From: Raul Parolari <raulparolari@...>
Date: 2007-11-27T10:36:20+09:00
Subject: Re: False positives in editing data

RichardOnRails wrote:

> 
> I forgot to tell you that I finally understand your second example.
> 
>> md = s.match( /^ (.*) [a-zA-Z] /x )
>> md[1]  # => "2.1Topi"
> 
> Without the question mark, in principal, the ".* initially consumes
> all the characters,  but then it sees the match fails, because there's
> no match for the "[a-zA-Z]".  So the ".*" sort of "backs off" and
> satisfies it self with "2.1Topi", leaving the "c" to satisfy "[a-zA-
> Z]".

Very good, Richard!
It is a question on when one is 'content'; if you need a metaphor to 
remember it, think of a WallStreet banker (.*, .+) vs a Franciscan monk 
(.*?, .+?)
:-)

> The one I like settled on is:
> 
> s="2.1Topic 2.1"
> md = s.match( /^ ([\.\d]*) [^\.\d] /x )
>   #md[0]=2.1T
>   #md[1]=2.1

I see that you have solved the problem in your previous post (that I 
could not reply to), when you wrote (removing all other code):

s = "2.002.1Topic 2.2.1"

s =~ /^ (\d+[.]?)+  [^\.\d] /x

I must confess: I was stunned myself that it did not work; foolish of 
us, in fact it was working, but you failed to collect the bounty! you 
needed parenthesis to include the '+'!

s =~ /^ ((\d+[.]?)+)  [^\.\d] /x

p $1, 2 # => "2.002.1", "1"

However it is better to avoid collecting also the inner results as they 
overwrite each other in $2 and then confuse us (that's the reason that 
you saw the last digit captured above..); so let's use the '?:' trick, 
to avoid writing in $2, where instead we will capture the 'non 
digits/dots' that come after:

s =~ /^ ((?:\d+[.]?)*)  ([^\.\d]+) /x

p $1, $2  # => "2.002.1", "Topic "

Do you see it? I think you do. Now, to finish, let's examine how you 
solved the problem in this post:

> s="2.1Topic 2.1"
> md = s.match( /^ ([.\d]*) [^\.\d] /x )

Ah, you resorted to 'pragmatism'.. you said: "the bloody '\d+[.]?)+' 
does not work, so I will change it". This was ok, but do you see the 
difference between:

   ((?:\d+[.]?)*)   # I changed + -> * to compare

   ([\d[.]]*)

aside that the second one is easier to read? (you may want to stop 
reading and think about this as this is your test to graduate from 
"intermediate level regexp" :-)

Ok: if they could speak, they would say respectively:
1) I want 0 or more sequences of (digits followed optionally by a dot)
2) I want 0 or more combinations of digits and dots as they come

Do you see?
both would match: "2.002.1" but the 2nd would also match "...1..37"!

The last question you had was: how do I pick up the digits once I 
collected the "2.002.1"? Study scan in Pickaxe and then do:

str = "2.002.1"

str.scan(/ (\d+) /x) # => [["2"], ["002"], ["1"]]


All right, let's call it a Regexp day,

Raul


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