the best description.... hope it helps.

      -Blake

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