Regular expressions

[Seymore4Head]

On Thu, 05 Nov 2015 11:54:20 +1100, Steven D'Aprano
<steve at pearwood.info> wrote:

>On Thu, 5 Nov 2015 10:02 am, Seymore4Head wrote:
>
>> So far the only use I have for regex is to replace slicing, but I
>> think it is an improvement.
>
>I don't understand this. This is like saying "so far the only use I have for
>a sandwich press is to replace my coffee pot". Regular expressions and
>slicing do very different things.
>
>Slicing extracts substrings, given known starting and ending positions:
>
>
>py> the_str = "Now is the time for all good men..."
>py> the_str[7:12]
>'the t'
>
>
>Regular expressions don't extract substrings with known start/end positions.
>They *find* matching text, giving a search string with metacharacters. (If
>there are no metacharacters in your search string, you shouldn't use a
>regex. str.find will be significantly faster and more convenient.)
>
>Slicing is not about finding text, it is about extracting text once you've
>already found it. So they are complementary, not alternatives.

Here is an example of the text we are slicing apart.

>From stephen.marquard at uct.ac.za Sat Jan  5 09:14:16 2008
Return-Path: <postmaster at collab.sakaiproject.org>
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From: stephen.marquard at uct.ac.za

The practice problems are something like pull out all the email
addresses or pull out the days of the week and give the most common.