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From: rh <richard_hubbe11@lavabit.com>
Subject: Re: regex walktrough
Date: Sat, 8 Dec 2012 14:19:40 -0800
References: <20121208094813.5560b42fb233329a44b05dee@lavabit.com> <50C38224.3040201@mrabarnett.plus.com>
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Xref: csiph.com comp.lang.python:34504

On Sat, 08 Dec 2012 18:08:36 +0000
MRAB <python@mrabarnett.plus.com> wrote:

> On 2012-12-08 17:48, rh wrote:
> >   Look through some code I found this and wondered about what it
> > does: ^(?P<salsipuedes>[0-9A-Za-z-_.//]+)$
> >
> > Here's my walk through:
> >
> > 1) ^ match at start of string
> > 2) ?P<salsipuedes> if a match is found it will be accessible in a
> > variable salsipuedes
> > 3) [0-9A-Za-z-_.//] this is the one that looks wrong to me, see
> > below
> > 4) + one or more from the preceeding char class
> > 5) () the grouping we want returned (see #2)
> > 6) $ end of the string to match against but before any newline
> >
> >
> > more on #3
> > the z-_ part looks wrong and seems that the - should be at the start
> > of the char set otherwise we get another range z-_ or does the a-z
> > preceeding the z-_ negate the z-_ from becoming a range?  The "."
> > might be ok inside a char set. The two slashes look wrong but maybe
> > it has some special meaning in some case? I think only one slash is
> > needed.
> >
> > I've looked at pydoc re, but it's cursory.
> >
> Python itself will help you:
> 
>  >>> re.compile(r"^(?P<salsipuedes>[0-9A-Za-z-_.//]+)$",
>  >>> flags=re.DEBUG)
> at at_beginning
> subpattern 1
>    max_repeat 1 65535
>      in
>        range (48, 57)
>        range (65, 90)
>        range (97, 122)
>        literal 45
>        literal 95
>        literal 46
>        literal 47
>        literal 47
> at at_end
> 
> Inside the character set: "0-9", "A-Z" and "a-z" are ranges; "-", "_",
> "." and "/" are literals. Doubling the "/" is unnecessary (it has no
> special meaning). "-" is a literal because it immediately follows a
> range, so it can't be defining another range (if it immediately
> followed a literal and wasn't immediately followed by an unescaped "]"
> then it would, so r"[a-]" is the same as r"[a\-]").

Handy tip there, thanks.

re.compile(r"^(?P<salsipuedes>[-\w./]+)$", flags=re.DEBUG)
at at_beginning
subpattern 1
  max_repeat 1 65535
    in
      literal 45
      category category_word
      literal 46
      literal 47
at at_end

I reduced the expression too. Now I wonder why re.DEBUG doesn't unroll
category_word. Some other re flag?

> 
> As for "(?P<salsipuedes>...)", it won't be accessible in a variable
> "salsipuedes", but will be accessible as a named group in the match
> object:
> 
>  >>> m = re.match(r"(?P<foo>[a-z]+)", "xyz")
>  >>> m.group("foo")
> 'xyz'
> 

Ok, "named group" it is.