a list/re problem

[Ed Keith]

--- On Fri, 12/11/09, Peter Otten <__peter__ at web.de> wrote:

> From: Peter Otten <__peter__ at web.de>
> Subject: Re: a list/re problem
> To: python-list at python.org
> Date: Friday, December 11, 2009, 4:24 PM
> Ed Keith wrote:
> 
> > I have a problem and I am trying to find a solution to
> it that is both
> > efficient and elegant.
> > 
> > I have a list call it 'l':
> > 
> > l = ['asc', '*nbh*', 'jlsdjfdk', 'ikjh',
> '*jkjsdfjasd*', 'rewr']
> > 
> > Notice that some of the items in the list start and
> end with an '*'. I
> > wish to construct a new list, call it 'n' which is all
> the members of l
> > that start and end with '*', with the '*'s removed.
> > 
> > So in the case above n would be ['nbh', 'jkjsdfjasd']
> > 
> > the following works:
> > 
> > r = re.compile('\*(.+)\*')
> > 
> > def f(s):
> >     m = r.match(s)
> >     if m:
> >         return
> m.group(1)
> >     else:
> >         return ''
> > 
> > n =  [f(x) for x in l if r.match(x)]
> > 
> > 
> > 
> > But it is inefficient, because it is matching the
> regex twice for each
> > item, and it is a bit ugly.
> > 
> > I could use:
> > 
> > 
> > n = []
> > for x in keys:
> >     m = r.match(x)
> >         if m:
> >         
>    n.append(m.group(1))
> > 
> > 
> > It is more efficient, but much uglier.
> 
> It's efficient and easy to understand; maybe you have to
> readjust your 
> taste.
> 
> > Does anyone have a better solution?
> 
> In this case an approach based on string slicing is
> probably best. When the 
> regular expression gets more complex you can use a nested a
> generator 
> expression:
> 
> >>> items = ['asc', '*nbh*', 'jlsdjfdk', 'ikjh',
> '*jkjsdfjasd*', 'rewr']
> >>> match = re.compile(r"\*(.+)\*").match
> >>> [m.group(1) for m in (match(s) for s in items)
> if m is not None]
> ['nbh', 'jkjsdfjasd']
> 

I am going to use string slicing, re is the wrong tool for the job. But this is what I was looking for when I posted. Simple, elegant and efficient.

Thanks all,

   -EdK

Ed Keith
e_d_k at yahoo.com

Blog: edkeith.blogspot.com