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From: Tim Chase <python.list@tim.thechases.com>
Newsgroups: comp.lang.python
Subject: Re: Regular expressions
Date: Wed, 4 Nov 2015 20:48:15 -0600
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On 2015-11-05 13:28, Steven D'Aprano wrote:
> > I tried Tim's example
> > 
> > $ seq 5 | grep '1*'
> > 1
> > 2
> > 3
> > 4
> > 5
> > $  
> 
> I don't understand this. What on earth is grep matching? How does
> "4" match "1*"?

The line with "4" matches "zero or more 1s".  If it was searching for
a literal "1*" (as would happen with fgrep or "grep -F"), it would
return no results:

  $ seq 5 | fgrep '1*'
  $

-tkc