hi Danny,

ah yes, I have seen Ping at various parties (and wearing a PythonLabs
shirt no less!).  But I digress.  I am still confused why you provided for
a negative lookahead.  I looked at amk's definition of a negative lookahead,
and it seems to say the regex will not match if the negative lookahead
condition is met.  So:

> >> testsearch = re.compile('tetsuro(?!hello)', re.IGNORECASE)
> >> testsearch.search('tetsurohello')
> >> testsearch.search('hitetsuroone')
<_sre.SRE_Match object at 0x860e4a0> 

Now in the case of:

> >> myre = re.compile(r'http://[\w\.-]+\.?(?![\w.-/])')

you are looking for 'http://' then one or more word characters, periods
and hyphens, and an optional period and then a negative lookahead of a
word character, any character, a hyphen and a forward slash.  Granted,
your regex may have been sloppy example and you might have meant a
negative lookahead of a word character, a period, a hyphen and a forward
slash.  I still do not understand why you provided for one, and if you had
a good reason, why the sfgate url would match at all, since you clearly
had a word character, period, or hyphen following the set of characters
you were allowing for, including the optional period.  Here is an example
of something similar that perplexes:

> >> testsearch = re.compile(r'tetsuro\.?(?!hello)', re.IGNORECASE)
> >> testsearch.search('tetsurohello')
> >> testsearch.search('tetsuro.hello')
<_sre.SRE_Match object at 0x8612028> 
> >> match = testsearch.search('tetsuro.hello')
> >> match.group()
'tetsuro'
> >> match = testsearch.search('tetsuro..hello')
> >> match.group()
'tetsuro.'

Why wasn't the first period caught ?

On Mon, 30 Jun 2003, Danny Yoo wrote:

> 
> 
>  On Mon, 30 Jun 2003 tpc@[...].edu wrote:
> 
>  > hi Danny, I had a question about your quick intro to Python lesson sheet
>  > you gave out back in March 2001.
> 
>  Hi tpc,
> 
> 
> 
>  Some things will never die.  *grin*
> 
> 
> 
>  > The last page talks about formulating a regular expression to handle
>  > URLs, and you have the following:
>  >
>  > myre = re.compile(r'http://[\w\.-/]+\.?(?![\w.-/])')
> 
> 
> 
>  Ok.  Let's split that up using verbose notation:
> 
> 
>  ###
>  myre = re.compile(r'''http://            ## protocol
>                        [\w\.-/]+          ## followed by a bunch of "word"
>                                           ## characters
> 
>                        \.?                ## followed by an optional
>                                           ## period.
> 
>                        (?!                ## Topped with a negative
>                                           ## lookahead for
>                              [\w.-/]      ## "word" character.
> 
>                        )''', re.VERBOSE)
>  ###
> 
> 
>  The page:
> 
>      http://www.python.org/doc/lib/re-syntax.html
> 
>  has more details about some of the regular expression syntax.  AMK has
>  written a nice regex HOWTO here:
> 
>      http://www.amk.ca/python/howto/regex/regex.html
> 
>  which you might find useful.
> 
> 
> 
> 
>  > I understand \w stands for any word character, and \. means escaped
>  > period, and ? means zero or one instances of a character or set.  I did
>  > a myre.search('http://www.hotmail.com') which yielded a result, but I am
>  > confused as to why
>  >
>  > myre.search('http://www.sfgate.com/cgi-bin/article.cgi?f=/gate/archive/2003/06/29/gavin2
9.DTL')
>  >
>  > would work, since there is a '=' and you don't provide for one in the
>  > regular expression.
> 
> 
>  Very true.  It should, however, match against the negative lookahead ---
>  the regex tries to look ahead to see that it can match something like:
> 
>      "This is an url: http://python.org.  Isn't that neat?"
> 
> 
>  The negative lookup should match right here:
> 
>      "This is an url: http://python.org.  Isn't that neat?"
>                                         ^
> 
>  In your url above, the negative lookahead should actually hit the question
>  mark first before it sees '='.  That regex was a sloppy example; I should
>  have been more careful with it, but I was in a hurry when I wrote that
>  intro...  *grin*
> 
> 
> 
>  If you're in the Berkeley area, by the way, you might want to see if
>  Ka-Ping Yee is planning another CS 198 class in the future:
> 
>      http://zesty.ca/bc/info.html
> 
> 
> 
> 
> 
>  Anyway, we can experiment with this more easily by introducing a group
>  into the regular expression:
> 
>  ###
>  myre = re.compile(r'''
>                      (                  ## group 1
> 
>                        http://            ## protocol
>                        [\w\.-/]+          ## followed by a bunch of "word"
>                                           ## characters
> 
>                        \.?                ## followed by an optional
>                                           ## period.
> 
>                      )                  ## end group
> 
> 
>                        (?!                ## Topped with a negative
>                                           ## lookahead for
>                              [\w.-/]      ## "word" character.
> 
>                        )''', re.VERBOSE)
>  ###
> 
> 
> 
>  Let's check it now:
> 
>  ###
>  >>> match =
>  myre.search('http://www.sfgate.com/cgi-bin/article.cgi?f=/gate/archive/2003/06/29/gavin29.
DTL')
>  >>> match.group(1)
>  'http://www.sfgate.com/cgi'
>  ###
> 
> 
> 
>  Oiii!  The regular expression is broken.  What has happened is that I've
>  incorrectly defined the hyphen in the character group.  That is, instead
>  of
> 
> 
>      [\w\.-/]+
> 
> 
>  I should have done:
> 
>      [\w./-]+
> 
> 
>  instead, to keep the regex engine from treating the hyphen as a span of
>  characters (like "[a-z]", or "[0-9]").  You can then introduce the other
>  characters into the "word" character class, and then it should correctly
>  match the sfgate url.
> 
> 
> 
>  I hope this helps!
> 
> 


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