From: Raul Parolari <raulparolari@...>
Date: 2007-11-22T17:34:34+09:00
Subject: Re: False positives in editing data

RichardOnRails wrote:

> Hi Raul,
> 
> I like your "battle plan"..
> I especially appreciate your showing me how a regex can be written to
> handle an arbitrary number of dot-separated numbers (rather than hard-
> code distinct sub-expressions).
> 
>>   if  line =~ /^ (.*?) [a-zA-Z] /x
> 
> I thought I could simply remove the question-mark. 
> So, your question mark is clearly working,  but HOW?
> 

Richard

I saw that Gavin has given you (in another thread) a general tutorial on 
this. I add a simpler explanation just in the context of the problem we 
treated;

  .*  means 'as many characters as possible'

Now, the point 1 of the 'battle plan' was (I quote):
"1) we first collect everything until the first letter (not included); 
we
will consider this the Prefix."

So we want to tell the Regexp Engine: "as few characters as possible 
until you see a letter (a-zA-Z), then stop right there!".

Let's examine the 2 expressions, with and without the question mark:

           (.*?)                      [a-zA-Z]
 minimal nr of chars needed until ..  1st letter

           (.*)                       [a-zA-Z]
 as many chars you can get
 possible get away with,  and then ..  a letter

An example:

s="2.1Topic 2.1"

md = s.match( /^ (.*?) [a-zA-Z] /x )
md[1]  # => "2.1"

md = s.match( /^ (.*) [a-zA-Z] /x )
md[1]  # => "2.1Topi"

Have you seen? Both expressions were satisfied, but in different ways:
a) the first (with .*?) tried to find the minimal number of characters 
until the first letter, and so it stopped when it found the 'T' of 
Topic.

b) the second expression tried to find as many characters as possible, 
only bounded by having to then find a letter, so it stopped at the 'c' 
of Topic.

With sense of humour, somebody observed that ".*? values contentment 
over greed"; and since then the ".*?" were called "not greedy", while 
the ".*" were called "greedy".

[I stop here as Gavin described to you '.+" & co].

One advice: the key to learn the regular expression is to read a good 
book (just trying them drives one insane) while experimenting (just 
reading drives one insane too). The time spent pays you back very 
quickly at the first serious exercise (as you can develop a 
'battle-plan' rather than a 'guerrilla war' with the regexps).

I am glad that you found the script useful, and I hope that this helped 
too

Raul
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